GEOLOGY OF SAIPAN MARIANA ISLANDS PART 2. PETROLOGY AND SOILS

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• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP81-01043R002500120004-3
• Release Decision: RIPPUB
• Original Classification: K
• Document Page Count: 66
• Document Creation Date: December 27, 2016
• Document Release Date: October 22, 2013
• Sequence Number: 4
• Publication Date: January 1, 1957
• Content Type: REPORT

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Geology of Saipan
Mariana Islands
Part 2. Petrology and Soils:
STAT
GEOLOGICAL SURVEY PROFESSIONAL PAPER 280-B-D
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Geology of Saipan
Mariana Islands
GEOLOGICAL SURVEY PROFESSIONAL PAPER
Chapter B. Petrology of the Volcanic Rocks
By ROBERT GEORGE SCHMIDT
Chapter C. Petrography of the Limestones
By J. HARLAN JOHNSON
Chapter D. Soils
By RALPH J. McCRACKEN
UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1957
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UNITED STATES DEPARTMENT OF THE INTERIOR
Fred A. Seaton, Secretary
GEOLOGICAL SURVEY
Thomas B. Nolan, Director
For sale by the Superintendent of Documents, U. S. Government Printing Office
Washington 25, D. C.
CONTENTS OF PART 2
Page
Chapter B. Petrology of the Volcanic Rocks 127
Chapter C. Petrography of the Limestones 177
Chapter D. Soils 189
In
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0.11L le-at
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;
Abstract
Introduction
Previous petrologic investigations
Acknowledgments
Classification of rocks
Mineralogy
Primary minerals
Plagioclase feldspar
Alkali feldspar
Silica minerals
Pyroxenes
Hornblende
Accessory minerals
Biotite
Magnetite
Ilmenite
Hematite
Rutile
Apatite
Zeolites
Alteration minerals
Petrography
Dacites
Dacite
Daeite vitrophyre and perlite
Hornblende-bearing dacite porphyry
Andesites
Augite-hypersthene andesite
General features
Phenocrysts
Groundmass
Alteration
CONTENTS
Page
146
146
147
148
149
150
153
153
154
156
158
160
161
162
163
163
163
163
165
165
165
166
170
172
172
173
175
Page
127
127
130
131
131
132
132
132
133
134
135
138
138
138
138
138
138
138
138
138
139
139
139
139
141
142
143
143
143
143
144
145
Petrography?Continued
Andesites?Continued
Augite-hypersthene andesite?Continued
Secondary rock types
Quartz-bearing augite-hypersthene an-
desite
Quartz-bearing augite-hypersthene an-
desite porphyry..
A ugite andesite
Hypersthene andesite
Chemical composition of the major rock types
Comparison with volcanic rocks of other Pacific islands
and with Daly's average rock types
Tinian, Rota, and Guam
Palau, Yap, and Bonin Islands .
Northern Mariana and Volcano Islands
Izu Peninsula region of Japan and Izu Islands_
Hawaiian Islands
Daly's average rock types
Summary and conclusions
Petrogenesis
Compositional variation of the rocks
Variations between and within major rock types_
Comparison between bulk and groundmass com-
position of porphyritic andesites and dacites_
Evidence of contamination
Origin of the rocks
Nature of a parent magma
Fractional crystallization and assimilation
Relationship of volcanism to the development
of the Mariana arc_
Conclusions
The petrogenetic significance of the andesite line
Literature cited
Index
1
,
ILLUSTRATIONS
Plates 2, 4 In pocket, plates 26-30 follow Index]
Pt,ATR 2. Generalized geologic map and sections of Saipan, Mariana Islands.
4. Locality-finding map of Saipan.
26-27. Photomicrographs of dacites from Saipan.
28-30. Photomicrographs of andesites from Saipan.
FIGURE
11. Index map of the western north Pacific Ocean
12. Simplified bathymetric chart of the Mariana arc_
13. Composition diagram of normative feldspar of analyzed andesites from
and Daly's average rock types
14. Composition diagram of phenocryst pyroxenes of andesites from Saipan, groundmass pyroxene of andesites from
Saipan, and normative pyroxene of analyzed andesites from Saipan
Saipan,
analyzed
dacites from Saipan,
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VII
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128
129
134
136
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CONTENTS
FIGURE 15. Composition diagram of phenocryst pyroxenes, groundmass pyroxene, and normative pyroxene of analyzed
basalts and andesites from the Izu Peninsula region of Japan and the Izu Island
16. Harker variation diagram of andesites and dacites from Saipan
17. Triangular ACF and SKM diagrams of andesites and dacites from Saipan
18. Triangular ACF and SKM diagrams of andesites and dacites from Saipan and volcanic rocks from Guam, the
Palau Islands, and the Bonin Islands
19. Triangular ACF and SKM diagrams of andesites and (Incites from Saipan and volcanic rocks from the northern
Mariana and Volcano Islands
20. Triangular ACF and SKM diagrams of andesites and dacites from Saipan, volcanic rocks from the Izu Peninsula
region of Japan and the Izu Islands, and average rocks of the Hawaiian Islands
21. Triangular ACF diagram of average andesite and dacite of Saipan, average groundmass of andesite from Saipan,
average basalt of the Izu Peninsula region of Japan, and Daly's average rock types
22. Triangular SKM diagram of average andesite and dacite of Saipan, average groundmass of andesite and dacite
from Saipan, average basalt of the Izu Peninsula region of Japan, and Daly's average rock types
23. Composition diagram of normative feldspar of average andesite and dacite of Saipan, average basalt of the
northern Mariana Islands, average olivine basalt of the Hawaiian Islands, and average basalt of the Ian
Peninsula region of Japan 166
24. Position of dacites of Saipan with respect to the low-temperature trough of the system nepheline-kaliophilite-
silica 170
Page
137
150
152
155
156
161
162
162
TABLES
Page
TABLE 1. Volcanic formations of Saipan 130
2. Specific gravity and composition of plagioclase feldspar phenocrysts from various andesites of the Hagman forma-
tion, Saipan 132
3. Optical properties and composition of pyroxenes in andesites from Saipan 136
4. Estimated mineral composition of the principal volcanic-rock types of Saipan 110
5. Chemical analyses and norms of volcanic rocks from Saipan and Guam 151
6. Chemical analyses and norms of volcanic rocks from the northern Mariana Islands 157
7. Sequence of Tertiary volcanic rocks of the Izu Peninsula region, Japan.. 159
8. Sequence of Quaternary volcanic rocks of the Izu Peninsula region, Japan _ 160
9. Volume percent of phenocrysts, bulk chemical composition, and calculated composition of the groundmass of
analyzed porphyritic andesites and dacites from Saipan 16-1
10. Average chemical composition of olivine basalt from the Hawaiian Islands and basalts of -.1 holeiitic magma type
from various parts of the world .. ? 167
11. Composition of material subtracted from average basalt of Izu to yield average andesite and dacite of Saipan, and
composition of material added to average basalt of Izu to yield average andesite of Saipan 167
12. Composition of material subtracted from and added to average andesite of Saipan to yield average dacite of
Saipan
_ 169
Summary of the geologic units of Saipan
3r;
CHART
Page
in pocket
GEOLOGY OF SAIPAN, MARIANA ISLANDS
PETROLOGY OF THE VOLCANIC ROCKS
By ROBERT GEORGE SCHMIDT
ABSTRACT
The rocks that comprise the volcanic formations of Saipan
are of two principal types: dacites, which are characteristically
glassy, and andesites, which are comparatively crystalline. The
(Incites consist primarily of snide glass, oligoclase, and silica
minerals (quartz, tridymite, cristobalite, chalcedony, and opal).
Minor constituents in these rocks are green hornblende, biotite,
magnetite, and hematite. The andesites are composed princi-
pally of labradorite, hypersthene, augite, and subcalcie augite.
Minor but also characteristic constituents of the andesites are
quartz, tridymite, cristobalite, anorthoclase, and accessory mag-
netite, ilmenite, rutile, and apatite. Nine varieties of dacite
and andesite are recognized on the basis of chemical composi-
tion, mineralogy, and texture. These are dacite, dacite vitro-
phyre, dacite perlite, hornblende-bearing dacite porphyry, aug?-
ite-hypersthene andesite, quartz-bearing augite-hypersthene
andesite, quartz-bearing augite-hypersthene andesite porphyry,
augite andesite, and hypersthene andesite.
Chemically, the volcanic rocks of Saipan are characterized
by a high silica and alumina content and a low potash, tita-
nium dioxide, and phosphorus pentoxide content. Quartz is
universally present in the norm, attaining as much as 49 per-
cent in the dacites. The andesites are extremely calcic and con-
tain a large excess of lime over alkalies.
The andesites and da-cites of Saipan generally are close in
composition to volcanic rocks of other islands in the system of
arcs extending from Japan to the Palau Islands. Apparently
the great bulk of the volcanic rocks in this region belong to a
characteristic calc-alkaline suite and form a well-defined petro-
graphic province. The general uniformity of composition of
the rocks throughout the province is a reflection of origin under
similar geological conditions.
Many features of the andesites and dacites of Saipan, espe-
cially the high silica content and peraluminous nature of the
dacites, are difficult to reconcile with simple differentiation of
a primary basaltic magma. Providing these rocks are related
to ancestral basalts, it seems necessary to assume assimilation
of important amounts of siliceous and Ominous crustal mate-
rial to account for their composition. The absence of basalts
on Saipan, and the wide compositional gap between the andes-
ites and dacites, may indicate that the andesitic and dacitic
magmas originated independently.
Volcanism is n normal accompaniment to the structural de-
velopment of the island arcs which border the western and
northern Pacific Basin, and this suggests that igneous activity
and structural evolution of the arcs are interrelated phenomena.
The andesites and da-cites of Saipan lie within the western
part of the circum-Pacific province in which the characteristic
volcanic association is basalt, andesite, dacite, and rhyolite or
388406-57-2
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some combination of these types. This rock kindred is in
marked contrast to that of the adjacent intra-Pacific or Pacific
Basin province in which the typical rock association is de-
cidedly more alltalic and consists of olivine and picrite basalt
and smaller amounts of their differentiation products such as
oligoclase andesite and trachyte. Around much of the Pacific
margin the transition between the circum-Pacific and intra-Pa-
chic rock provinces seems to be across a relatively narrow zone,
and it is this narrow transition zone that has been called the
andesite line. The significance of the andesite line, from the
standpoint of petrogenesis, is that it marks a combined petro-
logic, structural, and physiographic boundary separating a con-
tinental-type region (the circum-Pacific province) in which rock
evolution and rock composition are related to orogeny and the
presence of a sialic layer, from an oceanic-type region (the
intra-Pacific province) in which rock evolution and rock com-
position are related to crustal stability and the absence of a
sialic layer.
INTRODUCTION
This report presents the results of laboratory studies
carried on from 1950 to 1952 as part of a general investi-
gation of the geology of Saipan. Its purpose is to de-
scribe the physical and chemical characteristics of the
volcanic rocks, to discuss their relationship to rocks of
adjoining regions, and to make deductions and sugges-
tions as to their origin.
Laboratory investigation of the volcanic rocks has
involved microscopic examination of 350 rock sections,
X-ray studies of the groundmass of dacitic flow rocks,
and microscopic study of rock-forming minerals. Point-
counter analyses (see Chayes, 1949, p. 1-11) were made
on sections of chemically analyzed rocks to obtain the
volumetric mineral composition of principal rock types
and the composition of their groundmass. The average
chemical composition of plagioclase feldspar pheno-
crysts in several varieties of andesite was determined by
specific-gravity measurements. Chemical analyses were
obtained for 10 samples of volcanic rocks from Saipan,
1 of andesite from Guam, and 5 of basalt from the
islands of Alamagan, Pagan, and Agrihan (Agrigan).
Saipan lies about midway between Japan and New
Guinea in the western Pacific Ocean (fig. 11). It is one
of the larger of the Mariana Islands and is situated
near the center of that island chain, about 120 miles
127
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1".
f
128
GEOLOGY OF SAIPAN, MARIANA ISLANDS
north of Guam. On the west the Marianas are bounded
by the Philippine Sea; the Pacific Ocean proper lies on
the east.
The Marianas form a principal link in the system of
120'
130'
140'
150'
island arcs that extends southward from the Izu Penin-
sula of Japan to the southern limit of the Palau Islands,
along the east border of the Philippine Sea. The salient
features of the Mariana arc are shown in figure 12. The
160'
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PHILIPPINE SEA
PHILIPPINE ISLANDS
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IZU IS
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MARIANA
ISLANDS ?.
:Saipan
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MARSHALL ISLANDS
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Tarawa GILBERT IS
........
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CP ? ........ Ocean
New Ireland
? ??????.
11?::1 SO4????`?0 .? .4........1
Britain .:, 4'04, ????!..14Fe
isr0 % Z>i . The refractive index, y, ranges from about
1.709 to 1.714 and seems to be most commonly between
about 1.710 and 1.713. The hypersthene is optically
Wo
EXPLANATION
?
ftemoYst Prosems
Groundmass pyrozane
Nosmatme mono.
Numbers Wet to analyses sod
amvstmodmgmcksoecumms
m table 5
0, +f-9
0706
Nonnative prose.
60 40
Molecular percent
FIGURE 14.-ComposItIon diagram of phenocryst pyroxenes of andesites
from Saipan, groundmass pyroxene of andesites from Saipan, and
normative pyroxene of analyzed andesites from Saipan.
Fs
negative with an optic angle 217 ranging from about
60? to 65?, and the dispersion is distinct with r>v.
The most iron-rich hypersthene is found in a specimen
of augite-hypersthene andesite (specimen S3D) from
the breccia facies of the Hagman formation. The opti-
cal properties and molecular composition of phenocryst
hypersthene from various specimens of andesite are
given in table 3, and the average molecular composition
is plotted on the diagram of figure 14, together with
phenocryst and groundmass augite and the normative
pyroxene composition of analyzed rocks.
Like the augite phenomysts, the composition of hy-
persthene varies somewhat in different grains within
individual hand specimens as well as in single crystals.
This latter variation is not evident in section but is
apparent in crystals powdered and examined in oil im-
mersion, and probably it indicates a weak compositional
zoning of the hypersthene phenocrysts. The widest
range of composition found in a single rock is from
En07Fs33 to En33Fs37 and in a single crystal from about
En33Fs34 to En64Fs30.
The groundmass pyroxene of the andesites of Saipan
appears to be predominantly augite and subcalcic au-
gite, though in most rocks these elements are in such a
fine-grained form that they cannot be separated from
the rocks for optical analysis. In addition, it proved
to be impossible to prevent contamination of the ground-
mass pyroxene with phenocryst pyroxene in the separa-
tions that were attempted on porphyritic rocks. How-
ever, several specimens contained groundmass pyroxene
coarse enough, to permit a rough determination of the
optic angle of larger grains, and groundmass pyroxene,
perhaps only slightly contaminated with phenocryst
V *a
t
PETROLOGY OF THE VOLCANIC ROCKS
augite, was separated from 2 specimens (S43 and
S135, table 3) of augite-hypersthene andesite. The
optic angle 217 of the groundmass pyroxene in these
rocks, as measured on the universal stage, ranges from
about 40? to 48?, and the average is probably about 44?
or slightly more. The fi index is between about 1.698
and 1.700, and the compositional range is between ap-
proximately Wo21En47Fs32 and Wo27En44Fs29.
Pigeonite was not recognized and is probably not
present in the andesites of Saipan. In figure 14 the
normative pyroxene composition of various andesites
lies generally on a line between the grouping of points
representing phenocryst augite and hypersthene and is
displaced slightly toward the MgSiO3 side of this line.
However, the normative composition should fall on the
FeSiO, side of the line and is probably displaced to the
left because of the inherent error in the norm calcula-
tion. Specifically, the assumption that all ferric iron
and titanium are in magnetite and ilmenite is not cor-
rect and means that normative ferrosilite is too small
by a significant amount. Normative wollastonite is
also a bit low because of the assumption that all the
alumina is in feldspar.
Although the real normative pyroxene composition
of the andesites of Saipan is therefore believed to be
slightly higher in ferrosilite than the phenocryst py-
roxenes, there is a sharp contrast between the normative
pyroxene of the andesites of Saipan and that of the
majority of basalts and andesites of the Izu Peninsula
region of Japan, a plot of which is shown in figure 15.
In the rocks of the Izu Peninsula, iron-rich pigeonite
is the common pyroxene of the groundmass, and the
augite phenocrysts tend to be less calcic and slightly
more magnesian rich than those in the andesites of
Saipan. The normative pyroxene composition of the
rocks of the Izu Peninsula region therefore falls con-
siderably to the right and on the FeSiO3 side of the
line between the grouping of points representing pheno-
cryst augite and phenocryst hypersthene, about mid-
way between the line and the grouping of points repre-
senting groundmass pigeonite.
The absence of pigeonite in the groundmass of the
andesites of Saipan, and the apparent slight increase
in iron content of the subcalcic groundmass pyroxene
of these rocks, is not in general accord with pyroxene
relationships within the andesites of Japan, nor does
it entirely agree with what would be expected from the
physical chemistry of pyroxene crystallization in
lavas. However, the andesites of Saipan bear a strong
resemblance to rocks of the Hakone region of Japan
that belong to the hypersthenic rock series as defined
by Kuno (1950b, p. 992-993). In these rocks the
groun d m ass pyroxenes are characteristically hyper-
Wo
137
EXPLANATION
?
P8wnocnnte9r000041
0.4.1f1Slt5 prow,*
0
lectrnstunt pruseoe
*arts pnaccayt?
IS ?
C0 0 +
0 Of ttGroundrroos
o
+ Draws
61000160 0900000? 00 0
6'
o
+
Itscrerstbene
p e n o c b
60 40
Molecular percent
FIGURE 15.-Composition diagram of phenocryst pyroxenes, groundmass
pyroxene, and normative pyroxene of analyzed basalts and andesites
from the Izu Peninsula region of Japan and the Izu Islands. Com-
puted from data in Tsuya, 1937, p. 234-315.
sthene and augite; more rarely hypersthene, augite,
and pigeonite; and the ratios of FeSiO3 to MgSiO3
in the groundmass pyroxenes are rarely higher than
unity. These relationships generally agree with those
in the andesites of Saipan, many of which contain
hypersthene and augite or augite and subcalcic augite
in the groundmass. The absence of pigeonite in the
lavas (andesites) of Saipan is probably the result of
the low ratio of Fe to Mg in these rocks, with crys-
tallization therefore prevailing in a relatively mag-
nesian-rich system at temperatures below the clino-
pyroxene-orthopyroxene inversion curve. Under these
conditions hypersthene, rather than pigeonite, crystal-
lized along with augite.
In many instances hypersthene and augite pheno-
crysts, as well as small lath-shaped hypersthene crystals
in the groundmass of the andesites of Saipan, have
been resorbed by reaction with the groundmass and are
bordered by reaction rims. Hypersthene and augite
crystals are commonly surrounded by narrow to broad
opaque rims of finely divided hematite, monoclinic
pyroxene, and plagioclase feldspar ( ?) , and small
hypersthene crystals in the groundmass are completely
resorbed, their former position now occupied by a
pseudomorphic replacement of finely divided hematite.
In several rocks, hypersthene crystals are surrounded
by narrow irregular rims of subcalcic augite (pl.
29A, B), and in these same rocks augite is marginally
zoned with a broad outer zone of augite of less calcic
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GEOLOGY OF SAIPAN, IVIARIANA ISLANDS
composition than the core and with a probable composi-
tion tending toward subcalcic augite.
Hypersthene is always resorbed to a greater degree
than augite in the same rock, and in the majority of
rocks in which hypersthene is strongly resorbed, augite
commonly shows no effects of reaction. This contrast
in stability of augite and hypersthene is also reflected
in the secondary alteration of the pyroxenes, hyper-
sthene commonly altering to serpentine (bastite)
whereas augite in the same rock is unaltered.
nonNBLENDE
Green hornblende is a rare mineral in the volcanic
rocks of Saipan and is found sparingly in dacite perlite
and in hornblende-bearing quartz dacite porphyry. It
is present in these rocks as narrow, acicular crystals
(microphenocrysts) as much as 1 millimeter in length,
but most less than 0.1 millimeter in length, scattered
throughout the groundmass. The crystals are euhedral
and prismatic and show no indication of resorption.
ACCESSORY MINERALS
BIOTITE
Biotite is confined to silicic &cites, where it is present
as tiny plates (0.001 to 0.020 mm in width) embedded
in the glassy portions of the groundmass and asso-
ciated with silica minerals and microlites of oligoclase
feldspar. The biotite crystals have a high birefrigence,
are strongly pleochroic, and are perhaps of a phlog-
opitic variety and similar to biotite described by Kuno
(1950b, p. 982) in the groundmass of silicic dacites
from the Hakone Volcano of Japan.
MAGNETITE
Magnetite is present in the groundmass of all the
various types of andesite and dacite, and in the dacites
it forms small microphenocrysts that are commonly
either perfectly formed or slightly distorted octahedra
which average slightly less than 0.5 millimeter in dia-
meter. Magnetite forms less than 0.5 percent of the
dacites by volume. In andesites, magnetite is essen-
tially confined to the groundmass and forms small
subhedral to anhedral grains interstitial to groundmass
feldspar. Finely granular magnetite is present as in-
clusions in feldspar and is also produced at the borders
of resorbed pyroxene phenocrysts. Skeletal crystals
and dendritic growths of magnetite are present in the
interstitial glass of certain andesites. In reflected light
the magnetite grains are grayish white.
ILMENITE
Ilmenite was noted in the form of small flakes and
equant crystals in the groundmass of a few andesites
and has a dark brown to nearly black color in reflected
light. Ilmenite is apparently not an abundant con-
stituent in the andesites, however, and this is correlated
with the generally low content of titanium in the rocks.
IIE5IATITE
Hematite forms small microscopic flecks scattered
throughout the groundmass of dacite flows, but it is a
very minor constituent of these rocks. In part, at least,
the dacite flows may owe their reddish color to the
included hematite. In andesites, finely granular hema-
tite is found with magnetite as reaction rims around
large hypersthene phenocrysts. In some of these rocks,
smaller hypersthene grains are completely resorbed and
replaced by finely granular hematite.
RUTILE
Rutile is a rare accessory mineral of the andesites
but is generally present in small amount as small, short,
needlelike crystals embedded in interstitial groundmass
glass or interspersed in finely crystalline interstitial
material.
A single specimen of augite-hypersthene andesite
(specimen S3B) contains relatively large crystals of
what is believed to be rutile of a decidedly different
habit. In section, the crystals have a dark amber color
and are embedded in a cryptocrystalline and nearly
opaque groundmass clouded with dust-sized magnetite
and ilmenite( ?) grains. They are equant, euhedral, as
much as 0.4 millimeter in diameter, and appear to be
uniaxial, with a high refractive index and high bire-
fringence. Most of the crystals have a well-developed
prismatic cleavage, and basal ( ?) sections exhibit a tri-
angular twin pattern, with dark bands and irregular
inclusions of opaque ilmenite ( ?) traversing the mineral
parallel to and along the twin lamellae.
APATITE
Apatite is confined to the groundmass of andesites
and generally forms needlelike crystals less than 0.05
millimeter in length set in finely crystalline or glassy
material interspersed between feldspar laths. Apatite
is most abundant in flows of augite andesite which con-
tain a relatively large amount of P205.
ZEOLITES
Zeolites, largely of deuteric origin but in part the re-
sult of weathering, form rounded and irregular aggre-
gates in the groundmass of the andesites and are par-
ticularly abundant as coatings on the walls of vesi-
cles in flows of augite andesite. The common zeolites
are chabazite (gmelinite ?), heulandite, analcite, and
stilbite.
PETROLOGY OF 'lat. VOLCANIC ROCKS
ALTERATION MINERALS
Secondary alteration minerals include zeolites (prin-
cipally analcite), calcite, serpentine (bastite), chlorite,
sepiolite( ?), kaolinite, opal, chalcedony, and quartz.
Highly weathered rocks are altered to various clay min-
erals and hydrous iron oxides, chief among which are
kaolinite, montmorillonite, nontronite( ?), goethite,
limonite, and hematite.
PETROGRAPHY
The dacites of Saipan are primarily restricted to
flows and fragmental pyroclastic materials of the
Sankakuyama formation and to 1 of 2 small volcanic
plugs presumably related to the dacitic succession. An-
&sites are the chief rock component of the various
facies of the Hagman, Densinyama, and Fina-sisu for-
mations. Locally, however, accessory fragments of
dacite are fairly common in parts of the Densinyama
formation, a few fragments of (incite are present in
andesitic sandstone and conglomerate beds of the Hag-
man formation, and accessory inclusions of andesite
are found in dacitic breccias of the Sankakuyama for-
mation. The classification, texture, and mineral com-
position of the principal volcanic rock types of Saipan
are given in table 4.
DACITES
The chief textural varieties of dacite in the volcanic
formations of Saipan are dacite, dacite vitrophyre,
dacite perlite, and hornblende-bearing dacite porphyry.
DACITE
Dacite forms the tabular flows and irregular masses
of rock that comprise the flow-rock facies of the San-
kakuyama formation, and small fragments of dacite
also are found in dacitic breccias and tuffs of the Sanka-
kuyama formation and in andesitic breccia and con-
glomerate beds of the Densinyama formation. The
dacite fragments in the Densinyama formation are be-
lieved to be accessory inclusions derived from the older
flow rocks of the Sankakuyama formation.
The typical rock is grayish red, pale red, pale brown,
brownish gray, and light gray and is composed of a
glassy groundmass enclosing small scattered pheno-
crysts of oligoclase and quartz and rare euhedral crys-
tals of magnetite. It is massive to highly vesicular,
glassy, rarely cryptocrystalline, foliated (flowbanded),
and finely porphyritic. The measured specific gravity
of the more massive varieties of dacite ranges from 2.26
to 2.45 and averages about 2.30. However, these values
do not take into account the pore spaces (vesicles) in
the rocks and are therefore somewhat low. The true
specific gravity of the typical rock is probably close
to the maximum value of 2.45.
139
The groundmass is generally glassy and is only rare-
ly cryptocrystalline where the dacitic glass of the
groundmass is nearly or completely devitrified. Or-
dinarily the groundmass is highly vesicular (pl. 260),
but the vesicles of some of the rocks are filled with silica
minerals (mainly tridymite, opal, and chalcedony), and
such rocks have a massive, flintlike texture. The
groundmass is generally composed of small microlites
and crystallites of oligoclase feldspar (Ani0-15) less
than 0.5 mm long, equant grains of oligoclase and quartz
between 0.05 and 0.1 mm across, small irregular patches
and elongate crystals of tridymite, small needlelike
crystals of cristobalite less than 0.05 mm long, tiny
plates of biotite (perhaps phlogopite) between about
0.001 and 0.02 mm in width, small euhedral grains of
magnetite less than 0.1 mm in diameter, and small scat-
tered flecks of hematite set in a mesostasis of clear or
partly devitrified glass. In some rocks, small rounded
spherulites less than 0.1 mm in diameter are abundant
and form about 5 percent of the rock, but generally
they are rare or altogether absent. They are formed
of a radiating intergrowth of silica minerals and feld-
spar( ?).
The dacite flows of the Sankakuyama formation are
mostly highly vesicular, and some are pumiceous. The
vesicles are narrow and elongate, average about 1 mm
in length, and are generally about one-fifth as broad.
They are commonly lined or filled with silica minerals,
the most common of which are tridymite, opal, and
chalcedony. Many individual vesicles have a lining of
opal and a center filled with chalcedony, the opal always
forming the innermost lining against the walls of the
vesicles. Other vesicles are entirely filled with opal. In
some rocks, small irregular aggregates of tridymite
form the lining of vesicles (pl. 260, D). The tridy-
mite is weakly birefringent and has an index of refrac-
tion of about 1.48.
The principal constituent of the groundmass of the
porphyritic dacites is a pale-red or light-grayish-red
dacitic glass. In some of the flow rocks the glass is part-
ly devitrified, and small recrystallized patches of glass
alternate with areas of clear glass on a microscopic
scale. The groundmass glass has an index of refrac-
tion between 1.49 and 1.50; the average specific gravity
of the glass is about 2.30.
All the flow rocks contain small subhedral to rounded
phenocrysts of oligoclase feldspar and quartz, and
scarce euhedral crystals of magnetite, most of which
properly fall into the category of microphenocrysts.
The phenocrysts of oligoclase and quartz form about 5
to 8 percent of the rock and are less than 2 mm in di-
ameter, with an average of about 1 mm. Oligoclase
phenocrysts are weakly zoned, generally subhedral and
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140
TABLE 4.?Estimated mineral composition of the principal volcanic-rock types of Saipan
Phenocrysts (volume percent)
.?71. (4,
Cl
tzi
???
GEOLOGY OF SAIPAN, MARIANA ISLANDS
o o F.' 7
7 7.
Cl
N FT F2
a a .4.aa a2,0'.4a
-4. e
Microscopic texture
Holocrystallino to hyalopilitic
llolocrystalline to hyalopilltle
Alegaseopic toxturo
Finely porphyritic, aphanitle to glassy__
Finely porphyritic, glassy
Coarsely porphyri tic, aphanitic to glassy.
Porph yritie, plienocrysts generally largo-
Porphyritic, phonocrysts largo
C4 a:
c7iF
0
1 1 1
00000060
00000000
r$'
Dacito vitrophyre and pulite
Hornblende-bearing dacito porphyr
Ilypersthene andesito
Groundmass (volume percent)
.54
gg
oo
Fr 2
C.
Et Et
Op
0 0
V V V 02 '2
T T
t?1
7.1
.c;652
1:744-8
'a=
T
CI
C.
0
0 0
-
Rock Quartz,
Plagio- Anorthoclase choked-
claso any
Docile. _ Anio-2o Present(?) 0-5
5-15
Docile vitrophyro and per- ATI10-20 ---- (10
1110. 5-10
nornblendo-bearing daelte Ams-23 - do 1-5
porphyry. 20-30
Aticito-hypersthene antic- Amo-60 0-10 0-2
site. 5-35
Quartz-bearing auglte- Am3-33 0-10 2-5
hyperstheno andesite. 10-35
Quartz-bearing Anco-00 Present()
hypersthene andesito 30-10
porphyry.
Augitc andesite Ams-es Rare- -
50-60
ITyperstheno andesitei An51-10 Present(?)
20-25
1 .Estimato based on description by Tsuboya, 1032, p. 208-211
PETROLOGY OF THE VOLCANIC ROCKS
slightly elongate, and range in composition between
about An. (cores) and An15 (rims). Quartz pheno-
crysts are subhedral to rounded in outline, generally un-
broken, clear, and show no strain shadows. Only a few
of the oligoclase and quartz phenocrysts examined in sec-
tion have irregular borders against the groundmass, but
occasional crystals of both quartz and oligoclase are sur-
rounded by narrow rims of fibrous and cryptocrystalline
intergrowths of quartz and feldspar (potash oligoclase?
or perhaps anorthoclase ?). Small euhedral to sub-
hedral crystals of magnetite, between 0.01 mm and 0.5
mm in diameter, are the common accessory mineral of
the dacites, and many of the magnetite crystals are per-
fectly formed octahedra.
X-ray powder photographs of the groundmass of five
specimens of porphyritic dacite were made in an at-
tempt to determine the qualitative mineral composition
of the groundmass. All the samples gave the same
X-ray powder pattern. The d-spacing (atomic spac-
ings) of the lines on the groundmass photograph, and
thus the lines themselves, correspond closely to the lines
of higher intensity for a-cristobalite, a-tridymite, and
oligoclase. The d-spacings of the high-intensity lines
for quartz do not correspond well with the d-spacings
of the lines on the groundmass photograph. This ap-
pears to indicate that crystalline quartz is probably a
very minor constituent of the groundmass and that silica
is mainly in the form of opal, cristobalite, and tridymite
and is also occult in the groundmass glass.
The estimated mode of typical dacite is given in table
4, and the chemical composition of a type specimen of
the rock is given in table 5.
DACITE VITROPHYRE AND PERLITE
Dacite vitrophyre and perlite are the chief rock com-
ponents of pyroclastic breccias, flow breccias, and tuffs
of the Sankakuyama formation. They are medium- to
light-gray glassy pitchstonelike finely porphyritic rocks
containing small scattered anhedral to subhedral pheno-
crysts of oligoclase and quartz and microphenocrysts of
magnetite and green hornblende (rare). The pheno-
crysts and microphenocrysts form approximately 5 to 8
percent of the rock and are enclosed in a light- to dark-
gray glassy groundmass.
The quartz and oligoclase phenocrysts are as much
as 3 mm in diameter and have an average diameter of
about 1 mm. Oligoclase phenocrysts are generally
weakly zoned, subhedral in outline, equant to some-
what elongate and tabular, commonly broken, and are
occasionally somewhat embayed by the groundmass.
The range in composition is from about An. (cores) to
An. (rims). Quartz phenocrysts are subhedral to an-
hedral in outline, clear, often broken, commonly show
141
strain shadows, are only rarely embayed by the ground-
mass, and show no other effects of resorption. Small
euhedral to subhedral crystals of magnetite, between
0.01 and 0.3 mm in diameter, are the common accessory
mineral of the dacite vitrophyre and perlite. Many of
these small magnetite crystals are perfectly formed
octahedra. Small scattered acicular to equant crystals
of green hornblende, as much as 0.3 mm in length, were
noted in 1 section of perlite. The magnetite and horn-
blende together constitute less than 1 percent of the
rock.
The groundmass of dacite vitrophyre and perlite (pl.
26A, B) is dominantly a light- to dark-gray (colorless
in section) transparent dacitic glass containing numer-
ous tiny acicular microlites and crystallites of oligoclase
(An.-15). The crystallites are generally less than 0.01
mm in length, their long axes are parallelly oriented,
and they are concentrated in flow lines. Small rounded
spherulite,s, less than 0.2 mm across, are common in
some rocks but are not abundant. The spherulites have
a radiating structure and, like the small spherulites in
the dacitic flow rocks, are probably composed of an
intergrowth of oligoclase and silica minerals (quartz,
tridymite, and cristobalite).
The groundmass of the vitrophyre and perlite has a
specific gravity that ranges between 2.28 and 2.32, with
an average of about 2.30. The specific gravity of the
rock itself is probably close to the value. The index
of refraction of the groundmass glass is about 1.498.
Commonly the vitrophyre and perlite fragments ex-
hibit a fine, almost microscopic banding of alternate
light and dark laminae which are from a fraction of a
millimeter to about 2 millimeters across. The banding
is best seen in section and is produced by a concentra-
tion of microlites and magnetite grains into thin layers
that are separated by alternating layers of clear glass.
Banding of the vitrophyre and perlite is also produced
by an alternation of vesicular and massive layers,
though this textural banding is generally somewhat
coarser than the mineral banding. The oriented micro-
litic bands and elongate vesicles wrap around larger
phenocrysts and around small knotlike fragments of
glass that have become detached in the groundmass.
The vitrophyre and perlite fragments are commonly
extremely vesicular and pumiceous, containing closely
spaced long, slender, tubelike vesicles that give the rock
a fibrous texture. The tubular vesicles are from less
than 1 mm to as much as 2 cm in length and are alined
with their long axes parallel. They have an average
width of about 0.2 mm, about % to lho their length.
Only rarely do the vesicles contain secondary minerals,
but in some rocks the vesicles are lined with narrow
(about 0.01 mm in width) coatings of a weakly hire-
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142
GEOLOGY
OF SAIPAN, MARIANA ISLANDS
fringent silica mineral of low index of refraction that
is believed to be tridymite or cristobalite (pl. 26A).
Many vitrophyre fragments are intricately fractured
and traversed by curving cracks that pass through
groundmass glass and phenocrysts alike. These cracks
are not to be confused with the concentric cracks that
characterize perlite. though they probably have the
same origin. The groundmass of the perlite has a shot-
like appearance, being made up of small ball-like
aggregates of glass bounded by arcuate cracks that have
formed by contraction of the glass upon cooling (pl.
26B). Commonly several sets of these spherical cracks
develop around phenocrysts of quartz and oligoclase in
the perlite.
The estimated mineral mode of typical dacite vitro-
phyre and perlite is given in table 4, and the chemical
composition of a type specimen of the rock is given in
table 5.
HORNBLENDE-BEARING DACITE PORPHYRY
Fragments of hornblende-bearing dacite porphyry
are found in a small clacitic volcanic plug, are widely
scattered throughout conglomerate beds of the Densin-
yama formation, and are sparsely distributed in ande-
site conglomerate beds of the Hagman formation.
The typical rock is light gray, massive, and coarsely
porphyritic and contains large phenocrysts of plagio-
clase feldspar and quartz and scarce acicular crystals of
green hornblende enclosed in a microcrystalline
groundmass. The phenocrysts form about 20 percent
of the rock.
Plagioclase phenocrysts are subhedral, rarely euhe-
dral, generally equant or slightly elongate, and weakly
zoned. They are as much as 1 cm in length, average
about 4 mm, and comprise about S to 12 percent of the
rock. The cores of the plagioclase phenocrysts are
oligoclase-andesine (An5..35), the rims are medium
oligoclase (about An,o_n) , and the average composition
of the phenocrysts is about Ann. A few of the plagio-
clase phenocrysts show carlsbad and albite twinning,
but most are untwinned. The plagioclase phenocrysts
are generally clear and without inclusions, but a few
contain small regularly oriented inclusions of ground-
mass material. Some of the plagioclase phenocrysts
show ragged edges against the groundmass, but there
are no other noticeable effects of resorption. In the
fragments from the dacitic volcanic plug, plagioclase
phenocrysts are shattered and appear to have been
crushed by shearing stress, and groundmass material
fills the areas between broken parts of the crystals (pl.
27A). A few phenocrysts are broken into small frag-
ments that have become widely separated in the ground-
mass, indicating that crushing somehow occurred while
part of the groundmass of the rock was still liquid.
Quartz phenocrysts are nnhedral to subhedral, gen-
erally equant, and decidedly rounded. They are as
much as 8 inm in diameter, average about 3 mm, and
form about 5 percent of the rock. The quartz pheno-
crysts are clear and without visible inclusions, but they
show pronounced strain shadows in. polarized light.
Like the plagioclase phenocrysts, many of the larger
quartz grains are crushed and broken (pl. 27B), and
groundmass material fills the spaces between the crystal
fragments. The quartz phenocrysts are rounded and
generally somewhat resorbed and embayed by the
groundmass.
Hornblende phenocrysts are acicular and prismatic,
are as much as 2 mm in length but average less than 1
mm, and form less than 1 percent of the rock. They
show no effects of resorption. Commonly the horn-
blende phenocrysts are altered to a dark green fibrous
serpentine or chlorite.
The groundmass of the rock is light gray and micro-
crystalline and is composed of randomly oriented pla-
gioclase microlites (oligoclase, approximately Ari2o)
generally less than 0.1 mm in length, small equant
grains of quartz with a diameter less than 0.05 min,
and tiny acicular crystals of green hornblende scattered
throughout a devitrified glass base. Small rounded
spherulites, as much as 0.1 mm across, are present in the
devitrified portions of the groundmass and are prob-
ably radial intergrowths of quartz, tridymite, and feld-
spar. The devitrified glass of the groundmass is gener-
ally clear and weakly birefringent and contains a scat-
tering of dark submicroscopic grains. Small patches
of fibrous chalcedony are present in the groundmass
and may be largely of secondary origin. Tridymite and
cristobalite were not recognized in the groundmass but
are probably present as submicroscopic grains and in
spherulites.
The groundmass of some of the rocks, particularly
those from the dacitic volcanic plug, is traversed by nu-
merous randomly oriented fractures filled with finely
crystalline quartz and fibrous chalcedony. The frac-
tures pass through phenocrysts and groundmass alike,
and the groundmass is crushed and shattered in much
the same manner as the quartz and feldspar pheno-
crysts.
A single specimen of dacite porphyry, containing
phenocrysts of sodic andesine (An30 1, was collected
-40,
from the conglomerate and sandstone facies of the Hag-
man formation. This rock is somewhat more calcic
than other rocks of this general type.
The estimated mode of typical hornblende-bearing
dacite porphyry is given in table 4, and the chemical
composition of a type specimen is given in table 5.
PETROLOGY OF THE
ANDESITES
The various types of andesite from Saipan differ
widely in texture, volumetric mineral composition, and
color, but chemically they are all much alike, as is indi-
cated by a close correspondence in chemical composi-
tion (table 5). Although the andesites exhibit a fairly
wide textural and mineral variation (chiefly with re-
gard to accessory minerals and texture of the ground-
mass), they may be conveniently grouped into a reason-
ably small number of major rock types for purposes of
petrographic description.
AUGITE-HYPERSTHENE ANDESITE
GENERAL FEATURES
This is the most abundant kind of andesite in the
Hagman and Densinyama formations and forms ap-
proximately 50 to GO percent of the larger fragments
in the pyroclastic deposits. The general type also com-
prises four small massive andesite flows of the Hagman
formation. The color of the rocks ranges through light
gray, light greenish gray, light olive gray, brownish
gray, reddish brown, and medium and dark gray. The
light and dark rocks are of about equal abundance.
The wide variation in color from light to dark gray
to nearly black is a particularly deceiving aspect of
the andesites, and color (as distinct from color index)
is of no practical use as a criterion for estimating com-
position or as a basis for field classification. Color in
these rocks is apparently more a function of texture
(principally grain size) than of composition. The
light-colored rocks are generally coarsely porphyritic,
containing large feldspar and pyroxene phenocrysts but
only sparsely scattered femic constituents in the
groundmass. The dark rocks, on the other hand, are
ordinarily finer grained, the pyroxene phenocrysts are
smaller, and the groundmass contains a greater density
of femic constituents, giving the rock a darker color.
The chemical composition of the light and dark rocks,
however, is nearly identical, though the darker rocks
commonly contain a slightly higher percentage of iron
and magnesia.
In general, the augite-hypersthene andesites are mas-
sive, highly compact, coarsely and profusely porphy-
ritic, and contain abundant large phenocrysts of labra-
dorite and fewer large phenocrysts of hypersthene and
augite. The phenocrysts form about 30 to 55 percent
of the rock and are generally enclosed in a light to dark
microcrystalline groundmass. The proportion of hy-
persthene and augite phenocrysts is variable. Com-
monly hypersthene phenocrysts are more abundant than
augite phenocrysts, but the rocks range to types con-
taining a greater proportion of augite phenocrysts than
hypersthene phenocrysts. The two extremes are grada-
VOLCANIC ROCKS 143
tional through rocks containing nearly equal propor-
tions- of augite and hypersthene phenocrysts, although
the medial rocks are rare.
PHENOCRYSTS
Plagioclase phenocrysts form about 20 to 45 percent
of the rock, are generally subhedral to euhedral, are
equant to slightly elongate, and are commonly moder-
ately to highly zoned (pl. 30A, B) . They are as much
as 1 cm in length, but the maximum length is ordinarily
about 5 or 6 inm, and the average length is about 2 to
3 mm. Commonly the plagioclase phenocrysts are
formed of an intergrowth of several individual crystals.
The cores of some of the larger phenocrysts are sodic
bytownite (An70-80), but more commonly the cores are
calcic labradorite (An00-70), and the rims are sodic
labradorite (An80-88). The average composition of the
plagioclase phenocrysts in these rocks is between
approximately An55 and AnGa, as determined by spe-
cific-gravity measurements (table 2). The plagioclase
phenocrysts are usually complexly zoned, and com-
monly the zoning is oscillatory or repetitive and rarely
normal. In many rocks of this general type the plagio-
clase phenocrysts contain abundant small inclusions of
dark-brown groundmass material (chiefly altered
glass) and small microscopic grains of monoclinic
pyroxene and magnetite. The inclusions generally are
oriented in regular zones parallel to the internal zone
boundaries of the phenocrysts. The larger phenocrysts,
and many of the smaller feldspar grains in the ground-
mass, commonly show albite and carlsbad twinning.
Pericline twinning is infrequently developed in the
plagioclase.
In general, the plagioclase phenocrysts show only
minor effects of reaction with the groundmass. Occa-
sional phenocrysts in some of the rocks have ragged,
serrate boundaries and are slightly rounded and em-
bayed by the groundmass. Commonly the ground-
mass fills cracks and irregular openings in broken
phenocrysts.
Hypersthene phenocrysts form about 1 to 12 percent
of the rock, are generally elongate, prismatic, and
euhedral, and in some of the lighter colored rocks are
as much as 1 cm in length and 4 mm in width. In
darker rocks the hypersthene crystals ordinarily have
a maximum length of 5 to 6 mm and an average length
of 2 or 3 mm. The hypersthene phenocrysts are gen-
erally unzoned and show no resorption effects with the
groundmass. A few rocks, however, contain hyper-
sthene crystals with narrow border rims of subcalcic
augite (pl. 29A, B) . This augite has an optic angle 2V
of approximately 45? and a composition that is identi-
cal or nearly identical to that of the groundmass pyrox-
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144
GEOLOGY
OF SAIPAN, MARIANA ISLANDS
one. One of the textural varieties of augite-hyper-
stheno andesite (specimens S654B, S679) , which
comprises 2 thin flows in the Hagman formation, con-
tains hypersthene grains that are surrounded by nar-
row to wide (about 0.01 to 0.20 mm) irregular reaction
rims of a mixture of finely granular hematite and small
amounts of monoclinic pyroxene and possibly plagio-
clase feldspar. Small phenocrysts and groundmass
grains, formerly hypersthene, are completely altered to
granular hematite.
In section, the hypersthene phenocrysts are generally
colorless or pale pink and weakly pleochroic. In thick
sections the pleochroism is X= light brown or pale
pink, Z = colorless or pale green, and 17= pale brown.
Nine samples of hypersthene from varieties of augite-
hypersthene andesite were studied in oil immersion and
with the universal stage (table 3). Their composition,
as determined from curves published by Kennedy
(1947, p. 564), ranges between approximately En67Fs,3
and En03Fs37.
Augite phenocrysts are dark green to greenish black,
form about 1 to 10 percent of the rock, are elongate to
equant, euhedral to subhedral and rarely anhedral, and
are as much as 1 cm in length in the light-colored rocks.
In the darker rocks the augite phenocrysts are generally
between about 1 to 4 mm in longest dimension and
average about 2 or 3 mm. Finely porphyritic augite-
hypersthene andesites contain augite phenocrysts with
lengths averaging about 1 mm. The larger phenocrysts
are commonly elongate, prismatic, and euhedral with
well-developed prismatic and pinacoidal faces termi-
nated by (111) and {001}. In the majority of the rocks
examined the augite phenocrysts are unzoned, but some
of the rocks contain augite phenocrysts with narrow
outer zones of augite and subcalcic augite having a com-
position approximating that of the groundmass augite?
less calcic and probably slightly higher in iron content
than the augite of phenocrysts and the cores of zoned
phenocrysts. Hourglass zoning was not observed in the
augite phenocrysts. In most of the rocks augite pheno-
crysts show no resorption effect other than occasional
ragged, serrate edges, but in rocks in which hypersthene
phenocrysts are strongly resorbed the augite crystals
possess narrow reaction rims of an opaque mineral
which is probably hematite.
In section the augite phenocrysts are colorless and
nonpleochroic. Within single rocks and even within
single crystals they are somewhat variable in composi-
tion, but the range in composition in rocks of the general
type is not large. Nine samples of phenocryst augite
from varieties of augite-hypersthene andesite were
studied in oil immersion and with the universal stage
(table 3). Their composition, as determined from opti-
cal property curves published by Kennedy (1947, p.
568), ranges from W037En41Fs22 to Wo33Ens7Fsao.
GROUNDMASS
The groundmass of the augite-hypersthene andesites
is of variable texture and mineral composition. In gen-
eral, the groundmass is light gray or light greenish gray
to dark gray or dark greenish gray and, rarely, black;
aphanitic, microcrystalline or crytocrystalline to hypo-
hyaline and glassy; and microlitic and pilotaxitic.
Only a few rocks were observed to have flow texture
developed in the groundmass.
The commonest rocks have a groundmass of small
crystals of sodic labradorite (more rarely, calcic an-
desine An45-55) augite, subcalcic augite, hypersthene,
magnetite, ilmenite ( ?), tridymite, cristobalite( ?), and
anorthoclase, and between these mineral grains there is
generally a small amount of partly devitrified glass.
Plagioclase is the principal constituent of the ground-
mass and in holocrystalline rocks comprises about 40
percent of the groundmass. The plagioclase forms
small elongate microlites mostly from about 0.1 mm
to 0.01 mm in length but ranging down to submicro-
scopic size. The compositional range is from about
An40-15 (rare) to AnGo and most commonly is between
An and An55.
Augite and subcalcic augite are the common pyrox-
enes of the groundmass, and generally they form small
equant grains ranging from about 0.5 nun across to
submicroscopic dimensions, but in some rocks they are
of uniform size and less than 0.05 mm in diameter. The
groundmass augite has a variable composition, even in
a single rock specimen. Among the various sections
examined, the optic angle 2-17 ranges between approxi-
mately 40? and 50?, with the majority of grains (sub-
calcic augite) having optic angles between 400 and 45?.
Hypersthene is a common constituent in the ground-
mass of some of the augite-hypersthene andesites and
forms small elongate prismatic crystals from about 0.02
to 1.0 mm in length. In a few rocks the small ground-
mass hypersthenes have reaction borders of magnetite
and hematite or are entirely replaced by a dust-sized
granular aggregate of magnetite and hematite.
Silica minerals, the most common of which are tridy-
mite, quartz, and fibrous chalcedony, are present in
the groundmass of the majority of the augite-hyper-
sthene andesites. Tridymite is almost universally pres-
ent and forms isolated crystals and aggregates of small
wedge-shaped crystals commonly closely associated with
small patches of intergrown granular quartz and chal-
cedony. The isolated crystals of tridymite are elongate
and tabular and are generally less than 0.1 mm long.
?
0
PETROLOGY OF THE VOLCANIC ROCKS
Ordinarily they are embedded in the recrystallized glass
of the groundmass. However, in many rocks the tridy-
mite crystals are closely associated with a mineral of
low index of refraction and low birefringence that is
probably anorthoclase. In some rocks the tridymite
crystals project into or are entirely included within
small prisms of anorthoclase( ?), and in others they
appear to surround irregularly shaped interstitial fill-
ings of this mineral (pl. 28A, B.). Tabular crystals of
tridymito forming small microscopic patches in the
groundmass commonly exhibit a characteristic wedge-
shaped twinning (pls. 2W, 29C). Small anisotropic
needlelike crystals embedded in groundmass glass of
many andesites are probably cristobalite, although they
cannot be positively identified as such. Quartz and
chalcedony, although not abundant, are commonly pres-
ent in finely crystalline and fibrous intergrowths with
feldspar and are also intergrown with zeolites formed
front the alteration of plagioclase. Finely crystalline
quartz, plagioclase, and possibly anorthoclase( ?) form
small microscopic patches in and around feldspar
phenocrysts. Opal and chalcedony are commonly
found in the altered portions of the groundmass and
may have developed largely from the alteration of
interstitial glass.
Microscopic prisms and irregular interstitial fillings
of anorthoclase (possibly in part potash oligoclase ?)
are present in the groundmass of some and possibly
most of the andestites. Most of the grains and irregular
fillings of this mineral are less than 0.05 mum in diame-
ter, have a 7 index of refraction considerably below 1.54,
and have a low birefrigence. Commonly the grains en-
close tiny needlelike crystals of tridymite, or they form
an interstitial filling between elongate tridymite crys-
tals. More rarely, microcrystalline grains of anortho-
clase are associated with finely granular quartz and
plagioclase at the borders of large plagioclase pheno-
-crysts.
Accessory minerals of the groundmass include small
equant and generally subhedral grains of magnetite and
ilmenite, small elongate prismastic grains of apatite,
and small elongate to equant grains of rutile. Equant
crystals of a dark amber-colored mineral, believed to be
rutile, are abundant in a single specimen of augite-
hypersthene andesite (specimen S3B) , where they are
embedded in a cryptocrystalline and nearly opaque
groundmass clouded with dust-sized particles of magne-
tite and ilmenite ( ?) . The crystals are as much as
0.4 mm in diameter.
"Within the general rock type the groundmass ranges
from clear to pale-brown and darkly clouded glass to
a felted or pilotaxitic mixture of plagioclase, pyroxene,
tridymite, anorthoclase ( ?) , magnetite, and ilmen-
145
ite( ?), with variable amounts of interstitial glass.
Commonly the interstitial glass is partly or wholly de-
vitrified and altered. Rounded microscopic patches of
finely crystalline and radiating fibrous intergrowths,
believed to be quartz and feldspar, are common. In
some rocks the groundmass is composed of a weakly bi-
refringent aggregate of devitrified glass containing
randomly oriented submicroscopic grains of pyroxene
and magnetite. In other rocks, especially fresh rocks
unaffected by weathering, the interstitial material is a
light-brown to yellow or nearly colorless glass, gener-
ally containing randomly oriented to parallel micro-
scopic inclusions of monoclinic pyroxene and mag-
netite. A few rocks possess a groundmass of light-
brown interstitial glass enclosing dark microscopic
dendrites and crystallites of magnetite and possibly
monoclinic pyroxene.
The typical mineral composition of augite-hyper-
sthene andesite is given in table 4, and chemical com-
positions of type specimens of this rock are given in
table 5.
ALTERATION
The majority of the rocks examined show some de-
gree of alteration, part of which may be hydrothermal,
but mostly the result of weathering. Feldspar pheno-
crysts are altering to kaolinite, calcite, and occasionally
to zeolites (principally analcite). The alteration to
kaolinite is most intense at the borders of crystals and
along transverse cracks. Calcium carbonate is com-
monly present along with kaolinite, and the cores of the
feldspar phenocrysts are preferentially altered to this
mineral. However, kaolinite is the chief alteration
product of the plagioclase feldspars, and in deeply
weathered rocks the feldspar phenocrysts are com-
pletely altered to kaolinite or to a mixture of kaolinite
and gibbsite ( ?).
Phenocrysts and smaller groundmass crystals of by-
persthene are generally altered to light- and dark-green
serpentine and chlorite ( ?) minerals. The alteration
proceeds along transverse fractures and along crystal
boundaries, and even the hypersthene of fresh rocks is
commonly altered at the borders and has a core with rem-
nant sections surrounded by green serpentine. The com-
monest alteration mineral is light green in section and
has a low birefringence. This is most likely the anti-
gorite variety of serpentine. A less common alteration
mineral is pleochroic, grass green to dark green and
yellowish brown in section, and has a higher birefring-
ence. This may be a ferriferous chlorite. In some
rocks the alteration is a light-green mineral with an
extremely low birefringence?probably sepiolite ( ?).
Dark-brown birefringent goethite is found with the
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146
GEOLOGY OF SAIPAN, MARIANA ISLANDS
serpentine in a few rocks. Some rocks contain large
hypersthene phenocrysts altered to a mixture of light-
green serpentine, goethite, and calcite.
An interesting aspect of the alteration of pyroxenes
in these rocks is the marked stability of augite. Augite
in the fresh rocks is unaltered and even in the more
highly weathered rocks is only slightly altered to ser-
pentine. It is common to find unaltered augite in rocks
in which hypersthene phenocrysts are completely al-
tered to serpentine and in which the groundmass con-
sists of an aggregate of zeolites, quartz, opal, serpentine,
chlorite( ?), and calcite. This contrast in stability be-
tween augite and hypersthene is difficult to explain but
is probably related in some way to the difference in
structure and composition of the two minerals.
The groundmass of relatively fresh augite-hyper-
sthene andesite is ordinarily somewhat altered. Zeo-
lites are commonly present, and as nearly as can be de-
termined in section and in oil immersion they are
analcite, chabazite, gmelinite( ?), heulandite, phil-
lipsite, and possibly stilbite. In part the zeolites are
of deuteric origin and form small microscopic and
megascopic amygdules.
In the more highly altered rocks the groundmass
contains irregular patches of zeolites which are the
product of weathering, and commonly they are de-
veloped at the margins of feldspar phenocrysts. Clay
minerals also are present as an alteration product of
the glassy portions of the groundmass. Kaolinite and
possibly other kaolin minerals are present in the
weathered rocks, and a light-green weakly birefringent
clay, thought to be montmorillonite, is common.
In deeply weathered andesites the alteration of pri-
mary minerals is complete, and these rocks are com-
posed of clay minerals (chiefly kaolinite, montmoril-
lonite, and nontronite( ?), hydrous iron oxides ( goeth-
ite and limonite), hematite, and zeolites. Alkalies,
lime, and magnesium are removed except for small
traces; ferrous iron is oxidized to ferric iron to produce
hematite and hydrous ferric oxides; and some silica is
removed and a large amount of water is added in the
weathering process. The most notable concentrations
are in alumina, ferric iron, and water (OH). The fol-
lowing analyses illustrate the marked change in com-
position effected by weathering. Specimen S67A is
from the unweathered core of a spheroidally weathered
botilder of augite-hypersthene andesite from the brec-
cia-tuff facies of the Hagman formation, and specimen
S67B is a portion of the thick weathered shell enclos-
ing the fresh core. The boulder is about 5 to 6 feet in
diameter and the weathered shell about 21/2 feet thick.
The analyses were made by A. C. Vlisidis and S. M.
Berthold, U. S. Geological Survey.
BOA
SOD
SIO.
60. 95
54. 39
TIO.
.
.05
A1.03
18.06
24.83
Fe303
2.41
5.15
Fe?
2.61
.58
MnO
.08
.04
MgO
2.37
.58
CaO
8.16
.10
Na:0
3.10
.44
K.0
.58
.08
H:0 -
.76
2.42
MO+
.58
10.62
P:03
.10
.01
100.30
99. 92
*
04-
SECONDARY ROCK TYPES
QUARTZ?BEARING ALMITE-HYPERSTIIENE ANDESITE
This rock type is fairly common in the Hagman and
Densinyama formations and is similar in texture and
general mineralogy to the augite-hypersthene andesites
described above. It differs from the augite-hypersthene
andesites proper in containing visible grains of quartz,
which are mostly confined to the groundmass as isolated
crystals, probably of primary origin.
The rocks are light to dark gray and greenish gray,
massive, and coarsely porphyritic. Phenocrysts form
about 20 to 40 percent of the rock and are calcic labra-
dorite, elongate prismatic hypersthene, elongate to
equant diopsidic augite, and scarce small equant to
rounded quartz. The feldspar phenocrysts are as much
as 5 mm across, the pyroxene phenocrysts are as much
as 4 mm in length, and the quartz phenocrysts are as
much as 1 mm in diameter.
The groundmass is cryptocrystalline to microcrystal-
line, generally has an intergranular texture, and is com-
posed of plagioclase microlites, larger lath-shaped
grains of plagioclase, equant grains of subcalcic augite,
small prismatic grains of hypersthene, randomly scat-
tered grains of magnetite and ilmenite( ?), elongate
laths of tridymite and cristobalite ( ?), and small grains
of quartz. The mineral grains are enclosed in a crypto-
crystalline interstitial base of devitrified and altered
glass, which in part consists of secondary minerals-
zeolites, opal, and a patchy intergrowth of fibrous quartz
and feldspar. Small patches of finely granular quartz
are also present.
The small quartz phenocrysts and groundmass grains
are equant and rounded and have ragged, serrate edges
against the interstitial material of the groundmass.
They have evidently suffered strong resorption. The
quartz grains are as much as 1.0 mm in diameter, but
average mostly less than 0.1 mm.
The typical mineral composition of quartz-bearinn-
t,
augite-hypersthene andesite is given in table 4.
PETROLOGY OF THE VOLCANIC ROCKS
Inclusions of quartz-bearing augite-hypersthene an-
desite were found at one horizon in dacitic breccias of
the Sankakuyama formation. They have a consider-
ably different mineral composition and texture from the
quartz-bearing andesites of the Hagman and Den-
sinyama formations and warrant separate description.
The inclusions are brownish-gray massive finely por-
phyritic rocks composed of subbedral to euhedral phe-
nocrysts of highly zoned labradorite, quartz, augite,
and serpentine-replaced hypersthene ( ?) enclosed in a
cryptocrystalline groundmass. The phenocrysts range
in length from less than 1 mm to as much as 3 mm and
have an average length of about 1 mm. Plagioclase
phenocrysts form about 5 percent of the rock and are
highly zoned; the zoning is normal. Cores are medium
to calcic labradorite (Ana0-05) and rims are calcic an-
desine and andesine labradorite (An45-L0). The plagio-
clase phenocrysts are commonly intergrown composites
of many small elongate individual crystals. Euhedral
prismatic crystals, forming about 1 to 2 percent of the
rock and as much as 1 mm in length, are replaced by
weakly birefringent fibrous serpentine that is believed
to be pseudomorphous after original hypersthene.
Augite forms euhedral equant to slightly elongate pris-
matic crystals as much as 2 mm long and is not abun-
dant, representing only about 1 percent of the rock. The
mineral is a somewhat ferriferous augite with the ap-
proximate average composition Wo30En3IFs10 (speci-
men S583, table 3). Quartz phenocrysts represent about
1 percent of the rock, are commonly decidedly rounded
and embayed by the groundmass, and are generally
cracked and broken, but clear and without strain
shadows. They are as much as 2 mm in diameter.
The groundmass of .the rock is formed principally of
a felted aggregate of randomly oriented microlites of
andesine labradorite (An 15-50), submicroscopic grains
of monoclinic pyroxene less than 0.01 mm across, and
small euhedral to anhedral crystals of magnetite and il-
menite ( ?) 0.01 to 1.0 mm in diameter. Less abundant,
but nonetheless conspicuous, are minute slender needles
and tabular grains of tridymite and cristobalite( ?)
and isolated prisms of anorthoclase( ?) or perhaps pot-
ash oligoclase( ?) . Needles of cristobalite ( ?), and
needles and tabular crystals of tridymite are commonly
embedded in the interstitial glass, but in some rocks
they form small irregular patches and apparently fill
small interstices in the groundmass. The small needles
of tridymite commonly surround isolated prisms or ir-
regular patches of anorthoclase( ?), which appears to
have crystallized interstitially to the tridymite crystals.
Rarely, small needles of tridymite project into or are in-
cluded within the anorthoclase( ?) prisms. The anorth-
oclase ( ?) has an index of refraction (estimated 1.52)
147
considerably below that of balsam and a low birefring-
ence. Between the mineral grains there is a small
amount of colorless and light-brown interstitial vol-
canic glass charged with minute dark inclusions, and a
colorless isotropic material, opal( ?), with an index of
refraction between 1.45 and 1.47, forms an interstitial
filling in the groundmass and has partly replaced feld-
spar and hypersthene phenocrysts.
The estimated average mode of the quartz-bearing
augite-hypersthene andesite inclusions is given below.
Volume
Percent
Phenocrysts:
Labradorite 5
Hypersthene (serpentine) 3
Augite 1
Quartz 1
Groundmass :
Andesine-labradorite microlites 50
Monoclinic pyroxene (augite) 10
Tridymite and cristobalite 10
Anorthoclase
Opal( ?)
2
Ittgnetite and ilmenite 3
Volcanic glass 10
QUARTZ-BEARING AUGTTE-IIYPERSTIIE7%.7E ANDESITE PORPIIYRY
This rock was recovered from a dacitic volcanic plug
in association with fragments of hornblende-bearing
dacite porphyry. It bears a close compositional re-
semblance to quartz-bearing augite-hypersthene ande-
sites of the breccia facies of the Hagman formation.
However, the rock has a distinctive coarsely porphyritic
texture and contains a higher percentage of free quartz
than the breccia-associated rocks.
The rock is dark gray, massive, and coarsely porphy-
ritic. Phenocrysts of labradorite, augite, hypersthene,
and quartz comprise about 30 to 40 percent of the rock
and are enclosed in a dark partly glassy pilotaxitie
groundmass.
The plagioclase phenocrysts are subhedral and com-
monly equant, are as much as 1 cm in diameter, and
form about 15-20 percent of the rock. They are highly
zoned and the zoning is normal. Cores range from
bytownite (An-so) to labradorite (about An70), and
rims are sodic labradorite (An50-55). The average
composition, as determined by specific gravity measure-
Ments (specimen S141, table 2) is about Ana,. Many
of the larger plagioclase phenocrysts have a sievelike
texture and contain abundant small and generally elon-
gate inclusions of light-brown volcanic glass oriented
parallel to zonal boundaries. In some phenocrysts the
inclusions appear to be distributed throughout the en-
tire crystal, but in general they are confined to the
outermost zones. Plagioclase phenocrysts are generally
idomorphic toward the groundmass and show no effects
of resorption.
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148
GEOLOGY OF SAIPAN, MARIANA ISLANDS
Augite phenocrysts are euhedral, equant to elongate,
prismatic, as much as 3 mm long, and form about 5 to
10 percent of the rock. A few large augite phenocrysts
are of irregular shape and have poildlitic texture, en-
closing small rounded grains of feldspar and magnetite.
Many of the augite phenocrysts are zoned. The cores
of these are diopsidic augite. The outer rim of one
of these zoned phenocrysts has a positive optic angle
2V of about 42?, and its composition probably corre-
sponds closely with subcalcic augite of the groundmass.
Many of the augite phenocrysts are composed of aggre-
gates of several individual grains. The augite shows
no effects of resorption.
Hypersthene phenocrysts are rare in the porphyry.
They are small, euhedral, prismatic crystals averaging
about 1 mm in length, and form about 1 to 2 percent
of the rock. They exhibit no effects of resorption.
Quartz phenocrysts are subhedral to anhedral and
are commonly well rounded by resorption (pl. 29D).
Some are large and have a diameter of about 1 cm.
They form 1 to 4 percent of the rock. Several of the
quartz phenocrysts are red and may be stained with
iron oxides, although this was not evident in section.
In contrast to the shattered quartz and feldspar
phenocrysts and groundmass of the hornblende-bearing
dacite porphyry plug rock, the quartz-bearing augite-
hypersthene andesite porphyry fragments are unaf-
fected by shearing stress, and crushing does not appear
to be a general characteristic of the plug rocks as a
whole.
The groundmass of the andesite porphyry is dark
gray to nearly black and consists of a felty aggregate
of small plagioclase microlites (labradorite,
50-60)
with lengths less than 0.2 mm, equant grains of sub-
calcic augite less than 0.2 mm in diameter and with an
optic angle 2V of approximately 42? to 45?, and small
magnetite grains surrounded by a light-brown glass base
containing dark submicroscopic inclusions. This glass
forms about 5 to 10 percent of the groundmass. The
texture of the groundmass is intergranular tending
toward intersertal.
The estimated mode of the rock is given in table 4.
AUGITE ANDESITE
Augite andesite comprises flow rocks of the Fina-sisu
and Hagman formations, fragments in the pyroclastic
deposits of the Hagman and Densinyama forma-
tions, and inclusions in the mixed dacitic pyroclastic-
rock facies of the Sankakuyama formation.
Flows of augite andesite are light to dark olive gray,
brownish gray, and greenish gray, massive to highly
vesicular, and aphanitic to finely porphyritic. The
rock contains small acicular phenocrysts of labradorite
(average composition about An.) having a maximum
length of 3 mm and an average length of about 1 mm.
The phenocrysts, which form about 1 to 5 percent of the
rock, are subhedral to euhedral in outline, are elongate
parallel to the a crystallographic axis, are weakly zoned,
and show carlsbad and albite twinning.
The groundmass of the rock is aphanitic and micro-
crystalline and consists chiefly of small lath-shaped
crystals and microlites of labradorite ; less
abundant and generally equant grains of augite, mag-
netite, and ilmenite( ?) ; and exceedingly rare, small
prismatic apatite crystals. The interstitial material be-
tween these mineral grains is a light-brown (in section)
partly or wholly devitrified generally altered glass con-
taining swarms of dark crystallites of monoclinic py-
roxene( ?), plagioclase( ?), and tiny grains of magnetite
and ilmenite( ?). Silica minerals, common in the
groundmass of other andesites, are not present. The
texture of the groundmass is commonly intersertal, the
glass mesostasis filling the interstices between plagio-
clase grains (pl. 28D).
The plagioclase grains of the groundmass range in
length from about 0.2 mm to submicroscopic dimen-
sions, are randomly oriented, and form an estimated 50
to 60 percent of the rock. Augite grains form an esti-
mated 10 to 20 percent of the rock and are generally
equant and less than 0.05 mm across, although a few
elongate crystals of augite with lengths of as much as
1 mm are present in some rocks. They have the approxi-
mate composition Wo37En36Fs27 (specimen &HA, table
3). Small equant subhedral to anhedral grains of mag-
netite and ilmenite( ?) are scattered throughout the
rock. They are as much as 0.1 mm across, although gen-
erally less than 0.05 mm in diameter, and form about
5 percent of the rock. Apatite forms small prismatic
crystals embedded in the interstitial groundmass glass,
and generally these crystals are less than 0.01 mm long.
The flows of augite andesite are generally highly
vesicular at the top and moderately and minutely vesicu-
lar at the middle and base. The vesicles are ordinarily
spherical or slightly flattened and ovoid, and are as
much as 5 mm in maximum diameter at the tops of
flows. In the more massive flow rock (middle and basal
portions of flows) the vesicles are generally less than
1 mm in diameter and are rounded to uneven. They are
commonly lined with a narrow coating of white, pink,
and bluish-green zeolites, and a few were noted to be
lined with calcite and some with chalcedony. The most
common zeolites are chabazite (possibly gmelinite),
heulandite, and analcite. A fibrous zeolite, possibly
stilbite, is present in the vesicles along with the other
zeolites, and a light-bluish-green mineral, possibly
prehnite, forms thin coatings in the vesicles of some of
the rocks.
PETROLOGY OF THE VOLCANIC ROCKS
The augite andesite flows are deeply weathered at the
surface, and commonly to depths of tens of feet; no
fresh rock is exposed. The interstitial glassy portions
of the groundmass are readily altered to mixtures of
clay materials, zeolites, and secondary silica. Plagio-
clase grains are altering to kaolinite and more rarely to
a mixture of kaolinite and calcite. Augite grains are
ordinarily very stable in the zone of weathering, but in
deeply rotted rock they are altered at the borders to
fibrous serpentine. Opal and chalcedony of secondary
origin are present in several rocks and are mostly con-
fined to the altered interstitial glass of the groundmass,
but they also appear to be forming from the alteration
of plagioclase. In the upper portions of the weathered
zone original rock-forming minerals are completely de-
stroyed and the rock consists of a variety of clay min-
erals (chiefly kaolinite, montmorillonite, and nontro-
nite ? ), hydrous iron oxides (goethite, limonite), and
hematite, though relict igneous texture is still preserved
because of the differential alteration?plagioclase grains
alter to white kaolinite and give the rock a relict por-
phyritic appearance.
The estimated mode of typical augite andesite flow
rock is given in table 4, and the chemical composition
of a type specimen of the rock is given in table 5.
A second type of augite andesite, not found among the
flow rocks, is a light-gray to light-greenish-gray mas-
sive coarsely porphyritic andesite. Texturally the rock
is similar to light-colored varieties of augite-hyper-
sthene andesite, but it differs from these rocks in that
augite is present to the exclusion of hypersthene. Phe-
nocrysts are highly zoned calcic labradorite and euhe-
dral to subhedral diopsidic augite, the latter as much
as 8 mm in length. The groundmass has an intergranu-
lar texture and is composed of tiny microlites of feld-
spar between which are scattered grains of monoclinic
pyroxene, tridymite, magnetite, ilmenite ( ?), and apa-
tite. Interstitial to the mineral grains is a small amount
of partly devitrified colorless glass. Secondary min-
erals include zeolites, silica minerals (opal and chal-
cedony), and clay minerals.
Inclusions of augite andesite in the breccias and tuffs
of the Sankalcuyama formation are a fraction of an inch
to 6 inches across. They are dark grayish brown, mas-
sive, and finely porphyritic and are composed of pheno-
crysts of labradorite, diopsidic augite, and smaller crys-
tals of magnetite enclosed in a fine-grained ground-
mass. The phenocrysts comprise only about 5 percent
of the rock, are from less than 1 mm to as much as 3 mm
long, and have an average length of about 1 mm.
Plagioclase phenocrysts in the inclusions are sub-
hedral and highly zoned, with cores of bytownite (about
An75) and rims of labradorite (about An55-60) ; the zon-
149
lug is normal. Both albite and carlsbad twinning are
common. Augite phenocrysts are equant to somewhat
elongate and are as much as 2 mm long. The augite
phenocrysts are unzoned, slightly rounded, and a few
possess narrow reaction rims of a finely granular bire-
fringent mineral that is probably monoclinic pyrox-
ene. The augite phenocrysts (specimen S235, table 3)
are slightly pleochroic in section with 2= greenish
blue, 17?light brownish green, and X= light green.
Their approximate composition is Wo40En25Fs31. Sub-
hedral crystals and small equant grains of magnetite as
much as 0.5 mm across are scattered throughout the rock
and form approximately 3 or 4 percent of the rock
volume.
The groundmass of the augite andesite inclusions is
composed of a felted aggregate of randomly oriented
microlites and lath-shaped crystals of labradorite
(about An50-55) from submicroscopic size to about 0.1
mm in longest dimension, small elongate prismatic crys-
tals of monoclinic pyroxene with a length from about
0.01 to 0.1 mm, small grains of magnetite and ilmen-
ite( ?) generally less than 0.05 mm across, and small
elongate and tabular crystals of tridymite less than 0.1
mm in length. Minute needles of extremely low re-
frigence may be cristobalite. Anorthoclase is proba-
bly present in small amounts interstitially, although it
was not recognized in the groundmass. The mineral
grains are surrounded by a light-brown interstitial vol-
canic glass containing swarms of tiny dark opaque
inclusions which are probably magnetite.
The approximate average mode of the augite-ande-
site inclusions is given below.
vo win c
Phenocrysts: percent
Labradorite 4
Augite 1
Groundmass:
Labradorite microlites 55
Monoclinic pyroxene 15
Magnetite and ilmenite 3
Tridymite and cristobalite 2
Volcanic glass 20
/I YPER STLIE NE ANDE S ITE
A porphyritic hypersthene andesite from the shore
of Laulau Bay on Saipan (breccia facies of Hagman
formation) has been described by Tsuboya (1932, p.
208-211), but apparently this rock type is rare in the
volcanic formations of Saipan, for it was not found
among the many specimens of andesite collected by the
writer. Tsuboya's rock consists of phenocrysts of cal-
cic labradorite and hypersthene in a brownish aphanitie.
groundmass. The groundmass is composed of a glass
base containing lath-shaped plagioclase and hypers-
thene grains, the latter stained by brown iron oxides
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GEOLOGY OF SAIPAN, MARIANA ISLANDS
(hematite?). No mention is made of monoclinic pyro-
xene in the groundmass, and probably it is not present,
for the high alumina content of the rock would indicate
that most of the calcium probably went into the feld-
spars during crystallization. Plagioclase phenocrysts
in this rock are highly zoned, with cores of bytownite
(An28) and outer zones of calcic labradorite, and the
crystals exhibit both albite and pericline twinning.
Hypersthene phenocrysts are euhedral, prismatic, and.
strongly pleochroic (in section?), with Z =light green,
r=brownish yellow, and X=brown. Iron oxides
(hematite and magnetite?) are disseminated in the
margins of the hypersthene phenocrysts. An analysis
of this specimen is given in table 5.
CHEMICAL COMPOSITION OF THE MAJOR ROCK TYPES
The chemical compositions and norms of major types
of andesite and dacite of Saipan are given in table 5.
Columns 12 and 13 of the table give the average chemi-
cal compositions and norms of these rocks. All the
analyses are new except for two hitherto published
analyses (columns 4 and 10). Because it is uncertain
that the augite andesite described by Kaiser (1903, p.
120) is from Saipan, and because it does not conform
well with the modern analyses, it was omitted in com-
puting the average composition of andesite from Sai-
pan and is not included in the variation diagrams.
In general, the volcanic rocks of Saipan are charac-
terized by a high silica content, a high alumina content
with respect to the sum of the alkalies and lime, and a
low potash content compared with the average andesite-
dacite-rhyolite series of the world. The dacites are ex-
ceptionally high in silica and are peraluminous (the
molecular proportion of alumina in the rocks exceeds
the sum of the molecular proportions of soda, potash,
and lime). The most silicic dacite contains almost 50
percent quartz in the norm. The andesites of Saipan
are strongly oversaturated with silica, are moderately
aluminous or peraluminous, and have a high lime con-
tent (excluding the analysis given by Kaiser) com-
pared with average world andesite. Normative plagio-
clase in the andesites of Saipan is highly calcic, and no
normative composition is more sodic than Anso (again
excluding the augite andesite of Kaiser). Phenocrysts
of plagioclase are exceedingly abundant as compared
with those of mafic constituents, and this feature, to-
gether with the high anorthite content of the plagio-
clase, can be correlated with the high content of
A1203 and CaO in the bulk composition of these rocks.
All the andesites of Saipan contain an appreciable
amount of quartz in the norm, which is largely attribut-
able to the presence of silica minerals in the ground-
Analysis number in table 5
It 1.98 65
18
16
14
12
Li
6
4
e, 2
;c (Kuno, 1950b, p.
986, 1012; 1953, p. 269-270), which are actually some-
what oversaturated with silica. Kuno believes the un-
dersilicated olivine basalt magma gives rise to two dis-
tinct rock series of tholeiitic magma type: a pigeonitic
rock series formed chiefly through simple fractional
crystallization of olivine basalt magma, and a hyper-
sthenic rock series formed through reaction with and
assimilation of overlying or adjacent granitic material
of the sial in the olivine basalt magma. The andesites
and dacites of Saipan are more nearly akin to members
of the hypersthenic rock series.
On the other hand, undersilicated basaltic rocks of
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GEOLOGY OF SAIPAN,
the nature of average Hawaiian olivine basalt, as well
as other basalts with characteristics of the olivine basalt
magma typo of Kennedy, are not present among the
calc-allailine volcanic-rock associations of the Izu
Peninsula region of Japan and the island-arc system
that extends from Honshu to Palau. Consequently, it
appears doubtful that magmas of this nature were in-
volved in the genesis of the calc-alkaline volcanic rocks
of this area. This conclusion is further sustained by
the general absence of dacitic and rhyolitic lavas within
the oceanic area of the Pacific Basin, which strongly
suggests that lavas of this composition do not originate
by simple fractionation of undersaturated oceanic oli-
vine basalt. The di frerentiation of oceanic olivine basalt
in it sac direction leads, instead, to the formation of
oligoclase andesites and trachyt e or quartz trachyte.
Aside from the absence of undersilicated olivine ba-
salt in the calc-alkaline volcanic series of the Izu Penin-
sula region, and the absence of calc-alkaline rocks among
the lavas of the Pacific Basin, the case for rejecting a
magma. of this composition as the primitive magma from
which the tholeiitic basalt of Izu has developed is some-
what strengthened from other lines of evidence. It can
be seen from the SKM diagram (fig. 20) that separation
and removal of olivine and some proportion of hyper-
same from a liquid (magma) with the composition of
average Hawaiian olivine basalt causes the composition
of the liquid to move toward the alkali feldspar side of
the diagram, and in this way (assuming coordinate re-
moval of calcic plagioclase) the andesine and oligoclase
McstKular pecent
FIGURE 23.--Composition of normative feldspar of average andesIte of
Saipan. average basalt of the northern Mariana Islands, average
olivine basalt of the Hawaiian Islands, and average basalt of the Izu
'Peninsula region of 3apan.
MARIANA ISLANDS
andesites and trachyte of the Hawaiian Islands might
be derived, as is so postulated by Macdonald (1949, P.
1575). It also appears reasonable that the slightly
oversilicated basalt of the Hawaiian Islands, an average
analysis of which is plotted on the diagrams of figure
20, may originate by separation and removal of olivine
in more than its stoichiometric proportion. However,
as shown in figure 23, average Hawaiian olivine basalt
has a normative feldspar composition considerably less
calcic than the average basalt of Izu or the average
andesite of Saipan, which clearly rules out the possi-
bility of derivation of the basalts of Izu or the andesites
of Saipan from a magma with the composition of aver-
age Hawaiian olivine basalt either by fractional crystal-
lization or assimilation of rock material. Assimilation
of granitic material by a magma of the composition of
average Hawaiian olivine basalt, a material which by
definition would contain alkali feldspar and which
would therefor& enrich the olivine basalt magma in
alkalies, could never give rise to a differentiate with
the composition of the average basalt of Izu. The
derivation of average Izuian basalt from average
Hawaiian olivine basalt is thus primarily a problem of
alkali impoverishment and oversaturation with silica.
The latter condition might conceivably result by re-
moval of olivine in more than its stoichiometric propor-
tion by crystal settling, but the mechanics of alkali im-
poverishment cannot be satisfactorily explained by
crystal fractionation or assimilation.
The foregoing evidence strongly suggests that the
tholeiitic magma type of the Izu Peninsula region
probably developed independently of olivine basalt
magma, and this conclusion is reached by Turner and
Verhoogen (1951, p. 199) concerning the general rela-
tionship between olivine basalt and tholeiitic magmas.
The composition of average Hawaiian olivine basalt
and the composition of various basalts of tholeiitic type
are given in table 10. Among these types, the average
basalt of Izu, because of its low content of alkalies, par-
ticularly potash, corresponds more closely to a parent
type for the andesites and dacites of Saipan than any
of the others.
FRACTIONAL CRYSTALLIZATION AND ASSIN1ILATION
Assuming that the volcanic rocks of Saipan are
genetically related to a parent tholeiitic magma of the
nature of average Izuian basalt, it is possible to consider
whether the andesiles and dacites of Saipan are. the
result of fractional crystallization or contamination of
such a parent magma.
In the considerations that follow, the author has cal-
culated the composition of the smallest amounts of
material that must be subtracted from and added to
PETROLOGY OF THE
TABLE 10.-Average chenucat composition of olivine basalt from
the Hawaiian Islands and basalts of tholeiitic magma type from
various parts of the world
1
2
3
4
5
0
7
SiO2
48. 35
50. 53
50. 61
50. 79
51. 01
51. 07
53. 31
TiO2
2. 77
1. 49
1. 91
2. 65
. 94
. 94
1. 14
A1203
13.18
17.87
13.58
13.96
17.19
17.70
18.38
Fe2O3
2. 35
3. 58
3. 19
2. 11
3. 62
3. 23
2. 60
FeO
9. 08
7. 62
9. 92
11. 27
7. 32
8. 06
5. 35
MnO
.14
.21
.16
.22
.20
.21
.21
MgO
9. 72
4. 94
5. 46
4. 88
5. 11
4. 96
5. 18
CaO
10. 34
9. 74
9. 45
8. 48
10. 75
10. 56
8. 33
Na20
2. 42
2. 73
2. 60
2. 98
2. 59
2. 11
3. 63
K20
. 58
. 76
. 72
1. 24
. 72
. 37
. 79
1120
. 43
2. 13
. 83
. 31
. 85
. 62
P205
34
. 26
. 39
. 61
. 20
. 12
. 32
Total_ _
99. 27
100. 16
100. 12
100. 02
99. 96
100. 18
99. 91
1. Olivine basa t of Hawaii; average 0( 53 analyses (Macdonald, 1049, P. 1571)?
2. Basalt of lint, San (Fullyaina), average of 8 analyses (Tsuya, 1937, p. 307).
3. Basalt of the Deccan Plateau; average 0( 11 analyses (Washington, 1922. p.774).
4. Basalt of Oregon, plateau type; average 0(0 analyses (Washington, 1922, p. 779;
Thayer, 1937, P. 1032).
5. Basalt of northern Marianas; average of 7 analyses (table 6, this report).
0. Basalt of Izu; average of 29 analyses (Tsuya, 1937, p. 235-301; Kuno, 1950b, p.
1000-1002, 1004-1000).
7. Olivine basalt of Oregon, average of 9 analyses (Thayer, 1037, p. 1622, 1633).
possible parent magmas to form the andesitic and da-
citic lavas of Saipan. Basic to calculations of this sort
are the assumptions that potash is not removed from
the subtracted fraction and that magnesia is not intro-
duced in the added fraction, although neither of these
conditions is likely to obtain in natural processes of
magmatic differentiation. Moreover, the smallest
amount of material added or substracted is generally
found to be of very unusual composition and not such
as would be expected to separate as crystals from a
parent magma. The composition of an intermediate
amount of material, which may approach a reasonable
magmatic composition, can, of course, be calculated, but
because there are an infinite number of such inter-
mediate compositions obtainable, the meaning of such
calculations is questionable. For these reasons the sub-
traction and addition method of analysis of magmatic
differentiation must be used advisedly and with extreme
caution in attempting to reach definite conclusions
regarding the origin of lavas.
It should also be noted that throughout this theoreti-
cal treatment the porphyritic rocks of Saipan are
treated as representing magmatic liquids, and that the
bulk compositions of porphyritic rocks of Izu were used
in computing the average composition of the parent
Izuian basalt. Whether these rocks actually represent
the composition of magmatic liquids is open to ques-
tion, and probably a more accurate analysis would
result if groundmass compositions were used instead.
However, the calculated compositions of the ground-
mass of porphyritic andesites from Saipan (table 9) are
erratic and cannot be relied upon as being representa-
tive of the groundmass compositions. For this reason
VOLCANIC ROCKS
167
the bulk compositions of the porphyritic rocks are alone
considered in the treatment of fractional crystallization.
The compositions and norms of the smallest amount
of material which must be removed from and added
to the average basalt of Izu to yield the average an-
desite of Saipan is shown in columns 1 and 2 of table 11.
The composition and norm of the smallest amount of
material which must be removed from the average
basalt of Izu to yield the average dacite of Saipan is
given in column 3.
TABLE 11.-Composition of material subtracted from average basalt
of Izu to yield average andesite and docile of Saipan, and compo-
sition of material added to average basalt of Izu to yield average
andesite of Saipan 1
2
1 3
Compositions (weight percent)
SiO2
45. 60
70. 75
45. 60
A1205
17. 80
19. 00
19. 70
Total iron as FeO
14. 90
. 45
13. 40
MgO
6. 90
. 00
6. 20
CaO
13. 20
4. 40
12. 85
Na20
1. 40
4. 15
1. 90
120
.00
1.25
.00
Amount subtracted (per-
cent)
44
78
Amount added (percent) _
39
Norms weight percent)
Quartz
Orthoclase
32.
7.
98
23
Albite
12.
05
35.
11
16.
24
Anorthite
42.
26
21.
96
45.
04
Diopside:
Wollastonite
9.
74
7.
80
Enstatite
4.
90
3.
70
Ferrosilite
4.
62
4.
09
Hypersthene.
Enstatite
5.
30
.
60
Ferrosilite
4.
88
.
26
.
66
Fosterite
4.
90
7.
84
Fayalite
5.
10
9.
59
'Magnetite
6.
03
.
23
6.
50
Cordierite
2.
75
Feldspar:
Orthoclase
11
Albite
23
56
28
Anorthite
77
33
72
'Only the principal oxides have been used In the as culation, and all iron has been
calculated as FeO. However, in the norms, iron is distributed between FeO and
Fe303 in the same proportion as in the parent basalt, and magnetite is calculated as
a normative component.
1. Composition of smallest amount of material which, subtracted from average
basalt of Izu, yields average andesite of Saipan.
2. Composition of smallest amount of material which, added to average basalt of
Izu, yields average andesite of Saipan.
3. Composition of smallest amount of material which, subtracted from average
basalt of Izu, yields average dacite of Saipan.
The norms of the material removed from the basalt
to form the andesite and dacite consist of calcic plagio-
clase, diopside, hypersthene, olivine, and magnetite.
All of these minerals might be expected to crystallize
in the basaltic magma at high temperatures, although
the monoclinic pyroxene would be augite rather than
diopside. The proportion of mtiterial removed, which
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168
GEOLOGY OF SAIPAN,
represents the proportion of material crystallized from
the parent basalt, is 44 percent to yield the andesite and
78 percent to yield the dacite. This also appears rea-
sonable. The average composition of the feldspar is
A.n77 in the material subtracted to form the andesite
and An72 in the material subtracted to form the dacite.
The composition of plagioclase in the material sub-
tracted to form the andesite corresponds well to the cal-
cic plagioclase phenocrysts actually found in the ba-
salts of Izu. On the other hand, derivation of the an-
desite, and especially the dacite, by removal of a mini-
mal amount of material from the basaltic magma,
requires that olivine be subtracted in amount greater
than its stoichiometric proportion in the rock to yield
either the andesite or the dacite, and that both olivine
and diopside (augite) be removed in amounts greater
than their stoichiometric proportion in the rock to yield
the dacite. Removal of olivine in some amount greater
than its stoichiometric proportion might reasonably be
assumed to take place at elevated temperatures, but it is
doubtful that either olivine or diopside (augite) could
be removed from the parent basalt in the amount re-
quired to form the dacite.
Other difficulties involved in deriving the volcanic
rocks of Saipan by such a simple mechanism of frac-
tional crystallization are that normative diopside (au-
gite), hypersthene, and olivine in the subtracted mate-
rial are more iron rich than actual phenocrysts of these
minerals in the basalts of Izu, and the change in the
average feldspar composition from An77 in the mate-
rial subtracted to form the andesite to .A.n72 in the min-
eral subtracted to yield the dacite is not as great a
change in sodium content as would be expected in nor-
mal crystallization of the basalts of Izu. However, the
high iron content of the ferromagnesian minerals in the
material subtracted to form the andesite may be a con-
sequence of not assuming the right order of oxidation
state for iron, and if a change in the oxidation state is
postulated to put more ferrous iron into magnetite, the
iron content of the ferromagnesian minerals can be
made compatible with the actual composition of pheno-
crysts in the basalts of Izu.
The amount and composition of the smallest amount
of material which when added to the average basalt of
Izu yields a rock with the composition of average ande-
site of Saipan is given in column 2 of table 11. The
norm of this material consists essentially of quartz, an-
desinic plagioclase, and aluminous minerals. Material
of this composition might conceivably be derived by se-
lective assimilation of mineral material in andesitic
and dacitic rocks of a sialic crust through which the
parent basalt may have risen, but it is not such as would
be expected to crystallize from the basalt. The pro-
MARIANA ISLANDS
portion of added material (39 percent of the resultant
mixture or an amount equal to more than one-half the
volume of the original magma) assumed to have been
dissolved by the magma is, however, quite improbable.
Derivation of the dacites by solution of foreign rock
material in a basaltic magma would require assimilation
of even a more highly siliceous and aluminous material
in an amount many times greater than the volume of
the original magma. It thus appears that the andesites
and dacites cannot be derived solely through simple en-
richment of the basaltic magma, either by selective
fusion (assimilation) of contaminating wall-rock mate-
rial or by assimilation of crystals formed in another
part of the magma body.
In summary, the data given in table 11 appear to in-
dicate that the andesites of Saipan might have origi-
nated through simple fractional crystallization of a
magma with the composition of average Izuian basalt,
the dacites of Saipan can doubtfully be ascribed to a
pure differentiation process, and the andesites and da-
cites of Saipan could not have originated through sim-
ple assimilation of foreign rock material in a parent
basalt magma.
Assuming that the andesites were derived by fraction-
al crystallization of a parent basalt, the possibility may
be considered that the dacites originated through a
process of differentiation of a magma with the composi-
tion of average andesite. The compositions and norms
of the smallest amounts of material which must be
subtracted and added to the average andesite of Saipan
to yield the average dacite of Saipan are given in table
12. The material added consists of siliceous feldspathic
material of very unusual composition (quartz and po-
tassium-rich albitic feldspar), and it is very doubtful
whether any such mixture of known igneous or sedi-
mentary rocks could approach such a composition.
Moreover, even were it granted that such a mixture is
available, it would require assimilation to the extent of
93 percent of the resultant magma (nearly 13 times the
amount of the original magma) in order to produce the
required change. Any less siliceous material would
have to be assimilated in still greater amount. It is
therefore highly unlikely that the dacites could have
been derived solely by solution of foreign rock in a
magma with the composition of the average andesite of
Saipan.
Supposing that the change from andesite to dacite
was effected by subtraction of crystals (crystal frac-
tionation), the smallest amount of material that would
have to be removed from the andesite (table 12) is 01
percent of the melt and consists of basic plagioclase,
olivine, diopside, and hypersthene. All except olivine
are common as phenocrysts in the andesites of Saipan,
PETROLOGY OF THE VOLCANIC ROCKS
Luna.: 12.-Composition of material subtracted front and added
to average andesite of Saipan to yield average docile of Saipan 1
2
Compositions (weight percent)
SiO2 _
A'l2Pa3
Io1- iron as FeO. _ _
MgO _
CaO_ _ _
Na20._ _ _ .....
K20
Amount subtracted (percent)_
46. 00
23. 10
10. 50
1 90
12. 60
..... 2.50
61. 00
Amount added (percent) _
Norms (weight percent)
Quartz
Orthoclase_ _
Albite
Anorthit e.
Diopside:
Wollastonite _
I?mstatite
Ferrosilite _
Ilypersthene:
Enstat ite _
Ferrosilite_
Fosterite
Fayalite
Magnetite_
Cordierit
Feldspar:
On lioclase
Albite
Anorthite
20. 96
51.71
82. 50
10. 28
. 90
. 00
. 75
3. 75
1. 82
93
52. 26
10. .56
31. 44
3. 61
4.52
2.70
1.58
1.30
--
.79 .53
5.74 _____
3. 47
6.73 .70
. 92
22
30 70
70 8
T Only the principal oxides have been used in the calculation, and all iron has been
calculated as FeO However, in the norms, iron is distributed between FeO and
Fe201 in the same proportion as in the parent andesite, and magnetite is calculated
as a normative component.
1 Composition of smallest amount of material which, subtracted from average
andesite, yields average dacite.
2. Composition of smallest amount of material which, added to average andesite,
yields average dacite.
although the monoclinic pyroxene is actually an alumi-
nous augite rather than diopside. It is doubtful that
olivine could be removed from the andesite in the pro-
portion indicated, and it must be assumed that, if
olivine is not separated, the material removed must then
have a more siliceous composition, in which case the
subtracted fraction would comprise more than 70 per-
cent by weight of the melt.
Derivation of the peraluminous dacites by removal
of a minimal amotmt of material from the andesitic
melt also requires removal of diopside (augite) in an
amount greater than its actual stoichiometric (norma-
tive) proportion in the rock, the residual melt thereby
gaining an excess of alumina (relative to alkalies and
lime) by subtraction of lime from the system which
would otherwise combine with alumina to form an-
orthite. However, there is little basis for believing
that such a process is an important factor in the devel-
opment, of highly peraluminous rocks such as the dacites
of Saipan.
169
As shown in the ACF diagrams of figures 17, 18, and
20, the salic (felsic) members of calc-alkaline rock asso-
ciations are generally peraluminous. The peralumi-
nous character of the rocks is difficult to account for
by a process of simple fractional crystallization. For
example, it can be seen from the ACF diagrams that
if the anorthite-cordierite-hypersthene and anorthite-
diopside-hypersthene triangles represent separate ter-
nary systems, the anorthite-hypersthene join may co-
incide approximately with a thermal high on the
liquidus surface of these systems, in which case removal
of crystals could not cause the composition of the melt
to pass into the peraluminous triangles. However, the
fact that a specimen of augite andesite from Saipan
as well as several augite-bearing rocks from the Izu
Peninsula region are slightly peraluminous and fall
within the anorthite-cordierite-hypersthene triangle
may indicate that the anorthite-hypersthene join does
not correspond with a thermal high (at least not in the
vicinity of these peraluminous andesites) and that the
field of crystallization of diopside (augite) extends
across the join into the peraluminous triangle. Should
this be true, crystallization and subsequent removal of
diopside (augite) from a melt with the composition of
average andesite of Saipan would probably cause the
melt to change along a path in the direction of average
dacite of Saipan, toward a possible diopside-anorthite-
cordierite eutectic in the peraluminous triangle. The
melt might therefore become slightly peraluminous by
removal of diopside in slight excess of its normative
proportion. The position of the possible diopside-
anorthite-cordierite eutectic is not known, but if it
should lie close to the anorthite-hypersthene join, as
seems probable, then it is doubtful that other than
slightly peraluminous rocks could be produced by re-
moval of excess diopside (augite).
The mineral relations in the andesites of Saipan give
no indication that removal of diopside (augite) is the
means through which the rocks have become per-
aluminous, for it appears that calcic plagioclase has
mostly crystallized early along with augite in the ande-
sites and in some instances may have begun. to crystal-
lize before augite. It may also be noted that the cal-
culated groundmass compositions of analyzed andesites
from Saipan (table 9) are in every instance strongly
deficient in alumina relative to alkalies and lime. This
appears to indicate that the course of fractionation of
these rocks with respect to alumina is in a direction
away from the anorthite-hypersthene join, with crystal-
lization starting in the anorthite field and the melt sub-
sequently moving toward a possible eutetic in the anor-
thite-diopside-hypersthene triangle of the ACF dia-
gram. This relationship may be inferred from the ACF
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170
GEOLOGY OF SAIPAN, MARIANA ISLANDS
diagram of figure 21, on which the average groundmass
composition of the andesites of Saipan is plotted along
with the average bulk composition.
On the other hand, the groundmass compositions
listed in table 9 provide evidence that, at least to some
extent, simple strong fractionation of the andesitic
magmas might have played an important role in the
evolution of the dacitic magmas. The groundmass com-
positions of the andesites and dacites are decidedly more
silicic than the bulk compositions, and it may be seen
from the SICM diagram of figure 22, on which the aver-
age groundmass compositions are plotted, that the
course of fractionation of the average andesite of Sai-
pan is toward and approaches the composition of the
average dacite. In this connection, it is worth mention
that the course of fractionation of the average dacite
appears to be in the same direction (with respect to
silica) as that of the average andesite. Were the
groundmass of the dacites actually less silicic than the
phenocrysts (less silicic than the bulk composition) , a
course of fractionation of the dacites in a direction
opposite to that of the andesites would be indicated,
suggesting that these rocks bear little or no genetic re-
lationship to each other.
The chief property of the dacites of Saipan, and
of the petrogenic series of Saipan as a whole, which
is most difficult to account for through a mechanism
of simple fractional crystallization of a basaltic magma,
is the high silica content of the rocks. In general,
fractional crystallization of oversilicated basaltic
magma leads to residua rich in alkali feldspar plus
quartz, and Bowen (1937, p. 11-13) has stated that the
salic members of a differentiation series containing
more than SO percent of normative salic minerals (ex-
cluding anorthite) should have a composition that
approximates mixtures in the experimentally investi-
gated system nepheline-kaliophilite-silica. Under these
conditions, as a consequence of fractional crystalliza-
tion, the compositions of the residual magmas should
change toward those compositions represented by mix-
tures lying within the region of the low-temperature
trough of the system, and the maximum content of
normative quartz of even the most salic differentiates
should not be in excess of about 35 percent by weight.
However, the plotted positions of the dacites of Saipan
fall far outside (above and to the left) the trough of
this triangle (fig. 24).
On the basis of the foregoing analysis it appears
doubtful that the dacites of Saipan could have origi-
nated by pure differentiation of a magma with the
composition of average andesite of Saipan, and that
processes other than fractional crystallization probably
have contributed to their formation. To account for
NaAlS104
Vietsht pet
FIGURE 24.?Position of dacites of Saipan with respect to the low-
temperature trough of the system nepheline-kallophillte-silleu.
5A15?04
these highly silicic and peraluminous rocks, providing
they are indeed related to the andesites, it seems neces-
sary to assume some special process such as perhaps
extreme fractionation of an andesitic magma coupled
with assimilation of significant amounts of siliceous
and aluminous crustal material.
Objections may also be raised that salic magmas such
as the dacites of Saipan could be derived from basic
magmas without the development of rocks of inter-
mediate composition.
RELATIONSHIP OF VOLCANISM TO THE DEVELOPMENT
OF THE MARIANA ARC
It has long been recognized that volcanism is a normal
accompaniment to the structural development of the
island arcs which rim the Pacific Ocean, and this sug-
gests that volcanism and structural evolution of the arcs
are interrelated phenomena. The arcuate alinement of
the Mariana Island chain parallel to the bordering mar-
ginal deep known as the Mariana trench affords a good
illustration of this structural-volcanic relationship in
which igneous activity is probably contemporaneous
with orogeny (see fig. 12).
The doubly arcuate arrangement of the Mariana
Islands into an older outer (eastern) arc and a younger
inner (western) arc (fig. 12) implies that a shift in the
locus of volcanism has taken place, the locus having been
displaced from near the crest of the present Mariana
ridge westward to the backslope of the ridge in the
southern part of the chain. This shift WftS apparently
accompanied by little change in the chemical nature of
?
PETROLOGY OF THE VOLCANIC ROCKS
the eruptive products, except that the older eastern vol-
canic arc (southern Marianas) appears to have pro-
duced a greater volume of andesitic rocks and has
Produced dacitic rocks, whereas the younger western arc
(northern Marianas) has produced mafic basaltic and
andesitic rocks which are somewhat richer in potash
than the older lavas.
To some extent, the above relationships may be a re-
flection of changing conditions in the structural en-
vironment in which the volcanic rocks of the Mariana
Islands originated. The early Tertiary rocks?basalts,
andesites, and dacites?of the southern Mariana Islands
may relate to the initial development of a crustal down-
warp or tectogene, beneath the Mariana trench, a type
of structure which has been postulated to account for
the linear and arcuate oceanic deeps (see Cloud,
Schmidt, and Burke, 1956).
Westward shift of the locus of volcanism in the Mari-
anas to the backslope of the Mariana ridge may have
resulted through later migration of simatic material
away from the tectogene or downbuckle, on the concave
side of the evolving arc, in response to an increase in the
curvature of the arc, as suggested by Umbgrove (1947,
p. 191). The appearance of major amounts of basaltic
rocks in the northern Marianas (the inner volcanic
arc) may be a direct rewlt of the shift in the locus of
volcanism away from the downbuckle beneath the Mar-
iana trench. Such a shift would not only result in dis-
placement of the point of origin of the mafic magmas
laterally away from the downbuckle, but would also
place the environment at greater depth within the sima-
tic substratum, a position where there would, perhaps,
be less chance of contamination with sialic material.
The possible mode of origin of lavas in an environ-
ment of this sort has been described as follows by Turner
and Verhoogen in Igneous and Metamorphic Petrology
(1951, p. 222-224) :
In some provinces, and at some stage or other in the history of
most provinces, great volumes of andesite, and in some cases
(Incite and rhyolite, have been erupted over large areas with
little or no accompanying olivine basalt. This contrasts sharply
with the characteristically small volumes of trachytic, phono-
litic, or rhyolitic differentiates that accompany floods of basaltic
lavas in provinces of quite different character where basaltic
magma is generally believed to be the parent material. It would
be surprising if, in the tectonic environment provided by a zone
of active folding, great volumes of andesitic and rhyolitic differ-
entiates were habitually squeezed up from the depths without
considerably greater volumes of undifferentiated basaltic
magma welling up simultaneously. A second difficulty raised
by the pure differentiation hypothesis concerns the alternate,
and in some eases simultaneous, eruption of fine-grained olivine
basalt and glassy rhyolite in comparable amounts from the same
volcano, without appearance of lavas of intermediate (andesitic)
composition. This condition is illustrated by the Newberry
171
volcano. A distinct but much narrower compositional break
between the more basic and more siliceous members of volcanic
series is by no means uncommon, and could be explained by as-
suming that some special mechanism of fractional crystalliza-
tion, such as gas streaming, Is effective In separating the last
liquid fractions from the mass of early-formed crystals. Such
are the breaks between andesite and dacite in the present asso-
ciation, and between mugearite (or trachyandesite) and tracbyte
In the lavas of oceanic islands. To account for the basalt rhyo-
lite combination of such volcanoes as Newberry, it seems neces-
sary to assume either some drastic mechanism of differentiation,
c. g., unmixing of magmas into immiscible rhyolitic and basaltic
liquid fractions, or independent origin and uprise of the two
kinds of magma.
With such anomalies in mind, the authors suggest that some
of the features of andesites and rhyolites ascribed by orthodox
opinion to fractional crystallization of basaltic magma in the
depths may really be due to differential fusion of basaltic and
other rocks beneath or within the sial. Students of tectonic
geology have brought forward convincing evidence indicating
that downward thickening of a "granitic" upper layer (sial)
accompanies orogenic folding. If, following orthodox petrolo-
gical opinion, we were to assume that temperatures in an under-
lying basic layer are periodically raised sufficiently to allow
complete fusion, i. e., generation of basaltic magma, then
surely, in view of the relatively low temperatures required for
complete melting of granite, we would also be obliged to admit
the probability of local melting within the sial itself. Presum-
ably this would be particularly liable to occur within the thick-
ened portion of the sial, locally depressed below the level else-
where reached by the basic or ultrabasic substratum. Accord-
ing to this or any other current hypothesis of the deep struc-
ture of fold mountains, deep-seated generation of basaltic
magma is likely to be accompanied by nearby development of
andesitic and rhyolite magmas, either by complete or by par-
tial fusion of rocks of varied composition. Where folding is
active, there is ample opportunity for filter-pressing and segre-
gation of the magmas so formed and for mixing and blending
of magmas en route to the surface. These processes, modified
by differentiation wherever magma temporarily is held in a
closed chamber, are surely complex enough to account for the
wide variation observed in the products of eruption at the sur-
face Nor is it difficult to imagine why volcanic series in which
andesites and rhyolites are so conspicuous are confined to the
continents and attain their most spectacular development along
the Pacific margin, where for long ages the rocks of the sial
and adjacent underlying basic material have almost continu-
ously been kneaded together.
Varied processes such as the above may account for
the development of the extreme compositional gap be-
tween the andesites and dacites of Saipan without the
development of intermediate rock types, as well as the
observed absence of basaltic rocks on Saipan. Evi-
dence was presented in an earlier section of this report
to show that the volcanic rocks of Saipan probably
could not originate by simple differentiation alone, and
that some such process as assimilation of sialic material
by a basaltic or andesitic magma is necessary to form
magmas with the composition of silicic dacite.
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172
CONCLUSIONS
GEOLOGY OF SAIPAN, MARIANA ISLANDS
border of the Philippine Sea indicates that the rocks
have developed under similar geological conditions, and
that there is a close interrelation in this region between
volcanism and orogeny. In such an environment it is
not unreasonable to expect that petrogenetic processes
such as differential fusion and perhaps independent
origin of femic and salic lavas might result. These
processes, probably operating in conjunction with
differentiation (fractional crystallization) in the tec-
tonic environment, may explain the origin of the widely
variable calcic and silicic rocks of Saipan.
The close analogy of the volcanic rocks of Saipan
with volcanic associations in Japan and in the Palau
Islands-associations that comprise gradational series
ranging from basalts of tholeiitic type to silicic dacites
-suggests that the andesites and dacites of Saipan may
possibly represent members of a larger rock sequence
that includes ancestral basalts, even though the latter
rocks do not appear in the volcanic formations of
Saipan.
While it is not unreasonable to suppose that basic
magmas might have played a parent role in the evolu-
tion of the andesites and dacites of Saipan, many fea-
tures of the rocks are difficult to reconcile with simple
differentiation of a primary basaltic magma. On the
basis of elementary considerations regarding the na-
ture and amount of material which must be removed
from or added to parent rocks to yield average ande-
site and dacite of Saipan, and on the basis of graphical
analyses using chemical and petrographic data, the fol-
lowing inferences appear valid.
1. The andesites and dacites could not have originated solely
through simple assimilation of foreign rock material in a par-
ent basaltic magma.
2. The andesites of Saipan might have originated by simple
fractionation of a magma with the composition of average
tholelitic basalt of Izu, but the more reasonable mechanism
of origin from such a parent magma is by removal of pyrox-
ene and feldspar components coupled with assimilation of small
amounts of siliceous, feldspathic material.
3. The dacites of Saipan doubtfully could have originated by
simple fractional crystallization of basic magmas. Character-
istic properties of the dacites which cannot be correlated with
simple differentiation are the high silica content and peralumi-
nous nature of the rocks. On the other hand, the course of
fractionation of the andesites with respect to silica, as indicated
by the groundmass composition of the andesites, is toward and
approaches the composition of the dacites. This tends to sug-
gest that fractional crystallization of the andesitic magmas
might have been an important factor in the evolution of the
dacitic magmas.
4. Providing the dacites are derivatives of ancestral basaltic
or andesitic magmas, it seems necessary to assume assimila-
tion of significant amounts of siliceous and aluminous crustal
material to account for their composition.
Although the origin of the andesites and &cites of
Saipan may be explained by basaltic differentiation and
assimilation, there is no clear-cut evidence of contami-
nation of the rocks, suggesting that perhaps some spe-
cial process might have contributed to their formation.
The absence of basalts on Saipan, and the wide com-
positional gap between the andesites and dacites
without the development of rocks of intermediate com-
position, may indicate that the andesitic and dacitic
magmas originated independently.
The general similarity of the volcanic rocks through-
out the structural belt of island arcs along the eastern
THE PETROGENETIC SIGNIFICANCE OF THE
ANDESITE LINE
The andesites and (Incites of Saipan properly lie
within the western part of the circum-Pacific province
in which the characteristic volcanic-rock association is
basalt, andesite, dacite, and rhyolite or some combina-
tion of these types. The circum-Pacific province is
petrographically and geographically distinct from the
adjacent intra-Pacific or Pacific Basin province in
which the characteristic rock association of the island
groups (for example, the Hawaiian Islands) is olivine
basalt and smaller amounts of its differentiation prod-
ucts such as oligoclase andesite and trachyte. The
petrologic boundary between these two provinces is
well defined around much of the Pacific border, and
it is this boundary that has been called the andesite line
(see fig. 11) by various writers.
The already discussed differences between the area of
the Pacific Basin and the regions bordering it cannot
be overemphasized in terms of significance for petro-
genesis and the development of the two widely contrast-
ing rock suites of the circum-Pacific and intra-Pacific
provinces. The pyroxene basalt, pyroxene andesite,
dacite, and rhyolite association of the circum-Pacific
province and the picrite basalt, olivine basalt, trachyte
association of the intra-Pacific province are to a large
extent a reflection of the differing structural environ-
ments in which the two rock associations originated.
In the Pacific border region igneous activity has been
broadly contemporaneous with orogeny, and the vol-
canic rocks have developed under conditions and proc-
esses largely controlled by orogenic folding of a sialic
crustal layer. In such an environment normal evolu-
tion of contrasting volcanic rock types by differentia-
tion of magmas has been modified by assimilation of
sialic material or by special processes such as independ-
ent evolution of magmas of varying compositions by
complete or partial fusion of rocks of varied composi-
tion. Conversely, in the area of the Pacific Basin, a
sialic crust is presumably absent, igneous activity is not.
known to be related to orogenic folding, and the vol-
PETROLOGY OF THE
canic rocks have originated by fractional crystalliza-
tion of primary olivine basalt magma.
The significance of the andesite line, from the stand-
point of petrogenesis, is that it separates a region in
which rock evolution and rock compositions are related
to orogeny and the presence of a sialic layer (the circum-
Pacific province) from a region in which rock evolution
and rock compositions are related to crustal stability
and the absence of a sialic layer (the intra-Pacific or
Pacific Basin province).
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GEOLOGY OF SAIPAN, MARIANA ISLANDS
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Distribution of igneous and metamorphic rocks in
Page
Accessory minerals . . 138
ACF diagram__ . 151-152, 155, 156. 161,169
Acknowledgments............-. 131
Agrthan (Agrigan), northern Markma Islands 127,
130, 156, 157
Aguljan, southern Mariana Islands _ 130
Alamagen, northern Mariana Islands 127, 130, 156, 157
Alkali feldspar . . _ .. 133-134
Alteration minerals.... _ . 139, 145-10
Anatahan, northern AlurLana Islands... . 156
Anorthoclase. See Alkali feldspar.
Andesito, augite 148-149; pi. 28
at3gite-hypersthene . 143-146; pls. 23-30
basiltic------ 132
hypersthene 149-150
limo . ..... _ 132
quartz-bearing augite-hypersthone__ . 146-147
qua' tz-bearing augite-hypersthene andesite
porphyry 147-149; pl 29
Andesite line . 128, 160,172-173
A ndesites, ago _ ... 130, le3
chemical analyses and norms 150-153, 157, 16-1
classification _ 131-132
mineral composition, tables 140, 164
origin _ 166-170
Apatite 138
A mkabesm, Palau Islands . 154
As Lito, augite andesite . 151
Assimilation 156-170, 172
Augito 135-138, 144-145
See also descriptions of types of andesites.
install Islands . 161
Babelthaup, Palau Islands... . . 154
Bustlt, chemical analyses and norms . 151, 157. 167
Hawaiian Islands . 160-161, 165-165, 167
lzu Islands and Izu Peninsula region _ 158-
160, 166, 167
Berthold, S AI., analyses by . 146, 151, 157
Biotite . 138
Bonin Islands, northern Mariana Islands . . 155-
156, In 160, 162
Calcite 139
Caroline Islands . 161
Chalcedony - - 134
Chemical analyses, basalts of tholeiltic type _._ 167
olivine basalt of Hawaiian Islands _ . 167
volcanic rocks of northern Mariana Islands. 157
volcanic rocks of Saipan and Guam 151
Chlorite -------------139,143, 146
CU-cum-Pacific province . 160, 162, 172, 173
Classification of rocks. _ 131-132
Clay minerals. _ .. 139
Contamination of rocks 165
Cook Islands 161
CrLstobalito _ 134. 135
Melte. 139-141, p1.26
Daacciitteesi: goarleahyry, hornblende-bearing 142; pl. 27
D
M elte vitrophyre and porlita_ 141-142; pl. 26
130,163
..... _ 150-153, 157, 164
analyseseclailessmifliccaaltion.
and .....
s._ . 131-132
urIgnienral composition, tables 140, 164
o 166-170
Daly, R. A ,average rocks 131, 161-162
INDEX
Page
Mishima, Izu Islands . 158, 159
Noi them Mariana Islands 129.150-155
Opal 134
Ototo Jima, Bonin Islands 156
Pacific Basin. IGO. 1W, 162. 172, 173
Pagan, northern Mariana Islands 129,
130, 131,156,157
Pajaros. See Farallon de Pajaros.
Palau Islands 128, 151, IGO, 162
Parent magma, nature of ----------165- 166
Petrography . 139-150
Pigeonite 137
Plagioclase feldspar .. 132-133
See also descriptions of major rock types
Point Flores, hornblende-hearing (Incite por-
phyry151
Previous investigations 130-131
Primary minerals 132-138
Pyroxenes. See Auglte, Hypersthene, Pigeon-
ite, Subcaleic augite.
Quartz .. ....... 134, 135
Reaction rims 137, 158, 144
Rhyodacito. ..... ........ .... 132
Rhyolite 132
Rota, southern Afarlana Islands . 131, 133,160, 162
Rumong, Yap island group_ _ _ 154
Ru tile... _ _ ...... 138
Sabanan Talofofo, augite-hyporstlione andesite_ 151
Samoan Islands _ 161
Sankakuyama formation, andesites_ _._ 139,
147, 148, 149, 16,3,
dacites _ __ 139, 190,141, 151
general description _ ..... 130; pl. 2; chart
Sarlgan, northern Mariana Islands 156
Sepiolite 139, 145
Serpentine__ . .......... _ _ _ . _ 139, 145-146, 147
Serpentinized pet _ _ _ 130, 156
Shapiro, Leonard, analyses by 151, 157
Silica minerals. See Chalcedony, Cristoballte,
Opal, Quartz.
Sin Iwo Jima, Volcano Islands ----------153
SKAI diagram_ ....... _ 152, 153, 155, 156, 157, 158
Society Islands _
Subcaleic augite ............. 135, 136, 137
Talafole Creek, augito-hypersthene audesite 151
Tinian, southern Mariana Islands.... 130, 131, 153,160
Tonal' agglomerate, Yap island group -----155
Tridymite 134-135
Truk, Caroline Islands 161
Vlistdis, A. C., analyses by 146,151
Volcano Islands 158, 162
Von Wolff t! !angle 152
Yap, Yap island group 154
Yap island group 154-155. 160, 162
Zeolites 138, 130,145,146
Zoning 132-133,135, 136, 137-138
Page
Densinyama formation, andesites 140,
143. 146, 147, 148,163
dacites 139 142, 153
general description.... 130; pi. 2; chart
Firm sisu formation, andesites _ 140, 148, 151, 163
general description 130; pl. 2; chart
Farallon do Pajaros, northern Mariana Islands_ 129,
130, 131, 156. 157, 158
Fractional crystallization 166-170, 172
Fujiyama 157. 158, 160. 167
Ortgil-Tomil, Yap island group. ...... 154
Gambier Islands 161
Gonyer, F. A., analyses by 151
Guam, southern Mariana Islands 127,
12, 129, 151, 153-154, 162, 165
Hagman formation, andesites 132,
136,140, 143, 144, 146. 147, 198, 151, 153
(incites 139, 140
general description. _ _ ....... 130; pl 2; chart
Hahn Jima, Bonin Islands 155
liakone volcano, Japan 133,
134, 135, 137. 138, 158, 159, 165
Harker variation diagtam 150
Hawaiian Islands . _ 159. 160-161, 166, 167
Hematite 138
Hornblende _ ..... 138
Iluzism. See Fujiyama
ypersthene 135-138
See also descriptions of major rock types.
limenite 138
Inclusions, in plagioclase feldspar 133
Intra-Pacific province _ 160, 162, 172, 173
Iron oxides 139
Iwo Jima, Volcano Islands.. . ..... 158, 162
Izu Islands 158, 159, 160
Izu Peninsula region, Japan_ 132,
135, 137, 157,158-160. 162, 165, 166, 167
sequence of Quaternary volcanic rocks 160
sequence of Tertiary volcanic rocks. 159
Kaolinite 139,145,146
Kiln Iwo Jima, Volcano Islands 158
Kodushima, Izu Islands . 158,159
Koror, Palau Islands ..... 154
Kurose, Bonin Islands _ 156
Carolina Islands _ 161
Laboratory procedures ....... _ _ 127, 131, 141
Laulau Bay, hypersthone andeslto ...... _ 149, 151
Leucodacito 132
Location of the area 127-130
Magnetite 138
Magni, augito andesite 151
Alalakal, Palau Islands 154
Map, Yap island group 154
Map formation, Yap island group 155
Mariana arc, ridge, and trench _ 128, 129, 130, 170, 171
Marquesas Islands 161
Mang, northern Mariana Islands 131,156
Allnanal Iwo Jima, Volcano Islands 158
Mineralogy 132-139
Mount Achugau, dacito 151
special reference to the geology and petrology of Idu and
the Southern Islands (Nanpo Shoto) : Tokyo Imp. Univ.,
Earthquake Research Inst. Bull., v. 15, part 1, p. 215-357.
Turner, F. J., and Verhoogen, Jean, 1951, Igneous and metamor-
phic petrology: New York, McGraw-Hill Book Co., Inc.,
602 p.
1937,
the south sea islands under Japanese mandate: Imp. Acad-
emy Tokyo, Proc., v. 13, no. 3, p. 74-77.
Yoshiwara, S., 1902, Geological age of the Ogasawara Group
(Bonin Islands) as indicated by the occurrence of Nummu-
Wes : Geol. Mag., decade 4, v. 9, no. 7, p. 290-303.
175
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J???? -
It.
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PLATE 26
Dacite vitrophyre (specimen S448). Small spherulites (s) and phenocryst of
oligoclase (o) In groundmass of (bleak: glass enclosing oligoclase microlites.
Elongate vesicles are lined with cristobalite (c). Thin, massive layer in
breccia facies of Sankalcuyama formation.
Melte perlite (specimen S571). Phenocrysts of oligoclase (o) and quartz (q)
in groundtnass of fractured ductile glass enclosing oligoclase microlites.
Typical perlitic texture. Fragment in breccia facies of Sankalcuyama
formation.
GEOLOGICA L SURVEY
C. Dacite (specimen S293). Vesicular dacite with groundmass (dark) of
partly recrystallized glass enclosing oligoclase microlites and grains of
magnetite and hematite. Large, elongate vesicles are lined with tridymite
and cristobalite? (IC), and smaller vesicles are filled with these minerals.
Thin flow in Sankakuyama formation.
D. Dacite (specimen S230A). Tridyinite (t) in small cavities in groundmass
of dacitic glass, plagioclase microlites, and grains of magnetite and
hematite. Middle portion of thick flow in Sankakuyama formation.
? - A
PROFESSIONAL PA PER WA PLATE
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PHOTOMICROGRAPHS OF DACITES FROM SAI PAN
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GEOLOGICAL SURVEY
.?
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PROFESSIONAL, PAPER Zin PLATE 27
PHOTOMICROGRAPHS OF D 1CITES FROM SAIPAN
PLATE 27
A. Hornblende-bearing dad te porphyry (specimen S139). Shattered plienocryst of oligoclase (o) and phenocryst of hornblende (h)
in groundmass of partly recrystallized dacitic glass, spherulites, silica minerals, and magnetite grains. Groundmass
material fills interstices between broken feldspar fragments. Block in dacitic volcanic plug.
B. Hornblende-bearing dacite porphyry (specimen S139). Shattered pbenocryst of quartz with groundmass material filling
interstices between broken quartz fragments. Same rock as plate 27A.
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1,11ATII;
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11110TOM R.ROG RA PHS OF AN DESITES FRON1 SA ll'A
?
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GEOLOGICAL SURVEY
PROFESSION I P %PER 2tiO MATE 29
miffromll'IMGH API IS k1 twsrrEs FRom s111)1\
PLATE 29
A Augite-hypersthene andesite (specimen scosr) Phenocryst of hyper-
sthene (central part of crystal) with broad rim of subcalcic augite in
parallel intergrowth. Boulder in conglomerat e-sandstone facies of
Densinyama formation. Nicols crossed.
B. Augite-hypersthene andesite (specimen S43). Phenocrysts of labradorite
(I), augite (a), and hypersthene (h); the hypersthene (10 has a rim of
subcalcic augite. Block In breccia facies of I ragman formation. Nicols
crossed.
t :4 ? "*" t -fi?Z'''?",11-'::'4."'"41.:"
C. Augite-hypersthene andesite (specimen 821). Wedge-shaped crystals of tri-
dymite in small cavity between large labradorite phenocrysts. Block in
breccia facies of Hagman formation. Nicols crossed.
D. Quartz-bearing augite-hypersthene andesite porphyry (specimen S141).
Rounded xenocryst(?) of quartz and smaller phenocrysts of labradorite
(I), zoned augite (a), and hypersthene (h) in groundmass of labradorite
microlites, monoclinic pyroxene, tridyinite, anorthoclase, and andesitic
glass. Block in dacitic volcanic plug. Nicols crossed.
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PROFESSIONAL PAPER 280 PLATE 30
PLATE 30
Augite-hyperstheno andesito (specimen S37). Phenocrysts of zoned labradorite (/) and smaller grains of labradorite (p/)
and hypersthene (h) in microcrystalline groundmass of plagioclase microlites, grains of subcalcic augites, magnetite,
tridymite, anorthoclase, and andesitic glass. Hypersthene phenocrysts are altering to chlorite. Note density of plagioclase
phenocrysts. Block in breccia facies of Hagman formation. Nicols crossed.
B. Augite-hypersthene andesite (specimen S107). Phenocrysts of labradorite (/), hypersthene (h), and augite (a) in ground-
mass of nearly opaque glass enclosing crystallites (not observable) of monoclinic pyroxene and magnetite. Hypersthene
phenocrysts are altering to mixture of serpentine and chlorite. Note density of plagioclase phenocrysts. Block in breccia
facies of Hagman formation. Nicols crossed.
murromicRocH,1131IS OF 1\ DESITES FROM SAIPAN
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Petrography of the
Limestones
By J. HARLAN JOHNSON
GEOLOGICAL SURVEY PROFESSIONAL PAPER 280-C
A study of the composition, organic
constituents, and relative importance
of the limestones of Saipan
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CONTENTS
Abstract
Introduction _ _
Principal limestone-building organisms_ _
Coralline algae
Calcareous green algae
Foraminifera
Corals
Accessory limestone-building organisms_
Echinoderms
Mollusks__ _ _
Unidentified organic debris.
Groundmass
Fine organic debris
Calcareous paste__ _
Crystalline calcite
Fine sand and volcanic debri,_ _______ _
Page
177
177
177
178
178
179
179
179
180
180
180
180
181
181
181
181
Cementation
Recrystallization
Introduction of other minerals
Classification of the Saipan limestones _
Tuftaceous limestones and calcareous tuffs ____ _ _
Detrital limestones
13ioclastic limestones_ _ _ _________
Foraminiferal limestones
Algal-foraminiferal limestones
Coral-algal limestones
Algal limestones
Constructional limestones
Summary
Selected bibliograph3
Index_
Page
181
182
182
182
183
183
183
183
183
183
18-1
18-1
184
185
187
ILLUSTRATIONS
[Plates 2, 4 in pocket plates 31-35 follow index]
Plate 2 Generalized geologic map and sections of Saipan, Mariana Islands.
4 Locality-finding map of Saipan.
31. Rock-building organisms.
32-33. Saipan limestones.
34. Halimedu limestones.
35. Sections of Saipan limestones.
TABLE
Page
Organic constituents of Saipan limestones 185
CHART
Page .
Siumnitry of the geologic units of Saipan In pocket
III
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GEOLOGY OF SAIPAN, MARIANA ISLANDS
PETROGRAPHY OF THE LIMESTONES
BY J. I-IARLAN JOHNSON
ABSTRACT
The limestones of Saipan are elastic and consist mainly of
foraminiferal tests, debris of the lime-secreting algae, and cal-
careous shells and exoskeletons of animals. In a few, volcanic
material is present. The animals which contributed most to
limestone building are corals and Foraminafera ; the important
plants are coralline algae and Halimeda. Other organisms
represented are echinoids, mollusks, and dasycladacean algae.
The groundmass may consist of fine organic debris, calcareous
paste, crystalline calcite, or fine sand and volcanic debris.
The limestones are tuffaceous limestones and calcareous tuffs,
detrital limestones, bioclastic limestones, and constructional
limestones. The bioclastic limestones include foraminiferal,
algal-foraminiferal, coral-algal, and algal limestones. The algal-
foraminiferal and the coral-algal limestone are the most com-
mon The constructional limestones are old reefs or biostromes
that contain corals and algae in position of growth.
Recrystallization has altered some of the limestones, which
range in age from Eocene to Recent.
INTRODUCTION
The Saipan limestones are all elastic limestones of
Cenozoic age (see chart) . The larger elastic particles
are the tests of Foraminifera or fragments of the
calcareous skeletons of algae and other organisms
imbedded in a groundmass of fine mechanical debris,
calcareous precipitate, or crystalline calcite.
The following classification of grain sizes, used at
the Colorado School of Mines (Low, 1951, p. 17-18),
was used in this chapter: coarse, 2.00 mm; medium,
2.00-0.25 mm; fine, 0.25-0.05 mm. Below the range of
visibility with 12X power are two classes: sublitho-
graphic, dull luster, earthy, opaque; and lithographic,
porcelaneous, semitranslucent.
Field localities are shown on a special locality-finding
map (pl. 4). Locality numbers, arranged in numerical
order at the lower right corner of this map, may be
found by reference to grid coordinates. The letter pre-
fix of these numbers indicates the collector?B for
Burke, C for Cloud, S for Schmidt. A complete de-
scription of the field numbering system is given in Chap-
ter A, page 39. This locality-finding map is intended to
be used in connection with the generalized geologic map
(pl. 2) at the same scale.
38840G-57-6
The petrographic studies of thin sections of the lime-
stones were directed toward recognition of the organ-
isms and organic debris of which the limestone was
built, in order to determine their relative hnportance
and the conditions of the deposition and ecology. In
determining percentages, the ordinary crossgrid whip-
pie plate was used and actual counts were made of the
area of a slide occupied by the different types of
organisms.
PRINCIPAL LIMESTONE-BUILDING ORGANISMS
Calcareous algae and Foraminif era are the principal
builders of the Saipan limestones. The relative percen-
tage of each varies with time of deposition and location,
but together they commonly form as much as 75 percent
of the rock. The fossil calcareous algae of Saipan in-
clude representatives of 3 families and 15 genera which
are listed below.
Rhodophyta (red algae)
Family Corallinaceae (coralline algae)
Division 1.?Subfamily Melobesieae? (crus-
tose corallines)
Archaeolithothamnion
Dermatolithon
Goniolithon
Lithothamnion
Lithophyllum,
Lithoporella
llIesop/i?JZlulrt
P orolithon,
Division 2.?Subfamily Corallinae (articulate
eorallines)
Amphiroa
Arthrocardia
Cheilosporum
C orallina
J ania
Chlorophyta (green algae)
Family Codiaceae
Halimeda
Family Dasycladaceae
C ymopolia
177
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178
GEOLOGY
CORALLINE ALGAE
OF SAIPAN, MARIANA ISLANDS
occurs throughout but in minor numbers. Plants of
this type develop usually as small bushy structures, each
composed of clusters of delicate fronds which are made
up of numerous segmented portions. The fronds are
small, thin, and delicate, and the living plant is quite
flexible. With the death of the plant, they break into
individual segments. Most of the fragments seen in
the thin sections are separated segments. Occasionally,
pieces containing several connected segments occur.
With the aid of a magnifying lens, these segments can
be observed in many of the limestones, and they can be
separated and washed out of some of the more manly
and shaly facies. In thin section, they can be readily
recognized by the shape of the fragment and general
structure. Characteristically they are composed of
curved layers of relatively long, narrow cells. The
cells in general are much longer than in the crustose
corallines. The skeletal material shows dark in the sec-
tions. They commonly occur associated with certain
groups of Foraminifera and at places with HaTimeda.
Only rarely are they found in the same rock with
abundant remains of the crustose forms. In a few
samples of the limestones studied, they occur in suf-
ficient abundance to be the predominant rock-building
organism. However, in many instances where numer-
ous specimens were observed in a slide, they did not
form a large percentage of the rock.
The coralline algae comprise two distinct groups of
the red algae, the crustose and the articulate, which have
developed different growth structures and become ad-
justed to different environmental conditions.
The crustose corallines form solid, stony structures
which range from thin crusts to thick, massive branch-
ing forms. Some attain considerable size, specimens a
foot or more across having been observed in and around
the tropical reefs. On the other hand, the articulate
corallines ordinarily develop small, delicate, bushy
structures, seldom more than a few inches across. The
crustose coralline algae have developed many growth
forms. In Chaper E, Calcareous Algae, these have been
discussed under the following types: Thin crusts which
are attached to and cover or partially cover other or-
ganisms or organic debris; thin laminae which grow
loose or unattached on the ocean bottom; crusts which
develop warty protuberances, mammillae, or short, stub-
by branches; and strongly branching forms. In the
limestones, one commonly finds the thin crusts entire
and in position of growth. In some, small nodular
masses are formed of superimposed crusts or alterations
of these encrusting algae with encrusting Foraminifera
(pl. 31, fig 2). The other growth types are commonly
represented by broken and worn fragments. Their
form may be elongate, ovoid, or irregular. In hand
specimens they are easily recognized by their white
chalky to porcelaneous texture. They occur in all the
limestones. In thin section they may be recognized
by the fine cellular structure commonly in definite lay-
ers and by the dark color. The calcium carbonate
particles precipitated by the algae are so fine that they
show no crystalline structure and appear dark. The
characteristic structures of the common genera are
shown on plate 31, figure 2, and plate 32, figure 3.
Algae of this type are commonly associated with For-
aminifera and coral. The actual percentage of such
algae in the rock specimens studied varies greatly ac-
cording to the specimen, but in relative abundance they
commonly rank first or second and in volume second
or first among organisms present.
Remains of the articulate corallines are surprismgly
common and widespread in the Saipan limestones.
Their presence can be recognized in a large majority
of the slides studied, although only a moderate propor-
tion of the limestones contain them in large number.
In the Mariana and Tanapag limestones (Pleistocene),
they appear to be represented primarily by the genus
Amphiroa (pl. 35, fig. 4) ; although some contain ma-
terial identified as Jan/a. In the Miocene and Eocene
limestones, no Amphiroa were observed; Corallina is
the commonly represented genus in them, while Jania
CALCAREOUS GREEN ALGAE
In the Saipan limestones, two types of calcareous
green algae have been observed?the codiaceans, repre-
sented primarily by Halimeda, and the dasycladaceae,
represented by Cymopolia.
Halimeda occur in great numbers locally (pl. 34).
They grow attached to the bottom as small busby plants
several inches high. Each bush is composed of many
branches or fronds, each of which is segmented. Many
of the segments resemble small models of the prickly
pear (0 punt/a) leaf. The young and growing forms
are bright green. As they grow older, they become
more and more encrusted with lime and assume a gray-
ish appearance. After the death of the plant, the
branches tend to break into separate segments which
bleach white or light gray. These can be observed in
many of the hand specimens of limestones (pl. 34, fig.
2). In thin section, the segments may assume a num-
ber of outlines depending on the angle of the section.
Commonly they are long and slender, but occasionally
a section parallel to the flat surface may show wide, lo-
bate forms. The microstructure is distinctive. The
central portion of the leaf consists of coarse tubes which
branch into smaller and smaller tubes as the outer part
of the segment is approached, ordinarily ending in very
PETROGRAPHY OF itu, LIMESTONES 179
fine tubes perpendicular to the edge of the segment (pl.
34, fig. 1, 3). The calcification starts at the outer sur-
face and works inward. Where it is complete, the mi-
crostructure of the entire segment is preserved. If only
the outer rim is calcified, it, only, is preserved. Had-
meda is commonly associated with Foraminifera and
locally with the crustose coralline algae. More rarely
they may be associated with articulate coralline algae.
In certain facies, they are so abundant as to be the pre-
dominant rock-forming organism, and the rocks are
spoken of as Halimeda limestones (pl. 34, figs. 1, 2).
Dasycladaceans were recognized in a number of slides
of limestones of both Miocene and Eocene age, but only
the genus Cymopolia has been identified. The plants
develop as small, brushlike or club-shaped structures
from a fraction of an inch to several inches high. The
individual fronds consist of a series of beadlike, club-
shaped, or cylindrical segments. Each individual
frond consists of a relatively thick central stem from
which develop whorls of primary branches which are
arranged like the spokes of a wheel. From these may
develop secondary and even tertiary branches. Calci-
fication consists of a precipitation of calcium carbonate
around the central stem and primary branches. Occa-
sionally it becomes thick enough to enclose the second-
ary and tertiary branches, forming a moderately com-
pact crust. After the death of the plant, such crusts
may be preserved as external casts of the central stem
and branches which may or may not be filled with sec-
ondary calcite or fine calcareous paste. The fossils in
hand specimens can be recognized with a low-power
glass or the naked eye and they may be washed from
shaly, chalky, or manly facies. In thin section, they
have a very characteristic structure with a central stem
and radiating primary branches. Certain echinoid
spines in perpendicular section have a similar gross
structure. Among the dasycladacean algae, however,
the calcium carbonate is fine-grained and is not precipi-
tated in optical continuity as in the echinoids. The
dasycladacean algae may occur with Foraminifera and
Halimeda; they have been observed only rarely with
articulate coralline algae. In the Saipan limestones,
they are of no importance as rock builders.
FORAMINIFERA
Foraminifera, are abundant in most of the limestones
from Saipan; in bulk and abundance they rank first
to third among the rock-building organisms there.
They include large, moderately deep-dwelling benthonic
forms (pl. 32, fig. 4; pl. 35, fig. 1), small shallow-water
types (pl. 35, fig. 2), and planktonic species. Struc-
turally, they tend into two types. Most of the smaller
Foraminifera are made up of radial calcite fibers.
These fibers when oriented parallel to the crosshairs
extinguish under crossed nicols. The larger For-
aminifera are characterized by a compact shell struc-
ture which is nearly opaque under crossed nicols
(Cayeux, 1916, p. 352-375). The Foraminifera are de-
scribed and discussed in Chapters H and I.
The Foraminifera are associated with almost all of
the other types of organisms noted in the limestones of
Saipan. Many of the limestones are essentially algal-
foraminiferal limestones, the 2 groups of organisms
together at places making up as much as 75 to 90 per-
cent of the rock mass (pl. 35, figs. 1, 3).
CORALS
Rock-building corals are abundant, varied, and wide-
spread in many of the warm seas. They may be en-
crusting, branching, or may grow as compact heads.
The tropical limestone-building corals have a skeleton
of minute crystals of calcium carbonate (Vaughan and
Wells, 1943, p. 31-35). The majority of the reef-build-
ing corals secrete skeletons of aragonite. In the com-
mon tropical corals, the tabulae and dissepiments are
formed of parallel crystals which grow at right angles
to the surface (pl. 31, figs. 6, 7). The septa have more
complicated structures. Fibrous crystals form pris-
matic or cylindrical columns of tiny fibers which radiate
from a common axis, giving a feathery appearance in
longitudinal section. In most of the fossil corals, the
original aragonite has changed to calcite. The coral
skeletons recrystallize easily, and much of the fossil
material shows some degree of recrystallization (John-
son, 1951, p. 32-39).
Corals are commonly associated with calcareous algae
and Foraminifera (pl. 32, figs. 1, 5). It is difficult to
estimate their real importance in the Saipan limestones.
Large colonies and fragments of coral can commonly
be seen on outcrops of the post-Miocene limestones. In
thin sections of the nonreef facies, fragments of coral
are seen only occasionally (pl. 32, fig. 5). Also, as the
coral fragments are typically large in comparison to the
size of a thin section (pl. 34, fig. 3) , most samples con-
taining them were rejected in picking pieces for sec-
tioning. Thus, the amount of coral material in the
slides is not a fair indication of the amount in the rock.
It is clear from the field studies reported in Chapter A
that they have been important contributors to most of
the younger limestones and are dominant locally. They
are, however, relatively much less important than either
the calcareous algae or the Foraminfera in the Miocene
and Eocene limestones.
ACCESSORY LIMESTONE-BUILDING ORGANISMS
In addition to the important rock-building organisms
mentioned above, remains of the hard parts of a number
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180
of other animals are commonly observed in thin section.
However, these do not occur in sufficient quantity to
form an appreciable percentage of the rock. The most
important of these accessory rock builders are the echi-
noderms and mollusks, described below.
In addition, remains of several other groups of or-
ganisms were observed in trace amounts. These or-
ganisms included bryozoans, worms, ostracods, frag-
ments of crustaceans, and fish teeth and bones.
GEOLOGY OF SAIPAN, MARIANA ISLANDS
laminae extends little beyond the previous one. If thin
and crumpled, it gives a pearly luster to the shell. If
thick and relatively smooth, the shell surface has a
porcelaneous appearance.
Differences in the crystallographic organization of
the calcium carbonate shell are found between genera.
For example, among the oysters, both the prismatic and
laminated layers are composed of calcite, whereas in
some other groups, the layers are entirely aragonite; in
others the outer prismatic layer is calcite and the lami-
nated layer is aragonite. This original composition has
a (Treat deal to do with the manner in which the shell is
preserved, as aragonite is much less stable and more
easily dissolved than calcite. (See BOggild, 1930, and
Mackay, 1952, for details of the structure.)
Gastropod shells consist of an organic base impreg-
nated with calcite or aragonite. Ordinarily they, also,
show three or more distinct layers comparable in a
general way to those of the pelecypods. The calcareous
bulk of the shell is formed of very thin laminae com-
posed of microscopic prisms of calcium carbonate ori-
ented obliquely to the surface of the shell with a differ-
ent orientation in each lamination. In a few genera
the calcium carbonate is largely calcite, but in most
genera aragonite predominates. Many of the fossil
shells show evidence of secondary inversion of arago-
nite to calcite with considerable loss of the original
structure. Usually, the aragonite prisms are very small
and slender. The fact that the majority of gastropod
shells are formed of a high proportion of aragonite
probably explains their susceptibility to solution and
frequently poor preservation.
Many of the limestones contain small, usually badly
broken and worn fragments of molluscan shells, but
they are rarely abundant. However, at a number of
places shells are abundant in the late Pleistocene Tana-
pag limestone (pl. 33, fig. 3).
UNIDENTIFIED ORGANIC DEBRIS
Almost all of the thin sections studied contained some
organic fragments which, because of lack of distinguish-
ing structural features, recrystallization, or organic de-
struction, could not be identified. Recrystallization
and destructive organisms such as boring worms and
other animals and fine penetrating filaments of algae
tend to destroy the original structures.
ECHINODERMS
Echinoderms are common in the present seas around
Saipan. The phylum is there represented by echinoids,
holothurians, and several kinds of starfish. Remains of
all these groups have been recognized in the limestones.
The echinoids are the most abundant and most easily
recognized. The calcite of their spines is arranged in
optically oriented bands that show up prominently in
thin sections (pl. 31, figs. 3-5). Recognizable starfish
plates are relatively uncommon in thin sections of the
Saipan rocks. Holothurians have embedded in their
thick skin numerous tiny spines and peculiarly shaped
calcareous plates. These are occasionally recognized in
the thin sections of the limestone.
Remains of echinoderm tests have been observed in
all facies of all the limestones and may occur with any
or all of the other organisms recorded. They are most
commonly observed in the algal-foraminiferal lime-
stones. Their total volume is insignificant as compared
to the bulk of the limestone.
MOLLUSKS
The mollusks are abundantly represented in the pres-
ent seas around Saipan. In the limestones studied,
fragments of the shells of both pelecypods and gastro-
pods have been observed (pl. 33, fig. 3).
The pelecypod shell is composed of three or more dis-
tinct layers: commonly an outer scleraprotein or
conchin layer and calcareous middle and inner layers.
The outer layer is ordinarily thin and is generally worn
off in fossil shells. The middle layer is composed of
closely packed polygonal prisms of calcium carbonate,
disposed perpendicularly to the surface of the shell
(pl. 31, fig. 1). It is called the prismatic layer and
constitutes the outer layer in all the fossil fragments
observed. The prisms are secreted by the free edges of
the mantle lobes, hence growth takes place only on the
margins of the shell. The inner layer of the shell is
composed of thin laminae of calcite or aragonite, ar-
ranged roughly parallel to the surface of the mantle,
and is usually called the laminated layer. It is secreted
by the entire outer surface of the mantle, hence grows
continuously during the life of the shell, and each
GROUNDMASS
The fine groundmass between the coarse organic de-
bris makes up a large percentage of the rock, although
the amount varies considerably. It may range from
about 15 percent to as much as 85 percent. Four types
of groundmass were observed in different limestones:
PETROGRAPHY OF
very fine organic debris, calcareous paste, crystalline cal-
cite, and line sand and volcanic debris.
FINE ORGANIC DEBRIS
The very fine organic debris represents small, com-
monly minutely macerated fragments of foraminiferal
tests, shells, calcareous algae, and so on, the same mate-
rial as the large organic fragments but much more
finely triturated (pl. 35, figs. 2 and 4).
CALCAREOUS PASTE
The calcareous paste represents extremely fine parti-
cles of calcareous material which shows little or no
structure. Such a groundmass occurs in limestones of
all ages in many parts of the world. Its origin has been
discussed by many writers and there is no clear con-
census as to origin. The various suggestions on the
matter have been recently summarized by G. W. Crick-
may (1945, p. 233-235) in the report of the petrography
of the litnestones from Lau, Fiji. The suggested ori-
gins include altered fine organic debris, physicochemi-
cal precipitates (Johnson and Williamson, 1916),
biochemical precipitates (Bavendamm, 1913, p. 597;
Drew, 1014, p. 7-45), and extremely fine end products
from the abrasion of shells in the littoral zone. Quite
possibly some of the calcareous paste in the Saipan
limestones have been developed in all those ways.
However, the writer suggests that much of it may have
been deposited by algae, especially green and blue-green
types. Many such algae deposit calcium carbonate as
extremely fine particles so tiny that they appear dark
in thin sections. Such fine precipitate is found in most
limestones rich in algae. Wood (1941, p. 192) has called
it algal "dust."
In rocks formed largely of calcareous paste, the most
common fossils are Foraminifera and red algae. Echi-
noid fragments may be present, but in very small quan-
tity. Corals and molluscan debris, if present, are usually
fragmentary and badly worn. During studies around
Guam and Palau in 1952, the writer found that in
many places behind the outer part of the reef, green
algae occurred in considerable abundance with corals
and coralline algae. Among the limestone slides
studied, some showed vague suggestions of threads or
fibers, which the author interprets as indicative of algal
precipitation. These observations lead him to the be-
lief expressed above that algal precipitate is the source
of much of the material in the calcareous paste.
CRYSTALLINE CALCITE
Crystalline calcite is very common in the groundmass
of Saipan limestones. In some it is fine to medium
grained, and the whole groundmass has a more or less
granular appearance. Granular calcite may fill the
THE LIMESTONES
181
spaces between fossils and the coarser organic debris,
as well as the cavities in fossils. In other limestones, a
coarser, crystalline calcite surrounds fossils and similar
objects. In some specimens the optical axes are
oriented in phase with the prismatic material in the
shells. In others, the calcite forms aureoles or bands
around the outside of the fossils, with the calcite prisms
more or less perpendicular to the edges of the fossils.
Coarse crystalline material may fill vugs in the rock
and former cavities in fossils.
The crystalline calcite not only is of several types
but apparently was formed at several widely separated
times. One gets the impression that most of the granu-
lar calcite formed rather early, probably almost con-
temporaneously with deposition, as much of this calcite
coating as seen in section completely rings the elastic
particles. If these entire rings are assumed to represent
continuous sheaths, this suggests that the coating formed
while the particles could still be moved on the sea bot-
tom. If formed after the rock was well compacted, it
would not entirely sheath the elastic particles but would
fill the interspaces without coating the contact surfaces
of the particles.
The coarser crystalline calcite is later and probably
is related to the recrystallization of the rock which ap-
pears to be connected with the present chemical weather-
ing of the surface material. This will be considered
further in the discussion on recrystallization of the
limestones.
FINE SAND AND VOLCANIC DEBRIS
A few of the limestones contain appreciable amounts
of noncalcareous matter in the groundmass (pl. 32,
fig. 2). This is particularly true of the calcareous
bands in the Hagman formation and the limestones in
the Densinyama formation, but it is also true of some
of the limestones in the Donni sandstone member and
the tuffaceous facies of the Tag,pochau limestone. The
material ranges from pure silica sand to but slightly
altered volcanic sediments; much of it appears to be
weathered volcanic ash. Some has been altered to clay.
The rocks range from nearly pure limestones contain-
ing a small amount of pyroclastic material to calcareous
tuffs in which pyroclastic material predominates. The
tuffaceous limestones and calcareous tuffs range in color
from gray to brown, the shade depending largely on
the amount of pyroclastic material present.
CEMENTATION
The Saipan limestones vary greatly in the amount and
nature of the cementation, ranging from soft chalky
marls to aphanitic compact well-cemented limestones.
Typically, lithification of the limestone involved an in-
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GEOLOGY
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OF SAIPAN, MARIANA ISLANDS
nesium carbonate, which reduces solubility;. the form
of the calcium carbonate, whether aragomte or calcite;
and the nature of the shell structure, whether compact,
porous, coarsely prismatic, or very fine textured. Thus,
corals and many mollusks, particularly gastropods,
which are made up largely of aragonite fibers, are much
more soluble than those animals which have calcite
shells.
The shell structure largely determines the nature and
form of the recrystallization. Prismatic shells at first
become more coarsely prismatic. Later the prismatic
structure becomes more and more indistinct and is grad-
ually replaced by granular calcite. Compact shells or
skeletons, as of the larger Foraminifera and the cal-
careous coralline algae, become more transparent and
optically more distinctly birefringent. The echinoder-
mal material which is made up largely of crystal plates
becomes flecked with small grains of calcite and finally
changes to a granular aggregate of calcite, or, more
rarely, the individual crystals grow into very large
crystalline aggregates. Ultimately the fossils are so
altered that their indentification becomes impossible.
The calcareous paste usually is the first part of trte rock
to recrystallize as it is more susceptible to recrystalli-
zation than fossil fragments other than corals and cer-
tain of the mollusks, particularly gastropods. The orig-
inal paste is dark colored and extremely fine grained.
Commonly the grain size of the calcite particles is less
than 0.005 millimeters. The recrystallized granules are
from 10 to 100 times as large, frequently 20 to 25 times.
Recrystallization seems to be spotty and irregular. It
appears to be closely related to weathering, either pres-
ent or past.
troduction of calcium carbonate as fine granular par-
ticles. The carbonate is usually either quite fine (parti-
cles generally about 0.005 mm across), very rarely in
plumose crystalline masses, or in large crystals. The
latter imply secondary recrystallization. In some thin
sections there appears to have been a little recrystalliza-
tion of the paste along with the introduction of the gran-
ular calcite, but typically this does not happen. Com-
monly, well-preserved small fossils and fragments of
organic debris occur in the midst of granular calcite.
Most of the fossils surrounded by the granular calcite
show sharp, clean-cut outlines, quite different from the
more irregular and indefinite outlines found with the
recrystallized groundmass. In some specimens cemen-
tation locally was surprisingly complete, yet they con-
tain well-preserved fossils, as for example in a number
of specimens of the pink and white Eocene limestones.
RECRYSTALLIZATION
Many of the limestones show evidence of recrystalli-
zation, which ranges from very little to almost complete.
However, the amount of thoroughly recrystallized lime-
stone is small and is typically restricted to the weathered
surfaces. The recrystallization involves both the
groundmass, the fossils, and the coarse organic debris.
Characteristically, it starts in the groundmass and pro-
ceeds until most or all of the groundmass is replaced by
coarse crystalline calcite. Then the fossils are attacked
from the outer edges or from cavities within the mass.
At first crystals develop and grow along the margins
of the shells and foraminiferal tests and work forward
in optical continuity into the groundmass. Gradually,
the fossils become more and more indistinct until finally
they are indicated only by marginal lines of "dust," color
bands, or textural differences in the groundmass. Not
all of the organic remains are equally affected; some
alter more quickly and more thoroughly than others.
Roughly, they may be arranged in the following order
of decreasing susceptibility to crystallization: Corals,
mollusks, pelagic Foraminifera, beach-type Forami-
nifera, larger Foraminifera, echinoids, and calcareous
red algae. Among the green algae the Dasycladaceae
are much more susceptible to alteration than the red
coralline algae, but commonly Halimeda are less sus-
ceptible. Dasycladaceae commonly alter before any of
the Foraminifera. The larger Foraminifera, coralline
algae, and echinoids offer about equal resistance to re-
crystallization. Typically they are found with slightly
altered structures after most of the other fossils are re-
duced to the order of solubility of the shells and skeletal
fragments.
Solubility appears to be determined largely by the
chemical composition, particularly the presence of mag-
INTRODUCTION OF OTHER MINERALS
Accompanying recrystallization there is generally an
introduction of other minerals such as iron oxide,
silica, manganese oxides, and phosphate. However, the
total amount of such alteration is small, spotty, and very
localized. The most spectacular examples were ob-
served in some of the outcrops of the Mariana lime-
stone and some of the Halimeda-rich Mariana lime-
stone specimens. The Halimeda have been discolored
and in some cases largely replaced by iron oxide or more
rarely by phosphate.
CLASSIFICATION OF THE SAIPAN LIMESTONES
The Saipan limestones are broadly divisible into
four classes, though actually all graduations from one to
the other may be found. These classes are tuffaceous
limestones and calcareous tuffs (pl. 32, fig 2) ; detrital
limestones; bioclastic limestones (pl. 32, fig. 1-1) ; and
constructional limestone, commonly the coral-algal type
(pl. 32, fig. 5) .
?
N. ?
PETROGRAPHY OF THE LIMESTONES
TUFFACEOUS LIMESTONES AND CALCAREOUS
TUFFS
The Eocene rocks exposed on Saipan are mainly py-
roclastic rocks, some of which accumulated in marine
waters and contain calcareous material and even well-
preserved fossils. The amount of calcareous material
ranges from very low to very high, that is, from a vol-
canic tuff containing a slight amount of calcium car-
bonate to nearly pure limestones slightly contaminated
with volcanic material.
Much higher in the section are tuffaceous Miocene
limestones which may contain 12 to 15 percent or more
reworked volcanic material. However, in most of the
specimens selected for study the percentage of volcanic
material was low, 3 to 5 percent.
A typical representative is a specimen from locality
C132, which contains rounded fragments of corals,
larger Foraminifera, and crustose coralline algae. All
are considerably rounded and worn and are associated
with less worn fragments of articulated coralline algae
in a groundmass of clear crystalline calcite. The or-
ganic debris forms 45 to 52 percent of the rock, the
volcanic material forms 3 to 8 percent, and the rest is
clear crystalline calcite groundmass (see table).
DETRITAL LIMESTONES
The limestones here referred to as detrital contain
appreciable amounts of rounded particles of reworked
older limestones or previously deposited and partially
lithified sediments. The particles may range from small
sand grains to well-rounded pebbles, 3 or 4 centimeters
across. Commonly these occur in a groundmass of finely
macerated organic debris or of calcareous paste. Coarse
particles of organic debris and even unworn tests of
Foraminifera may occur between the detrital limestone
fragments. There probably was very little difference
in age between the detrital material and the ground-
mass in most of the Saipan limestones that are desig-
nated as detrital.
Relatively pure detrital limestones are found in the
various facies of the Eocene, Miocene, and Pleistocene,
but are most abundant in the Pleistocene and Miocene.
BIOCLASTIC LIMESTONES
Although nearly all the limestones of Saipan are
elastic limestones, those referred to as bioclastic lime-
stones are composed of fragments or whole tests of
Foram inifera, pieces of coral, and pieces of other types
of fossils, rather than pieces of older rocks. The ma-
jority of them are surprisingly free of terriginous sedi-
ments and many of them are very pure chemically. For
convenience in discussion, they are divided in groups
on the basis of the predominant rock-building organism
present.
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183
FORAMINIFERAL LIMESTONES
Foraminifera are present in all of the limestones of
Saipan. The calcareous algae and the Foraminifera to-
gether are the most important limestone-building or-
ganisms present. The percentage of Foraminifera
measured in the sections of rock studied ranges from 5
to 85 percent. Where the Foraminifera are present in
quantities over 50 percent, the rock may be referred to
as a foraminiferal limestone. Examples were observed
in the inequigTanular facies (pl. 35, fig. 2) of the Mio-
cene Tagpochau limestone; in the tuffaceous facies of
the same formation; and in the white facies in the
Eocene Matansa limestone, the last at places containing
GO to 75 percent Foraminifera (pl. 35, fig. 1) .
ALGAL-FORAMINIFERAL LIMESTONES
In the majority of the Saipan limestones, the Forami-
nifera and the algae together make up over 50 percent
of the recognizable organic debris (see table). In some
samples, the amount of the two organisms present is
about equal. In others, there is slightly more of one
than the other. These are collectively classed as algal-
foraminiferal limestones (pl. 32, fig. 3; pl. 34; pl. 35).
As they occur in rocks of all ages on Saipan and in al-
most all of the facies represented, it is not surprising
that the algal-foraminiferal limestones show consider-
able variety. The Foraminif era included may be large
or small. The algae may be crustose corallines, articu-
late corallines, Halimeda, or some mixture of these three
types.
The Foraminifera commonly stand out clearly in the
specimens, slides, or photographs. The algae may in-
clude crusts in position of growth, fragments of vari-
ous crustose types, numerous pieces of articulated algae,
or Halimeda segments.
CORAL-ALGAL LIMESTONES
In laboratory studies of specimens and thin sections
it is difficult to evaluate the importance of corals in
rock building because in collecting the specimens in the
field one more or less consciously avoids taking hand
specimens that are made up entirely of coral or which
contain very large pieces of coral. Similarly, in pre-
paring slices for thin sections one avoids pieces that
would be entirely coral. In the field, large heads of
coral or large rounded fragments of them are fre-
quently observed in the rocks. It is safe to say that
corals are more important than the study of the hand
specimens and sections would indicate. Certainly
coral-rich rocks are important in the Pleistocene and
Recent limestones, and a number of specimens indicate
that coralline algae and Foraminifera are abundantly
associated with them. The term coral-algal limestone
is generally employed for this group (pl. 31, fig. 2).
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GEOLOGY OF SAIPAN, 3IARIANA ISLANDS
Laboratory studies of specimens of these limestones in-
dicate two quite different. types. The firzt., made up en-
tirely of worn pieces of coral which may range in size
from more than a foot across to relatively coarse sand,
is a bioclastic limestone (pl. 33, fig. 4). The second,
containing either essentially whole colonies of corals
and algae which occur more or less in position of growth
or large relatively unworn fragments, is a construc-
tional limestone.
AL,G.tr., LIMESTONES
The algal limestones may be divided into three
groups on the basis of the type of algae present in the
largest amounts: crustose coralline limestones, articu-
late coralline limestones., and Halimeda limestones.
The crustose coralline limestones (pl. 32, fig. 2;
pl. 35, figs. 1, 2) are formed of or contain considerable
quantities of the crustose corallines. including. Archae-
olithot7ianzniont. LitIzothamniom, Litizophynum. .41e8o-
pItyllunz. Lithoporella, and Dermataithon. The lime-
stones may contain entire crusts in position of growth,
or they may consist of fragments of the plants, com-
monly worn and abraded.
The articulate coralline limestones (pl. 35, fig.. 4) in-
clude all limestones which contain an abundance of
fragments of the articulated coralline
Anzphiroa, T ania, and others. Algae of this type are
especially abundant in the Pleistocene Mariana lime-
stone and in certain beds of Miocene limestones, al-
though they do occur occasionally in some of the Eocene
limestones, but only in the Miocene and Pleistocene do
they occur in sufficient quantities to be of outstanding
importance. Even where the pieces are so abundant as
to cover a large area of the slide., the actual bulk per-
centage is commonly smaller than it would appear, be-
cause typical segments of these algae are so tiny.
Halimeda limestones (pl. 34) are the last type of
bioclastic algal limestone to be considered. As will be
noted in the table, Holimeda occur in limestones of all
ages present on Saipan, and beds may be found which
contain them in such quantilids that the limestones may
correctly be called Halinzeda limestones. These are
abundant and widespread in the Halimeda-rich facies
of the Pleistocene Mariana limestone, and they are lo-
cally abundant in both the inequigra,nular and the
manly fades of the Miocene Tagpochau limestone.
Halimeda is also fairly abundant at a few localities at
places within the white facies of the Eocene Matan.sa.
limestone.
The Halimeda limestones may be filled with joint
fragments which, on weathered surfaces, strongly sug-
gest some of the large platy Foraminifera (pl. 34, fig.
2) but are easily recognized by the differences of struc-
ture in thin sections. In other specimens the Ha/imeda
segments have been dissolved and the rock appears
porous and vuggy on account of the numerous molds of
Halimeda (pl. 34, fig. 4). In still other samples the
joints have been replaced by iron or phosphate.
CONSTRUCTIONAL LIMESTONES
These limestones represent reefy limestone masses
formed largely of corals, calcareous algae, or other or-
ganisms, enough of which are in the position of growth
to indicate constructional origin. Such deposits nor-
mally are massive and poorly bedded. Most can be
easily recognized in the field, but in hand specimens or
slides they cannot readily be separated from bioclastic
limestones, as they are built by the same organisms.
Many of the coral-algal limestones of Saipan below, in
this group.
SUMMARY
Most of the Saipan limestones consist of org-anic ma-
terial in an organic or finely crystalline matrix or,
rarely, are. detrital and derived from earlier formed
organic limestones. The results of the study of a series
of specimens are tabulated in the following table.
PETROGRAPHY OF THE LIMESTONES
Organic constitucnia of Saipan limestone8
(A p us sign (4-) indicates presence, without estimate of percentage]
185
Age
Formation
Fades
Slide no.
Organisms (percent)
Groundmass (percent)
For-
amin-
!fern
Corals
Mol-
luscan
shells
Edit-
not('
spines
or
plates
Mali-
meda
Crus-
Lose
coral-
line
algae
Artie-
ulated
coral-
line
algae
Other
algae
Untie.
ter-
mined
Fine
organic
debris
Fine
dark
calcar-
eous
precip-
Bate
Coarse
crystal-
line
calcite
Recent
Beach rock
COO
5
11
7
+
11
10
Pleistocene
Tanapag limestone .
No fades subdivi-
C50
4
... ....
.......
20
5
+
+
don.
C50.. ____
15
18
10
50
4-
Mariana limestone .
Massive
C67 (2)
5
7
4
+
5
7+
+
MLISSIVe (1)1111k) .
C65
8
8
15
+
60
C68
5
11
+
5
10
4-
Ilalimedagleh .
C36
6
+
8
5
4-
+
C49.. ...
8
13
... .....
8
5
+
C52
3
18
8
5
C52
4
12
14
+
+
+
C54
18
55
10
12
+
+
21
4
I
2
16
1
11
+
Miottme
Tagpochau lime-
Inegulgratmlar ..
1320. ....
22
2
5
30
4-
+
stone
13261
35
+
+
25
+
+
11281
27
4
15
6
+
+
13284
27
2
9
9 ^
..I
5
+
+
13295
15
2
44
7
+
+
13295
it
2
50
+
+
C4
50
24
----------------------+
+
+
C16
18
5
+
12
15
+
+
CIGa
11
10
+
+
8
---------------------+
+
C21
18
4
+
+
3
11
6
+
+
C55
15
+
12
4
+
C62
8
8
6
9
+
+
C62
10
30
+
9
5
+
+
C73
25
17
5
7
+
+
+
C73
20
40
8
11
+
+
C76.. .
35
10
14
+
S257.. ....
12
I
1
20
q
+
+
S243......
811
7
I
+
+
Marly
5
15
:1:
5
8
+
+
S62 ..._
9
7
8
... . .
+
Clio
9
+
+
4
+
+
Transitional .
C56
16
5
+
15
8
+
+
C56
21
0
4
22
5
+
+
Tuffaceous
C14
12
0
10
+
CI4
65
3
5
+
C132..
24
.
+
2
2
+
Rubbly .
C23
9
+
8
7
+
+
C23.. ____
13
11
+
5
10
+
C127.-. -.
6
15
I
I
.
21
2
.
S
+
Eocene Matrinsa limestone White .
1367(1)..
5
1
4
8
21
. ...
+
1367(2).
7
16
.._. .
8
2
16
. .
5
+
+
1307(3)....
6
7
1
11
3
18
.
+
1375
7
11
+
XI
4
+
+
1350
8
7
3
+
5
+
C13(1)--
85
0
+
7
+
C13(2) ....50
3
+
Bagman . _
Pink
S262 _ _ ..
14
12
. .
2
3
10
+
i
Conglomerate-
sandstone.
5
15
.
...
5
4
30
-------------+
--.- -.-
I
-
SELECTED BIBLIOGRAPHY
Bavendanim, Werner, 1931, The possible role of micro-organ-
isms ill the precipitation of calcium carbonate in tropical
seas: Science, new ser., v. 73, p. 597-598.
Briggild, 0. B., 1930, The shell structure of the mollusks: Mem.
de L'Acad. Royale des Sci. et des Lettres de Danemark,
Copenhague, Sec. des Sc., 9m0 ser., tome 2, no. 2.
Cayeux, Lucien, 1916, Introduction a l'etude petrographique des
rochs sedimentaries : Paris, Imprimerie Nationale, 525 p.
Crickmay, G. W., 1945, Petrography of limestones; geology of
Lau, Fiji: Bernice P. Bishop Mus. (Honolulu, Hawaii),
Bull. 181, p. 211-258.
Drew, G. H., 1914, On the precipitation of calcium carbonate in
the sea by marine bacteria: Carnegie Inst. Washington Pub.,
Papers from The Torugas Lab., v. 5, p. 7-45.
Johnson, J. H., 1943, Geologic importance of calcareous algae,
with annotated bibliography: Colo. School of Mines Quart.,
v. 38, no. 1,102 p.
1951, An introduction to the study of organic limestones
[revised ed.] : Colo. School of Mines Quart., v. 46, no. 2,
185 p.
Johnston, John, and Williamson, E. D., 1916, The role of inorganic
3SS-106-----57-7
agencies in the deposition of calcium carbonate: Jour.
Geology, v. 24, p. 729-750.
Low, J. W, 1951, Examination of well cuttings: Colo. School of
Mines Quart., v. 46, no. 4,48 p.
Mackay, I. H., 1952, The shell structure of the modern mollusks:
Colo. School of Mines Quart., V. 47, no. 2, p. 1-27.
North, F. J., 1930, Limestones, their origins, distributions, and
uses: London, Thomas INIurby & Co., 467 p.
Pia, Julius, 1926, Pflanzen als Gesteinsbilder : Berlin, Gebriider
Borntraeger, p. 362.
Revell?, Roger, 1938, Physio-chemical factors affecting the solu-
bility of calcium carbonate in sea water: Jour. Sed.
Petrology, v. 8, p. 103-110.
Sorby, H. C., 1879, On the structure and origin of limestone:
Geol. Soc. London Proc., v. 35, p. 56-95.
Twenhofel, W. H., 1950, Principles of sedimentation: New York,
McGraw-Hill Book Co., Inc., 2d ed., 673 p.
Vaughan, T. W., 1917, Chemical and organic deposits of the sea:
Geol. Soc. America Bull., v. 28, p. 933-944.
Vaughan, T. W., and Wells, J. W., 1943, Revision of the suborders,
families, and genera of the Scleractinia : Geol. Soc. America
Special Paper 44,363 p.
Wood, Alan, 1941, 'Algal dust' and the finer-grained varieties of
carboniferous limestone: Geol. 'Mag., v. 78, p. 192-201.
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INDEX
Page
Page
Page
Algal precipitate ("dust")
181
I fagman formation __
181,184, 1)1. 2; chart
Merophyllum.
177,178
Amphtroa
177, 178,181; p1.35
Hattmeda.... . 177, 178. 170, 182,18.1, 181, 185; pls. 31,34
Arehaeolithothamnion
177, 184
I iolothurians.
180
Naftali Peninsula
pl 34
Arthroearata
177
Iron oxide
182
Opuntia
178
Bryozoans
180
Janie
177, 178, 181
Ostracods
180
Calcite, crystalline
181
Limestone, algal
183-181; pls. 32,35
Pelecypods.
180
Chetlosporum.
177
constructional .
. 181
Phosphate
182
Coral:Ina ..
177, 178. 184
coral-algal..
183-18-1, p1.32
PorolUhon
177
Corals
179: pls. 31, 31
coral-foraminiferal
pls. 32, 33
Crustaceans
. 180
crustose coralline.
184
Recrystallization .
182
Cymopolla .
177,178, 179
detrital
. 183
Red algae, articulate..
178:1)1 35
fomtniniferal
183, pls. 32,35
crustose
178
Denslnyama formation
181; pl. 2; chart
fomminiferal-aleal
. p1.35
listed
177
Dermatolithon
177,184
184
tuffaceous
.183, pl 32
Silica
182
Echinolds _
ISO; pls. 31,34
Ltthophyllum
177.184
Starfish
180
LIthoporella
177,184
Field localities, location
177; pl. 4
Lithothamnton
. 177,184
Tagpochau limestone. ...
pl 2; chart
Fish, bones .
180
Manganese oxides..
. 182
Donni sandstono member
181; p1.32
teeth...
. 180
Mariana limestone...
178, 184, pl. 2; chart
inequigranular facies
183,181; pls. 32, 34,35
Foraminifera
170; pls. 31, 33, 34, 35
Halimeda-rich fades..
pls. 31,34
many fades
_ 184
massive fades
_ pls. 33,35
organic constituents listed
_ 185
Gastropods
180; pls. 33,35
organic constituents listed
185
tuffaceous facies .....
pl. 32
Goniolithon
177
Matansa limestone
pl. 2; chart
Tanapag limestone
178; pls. 2,31,32; chart
Grain size, classification
177
organic constituents listed_ .
_ 185
organic constituents listed .
185
Green algae, codlaceans
178-179
pink facies
pls. 31, 35
Tuff, calcareous ...... .
183
dasycladaceans
179
transitional fades...
. p1.31
listed
177
white fades .
_ 183, 181, 185; pl. 32
Worms.
180
187
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PLATE 31
(X 40 unless otherwise indicated)
FIGURE 1. Fragments of coral, molluscan shell, calcareous algae (black), and Foraminifera in Recent beach rock (X 15). Field
locality C66. USNM 109233.
2. Coral (left half) coated with a thin layer of encrusting Foraminifera, a thin algal crust, and a thick layer of encrusting Fo-
raminifera. Eocene, Matansa limestone, pink facies. Field locality S604. Specimen on USGS type-algae slide
a112-1 from paleobotanical locality D173.
3. Section of an echinoid plate. Eocene, Matansa limestone, transitional facies. Field locality S349. Specimen on
USGS type-algae slide a8S-1 from paleobotanical locality D226.
4. Section through two echinoid spines. A foraminifer at lower left. Pleistocene, Ilalinzeda-rich facies of Mariana lime-
stone. Field locality S691. USNM 109241.
5. Echinoid spine above fragment of molluscan shell. Recent beach rock. Field locality C66. USNM 109233.
6-7. Coral (X 15). Pleistocene, Tanapag limestone. Field locality C35 (with carbon-14 age 20,000 years). USNM
109229.
6. A perpendicular section.
7. A parallel section.
GEOLOGICAL SURVEY
PROFESSIONAL PAPER 280 PLATE 31
ROCK-BUILDING ORGANISMS
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EOLOGICA I. SURVEY
pitoFESSION A I. PA PER NI pLATE 32
SA I PAN LIM EST )\ ES
PLATE 32
[Natural size)
FIGURE 1. Coral-foraminiferal limestone. Eocene, Matansa limestone, white facies. Field locality B69. USNM 109225.
2. Tuffaceous Miocene Tagpochau limestone containing coarse debris of coral, coralline algae, molluscan shells, and some
Foraminifera. Field locality B375. USNM 109228.
3. Algal limestone, reworked fragment containing large pieces of coralline algae. Miocene, Donni sandstone member of
Tagpochau limestone. Field locality S129. USNM 109236.
4. Foraminiferal limestone. Weathered surface shows numerous larger Foraminifera. Miocene, Tagpochau limestone,
inequigranular facies. Field locality S536. USNM 109240.
5. Coral limestone, reworked fragment. Weathered surface shows large pieces of coral. Miocene, Donni sandstone
member of Tagpochau limestone. Field locality S127. USNM 109235.
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PLATE 33
[Natural size]
FIGURE 1. Eocene Matansa limestone, pink facies, composed largely of medium-grained to fine organic debris and Foraminifera.
Field locality S341. USNM 109238.
2. Typical Miocene Tagpocbau limestone, inequigranular facies. Field locality C78. USNM 109234.
3. Coral-foraminiferal limestone with gastropods. Pleistocene, Tanapag limestone. Field locality C50. USNM 109231.
4. Coral-foraminiferal limestone somewhat recrystallized. Pleistocene, Mariana limestone, massive facies. Field
locality B226. USNM 109226.
GEOLOGICAL SURVEY
PROFESSIONAL PAPER 280 PLATE 33
SAIPAN LIMESTONES
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GEOLOGICAL SUR VET
PROFESSION A I. PA PER 2811 PLATE 31
HA LI 'VEDA LIMESTONES
PLATE 34
[Natural size unless otherwise indicated
Fla rnE 1. Halimeda-rich Miocene Tagpochau limestone, inequigranular facies. Section (X 15), showing sections of Halinzeda
segments, pieces of coral, and shreds of large Foraminifera. Field locality B281. USNM 109227.
2. Halinieda-rich Miocene Tagpochau limestone, inequigranular facies. Weathered surface of band specimen showing
Halinzeda segments, an echinoid spine, and pieces of coral. Field locality C16. USNM 109230.
3-4. Halimeda-rich Pleistocene Mariana limestone from Naftan Peninsula, southeast Saipan. USNM 109224.
3. Slide (X 15). Halinteda segments, Foraminifera, and fragments of coral.
4. Band specimen showing pits left by Halimeda segments removed by weathering.
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FIGURE 1.
2.
3.
4.
PLATE 35
Foraminiferal-algal limestone. Eocene, Matansa limestone, pink facies. The black particles are pieces of crustose
coralline algae. Field locality B251. USNM Foraminifera type number 624471.
Foraminiferal limestone. Miocene, Tagpochau limestone, inequigranular facies. Both larger and smaller Foraminifera
present. Much of the groundmass consists of foraminiferal debris. The large light-colored pieces at the base of the
photograph are fragments of shells of mollusks. Field locality S257. USNM 109237.
Foraminiferal-algal limestone. Eocene, Matansa limestone, pink facies. The black particles are pieces of crustose
coralline algae. Most of the rest of the slide consists of tests and fragments of larger Foraminifera. Field locality
S345. USNM 109239.
Algal limestone. Pleistocene, Mariana limestone, massive facies. Numerous segments of articulated coralline algae
(Amphiroa) and a gastropod in a groundmass of fine organic debris. Field locality C65. USNM 109232.
?,E01.061CA I. St' E
PROFESSIOSA I. PAPER ZSII PLATE 35
SECTIONS OF SAIP.1\ LIMESTONES
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1
?
,
1
.
?
II $
Soils
By RALPH J. McCRACKEN
GEOLOGICAL SURVEY PROFESSIONAL PAPER 280-D
A classification of the soils of Saipan,
their distribution, extent, genesis, and
morphology
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.41
CONTENTS
Abstract
Introduction and acknowledgments
Factors influencing soil development
Climate
Parent materials
Slope and drainage
Time
Vegetation
Soil series and types
Soil profiles and descriptions
Soils of the uplands with complete A-B-C profiles
Akina series
Akina clay
Akina clay loam
Dago clay
Chacha clay
Saipan clay
Shallow soils of the uplands
Chinen clay loam
Page
189
189
190
190
191
192
192
193
194
194
194
194
195
195
195
196
196
197
197
Soil series and types?Continued
Soil profiles and descriptions?Continued
Shallow soils of the uplands?Continued
Dandan clay
Teo soils
Soils developing from slope wash and alluvium
Lito clay
Alluvial clays
Soils of the western coastal plain
Shioya loamy sand
Miscellaneous land types
Marsh
Rough stony land on dacite
Rough stony land on limestone
Rough broken land
Morphology and genesis
Classification
Selected bibliography
Index
Page
197
198
198
198
199
199
199
200
200
200
200
200
200
204
205
207
ILLUSTRATIONS
[ Plates In pocket]
PLATE 2. Generalized geologic map and sections of Saipan, Mariana Islands.
36. Generalized soil map of Saipan.
FIGURE 25. Mean monthly temperatures of Saipan
26. Mean monthly rainfall of Saipan
27. Percentage distribution of Saipan soil groups and land types
28. Cation-exchange capacity and percentages of organic carbon, clay,
29. Cation-exchange capacity and percentages of organic carbon, clay,
30. Cation-exchange capacity and percentages of organic carbon, clay,
31. Cation-exchange capacity and percentages of organic carbon, clay,
32. Cation-exchange capacity and percentages of organic carbon, clay,
33. Cation-exchange capacity and percentages of organic carbon, clay,
34. Cation-exchange capacity and percentages of organic carbon, clay,
35. Cation-exchange capacity and percentages of organic carbon, clay,
TABLES
Page
190
190
194
and base saturation?Akina clay 195
and base saturation?Akins clay loam 195
and base saturation?Dago clay 196
and base saturation?Chacha clay. 196
and base saturation?Saipan clay 197
and base saturation?Dandan clay 198
and base saturation?Teo clay 198
and base saturation?Lito clay 199
Page
TABLE 1. Physical and chemical characteristics of Saipan soils 201
2. Some constants of representative Saipan soils 202
3. Estimated mineralogical composition of clay from selected horizons of some Saipan soils 202
4. Chemical composition of the clay fraction of representative Saipan soils 203
CHART
Page
Summary of the geologic units of Saipan In pocket
UI
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GEOLOGY OF SAIPAN, MARIANA ISLANDS
SOILS
By RALPH J. McCrucKEN
ABSTRACT
This report describes factors affecting soil formation and the
morphology and distribution of the various soils on the tropical
island of Saipan. The annual rainfall of about 80-90 inches Is
fairly uniformly distributed throughout the year, with a slight
decrease in March and April. Mean monthly temperatures are
800-850 F.
A lithosol (shallow stony soil) underlain by limestone and
the miscellaneous land unit of rough stony land on limestone
are the two most extensively mapped soil units. Two soils that
are moderately deep or deep (3-6 feet or more) over limestone
and of intermediate depth (18-36 inches) are of limited areal
extent.
Volcanic rocks underlie a little less than one-third of the
island. With the exception of two small areas of (incite out-
crops that total less than a square mile, the volcanic rocks are
andesitic in composition. The (Incites give rise to little or no
soil owing to the rugged topography of their area of outcrop
and their extremely siliceous composition. Two soil series with
complete A-B-C profiles have developed in the areas of volcanic-
rock outcrop. A shallow lithosolic soil type and a miscellaneous
land unit, rough broken land, are also recognized.
Less extensive types of parent material are the limesands
of the western coastal plain and the colluvial and alluvial
materials.
The soils of the uplands with complete A-B-C profiles do not
possess some of the diagnostic characteristics of latosols, a fact
which might be considered anomalous because of the prevailing
climate, nature of underlying rocks, and duration of develop-
ment. These soils do not have low silica-sesquioxide ratios of
the clay fraction, do not have low cation-exchange capacities,
lack intense iron and aluminum accumulation, and do not have
the high degree of aggregate stability common in latosols. They
also contain relatively high amounts of 2: 1-layer silicate
minerals such as vermiculite, hydrous-mica mixed-layer mate-
rials, and montmorillonite. However, they are strongly weath-
ered, as indicated by the high content of clay and the low con-
tent of most primary minerals and soluble constituents. Their
commonly red color indicates intense oxidation.
INTRODUCTION AND ACKNOWLEDGMENTS
This report deals with the soils of Saipan (pl. 36), lo-
cated at latitude 15? N. in the Mariana Islands of the
* Tennessee Agricultural Experiment Station formerly with U. S.
Department of Agriculture.
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western Pacific Ocean. The area is tropical?about the
same distance north of the Equator as northern Hon-
duras and southern Guatemala?and is a few hundred
miles closer to the Equator than the Hawaiian Islands
or Puerto Rico.
Field studies and mapping of the soils were in con-
junction with geologic studies of the island as a part
of the Pacific islands geologic-mapping program of the
U. S. Geological Survey, carried out in cooperation
with the Corps of Engineers, U. S. Army. The soil-sur-
vey party was attached to the geologic field party which
was under the direction of Preston E. Cloud, Jr.
The purpose of this investigation was to classify the
soils, to determine their distribution and extent, and
to learn as much as possible about their genesis and
morphology. The study included establishing map
units defined largely in terms of soil series and types,
and collecting samples of profiles of the major soil
series for future laboratory analysis. Mapping was on
aerial photographs at a scale of 1: 20,000, a level of
cartographic generalization comparable to that com-
monly used in soil surveys in agricultural regions of
America and Europe.
Particle-size distribution was determined by the
pipette method (Kilmer and Alexander, 1949). The
pH was determined on a 1: 1 soil suspension using a
glass electrode. Neutral normal ammonium acetate
was used to extract the exchangeable bases, and barium
chloride-triethanolamine was used to determine exchangeable hydrogen (Peech and others, 1947). Or-
ganic carbon was determined by dry combustion. Free
iron oxides were determined by the V. J. Kilmer mod-
ification (written communication) of the Deb method.
For description and definition of soil-group names
used, see U. S. Department of Agriculture Yearbook
for 1938, and Thorp and Smith (1949).
The field party was fortunate in having access to the
report of earlier investigations of Saipan soils by Japa-
nese soil scientists (Kawamura, Tanaka, and Inagaki,
189
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?
190
GEOLOGY OF SAIPAN, MARIANA ISLANDS
1940). Reports on detailed soil surveys of other near-
tropical island groups (the Hawaiian Islands, Cline,
and others, 1955; Puerto Rico, R. C. Roberts and others,
1942), as well as reconnaissances of somewhat similar
tropical areas elsewhere (the East Indies, Mohr, 1944;
the Belgian Congo, Kellogg and Davol, 1949; East
Africa, Milne, 1936) were useful as background infor-
mation. Experience gained from similar soil surveys
of Palau Islands and Okinawa, initiated shortly before
the work on Saipan, was made available by personal
communication from soil-survey men working in those
areas, as well as from the work of the writer in the Palau
group. In addition, the soil-survey men were fortunate
in being able to consult with the members of the geologic
field party about the parent rocks and other parent
material as the geologic and soil mapping progressed.
The mapping and the collecting of samples were ac-
complished during the early part of 1949 by Ray E.
Zarza (U. S. Geological Survey) and Ralph J.
McCracken (U. S. Department of Agriculture). The
mapping was reviewed by E. H. Templin (U. S. De-
partment of Agriculture), who was technical consult-
ant on soils investigations for the Pacific islands
mapping program at that time.
Laboratory determinations reported and discussed
herein were performed in the U. S. Department of
Agriculture soil-survey laboratories of which L. T.
Alexander is in charge. Differential thermal and X-ray
analyses were made by R. S. Dyal ; exchangeable-cation
determinations by E. M. Roller and R. S. Clarke, Jr.,
and mechanical analyses, pH, and organic-carbon deter-
minations by B. J. Epstein and C. J. Scott. Determina-
tions of free iron oxide were made by V. J. Kilmer.
In addition to those already mentioned, it is a pleas-
ure to acknowledge the many helpful suggestions of
geologists Robert G. Schmidt and Harold IV. Burke,
both of the Geological Survey, during and after com-
pletion of field work. Suggestions by Prof. Marlin G.
Cline of Cornell University and by Roy W. Simonson
and Guy D. Smith, of the Soil Survey, U. S. Depart-
ment of Agriculture, during preparation of the manu-
script are also gratefully acknowledged. Acknowledg-
ment is also made to Prof. R. T. Endo for translation of
a number of journal articles from the original Japanese.
FACTORS INFLUENCING SOIL FORMATION
CLIMATE
Mean annual rainfall, its monthly distribution, and
whether it comes as heavy, sustained rain or frequent
showers are of particular interest to the student of trop-
ical soils. Mean monthly temperatures and the range
of diurnal variations are also of great interest. Rain-
100
t 90
6?
70
Mean
monthly
maxim,
Mean
0
0
0
0
0
Mem
munthly
nUnInt
_
Jan Feb Mar Apr May June July Aug Sept Oct Nor One
Monne 25.?Mean monthly temperatures for the period 1928-37, Saipan,
Mariana Islands.
15
Vto
:f
C 5
;1*
Mean annua total. 825 inches
Jen Feb Mar Apr May June My Aug Sept Oct Nov Onn
PIOUI1E 20.?Mean monthly rainfall for the period 192S-37, Sniper
Mariana Islands.
shadow effects are not significant in soil development
on Saipan. The highest peak on the island reaches an
elevation of only a little more than 1,500 feet. and, as
the axial ridge is oriented in a north-northeast direc-
tion and storms and winds in the rainier season often
come from the south or southwest, no differences in soils
were observed on the western (lee) slopes as compared
with the eastern slopes.
The climate of Saipan is discussed in Chapter A
(General Geology), but the importance of climate in
soil development makes advisable the graphic summa-
tion of the essential data here (figs. 25, 26).
scriptions including chemical analyses and the strati-
graphic relationships of the parent rocks are given in
discussed below.
In Puerto Rico seven rainfall belts (due to rain-
shadow effects) have been found to coincide with rather
distinct soil regions (Roberts and others, 1942, p. 57-58,
426-484). Latosolic soils with some red-yellow pod-
zolic soils were found to dominate in those regions
which received an average of more than about 75 inches
of rainfall per year and were generally not found in
regions receiving less than this amount of annual
Distribution of soils in the Hawaiian Islands (Cline
and others, 1955) is a striking example of the influence
of amount and distribution of rainfall on soil develop-
ment over mainly basaltic parent rocks in the tropics.
Those belts of the islands receiving 45-150 inches of
rain per year have brown forestlike soils or humic lato-
sols, if the rainfall is seasonally distributed. Soils des-
ignated as hydrol humic latosols are found in those re-
gions receiving more than 150 inches of rain per year.
SOILS
Descriptions of these soil groups and the changes in
morphological properties are discussed in detail.
Significant changes in chemical properties and min-
eralogical content of the Hawaiian soils with change in
annual rainfall have been demonstrated in the work of
Tamura, Jackson, and Sherman (1953) and Tanada,
(1951). These investigators postulate that with in-
creasing annual rainfall the content of bases and of
silica decreases, whereas gibbsite, iron oxides, and or-
ganic matter increase. However, the soils receiving
very high (more than 150 inches) annual rainfall are
an exception to this generalization since reducing con-
ditions resulting from this high rainfall cause a decrease
in iron oxide content. According to the above-men-
tioned investigators, under the conditions in Hawaii
the content of 2: 1-layer clays (such as vermiculite, hy-
drous-mica mixed-layer materials, and montmorillo-
nite) and of potassium in the soils increases with in-
creasing annual rainfall and reaches a maximum at
about 80 inches per year. Tamura, Jackson, and Sher-
man (1953) postulate that the increase of 2: 1-layer sili-
cates (such as illite and hydrous mica) can be explained
by the nature of the rainfall, which, as it increases in
amount, comes as frequent showers. These showers
probably maintain the soil moisture at near field capac-
ity. Under this condition, it is postulated that silica
is iwt completely lost by leaching and is available
for combination with alumina to form silicate clay. The
importance of these observations to the present study
lies in the fact that annual rainfall of Saipan is about
82 inches, and other soil-forming factors on the island
are roughly similar to those prevailing in the Hawaiian
Islands.
That distribution of annual rainfall is an important
factor in soil development in warm regions has been
postulated by Mohr (1944, P. 55-67), Humbert (1948),
and Sherman (1949). In Mohr's classification of tropi-
cal climates according to the number and distribution of
wet and dry months, dry months have an average of
less than 2.4 inches (60 mm) of rain, wet months 4 8
inches, and very wet months more than 8 inches.
Sherman (1949) has presented data for Hawaiian
soils which indicate that the proportion of dry, wet, and
very wet months, as defined by Mohr (19-14), is impor-
tant in determining the nature of free oxides which be-
come stabilized and accumulate in the soil solum (A and
B horizons) . He generalizes that in warm climates
with alternating wet and dry seasons (2 or more con-
secutive months receiving less than 2.4 inches of rain-
fall), soils exhibit a different course of development
than those developing without a definite dry season.
The iron and titanium oxide content of those which
are intermittently dry is postulated to be increasing,
with an iron-rich laterite crust as an end product. The
191
iron oxide content of the continually moist warm soils,
on the other hand, is postulated as decreasing as the an-
nual rainfall increases, with a bauxite laterite as an
end product. The observations of Humbert (1948) in
British New Guinea tend to confirm these generaliza-
tions.
Saipan can be classed as having no dry months ac-
cording to Mohr's criteria and therefore no significant
dry season. However, a drier season does occur dur-
ing March and April, as can be seen in figure 26,
although actually more than 2.4 inches of rain falls dur-
ing these months.
PARENT MATERIALS
Parent material is perhaps best simply defined
as partly weathered and unconsolidated rock from
which soil is developing. Soil parent material, accord-
ing to a definition by Jenny (1941, p. 52-53), is the
initial state of the system at the inception of soil for-
mation.
The nature of the parent rocks, which when weathered
act as soil parent material, influences soil genesis and
soil distribution. The mineralogy, age, and special
weathering features of the parent material are of spe-
cial importance. Characteristics of each of the main
types of parent material are discussed in the following
paragraphs in so far as they are relevant to soil genesis
(see also pl. 2; chart). Complete mineralogical de-
scriptions including chemical analyses and the strati-
graphic relationships of the parent rocks are given in
other chapters of this report.
Primary volcanic rocks and sediments derived largely
through marine reworking of volcanic source materials
underlie a little less than one-third of the soils. These
volcanic rocks, which are the oldest rocks exposed, make
up the central core of the island and comprise dacite
and andesite.
The andesitic rocks are assigned to the Hagman and
Densinyama formations of late Eocene age and to the
Fina-sisu formation of late Oligocene age. They crop
out chiefly in the east-central and northeast parts of
the island, above the dacites. The Hagman formation
consists of andesitic breccias, tuffs, conglomerates, tuf-
faceous sandstones, and minor andesite flows. The
Fina-sisu formation is made up of andesite flows and
marine andesite tuffs. The Donni sandstone and
Machedit conglomerate members, and much of the tuf-
faceous facies, of the Miocene Tagpochau limestone are
also andesitic rocks, which give rise to essentially simi-
lar soils. The andesitic rocks have a relatively high
content of alumina and calcium oxide and a low con-
tent of potash compared to average andesite. They pre
broadly similar, as parent rocks for soils, to the
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GEOLOGY OF SAIPAN, MARIANA ISLANDS
Hawaiian basalts. They give rise to a weathered zone
50 feet or more thick below the soil solum, where erosion
has not been severe. This thick weathered zone is
referred to by some authors as the saprolite or zersatz
zone.
The dacites, classified as the Sankakuyama forma-
tion, are also believed to be of Eocene age although older
than the overlying andesites. They crop out in two
small areas in northern Saipan. These rocks have an
unusually high silica content and low alumina, iron and
alkalis, and alkaline-earths contents (see Chapter A).
Soils are shallow or entirely lacking in these areas due
to the rugged topography, rapid erosion, and slow rate
of parent-material formation.
Limestones underlie a little more than two-thirds of
the soils of the island. The most extensive of these are
the Tagpochau limestone of early Miocene age and the
Mariana limestone of Pleistocene age. The other lime-
stones of Saipan do not significantly contribute to soil
parent materials.
Four soil conditions are found in the areas underlain
by limestone (and all are characterized by an abrupt
contact between the soil and underlying limestone) :
deep, firm, plastic, clayey soils of reddish hue with more
or less complete profiles that are more than 3 and com-
monly less than 6 feet deep; rather friable brown soils
18-42 inches deep; friable alkaline brown shallow stony
soils about 6-18 inches deep; and very shallow soil
mixed with stone and less than 6 inches deep, with
numerous small areas of limestone outcrops. The last
condition is a land type?rough stony land on lime-
stone?rather than a soil unit. The third and fourth
conditions are the most extensive map units on the
island.
The soils underlain by limestone have, in general,
developed from residuum remaining after solution of
the limestone. They therefore tend to differ from the
soils underlain by volcanic rocks in the following ways:
The contact between soil and unaltered limestone is
abrupt, although irregular, whereas the soils underlain
by volcanic rocks are commonly underlain by a sapro-
lite or zersatz zone of a few to many tens of feet in
thickness; soils underlain by limestone are influenced
to a great extent by the nature of the soil parent mate-
rial remaining after the limestone dissolution; and
soils underlain by limestone are generally free of the
variegated mottling and ghost crystals which 'charac-
terize the soils derived from andesitic materials.
A fourth soil parent material is the limesand of the
western coastal plain. This material consists of beach
and shallow lagoonal deposits lying a few feet above
sea level. Sufficient organic matter has accumulated to
darken the upper foot or so of this material, and it
increases in amount between the coast and the inland
edge of the coastal plain. The soils are moderately to
strongly calcareous.
Alluvium and slope wash (local alluvium) constitute
a fifth parent material, which is of limited areal extent.
Soils beginning to develop on the alluvium have little
or no profile differentiation. They are found in small
valleys cut in limestone, in sinks, and on the coastal
lowlands. Some of the slope-wash material has given
rise to soils with recognizable profile development.
This is especially true on low inland slopes of south-
western Saipan.
SLOPE AND DRAINAGE
Topography has influenced the course of soil devel-
opment in two ways. The steep slopes of the dissected
landscape, as well as the convex moderate slopes that
are being actively eroded, undergo such rapid erosion
that the development of complete soil profiles is inhib-
ited; the acreage of this rough broken land exceeds that
of deep soils. Also, moisture relations in the nearly
level land underlain by limestone differ from those in
the sloping land underlain by limestone to such an ex-
tent that properties of soils developing in the two sit-
uations differ. The level soils are more nearly satu-
rated during wet periods and remain moist longer. As a
consequence, they are less well oxidized and exhibit yel-
lowish rather than reddish hues as in the sloping soils.
According to current American soil-survey terminol-
ogy, the former would be considered as somewhat poorly
drained, the latter moderately well to well drained.
However, toposequences, or groups of soils develop-
ing from similar parent materials but differing in prop-
erties due to natural drainage, were not found on Sai-
pan, with the above-described exception of deep soils
underlain by limestone. The rolling to hilly dissected
terrain, the porous limestones with good drainage, and
the lack of a water table near the soil surface probably
all help to explain the absence of this drainage relation-
ship which is commonly found in continental areas.
Poorly chained soils are found in several marshy
areas on the western coastal plain. These areas are con-
tinually wet and are often covered with water, so that
they are mapped as marsh rather than as a distinct soil
type. Some of the inextensive soils included with the
association of alluvial clays are also poorly drained and
unoxidized.
TIME
Time in soil studies means the elapsed time of soil
development. Time zero is the time at which the parent
material is introduced into a zone where it can be
acted upon by climatic factors and influenced by vege-
tation and other organisms to start soil development.
Geologic evidence suggests that weathering in the up-
lands has been proceeding without interruption (except
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SOILS 193
for such as caused by changes in rate of erosion due to
uplift or eustatic changes in sea level) since at least
late Pleistocene time. Terrain above an elevation of
about 500 feet may have been emergent since Pliocene
time.
This does not mean that the upland soils with com-
plete A-B-C profiles are indicative of the degree of soil
development attained under action of soil-development
factors for the indicated elapsed time of tens of thou-
sands to a million years or more. Where soils are
underlain by limestone, soil material has continually
been moving across the limestone bench and platform
surfaces by slope wash and colluviation. Some soil has
accumulated in pockets, where there has not been oppor-
tunity for complete development due to continual addi-
tion of fresh soil material. Despite unequal periods of
weathering on the various bench surfaces cut in the
Miocene and Pleistocene limestones, no appreciable soil
differences were observed on them. Where soils develop
in residuum from volcanic rocks and tuffaceous sedi-
ments, rate of removal of soil material by erosion has
almost exceeded the soil-development rate.
The upland soils of Saipan cannot be considered as
representative of old soils, since fresh soil parent mate-
rial is continually being exposed by erosion. Observa-
tions of soils under similar weathering conditions else-
where (Sherman, 1949; Tamura, Jackson, and Sherman,
1953) as well as generalizations on mineralogical
changes in soils with time of weathering (Jackson,
Tyler, and others, 1948) indicate that relatively high
concentrations of aluminum or iron and titanium
minerals occur in more mature soils weathered under
tropical climates. Since concentrations of these min-
erals are lacking in Saipan soils (table 2), their lack
of maturity seems to be confirmed.
VEGETATION
Extensive clearing for sugarcane during the period
of Japanese control (1914-44) and earlier clearing for
copra production, as well as introduction of exotic
plant species, makes it difficult to infer what the original
composition of the vegetative cover on Saipan was. As
deduced from scattered primary-forest remnants, sec-
ondary forests, and disturbed areas, the vegetation
before cultivation seems to have consisted of fairly
dense forests and some small savannalilce areas. To
what extent the savannas are manmade is unknown;
on many tropical islands of the Pacific where vegeta-
tion has been relatively undisturbed, the presence of
savanna coincides with areas of laterized volcanic rocks,
generally highly eroded. This is true on Saipan,
although some areas of savanna growing on rough
stony land on limestone were observed on the southern
slopes of the central ridge.
Among the tree species present in the primary forests
were daog, Calophylbuim.inopityllunt Linn?the legume
ifil, Intsia bijuge (Colebrooke) 0. Kuntze; breadfruit,
Artocarpus sp.; and several species of Pandanus. The
secondary forests and areas on to which trees are read-
vancing appear to be dominated by the legume Leuca-
ena glauca (Linn?Bentham and the Formosan kost
Acacia confvsa Merrill.
That there are no significant differences in influence
of vegetation on different soils (exclusive of savanna
areas, where no samples were taken because of extreme
erosion or shallowness) seems indicated by the fact that
the surface horizons of 7 of the 8 profile samples col-
lected contained 31/0-51/0 percent organic carbon. The
eighth profile contained about 7 percent organic carbon
in the surface horizon, which can probably be explained
by the more favorable physical properties of this soil.
This conclusion also seems to be confirmed by the fact
that ratios of carbon to nitrogen, as determined by
Kawamura, Tanaka, and Inagaki (1940), do not differ
significantly among soils.
Some of the earlier investigators reasoned that the
organic-matter content of latosolic soils must necessarily
be low (for example, less than 2 percent in the surface
horizon) due to increased rates of oxidation and of bac-
terial decomposition under year-round high tempera-
tures (Mohr, 1922; Corbet, 1935). Recent studies indi-
cate, however, that the content of organic matter and
nitrogen within many latosolic profiles is relatively high
?significant amounts have been found at depths of 2 or
3 feet?although there may be little or no surface litter.
This has been reported for Puerto Rican soils by Smith,
Samuels, and Cernuda (1951) ; for Hawaiian soils by
Cline and others (1955) and by Dean (1937) ; for Co-
lombia by Jenny (1950) ; and for certain soils of the
Belgian Congo by Kellogg and Davol (1949). The
Hawaii and Puerto Rico investigators suggest that the
luxuriant vegetation formed by year-round high tem-
peratures favors accumulation of organic matter and
nitrogen at a greater rate than oxidation and bacterial
decomposition. Jenny (1941) suggested that in Co-
lombia the relatively high incidence of leguminous
species in the flora with correlative nitrogen fixation
may be the main causative agent of the relatively high
content of nitrogen and organic matter observed in the
soils.
The organic-matter content of the deep and moder-
ately deep Saipan soils, 31A-51/2 percent in the upper 6
inches, is relatively high. These soils do not contain as
much organic matter below the surface 6 inches as the
lnunic latosols of Hawaii or many of the latosolic soils
of Puerto Rico and the Belgian Congo. A Saipan flora
recorded by Kawafrre (1915) and discussed in a U. S.
Navy civil-affairs handbook (1944), but not seen by the
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GEOLOGY OF SAIPAN, MARIANA
author, lists 10 species of legumes; how many are ar-
borescent is not specified. Although the incidence of
legumes may be relatively high in the Saipan flora,
symbiotoc nitrogen fixation does not necessarily follow.
The discussion of Smith, Samuels, and Cernuda (1951)
seems applicable to Saipan; that is, the frost-free year-
round growing season encourages luxuriant vegetation
which causes organic matter and nitrogen accumulation
in the soil profile.
SOIL SERIES AND TYPES
The map units were established as phases of soil
series and types wherever possible (see pl. 36).
The soil series is defined (II. S. Dept. of Agriculture,
1951) as "a group of soils having soil horizons similar in
differentiating characteristics and arrangement in the
soil profile, except for the texture of the surface soil, and
developed from a particular type of parent material."
The soil type is defined as "a subdivision of the soil series
based on the texture of the surface soil," and the soil
phase refers to subdivisions of the series and type ac-
cording to degree of slope (certain ranges of slope con-
stitute a slope phase) or the degree to which erosion has
truncated the profile.
However, owing to the limited number of phases es-
tablished and the relatively minor differences among
phases of a given series and type, for brevity the written
descriptions are in terms of soil series and types, rather
than the phases.
Differing soils in some small areas form intricate pat-
terns. It was not possible to differentiate a landscape
unit as homogeneous as the soil series in these places,
and soil complexes (intricate geographic associations of
different soils) were established.
The description of each of the soil series includes an
outline of the more important properties and the range
of those properties within the series, how they are dif-
ferentiated from related series, the parent rock from
which they were derived, and the position in the land-
scape which they occupy. (The clay loam and clay
types of the Akina series are separately described, but
all other series are monotypes.) This is followed by a
detailed description of a profile which is near the cen-
tral Concept of the series.
Numerical notations in parentheses in the soil pro-
files describe the moist-soil colors according to the Alun-
sell color system (see Soil Survey Staff, 1951, p. 194-
203) , and color names generally conform to those listed
and described in that publication.
The percentages of clay (