Petrology of the 

Limestones of Guam 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 403-D 
Petrology of the 

Limestones of Guam 

By SEYMOUR 0. SCHLANGER 

With a section on PETROGRAPHY OF. THE INSOLUBLE RESIDUES 

By J. C. HATI-IAWAY and DOROTHY CARROLL 

GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 403-D 

Field and laboratory study of reefassociated limestones of Pleistocene 

and Tertiary age 

LIBRARY 

BUREAU Of MINEI 

LIBRARY 

SPOkANI. INASH. 

J.t4 31971 

PltASf lfTtJIN 

ro lte•~•· 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1964 
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary 

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director 

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 
CONTENTS 

Abstract------------------------------------------- Introduction ______________________________________ _ 

Purpose and scope of study _____________________ _ 

Geologic setting _______________________________ _ 

~ethods--------------------------------------- Acknowledgments ______________________________ _ 

Limestone petrology _______________________________ _ 

Classification. _________________________________ _ 

~ineralogy and chemical composithn ___________ . __ 

Primary and secondary carbonate minerals ____ _ 

Calcium carbonate, magnesium carbonate, and 

strontium carbonate content ______________ ..: 

N oncarbonate minerals _____________________ _ 

Postdepositional alteration. _____________________ _ 

Recrystallization of original matrix and primary skeletal materiaL ________________________ _ Fractures _________________________________ _ 

Deposition of chemical calcite _______________ _ 

Silicification _______________________________ _ 

~anganese minerals in the Bonya Limestone __ _ 

~icrostylolites in the Alifan Limestone _______ _ 

N ordstrandite deposits in the Alifan Limestone_ 

Dolomite in the Ali fan Limestone ____________ _ 

Phosphate deposits in the ~ariana Limestone_ 

Petrography __________________________ _ 

Comparison with phosphatic limestone from 

Anguar, Palau Islands ________________ _ 

Genesis of the pellets ___________________ _ 

Limestone facies ___________________________________ _ 

Facies terminology and classification _____________ _ 

Recognition of facies ___________________________ _ 

Fossils as facies indicators ______________________ _ 

Calcareous algae ___________________________ _ 

Foraminifera ______________________________ _ 

Page 

D1 

2 

2 

2 

2 

5 

5 

5 

8 

8 

8 

9 

10 

10 

11 

12 

12 

12 

13 

14 

15 

16 

16 

17 

17 

18 

18 

19 

19 

20 

20 

Limestones facies-Continued 

Fossils as facies indicators-Continued 

Corals ____________________________________ _ 

~ollusks---------------------------------- Limestone formations of Guam ______________________ _ 

Limestone of Eocene (b) age from the Alutom For- mation _____________________________________ _ 

~ahlac ~ember of the Alutom Formation ________ _ 

~aemong Limestone ~ember of the Umatac Formation _____________________ • _______________ _ 

Bonya Limestone ______________________________ _ 

Alifan Limestone ______________________________ _ 

Barrigada Limestone ___________________________ _ 

Janum Formation ______________________________ _ 

~ariana Limestone ____________________________ _ 

Summary and conclusions ___________________________ _ 

Petrography of the insoluble residues, by J. C. Hathaway and Dorothy CarrolL ________________________ _ 

Laboratory procedures _________________________ _ 

Separation ____ ------- __________________ . ___ _ 

~ineralogical analysis ______________________ _ 

Clay and silt minerals __________________ _ 

~inerals in the sand grades _____________ _ 

PetrographY----------------------------------- Quantity of insoluble residue ________________ _ 

~ineralogY-------------------------------- Clay and silt grades ____________________ _ Sand grades ___________________________ _ 

~inerals present_ ______________________ _ 

General relationships of the minerals _____________ _ 

Origin of nordstrandite and woodhouseite-like min- eral ________________________________________ _ 

~ineralogical relationships in the sand grades _____ _ References cited ___________________________________ _ 

Index·--------------------------------------------

ILLUSTRATIONS 

[Plates follow index] 

PLATE 1. Secondary features in limestone from the Alutom Formation. 

2. Silicified limestone from the Alutom Formation. 

3. ~icrostylolites and quartz in the Alifan Limestone. 

4. Recrystallized mud and nordstrandite in the Alifan Limestone. 

5. Dolomite in the Alifan Limestone. 

6. Phosphatic limestone from Guam and from Anguar, Palau Islands. 

7. Limestone from the Alutom Formation. 

8.. Limestone from the Alutom Formation. 

9. Encrusting algae, encrusting Foraminifera, and recrystallized coral from the Alutom Formation. 

III 

D21 

21 

21 

21 

23 

23 

26 

28 

30 

31 

32 

35 

37 

37 

37 

37 

37 

37 

38 

38 

38 

38 

40 

41 

43 

44 

44 

48 

51 
IV 

PLATE 10. 

11. 

12. 

13. 

14. 

15. 

16. 

17. 

18. 

19. 

20. 

21. 

FIGURE 1. 

2. 

3. 

4~. 

5. 

CONTENTS 

Samples from the reef-wall and lagoon facies of the Maemong Limestone Member of the Umatac Formation. 

Samples from the forereef and basin facies of the Maemong Limestone Member of the Umatac Formation. 

Samples of the Bonya Limestone. 

Samples of the Bonya Limestone. 

Samples of the Alifan Limestone. 

Samples of the Barrigada Limestone. 

Samples of the Janum Formation. 

Samples of the Mariana Limestone. 

Samples of the Mariana Limestone. 

Limestone from southern Louisiana. 

Limestone of Oligocene age from Kirkuk, Iraq. 

Insoluble residues from selected Guam Limestone. 

Page 

Columnar stratigraphic sections for Guam_____________________________________________________________ D3 

Sample locality map _______________________________________________________________________ .. ________ 4 

Facies classification and typical rock associations in a reef complex ____ ""'___________________________________ 19 

Size distribution of the insoluble residues____________________________________________________________________ 39 

Distribution of minerals in the sand-grade fraction in insoluble residues____________________________________ 45 

TABLES 

Page TABLE 1. Classification of the pre-Recent limestone of Guam___ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ D5 

2. Attributes of particulate limestone_____________________________________________________________________ 8 

3. Carbonate contents of representative samples of Guam limestones ___________________________ .______________ 9 

4. Summary of composition and fossil content by volume percent of limestone formations_______________________ 22 

5. Weight percent of insoluble residues of Guam limestone--------------~-~--------------------------------- 38 

6. Sand, silt, and clay in insoluble residues of Guam limestone----------------------------------------------- 38 

. 7. Minerals in the sand fractions (greater than 62 microns) of insoluble residues of Guam limestone______________ 41 

8. Insoluble residues and size distributio~ in fivela~ge samples of Guam limestone______________________________ 41 

9. Amount of heavy residue (greater tha·n 2.9) in five large samples of Guam limestone_________________________ 41 

10. Comparison of the stratigraphic position and mineralogic classification of samples of Guam limestone__________ 43 · · 11. Samples studied ___________________ . ___________ ---- ________ ----- ________________ ----__________________ 46 
GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS 

PETROLOGY OF THE LIMESTONES OF GUAM 

By SEYMOUR 0. ScHLANGER 

ABSTRACT 

Limestones characteristic of the so-called reef facies are 

found in all the formations that crop out on the island of 

Guam. The mountainous central part of the island is underlain by the Alutom Formation of Eocene and Oligocene agea succession of more than 2,000 feet of marine tuffaceous 

sandstone, shale, pillow lava containing beds of Eocene limestone, and breccia of Oligocene age containing fragments of 

the Eocene limestone. The Mahlac Member of the Alutom 

Formation Is a marine calcareous shale of Oligocene age. 

The northern part of the island Is a limestone cap that rests 

unconformably on the Alutom Formation and is made up of 

the Bonya Limestone of Miocene age; the Allfan Limestone, 

the Barrigada Limestone, and the Janum Formation, all of 

Miocene or Pliocene age ; and the Mariana Limestone of Pliocene and Pleistocene age. The southern part of the Island Is 

underlain by the Umatac Formation of Miocene age, a succession of volcanic rocks and limestone approximately 2,200 

feet thick. This formation is made up of four members: the 

Facpi Volcanic, the Bolanos Pyroclastic, the Maemong Limestone, and the Dandan Flow Members. 

All the limestone on the island had, as primary constituents, various combinations of coral, coralline algae, Halimeda, 

Foraminifera, mollusks, echinoids, minor amounts of worm 

tubes and bryozoans, fine limemud, and volcanic material in 

the form of clay, single crystals of various minerals, and rock 

fragments. The limestones are classified into two main 

groups: incrustate and particulate. Incrustate limestones 

are those that have been built by incrusting or attached organisms such as colonial corals, coralline algae, or incrusting 

Foraminifera. Particulate limestones are those that have been 

formed by the accumulation of individual foraminiferal tests 

or fragmental skeletal debris. The particulate limestones are 

subdivided by considering four rock attributes: (1) the 

amount of intergranular mud matrix, (2) degree of wear of 

the fossils, ( 3) degree of sorting of the fossils, and ( 4) size 

of the fossils. A third, minor group of metasomatic limestones includes those in which replacement of the original 

carbonate has taken place. 

Although these limestones occur in the reef facies-as used 

in the stratigraphically unrestricted sense-it was possible to 

distinguish rock assemblages characteristic of subdivisions 

within this blanket term. Evidence bearing on the assignment of any given rock type or rock association to a facies 

includes: (1) Comparison of the rocks in question with modern sediments by the use of individual faunal and floral elements as ecologic indicators and by matching lateral lithologic 

changes in the rocks with areal distribution patterns of modern reef complexes; (2) comparison of individual formations 

with other well-known fossil reef complexes; (3) reconstruc-· 

tion of the original configuration of ·partly destroyed reer' 

complexes, on structural and stratigraphic evidence; and (4) 

comparison of thin-section attributes of rocks from a known 

facies with those of the rock in question. 

Systematic study of the limestone on a formational basis 

showed that rock associations characteristic of individual 

facies are present throughout the geologic column on Guam. 

The limestone of the island was formed as a series of reef 

complexes intermittently destroyed or buried by outbreaks of 

volcanic activity. Comparison of rocks from several facies of 

the Maemong Limestone Member with lower Tertiary lime5tones from Louisiana and Iraq shows that striking. textural 

similarities exist from one area to another. Lower Tertiary 

limestones throughout much of the world show similar microfacies attributes. 

Secondary features of the limestones include: extensive 

recrystallization of the original constituents in all formations; 

silicification as the result of burial by lava flows, in the Alutom Formation and in the Maemong Limestone Member of the 

Umatac Formation; replacement by several manganese minerals in the Bonya Limestone; local dolomitization in the 

Alifan Limestone; . and phosphatization in the Mariana Limestone. Microstylolites are well formed in the argillaceous 

. parts of the Alifan Limestone. The microstylolites formed 

under loads of less than 300 feet of superincumbent limestone. Inspection of similar limestone from cores taken at 

depths below 4,000 feet in Eniwetok reveals only minor pressure-solution effects. It is concluded that pressure alone may 

not cause the formation of microstylolites. 

Analyses of 29 samples for CaC03, MgC03, and SrC03 content show ranges of 52.7 to 99.2 percent CaC03, 0.52 to 2.72 

percent MgC03 (no dolomitic limestones were analyzed), and 

0.01 to 0.36 percent SrC03. The low SrC03 content correlates with the generally highly recrystallized condition of the 

limestones. 

In addition to calcite-the dominant mineral-aragonite, and 

dolomite, many minerals were identified in insoluble residues 

from the various formations by X-ray, microscopic, and chemical tests. In t~e clay and silt fractions are montmorillonite, 

chlorite, mixed-layer montmorillonite-chlorite, mica, kaolinite, 

halloysite, quartz, cristobalite,. plagioclase, zeolites, gibbsite, 

nordstrandite, magnetite, hematite, goethite, lithiophorite, pyrolusite, apatite, and a woodhouseitelike mineral. In the sand 

fraction are magnetite, ilmenite?, augite, hypersthene, greenish-brown amphibole, basaltic hornblende, blue-green amphibole, sphene, zircon, apatite, plagioclase, quartz, beta-cristobalite, and zeolites. These minerals show a wide range of 

relative abundances in the various formations. Most of the 

Dl 
D2 GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS 

samples, however, are characterized by either a feldspar-zeolite-montmorillonite assemblage indicative of contributions by 

ash falls, or an iron oxide-nordstrandite-halloysite assemblage 

indicative of contributions by subaerial erosion of volcanic 

terrain. Tabulation of these assemblages by stratigraphic succession shows that ash falls contributed to limestones in the 

Alutom Formation, the Umatac Formation, and the Bonya 

Limestone. The Janum formation and the Alifan Limestone 

received argillaceous material mainly from subaerial erosion. 

The Mariana Limestone contains the ashfall assemblage. The 

classification of insoluble residues by this grouping shows that 

periods of active volcanism can be differentiated from periods of subaerial erosion. 

Two dominant geologic processes, reef growth and volcanic 

· activity, combined to build up the island of Guam. This study 

shows that characteristic limestone facies recur through time 

in the various formations. A comparison of the fossil content of the formations shows that the same groups of organisms contributed rock-making material from Eocene through 

Pleistocene time. 

INTRODUCTION 

PURPOSE AND SCOPE OF STUDY 

Field study of reef and associated limestones of 

Recent and Tertiary age on the island of Guam (July 

1951 through July 1953) led to a supplementary thinsection study of the limestones of Tertiary age. The 

main purpose was to augment field data. As work 

progressed, it became evident that certain lithologic. 

attributes, apparent in thin section only, were diagnostic in rock and facies classification. Thus, the 

subject was approached somewhat after the fashion 

of Henson ( 1950), in his regional study of limestones 

in the Middle East. Limestones of Miocene (e) age 

from Guam already have been compared to reef and 

associated limestones from the H eterostegina zone of 

southern Louisiana (Forman ~nd Schlanger, 1957). 

Chapter A of this series (Tracey and others, 1963) 

covers the general geology of Guam ; the field relations and gross lithologic aspects of the limestones 

are fully treated in the section on stratigraphy in 

chapter A, and are discussed in the present chapter 

only where necessary to augment the laboratory investigation. The Recent calcareous deposits on and 

around Guam, such as those of present-day beaches, 

reefs, and lagoons, are discussed in chapter B (Emery, 

1962), and in less detail in chapter A. 

Age designations used in this report are based on 

diagnostic larger Foraminifera identified by Cole 

( 1963) and correlated with the standard section of 

the Indonesian region (van Bemmelen, 1949, p. 79-

108). In the Indonesian region letter designations 

are used to differentiate faunal zones. The relationship between these Indonesian faunal zones and the 

standard Tertiary series subdivisions of the United 

States is shown on figure 1. In this paper the age of 

a limestone under discussion is designated by two 

terms, the standard Tertiary series name followed by 

the Indonesian zonal letter in parenthesis; that is, 

Eocene (b) , Miocene (e) , and so forth. 

GEOLOGIC SETTING 

Limestone of Eocene (b), Oligocene (c), Miocene 

( e, j, and . g), and Pliocene to Pleistocene age are 

known on Guam (fig. 1; Tracey and others, 1963, pl. 

1) . Limestone of Eocene (b) age is found interbedded within marine tuffs and pillow lavas and as fragments in beds of breccia of Oligocene (c) age within 

a succession of more than 2,000 feet of folded and 

faulted volcanics. This succession-the Alutom Formation-forms the mountainous central core of the 

island. It includes a marine calcareous shale, the 

Mahlac Member of Oligocene (c) age. The northern 

plateau of the island is a limestone cap that rests unformably on the Alutom Formation and is made up 

of the relatively flat-lying Bonya Limestone of Miocene (f) age, Alifan Limestone, Barrigada Limestone, 

and Janum Formation, all of Miocene (/ to g) and 

Pliocene ages, and the Mariana Limestone of Pliocene 

and Pleistocene age. The southern third of the island is made up of a succession of volcanic rocks and 

limestones of Miocene (e) age approximately 2,200 

feet thick, named the Umatac Formation. This formatiOn is composed of four members: (1) the Facpi 

Volcanic Member; (2) the Bolanos Pyroclastic Member, which contains limestone fragments from (3) the 

Maemong Limestone Member; and ( 4) the Dan dan 

Flow Member. The field relations of each formation 

are discussed in the unit descriptions. 

METHODS 

Limestone samples collected during the fieldwork 

and used for the laboratory studies are listed in table 

11 and locations are shown in figure 2. Laboratory 

study consisted largely of examination of thin sections under a binocular microscope fitted with a polarizing attachment, at magnifications ranging from 

X 9 to X 45. Details were studied at higher magnification with a standard petrographic microscope. A 

few specimens were ground, polished, and stained 

with Meigen's solution to distinguish aragonite from 

calcite, and chemical and mineralogical analyses were 

made (p. 8-10). 

The percentages of fossils were determined by visual estimation in thin section. The visual-estimation 

percentage charts of Folk (1951), Allen (1956), and 

Terry and Chilingar ( 1955) were used, and the writer 

also prepared a set of charts that showed percentages 

of shapes within the field that wen~ characteristic of 

fossils common in the limestones of Guam. 
INDONESIAN UNITED 

FAUNAL ZONES STATES 

(after van TERTIARY 

Bemmelen, 1949 SERIES 

p. 73-108) 

Recent 

Pleistocene 

h Pliocene 

g 

f Miocene 

e 

d 

Oligocene 

c 

b 

Eocene 

a 

""' 

t; 

"' 

a: ""' 

"-' 

t 

a: 

c 

A 

NORTH GUAM 

(Vicinity of Mount Santa Rosa) 

Mariana 

Limestone 

--7 

. / ? 

Janum Formation '"/-. 

________ / / 

AI utom Formation 

j 

0... 

::: 

""' 

~ 

0: 

B 

CENTRAL GUAM 

(Vicinity of Fena basin) 

Agana 

Argillaceous 

Member 

l AU•n Umestone ~~~~~/ t- ~ ~ / / T ,,;.,y Membe' 

~ ~ -= Bony• Umestone 

1 

'=:: 

:::: "" 

" ', Mahlac 

' Member ,_' Alutom Formation '-. ... 

..... ~:\. 

'""" ' ' ' ' ' ' 

""' 

~ 

·i 

a:: 

:::: 

""' 

~ 

0... 

I 

~ 

c 

SOUTH GUAM 

(Umatac to Nomna Bay) 

Agana 

Argillaceous 

Member 

Mariana 

Limesto!le 

Alifan Limestone 

Bolanos 

Pyroclastic 

Member 

Facpi Volcanic Member 

FIGUB& !.-Columnar stratigraphic sections from north, central, and south Guam. Each diagrammatic section Is a local composite of strata arranged to illustrate formational 

relationships and is not to true vertical scale. 

'"d 

t::1 

1-3 

~ 

0 

t"4 

0 

0 

~ 

0 

Jo%j 

~ t::1 

t"4 

Joo--4 

~ t::1 

~ 0 

z s:: t::1 0 r:/1 ·.;; 

ell 

E 0 

l%j 

& 

<.) 

~ ~ ell 

E 

:::J 

tJ 

~ 
GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS 

eJI 3 

.Ek 7 

.Fk 4 

·.Hn. 

Gj 15. •G· 1 lj. 2. Jt' 1 

.oi 2 

Gj 11•. • . J Hi 3. · . Fi 5 G' 6 Gt 8 . 1 Ht4 JJ ~~J 9 • J . ~i 2 ••. • . JJ 6 

Gi 1 ~ ~ •Hi 7 • .. h 6 

Gi 4·Hi 5 · II 5 

.Eh 3 ..• li 8 Jh 1 

.Ee 2 

Ee 6-Ee 8 

Ee 7 

Ed 1 • .Fd 3, 4, 5 

.Ed 3, 4, 5, 6, 7 

. 

Ov ~ .Pv 10 

Pv9 

.ov 7 

.Pr 3 

N 

EXPLANATION 

.Ts 2 · 

Sample locality and 

number 

.Ru 8 

.Qu 4 

0 3 MILES 

FIGURE 2.-Sample locality map. (See table 11.) 

.sv 5 
PETROLOGY OF THE LIMESTONES OF GUAM D5 

ACKNOWLEDGMENTS 

Much of the information relating to field occurrence, stratigraphy, and gross lithology of the Guam 

limestones has come from the fieldwork and preliminary reports of other members of the Guam field 

party: J. I. Tracey, Jr., David B. Doan, Harold G. 

May, and John T.'· Stark. Discussion of the paleoecology of the Guam limestones and the ecology of 

extant forms with the above mentioned coworkers and 

the cooperating paleontologists, W. S. Cole, Ruth 

Todd, and J. Harlan Johnson, has been invaluable. 

Norman D. Newell, Francis J. Pettijohn, and David 

M. Raup read the manuscript critically. F. R. S. 

Henson of the Iraq Petroleum Co., Ltd., loaned the 

writer thin sections of limestones from representative 

Middle East formations for comparative purposes. 

M. J. Forman of Atwater, Cowan, and Associates, 

cooperated in a study of the H eterostegina zone of 

southern Louisiana. James A. Denson of the U.S. 

Geological Survey made some of the photomicrographs. Work done by Schlanger on the limestones 

was used in a dissertation submitted to the Faculty 

of Philosophy of the Johns Hopkins University in 

partial fulfillment of the requirements for the degree of Doctor of Philosophy. 

LIMESTONE PETROLOGY 

CLASSIFICATION 

In establishing the classification and terminology to 

cover the pre-Recent limestones of Guam, an attempt 

has been made to keep rock names distinct from facies 

names, to coin as few new terms as possible, and to 

use terms that have textural rather than genetic connotations. Thus in this report, "reef," "lagoon," and 

other terms connoting specific environments of deposition are reserved for facies designations. Rock terms 

defined in standard works are used where possible. 

The philosophy behind the classification of limestones 

used in this paper might well be described, perhaps 

somewhat hopefu11y, by quoting Pettijohn's description ( 1957, p. 293) of his classific.ation of sandstones: 

Although the classification here presented is based on what 

are believed to be genetically significant properties, the classification is not genetic. A knowledge of the origin is not necessary to name or classify the sandstone. The classification is 

based upon simple observable characters. 

Table 1 lists the main limestone classes defined and 

their various subgroups. All these rocks are limestones. Such terms as "lime-mudstone" (Rodgers, 

1954, p. 230) are redundant in a paper dealing with 

limestone petrology. These limestones may contain 

admixtures of volcanically produced material. Where 

the noncarbonate fraction is dominantly clay, the 

modifier "argillaceous" is added to the rock name. 

"Vhere the volcanic fraction consists of silt-size or 

coarser mineral and rock grains, the term "volcanic" 

is used. 

Any discussion of limestones from a reef complex 

involves the differentiation of the class of deposits 

made up of whole remains of sessile organisms, such 

as crustose coralline algae, incrusting Foraminifera, 

and hermatypic (reef-building) corals, from the class 

made up of discrete but aggregated tests of Foraminifera, mollusk shells, disarticulated or fragmented 

algal debris, and broken corals. The terms "incrustate" and "particulate" are proposed to denote, respectively, these two main classes. 

TABLE !.-Classification of the pre-Recent limestone of 

Guam 

Class Limestone type 

L __________ Incrustate: 

A, algal 

B, foraminiferal 

C, algal-coral 

D, E, and so forth, combinations of 

A-C 

II___ _ _ _ _ _ _ _ _ Particulate: 

A, coquinite (and microcoquinite) 

B, paracoquinite 

C, breccia (and microbreccia) 

D, coquinoid (and microcoquinoid) 

E, mudstone 

III __________ Metasomatic: 

A, dolomitic 

B, siliceous 

C, manganiferous 

D, phosphatic 

Class I, inorustat~ .. limestones.-Incrustate, an adjective, is defined bY' Merriam-Webster as "formed 

into or like a crust.; h~·vl.ng a crust." Rocks of this 

class are texturally '·:.4ominated by extensive in situ 

deposits of either crustose coralline algae or incrusting Foraminifera, or both. The algae· are members of 

the subfamily Melobesieae (Johnson, J. H., 1954, p. 

12), which includes the genera Lithothanvnium, Lithophyll!um, and others. The Foraminifera are represented by various species of such genera as 0 arpenteria, JI.omotrema, Gypsina, and others. Skeletal 

crusts of these organisms coat and bind coral heads 

in position of growth, and thus form the framework 

that traps tests of Foraminifera, coral debris, mollusk shells, and fragments of the crustose forms. In 

outcrop the structure of the rock is distinctive: The 

limestone is unstratified. In thin section the binding 

nature of the crustose forms is obvious also. 

This class of limestones is considered equivalent to 

the reef limestone of Hatch and Rastall (1923, p. 30) : 

"Reef limestone differs from the majority of frag-
D6 GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS 

mental and organic deposits in the fact that it is 

largely built up in a solid coherent form from the 

first, and therefore constitutes a rock mass in the 

strict sense, without any process of cementation." 

Subclasses of the incrustate limestones are based on 

the relative abundance of the main fossil contributors. 

Where algae is dominant, the rock is termed an "algal 

incrustate limestone"; the term "algal-coral incrustate limestone" indicates the presence of coral heads 

in situ in the algal matrix. 

Olass I I, particulate limestones.-Particulate, an adjective, is defined by Merriam-Webster as "of, or pertaining or relating to, distinct particles; existing as 

minute separate particles." This class includes limestones made up of lithified aggregates of discrete particles. These particles are whole and broken tests of 

benthonic and planktonic Foraminifera, fragments of 

tests of encrusting Foraminifera, disarticulated segments of articulate coralline algae and Halimeda, 

fragments of crustose coralline algae, whole and fragmental mollusk shells, coral debris broken from sessile 

colonies, echinoid spines and plates, other minor fossil constituents, and fine-grained carbonate mud. These 

limestones lack the laminar crustose framework of the 

incrustate limestones. This strictly textural difference, based on simple observable characters, is a more 

valid criterion for classifying limestones of the reef 

facies than inferred genesis implied by such word 

pairs as "mechanical versus nonmechanical" (Rodgers, 

1954, p. 231). Subclasses of the particulate limestones 

shown on table 1 are based on the following rock 

attributes: ( 1) amount of the intergranular mud matrix, ( 2) condition of the recognizable fossil fraction, 

( 3) degree of sorting of the fossil fraction, and ( 4) 

size of the fossil fraction. 

1. Amount of the intergranular mud matrix: The 

term "mud" denotes size (less than 0.125 mm) 

rather than composition, and covers the fine-sand-, 

silt-, and clay-sized carbonate material. This 

material is probably triturated or disaggregated 

skeletal debris. The volume of the mud matrix 

is important, for it determines whether the limestone has an intact or disrupted framework. The 

term "framework" is used in connection with 

particulate limestones to denote the fraction having grains greater than 0.125 mm in size, in contrast to the mud-matrix fraction. An intact 

framework is one in which each particle larger 

than 0.125 mm "is in contact with its neighbor 

so that the whole framework is a mechanically 

stable structure in the gravitational field" (Pettijohn, 1957, p. 283). A disrupted .S.:ramework is 

defined as one in which particles larger than 

0.125 mm float in the mud matrix. These observable relations have genetic implications: 

Where fossils or fragments float in mud, the assumption is that both were deposited together 

in the absence of currents; where packed fossils 

lack a mud matrix, the assumption is that washing by current action took place. 

2. Condition of the recognizable fossil fraction: 

Breakage in the fossil fraction indicates possible 

transportation of the sediment. Actual fracturing of a skeletal element is differentiated from 

disarticulation of organisms. 

3. Degree of sorting of the fossil fraction : The terminology used to describe sorting is taken from 

Pettijohn (1957, p. 284) : "A well sorted sand 

has three or fewer U dden size classes, and the 

maximum diameter therefore is 8 or fewer times 

the smallest. Sands with 4 to 6 grades, inclusive, may be said to show fair sorting. In these, 

the diameter of the largest grain does not exceed 

that of the smallest by more than a factor of 64. 

Sands with a size range in excess of this have 

7 or more grades and are poorly sorted." True 

current sorting must be distinguished from what 

is here termed "biologic" sorting that is due to 

the inherent size of dominant organisms. Thus, a 

Oalcarirui-rich reef-flat deposit may be well sorted 

even though the foraminiferal sand has not been 

moved at all. In the same general locality, however, coarse reef debris may be accumulating by 

entrapment along with mud in reef pools. This 

material has been subject to some current action 

but is unsorted. In distinguishing current sorting from biologic sorting, the natural growth 

form of the constituents must be considered. 

4. Size of the recognizable fossil fraction : If most 

or all of the fossil fraction falls in the sand-size 

grades coarser than the mud fraction-0.125 to 2 

mm-the prefix "micro" is added to the subclass 

name as shown on table 1. 

Olass II A, coquinite.-As defined by Rodgers (1954., 

p. 229), this term denotes. "a fully cemented mass of 

mechanically deposited shells or shell fragments." In 

this report, the term "coquinite" refers to a lithified 

aggregate of moderately to well-sorted fossil debris of 

coarser than sand size. The fossils generally consist 

of whole shells and fragments of shells. The rock has 

an intact framework. The cement of some coquinites 

is clear, chemically precipitated calcite; this calcite 

may be accompanied by some mud matrix. Definition 

of any exact percentage limits on the amount of mud 

matrix in any subclass has been avoided. Rocks 

classed as coquinites and microcoquinites generally have 
PETROLOGY OF THE LIMESTONES OF GUAM D7 

less than 25 percent intergranular mud matrix. 

Emery, Tracey, and Ladd (1954, p. 65, table 10) have 

shown that sands of Recent age from Bikini Atoll 

have intergranular pore space of as much as 50 percent. Thus a current-sorted sand may later be infiltrated by mud to this extent. A matrix value of 50 

percent might also indicate a disrupted framework. 

Therefore, approximately one-half the maximum observed intergranular pore space of modern atoll sands 

is ta.ken as the rough limit for the mud matrix of coquinites. This procedure follows that of Pettijohn (1957, p. 284, table 48) in the classification of 

terrigenous sandstones; the normal porosity of a 

quartz sand is 30 to 35 percent, and the division between graywackes as distinguished from arkosic and 

lithic sandstones is taken at 15 percent detrital matrix-a significant percentage. Varietal names for 

coquinites depend on the dominant fossil types; for 

example, foraminiferal, algal, and molluscan coquinites. 

Glass II B, paracoquinite.-This term refers to 

rocks that are texturally close to coquinite. Here the 

prefix "para" denotes a rock that is near to-almost, 

but not quite-a coquinite. The distinguishing characteristic of this subclass is an intact fossil framework of whole and fragmental shells that generally 

show only a fair degree of sorting and are imbedded 

in a conspicuous mud matrix generally greater than 

25 percent. The matrix may run as high as 60 percent. Emery, Tracey, and Ladd (1954, p. 65, table 

10) have shown that packed Halimeda debris has an 

intergranular porosity of as much as 57 percent. Paracoquinites are somewhat anomalous· rocks; the matrix 

may be due in part to postdepositional mud infiltration of a current-deposited sand. Some specimens 

show an obviously packed fossil framework in which 

mud clogs every intergranular and intercellular interstice. The somewhat clumsy but descriptive term 

"microparacoquinite" may be used to describe rocks 

having predominant sand-size fossil debris. Varieties 

of the paracoquinites are named in the same fashion 

as those in the coquinite subclass. 

Glass I I G, b1·eccia.-Rocks of this subclass are lithifled aggregates of poorly sorted fossil debris in a prominent mud matrix. The fossils are largely fragmental. It would be difficult to separate accurately the 

rocks of this subclass into breccias and conglomerates 

on the basis of rounding of the debris; the initial 

shapes of the fossils precludes this division. These 

limestones display both intact and disrupted frameworks. Breccias are much more common than microbreccias. The paracoquinites, which contain two distinct orders of size fractions (the framework, and the 

mud matrix), and the breccias, which commonly show 

a complete gradation from mud through cobble-size 

fragments, may be likened to graywackes in texture. 

These limestones, therefore, may be thought of as 

limewackes or calcwackes, just as some of the microquinites may be calcarenites as defined in the strict 

sense by Pettijohn (1957, p. 401). 

Glass I I D, co quinoid lim-estones.-This subclass refers to limestones having a promine~1t to dominant 

mud matrix in which are imbedded generally whole, 

unsorted fossils. These fossils may form either an 

intact or a disrupted framework; the latter is more 

common. Thus the characteristics of these limestones 

are in accord with those of coquinoid limestones as 

defined by Pettijohn (1957, p. 402) : "coquinoid limestones are autochthonous deposits consisting of coarse 

shelly materials which have accumulated in place and 

generally with a fine-grained matrix." The upper 

limit of mud matrix for coquinoid limestones is taken 

at 75 percent. This arbitrary limit follows the classification of terrigenous sediments, 75 percent marking the line between graywacke and mudstone (Pettijohn, 1957, p. 284). 

Glass I I E, mudstone.-Limestones in this subclass 

consist of approximately 75 percent or more mud containing generally whole fossils; some samples, owing 

possibly to compaction, show postdepositional fragmentation of fossils. No size division based on the 

fossil fraction is made in this subclass. Varieties are 

named after the dominant fossil type, such as foraminiferal mudstone. 

Use of this classification of particulate limestones 

implies the possibility. of differentiating a recrystallized mud matrix from a clear, chemically precipitated 

calcite cem.ent. Study of the limestones of Guam has 

shown that an original mud matrix can recrystallize 

into a deceptively clear granular calcite mosaic. Study 

of a number of thin sections from a single sample often 

revealed slight traces of original mud in otherwise 

generally clear cements. Not all clear cements were 

entirely due either to recrystallized mud or to a primary precipitate. 

Crystalline matrices that have formed from original 

mud matrices generally show some relict mud scattered 

throughout; the contact between the granular calcite 

crystals and the mud is usually gradational. In some 

rocks the mud has only partly recrystallized owing to 

the growth of clear calcite around the perimeters of 

the fossil framework .... Thus the centers of interstices 

between fossils are still occupied by semiopaque mud. 

Some rocks show a disrupted framework now packed 

in a clear calcite matrix, and these fossils probably 

were originally held apart by mud before recrystalli-
D8 GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS 

zation obliterated this original matrix. Rocks having 

crystalline calcite matrices that precipitated from interstitial solutions all show intact frameworks. The 

cement generally fails to fill completely the original 

interstices, and the rocks show euhedral crystals that 

project from the fossil walls into the interstices. Chemical cements commonly show a laminar structure, concentric with the fossils; recrystallized muds show no 

such structure. Table 2 summarizes the characteristics 

of the particulate limestones. 

TABLE 2.,....-Attributes of particulate limestone 

Type 

Size of 

Condition of fossils fossils Sorting of fossils 

(milli· 

meters) 

Type .of cement or matrix and 

abundance relative to fossils 

Condition of 

framework 

Faunal and floral varieties 

Microcoquinite........ Fragmental and whole.___ <2 Good to fair....... Chemical and little or no mud.... Intact ________________ _ Foraminiferal, algal, and so forth. Coquinite •..• __ •• ---- ______ do ..•..• ------_________ >2 ___ -~do __________________ do _________ ----- __ ------------ ______ do ________________ _ Coral, algal, and so forth. 

Coral, foraminiferal, algal, and so forth. 

Paracoquinite ______________ dO--------------------- < >2 Generally fair _____ Conspicuous mud ______________________ do ________________ _ 

Microbreccia __________ FragmentaL______________ <2 Generally poor _________ do _____________________________ Intact to disrupted ___ _ Coral, foraminiferal, and so forth. Breccia _____________________ do_____________________ >2 _____ do __________________ do ________ --------------------- _____ do ________________ _ 

Microcoquinoid __ ----- Whole.------------------- <2 Unsorted _______________ do _____ -------------------- _________ do ___________ ------ Do. 

Coquinoid _ -------- _____ •.. do ______________ ------- >2 _____ do __________________ do ••• -------------------------- _____ do _____ ---------- __ Mudstone ________________ .. dO--------------------- < >2 _____ do __________________ dO----------------------------- Disrupted ____________ _ 

Foraminiferal, and so forth. 

Molluscan, and so forth. 

Foraminiferal, molluscan, 

Class III, metasomatic limestones.-This class includes limestones that have been replaced, wholly or 

in part, by minerals other than calcite. "Dolomitic 

limestone" refers to rock that contains both calcite 

and dolomite, the dolomite being subordinate, as discrete mineral phases; the magnesium content of the 

rock is not considered in this classification. If dolomite is predominant, the rock is termed "calcitic dolomite." This terminology follows that suggested by 

Rodgers ( 1954). "Siliceous limestone~' refers to limestone that contains silica introduced from solutions; 

replacement of the host rock and void filling are criteria for this subclass. "Manganiferous limestone" 

covers those rocks that show replacement by pyrolusite 

wad, and lithiophorite. "Phosphatic limestones" are 

also placed in this class. Metasomatic limestones are 

rare on Guam; they are described in detail in the sec-

. tion on postdepositional alteration. 

MINERALOGY AND CHEMICAL COMPOSITION 

PRIMARY AND SECONDARY CARBONATE MINERALS 

The limestones of Guam are made up largely of 

skeletal carbonate produced mainly by calcareous algae, Foraminifera, echinoids, corals, and mollusks; 

some less important groups such as bryozoans and 

worms contributed traces of material. Originally this 

skeletal material was in the form of aragonite and 

calcite. The corals and one form of algae-the genus 

H alimeda-contribute aragonite containing generally 

less than 1 percent MgC03 and from 1 to 1.5 percent 

SrCOa. Mollusk shells contain both aragonite and 

calcite in various proportions. Most Foraminifera, all 

the echinoids, and all the coralline algae precipitate 

magnesium-rich calcite. Chave (1954, table 1) lists 

MgCOa contents of as much as 15.9 percent for echinand so forth._ 

oids and Foraminifera and 28.6 percent for coralline 

algae. Thus the primary carbonate minerals of these 

limestones were magnesium-bearing calcite and aragonite low in magnesium but containing a significant 

amount of SrC08 • Almost all the primary aragonite 

has been destroyed through solution and replacement 

by calcite. Dolomite in the Alifan Limestone has 

locally replaced calcite. Thus the rocks contain secondary calcite and dolomite, and chemically deposited 

calcite fills many interstices in all formations. 

CALCIUM CARBONATE, MAGNESIUM CARBONATE, AND 

STRONTIUM CARBONATE CONTENT 

Analyses to determine the CaC03, MgCOa, and 

SrC03 content of a number of limestone samples representative of the Guam suite were made by members 

of the U.S. Geological Survey (table 3). One reason 

for making these analyses was to use the MgC08 content as a guide to dolomitic zones; another was to see 

if the SrC03 content varied significantly with age or 

recrystallization. The low MgC03 content of these 

limestones indicated a lack of widespread dolomitization; this lack was confirmed by the examination of 

thin sections from each analyzed sample. The present 

MgC03 content is probably a function of magnesium 

in the calcite structure. However, a low MgCOs content 

cannot be taken as a criterion for assuming complete 

lack of dolomite. By staining the calcite matrix with 

2N copper nitrate, Schlanger (1956) found scattered 

dolomite euhedra in limestone from the subsurface of 

Kita-daito-jima that contained only 2.74 percent 

MgC03• Kulp and others (1952, p. 715) studied a 

large number of limestones of various ages and came 

to the conclusion that "Finally recrystallization always 

lowers the Sr/Ca ratio in varying amounts so that, 
PETROLOGY OF THE LIMESTONES OF GUAM D9 

after recrystallization, the strontium content cannot be 

used as an index of jnitial conditions." 

TABLE a.-Carbonate contents of representative samples of Guam 

limestones 

[Annlysos by J. I. Dinnin, P. W. Scott, and H. F. Phillips. CaCOs and MgC03 

soluble In 1:3 acetic acid; SrCOs soluble In 1:3 nitric acid; residue Insoluble in 1: 3 

acotlc acid. In sample Dr 0-1a tho Insoluble residue includes cristobalite in the rorm or Internal casts or planktonic Foramlnlrera] 

InFormation or mombor sample CaC03 MgC03 SrC03 soluble Total 

residue 

------------ Mariana Limostono: 

Kr 2-1.·--------·---------- 98.3 0.96 0.14 0. 9 100.3 

Kr 2-2----------·---------- 98.8 .92 .11 .6 100.4 

Qu 4-2-------------------- 98.5 1.00 .17 .5 100.2 

Bx 2-L-------------------- 97.6 2.09 .03 1. 2 100.9 

Po 1-2. --------·---------- 97.0 1. 59 .36 2. 2 101.1 

Sw 2-4---------·---------- 65.8 1.36 .09 31.2 98.5 

IJ 2-1. ••••••••••••• -------- 96.1 1.09 .04 3.5 100.7 

Pr 8-3---·------·---------- 98.5 1.05 .13 .9 100.6 

Janum Formation: 

1's 5-4 ••••• ____ • ____ ------- 81.8 1.63 .11 14.2 97.7 

1's 5-0------------------·-- 97.6 1. 51 .06 1. 0 100.2 

Rr 23-1 ••••• ----·---------- 88.5 1. 21 .04 9.8 99.6 

Rr 5-l. •.••••••.••••••.••.. 95.0 . 98 .09 3.6 99.7 

Barrlgadn· Llmostono: 

Bv 5-L---------·---------- 08.8 1.05 .04 .5 100.4 

AIUnn Llmostono: 

Di 2-1. •••• -------·----•• __ 08.2 1.05 . 07 1.1 100.4 

Dl2-2 ••••••••••••••••••••• 91.8 1. 74 . 14 7. 5 101.2 Jl8-2 ______________________ 08.8 1. 05 . 01 .3 100.2 

'l's o-1.---------------- ____ 96.7 1. 76 .04 1.9 100.4 

Ru 8-3-------------------- 88.9 1. 05 .18 10.0 100.1 

Bonyn Llmostono: 

H.r 13-1. •••••••••••••.••••• 07.6 1. 38 .03 1.4 100.4 

01 2-L---------·---------- 98.2 1. 59 .11 . 5 100.4 

Ut 6-L-------------------- 08.2 1. 69 .04 .9 100.8 

Maomong Llmostono Member or Umatnc Formntlon: 

II 5-2 ••• -----•••••••••••••• 00.2 .86 . 01 . 7 100.8 

Er 1-1. •••••••••••••••••••• 98.2 1. so .03 .3 100.3 

Oi1-2n Ed 1-1b ••••••••••••••••••• 96.3 2. 72 .03 1.6 100.6 ____________________ Dr 91.0 1. 57 .01 7. 4 100.0 o-1n ____________________ 78.3 1. 27 .06 20.8 100.4 

Eo 2-L-------------------- 74.1 2. 51 .16 23.8 100.6 

Alutom Formntlon: 

l~k 4-11.------------------ 97.8 1. 84 .03 1. 0 100.7 

Mal1lnc Mombc.r: 

OJ 11-1-------------------- 52.7 .52 .17 42.4 95.8 

I\:ulp and others (1952, table 11) also give the average SrC03 content of 155 samples of limestones as 

being close to 0.1 percent. The 29 samples analyzed 

from Guam averaged 0.09 percent SrC03• Thus, the 

SrCOa content of the limestones of Guam is consistent 

with other limestones. The generally low SrCOs content of all the Guam samples indicates a high degree 

of recrystallization and replacement of primary aragonite by secondary calcite. In a few samples the degree of strontium expulsion or loss may be inferred 

because the rocks contain abundant fossils of a single 

group. Sample Po 1-2, for example, which contains 

0.36 percent SrCOs, contains 40 percent H alim,eda segments. Pure, unaltered H ali1neda segments carry approximately 1.27 percent SrCOa (Emery, Tracey, and 

Ladd, 1954, p. 67). If the non-Halimeda part (60 

percent) of Po 1-2 contains 0.08 percent SrC03, which 

is the 29-sample average less the SrC03 in Po 1-2 the 

segments themselves contain approximately 0.76 percent SrCOa and the SrCOs loss in the partly recrystallized aragonitic segments amounts to approximately 

40 percent. 

690-219 0-63-2 

NONCARBONATEMINERALS 

:M~any different noncarbonate minerals have been 

identified in the limestones of Guam by Dorothy Carroll and J. C. Hathaway of the U.S. Geological Survey. These minerals are listed by formation and are 

discussed on pages 37-46. 

Most of the minerals are traceable to the volcanic 

rocks exposed on the island, and they reached the areas 

of limeston~ deposition as airborne eruption products 

and as subaerial erosion products; some minerals are 

the result of alteration of volcanic material following 

its incorporation in the limestones. The insoluble-residue content of 23 samples ranges from 0.1 percent in 

parts of the Alifan Limestone to 42.1 percent in the 

Mahlac Member of the Alutom Formation. The residues, mainly silt and clay, consist of clay minerals 

and of nonclay-mineral groups. The clay minerals 

found are: montmorillonite, chlorite, mixed-layered 

montmorillonite-chlorit~, mica, and minerals of the 

kaolinite group. The nonclay minerals are divided 

into two groups: a group of silicate minerals including 

quartz, cristobalite, plagioclase feldspar, and zeolites; 

and a nonsilicate group including gibbsite, nordstrandite, magnetite, hematite, goethite, lithiophorite 

and pyrolusite, apatite, and a woodhouseitelike mineral. Most of the samples studied contain either a 

nonclay group characterized by feldspar and zeolites, 

or a group characterized by iron oxide minerals and 

nordstrandi te. 

The relative instability of zeolites to weathering indicates that the noncarbonate fraction of limestones 

containing the feldspar-zeolite assemblage originated 

as ash falls. Montmorillonite, which accompanies this 

assemblage, also commonly occurs as the dominant 

mineral in bentonite, which is the product of volcanic 

ash devitrification. Those samples containing the iron 

oxide-nordstrandite assemblage received their noncarbonate fractions from erosion of soil formed on volcanic rocks. The formations containing the erosion 

assemblage are the Agana Argillaceous Member of the 

~1ariana Limestone, the Alifan Limestone, and the 

J anum Formation. The Mahlac Member of the Alutom Formation, limestones of the Alutom Formation, 

the Maemong Limestone ~1ember of the Umatac Formation, the Bonya Limestone, and parts of the Mariana Limes~one contajn the ash-fall assemblage. 

Heavy ID:inerals of sand-size grades make up only a 

trace of the limestones. They include magnetite, ilmenite~, augite, hypersthene, greenish-brown amphibole, basaltic hornblende, bluish-green amphibole, 

sphene, and zircon. Minerals of the light fraction include plagioclase feldspar, quartz, clay aggregates, 

opaline silica (beta-cristobalite), zeolites, and apatite. 
DIO GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS 

Volcanic material easily identified in thin section, in 

hand specimens, and in outcrop is present mainly in 

three forms: ( 1) scattered angular fragments of single 

crystals of plagioclase and ferromagnesian minerals, 

(2) bands and lenses of fine detritus within calcareous interstitial fills or as pockets, and (3) wellrounded rock fragments. The rock fragments are now 

largely clay, but their shapes are suggestive of waterworn gravel formed during subaerial erosion of volcanic terrane. l\iany of the present-day beaches 

around south Guam are rich in this material. The 

angular single crystals are probably the result of ash 

falls. All three types of material are present in the 

Alutom Formation and the l\iaemong Limestone Member of the Umatac Formation. A few beds of the 

Maemong l\iember in the Gens River contain as much 

as 50 percent of volcanic detritus in the form of wellrounded pebbles, cobbles, and boulders of pyroclastic 

and crystalline rocks in a limestone matrix. 

POSTDEPOSITIONAL ALTERATION 

RECRYSTALLIZATION OF ORIGINAL MATRIX AND 

PRIMARY SKELETAL MATERIAL 

Both matrix and fossils in the limestones have been 

greatly altered. In the matrix the main change has 

been the conversion of much of the ubiquitous dark 

semiopaque mud to clearly transparent fine- to coarsegrained mosaics of calcite. This recrystallization follows two patterns. In some places, the mud first 

crystallized to clear calcite around the enclosed fragments. In the initial stages of this process each fragment is surrounded by a halo of clear calcite in which 

the crystals are perpendicular to the rim of the fragment. These halos coalesce to make the entire matrix 

crystalline. In other places, the fragments had no 

effect on the recrystallization o:f the mud; patches of 

clear calcite grade outward into dark mud. One effect 

of this second type of recrystallization is a spotted appearance strikingly similar to a leopard-skin pattern. 

In such rocks small black specks occur in a clear calcite 

mosaic. Some o:f these patches are thought to be fine 

algal :fragments that have resisted recrystallization. 

The sharpness o:f the contact between the recrystallized 

mud and the :fossil depends on the type o:f organism; 

in some places the calcite crystals :formed from the 

mud are continuous 'vith recrystallized mollusk shells 

and coral remains. This occurrence is especially true 

in advanced stages of recrystallization. The Foraminifera tests and the articulate and encrusting algae, 

however, except :for the fine algal material already 

mentioned, have sharp contacts with the calcite. 

This mosaic-fossil relationship depends on ·the varying susceptibility to recrystallization of different fossil 

groups. Crickmay ( 1945) arranged the fossil remains 

in the limestones o:f Lau, Fiji, in the following order 

of decreasing susceptibility o:f recrystallization: corals, 

mollusks, pelagic Foraminifera, beach-type Foraminifera, larger Foraminifera, echinoids, and calcareous 

red algae. In this report Ii alimeda is added between 

coral and mollusks. Crickmay ( 1945, p. 239), in discussing this relative solubility, states: 

* * * The factors which determine solubility appear to be: 

(1) chemical composition-the presence of magnesium carbonate (calcareous red algae) reduces solubility; (2) form of 

calcium carbonate-aragonite shells (corals, most mollusks) 

are more soluble than calcite shells (Foraminifera) ; (3) shell 

structure-compact nearly opaque shells (larger Foraminifera, 

calcareous red algae) are much less soluble than prismatic 

transparent shells (pelagic and beach-type Foraminifera). 

There is general agreement with Crickmay's order 

o:f susceptibility and the factors controlling it, but the 

stages and mode o:f recrystallization of shell material 

described by Crickmay differ in some respects from 

those seen in the Guam material. In discussing shell 

material, apparently molluscan, Crickmay (1945, p. 

238) states : 

shell molds filled with granular calcite resemble shells which 

have undergone recrystallization, but can be distinguished by 

the following criteria: (a) filled molds (casts) have sharp 

margins ; recrystallized shells have irregular and fuzzy margins; (b) a rock with casts generally contains some shells of 

the same species showing unaltered structure; in a recrystallized rock all shells of similar character are more or less 

equally altered ; (c) insofar as the calcareous paste is generally more susceptible to alteration than calcite shells, 

coarsely granular shells in unaltered paste indicate casts and 

not recrystallization. Shells originally made up entirely or in 

part of aragonite (mollusks, corals) are much more commonly represented by molds or casts than calcite shells. 

Crickmay ( p. 240-41) :further states : "The paste 

surrounding radially prismatic shells commonly recrystallizes in optic continuity with the shell fibers, and 

thus structurally there is a breakdown of the shell 

margin. * * * Further recrystallization leads to a replacement of prismatic structure by granular calcite, 

but before that occurs the shell margin has already 

been rendered obscure." 

The slides studied, particularly those from sample 

Fk 4-5 (pl. 1A, B), contain :fragments of molluscan 

shells showing a sequence o:f recrystallization that differs in process :from Crickmay's generalizations but 

which yields the same final result. This sequence in 

the Guam rocks o:f Eocene age is : 

1. Almost unaltered fibrous yellow-brown laminar 

shell fragments occur in sharp contact with the 

paste matrix. 

2. Formation of a patchwork of coarse well-defined 

polygonal calcite crystals within the shell frag-
PETROLOGY OF THE LIMESTONES OF GUAM Dll 

ment. These crystals are clearly apparent only 

under crossed nicols; they terminate sharply at 

the edge of the shell. 

3. The crystal patchwork becomes easily visible in 

plain light; shell structure is still distinct. 

4. The crystals become a clearer mosaic that partly 

obscures the original structure. 

5. The crystals transcend the shell boundary and grow 

into the surrounding paste, but the original shell 

structure can still be vaguely seen as yellowbrown bands and fibers. In only a few fragments 

is all structure completely effaced. 

Crickmay's criteria for distinguishing shell recrystallization from replacement and mold filling may 

not hold true in all coral limestones. In the material 

of Eocene age from Guam, numerous shell fragments 

show sharp edges even though internal recrystallization is in an advanced stage. The encroachment on 

the matrix by crystals in the shell may be a late 

stage of recrystallization. The distinction between 

shell recrystallization and solution and secondary filling is important because the solutional history of a 

limestone can sometimes be determined accurately 

only if this distinction is made. 

Coral fragments, originally aragonite, have been 

completely recrystallized into fine- to coarse-grained 

calcite mosaics and are recognizable as coral remains 

by the following features: ( 1) original interseptal 

voids filled with dark-gray mud that has preserved 

the pattern of the coral structure (pl. 9D) ; (2) fine 

dark lines that preserve a coral pattern even when 

the dark fill is missing; and ( 3) algal coatings on the 

fragments that preserve the original outline of the 

corallum but prevent the recrystallization of the coral 

material from proceeding into the matrix. 

Foraminifera are in general well preserved and 

show encroachment by crystalline calcite and then 

along cracks or as enlargments from calcite fillings 

in cells. The larger Foraminifera appear yellow and 

fibrous in section, but the small thin-walled types, such 

as miliolids, appear black. 

Calcareous algae, except Il alim.eda., are well preserved and show calcite replacement in only a few 

places. The algae appea.r dark and have well-defined 

cellular structures in thin section. 

Halimeda segments are recognizable by their characteristic outline and sparse remnants of internal 

structures. In one locality, the ~1ariana Limestone 

(sample Po 1-2) contains primary aragonite in the 

form of irregularly altered Il alinwda segments. Almost unaltered segments in sample Po 1-2 appear as 

amber, yellow, and brown masses of cryptocrystalline 

aragonite that show peculiar low relief against the 

surrounding calcite. The central tubes are filled with 

clear granular calcite. In early stages of replacement 

the segment retains its yellow color but has been 

transformed into a fine-grained mosaic of anhedra 

that shows normal calcite relief. In the final stage 

of recrystallization the segment is completely occupied by clear calcite anhedra. 

Solution has dissolved many of the segments and 

left H alimeda-shaped voids. Segments in all stages 

of recrystallization have been dissolved, but those 

having a high percentage of original aragonite are 

most affected. Parts of the rock have, in effect, undergone a textural reversal from what must have been 

a loosely packed Halimeda gravel to a porous network of banded aragonite and granular calcite. 

The abundance of unaltered aragonite shells and 

skeletons in cores from Bikini Atoll has been assumed 

to indicate a history of nonemergence for the limestones containing them. Emery, Tracey, and Ladd 

(1954, p. 2) state: "Recrystallization of aragonitic 

shells and skeletons to calcite is limited to irregularly 

consolidated intervals, and is believed to be due to 

emergence of the rocks above sea level." Johnston 

and others ( 1916) make the point that impure aragonite (that which contains some lead, zinc, or strontium in solid solution) might be stable in contact 

with the "natural waters in which it formed." They 

further state, in connection with the transformation 

of aragonite to calcite (p. 509) : "thus it may be 

that chemically precipitated aragonite, which apparently persists under some circumstances in saline 

waters, is soon transformed to calcite when exposed 

to the action of meteoric waters." The presence of 

aragonite in limestones of ~1iocene age in the subsurface of Bikini (Emery, Tracey, and Ladd, 1954, p. 2) 

certainly supports the idea of the stability of aragonite in normal marine saline waters over the period 

of some tens of millions of years. However, the aragonitic limestones from Guam indicate that the rate of 

change of aragonite to calcite under subaerial conditions may be slower than generally assumed. The 

persistance of small pockets of aragonite-rich limestone in the otherwise almost completely recrystallized 

mass of the ~1ariana Limestone, which is several hundreds of feet thick and several miles in lateral extent, 

indicates that the mere presence of aragonite in a core 

should not be taken as unequivocal evidence for a 

history of nonemergence. 

FRACTURES 

In thin section many of the limestone fragments 

from the Alutom Formation display intersecting sets 

of fine fractures, generally less than 0.25 mm wide, 
D12 GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS 

healed by finely granular calcite. These fractures 

are best developed in sample Hj 1-5 (pl. 1D). In 

slides from this sample, two distinct well-developed 

sets of fractures intersect at approximately 60°. The 

amount of offset along these fractures w~.s determined 

by matching algal fragments across the cracks. Offsets in the plane of the slide were as much as 0.5 mm. 

Offsetting at an angle to the slide no dot1bt took place 

but could not be measured. In slides from sample 

Hj 1-7, a fracture system of two sets intersecting at 

approximately 70° was poorly developed. In this 

sample the calcite-filled veins were as much as 1 mm 

wide. These fractures are well-developed on Guam 

only in limestones of Eocene (b) age. 

DEPOSITION OF CHEMICAL CALCITE 

Chemically depos1ted calcite is in two forms. The 

dominant type is represented by coarse-'grained clear 

calcite that fills cells in tests of Foraminifera and 

original interstices between large detrital fragments. 

Some of these original voids are incompletely filled 

and show geodelike incrustations of calcite. The second form of secondary calcite is represented by fine 

fibrous crystals perpendicular to the "ralls of large 

solution voids (pl. 10). These bands of fibrous calcite reach a maximum thickness of 1 mm. The remaining interstices, some of which are 5 mm in diameter, are completely filled with gray mud. 

SILICIFICATION 

Limestones, interbedded with tuffs and flows of the 

Alutom Formation, show extensive silification both 

as vein fillings and as metasomatic replacement (pl. 

2A-D). In one sample (Ek 7-4), the ~ilicified limestone is directly overlain by a pillow lava flow of 

considerable but undetermined thickness. Prior to 

the silicification, the limestone underwent a period of 

brecciation, subaerial solution, and deposition of calcite. However, inasmuch as the overlying pillow 

lava suggests a submarine flow, the limestone may have 

been silicified in a marine environment after becoming submerged following a period of subaerial erosion and solution. The flow was probably the source 

of the silica that gained access to the limestone 

through fractures and solution fissures. Evidently 

the cavity filling and the metasomatic replacement of 

]jmestone took place contemporaneously. 

Limestones of l\1iocene (e) age int~rbedded with 

pillow lava near Umatac show silica filling of globigerinid tests by beta-cristobalite, probably a result 

of a postdepositional history similar to that of the 

Eocene (b) limestone just described. 

MANGANESE MINERALS IN THE BONYA LIMESTONE 

Large parts of the Bonya Limestone in the UgumTalofofo area show patchy replacement by earthy 

black manganese minerals. Lithiophorite, lithiumbearing manganese wad, and pyrolusite were identified in this formation by X-ray diffraction techniques. 

On Guam, replacement by manganese is apparently 

confined to the Bonya Limestone, though replacement 

is widespread within this formation. Sample Ut 6-1, 

a cobble of Bonya Limestone found in the underlying 

J anum Formation, contained lithiophorite. Thus the 

time of manganese replacement of the Bonya Limestone can be dated as pre-J anum in age. Evidently 

a small amount of these manganese minerals in a limestone can impart a dark color; sample Ut 6-1 contains less than 1 percent insoluble residue, yet it is 

dark slatB gray in hand specimen. 

Study of these manganiferous limestones in thin 

section reveals a characteristic mode of occurrence 

(pl. 130, D). Most of the manganese occurs in the 

microcrystalline and fine-grained matrix. It is finely 

disseminated as small distinct flecks and blebs. In 

transmitted light as well as in reflected light the 

manganese is coffee brown to black. In scattered areas 

these flecks and blebs coalesce, becoming blacker and 

more opaque as they become more concentrated, and 

finally merge into black massive patches of pyrolusite 

that show a metallic luster in reflected light. Fossil 

debris serves as centers for deposition. Almost all 

the Foraminifera show fine coatings of black and 

brown minerals on both their internal and external 

surfaces. On these tests there is a tendency for the 

manganese minerals to· take the form of stubby, tabular crystals oriented perpendicular to the test surface. 

In fragments of encrusting Foraminifera the perforations are filled with these minerals. Coralline algae 

are more resistant to replacement and are only locally 

infiltrated by manganese. Late-stage calcite mosaics 

fill, or almost fill, many secondary solution voids. In 

a few of these voids the final deposit is black massive 

manganese oxide that shows metallic lustre. Most of 

these late calcite mosaics lack manganese replacement minerals. However, small radiating and sheaflike clusters of stubby, acicular, and rhombohedral 

crystals have formed in some mosaics. These crystals 

are coffee colored and. show weak pleochroism or absorption in shades of light yellow brown. Absorption 

is greater parallel to the long axis of the crystals. 

These crystals show high birefringence and indices 

of refraction greater than calcite; they have not been 

identified. 

The source of the manganese just described was 

undoubtedly the pre-Bony a volcanic rocks of Guam. 
PETROLOGY OF THE LIMESTONES OF GUAM D13 

Chemical analyses of fresh basalt from Guam show 

a range in Mn02 content of 0.05 to 0.20 percent 

(Stark, 1963). Slightly acidic surface waters draining these volcanics during Bonya time probably car- . 

ried the manganese (as manganous ions) to the sea, 

where slightly alkaline conditions caused its precipitation as manganese hydroxide Mn ( OH) 2• According to Pauling (1953, p. 524), manganese hydroxide, 

a white precipitate, rapidly changes in the presence 

of oxygen to the brown manganic compound 

MnO (OI-l). The presence of the finely disseminated 

dark-brown to black manganese minerals in the finegrained matrix and the crusts of these minerals on 

and in tests of Foraminifera suggest that the manganese hydroxide may have been precipitated and 

oxidized soon after the deposition of the Bonya 

Limestone; possibly the precipitation was contemporaneous with the accumulation of the limestone. 

Later, further oxidation converted much of these compounds to black metallic manganese oxides such as 

pyrolusite. Subaerial solution of the limestone by 

percolating water may have resulted in the redistribution and concentration of the minerals. 

The conditions that would cause transport and precipitation of manganese compounds would also operate for iron compounds. However, as Mason ( 1952, 

p. 147) and Clarke (1920, p. 553-554) point out, 

iron compounds tend to precipitate under weaker 

oxidizing conditions than equivalent manganese compounds. Thus the manganese may be transported 

longer distances, or into a different environment, 

which would effect a separation of manganese from 

iron. The presence of lithium in the manganese minerals may be due to the strong absorptive power of 

compounds such as manganese dioxide hydrate, which, 

according to ~1ason ( 1952, p. 154), causes oxidate 

sediments to be relatively rich in minor elements. 

In summary, field and chemical data indicate that 

a period of subaerial erosion during which the streams 

were slightly acidic affected the pre-Bonya volcanic 

rocks during the deposition of the Bonya. That this 

erosion was island-wide is indicated by the lithiophorite in sample Ut 6-1 from north Guam. 

In addition to the manganese replacement described, the Bonya Limestone carries earthy black 

coatings of manganese minerals that may be related 

to the concentration of manganese oxides in the nearby 

volcanic rocks during later erosion cycles. Abundant 

nodules and stringers of manganese minerals are 

found in the deeply weathered volcanic rocks throughout the island. 

MICROSTYLOLITES IN THE ALIFAN LIMESTONE 

Thin sections from samples Eh 3-5 (pl. 3A) and 

Di 2-1 (pl. 3B, 0) show well-formed microstylolites. Closely spaced bands of subparallel, bifurcating, and intersecting seams produce reddish-brown 

streaks in the limestone that show up well in reflected 

light. In transmitted light these color streaks are 

resolved into individual microstylolite seams, each of 

which is marked by a film of reddish-brown clay. 

The seams range in width from 0.1 to 0.25 mm and 

are closely spaced; in sample Eh 3-5, 14 were counted 

over a distance of 10 mm. These truncate tests of 

Foraminifera as well as the fossil debris and the finegrained material of the matrix. Some fossil debris, 

particularly sand-size echinoid fragments, are completely bounded by such seams and, in addition, have 

sutured contacts with adjacent fossil fragments. Many 

molluscan shell fragments show well-formed sawtooth contacts with the matrix (pl. 3B) . 

The limestones in which the microstylolites are 

found retain some primary porosity in the form of 

empty cells within tests of Foraminifera, and also 

show secondary voids due to solution. Interstitial 

residual primary porosity is low owing to the tight 

packing of the grains having concomittant suturing 

and to the dense microcrystalline matrix. The areas 

most affected by secondary porosity are those occupied by bands of microsty lolites. The formation of 

secondary porosity post-dates the microstylolites, and 

the seams are cut and bounded by irregular solution 

voids. 

The microstylolites discussed in this paper are 

similar to those described by Dunnington ( 1954) from 

subsurface rocks of north Iraq. Dunnington reviewed 

the evidence for and against the postlithification 

growth of stylolites and came to the conclusion that 

these seams postdate lithification of the enclosing sediments. Study of the meager Guam material may not 

resolve the prelithification verus postlithification controversy, and it is not intended here to attempt such 

a resolution. The Guam occurrence, however, may 

shed additional light on the importance of differential pressure as a factor in stylolite formation. Dunnington (1954, p. 47) studied thousands of thin sections from representative rock suites of Carboniferous 

to Miocene age and came to the following conclusion: 

* * * Other factors being equal, it would appear that there 

must be a critical rock pressure below which stylolites cannot 

form, and above which they must. This necessity is merely a 

vague and theoretical one, but it is supported by the finding 

that in the more recent or never deeply buried formations 

studied in the Middle East, stylolites are rare, whilst in sin-
D14 GEOLOGY AND HYDROLOGY OF GUAM, MARIA::\A ISLANDS 

gle formation stylolites are found in increasing abundance in 

rough proportion as they have been subjected to increasing 

overburden. Other factors, which must in fact vary, render 

difficult any investigation of the significance of overburden, 

and any attempt to determine the critical pressure value (if 

such exists) . 

Samples Eh 3-5 and Di 2-1 are from the base of the 

Alifan Limestone, which now forms a cap on the 

Mount Alifan-Mount Lamlam ridge. The site from 

which sample Eh 3-5 was collected is, at present, overlain by perhaps 200 feet of limestone. Sample Di 2-1, 

however, taken from the crest of the ridge, was overlain by only a few feet of limestone. No doubt the 

upper part of the Alifan Limestone has been removed 

by subaerial solution, but it is doubtful whether the 

site of sample Di 2-1 was ever buried below more 

than perhaps 300 feet of limestone. In connection 

with studies of the subsurface geology of Eniwetok 

(Schlanger, 1963), thin sections of limestone taken 

from a depth of 4,530 to 4,550 feet below the atoll 

have been examined. A coarse-grained foraminiferalalgal dolomitic limestone from this depth retains a 

porosity of approximately 5 to 10 percent and shows 

only slight signs of intergranular penetration; no 

well-formed microstylolites were seen. The limestone column below Eniwetok has not been subjected 

to any tectonic pressure; the differential pressure on 

the sample studied is due to simple static loading. 

The presence of microstylolites in the lightly loaded 

Alifan Limestone contrasts with the lack of such pressure-solution features in the Eniwetok material. Dunnington (1954, p. 47) was careful to state that his 

critical pressure concept assumed "Other factors being equal * * * ." One factor that is not equal in 

the Guam-Eniwetok comparison is argillaceous contamination ; the basal part of the Alifan Limestone 

contains enough insoluble clay to color the limestone 

red, whereas the Eniwetok sample is pure. The solution histories of the two limestones are also different. 

The Alifan Limestone has been subject to considerable subaerial exposure and concomitant ground-water 

percolation. The basal part of the Alifan Limestone 

which directly overlies relatively impermeable volcanic rocks was and is subject to lateral ground-water 

movement at and above the limestone-volcanic contact. 

The Eniwetok material appears to have undergone 

some subaerial solution, but probably has a longer 

history of immersion in sea water. It is suggested 

that ground water movement along clay seams, rather 

than merely differential pressure, was the controlling 

factor in microsty lolite development in the Alifan 

Limestone. 

NORDSTRANDITE DEPOSITS IN THE ALIF AN 

LIMESTONE 

N ordstrandite (AbO a • 8H20), a polymorph of 

gibbsite-bayerite, is present in a number of widespread 

samples from the Alifan Limestone. The petrography 

of nordstrandite is discussed in the following paragraph; the chemistry of its deposition and theoretical 

considerations involved are discussed on page 44. 

Detailed chemical, physical, optical, crystallographic, and X-ray diffraction data are published 

elsewhere (Hathaway and Schlanger, 1962, p. 265-

266; Hathaway and Schlanger, 1963). A simultaneous 

discovery of nordstrandite from Borneo is described 

by Wall and others ( 1962, p. 264-265) ; the Guam and 

Borneo nordstrandite finds are the first known natural 

occurrences of this mineral. 

X-ray diffractometer study of the Alifan Limestone 

revealed nordstrandite in samples Di 2-1, Ts 9-1, and 

Ru 8-3. Thin-section study showed that the mineral 

is present in samples Dh 12-1, Eh 3-2, and Ts 16-13. 

Thus, nordstrandite is characteristic of the lower part 

of the Alifan Limestone from both north and south 

Guam. In all these samples, nordstrandite occurs 

in secondary solution voids. This mode of occurrence 

is best seen in sample Dh 12-1 (pl. 40, D) ; the 

crystals of nordstrandite completely fill a former solution void approximately 0.5 mm in diameter. The 

nordstrandite fills spaces between previously deposited 

calcite; it does not replace the calcite. 

In plane polarized light (pl. 40), nordstrandite shows 

mark.ed negative relief against abutting calcite; the 

crystals are clear and barely visible. Between crossed 

nicols the crystals are arranged radially from discrete 

points or areas on the wall of the original void (pl. 4D); 

from these centers the crystals fan out toward the opposite wall as bladed to flamboyant aggregates. In 

some filled voids, nordstrandite shows up as a mosaic 

of intricately intergrown twinned anhedra. Several 

crystals do not impinge on the void walls: these crystals 

are tabular in form and terminate in sharp, well-defined 

pyramidal(?) faces. Many of the crystals show polysynthetic twinning parallel to their length. The optical 

properties of the mineral are as follows (Hathaway and 

Schlanger, 1962): a=l.580±0.004 colorless; {3=1.580 

±0.004 colorless; 'Y=1.596±0.004 colorless; 'Y-a 

=0.016. Optic sign=positive, 2V=low, elongation 

=negative, extinction =inclined, 2/\C ~ 34 °. 

The original voids appear to be the result of structurally controlled, probably subaerial solution of the 

limestone. The three converging linear areas of clear 

calcite leading into the central clear area, shown on 

plate 40, D, are healed fractures that traverse the rock 

and connect with other former voids now filled by 

nordstrandite. The initial deposit, or residuum, on the 
PETROLOGY OF THE LIMESTONES OF GUAM D15 

walls of tl1ese solution openings is a thin film of reddish-brown clay. Similar-appearing argillaceous material is finely disseminated in the aolid limestone 

around the voids. A few of these particles are clear 

and appear dark red to brown in transmitted light, 

showing extinction between crossed nicols. The argillaceous layer shows up as dark lines in the photomicrographs. Directly overlying this clay film over most of 

its area is a deposit of clear calcite, in the form of 

subhedral crystals, which fills smaller original voids 

and fractures but merely lines the larger openings. 

The final deposit is nordstrandite. The nordstrandite 

growth centers from which the crystals radiate, are 

commonly the places along the original void wall that 

lack a deposit of clear calcite. At these points the 

initial part of the nordstrandite crystals are in contact 

with the clay film and contain scattered, possibly residual, blebs of the reddish-brown clay. In connection 

with this nordstrandite clay contact, it is noted that 

the voids that are thickly and completely coated with 

calcite lack nordstrandite deposits. 

These nordstrandite clay relations suggest that the 

growth of the clear nordstrandite was initiated by recrystallization of the clay. However, the volume of 

nordstrandite certainly exceeds that which could have 

been produced by recrystallization. Some deposition 

of nordstrandite from solution must have taken place. 

The time of deposition of the nordstrandite may be 

inferred from the textural relations seen in thin section. Emergence, lithification, and fracturing of the 

Alifan Limestone must have taken place before the 

subaerial solution that produced the original voids 

could have operated preferentially along fissures. A 

period of precipitation of calcite from solution followed; the precipitation produced the void linings and 

fracture-healing mosaics. The precipitation of nordstrandite was a late-stage event in the history of the 

limestone. 

N ordstrandite occurs in the J anum Formation according to X-ray diffractometer data, but it was not 

recognized in thin sections. The nordstrandite in the 

J anum Formation may be entirely the result of recrystallization of detrital gibbsite. (Seep. 44.) 

DOLOMITE IN THE ALIFAN LIMESTONE 

Dolomite "·as tentatively identified in thin sections 

from several samples of the Alifan Limestone on the 

basis of crystal habit and textnral similarity to known 

dolomitic limestones from the Pacific area. A thin 

section of sample Tt 7-4, cover glass removed, was 

subjected to X-ray diffractometer testing that confirmed the preliminary identification. The occurrence 

of dolomite was not noted during fieldwork on Guam 

and, therefore, its distribution can only be inferred 

from its frequency of occurrence in sample suites. 

In 13 closely spaced samples of the Ts 16 set, only 

Ts 16-8 and possibly Ts 16-7 contain dolomite. In 

four samples of the Tt 7 set, collected within a distance 

of 100 yards along the volcanic-limestone contact 

northeast of l\1ount Santa Rosa, only Tt 7-4 contains 

dolomite. Other samples from south Guam lack even 

these scattered euhedra. Thus, dolomitic limestones 

are known to be sparsely distributed within a part of 

the Alifan Limestone on the north plateau. 

Sample Ts 16-8 is an intensely recrystallized dolomitic limestone that contains relict ghost structures 

indicative of original texture and fossil content (pl. 

5A, B, 0). Recognizable fossil ghosts include fragments of coral, detrital coralline algae, whole and 

broken Halimeda segments, whole tests of Rotalia and 

smaller, thin-walled Foraminifera, fragments of encrusting Foraminifera, and echinoidal debris. These 

are packed in a fine-grained matrix. The fossil ghosts 

are invested, in many places, in a clear coarse-grained 

calcite mosaic. Under high magnification the outlines 

of these ghosts show up as very fine grained mixtures 

of microcrystalline calcite and barely visible dolomite 

rhombs. These now occupy what were original cell 

openings and perforations in Foraminifera, calcareous 

algae, and echinoids and line the septa of coralla. The 

small dolomite crystals show darker centers that indicate possible growth on nuclei of microcrystalline 

calcite. The form now preserved as darker materialthe relative opacity of this type of deposit may be 

due to its fine grain-is the original void pattern of 

skeletons and tests. This reversal of fossil material, 

first described by Cullis (1904, p. 418) from Funafuti 

Limestone, is a common phenomenon in Pacific limestones and dolomitic limestones. The fine-grained calcite and dolomite that preserve the fossil outlines are 

coated throughout with deposits of larger euhedral 

crystals of clear dolomite (pl. 5B, 0). Many of these 

larger crystals show one or more zones, generally near 

their outer edges. The clear background investing the 

coral ghost on plate 5A is a mosaic of calcite that 

forms the final cementing deposit in the rock. The 

deposition of this calcite evidently proceeded inward 

from the enclosing walls, inasmuch as a few voids 

show converging crystals that fail to merge. Calcite 

crystals within any single area bounded by dolomitic 

fossil ghosts generally show optical continuity; some 

of these crystals are continuous over several fossils. 

The matrix of the rock is a fine-grained mixture of 

dolomite crystals, which are euhedral to subhedral, and 

microcrystalline calcite. The matrix also is saturated 

with clear calcite that fills all former secondary solu-
D16 GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS 

tion voids. Sample Tt 7-4 has an original texture 

and postdolomitization aspect that differs somewhat 

from that in sample Ts 16-8. Although more intensely dolomitized, sample Tt 7-4 shows many fossils, 

particularly coralline algae, that are fairly well preserved. These fossil remains and jigsaw-shaped areas 

of dark microcrystalline mud are in a matrix of euhedral dolomite crystals, which are in turn loosely 

packed in microcrystalline calcite (pl. 5D, E). Dark 

mud areas are made up of scattered flecks of opaque 

material in a microcrystalline matrix; dolomite euhedra are not obvious in these areas. 

Some of the large calcite crystals that saturate the 

rock show a peculiar texture. The originally optically continuous crystals have apparently undergone 

recrystallization or exsolution and a complementary 

set of crystals has been formed; the secondary crystals 

all show the same morphologic and crystallographic 

orientation. These are set in a single void-filling calcite crystal. In other areas marked by this type of 

secondary texture the rhombohedra are randomly oriented and show individual extinction against the still 

optically continuous background crystal. Many of 

these randomly oriented rhombohedra appear to be 

floating in the original calcite. These secondary crystals are interpreted as possible dolomite or incipient 

dolomite crystals on the basis of their crystal form 

and their evident alteration from original calcite. 

The lithologic reversal shown by these rocks indicates that a period of intense solution preceded dolomitization. Fo11owing this solution, a deposit of dolomite euhedra was laid down on the fossil ghosts; and 

the calcite matrix was, possibly at the same time, the 

site of formation of floating and' loosely packed dolomite crystals. These matrix crystals may be the result of replacement of calcite. The deposition of clear 

saturating calcite followed; this deposit then underwent recrystallization, and possibly dolomitization. 

The predolomite solution of the original limestone 

probably took place under subaerial conditions, and the 

rock became highly porous. The dolomite that coats 

the ghosts appears to have been deposited from solution in voids. The original algal content of the Alifan 

Limestone was not high enough to provide significant 

amounts of magnesium-rich carbonates by preferential 

leaching of the algal calcite. The algae do not serve 

as centers of dolomitization as they do in dolomitic 

limestones of Eocene age from below Eni wetok Atoll 

(Schlanger, 1957). The initial dolomitization of the 

Alifan Limestone probably took place below sea level 

or in the intertidal zone, where magnesium was available, after a period of subaerial exposure. The final 

cementation required a considerable volume of calcite. 

If this cementation took place subaerially, the limestone nearby was dissolved, then transported and reprecipitated in the porous dolomitic host rock. Skeats 

(1903, p. 115-118) described a virtually identical sequence of solution, dolomitization, and cementation in 

emergent dolomitic limestones from the islands of 

N gillangillah and Mango in the Fiji group. 

Dolomite from below sea level at Funafuti ( Cullis, 

1904, p. 407-408, figs. 41 and 42) , however, is identical 

with the reversed dolomitic limestones from Guam except for the final cementing calcite deposit. If this 

deposit is a typical submarine phenomenon, it should 

be in the Funafuti material; its presence in the 

emerged Fiji and Guam material suggests a final subaerial stage. The recrystallization of the cement along 

with its possible conversion to dolomite, may then be a 

subaerial process. 

This sequence of textural evolution suggests that the 

Alifan Limestone, which was originally a shallowwater deposit, underwent postdepositional uplift and 

solution, postsolution submergence and dolomitization, 

and finally, postdolomitization emergence and cementation. The depth at which dolomitization took place 

was not great. Skeats (1903) made a strong case for 

shallow-water dolomitization which he based on the 

study of many subaerial sections of dolomitic limestones exposed throughout the Pacific and Indian 

Oceans. Schlanger (1956) compared the vertical distribution of dolomite beneath Funafuti (Judd, 1904, 

p. 364-365) with that below Kita-daito-jima (Sugiyama, 1936) and came to the same conclusion as 

Skeats, that these atoll dolomites formed in shallow 

water. The textural similarity of the dolomitic limestone of Guam to those from Funafuti, the Fiji group, 

and Kita-daito-jima suggests a shallow-water origin 

for the Guam material. 

PHOSPHATE DEPOSITS IN THE MARIANA LIMESTONE 

A local veneer of phosphatic rock covers the Mariana Limestone in the Tarague embayment. This single 

outcrop is at an altitude of approximately 100 feet 

above sea level, bears E. 15°, and lies about 5,500 feet 

from the roadcut through the cliff edge overlooking 

Tarague Beach. The outcrop is on the east side of a 

box reentrant in the cliff, at the juncture between the 

marine terrace and the landward cliff. A relict sea 

nip is present 10 to 12 feet above the general level of 

the phosphate-veneered surface of the outcrop. 

PETROGRAPHY 

The phosphatic limestone is a dense, strongly cemented aggregate of hard coffee-brown spherical to 

discoidal biconvex pellets ranging from 1 to 5 mm and 

averaging 3 mm in diameter. On the weathered sur-
PETROLOGY OF THE LIMESTONES OF GUAM D17 

face the pellets stand out in high relief owing to the 

extremely porous texture of the interstitial phosphatic 

limestone; they show no tendency toward preferred 

orientation and are loosely packed, some apparently 

floating in the matrix. The rock breaks across the 

pellets which are more brittle, denser, and harder than 

the matrix. On a broken or smoothly sa wed surface 

the centers of the majority of the pellets are exposed as 

fragments of pelecypod shelJs and, rarely as tests of 

Foraminifera. The shape of the individual pellets is 

controlled by the shape of the nuclear fragment. The 

smaller, more spherical bivalve fragments are enclosed 

by spherical pellets, whereas larger tabular shell fragments are now within flattened disc-shaped, biconvex 

pellets. Some of these pelecypod fragments, after being boiled in l\1eigen's solution, were found to be 

aragonite and therefore probably were original shell 

material. Chemical analyses and mineralogical determinations were made on two pieces of sample Sw2-4 

(seep. 9/38/40). One piece showed 31.2 percent insoluble residue after treatment in 1 :3 acetic acid, and the 

second piece showed 16.4 percent after treatment in 

1 :4 acetic acid. Approximately 98 percent of the second residue was clay- and silt-size apatite. 

The phosphatic pellets are seen in thin section (pl. 

6A, B) to be lightly in contact with one another and 

cemented by a thin film of clear fibrous to granular 

calcite. Approximately 50 percent of the remaining 

interstitial void space is filled with light-brown microcrystalline phosphate. The internal structure of the 

pellets shows common pelecypod shell nuclei; most pellets contain fine- to medium-grained shell detritus. 

The more tabular of these fragments are usually oriented parallel to the edge of the pellet. In some 

pellets these fragments are concentrated into concentric zones that parallel color changes in the more 

phosphatic bands. These relations suggest intermittent 

accretion of both phosphate and shell material. Most 

of the pelletal phosphate is microcrystalline and does 

not change in appearance under polarized light. Some 

pellets, however, are partially or wholly amorphous 

and appear as light-yellow transparent colloform 

structures in which ghosts of fossil shell material are 

present. An apparent second generation of pellets 

is represented by a microcrystalline pellet that has a 

large nucleus consisting of an originally separate 

amorphous pellet. 

COMPARISON WITH PHOSPHATIC LIMESTONE FROM ANGUAR, 

PALAU ISLANDS 

Large deposits of oolitic phosphate ore on Anguar 

Island in the Palau group have been decribed by 

Irving (1950, p. 69), who writes: 

* * * One of the most typical occurrences is over karrenfeld 

topography on the 80-foot terrace within the amphitheater. 

Owing to the presence of a low saddle at the south end of. the 

amphitheater, this terrace could never have been submerged 

beneath a lake. As the terrace apparently represents a former 

level of the sea, it is obvious that all lower areas were submerged beneath the sea. Nevertheless, it is believed that the 

deposits did not form in the sea, because stratification and 

organic remains would then be present. It is concluded that 

the oolites did not form in either a lacustrine or marine 

environment. 

The prevalence of amorphous phosphate, and the tendency 

towards colloform structures in the oolites, suggest that the 

oolites may be the product of intermittent colloidal precipitation. 

The writer visited and saw these deposits during field 

work on Guam and nearby islands. Gilbert Corwin of 

the U.S. Geological Survey field party in the Palau 

Islands, supplied thin sections of the material (pl. 

60, D). 

In thin section the phosphatic limestone of Anguar 

jg seen to be a more advanced alteration of a rock 

originally like the pelletal limestone of Guam. In the 

Palau material all the pellets are now completely 

amorphous. These materials are the same size and 

shape as their Guam counterparts. Relict fossil 

structures in these pellets show that they originally 

had the structure and mechanical banding characteristic of the Guam pellets. As in the Guam material, 

phosphatized mollusk shells are common nuclei. The 

sequence of cementation and void filling in the Palau 

rock is different than that of the Guam phosphate. 

The Palau pellets are imbedded in: (1) a gray, microcrystalline calcite matrix that only partly fills the 

original primary voids; followed by ( 2) a thin deposit of clear grant~lar calcite; followed by (3) a dark 

detrital mud fill (pl. 60, D) ; and ( 4) a final layer of 

calcite. 

GENESIS OF THE PELLETS 

The field occurrence on terraces, and the similarity 

in the initial internal structure of the pellets-secondary solution and deposition effects notwithstanding 

-suggest a common mode of origin of the phosphatic 

limestones of Guam and Palau. The presence of the 

sea nip 10 to 12 feet above the flat on which the 

phosphate is found indicates that the local terrane in 

Guam is close to its original configuration, except for 

approximately 100 feet of relative emergence. Thus, 

if the nip represents the approximate sea level during 

the formation of the terrace, the site of deposition was 

in shallow water, 12 feet or less, directly below a steep 

cliff. The internal structures in the pellets are probably the result of mechanical accretion of finely divided phosphate and pelecypod shell fragments rather 
DIS GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS 

than of chemical precipitation of colloids. The ultimate source of the phosphate, on an island such as 

Guam, was probably bird excrement-guano; the size 

of the individual pellets is well within the fecal-pellet 

range. The high proportion of bivalve fragments 

present as nuclei is suggestive of an original sediment 

similar to the molluscan-rich coquinoid limestones described from the molluscan facies, which is a lagoonal 

deposit. 

The preceding physiographic and petrographic evidence suggests the fol1owing interpretation of pellet 

origin. The local environment was one of a shallow 

back-reef flat or lagoon bounded to the landward by 

a steep cliff. The bottom fauna was dominated by 

pelecypods and mud-ingesting animals, possibly holothurians, living in and on a fine-grained carbonate 

bottom. Bird excrement, rich in phosphorous compounds, deposited from cliff rookeries was concentrated and molded by the mud-ingesters, which also 

ground up large amounts of pelecypod-shell material. 

The lack of a fine-grained carbonate matrix in the 

Guam material is possibly due to the washing out of 

fines and the concentration of pellets by water action. 

Evidently most of the Anguar pellets remained in the 

carbonate mud matrix. 

Thus, the situation during the deposition of the 

Guam phosphate may have been much like the existing situation on the reef of Anao Point, where hundreds of sea birds congregate in a few acres on large 

limestone blocks that have fallen from the nearby landward cliffs and are lying in shallow back-reef waters. 

Since the formation of the pellets, the Guam material has undergone cementation by calcite and some 

later solution that has resulted in the partial filling of 

primary interstices with phosphate. These processes 

were probably subaerial. The Anguar phosphate 

shows the effects of more than one period of calcite 

deposition. The gray microcrystalline deposit, closest 

to the pellets, is possibly the slightly altered original 

mud matrix. The second layer, of clear granular calcite, suggests a cycle of subaerial solution and deposition. The third layer, or filling, of calcite detritus, 

indicates a cycle of submarine deposition. The fourth 

and final layer, of clear calcite, may be a result of the 

present cycle of subaerial processes. 

LIMESTONE FACIES 

FACIES TERMINOLOGY AND CLASSIFICATION 

In order to go beyond the definition and classification of these limestones as rock types and discuss 

their depositional histories, the genetic relationship of 

individual specimens both to contemporary, laterally 

contiguous limestones and to limestones of differing 

ages in the local column must be considered. This 

consideration involves the use of the term "facies." 

Pettijohn ( 1957, p.611) uses "facies" as equivalent to 

the term "consanguineous association," defined as "a 

natural group of sedimentary rocks related to one 

another by origin * * * Such an association is called 

a facies, such as the black shale facies * * *." 

According to this definition however, the limestones 

discussed in this paper belong to the reef facies. The 

most striking attribute of the reef facies, is the 

short-range persistence, both laterally and vertically, 

of a given association of rock types. Detailed studies 

of well-exposed reef deposits led workers to divide the 

reef complex into a series of facies, or subfacies 

(Newell and others, 1953; Henson, 1950; Adams and 

Frenzel, 1950, p. 302-307). The term "reef complex" 

is used in the sense of Henson (1950, p. 215-216) and 

r~fers to an aggregate not only of reef limestone in the 

restricted sense-incrustate limestone of this paperbut of all "genetically associated sediments." In this 

report the "genetically associated sediments" are restricted to reef-derjved and reef-influenced limestones. 

The entire limestone column that forms the base for 

Bikini Atoll (Emery, Tracey, and Ladd, 1954) may 

be thought of as the product of a series of reef complexes; reef, lagoons and forereef deposits are all part 

of the complex. Some limestones described in the 

foregoing rock classification may form independently 

of a reef; for example, globorotalid-rich mudstone, although not in the strict sense part of the reef complex, does grade into it directly and is considered a 

reef-complex rock in this paper. 

The facies recognized in the Guam limestone section are listed in figure 3. The dominant limestone 

associations characteristic of each facies are given, 

and idealized facies relations in several types of reef 

complexes are shown. This terminology and classification of facies is derived from Henson (1950, table 

1) for ancient reef complexes in the Middle East. 

Adams and Frenzel (1950, p. 302-307) used a fourpart facies division: reef wall, back reef, forereef, and 

basinal, for the Capitan Barrier Reef. Not all the 

formations mapped exhibit all the facies listed, but 

all the facies are represented in more than one formation. 

Because both a rock name and a facies name must 

be applied to fully describe these limestones, such 

names as "molluscan coquinoid limestone of the lagoon 

facies" and "foraminiferal-algal microcoquinite of the 
PETROLOGY OF THE LIMESTONES OF GUAM D19 

1-------------------------------------------REEF COMPLEX----------------------------------------------1 

1------------------Logoon ---Reef wall-- --Forereef-- --Forereet--Bosintronsitional 

Facie8 Typical rock a88ociation8 Facie8 Typical rock a880ciation8 Facie8 Typical rock a880ciation8 

Lagoon _________ 1. Foraminiferal-algal mlcroparn- ForereeL _______ 1. Foraminiferal and foraminiferal- Basin ___________ 1. Foraminiferal (benthonic plus 

coquinlte. 

2. Halimeda parncoquinlte. 

algal coquinite and mlcrocoqulnite. 

planktonic) microparacoquinite. 

3. Molluscan coqulnold and mlcrocoquinold limestone and mudstone. 

2. Fornmlniteral and foraminiteralalgal paracoquinlte and mlcroparacoquinite. 

2. Foramlniteral (planktonic) 

mudstone. 

Off-reef (deep)_ 1. Foraminiferal (benthonic) paracoquinite and mudstone. 

(shallow.). 2. Foraminiferal-algal paracoquinltc 

and mlcroparacoquiaite. 

4. Coral breccia. 

Root-walL _____ 1. Incrustate limestone of all 

3. Foraminiferal-algal breccia and 

micro breccia. 

varieties. 4. Coral breccia. 

2. Coral breccia (restricted to Forereef 

pockets). 

3. Fornminiteral-algal paracoqulnito and microparacoqulnlte 

(restricted to pockets). 

transitionaL_ 1. Foraminiferal (benthonic plus 

planktonic) mlcroparacoquinite. 

2. Foraminiferal mlcrobreccla. 

3. Foraminlteral (planktonic) 

mudstone. 

FtOURE 3.-Facles classification and typical rock associations In a reef complex. The ofi-reet facies, not shown, is also recognized in the limestone of Guam. It is not closely 

associated with limestone of the reef complex, but fossil content allows some ecologic interpretation. 

forereef facies" are used In the description of individual specimens. 

RECOGNITION OF FACIES 

A specimen of limestone from Guam can be given 

a name from table 1. However, the assignment of a 

specimen, or outcrop, to one of the facies listed in 

figure 3, involves the use of one or more of the following four lines of evidence : 

1. Comparison of pre-Recent rocks with modern sediments including: 

a. Use of individual faunal and floral elements 

as ecologic indicators, and 

b. matching lateral lithologic changes in ancient deposits with areal distribution patterns of modern reef complexes. 

2. Comparison of individual formations with other 

well-known fossil-reef complexes. 

3. Reconstruction of the original configuration of 

partly destroyed reef complexes on structural and 

stratigraphic evidence. 

4. Matching thin-section attributes of rocks from a 

known facies with those of the rock in question. 

FOSSILS AS FACIES INDICATORS 

The first line of evidence is direct; it depends on the 

fact that many of the rock-forming organisms, at 

least on the generic level, range from Recent to 

Eocene. Where specific types do not have this range, 

extinct fossil groups of similar form, and perhaps 

habitat, make up a part of the older rocks. In order 

to take advantage of the numerous recognizable fossil 

remains in these limestones, the main rock-making fossil elements are arranged in groups that are easily 

identified and have well-known habitats in modern 

reef complexes. Other organisms such as bryozoans, 

echinoids, and worms also contribute to the limestones, 

but they are not major rock formers. 

The main groups considered useful as ecologic indicators are discussed in the following paragraphs. The 

synopsis is taken from the extensive literature published on the subject, particularly from studies made 
D20 GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS 

since World War II in the Pacific Ocean area. Additional information on the ecology of these rockmakers is available in Emery, Tracey, and Ladd (1954, 

p. 79-80), Ladd and others ( 1950, p. 410-425), and 

Cloud (1952, p. 2125-2147). 

CALCAREOUS ALGAE 

Halimeda.-Taylor (1950, p. 77-78) figures and describes this genus in detail and states, concerning its 

habitat: 

• • • The Halimeda beds familiar in the Florida Keys and 

the West Indies were quite absent from the shallow reef area 

in the Marshall Islands. Occasionally similar species filled 

crevices, but the only really lusty Halimeda growth was on 

the sides of the deep holes and pools of the outer reef. The 

dredgings and bottom samplings, however, showed that the 

deeper parts of the lagoon floor were in some areas abundantly populated with Halimeda, and the segments from dead 

plants persisted and formed a substantial superficial layer on 

the bottom. 

In general, substantial amounts o:f Halimeda segments are indicative of shallow or lagoonal sediments 

deposited as deep as 20 to 30 fathoms (Emery, Tracey, 

and Ladd, 1954, p. 79). Disjointed Halimeda, however, are often swept over the reef front into deeper 

waters. In addition, some species grow on the outer 

slopes and contribute to :forereef deposits. Recent 

samples from as deep as 135 fathoms off Guam contain numerous Halimeda segments, (Emery, 1962). 

Emery, Tracey, and Ladd (1954, p. 79) describe samples rich in H alirneda debris :from sea ward slopes off 

Bikini Atoll. 

Articulate coralline algae.-J. H. Johnson (1954, p. 

45) describes these plants as "very abundant in the 

regions· having tides of considerable amplitude where 

they usually occur in the lower part of the littoral 

stage and the upper part of the sublittoral zone, particularly on long, rocky coasts." These algae disarticulate easily and contribute fine rod-shaped detritus 

to the sediments :formed at shallow depths. The :fragments of both crustose and articulate coralline algae 

are also susceptible to transport down and the steep 

outer slope common around reefs. 

Orustose coralline algae.-According to J. H. Johnson (1954, p. 12), this group of algae grows as crusts, 

as branching colonies, and as leaflike masses. Several 

genera are included under the term Litlwthan1nium. 

The crustose forms are particularly effective in :forming much of the cement of reef rock. The branching 

colonies such as Goniolithon, which are brittle and 

fragment easily, contribute large amounts of detritus 

to shallow deposits. David, Edgeworth, Halligan, and 

Finckh (1904, p. 157) differentiate between laminar, 

encrusting Lithotha1nnitt111 and nob by, branching Lithothamnion, stating: "the branching and knobby varieties of Lithothamniun1 are exclusively shallow 

water in habitat at Funafuti, ·whereas the encrusting 

form is both shallo'v and deep water in habitat." 

These algae have been dredged alive from depths as 

great as 156 meters in the tropical Pacific, but they 

reach their greatest abundance in the zone from lowtide level to depths of 30 to 70 feet. Below this 

depth they exhibit a marked decrease in size and 

abundance. 

Dasycladacean algae.-According to Cloud (1952, 

p. 2134), these plants thrive only in water 3 to 5 

meters deep. Thus, although they are rare, they are 

good ecologic indicators. 

FORAMINIFERA 

Large benthonic types such as Cycloclypeus and 

Heterostegina.-H eterostegina, ·where abundant, is indicative of off-reef deposits. In Bikini lagoon Heterostegina outnumbered, in some samples, the usually 

dominant Amphistegina (Cushman, Todd, and Post, 

1954). Cole (1957, p. 750), in summarizing the depth 

ranges of living H eterostegina, gives average depths 

of 25 to 32 fathoms for their frequent or common occurrence. Oycloclypeus, which occupies a :forereef 

habitat exclusively, occurs from 24 to 565 fathoms 

(common or :frequent), and an average depth of 181 

fathoms is given (Cole, 1957, p. 750). Myers (1943, p. 

30), in a study of the habitat of large Foraminifera 

states: 

* * *Most Foraminifera provided with large planospiral, fusiform, or discoidal tests are limited to firm sand or sandy-mud 

bottoms within the sublittoral zone at depths that do not exceed 

that at which photosynthetic organisms thrive and are most 

numerous in areas adjacent to reefs. • * * Strong currents are 

required to transport these large tests and under these conditions one often finds them mixed with coarse detrital reef 

material including broken coral or even gravel from previous 

horizons. 

Small, thin-walled benthonic types such as the miliolids.-These types, where abundant enough to show 

up significantly in thin sections, are considered indicative o:f shallow-water conditions, usually reef or backreef (Henson, 1950). Investigations of Recent Foraminifera from Pacific Ocean reefs confirm this habitat. 

Small, thick-walled benthonic types such as Amphistegina.-Concerning the habitat o:f A mphistegina 

in the Marshall Islands, Cushman, Todd, and Post 

( 1954, p. 362) state: "it occurs abundantly in the lagoons and down to moderate depths on the outer slopes 

o:f the reefs, comprising in some samples as much as 

80 percent of the Foraminifera present. In the deepwater samples the species is present but much less 
PETROLOGY OF THE LIMESTONES O:F GUAM D21 

abundantly and the specimens are very much smaller." 

Thus, A1nphistegina alone might not be ecologically 

diagnostic, whereas Amphistegina plus miliolids would 

indicate shallow-water environments as opposed to 

deeper water conditions, which would be indicated by 

Ampistegina plus Oycloclypeus or globigerinid types. 

Tests of Rotalia are abundant in some Guam limestones. This genus thrives "only in rather stagnant, 

warm waters at the Tortugas" (Cushman, 1926, p. 76); 

according to Hedberg (1934), Rotalia is characteristic of brackish-water conditions. 

Enmvusting types such as Homot.rema and Carpenteria.-In habitat and function these Foraminifera resemble encrusting calcareous algae. They are most 

abundant in shallow reef waters and serve to coat 

and bind detritus. Laminae of encrusting Foraminifera often alternate with algal layers on coral masses. 

Loose nodules and encrustations of Foraminifera are 

also found to considerable depths on the outer slopes 

of atolls (Emery, Tracey, and Ladd, 1954, p. 80). 

Chapman (1900) describes in detail the structure and 

habitat of these encrusting types at Funafuti Atoll. 

Planktonic ty]Jes such as Globigerina and Globorotalia.-The mass of evidence indicates that where tests 

of these types dominate a deposit, deep-water conditions prevailed. Studies by l\1. ,V. Johnson ( 1954, p. 

306, fig. 107) in the Marshall Islands show that the 

concentration of Globige'rina tests in lagoonal waters 

may actually exceed the concentration in parts of the 

surrounding open ocean. However, examination of 

lagoon-bottom samples reveals only rare Globigerina 

tests, for the high benthonic sedimentation rate in the 

lagoon masks the small planktonic contribution. Globorotalia do not form a significant part of bottom deposits above 4,000 feet in the Bikini area (Cushman, 

Todd, and Post, 1954, table 5). 

CORALS 

The use of corals in thin section as paleoecologic indicators is difficult. The growth form of a single 

species may differ greatly owing to its position with 

respect to the reef front. Also, very similar species, 

difficult to differentiate in thin section, have various 

ecologic niches. Outcrops and long cores, in which 

the size, shape, and orientation of the corals and 

their relations to the rest of the sediment can be seen, 

are necessary for reliable paleoecologic work. The 

recognizable sediment contributions of corals range 

from single massive heads 4 to 5 feet in diameter, to 

sand-size detritus. 

MOLLUSKS 

"Tell-preserved pelecypod and gastropod shells are 

found in varying amounts in deposits at. all depths. 

690-219 0-63-3 

This group, however, like corals, is best used for paleoecologic work when outcrops or large cores are available; their use in thin-section work is limited because 

of the difficulty of identification. Studies of the 

Florida reef tract by Ginsburg (1956) show that molluscan-rich muds are characteristic of back-reef or 

lagoonal areas. The distribution of the mapped molluscan facies in the l\1ariana Limestone also shows the 

back-reef nature of these deposits (Tracey and others, 

1963, pl. 1.) 

To summarize by example : 

1. An incrustate limestone consisting of masses of matlike algae investing coral heads in growth position would be from the reef-wall facies. 

2. A paracoquinite containing abundant Halimeda 

segments, miliolid Foraminifera, and dasycladacean algae would be from the lagoon facies. 

3. A coquinoid limestone made up of mollusk shells 

and mud would also be from the lagoon facies. 

4. A paracoquinite or breccia containing Oycloclypeus 

would be from the forereef facies. 

5. A mudstone containing abundant globigerinid and 

globorotalid Foraminifera would be from the 

basin facies. 

6. If the above limestone contained an admixture of 

benthonic larger Foraminifera and algal debris it 

would be from the forereef transitional facies. 

Other examples and the reasoning behind their 

placement in a facies are discussed in the descriptions 

of the individual formations. 

Table 4 summarizes the contributions of each ecologic fossil group to the limestone formations of 

Guam. The number of samples and the number of 

thin sections from each sample studied are given in 

table 11. 

LIMESTONE' FORMATIONS OF GUAM 

LIME~TONE OF EOCENE (B) AGE FROM THE 

ALUTOM FORMATION 

Limestones of Eocene (b) age are present on Guam 

mainly as fragments in volcanic breccias and, to a lesser extent, as discrete beds within the Alutom Formation. The following rock types have been recognized 

fron1 the formation: molluscan-algal paracoquinitemolluscan and foraminiferal-algal microparacoquinite 

from the lagoon facies; algal incrustate limestone from 

the reef-wall facies; foraminiferal-algal coquinite, 

foraminiferal-algal microcoquinite, and foraminiferal 

paracoquinit~ from the forereef facies; foraminiferalalgal paracoquinite from the shallow off-reef facies; 

and foraminiferal mudstone from the basin facies. 
D22 GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS 

TABLE 4.-Summary of composition and fossil content, by volume . percent, of limestone formatio'YLs 

[Numbers represent estimated percentage of each group seen in thin sections. Tr. means less than 1 percent. Volcanics include only material seen in thin section. Matri~ include~ comminuted fossil debris too fine to be placed in a definite group but st1ll recogmzable as skeletal, carbonate mud, chemical deposits of calcite and 

recrystallized mud) ' 

Mac- Barri- MariAlutom mong Bonya Janum Alifan gada ana Lime- For- stone Lime- For- Lime- Lime- Limematlon Mem- stone mation stone stone stone 

ber ------------ Fossil group: 

Halimeda ___ ------- Tr. 3 Tr. Tr. Tr. 4 Encrusting algae ___ 8 6 6 5 Tr. 8 

Articulate algae ____ 14 5 4 ----Tr~- 3 Tr. 2 Larger Foraminif- 11 5 5 15 2 1 era. 

Smaller Foraminif- 2 2 Tr. 2 era. 

Encrusting Foramin if era. MUlolids, and so Tr. Tr. Tr. Tr. Tr. Tr. 

forth. 

Planktonic For- Tr. Tr. 39 ....................... Tr. Tr . aminifera. 

Coral._------------ 8 6 2 4 5 8 

Pelecypods.------- Tr. Tr. 5 Tr. 2 

Gastropods ___ ----- Tr. ----i'i-~- ------i- -------- 2 Tr. 1 Echinoids __________ Tr. -------- Tr. Tr. Tr. -------------- Total (rounded)_ 50 32 34 40 43 17 33 

Volcanics._------------ Tr. 1 Tr. 7 Tr. Tr. Tr. Matrix __________ ----- __ 47 64 60 53 51 75 57 Pore space. ____________ 2 3 6 -------- 5 8 10 

-------------- TotaL----------- 99 100 100 100 99 100 100 

ROCK DESCRIPTIONS 

Limestones from the reef-wall facies (pl. 7 A).-

These rocks are algal incrustate limestones. The incrusting organisms-incrusting algae and Foraminifera combined make up 49 percent of the organic remains-bind and coat much finer grained detritus as 

well as larger coral fragments. In some specimens, 

a single lamina of algae can be traced for over an 

inch along a convoluted course. The incrusting algae 

provide both a framework and detritus trap for two 

kinds of debris: a fine-grained dark mud, and scattered remains of Halimeda, Foraminifera, and articulate coralline algae. Coral-rich specimens present a 

different aspect, these rocks appear brighter in thin 

section owing to the large areas of clear calcite that 

are recrystallized coral. All the coral shows mud 

filling and algal-foraminiferal incrustations. 

Limestones from the lagoon facies· (pl. 7B, C).-

Two rock types are represented: a miliolid-bearing 

foraminiferal-algal m1croparacoquinite and a molluscan -algal parocoquinite. The first type is characterized by numerous thin-walled tests of miliolid, uniseriaJ, biserial, and coiled smaller Foraminifera and a 

flood of fine articulate algal debris. These fossil elements are in a matrix of finely crystalline clear calcite 

in which patches and clouds of the original mud 

matrix are visible. The rock is well-sorted, probably 

owing mostly to the inherent size of the detritus; 

the small tests are probably a life assemblage. The 

second type is characterized by significant amounts of 

molluscan remains ( 5 percent) in the form of whole 

and disarticulated pelecypod valves and fragments of 

gastropod shells set in algal debris. 

Limestones from the off-reef shallow-water facies 

(pl. 7D) .-Limestones of this type are dense algalforaminiferal paracoquinite. Fine debris of articulate 

coralline algae, most of which is less than 1 mm in 

long dimension, makes up 42 percent of the organic 

remains. The next dominant group, the larger Foraminifera, are mainly Fabiana, Biplanispira, and Pellatispira. Together these forms make up as much as 

50 percent of the rock. In some places in the slides 

studied, this detrital material is closely packed, whereas in other areas organic detritus floats in a dark mud 

matrix. In sample Hj 1-6 this mud has largely recrystallized into cloudy calcite. Framework organisms like encrusting algae are lacking except as fine 

detritus. The abundance of algal detritus suggests a 

reef or near-reef source for much of the detritus. On 

negative evidence it might be said that these limestones are not from the reef-wall or lagoon facies. 

Texturally and in organic content they most closely 

resemble limestones from the forereef facies. 

Limestone8 from the forereef facies (pl. SA, B, D). 

-Limestones from this facies are foraminiferal-algal 

coquinites, microcoquinites, and paracoquinites. A 

typical coquinite is dominated by broken tests of 

larger discoidal Foraminifera, some of which are as 

much as 6 mm in diameter (sample Ek 7-2). Subrounded fragments of fine-grained pre-extant limestones are present. However, the algal and foraminiferal debris is striking in its angularity. These 

organic remains are in a porous matrix of crystalline 

calcite. In a finer grained microcoquinite there is 

little fine-grained matrix; the rock is a tightly packed 

mass of angular and well-sorted algal-foraminiferal 

debris (sample Ek 7-3, pl. SB). A trace of wellpreserved globigerinid tests and some tests of smaller 

discoidal Foraminifera distinguish this type from the 

coarser coquinite. A limestone fragment from the 

Bolanos Pyroclastic Member of the Umatac Formation shows the effect of intense water-wearing of its 

components, mainly tests of Biplanispira (sample Ii 

8-1, pl. SD). These normally papillate tests have 

been worn smooth and deposited with w·ell-rounded 

pebbles of finer grained limestone; they show a high degree of orientation. The abundance of larger Foraminifera, the tendency to preferred orientation and 

sorting, and the evident strong influence of transport 

and wear indicate a site of deposition on a forereef 

slope. The globigerinid tests in the microcoquinite 

suggest deeper water conditions than those which 

obtained during the deposition of the coquinite. 
PETROLOGY OF THE LIMESTONES OF GUAM D23 

Limestones jr01n the basin facies (pl. BC) .-These 

limestones are fine-grained globigerinid-bearing mudstones. The exceptionally well-preserved fragile 

Foraminifera tests are set in a matrix of fine mud 

that shows no signs of recrystallization and appears 

dark brown in transmitted light and chalky white in 

reflected light. Volcanic material is present as angular fragments of crystals of magnetite, plagioclase, 

and ferromHgnesian minerals. Some of the globigerinid tests are filled with fine matrix; others are 

empty or filled with silica. The fine-grained mud 

matrix, the well-preserved tests, and the angular volcanic crystals points to a deposition in a quiet water 

far from the reach of reef detritus. 

ENVIRONMENTAL RECONSTRUCTION 

The presence, as fragments and bedded limestones, 

of a complete array of limestones from several facies 

within the Alutom Formation shows that during 

Eocene (b) time a widespread and highly evolved 

reef complex probably existed. Included volcanic material in the form of well-rounded gravel and single 

crystals indicates that a volcanic island existed and 

was subject to erosion, at least temporarily, during 

late Eocene time and that some volcanic activity may 

have produced ash falls. 

The few remaining beds of limestone of the forereef 

and basin facies near Santa Rita are the relics of a 

larger reef destroyed by post-Eocene (b) volcanism ; 

much of this former reef complex is now incorporated 

as limestone fragments in beds of volcanic. breccias 

over central Guam. The structural geology of centra] Guam (Tracey and others, 1963) and the northeastward dip of the forereef talus and basin limestone 

remnants suggests that the Eocene (b) reef complex 

was mostly west of the present site of central Guam. 

MAHLAC MEMBER OF THE ALUTOM FORMATION 

The Mahlac Member is made up of buff, friable to 

chalky, fine-grained, argillaceous mudstone that fractures both conchoidally and around conspicuous sandsize tests of Foraminifera. Because of its fossil content and the lithologic similarity to planktonic mudstones from the J anum Formation and the Maemong 

Limestone l\1ember of the Umatac Formation, it is 

placed in the basin facies. The small Foraminifera 

from the l\1ahlac l\1ember indicate that it is of Oligocene (c) age, younger than the limestone of Eocene 

(b) age near Santa Rita. 

Thin sections of this member appear dark in transmitted light owing to the fine-grained calcareous clay 

matrix. Floating or loosely packed in irregular 

patches in this mn.trix are well-preserved tests of 

Foraminifera, largely globigerinids, that are as much 

as 0.5 mm in diameter. Some of these tests are filled 

with opaline silica; others are empty. These organic 

remains account for perhaps 1 percent of the rock. 

The volcanic material, greater than silt size, is dominated by clear euhedral tabular crystals of plagioclase 

that are as much as 0.25 mm in length. Black opaque 

minerals and several species of ferromagnesian minerals are present in various stages of weathering; most 

of this material is angular. 

l\1echanical analysis of an insoluble residue, which 

constituted 42 percent of the sample by weight, showed 

1.8, 42.1, and 56.1 percent in the sand-, silt-, and claysize grades, respectively. Thus the Mahlac l\1ember 

may be considered to be more nearly an ·argillaceous 

limestone than a calcareous shale. 

M. R. Todd (written communication, 1955) interpreted the Mahlac l\1ember of the Alutom Formation 

as being a moderately deep marine deposit, probably 

around 100 fathoms, as suggested by the composition · 

of the foraminiferal fauna. There is a preponderance 

of planktonic specimens over benthonic specimens, and 

an abundance of species in the Lagenidae, Ellipsoidinidae, Cassidulinidae, and Chilostomellidae. 

The presence of angular fine-grained-all less than 

0.25 mm in length-volcanic contamination in the form 

of single crystals suggests ash-fall contributions rather 

than subaerial erosion of volcanic terrane. 

MAEMONG LIMESTONE MEMBER OF THE UMATAC 

FORMATION 

The l\1aemong Limestone l\1ember is exposed in two 

distinct geologic situations in central and southern 

Guam. In the Fena-l\1apao area, massive residual 

outliers of this member unconformably overlie the Alutom Formation. Along the west side of the Mount 

Bolanos-Mount Sasalaguan ridge, eastward-dipping 

limestones of this member, interbedded with the Facpi 

Volcanic l\1ember, crop out in stream beds. The erosional outliers in the Fena-Mapao area are made up 

of white to pink-white hard massive, partly recrystallized limestone in which coral heads in growth position are coated with encrusting algae and Foraminifera. These heads are packed in particulate reef detritus; volcanic contamination is lackin. This sections 

of these shallow-water limestones reveal Halimedarich paracoquini te of the lagoon facies. Limestone 

fragments from the overlying Bolanos Pyroclastic 

Member in this area are shown in thin section to be 

miliolid-rich microparacoquinite of the lagoon facies. 

One outcrop of the basal l\1aemong Limestone Member 

in the Fena-Mapao area is well-bedded globigerinid 

mudstones of the basin facies. The particulate lime-
D24 GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS 

stones within the Facpi Volcanic Member show a wide 

range of structures and textures indicative of forereef deposits. In the Geus River section, volcanically 

contaminated coral breccias and foraminiferal-algal 

breccias are interbedded with foraminiferal-algal coquinites and microcoquinites. Elsewhere in the Facpi 

Volcanic Member, microbreccias contain~ng abundant 

tests of Foraminifera and algal debris ar~ interbedded 

with mudstones carrying globigerinid Foraminifera 

only. In general the decrease in grain ·size is correlated with decrease in the thickness of bedding. 

ROCK DESCRIPTIONS 

Limestones of the reef-wall facies.-Fossil remains 

make up more than half the limestone in this facies. 

Coral, encrusting algae, and incrusting .Foraminifera 

form the rock framework. The coral inv~riably shows 

some incrustation by algae or Foraminifera, or both 

(pl. lOA). These incrustations are not limited to 

coatings on the framework organisms but wind through 

the particulate matrix. In many places; the laminae 

show bulges that enclose detritus; fragments of incrusting algae, segments of Halimeda, and tests of 

Foraminifera are loosely packed in between the coralla. 

This fossil debris is packed in a carbonate mud matrix 

that fills in almost all the interstices between particulate elements and much of the origin~} void spaces 

within the incrustate organisms. Th~ interseptal 

spaces in the originally porous coral are~ choked with 

dark fine-grained carbonate. Some of th¢ larger original voids are only partly filled with :rpud and are 

coated with a later deposit of clear grapular calcite. 

Much of the mud matrix has recrystallized, starting 

from the surface of fossil material and proceeding inward, into a mosaic of medium-grained ~nhedral calcite (pl. 100). In addition, much or~ginally void 

space has been filled by clear, coarse-gr,itined calcite, 

probably directly precipitated. The present porosity 

is approximately 10 percent, it is almost all secondary 

and due to solution. 

Limestones of the lagoon facies.-Two types of limestone are present: Halimeda-rich paracoquinite (pl. 

lOB) and miliolid-rich microparacoquinite (pl. lOD). 

The Halimeda-bearing limestone was coHected a few 

feet from the reef-wall limestone just de~cribed. The 

miliolid limestone is from a boulder in the Bolanos 

Pyroclastic Member in the Fena-Mapao ·~rea. In both 

limestones the size of the fossil remains determines the 

texture. The Halimeda segments rang~ from 1 to 3 

mm in length and are associated with .·fragments of 

both articulate and encrusting algae as much as 2 mm 

in diameter. The thin-walled tests of miliolids and 

other sma11 benthonic types range from less than 0.25 

mm to, rarely, 1 mm in size; they average approximately 0.5 mm in maximum dimension. The large 

majority of these delicate tests are well preserved. Also 

present in this limestone are dark rodlike segments 

contributed by articulate coralline algae. 

The matrix of both rock types is a recrystallized 

mud. Fossil elements in the Halimeda limestone are 

invested in coatings of fine dusty calcite having 

crystals oriented perpendicular to the fossil surface, 

which grades into clear subhedral mosaic toward the 

·centers of interstices. Porosity is residual after these 

incomplete void fillings. The miliolid tests are filled 

with fine-grained calcite identical to the rest of the 

matrix indicating that the test fillings were originally 

fine mud rather than a later precipitate. The H alimeda segments probably represent a local accumulation 

in close association with the reef wall. The miliolid 

limestone is interpreted as being a lagoonal limestone 

by virtue of its high miliolid-peneroplid content, which 

is characteristic of lagoonal deposits (Henson, 1950). 

The well-preserved tests suggest a life assemblage. 

Limestones of the fore'l'eef facies.-Limestones of 

this facies are coral breccia (pl. llA), foraminiferalbreccia (pl. liB), and foraminiferal-algal microcoquinite (pl. 110). The breccias are poorly sorted and 

contain tests of larger discoidal Foraminifera as much 

as 13 mm in diameter, as well as fragmental fossils 

approximately 0.1 mm in diameter. These broken, 

chipped, and worn tests are randomly oriented, along 

with better preserved smaller discoidal tests and an 

accumulation of fragmental encrusting and articulate 

algae, in a dense fine-grained mud matrix. Rare subrounded fragments of coral are present. A trace of 

small, delicate planktonic tests of globigerinid Foraminifera is in the fine matrix. The fossils have evidently undergone considerable transport before deposition in an environment also capable of retaining the 

fine carbonate mud that forms the present matrix. 

The microcoquinites, by contrast, are better sorted and 

are characterized by an abundance of angular fragments of coral1ine algae, which dominate the fossil 

remains, evenly interspersed with tests of both thickwalled benthonic and thin-walled planktonic Foraminifera; the benthonic types are dominant. Volcanic contamination is present as sand-size fragments 

of fine-grained rocks and as single crystals of angular 

plagioclase. 

Limestones of the forereef transitional facies.-

Foraminiferal microbreccia and foraminiferal mudstone make up the limestone suite of this facies. Thin 

sections (pl. llD) of the micro breccia show lenses and 

strings of packed tests of discoidal benthonic Foraminifera and algal debris alternating with fine-
PETROLOGY OF THE LIMESTONES OF GUAM D25 

grained carbonate mud in which scattered tests of globigerinid Foraminifera float. The benthonic fossil 

elements are commonly fragmental; the planktonic 

contributions are well preserved despite the extreme 

fragility of the tests. Fragments of the benthonic 

material rarely exceed 1 mm in long dimension; most 

are less than 0.5 mm. Lenses of this detritus are as 

narrow as 2 mm. The fine matrix is dense; pore space 

as seen in thin section is less than 1 percent. The 

lenses of benthonic debris probably represent detritus 

swept downslope into a normally quiet water environment in which fine mud and planktonic tests were 

slowly accumulating. 'Vhere these two rock types 

are interbedded, the outcrop is assigned to the forereef 

transition facies. 'Vhere thick sequences of globigerinid-globorotalid mudstones occur, the outcrop is assigned to the basin facies. 

Limestones of the basin facies.-Limestones of this 

facies contain tests only of planktonic Foraminifera 

(pl. 11E) scattered through a carbonate mud matrix. 

Both globigerinids and globorotalids are represented. 

The tests are crudely concentrated in vague bands. 

The orientation of the slightly discoidal tests of the 

globorotnJids is random. A few large globigerinid 

tests, have diameters of 1.5 mm, but the average test 

size is close to 0.5 mm, and some individuals are as 

small as 0.1 mm. The tests are perfectly preserved 

and, in polarized light, show pseudo uniaxial crosses 

indicative of unrecrystallized shell material. The tests 

are largely free of any infillings of the fine-grained 

matrix. Some tests, however, have been filled with 

clear silica in the form of bet.a-cristobalite which displays undulatory extinction in polarized light. (See 

p. 42.) This limestone is directly overlain by submarine lava beds. 

The presence of Globo1·otalia tests in places (sample 

Df 9-1a) makes possible an interpretation of the depth 

nt which the limestone containing them was deposited. 

According to Todd (written communication, 1956), 

the Glob01·otaUa tests are probably G. menardii and 

G. t~t?nida. 'Vhile clear-cut specific identifications were 

not made, the possibility is strong that these tests are 

G. 'lne'IW?Ylii and G. t~tmida, inasmuch as both species 

are abundant in beds of the Donni Sandstone ~iember 

of ~Eocene (e) age on Saipan, 100 miles north of 

Guam (Todd, 1957). Study of Recent Foraminifera 

in the Bikini area by Cushman, Todd, and Post (1954) 

showed that only two specimens of G. menardii and 

none of G. t'lt?nida were found in 59 lagoon samples. 

Globm·otalia, however, is abundant in the deep outer 

slopes around Bikini at depths of several thousands 

of feet. These rocks, therefore, were probably deposited in water deeper than 1,000 feet. 

ENVIRONMENTAL RECONSTRUCTION 

The conditions under which the various facies of 

the ~Iaemong Limestone Member were deposited were 

mainly controlled by uplift of the preexisting deformed Alutom Formation and the growth of a submarine volcano in early ~Eocene time to the southwest of the older volcanic ridge. The structure of 

south Guam (Tracey, and others, 1963) indicates that 

the Umatac Formation, as presently exposed, is the 

northeastern sector of a remnant of the Miocene volcano. This formation was deposited, from a southwestern source, against the Alutom Formation in central Guam. The oldest limestones of Maemong age 

are the deep-water, globorotalid-bearing mudstones. 

The presence of these rocks in both the south Guam 

section and in the Fena-~iapao area suggests that the 

south and central parts of Guam were deeply submerged in early Miocene time. Uplift followed and 

massive reef-wall growth was initiated on the de-. 

formed Alutom Formation in central Guam, which by 

then formed a shallow submarine ridge. The thick 

section of forereef limestones in the Geus River exposure indicates a lull in volcanic activity, during 

which a reef, probably of the fringing variety, formed 

on the flanks of the cone. This reef fed debris into a 

forereef basin between the present site of Umatac and 

the Fena-Mapao area. Deeper water planktonic limestones interbedded with forereef transitional limestones in the caldera wall section also accumulated in 

this basin. These forereef deposits were later covered 

by lava flows. A later explosive phase of volcanism 

gave rise to the Bolanos Pyroclastic ~iember and probably was followed by caldera collapse that destroyed 

the shallow-water reef-wall facies west of the present 

forereef outcr~ps. 

Fragments of reef-wall limestone, however, are 

found in the Bolanos Pyroclastic ~iember along the 

Mount Bolanos-Mount Sasalaguan ridge line. This occurrence confirms the presence of a shallow-water reefwall updip from the forereef limestones found in the 

Facpi Volcanio Member. Thus, during early Miocene time there existed two' independent centers of 

reef-wall formation, one on a submarine ridge and 

one on a volcanic cone, separated by a basin receiving 

reef debris and accumulating planktonic tests of Foraminifera to form forereef and basin limestones. 

PRESENT-D.A. Y .AN .A. LOGUE 

The early ~i)ocene paleogeology of south Guam may 

be compared to the present-day configuration and 

geology of the island of Borabora in the Society 

Islands as described by Stark and Howland (1941). 

The geologic map of Borabora shows it to be a vol-
D26 GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS 

canic cone, greatly dissected, that rises to a height of 

2,380 feet above sea level. The center of the cone 

has collapsed, the crater forming part of the present 

lagoon. The island and lagoon are almost completely 

surrounded by a roughly circular barrier reef 6 to 8 

miles in diameter. Stark and Howland (1941) describe the growth of a lava cone approximately 5 miles 

in diameter in late Pliocene or early Pleistocene time, 

followed by caldera collapse, erosion, subsidence, and 

reef growth. The present conditions at Borabora 

probably resemble conditions that existed during the 

early Miocene lull in volcanism in the area around 

and to the southwest of the present caldera-wall 

section on south Guam. A second growth cycle at 

Borabora, including large-scale volcanism, would result in the destruction of the barrier reef and its reefwall and lagoonal limestones, and in the burial of the 

forereef limestones under lava flows and conglomerates. Collapse, uplift, and erosion would produce 

a caldera-wall section such as the one on Guam. Borabora, however, lacks a nearby ridge of older volcanic 

rocks such as the submarine ridge of deformed Alutom 

Formation on Guam. 

BONYA LIMESTONE 

The Bonya Limestone crops out extensively in the 

low areas of Fena basin, the Mapao-Maagas River 

area, and along the Talofofo, U gum, and Togcha 

River scarps. Smaller scattered outcrops of the formation are exposed on terraces and in coastal' reentrants from Lujuna Point to Anao Point. Between 

these two widely separated areas no Bonya Limestone 

has been found. In the Fena basin-Ugum-Togcha 

area the formation is exposed principally as a series 

. of eroded outliers and, to a smaller extent, interstratified between other formations. In this area the Bonya 

Limestone is generally unconformable on the Bolanos 

Pyroclastic Member of the Umatac Formation; the 

Bonya Limestone locally appears to grade down into 

the Bolanos Pyroclastic Member of the Umatac Formation. The basal part of the Bonya Limestone carries pebbles and cobbles of the l\1aemong Limestone 

Member eroded from the Bolanos Pyroclastic Member. 

One outcrop of the Bonya Limestone in Fena basin 

directly overlies the Maemong Limestone Member of 

the Umatac Formation. Near the east coast of south 

Guam the Barrigada Limestone conformably overlies 

the Bonya Limestone. The thickness of the Bonya 

Limestone in outcrop in south Guam ranges from less 

than 40 feet to 120 feet. In north Guam the Bonya 

Limestone overlies, with angular unconformity, the 

Alutom Formation. The J anum Formation generally 

overlies the Bonya Limestone in this area. The Janum 

Formation carries pebbles and cobbles of the Bonya 

Limestone in the same fashion as the Bonya carries 

material from the Maemong Limestone Member. 

Where the Janum Formation is missing, the Mariana 

Limestone lies unconformably on the Bonya Limestone. 

The bulk of the Bonya Limestone is well bedded. In 

south Guam, porous to dense friable to indurated foraminiferal-algal coquinites and paracoquinites (and 

their micro-varieties) predominate. Dips, probably 

initial, as much as 15° have been observed. Significant concentrations of coral, mollusks, and coarse algal debris are only locally present as massive breccia. 

Volcanic contamination, in the form of rounded grains 

of clay, and interlayered fine-grained crystalline rocks, 

is widespread. The limestones range from buff white 

and white to brown, gray, and gray black in color. 

The gray and black shades are caused by replacement 

of the limestone by manganese ·oxides. This replaced 

limestone is mottled in shades of gray and black. In 

north Guam the Bonya Limestone is uniformly a 

dense, massively bedded pure foraminiferal mudstone. 

The amount of algal and coral detritus ranges from 

a trace to none; the main bulk of the rock is a finegrained lithified mud in which the fossil remains float. 

The overlying J anum Formation, however, carries 

cobbles of manganiferous foraminiferal limestone 

strikingly similar to samples of Bonya Limestone from 

the Togcha River area. Apart from these cobbles, 

the limestones from north Guam are lithologically_ 

unlike those from the Bonya Limestone of south 

Guam. 

ROCK DESCRIPTIONS 

Limestones from the forereef facies.-These deposits 

include moderately well sorted to well-sorted foraminiferal-algal coquinites, paracoquinites, and their microequivalents. Worn tests of Foraminifera are predominant, mostly small thick-walled types like amphisteginids. Fragments of both incrusting and articulate algae are abundant. The size range of the 

originally incrustate algal material approximates that 

of the other primary particulate debris in the slides 

studied. Fragments of incrusting Foraminifera, also 

sorted in accordance with the other debris, are conspicuous contributors to some samples. This sorting 

relationship among material of different original size 

and shape indicates considerable sorting of heterogeneous elements before their final incorporation into 

the sediment. Fragments of echinoids, largely spines, 

also occur in unusual abundance, locally up to 5 percent. These fragments show optically continuous overgrowths of calcite. The debris is locally closely packed 

in a mud matrix. Recrystallization of the original 
PETROLOGY OF THE LIMESTONES OF GUAM D27 

matrix is seen in all stages including clear mosaics of 

calcite, from which all traces of the dark mud have 

disappeared. Packing appears to be closer than 

normal ; many of the fossil fragments show sutured 

contacts against adjacent grains. This contact suturing is more evident in finer grained deposits. In 

many samples the long algal rods and discoidal Foraminifera show distinct alinement parallel to the bedding. 

Breccias are also included in this facies. These are 

typically poorly sorted and carry a wide size range 

of fossil debris, mainly coral, algae, and Foraminifera, randomly packed in a mud matrix. All the fossil 

material is fragmental, except wear-resistant tests of 

thick-walled Foraminifera and some thin-walled globigerinid tests. Both the flat laminar and knobby 

types of encrusting algae are present; continuous laminae of encrusting algae are rare to lacking. Many 

of the lar~er fragments, such as stems of coral, have 

coatings of encrusting Foraminifera, which grew prior 

to the breaking and transporting of the host. Interseptal spaces of c01·alla are mud filled, and the original skeletons have recrystallized to mosaics of calcite. 

The trace of well-preserved tests of globigerinid Foraminifera and angular fragments of Oycloclypeus is 

an off-reef, deeper water contribution. Thus, these 

breccias were probably deposited in front of a reef 

rather than within it. A large part of the mud matrix 

has recrystallized, imparting a grumose texture to the 

rock. Solution cavities represent secondary porosity; 

the primary porosity of these limestones was low. 

The walls of many secondary voids are lined with 

clear calcite deposited from solution. Rounded pebbles of pre-Bonya limestone are present (pl. 13B). 

Limestones f'rom the forereef transitional facies.-

These limestones are foraminiferal-algal microparacoquinites (pl. 120). Planktonic tests are evenly scattered throughout, as are the other fossil constituents, 

rounded fragments of articulate and encrusting algae, 

'vorn tests of small thick-walled Foraminifera, fragments of encrusting Foraminifera, and echinoidal 

debris. Although these organic remains had, originally, a wide range of inherent growth sizes and 

shapes, their fragments are present now in approximately the same size grades. This sorted debris loosely 

packed in a mud matrix. In places (sample, Jj 9-1) 

the dark mud has recrystallized to finely granular 

clear calcite. Various fossil types show different contact relations to the matrix. The globigerinid tests 

show pseudouniaxial crosses and both internal and 

external overgrowths in the cell walls ; the crosses pass 

undisturbed through all three layers. These optically 

oriented overgrowths are at least as thick as the cell 

wall. Amphisteginids, which also show a fibrous calcite test, have similar, but relatively thinner, overgrowths. The calcareous algae lack these recrystallized halos and have sharp contacts with the matrix. 

Echinoidal debris shows clear single-crystal overgrowths. The original structure of the fossil shell influences the type of recrystallization in the matrix 

adjacent to the fossils themselves. 

Limestones from the off-reef deep-water faoies.-

Representatives from this facies in south Guam are 

foraminiferal-algal' paracoquinites (pl. 12D). Flat 

tests of large Foraminifera are characteristic. Operculinid and amphisteginid tests are common, and miliolids and globigerinids are rare. Fragments of coralline algae and encrusting Foraminifera are ubiquitous. 

This heterogeneous mixture is in a matrix that varies 

from one thin section to another, fine-grained sand 

and mud dominating. Recrystallization has produced 

grumose texture in most specimens. The organic 

debris locally shows some degree of preferred orientation. Volcanic material is present as fine-grained 

angular detrital quartz and disseminated clay. 

The off-reef facies in north Guam is represented by 

dense, massively bedded foraminiferal mudstones in 

which fragments of Oycloclypeus tests are common. 

In a typical specimen, these tests, and amphisteginid 

and operculinid Foraminifera, rare Lepidocyclina 

tests, and a trace of fragmental pelecypod shells make 

up the fauna and show a tendency toward a preferred 

orientation parallel to the bedding (pl. 13A) . Those 

fossils and common nodules of encrusting Foriminifera, probably Gypsina, are in a dense mud matrix. 

Thin beds of this matrix lack fossils. In some specimens, fragments of recrystallized and reversed coral 

are seen. Some thin sections show irregular patches 

of matrix that are, in themselves, lithologically similar to limestones from the off-reef facies of south 

Guam. These patches show irregular blending contacts-or schlieren-with the surrounding mud. This 

type of contact suggests deposition of poorly lithified 

or semisolid sediment bodies into the mud host. In 

the specimen figured (pl. 13A) , the Oycloclypeus tests 

commonly show breakage but only slight separation 

of the resultant fragments; fracturing after burial is 

suggested. Parts of the mud matrix also show 

smeared structures and inclusions of darker matrix 

that appear to have been originally a matrix. These 

relationships indicate periodic shifting of semilithified 

sediments. 

A cobble (sample Ut 6-1) from the overlying Janum 

Formation is lithologically identical with the off-reef 

facies (sample Jj 9-1) from south Guam, except for 

some minor replacement by manganese oxides in the 
D28 GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS 

cobble. The westernmost and highest part of the 

Bonya Limestone mapped in the window at Lujuna 

Point is very dolomitic. I-Iere the Bonya directly underlies nondolomitic beds of the J anum Formation. 

The texture of the dolomitic limestone is virtually 

identical with that described from the flanks of Mount 

Santa Rosa in the Alifan Limestone. These relations 

suggest that the dolomitization took place before the 

deposition of the J anum Formation. 

ENVIRONMENTAL RECONSTRUCTION 

During Bonya time the Fena basin-Ugum RiverTogcha River area must have been an embayment 

bordered by exposed volcanic rocks. Local concentrations of coral-algal breccia, such as that found on the 

south bank of the Talofofo River just west of its confluence with the Ugum River (locality Ih 5), indicate 

that small reefs flourished in the shallow peripheral 

waters of the embayment. Volcanic contamination of 

these limestones and the associated limestones of the 

forereef facies shows subaerial erosion of the volcanic 

rocks. These reefs fed detritus into the embayment in 

which the forereef facies was being deposited. The 

sorting displayed by many samples from the Bonya 

limestone reflects a high degree of current action and 

consequent movement and winnowing of material. In 

north Guam, reefs appear to have been subordinate 

or lacking. The situation is interpreted as dominantly 

shoal to deep-water deposition on a bottom of sufficient 

relief to allow movement of coarse fossil debris. The 

abundant mud that now makes up the matrix in this 

area suggests deposition in quiet water of sufficient 

depth to allow the growth of (Jycloclypeus in abundance. · 

ALIF AN LIMESTONE 

The Alifan Limestone crops out ·on the Mount Alifan-Mount Lamlam ridge, where it forms a cap on 

older volcanics, in scattered small exposures in central 

Guam, and in the vicinity of Mount Santa Rosa in 

north Guam. This formation is best exposed in the 

Ali fan quarry. There the basal part of the Ali fan 

Limestone is a dense pink-red fine-grained mudstone 

recrystallized to a sedimentary marble that shows 

luster mottling on a fresh fracture (pl. 4A, B). It is 

pierced by numerous crooked and nearly vertical tubes, 

an inch or so in diameter and 1 to 3 feet long, produced by boring mollusks or worms. The upper part 

of the limestone is crudely bedded, white to buff, 

porous, and fossiliferous. A typical specimen is made 

up of randomly oriented pencil-like sticks of Porites 

and Acropora as much as 3 inches long and 1 inch in 

diameter, casts of articulate bivalves as much as 2 

inches across, and finer molluscan debris floating in a 

matrix of sand- and granule-size skeletal debris and 

fine-grained carbonate mud. Original aragonite shell 

material has been partly removed by solution or recrystallized to calcite. Casts and molds of coral predominate; mollusks are subordinate. The rock may be 

classified as coral-molluscan coquinoid limestone. The 

section exposed in the Alifan quarry is part of the 

lagoon facies. 

The Alifan Limestone near Yona is a dense white 

foraminiferal microparacoquinite made up almost entirely of tests of Rotalia in a matrix of fine-grained 

carbonate and that has largely recrystallized to clear 

crystalline calcite. On a fractured surface the tests 

appear chalk white against the light-tan calcite matrix. 

The tests are normally packed and range from 1 to 2 

mm in diameter. Owing to preferential solution of 

the tests, weathered surfaces of the rock have a honeycombed appearance and a raised matrix. 

In the vicinity of Mount Santa Rosa, the basal part 

of the Alifan Limestone, which directly overlies the 

Alutom Formation, is a mottle pink, yellow and orange 

dense massive Rotalia-rich paracoquinite (pl. 4B) and 

coral breccia containing some small included masses 

of algal incrustate reef-wall rock (pl. 14A). Within 

a few feet of the limestone-volcanic contact, this colored limestone gtades up into white limestone that 

lacks volcanic contamination. In this area the Alifan 

Limestone is locally dolomitic. Limestone of the reefwall facies forms only a small part of the Alifan in 

this vicinity. Rotalia-rich microparacoquinites are 

here classed as off-reef shallow-water limestone. 

ROCK DESCRIPTIONS 

Limestones from the reef-wall facies.-This facies 

(sample Tt 7-3, pl. 14A) is characterized by thick intergrown laminae of encrusting algae and Foraminifera. The limestone is directly comparable to those 

from the reef-wall facies in the Alutom Formation 

(pl. 7 A), the Maemong Limestone Member of the 

Umatac Formation (pl. lOA), and the Mariana Limestone (pl. 18B). The encrusting Foraminifera are the 

thick-walled punctate type and the thin-walled type. 

These encrusting elements coat coral and bind finer 

grained debris, which is composed largely of foraminiferal and algal fragments, within a mud matrix. 

Fragments of a nodular form of encrusting algae are 

also present. The fine-grained matrix contains disseminated clay of red-brown color that sets it apart 

from the pure white algal and foraminiferal calcite. 

Branching and pinching fractures that break and 

separate originally continuous algal laminae are 

numerous. These show separations of as much as several millimeters in the plane of the thin section. The 
PETROLOGY OF THE LIMESTONES OF GUAM D29 

fractures are filled with fine-sand-size foraminiferalalgal debris. vValls of the separate algal masses are 

coated with a thin film of red clay. The fracturing 

appears to have taken place where there was an available source of soft sediment to fill the open fractures. 

In characteristic fashion, parts of' the mud matrix 

have crystallized to clear mosaic calcite. Coral remains are now mosaic calcite in which ghost skeletal 

structures are seen. 

Coral-rich breccias are also placed in this facies. 

Thin sections of this rock show encrusting elements 

only as detritus. A few slides (sample Ts 16-1), however, show extensive encrustations of Foraminifera 

that resemble incrustate reef-wall limestone. Thus, the 

breccia is closely related to the reef-wall rock. The 

breccia is poorly sorted; it contains coral fragments as 

much as 1 inch across in association with fine-sandsize foraminiferal and algal debris, tests of Rotalia 

from 1 to 3 mm in diameter, and fragments of encrusting algae-including the nodular types-as much 

as one half inch long. These fossils are randomly 

packed in a fine-sand-silt matrix of triturated fossil . 

debris containing local patches of mud. Primary porosity is low, but some pores were produced during a 

period of subaerial solution, which was followed by 

deposition of a thin layer of clear carbonate crystals 

on the void walls. The fine-grained parts of the 

matrix carry some finely disseminated reddish-brown 

clay contamination. 

Lirnestones from the lagoon facies.-The three lithologic types included in this group occur in close association and are probably the result of local variations 

in conditions of deposition; they are described below. 

Cor'al-molluscan coquinoid limestone makes up the 

main mass of the Alifan Limestone, as represented 

in the Alifan quarry. In thin section (pl. 140) the 

characteristic poor sorting of this rock is obvious; 

pelecypod and gastropod shells and fragments of 

coral' are loosely packed, in a finer grained matrix. 

There are three distinct size grades present: granulesize and larger fossil fragments, sand-size debris derived from shells, and carbonate mud. The relative 

proportions of these size grades vary from sample to 

sample. The degree of preservation of the fossils also 

varies from sample to sample, except for the coral 

remains that are present only as ghosts. Gastropod 

shells shmv up mainly as calcite mosaics. Pelecypod 

shells are represented by fragments that retain their 

original fibrous structure and by recrystallized mosaic calcite that grade outward into the fine-grained 

matrix. 

Foraminiferal-algal microparacoquinites are limestones composed of foraminiferal and algal debris 

randomly oriented and packed in a partly recrystallized mud matrix that is cloudy in patches. Much of 

the fossil debris is between 0.1 and 0.5 mm in diameter. 

Scattered throughout this even-grained material are 

whole tests of Rotalia as much as 2 mm in diameter 

and occasional fragments of encrusting algae as much 

as 5 mm in diameter. The finer-grained fossil debris 

is a mixture of encrusting algae, articulate algae, encrusting Foraminifera, and mollusk shells. Also present are tests of small Foraminifera, among which are 

amphisteginids, miliolids, and uniserial, biserial, and 

coiled forms. Subaerial solution has produced a fine 

irregular secondary porosity. Primary pore space in 

the form of open cells of Foraminifera tests remains. 

Angular fragments of quartz, and rare euhedra (pl. 

3D), are present in the Alifan Limestone (Gj 8-1) 

from the Fena basin. These fragments range from 

0.1 to 0.5 mm. The quartz-calcite contacts are sharp, 

and there is no evidence of replacement of the quartz 

by calcite. Hathaway and Carroll (see p. 42) describe these euhedra as squat doubly terminated crystals similar to those described from a volcanic ash in 

Java. 

Recrystallized mudstones in thin section reveal the 

intricate granoblastic texture formed during the recrystallization of what probably was an original limemud. In plain light (pl. 4A) the inter grown crystal 

boundaries are not readily apparent; however, under 

polarized light (pl. 4B) individual crystals are seen. 

No recognizable coarse fossil debris is present except 

a few fragments of pelecypod shells in one thin section. Fine, hairlike streaks of reddish-brown clay 

suggestive of bedding planes extend through parts of 

the matrix. Irregular patches of coarser grained 

crystals are separated from the finer grained matrix 

by sharp dividing lines. These coarser grained areas 

may represent filled burrowing tubes now distinct because of an original difference in grain size between 

the burrow filling and .the original matrix. In one 

thin section an offset of one such relict tube along a 

warped surface suggests prelithification slumping of 

soft sediment. The type of recrystallization occurring 

in this rock is unique to the Alifan Limestone. The 

original mud seems to be in the process of being reorganized. Individual crystals are very intricately interlocked and intergrown, quite unlike the normal 

mosaic calcite in most of the Guam limestones. Each 

crystal retains numerous inclusions of original finegrained material and the present size of the crystals 

appears to be dependent on the original grain size. 

Limestones from the off-reef shallow-water facies.-

The dominant fossil element in these microparacoquinites is Rotalia, tests of which make up from 35 
D30 GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS 

to 60 percent of individual samples. The tests range 

from 1 to 2 mm in diameter ; rare specimens are as 

much as 4 mm in diameter. Similar-size debris of encrusting algae and Foraminifera and articulate algae 

make up 5 to 10 percent of the limestone. The degree 

of sorting shown by the Rotalia tests may be biologic; 

however, the algal debris shmYs the same size characteristics; this similarity suggests some true hydraulic 

sorting of both the Foraminifera and the algae before 

deposition. These fossil elements are closely packed 

in a fine-sand and mud matrix. The fact that the 

Foraminifera tests and the algae show some minor 

interpenetration. at some contact points suggests possible compaction. Perhaps the fossil elements were 

originally floating in the mud matrix. The matrix 

contains some fine-grained recognizable algal debris. 

In some samples (Ts 16-11) (pl. 14B), this matrix has 

undergone a partial recrystallization to clear calcite. 

The recrystallization begins from the surfaces of the 

fossil debris and proceeds into the mud-filled interstices, in the form of acicular crystals of clear calcite 

that grow perpendicular to the fossil walls. These 

peripheral growths gradually coalesce to form a clear 

interstitial cement. The lower, central part of plate 

14B shows such a completely recrystallized area. Most 

of the thin section retains the original mud matrix. 

Because certain areas have been affected by solution, 

almost all the mud fill has been removed and the resultant void lined with clear calcite. Fossils surrounding such voids show three encrusting layers: ( 1) a 

coating of secondary calcite crystals, (2) a layer of 

residual dark mud, and ( 3) a clear primary deposit 

of calcite. In some thin sections all traces of the 

original mud has disappeared; the rock shows a grumose texture. 

ENVIRONMENTAL RECONSTRUCTION 

The Alifan Limestone accumulated under a variety 

of conditions. The deposits around Mount Santa Rosa 

include incrustate reef-wall rock and coral breccia 

that are argillaceous at the base. These relations suggest that during early Alifan time Mount Santa Rosa 

stood as an island around which grew a shallow-water 

reef complex. This island contributed clays to the 

reef and associated sediments. The transition upward 

to pure limestones in north and south Guam indicates 

a cessation of argillaceous contamination, probably 

due to a transgressive sea that covered Mount Santa 

Rosa and the south Guam volcanics and eliminated 

them as sources of sediments. In southern and central Guam no reef-wall limestones have been found; 

the bulk of the Alifan Limestone accumulated under 

lagoonal conditions. The articulated pelecypods and 

abundant long sticks of delicate branching coral indicate inplace accumulation. These deposits closely resemble modern deposits in Cocos Lagoon, where similar delicate branching corals contribute large amounts 

of similar sticks to a shallow sand bottom on which 

mollusks are common. Much of this unsorted coralliferous material is accumulating in protected lagoonal 

areas in water less than 5 feet deep. The local abundance of Rotalia in some rocks from the Alifan Limestone suggests time of adverse conditions for normal 

reef and lagoonal communities. According to Hedberg (1934, p. 475), Rotalia beccarii has a high tolerance for fresh water; where it occurs alone, brackish 

water is indicated. Thus the local accumulation of 

Rotalia tests may be the result of deposition under 

conditions of high fresh-water inflow very near shore, 

possibly just below the littoral zone. 

BARRIGADA LIMESTONE 

The Barrigada Limestone is well exposed on north 

Guam. It crops out in a large ring-shaped area 6 

miles in diameter; the width of the outcrop averages 

approximately 1 mile. The formation, as exposed, is 

made up of massive gray to white indurated to friable 

dense to porous foraminiferal mudstones, microcoquinites, and microparacoquinites of the off-reef deepwater facies. The degree of induration varies; in 

zones of faulting the rock is a dense yellow, finely 

crystalline limestone that forms sharp, sheetlike ridges. 

On rolling or flat terrain the limestone is commonly 

friable, possibly owing to solution by percolating 

water. 

ROCK DESCRIPTIONS 

The Barrigada Limestone, unlike the limestones of 

Eocene (b) and Miocene (e) age previously described, 

does not display a complex suite of rock types. 

Rather, it is a monotonous expanse of foraminiferal 

limestone. Variations in fossil size, type of matrix, 

and porosity constitute the only lithologic differences 

over large areas. 

The largest foraminiferal tests are associated with 

the finest matrix. Some rocks (sample Pv 10-2, pl. 

15A) are foraminiferal mudstones containing abundant large tests in a fine-grained matrix, a strongly 

bimodal size distribution. Other rocks (sample St 

8-1, pl. 150) , which are better sorted, are microparacoquinites containing small amphisteginid tests in a 

matrix that is not as fine grained as the mudstones. 

Still other rocks (sample Nr 1-7a, pl. 15B) are well 

sorted microcoquinites showing a texture close to that 

of a normal detrital sandstone. 

All the tests show strong effects of abrasion and 

fragmentation; preferred orientation is lacking even 
PETROLOGY OF THE LIMESTONES OF GUAM D31 

among the flat tests. The high content of encrusting 

Foraminifera, all in fragments, suggests considerable 

reworking before deposition in the fine-grained matrix. 

Samples Ov 7-1, 7-2, and 7-3, from depths of 94, 

404, and 543 feet, respectively, in a well drilled for 

the transformer house at Naval Communications 

Station Finaguyen, show a considerable vertical variation in lithology. Sample Ov 7__:1 is a foraminiferal 

microcoquinite similar to sample Nr 1-7a; porosity, 

mainly secondary, is low and in the form of fine irregular openings in the matrix. Sample Ov 7-2 is a 

porous breccia containing Halimeda segments, debris 

of coralline algae, and fragments of pelecypod shells; 

porosity is high, mainly secondary; voids take the 

shape of fossil fragments. Sample Ov 7-3 shows a 

fine-grained dark mud matrix in which are imbedded 

recrystallized Halimeda segments, coralline algae, and 

a few tests of large discoid Foraminifera; porosity is 

variable. One slide shows dissolution of numerous 

Halimeda segments and consequent high porosity. Another slide shows the matrix to be almost entirely fine 

grained and of low porosity. Under high magnification the r9ck shows a trace of red-brown detritus that 

closely resembles the fine-grained weathered volcanic 

detritus seen in more argillaceous rocks. In reflected 

light, patches of this coloration are seen. The material 

is very finely disseminated throughout the fine-grained 

dark matrix. 

ENvmONMENTA.L RECONSTRUCTION 

The fauna of the north plateau outcrops of the Barrigada Limestone are indicative, according to W. S. 

Cole (written communication, Mar. 26, 1953), of a 

bank (or forereef) deposit, undoubtedly on the outside 

of a reef, and a water depth of about 500 feet. The 

abundance of Oycloclypeus tests certainly precludes 

a reef or lagoonal origin for these sediments. The 

sporadic occurrence of coral and other shallow-water 

elements suggests that the limestones containing them 

were deposited on local highs on a generally deeper 

submarine bank. The worn and broken Oycloclypeus 

tests in a fine-grained matrix indicate transport of 

these tests before deposition in a quiet-water environment. The abundance of fragmented encrusting Foraminifera supports the idea of much predeposition 

transport. The general absence of reef contributions 

may be due to lack of reef-wall deposition during the 

late Miocene in the present north Guam area. 

JANUM FORMATION 

The J anum Formation crops out in seven places 

along the northeast coast of Guam between Lujuna 

Point and Anao Point. Sections exposed north and 

south of the type section at Catalina Point thin from 

a maximum of 70 feet; at Anao Point 6 feet is exposed, at Lujuna Point 4 feet. In outcrop the J anum 

Formation exposes compact to friable red-orange, pink, 

brown, yellow, and gray-white tuffaceous to relatively 

pure thin-bedded globigerinid -rich mieroparacoquinites. No scour and fill structures are seen, and beds a 

few inches thick can be traced across the outcrop. 

Rounded to subangular pebbles and cobbles of the 

Bonya Limestone are common, especially in the basal 

part of the J anum Formation. The scattered exposures may be due to lensing of the formation and 

pinching out between outcrops. At Lujuna Point a 

thin bed of the J anum pinches out between Bonya 

Limestone and Mariana Limestone in a well-exposed 

cliff. 

ROCK DESCRIPTIONS 

Three major sedimentation units are present in the 

J anum Formation at the type section. These units 

show variations in faunal content, volcanic contamination, and texture, all of which taken together indicate 

a shift upward in the section, from shallower water, 

relatively pure limestone to argillaceous, deeper water, 

basin sediments. 

The uppermost 15 feet of the type section is made 

up of medium-grained argillaceous foraminiferal microparacoquinites (pl. 160). The distinguishing characteristic of this unit is the high content of volcanic 

material, largely in the form of clay minerals weathered from single crystals and sand -size volcanic rock 

fragments. This clay content causes pink, brown, and 

orange-red coloration in reflected light and adds a 

dark-brown cast to the thin sections. The character 

of the volcanic fraction varies (see p. 37-48) . In some 

samples (Ts 5-4 and Ts 4-4a), much sand-size material is present, but in a sample taken 15 feet lower 

in the section, volcanic contamination takes the form 

of evenly disseminated silt- to fine-sand-size, angular 

fresh magnetite. Tests of globigerinids and other 

smaller Foraminifera form as much as 50 percent of 

the rock; the remainder of the carbonate fraction is 

in the form of a mud matrix. 

The middle sedimentation unit is made up of finegrained, relatively pure limestone taken from 25 to 

55 feet below the Janum-1\{ariana contact. This limestone (pl. 16A) is characterized by a high content 

(50 to 70 percent) of globigerinid and other smaller 

Foraminifera tests. These tests are as large as 0.25 

mm in diameter, but average approximately 0.1 mm 

in section; the packing is unusually dense and many 

tests have apparently been crushed in place, not in 

subsequent transport. In one sample, many of the 

unbroken tests remain empty, but in another, similar 

tests are filled with fine mud. Volcanic contamina-
D32 GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS 

tion amounts to only a few percent, largely of clay 

minerals. 

The lowermost sedimentation unit is a pure foraminiferal microparacoquinite (pl. 16B). Fossil remains (sample Ts 5-9) from this unit are of ( 1) 

globigerinids as much as 1 mm in diameter, biconvex, 

(2) discoidal amphisteginids of slightly larger diameter, and (3) a few fragments of Cycloclypeus 

tests. The amphisteginid tests show signs of wear 

and breakage. These fossils are all in a carbonate mud 

matrix that contains approximately 1 percent volcanic material, largely clay minerals. The total combined faunal content amounts to approximately 30 

percent of the rock. 

ENVmONMENTAL RECONSTRUCTION 

The three sedimentation units described from the 

Janum Formation are the products of three distinct 

depositional environments. The lowermost stratum 

at the type section contains a foraminiferal fauna suggestive of a forereef or bank environment. The 

abundant amphisteginid tests in both this stratum and 

the lowest sedimentation unit of the J anum Formation, which directly overlies it, indicate a depth of 

deposition probably between 40 and 100 fathoms (Ruth 

Todd, written communication, Aug. 22, 1952). The 

fauna shows the effects of vigorous transport. The 

basal part of the J anum at Catalina Point is a moderate-depth deposit unaffected either by volcanic activity or by the erosion of a volcanic terrane. The 

middle sedimentation unit, however, lacks all shallowwater faunal elements as well as significant volcanic 

contributions. In referring to samples from this 

middle unit, Ruth Todd (written communication, Oct. 

30, 1952) states: "The benthonic population, represented by a good variety of species but few individuals, 

is indicative of depths between 100 and 1500 fathoms, 

more likely toward the shallow end of this range." 

The uppermost sedimentation unit reflects a decrease 

in the rate of sedimentation of globigerinid tests and 

an increase in the deposition of volcanic detritus. 

Higher in the section (samples Ts 5-4 and Ts 5-4a) 

the volcanic detritus is abundant and containing rock 

fragments as well as single crystals suggestive of subaerial erosion of volcanic terrane. The inclusion of 

Bonya Limestone pebbles indicates that erosion of the 

Boyna Limestone was in progress in north Guam during Janum time. 

MARIANA LIMESTONE 

The Mariana Limestone, which forms approximately 

80 percent of the exposed limestone of Guam, is an 

emerged reef and was mapped in the field as five 

lithologic units (Tracey and others, 1963). The reef 

facies, largely made up of coral-algal incrustate limestone, forms a discontinuous peripheral belt at or 

near the present sea ward-facing limestone cliffs and 

encloses the detrital facies and the molluscan facies, 

both of which are particulate limestone types of lagoonal origin. The forereef facies of the Mariana 

Limestone is a deposit of particulate limestone that is 

exposed only on the sea ward slopes of the bounding 

limestone cliffs. The Agana Argillaceous Member is 

restricted to a fringe around the older volcanic rocks 

that were the source of the clay contamination in this 

member. 

The areal distribution of mapped facies and members suggests that the surface of the north limestone 

plateau is a relatively undisturbed depositional surface, hence much can be inferred about the petrology 

of limestones of the Mariana from the field relations. 

The similarity of these limestones to present-day deposits in shallow-water environments around Guam 

also makes profitable a comparison of the facies relations deduced from field mapping with those deduced 

from the petrologic study. 

The petrologic classification of limestone facies as 

used in· this study-reef wall, lagoon, and forereefmay be recognized within the Mariana Limestone. 

Limestones of the reef-wall facies are represented 

within the mapped reef facies. Limestones of the 

lagoon facies occur within the mapped detrital facies 

and the mapped molluscan facies. Limestones of the 

forereef facies include the mapped forereef facies and 

a foraminiferal breccia of the mapped detrital facies. 

Rock types from all three limestone facies occur as 

lenses within the Agana Argillaceous Member of the 

Mariana Limestone. 

The correlation of the limestone facies classification 

with the mapped facies of the Mariana Limestone follows. 

Mappedfacie6 of the 

Mariana Lime6tone 

Lime6tone-facie6 (Tracey and other6, 

cla66ijication 1989, pl. 1) 

Reef-wall facies ____ Reef facies __________ _ Lagoon facies______ Detrital facies _______ _ 

Molluscan facies ____ _ Forereef facies_____ Forereef facies _______ _ 

Detrital facies _______ _ 

Rock type6 

Incrustate: Coral-algal. Particulate: 

Coral-rich breccia. 

Halimeda-ricb paracoquinite. Molluscan coquinoid. Particulate: 

Foraminiferal microcoquinite. Foraminiferal-algal microcoquinite. Foraminiferal breccia. 

Foraminiferal breccia. 

ROCK DESCRIPTION'S 

Incrustate lim.estones from the reef-wall facies.-In 

the field the rocks from the reef-wall facies occur 

within the mapped reef facies of the ~1ariana Limestone and are characterized by large numbers of coral 

heads in growth position. Most are of the massive 

reef-building types such as Favia, blunt stubby Acropora and P ocillopora, and meandrine or brain corals. 
PETROLOGY OF THE LIMESTONES OF GUAM D33 

Algal growths cover many of the heads. These laminae were produced for the most part by algae of the 

genus Po1·olithon. Reef mollusks such as Trochus, 

T'l.t?•bo, and elongate coral borers are abundant in 

places. The spaces between coral heads are filled with 

fine-grained white dense limestone containing lenses 

and pockets of Halimeda plates, algal detritus, gastropod shells and, rarely, pelecypod shells. Thinsection study shows that the corals are largely recrystallized and retain their coarse structure, but are 

composed of yellow-brown translucent, coarsely crystalline calcite. Th1any of the primary void spaces and 

some secondary solution voids are partially or wholly 

filled with yel1ow drusy calcite. The mass, or vuggy, 

porosity amounts to several percent, perhaps as much 

as 10 percent locally. The rock weathers to a gray, 

sharply pinnacled surface, upon which many relict 

coral structures are visible. In outcrops where a high 

percentage of algal material is present, the rock is extremely dense and appears dead-white. The weathered 

surface of algae-rich rock is smoother and lighter in 

color than the coraliferous variety. These rocks of 

the Th1ariana Limestone in thin section show typical 

characteristics such as mud-filled coral and a high 

content of in place encrusting algae (pl. 18B). 

A1nphisteg1:na dominates the Foraminifera fossil 

group along with encrusting types such as Oarpenteria. 

Pockets of sand-size, highly concentrated angular 

debris of articulate coralline algae occur in some samples but are subordinate to laminar algal growths. 

The effectiveness of the reef wall as a trap for fine 

sediment within a turbulent marine environment is 

shown by the high percentage of interstices filled with 

fine dark mud and fine-sand-size debris. 

The areal distribution pattern, outcrop lithology, 

and characteristics in thin section of these rocks indicate that the mapped reef facies is the in place remnant of a shallow-,vater to surface-breaking, constructional reef front that rimmed the shoal-water detrital 

deposits of the Th1ariana Limestone. 

Partimdate limestones of the lagoon facies.-The 

detrital facies, as mapped in the field, is made up of 

two rock types of lagoonal 01·igin: coral-rich breccias, 

which dominate volumetrically, and Halimeda-rich 

paracoquin:ites. These rocks, plus molluscan coquinoid 

limestones "·ithin the mapped molluscan facies, cover 

all the lagoonal deposits of the Mariana Limestone. 

The breccias are dense to porous, indurated to friable, white, pink, and buff limestones that contain 

scattered remains of corals, gastropods, echinoid debris, 

and pelecypods. The degree of lithification is irregular. In some areas the rock is friable and white to 

pink, and contains few recognizable fossils. In other 

690-219 0-63-4 

areas it is a dense recrystallized light-buff, slightly 

translucent rock in .which an occasional echinoid spine 

is found. 

Lenses containing approximately 10 percent coral 

are present. These are made up of white to lightbrown, coarse-grained porous limestone and bouldersize coral remains in a poorly sorted matrix. 

The Halimeda paracoquinites locally contain high 

concentrations of I/alimeda segments (pl. 170) within 

the larger mass of limestones just described. Staining 

of these limestones (samples Po 1-2 and Tw 8-1) by 

boiling in Th1eigen's solution revealed that most of the 

original carbonate produced in the algal segments was 

still present as aragonite. On a stai~ed polished surface the original aragonite showed sharp boundaries 

against the enclosing calcite matrix. The internal 

tubes characteristic of Halimeda (segment sections on 

pl. 17 0) were filled with calcite. Almost all the segments are entire and show some degree of local packIng. 

Three sets of carbonate deposits make up the matrix 

of the Halilmeda paracoquinite (sample Po 1-2). Each 

segment is coated with a thin film of granular calcite. 

This coating appears as a bright band (pl. 170), 

which is succeeded by a granular calcite layer. In 

other sections, however, the second layer is fibrous 

aragonite, as shown by the :Meigen's solution test. 

Between thi's layer and a final fill of medium-grained 

clear anhedral calcite, lies a fine-grained dark mud 

fill. The fibrous aragonite shows as many as 5 distinct bands of clark lines which are probably a result 

of breaks in carbonate deposition. 

The coquinoid limestones contain two rock types, a 

coraliferous-molluscan limestone and a pelecypod- and 

gastropod-bearing limestone that lacks corals. The 

first type is characterized by coraliferous and molluscan remains in a dense to porous, white to tan, finegrained matrix. Casts, empty molds, and a few shell 

fragments are all that remain of the original shell 

material. The corals have been largely recrystallized 

into yellow translucent masses of calcite that show 

original structure. The rock weathers to light-brown 

to gray and has an irregular surface upon which corals 

and mollusk casts stand out in bold relief. The second 

type is a d~nse, fine-grained to sandy, light-tan rock 

in which randomly oriented casts and molds of large 

pelecypod valves locally occupy a large part of the 

rock. Gastropod-rich limestones are locally abundant. 

The typical gastropod-bearing areas within the 

molluscan facies contain whole and fragmented small 

gastropod and pelecypo<;l shells in a matrix of finesand- and silt-size calcite detritus (pl. 17A). A small 

percentage of this matrix is sand-size algal debris. The 
D34 GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS 

shells have recrystallized into mosaics of clear calcite 

that have both gradational and sharp boundaries 

with the matrix. Darker, finer grained streaks through 

the matrix are the only suggestion of bedding in thin 

section. The pelecypod-rich molluscan facies of the 

north plateau has not only the same Jrind of matrix 

as the detrital facies (sample Jj 6-1) from the Togcha 

River area but also traces of Halimeda segments and 

tests of miliolid Foraminifera. Many of the pelecypod valves have been:'dissolved, leaving disconnected 

pore spaces. However, original fibrous structure can 

still be seen in some shell fragments. 

The relative abundance of gastropod and pelecypod 

remains and the traces of algae and tests of miliolids 

suggest a lagoonal fauna unaffected by large reef contributions. The fine-grained matrix indicates deposition in quiet water. The outcrop pattern of the molluscan facies, as mapped, supports the interpretation 

of these beds as lagoonal deposits, for they are localized within the rimming reef and detrital facies (Tracey and others, 1963, pl. 1) . 

Particulate limestones of the forereef facies.-The 

forereef facies type of limestone occurs within two 

mapped units of the Mariana Limestone, the forereef 

facies and the detrital facies. The forereef facies as 

mapped in the field forms apronlike wedges that dip 

a way from and lie sea ward of and topographically 

below the mapped reef facies. At Lafac Point the 

rock is a thin-bedded well-sorted friable porous foraminiferal microcoquinite (pl. 17 B). A second area 

of exposure, flanking the mouth of the Togcha River, 

displays some breccia composed of pebbles and boulders of Bonya limestone in a matrix of weakly cemented foraminiferal tests. This matrix is like the 

foraminiferal limestone of the Lafac Point exposure. 

Some of the Togcha River exposures are well-sorted 

foraminiferal-algal microcoquinites. 

The foraminiferal microcoquinite of the exposure at 

Lafac Point is characterized by an abundance of shallow-water and reef foraminiferal tests in unusual concentrations. Oalcarina and Amphistegina predominate 

over lesser amounts of lJ! arginopora. The normally 

spiny tests of Oalcarina are smooth, probably owing to 

wear during transport from their usual reef habitat. 

Both the thick-walled 0 alcarina and the thinner Amphistegina tests show some breakage. These tests, and 

traces of echinoid spines, are loosely packed. The only 

matrix is a thin layer of fine-grained calcite that coats 

each piece of detritus evenly and serves as a weak 

cement. Hand specimens can be easily reduced to 

loose sand by finger pressure. The few echinoid spines 

have a thick growth of clear calcite in optical continuity with the typically monocrystalline spine. The 

porosity of this rock is high, approximately 30 percent, 

owing to the large intertest primary voids. 

The physiographic and stratigraphic position of 

the mapped forereef facies, as well as the seaward 

dip and bedding at Lafac Point, demonstrate its forereef origin. The abundance of reef and shallow-water 

Foraminifera suggests the selective transportation, 

possibly as a distinct size grade, off the reef shallows 

to be deposited as forereef talus. The Togcha River 

exposures show, by virtue of the included Bonya Limestone detritus, active erosion of older limestones. 

Parts of the mapped detrital facies within the Mariana Limestone are considered breccias of forereef origin because of included diagnostic Foraminifera. This 

rock shows the characteristic forereef Foraminifera, 

Oycloclypeus, in a poorly sorted matrix containing 

angular reef debris (pl. 17 D) . Almost all the large 

tests are partly fragmented. Some of the tests have 

worn and broken coatings of an encrusting Foraminifera; these tests must have come to rest once prior to 

their final incorporation in this breccia. 

Agana Argillaceous lJtlember.-This unit was differentiated in the field from the main mass of pure 

Mariana Limestone on the basis of its clay content. 

(See 37-45.) This clay is present in two forms: 

finely diBseminated material in the limestone matrix 

where it amounts to several percent of the rock by 

weight; and slightly calcareous pockets and lenses, 

which in a few outcrops amount to as much as 50 

percent of the rock by volume. Lenses of all these 

rock types occur in the Agana Argillaceous Member; 

faunally and texturally these lenses are correlative to 

rock types in the pure limestones. 

Molluscan-rich coquinoid limestones of the Agana 

Argillaceous Member contain numerous casts and 

molds of the high-spired gastropod Turritella filiola, 

which are from lj8 to lj2 inch in length and randomly 

oriented in a dense to friable yellow fine-grained 

matrix. Thin sections show a matrix similar to that 

described for the pure molluscan facies. However, a 

strong yellow-orange stain seen in reflected light is 

due to the clay contamination, especially around the 

shells. Most of the shell material has recrystallized 

to medium-grained mosaic calcite. The clay contamination must have been brought into the Mariana Limestone by streams flowing off the southern volcanic 

rocks, as shown by the areal limitation of the argillaceous member. 

ENvm.ONMENTAL RECONSTRUCTION 

The Mariana Limestone was deposited under a wide 

range of environmental conditions. Around the volcanic mass of central and south Guam, stream erosion 
PETROLOGY OF THE LIMESTONES OF GUAM D35 

contributed large quantities of volcanic clay to the 

near-shore limestone. '¥idespread shallow lagoons, 

bottomed with clay and fine argillaceous sand, were 

the habitat of a large population of mud-burrowing 

gastropods such as gave rise to the Turritella filiolabearing coquirioid limestones. The present north 

plateau was the site of reef, lagoon, and forereef environments relatively uncontaminated by volcanic material. 'Vhile the reef-wall limestone accumulated in 

place as a solid constructional reef front, Oyoloolypeusbearing breccias formed on the forereef slopes. Backreef lagoons accumulated reef and shallow-water detritus that resulted in coral breccia. Protected areas in 

the lagoon \vere collection sites for fine-grained sediments in which bivalves and gastropods flourished. 

Local concentrations of Halimeda were common. During the late stages of l\1ariana time, quantities of 

shallow-water Foraminifera tests were swept from 

their reef habitat and deposited as talus to form the 

mapped forereef facies. 

SUMMARY AND CONCLUSIONS 

The dominant rock-forming processes that built 

Guam were volcanism and reef growth accompanied 

by sedimentation in lagoon and forereef areas. The 

recurrence of :field relations and of typical rock associations containing diagnostic fossils shows that the 

reef complexes tended to evolve along similar patterns 

through time. The older reef complexes are partially 

destroyed by volcanic explosions and erosions. The 

Mariana Limestone, on the other hand, postdates active volcanism and hence retains its original depositional pattern. The reefs of Eocene (b) age were 

largely destroyed, they now occur mainly as limestone 

fragments in volcanic breccia; other reef complexes 

of intermediate age are in various stages of preservation. Comparison of the limestones of Guam with 

material from reef complexes in Louisiana and Iraq 

shows that Tertiary reef complexes throughout the 

world are lithologically much alike. 

Reef and associated limestones from the H ete'rostegina zone (late Oligocene or early l\1iocene in age) 

of southern Louisiana and from strata of Oligocene 

age in Iraq bear striking faunal and textural resemblances to the limestones of Eocene (b) and, particularly, l\1iocene (e) age described above (see pls. 

19 and 20). A detailed comparison of limestones from 

these three areas \Yas made by Forman and Schlanger 

(1957). The Louisiana reefs. were of two types: 

scattered individual reefs that formed on topographic 

highs over submarine salt domes, and a single regional shelf reef. Conditions during H eterosteginazone time are considered somewhat analogous to the 

present situation on the Gulf Coast continental shelf, 

where a number of small topographic prominences, 

having a relief of as much as 600 feet, reach to within 

50 feet of sea level. 'Vork by Stetson ( 1953) revealed 

living colonies of reef organisms on the crests and 

shoulders of these inferred domes. In Iraq, numerous 

reefs of various types contain a complete suite of reefwall and associated limestones (Henson, 1950). The 

areal distribution and microfacies characteristics, such 

as faunal and floral content and texture, of limestones 

in all three areas enabled Forman and Schlanger 

(1957) to come to the following conclusions regarding 

reef growth during the early part of the Tertiary: 

(1) Lower and middle Tertiary reef complexes, perhaps throughout the world, are characterized by basically the same faunal and floral assemblages; (2) these 

assemblages produced diagnostic limestone facies; ( 3) 

these limestone facies maintain predictable stratigraphic relations in the reef complex; ( 4) the nature 

of the substrata and the local geologic setting do not 

appear to be dominant factors in the formation of 

these reef complexes; (5) once favorable ecologic conditions are met, under widely differing circumstances, 

similar reef complexes will form. 

Thus, partial preservation of a complex allows interpretation of the original position of the unpreserved parts. Reconstruction of depositional conditions for the l\1aemong Limestone l\1ember is an example of such a reconstruction. The direction of 

dip of forereef deposits combined with the structural 

setting of south Guam indicates the existence of a 

former reef wall to the west. The application of thinsection studies to other ancient reef complexes, as first 

suggested by Henson (1950), will probably make possible more detailed interpretations of the geologic history of many limestone-volcanic islands in the open 

Pacific. 

In all the reef complexes studied, the ratio of particulate limestone to incrustate reef-wall rock is large; 

in the l\1ariana, incrustate limestone amounts to perhaps 5 to 10 percent of the total volume of the formation. As Ladd ( 1950, p. 204) points out, the reef wall 

is "* * * like the walls and rim of a pail that holds 

water * * *". Yet this wall controls sedimentation 

around it by forming a barrier to oceanic circulation 

and maintaining shallow-water conditions, thereby 

producing a lagoonal environment that is in sharp 

contrast to the forereef environment. In the backreef areas sediments accumulate largely in place, and 

there is only local small-scale movement. The forereef 

deposits, on the other hand, show the effect of considerable transport and breakage. The sediments tend 

to decrease in grain size away from the reef wall, 
D36 GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS 

concomitantly with . the decreasing ratio of debris 

transported from sha1low water to indigenous Foraminifera, particularly planktonic. Submarine slumping from overloaded forereef slopes probably carries 

material into basins where the normal sedimentation 

is due to planktonic contributions. Limestones in the 

Maemong Limestone Member show streaks and lenses 

of foraminiferal-algal debris in thin fine-grained globegerinid beds. Beds of pure globigerinid mudstone 

a few inches thick alternate with beds of fine-grained 

foraminiferal-algal microbreccia to build up one section known to be at least 50 feet thick. These numerous alterations can hardly be ascribed to rapid changes 

in sea level or lateral shifting of the far a way reef 

wall. The beds of algal-foraminiferal debris are probably composed of slumped material from upslope. 

Comparisons of the various limestone units show that 

all the major fossil groups considered in the rock classification occur in both the oldest and the youngest 

limestones. Some organisms, such as coral and echinoids, contributed material in fairly steady amounts 

through time. Articulate coralline algae, however, appear to have been more abundant in the older limestones, but diminished as rock makers through time. 

Field observations indicate that this trend is real and 

study of limestones from other islands in the Pacific 

also tends to confirm this trend; limestones of Eocene 

(b) and Miocene ( e-f) age carry large amounts of 

articulate coralline algae. Emery (1956, p. 1511) 

shows that coralline red algae reduced to sand size is 

the dominant constituent of modern submarine sediments in the surrounding shallows off Johnston Island. 

This high algal content is in contrast to the lower 

algal content of lagoon sediments from Bikini Atoll 

and Guam. The abundance of algae may be a function of the degree of enclosure of a lagoon by shoaling reef walls and islands. At both Bikini and Guam, 

lagoons are largely encircled by shoal reefs and islands, 

whereas at Johnston Island the surrou~ding shallows 

are largely open to the sea (Emery, 1956, pl. 1) . Thus 

the possibility exists that the lower Tertiary reef complexes were more open to the sea and less bounded 

by reef walls and islands than the reefs of Mariana 

age, for example, that must have restricted circulation in the lagoon; the geologic map (Tracey and 

others, 1963, pl. 1) shows that the Mariana reef walls 

almost completely encircled the lagoon. 

One of the most prevalent types of recrystallization 

is the conversion of original mud matrix to granular 

clear mosaic calcite. All gradations from relatively 

opaque mud to clear calcite have beeh noted. This 

point is important in interpretation of original textures 

inasmuch as a limestone that was deposited with a 

mud matrix might be mistaken for a coquinite that 

was later filled with precipitated calcite. The various fossil groups differ in their susceptibility to recrystallization. Coralline algae show a wide range of 

recrystallization; generally material from limestones 

of Eocene (b) and Miocene (e) age are better preserved than any other fossil group present. Yet in 

younger limestones, such as parts of the Alifan and 

Barrigada Limestones, algal fragments are bleached 

and dissolved while Foraminifera are well preserved. 

Halimeda segments show a change from microcrystalline aragonite to granular calcite; aragonitic Halimeda 

were found only in the Mariana Limestone. Because 

the persistence of aragonite in exposed limestones, as 

discussed above, may continue for long periods, the 

presence of aragonite is ruled out as unequivocable 

evidence of nonemergence. Corals are everywhere recrystallized; ghosts of original septal outlines are in 

mosaic calcite. These dust-line ghosts are tenacious 

and are present in the oldest limestones. 

Recrystallization is accompanied by a reduction in 

the amount of strontium in the resulting limestone. 

Dolomite is localized in the Ali fan and Bonya ( ~) 

Limestones; even within these formations, its distribution is spotty. If dolomitization was a normal diagenetic effect, much more of the Guam limestone section should be dolomitic. The local occurrence of 

dolomite suggests that rather special conditions are 

necessary for its formation. 

On Guam microstylolites occur only in the basal 

part of the Ali fan Limestone. Except for minor interpenetration of foraminiferal tests in the Bonya 

Limestone, the microstylolites are the only· features 

than can be ascribed to solution along preferred surfaces. The lack of microstylolites in samples from 

the deep cores taken below Eni wetok indicates that 

simple static loading is not the controlling factor in 

microstylolite formation. 

Introduction of noncarbonate material into the pure 

limestone takes place in several ways, all traceable 

back to primary volcanic rocks as the original source. 

Silification of limestones owing to the eruption of 

immediately superjacent lava flows took place after 

deposition of limestones of both Eocene (b) and 1\1iocene (e) age. The older limestones show both fissure 

fillings and metasomatic replacement by chalcedonic 

quartz. The globigerinid mudstones of the 1\1aemong 

Limestone Member show numerous tests filled with 

beta-cristobalite. Lava flo~·s directly overlie both of 

the silicified limestones and must have been the source 

of the silica. The mode of emplacement of the silica 

apparently depends on the condition of the limestone. 

The empty tests of planktonic Foraminifera provided 
PETROLOGY OF THE LIMESTONES OF GUAM D37 

convenient space for the siliceous solutions. In the 

limestones of Eocene (b) age, open fissures provided 

growth space. The absence of metasomatic replacement in the globigerinid mudstone from the ~1aemong 

Limestone ~1ember is difficult to reconcile with such 

replacement in the limestones of Eocene (b) age. 

The differentiation of the insoluble residues into two 

groups (see p. 43), one indicative of ash falls .an.d 

pyroclastic contributions and the other charactenst1c 

of contamination by erosion of volcanic terrane, shows 

that volcanic activity in the immediate Guam area 

ceased after the deposition of the Bonya Limestone. 

The presence of primary volcanic material in the 

~1n.riana limestone indicates pyroclastic contributions. 

I-Iowever, this contribution is light and is inte~preted 

as airborne material from afar. Independent hnes of 

evidence indicate that a period of intense surface 

w·eathering of volcanic rocks took place during the 

deposition of at least part of the Bonya Limestone and 

continued long enough to supply considerable clay to 

the basal part of the Alifan Limestone and most of 

the Janum Formation. Transport of manganese occurred during and perhaps shortly after the deposition 

of the Bonya Limestone. This transportation indicates 

the development of a drainage pattern and the resultant leaching of manganese on an island-wide scale. 

The presence of nordstrandite in the Alifan Limestone 

and the J anum Formation points to the movement of 

alumina either in solution or as detrital gibbsite or 

both; the occurrence of gibbsite itself indicates advanced weathering of volcanic terrane. 

This paper may best serve as an introduction to 

study of reef and associated limestones from the western Pacific. Future work in this area should include 

comparison of rock associations both within island 

chains, such as the ~1arianas, and between widespread 

c.hains. Limestones such as these are sensitive indicators of uplift and emergence; studies of areal shifts 

from shallow to deep-water facies within correlated 

stratigraphic units can show whether tectonic activity 

was local or regional. In addition, study of insoluble 

residues may lead to an understanding of erosion 

cycles, through time, in the island arcs of the western 

Pacific. 

PETROGRAPHY OF THE INSOLUBLE RESIDUES 

By .J. C. HATHAWAY and DOROTHY CARROLL 

LABORATORY PROCEDURES 

Laboratory procedures were designed to extract 

sand, silt, and clay grades of insoluble material from 

the limestones without altering individual minerals. 

For this reason, acetic acid ( 1 :4) was used in place of 

the usual hydrochloric acid for removing calcium carbonate. Acetic acid has little effect on clay minerals. 

SEPARATION 

Thirty-one samples of limestone were crushed in a 

jaw crusher and rolls to pass a 60-mesh (0.25 mm) 

sieve. Approximately 100 grams of crushed limestone 

was dissolved in 1 :4 acetic acid to remove the calcium carbonate from the sand, silt, and clay materials. 

Calcium carbonate was dissolved most rapidly when 

the acetic acid was kept dilute and the limestone-acid 

mixture was agitated with a magnetic stirrer. A temperature of about 50°C also increased the rapidity with 

which the calcium carbonate was dissolved. When 

the samples had dissolved to the point at which no 

reaction was obtained by treating a small portion of 

the sample with dilute hydrochloric acid (N/10), they 

were removed from the beakers and washed by centrifuging with distilled water. The washing was continued until the residue remained partly in suspension 

after 10 minutes at 2,000 rpm. The washed samples 

were dried and weighed as insoluble residue. 

The insoluble residues were dispersed in distilled 

water to which a little sodium metaphosphate was 

added. The sand was removed by wet sieving with a 

230-mesh sieve (62 microns). The material finer than 

62 microns was further dispersed in water in 1,000-

milliliter cylinders in an end-over-end shaker. These 

suspensions were fractionated by repeated centrifuging 

to separate the silt (2 to 62 microns) and clay (less 

than 2 microns) fractions. The excess water was removed from the resulting clay suspensions by filter 

candles under vacuum. The sand, silt, and clay fractions were dried and weighed and expressed as a percentage of the total insoluble residue. 

MINERALOGICAL ANALYSIS 

CLAY AND SILT MINERALS 

The minerals in the clay and silt fractions were 

identified by X-ray diffraction using Cul(a nickel 

filtered radiation. Various preparatory techniques requiring several X-ray diffraction patterns were used 

to identify the clay minerals. The minerals in the 

silt fractions were identified from X-ray diffractometer 

patterns of unoriented minerals. Powder photographs 

were made of samples that were too small to make 

satisfactory mounts on glass slides for diffractometer 

patterns. 

MINERALS IN THE SAND GRADES 

A weighed quantity of the sand from each sample 

was separated in bromoform (sp gr, 2.9) to obtain the 

light and heavy minerals for microscopic examination. 

In 14 samples the quantity of material of sand size was 
D38 GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS 

too small for bromoform separation. Magnetite was 

removed from all the samples with a small horseshoe 

magnet before the remaining heavy minerals were 

mounted on glass slides. If magnetite was present in 

appreciable quantities, it was weighed and expressed 

as a percent by weight of the sand fraction. The minerals were identified with a petrographic microscope. 

PETROGRAPHY 

QUANTITY OF INSOLUBLE RESIDUE 

The quantity of insoluble residue in the limestones 

examined ranges from less than 1 percent to more than 

42 percent (table 5). No systematic variation is apparent among the limestones, except that in the three 

samples of Bony a Limestone the quantity is small; it 

averaged 0.6 percent. In the Mariana Limestone the 

insoluble content ranges from 0.6 to 16.4 percent, in 

the J anum Formation from 0.8 to 18.1 percent; one 

sample of the Maemong Limestone Member of the 

Umatac Formation has 11.5 percent, but three others 

have less than one percent; two samples of the Maemong Limestone Member have 20.9 and 24.0 percent, 

but in these samples silica fillings of foraminiferal 

tests account for a large percentage of the insoluble 

re~idue. 

TABLE 5.-Weight percent of insoluble residues of Guam limestone 

[Analyst, Hildreth Schultz, U.S. Geological Survey] 

Sample 

Mariana Limestone: Sw 2-4 _______________________________________________ _ 

Pr 3-3-------------------------------------------------

lj 2-L------------------------- __ ------------ _ ----- ___ _ Allfan Limestone: 

Ru 8-3. _ ----------------------------------------------

Di Di 2-L--------------------------------------------- __ _ 2-2 ________________ - _________ --- __ ---- ____ -------- __ 

Ts 9-t_ ______ ---------------- ___ -----------------------

Jl 3-2- ---------------------- -~ ------------------------ Janum Formation: Ts 5-4 __________________________________ -------- ______ _ Ts 5-9 ________________________________________________ _ Rr 23-1. _____________________________________________ _ 

Rr 5-1. _________________ ---- _____________ --- _________ _ 

Bonya Ut Limestone: 6-1. ______________________________________________ _ 

Rr 13-1. _____________________________________________ _ 

Gi 2-L. ___ ... ---------------------------------- _______ _ Maemong Gi Limestone Member of Umatac Formation: 1-2a .. _____________________________________________ _ 

Ef 1-L- _ ------------------------------------ __ ------ __ Ed Ee 2-1 1-lb. _______________________________________________ ___ -------- ---------------------------------_ 

DC 9-1a __ --------------------------------------------- Ii 5-2.------------------------------------------------- Alutom Formation: Fk 4-11 ___________________________ ---- _______________ _ 

Mahlac Member: 

Gj 11:1.--------------------- _ -------------------------

Insoluble residue 

Weight 

(grams) 

22.3 .8 

3.5 

9.9 

. 5 

9.8 

2.0 

.15 

9. 7 

.9 

13.1 

3.2 

.7 

.6 

.5 

11. 1 

.075 

.9 

17.5 

26.7 . 7 

1.8 

36.6 

Percent 

16.4 

.6 

3.0 

8. 5 

. 5 

7.8 

1.3 

.1 

18.1 

.8 

11.6 

2.4 

.5 

.5 

.4 

11.5 .1 

.9 

24.0 

20.9 

.8 

1.3 

42.1 

The variation in insoluble-residue content must be 

explained by the variation in the amount of detrital 

material available in the area· in which calcium carbonate was accumulating. Wentworth and Ladd 

(1931) and Bramlette (1926) have noted a similar 

variation in the rocks on Vitilevu and in the sediments 

being deposited in Pago Pago Harbor, Samoa. 

The percent by weight of sand, silt, and clay grades 

in the insoluble residues of the limestones is given in 

table 6 and in figure 4. The mineral residues are 

concentrated in the silt ( 2 to 62 microns) and clay 

(less than 2 microns) grades; clay is more abundant 

than silt. Only a few samples contain much sand 

(greater than 62 microns). 

TABLE 6.-Sand, silt, and clay in insoluble residues of Guam 

limestone 

[Analyst, Hildreth Schultz, U.S. Geol. Survey. Results given ln percent by weight; Tr., less than 0.01 gram] 

Sample 

Mariana Limestone: 

Sw 2-4 __________________ ------------------ Pr 3-3. _______ ---------- _____________ -- __ _ 

Ij 2-L ----------------------·------------- Aiifan Limestone: Ru 8-3 ___________________________________ _ Di 2-1 ___________ -- ________ -- ____________ _ 

Di 2-2_ ----------------------------------- Ts 9-L ----------------------------------- Jl 3-2 __ ----------------------------------- Janum Formation: 

Ts 5-4.-----------------------------_----- Ts 5-9.----------------------------------- Rr 23-L---------------------------------- Rr 5-1·---------------·-------------------

Bonya Limestone: 

Ut 6-1 '-----------------·----------------- Rr 13-L------------------------------. ---

Gi 2-L----------------------------------- Maemong Limestone Member: 

Ef Gi 1-1. 1-2a_ _ ---------------------------------- --- ___ ----- ______________________ _ 

Ed 1-1b_ ---------------------------------

Df Ee 2-L _______ ---------------------------- 9-1a ________ ----- _____________________ _ 

Ii 5-2_------------------------------------ Alutom Formation: · Fk 4-11_ __ -------- ___ -- _ -- __ -------------- Mahlac Member: 

Gj 11-1.----------------------------------

Sand>62~ 

2.1 

Tr. 

.3 

2.6 

8.0 

.2 

.5 

Tr. 

1.8 

5. 6 13.4 

2. 5 

10.0 

1.7 

Tr. 

18.3 

Tr. 

2. 2 

3.8 

14.9 

2. 8 

23.9 

1.8 

Silt (2-62~) Clay(<2~) 

50.0 48.0 

37.5 62.5 

12.0 87.7 

5.6 91.9 

26.0 66.0 

7.6 92.1 

11.5 88.0 

26.7 73.3 

12.2 86.0 

16.7 77.8 

16.4 70.2 

25.3 72.2 

35.5 54.5 

5.0 93.3 

4.0 96.0 

40.5 41.2 

Tr. 100 

21.1 76.7 

55.1 41.1 

67.4 17.7 

20.0 77.2 

38.9 37.2 

42.1 56.1 

1 Sand fraction of residue accidentally contaminated; percentages given for sample 

represent a careful estimate. 

MINERALOGY 

CLAY AND SILT GRADES 

The minerals identified by X-ray examination are 

listed below; they are placed in two main groups: clay 

minerals and nonclay minerals. The clay minerals are 

montmorillonite, chlorite, mixed-layered montmorillonite-chlorite, mica, and minerals of the kaolinite 

group (kaolinite, fire clay, halloysite, and undifferentiated minerals). The nonclay minerals are divided 

into two groups, silicate minerals (quartz, cristobalite, 

plagioclase feldspar, and zeolites) and a nonsilicate 

group that contains gibbsite, nordstrandite, magnetite, 

hematite, goethite, lithiophorite (lithian manganese 

oxide) and pyrolusite, apatite, and a woodhouseitelike mineral (an alunite-type mineral in which calcium proxies for the potassium and phosphate proxies 

for some of the sulfate of alunite). 
PETROLOGY OF THE LIMESTONES OF GUAM D39 

SAND 

PERCENT INSOLUBLE RESIDUES 

oe >25 

D • 15-25 

D • 5-15 

~ D • 1-5 

D. 0 • <1 

D 

D 

D 

0 

D 

f:j. 

• 

0 

6 

SILT CLAY 

FIGURE 4.-Size distribution or the Insoluble residues from selected limestones. The open squares represent samples containing a group 1, or fresh assemblage; the black · 

circles represent samples containing a group 2, or weathered assemblage (table 10). The open triangles represent samples that fit neither category. 1'he size of the 

symbol Indicates tho relative amount of insoluble residue according to the scale shown. 

Estimates of the amounts of the various minerals in 

the samples are derived from the relative intensities 

of the diffracted lines. Inasmuch as many factors, 

such as variations in degree of preferred orientation, 

differences in degree of crystallinity, and variations in 

isomorphous substitution, in addition to quantity of a 

mineral, affect diffraction intensity, these estimates can 

give only a general indication of the relative amounts 

of the minerals present. 

Of the minerals identified, two should receive special mention, nordstrandite and halloysite. Nordstrandite is a probable polymorph of gibbsite-bayerite. It has recently been reported from artificial preparations by Van N ordstrand, Hettinger, and Keith 

( 1956), but had never been reported in a natural occurrence until 1962 (I-Iathaway and Schlanger, 1962, 

p. 265-266; vV all, and others, 1962, p. 264-265) . 

Identification of halloysite was based on the presence of broad, diffuse, and weak 001 reflections for a 

kaolinite-group mineral, and strong but broad hld 

bands with no resolution of hkl reflections. A further 

criterion wa.s the lack of distinct intensification of 

the 001 reflection in oriented aggregates of this mineral. Because electron micrographs were not made 
D40 GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS 

Minerals identified in the different samples 

Sample Fraction 

Mariana Limestone: 

Sw 2-4_ --------------- Clay ________ _ 

SilL--------- Pr 3-3----------------- Clay ________ _ 

Silt_ ________ _ 

Ij 2-L----------- ·----- Clay ________ _ 

SilL---------

Janum Formation: Ts 5-4 _________________ Clay ________ _ 

SilL ________ _ 

Ts 5-9 _________________ Clay ________ _ 

SilL---------

Rr 23-L _______________ Clay ________ _ 

SilL ________ _ 

Rr 5-L---------------- Clay ________ _ 

SilL---------

Al!fan Limestone: 

Ru 8-3. _ -------------- Clay---------

SilL---------

D12-L ________________ Clay ________ _ 

Silt_ ________ _ 

Di 2-2----------------- Clay ________ _ 

SilL---------

Ts 9-L---------------- Clay ________ _ 

SilL---------

Jl 3-2------------------ Clay_--------

SilL ________ _ 

Bonya Limestone: Ut 6-L ________________ Clay ________ _ 

Silt_ ________ _ Rr 13-L _______________ Clay ________ _ 

SilL---------

Gi2-L ________________ Clay ________ _ 

SilL---------

Maemong Limestone 

Member: Gi 1-2a ________________ Clay ________ _ 

SilL ________ _ 

Ef 1-L---------------- Clay ________ _ 

Ed 1-1b _______________ Clay ________ _ 

Silt_ ________ _ 

Ee 2-L---------------- Clay ________ _ 

SilL---------

Df 9-1a________________ Clay ________ _ 

SilL---------

Il5-2 __________________ Clay ________ _ 

SilL---------

Mineral& identified 

Apatite_---------------- ____ _ __ ... do ______________ ------- __ _ ____ do ______________________ _ 

Montmorillonite __ ----------- Quartz ______________________ _ 

Heulandite or clinoptilolite __ Gibbsite ____ ------- _________ _ 

Apatite.---------------------

Heulandite or clinoptilolite __ Quartz ______________________ _ 

Feldspar---------------------

Halloysite. ------------------ Goethite _____ --------- ______ _ 

Goethite __ ------------------- Halloysite ___________________ _ Gibbsite ____________________ _ 

Montmorillonite __ ----------- Halloysite __________________ _ 

Magnetite __ ----------------- Nordstrandite. -------------- Hematite __ ------------------ Halloysite __ ----------------- Montmorillonite ____________ _ 

Magnetite._-----------------

Hematite_-------------------

Halloysite. _ ----------------- Montmorillonite._-----------

Magnetite. __ ---------------- Halloysite. ------------------ Nordstrandite. -------------- Goethite __ ------------------- Woodhouseite-like mineraL __ 

Nordstrandite. -------------- Woodhouseite-like mineraL __ Goethite __ -------- ____ -------

Magnetite ___ ----------------

Fireclay mineraL ___________ _ 

Goethite __ -----_------------- Nordstrandlte. -------------- Goethite ___ ------------------ Montmorillonite_------------

Halloysite. ------------------ Nordstrandite. -------------- N ordstrandite ________ -- _____ _ 

Goethite _____ --------- ______ _ 

Montmorillonite._-----·····-

Halloysite. _ -----------·----- Goethite __ ------------------- Montmorillonite._--------- __ 

Halloysite_ ------------------ Montmorillonite-chlorite 

mixed-layer. Nordstrandite ________ ----- __ _ 

Woodhouseite-like mineraL __ Goethite __ ------ ___ --·------_ Quartz ______ ---------·~-----_ Halloysite _______ ------ _ --- __ 

Goethite __ ---------- ___ -----_ Nordstrandite ____ ------------ Woodhouseite-like mineraL __ 

Lithiopborite. _ -------------- Halloysite_ ------------------ Lithiophorite. _ ------------ __ Montmorillonite_------------

Heulandite or clinoptilolite. _ Heulandite or clinoptilolite __ Quartz ______________________ _ 

Magnetite. __ ---------------- Montmorillonite ____________ _ Chabazite __________ . ___ ------

~~a;st~ar ~~~= ~=:::: == == = ===== = 

Montmorillonite_------------ Halloysite _____________ ----- _ 

Montmorillonite __ -----·----- Halloysite. _ ----------------- Mica _____________ ------------ Montmorillonite _______ ------ Kaolinite ___________________ _ 

~~~rs~ar~ :::::::::::::::::::: Montmorillonite ____________ _ Mica ______________ --------- __ Plagioclase __________________ _ 

Montmorillonite_------------

Kaolinite ___ ----------------- Montmorillonite __ ----------- Heulandite or clinoptilolite __ Heulandite or clinoptilolite __ Montmorillonite __ ----------- Beta cristobalite. _____ ------- Montmorillonite __ ----------- Beta cristobalitc. ------------ Quartz ___ ------- ____ ----- ___ _ 

Feldspar_-------------------- Montmorillonite. ___________ _ 

Not determined. 

Part& 

in 10 

9 

10 

8 

Tr. 

Tr. 

Tr. 

Tr. 8 

1 Tr. 

Tr. 6 

3 

5 

1 Tr. 

8 Tr. 6 

3 

Tr. 9 Tr. 

Tr. 

Tr. 9 

Tr. 

8 

Tr. 4 

2 

3 

4 

4 

1 Tr. 

8 

2 

4 

4 

4 

4 

1 

8 

2 

9 

Tr. 

8 

1 

5 

4 

4 

3 

1 Tr. 4 

4 Tr. 

Tr. 

9 Tr. 

Tr. 9 Tr. 5 Tr. 

Tr. 

8 

7 

1 Tr. 

9 Tr. 9 

Tr. 2 

1 Tr. 

Tr. 

Tr. 9 

Tr. 7 

1 Tr. 9 Tr. 9 

1 

8 

1 

9 Tr. 

Tr. 

Tr. 

Minerals identified in the different samples-Continued 

Part& 

Sample Fraction Mineral& identified in 10 

Alutom Formation: 

Fk 4-11________________ Clay_-------- Montmorillonite __ ----------- 8 

Analcime____________________ 1 

Silt ___ ------_ Analcime._-----------------_ 9 

Feldspar_-------------------- Tr. 

Mahlac Member: Gj 11-L _______________ Clay _________ Montmorillonite_____________ 9 

Silt __ -------- Heulandite or clinoptilolite. _ 5 

Montmorillonite __ ----------- 4 

Quartz_______________________ Tr. 

for these samples, there is no assurance that a tubular 

shape is present. However, inasmuch as the use of 

the terms kaolinite or fire clay would imply that a 

material of a higher degree of crystallinity was present, the term "halloysite" is preferred to describe this 

mineral. 

SAND GRADES 

The minerals identified in the total sand of the insoluble residues are given in tables 7 and 8. The 

quantities of heavy minerals in these residues vary; 

some contain very few heavy fractions, whereas others, 

notably limestones of the Maemong ~1ember and the 

Janum Formation, have abundant heavy fractions; in 

some heavy fractions one mineral was present to the 

almost complete exclusion of others. The distribution 

of the various minerals is discussed at the end of this 

section. 

The heavy minerals found are magnetite, ilmenite(?), augite, hypersthene, greenish-brown amphibole, . basaltic hornblende, bluish-green amphibole, 

sphene, and zircon; sphene and zircon are very scarce. 

The minerals in the light fractions are plagioclase 

feldspar, quartz, clay aggregates, opaline silica (betacristobalite.), zeolites, and apatite; apatite should, 

however, belong in the heavy fraction. 

These minerals are common to the volcanic rocks 

of Guam as described by Stark (1963). The major 

well-crystallized constituents of these rocks are plagioclase feldspar and monoclinic pyroxene; the subordinate minerals are orthorhombic pyroxene, magnetite, 

occasional amphibole, and quartz. Zeolites are also 

present in some of the volcanics, but apatite and 

cristobalite have formed in the limestones. There are 

apparently only minor variations in the distribution 

of the volcanic minerals in the rocks and water-laid 

tuffs. 

In addition to the small samples, the insoluble residues and minerals "·ere examined in five large samples 

of limestone from the Janum Formation, ~1aemong 

Limestone ~1ember, Alutom Formation, and Mahlac 

~1ember, respectively. These samples yielded more 

information about the amounts of detrital minerals in 

the limestones. Table 8 gives the quantities of lime-
PETROLOGY OF THE LIMESTONES OF GUAM D41 

stone dissolved in acid to obtain the residues and size 

distribution of the residues. 

TABT .. E 7.-Minerals in the sandfractions (greater than 62 microns) 

of insoluble residues of Guam limestone 

['l'r., quantity too small to wel~h; Tr.•, sand fraction too small for bromoform sepa- ration, but heavy minerals are present in the sand. Minerals are given In order or abundance] 

Heavy Heavy minerals 

Sample residue >2.9 [sp gr] Light minerals <2.9 [sp gr] (percent) 

Mariana Lime- stono: 

Sw 2-4 •••••••.• 'rr. Magnotlte, augite, Clay mineral. 

hypersthene. Pr 3-3 •••••••••• Tr.• Magnetite, augite ..••. Plagioclase feldspar, zeoAgana lite, quartz, apatite. 

ArglJlaceous 

Member: 

IJ 2-1.. .•••••••• 'l'r.• Magnetite •. ---------- Apatite, plagioclase feld· 

Janum Formation: spar. 

'l's 5-4 .••••••••• 44. 1 Magn.:~tite, augite .•••• Altered grains, plagioclase 

Ts 5-9 •••••••••• Tr.• feldspar. Magnetite, apatite ..•• Plagioclase feldspar. Rr 23-1 ••••••••• 94.3 Magnetite, augite, Clay mineral, plagioclase hypersthene. feldspar, quartz (eubeRr 5-1 •••••••••• Tr.• dra). Magnetite, augite .•••. Plagioclase feldspar, opa- line silica, undetermined 

Ts 5-11 ••••••••• grains. 2.6 Amphibole, augite, Clay mineral, plagioclase 

'l's 5-12 ••••••••• magnetite. feldspar. 6. 7 Augite, amphibole, Plagioclase feldspar, altered 

Altran Limestone: magnetite. grains, day mineral. 

Ru 8-3 ••••••••• 11.5 Magnetite •••••••••••• Altered grains 1 plagioclase 

D12-l. ••••••••• feldspar. Tr.• ••••• do •.•••••••••••••• Apatite, quartz, plagioclase 

Di2-2 •••••••••• feldspar. Tr.• l\·fagnotlte, zircon ..••• Apatite, quartz, plagioclase 

Ts 9-L ••••••••• feldspar. Tr.• Magnetite •• ···------- Quartz, plagioclase foldspar, clay mineral. 

Jl3-2 ••••••••••• Tr.• ••••• do •• -------------- Quartz, altered grains. Bonya Limestone: nr 13-1. •••••••• Tr.• Magnetite, augite .•.•• Opaline silica, quartz, pla012-L ••••.•••• gloclase feldspar. Tr.• Magnetite .• ---------- Opaline silica, quartz, zeoMnemong Limo- lites, altered grains. 

stone Member: 

011-2 ..•••••••• 3. 9 Augite, magnetite. ____ Clay mineral, opaline silica, plagioclase fcldEf 1-1 •••••••••• spar. ~I'r. • Magnetite •• ---------- Opaline silica, quartz. l~dl-lb •••••.•. 'l'r.• Magnetite, augite .•... Plagioclase feldspar, opaline silica. 

Eo 2-1.. •••••••• Tr. • •••• do ••• ------------- Opaline silica. 

Dr 9-la ••••••••• 'l'r. Augite, magnetite ••••. Opaline silica. Ed 7-2 ••••••••• 24.4 Augite, hypersthene, Altered grains, plagioclase amphibole, magnet- feldspar, opaline silica, ito. zeolites. 

Alutom 

II 5-2 ••••••••••. Tr. Magnetite •.• --------- -----------.. ---------------- Formation: 

Fk 4-L •••••••• 'l'r. Magnetite, augite, Opaline silica, clay mineral. 

hypersthene, sphene 

(?). Ek 7 ••.•••.•••• 1.8 Amphibole, basaltic Plagioclase feldspar altered 

hornblende, mag- grains, opaline silica, netitc sphene. quartz, zeolites. Mahlac Member: 

OJ 11-L •••••••• 4. 4 Magnetite, hyper- Plagioclase feldspar, opal- stbene amphibole, inc silica, clay mineral. 

zircon. 

OJ 15-1.. ••.•••• .2 Amphibole, augite Clay mineral, plagioclase magnetite. feldspar, opaline silica. 

• Altered feldspar plus clay minerals. 

The +140 (0.25-0.105 mm) and the +270 (0.105-

0.053 mm) grades of the insoluble residues were separated in bromoform to obtain the heavy minerals, the 

amounts o:f: which are given in table 9. The minerals 

were identified by a petrographic microscope. 

The heavy minerals identified in these residues are 

listed in order of abundance; all the samples contain 

an appreciable amount of magnetite. 

Janum Formation (Ts 

5-11). 

Janum Formation (Ts 

5-12). 

Maemong Limestone 

Member (Ed 7-2). 

Mahlac Member (Gj 

15-1). 

Alutom Formation 

(Ek 7). 

Amphibole, brownish-green; augite. 

Augite; amphibole, brownish-green. 

Pyroxene; hypersthene; amphibole, 

brownish-green. 

Amphibole, brownish-green; basaltic hornblende; sphene. 

Amphibole, brownish-green; augite. 

TABLE 8.-Insoluble residues and size distribution in five large 

samples of Guam, limestone 

Sample Insoluble residue 

Weight Grade size, In percent 

Formation or member 

No. Weight Per- +60 +140 +270 

(grams) cent mesh mesh mesh -270 

Grams of (0.25 (0.105 (0.053 mesh sam- mm) mm) mm) ple 

--------------- Janum Formation •.•••• Ts &-11 .• 801 31 3.9 2. 9 25.9 16.3 54.8 

Janum Formation •.•••• Ts &-12 .. 378 113 30.0 Tr. 18.0 25.4 56.6 

Maemong 

Member. 

Limestone Ed 7-2 .• 257 27 10.5 1.1 34.5 25.3 37.5 

Mahlac Member ..••••• Gj 15-L. 626 131 21.0 2.1 11.2 16.4 70.1 Alutom Formation _____ Ek 7 .•.• 464 36 7. 7 .6 34.0 21.5 43.8 

TABLE 9.-Amount of heavy residue (sp gr greater than 2.9) in 

five large samples of Guam limestone 

Heavy residue 

Formation or member Sample Weight (grams) Percent Percent 

of insol- of whole 

uble sample 

+140 +270 residue 

--------- Janum Formation _______________ Ts 5-11 •••. 0. 56 0. 25 2. 60 0.10 Janum Formation _______________ 'l's 5-12 .••• 3. 40 4. 20 6. 73 2. 0 

Maemong Limestone Member_. Ed 7-2 ..•• 5. 76 2.80 24.40 2. 55 

Mahlac Member---------------- Gj 15-L. .• .04 . 21 .19 . 04 Alutom Formation ______________ Ek7 •.••.. . 61 .15 1. 84 .14 

MINERALS PRESEN·T 

l\iagnetite and pyroxene, present in varying 

amounts, are the most abundant minerals in the heavy 

fractions: 

Af agnetite.-l\iagnetite is quite abundant in most of 

the insoluble residues; it occurs in euhedral octahedra 

and in irregular grains, and there is no appreciable 

alteration to iron oxides. The sharpness of the small 

crystals is shown in plate 21E. 

Ilmenite.-Black opaque grains not as strongly magnetic as magnetite are considered to be ilmenite. Ilmenite is not as abundant in these residues as magnetite. 

Pyroxene.-Augite in stout prismatic grains is the 

principal pyroxene occurring in these heavy residues. 

l\iany grains are etched at the ends of the c crystallographic axis into low pyramidal forms. The grains 

are fresh and pale green to colorless, and exhibit the 
D42 GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS 

usual optical properties of augite, Z I\ c generally 

being 38° to 40°. Many grains contain small black 

inclusions that may be magnetite. Augite from the 

insoluble residue of the J anum Formation is shown 

on plate 21A. The small euhedral crystals in some 

residues occur and these may be zoned. 

Hypersthene.-Hypersthene is a minor constituent 

in many of the residues. It is variable in appearance, 

from somewhat corroded grains having etching along 

the prismatic cleavage to massive fresh ·prismatic 

grains, some of which are individual crystals having 

well-developed pinacoid and prism faces. Inclusions 

are common. The hypersthene is strongly to weakly 

pleochroic in green and pink. 

Amphibole.-Greenish-brown amphibole is abundant 

in the heavy residues of a number of the limestones, 

and is the principal mineral in the residue from the 

Mahlac Member of the Alutom Formation. This 

greenish-brown amphibole has the following optical 

properties: index of refraction 'Y = 1. 6 7-1.68 ; a and {3 

just below 1.67, hence low birefringence. Z/\c= 14 °. 

The pleochroism is Z=greenish-brown; X and Y = 

brownish-green. The grains are sharply angular cleavage fragments. Small inclusions of opaque grains 

(probably magnetite) and colorless grains, ~vhich may 

be sphene, are common. This amphibole is illustrated 

in plate 210. 

Dark-brown basaltic hornblende is associated with 

the greenish-brown amphibole in small quantities; it 

probably amounts to less than 5 percent of the total 

amphibole present. This hornblende has indices of 

refraction -y=just above 1.69, and a and {3 between 

1.67 and 1.68. The extinction Z/\c varies from 0° to 

a maximum of 9°. The pleochrism is strong with Z= 

deep brown; X and Y =yellow-brown. The birefringence is moderate. 

A small quantity of a bluish-green amphibole having 

low birefringence also occurs in some of the residues. 

When fresh, it is strongly pleochroic with Z=greenish 

blue, X and Y =pale brown. The extinction, Z/\c is 

24 °. Some of this amphibole is chloritized and it then 

has a very low birefringence. Many chloritized grains 

fail to extinguish completely. 

Sphene.-Sphene is present in pale-brown slightly 

pleochroic angular grains having typical pleochroism. 

Zircon.-A few grains of euhedral zircon occur in 

residues from the l\1ahlac Member, for example, Gj 

11-1. 

In the light fractions ( sp gr < 2.9) , feldspar and 

clay aggregate grains are the principal minerals; 

quartz is present in some residues, as is opaline silica 

and zeolites: · 

Feld8par.-The feldspar is plagioclase in lathlike 

grains of small size. Twinning and zoning commonly 

occur, but many grains show neither, and they appear 

much like quartz except for their good cleavage and 

higher indices of refraction. The feldspar is about 

labradorite in composition. The zoned crystals were 

apparently phenocrysts in the lavas or unconsolidated 

tuffs. In general the feldspar is fresh and shows no 

alteration to sericite or to clay minerals, although 

some of the clay minerals have the appearance of 

altered feidspar. 

Clay aggregates.-The clay grains are of indefinite 

aggregate appearance under the microscope; their 

birefringence is low, and most aggregates are isotropic. Associated with the clay are iron oxides, which 

produce a general light grayish brown color. Additional information about these minerals is given on 

p.38. 

Quartz.-Quartz grains occur in some of the residues as angular fragments showing no rounding by 

abrasion and as euhedral crystals. The latter are 

squat crystals made up of trigonal trapezohedrons or 

rhombohedrons without prism faces, generally described as double pyramids. A characteristic feature 

of these quartz grains is the presence of gaseous inclusions in the form of numerous bubbles. Quartz 

of similar type was described from volcanic ash in 

Java by Loos ( 1924, p. 92, pl. 9). 

Opaline silica.-A small quantity of opaline silica 

was noted in a number of the residues, but it becomes 

an important constituent only in samples from the 

Maemong Limestone Member and from the Alutom 

Formation. In the limestone, this silica, which was 

identified in the silt and clay fractions as beta-cristohalite, forms internal casts of Foraminifera (pl. 21E). 

In some other samples, opaline silica is present as irregular fragments that have the appearance of having 

been part of the matrix of a rock or perhaps derived 

from volcanic glass. 

Zeolites.-Several zeolites were identified in the clay 

and silt fractions, and larger grains occur in some of 

the sand fractions of these residues. Some grains are 

small cubes having penetration twins. The index of 

refraction is just above 1.48, and the crystals exhibit 

weak uniaxial interference figures. Such crystals were 

identified as chabazite. Other zeolites are isotropic. 

A radiating fibrous zeolite, whose index refraction is 

just below 1.53, is heulandite or clinoptilolite, but 

there was insufficient material for complete optical 

identification. 

Apatite.-Brownish grains showing the structure of 

organic materials such as shell fragments, spicules, 

and Foraminifera occur in several of the residues. 
PETROLOGY OF THE LIMESTONES OF GUAM D43 

These grains are isotropic or feebly birefringent and 

are similar in appeaJ.'ance to the apatite in some phosphate deposits. 

GENERAL RELATIONSHIPS OF THE MINERALS 

l\1ontmorillonite is the most widespread of the clay 

minerals; it occurs in about 70 percent of the samples. 

Halloysite is the next most common mineral, occurring in 35 percent of the samples. l{aolinite, mixedlayered chlorite-montmorillonite, chlorite, and mica 

are the last abundant clay minerals. 

Of the nonclay minerals, no single group predominates. The zeolites, feldspars, goethite, and gibbsitenordstrandite each occur in 30-35 percent of the samples. Quartz is extremely scarce, and in no sample 

does it occur as the predominant mineral. The greater 

part of the samples can be divided, however, on the 

basis of the major nonclay minerals into two categories 

that are, for the most part, mutually exclusive. These 

are: (1) the feldspar-zeolite group, and (2) the iron 

oxide-nordstrandite group. Nineteen of the 26 samples examined fall into one or the other of these 

groups. The remaining seven samples are either principally amorphous, or contain only clay minerals, or 

exhibit some unusual mineral composition. All nine 

samples in the feldspar-zeolite group contain montmorillonite as their major clay mineral constituent. 

Only one contains more than a trace of kaolinite 

group minerals. Of the ten samples in the iron oxidenorstrandite group, seven contain halloysite or other 

kaolinite-group minerals as their major clay mineral 

constituent. Of the remaining three samples in this 

group, one contains montmorillonite as its major clay 

mineral, the second contains mixed-Jayered montmorillonite-chlorite, and the third contains no clay minerals. 

It seems justifiable, on the basis of the predominance of one type of clay or the other in these two 

groups, to expand their designations to include the 

most typical clay mineral. Group 1 would then be 

designated the feldspar-zeolite-montmorillonite group 

and group 2 the iron oxide-nordstrandite-halloysite 

group. As the nonclay assemblages have been shown 

most likely to be mutually exclusive, these are given 

precedence in placing. a sample in given category. 

'Vhere nonclay minerals are absent, the sample is 

classified on the basis of the predominant clay mineral. 

If the samples are thus classified and compared 

with their stratigraphic position, ·it can be seen that 

most of samples from a stratigraphic unit fall into a 

common category. Such a comparison is shown in 

table 10. 

TABLE 10.-Comparison of the stratigraphic position and mineralogic classification of santples of Guam limestone 

[The following samples are omitted from this table because or unusual mineral 

composition. as indicated: Sw 2-4, Mariana Limestone (apatite); Ut 6-1, Bonya Limestone (lithiophorite). X, present in sample] 

Group 1 Group 2 

Feldspar- Iron 

Sample zeolite- oxide· 

montmorll· nordstanlonite dite· 

halloysite 

Mariana Limestone: 1 

Pr 3-3................................................. X Agana Argillaceous Member: 

Ij 2-1. ..••• ____ •• -- ___ •••• _____ --- ____ -- _ --•••• -- __ --- ___ ••• -.--- _ _ X 

Janum Formation: 'l's 5-4 ____ • ________________________________ •. ___ •• _ ••• ____ ••••.•.•• _ X 

Ts 5·9·-··----··-·--·····-·-·-··----·---------·-------- ------------ X 

Rr 23-L.------------------------·-------·---------..••.•••.••.•. __ X 

Rr 5-L •• ------·-·---·---------------·--------------·-- -----·------ X Alifan Limestone: 

Ru 8-3 .... ·----·······----------------------·--------- ---·····---- X Di 2-L .•••••..• ·--------------------···------------·-- --···-----·- X Di 2-2.----•••• __ ••.••••••• ______ ••••••.• _ ••• __ • _ ••.• -_ . -- _ ••• ---.. X 

Ts 9-L .•• ---·---·------------------------·----·------- -------·---- X J 1 3-2. _. __ • __ • __ •• __ • ___ • --- ____ --- __ •••.•••• __ •.• __ .•• __ --- __ •• _. X 

Bonya Limestone: 

Rr 13-1. ___ ---·--------· -----------------------·-·---- X 

Gi 2-L----------·---------·--------·------------------ X Umatac Formation: Maemong Limestone Member: 

Gi 1-2a •• ---·------------------·---------· ·--------·--- X 

Ef 1-L •••••••• --··------·-----·----·-· ·---·---·------- X? 

Ed 1-1b •• ------------·------·-----·-·----------------- X 

Ee 2-L ..•.•••• ·----·· ••••••• ·-·---------·-------·----- X 

Ef 9-1a ••. -------·-·---·------· ----· -·---------·------- X Ii 5-2 .•..• --- __ • __ ••••••••• __ •••• -- __ • __ ----••• -----.-- X 

Alutom Formation: 

Fk 4-11. __ ·--------------··-----------·----·------·--- X Mablac Member: 

Gj 11-L ••••••••••••••••• -··-·-·--··----------·-----··- X 

1 Additional samples of Mariana Limestone were later examined and feldspar was identified. 

l\1ineralogic examination of residual soils formed on 

the volcanic rocks of Guam was made as a separate 

study (Carroll and Hathaway, 1963). However, it 

should be mentioned that the data evolving from this 

study show a predominance of halloysite, gibbsite, and 

iron oxide, particularly goethite, in the weathered 

products of the volcanic materials. :Hutton and Stephens (1956) have reported the occurrence of a similar 

clay-mineral assemblage in the soils of Norfolk Island 

that are formed on volcanic tuffs and basalts similar 

to those qf Guam. The occurrence of this same type of 

assemblage, with the exception of gibbsite, in many of 

the insoluble residues of the limestones suggests that 

the noncarbonate fractions of the limestones shown in 

table 10 as containing a group 2 assemblage are derived from the erosion of soil formed by the subaerial weathering of the volcanic rocks. On the other 

hand, the relative instability of feldspars and zeolites 

to weathering and the occurrence of zeolites as alteration products of volcanic ash (Bradley, 1930; Ross, 

1941; Coombs, 1952, 1954; Harris and Brindley, 

1954) suggest that the noncarbonate fractions of the 

limestones containing a group 1 assemblage have their 

origin in ash falls accompanying volcanic activity. 

The occurrence of montmorillonite, a characteristic of 

the grouP. 1 residues, supports a volcanic origin for 
D44 GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS 

this assemblage, as montmorillonite commonly occurs 

as the dominant mineral in bentonite, which are products of the devitrification of volcanic ash (Ross and 

Hendricks, 1945, p. 64-65; Grim, 1953, p. 361-364). 

The distinction, then, between a limestone containing 

.a :feldspar-zeolite-montmorillonite assemblage and one 

containing an iron oxide-nordstrandite-halloysite assemblage may well serve to distinguish periods of volcanic activity from those of quiescence and subaerial 

weathering in the source area of the noncarbonate 

fraction. 

Table 10 may be interpreted on the above-mentioned 

basis as indicating volcanic activity extending through 

deposition of the Bonya Limestone. After deposition 

of the Bonya Limestone, a period of subaerial weathering of the volcanic rocks is suggested by the occurrence of group 2 assemblages. A recurrence of volcanic activity, probably at some distance from Guam, 

during the time of deposition of the Mariana Limestone is suggested by the presence of group 1 assemblages. 

ORIGIN OF NORDSTRANDITE AND WOODHOUSEITELIXE MINERAL 

Nordstrandite was identified in five and possibly 

in six of the samples that contain group 2 mineral 

assemblages. W oodhouseite ( ~) was identified in two 

of the samples containing nordstrandite, and was tentatively identified also in the sample that contains 

nordstrandite ( ~). 

These minerals occur in an assemblage that contains 

weathering products, such as halloysite ·and iron 

oxides, derived from the alteration of . primary minerals. N ordstrandite has been fpund in soil in 

Borneo (Wall, et al, 1962, p. 264-265) but was not 

found in the soils on Guam. W oodhouseite ( ~) has not 

been identified in soils, but the related mineral alunite 

occurs in saline lake muds in Western Australia, 

where it has probably formed by oxidation of sui· 

fides in the lake sediments (Simpson, 1948, p. 45). 

I£ woodhouseite ( ~), like alunite, is stable in an acid 

environment (pH about 4 or less), then it might be 

expected to survive weathering under acid conditions. 

It could, therefore, be deposited as a detrital mineral 

accompanying goethite and hematite. A woodhouseitelike mineral has been identified by the authors in a 

gray marine clay from a drill core in Bermuda. In 

that situation the mineral is probably authigenic. 

N ordstrandite occurs as groups of radiating bladed 

crystals filling cavities in limestones. It is well shown 

in thin sections (pl. 40, D ; see also description of 

Ali fan Limestone). The nordstrandite crystals formed 

authigenically in fractures in the limestone. Thin 

sections show that nordstrandite crystallized after 

lithification and fracturing of the limestone. 

A possible origin for the development of nordstrandite in the limestones is as follows : Water draining 

from the volcanic rocks on Guam may pass through 

the limestones or along the interface of the volcanic 

rocks and limestones. While in the volcanic soil zone, 

this groundwater may have a pH as low as 4. The 

water can attack the calcium carbonate of the limestones and it can carry silica ( Si02) in solution, but 

not alumina (Ab03 ), which is insoluble between pH 

4 and pH 9. Removal of Si02 from clay minerals 

occurs if they are leached in a limestone environment 

(Carroll and Starkey, 1959, p. 83-87), but Al20a remains. The pH of water within a limestone, however, 

rises to between 8 and 9, and alumina then becomes 

insoluble in it. Slight lowering of the pH would cause 

the alumina to become insoluble again, and it could 

crystallize as nordstrandite or gibbsite. It is probable that the detrital halloysite found in many of the 

limestones is the source of the alumina that recrystallized to nordstrandite. 

Clay minerals in suspension in water moving 

through or over the volcanic rocks and deposited in 

the limestones may also be desilicated and contribute 

the alumina to the limestones. It is significant that two 

of the samples from Guam that contain appreciable 

quantities of nordstrandite (Di 2-1 and Ts 9-1) lie 

immediately superjacent to volcanic material. 

The occurrence of nordstrandite in limestones that 

are appreciable distances stratigraphically or geographically from contacts with volcanic rocks suggests 

that gibbsite may recrystalli~e to nordstrandite. The 

association of nordstrandite with minerals of probable detrital origin and its absence in limestones containing a group 1 (volcanic) assemblage of minerals 

suggest that detrital gibbsite may be an important, if 

not the only, source of nordstrandite in these limestones. 

MINERALOGICAL RELATIONSHIPS IN THE 

SAND GRADES 

The distribution of the ·principal minerals of the 

sand grades listed in table 7 and table 9 are arranged 

in figure 5, which indicates variations found in the 

limestones at ·various stratigraphic positions. The 

variations in occurrence and in quantity suggest that 

detrital material became available at different. geological times, but the picture is complicated by the 

fact that detritus has been available from two sources: 

directly as contributions from volcanic ash, and inby stream action from the. island to the surrounding 

directly as a weathering product that was transported 
PETROLOGY OF THE LIMESTONES OF GUAM 

MINERALS IN INSOLUBLE RESIDUES OF LIMESTONES, GUAM 

LLJ LLJ z 1- LLJ LLJ LLJ :::::; 1- ::X: ....J 0::: <( 

i= 1- 0 <( CD (/) 

(/) CD ~ LLJ LLJ LLJ a. 0 1- z 0::: t::: ::X: (/) 0::: 1- LLJ :::::; a. Q (/) (.!) a. (.!) ....J <( 

0:: 0 <( >- ::::> ::E LLJ :::::> LLJ H.R. PERCENT ::E ::X: <( <( u.. 0' (..) N 

Mariana Limestone: 

Sw 2-4 

Pr 3-3 Tr.• 

Agana A.,_ 

lj 2-1 

'" IVit:IIIUt:l 

Tr.• 

~ 

Alifan Li .... ~.v""'" 

1.5 

~ 1 Tr.• .. 

J 3-

Di2 ·1 • 

Janum :-..,, """""'' 

Ts 5-4 44.1 .. 

Ts 

Rr 

Ts 

Rr 

5-1 

23 

5-11 

5-12 

·1 

Tr.• 

6.7 

2.6 

~ 

~ 

Sonya Li , .. ~.v· ,,.. 

Gi 2 1 Tr.• 

Umatac Formation: ,. 

Maemong Limestone Member: 

.9 

Tr.• 

-

Of 9 1a Tr. 

24.4 

li 5-2 Tr. 

IAiutom ... v, r•auv• .. 

1 

Mahlac :\~.::;. ••u.:;• 

Ek 

Gj 11 

7 

1 

1.8 

4.4 

~ 

- _Gj 

FIGURE 5.-Distrlbutton of minerals in the sand-grade fraction (greater than 62 microns) in insoluble residues of selected limestones. 

Tr., a quantity too small to weigh; Tr.*, a sand fraction too small for bromoform separation, heavy minerals present in the sand; 

H.R. percent, weight percent or heavy minerals in insoluble residues. Quantitative estimates are from data given in table 8. 

690-219 0 - 63 - 5 

D45 
D46 GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS 

marine environment in which carbonates were accumulatil)g. ·_This process was in part described by Wentworth and Ladd ( 1931, p. 35) as characteristic of 

· ·Pacific Ocean islands that consisted of volcanic and 

nonvolcanic material. 

The evidence from the silt- and clay-size minerals 

together· with the character of the insoluble residue 

as shown in figure 4 can also be applied to the sand-

-grade minerals. Those samples that contain much 

·sand-grain material are the result of the direct deposi-

- tion of volcanic ash in the marine lime-depositing 

environment, whereas most of those containing much 

. silt and clay result from the addition of weathered 

material removed from soil surfaces by erosion and 

transportation to bays and inlets by rivers. Examination of soil profiles on Guam (Carroll and Hathaway, 

·1963') has indicated that sand grades are scarce in 

soils, as are unweathered grains of minerals, with 

the exception of magnetite. Thus, those samples in 

which the- heavy residue is very small (marked with 

as asterisk in fig. 5) are impoverished in ferromagnesian minerals that are directly derived from volcanic 

ash. 

The quantities of heavy minerals present in the insoluble residues vary in the limestones examined. 

·Those containing large quantities of heavy minerals 

are limestones of the Maemong Member and the 

J anum Formation (tables 7 and 9). The distribution 

of the individual heavy minerals is given in table 7 

and below table 9. Augite and magnetite are commonly present in all the residues, but some residues 

consist almost exclusively of augit~: or of amphibole. 

Hypersthene is present in small __ guantities in a number of the residues. Amphiboi~'s' ~ppear to be re-

. stricted in distribution and are ··ab$ent from the samples of Mariana Limestone e~amined~ Brownishgreen amphibole is much more abundant than brown 

basaltic hornblende, which was. £ound only in the 

residues of the Mahlac Member. It would seem that 

Eocene and Miocene limestones ·had a source of amphibole available to them, but that the Pliocene and 

Pleistocene limestones did not. -

Greenish-brown amphibole accompanied by a little 

basaltic hornblende is the principal ferromagnesian 

mineral in volcanic ash of the Mahlac -Member and 

t4e Alutom Formation, although· some augite and 

hypersthene are also present. In the Maemong Limestone Member, amphibole is subordinate to augite and 

only a little hypersthene occurs. Sample Ed 7-2 contains a large heavy residue. One: sample from the 

Maemong Member contains a concentrate of augite. 

The Bonya Limestone apparently .received volcanic 

ash and limestones of the overlying J anum Formation 

may have had additions of ash for most of the samples 

contain augite ·and two contain amphibole, although 

these minerals may be detrital. No amphibole was 

found in beds younger than the Janum. Only one 

sample of the J anum contains hypersthene. The 

Alifan Limestone and the Agana Argillaceous Member of the Mariana Limestone contain practica1ly no 

heavy minerals. Some limestones of the Mariana contain augite, and one sample, Ru 8-3 from the Alifan 

Limestone, has very abundant magnetite; both samples 

indicate a possible occurrence of volcanic ash additions, although both augite and magnetite persist as 

stable minerals in some of the soils and therefore 

could be detrital. The abundant hornblende and augite in samples of Eocene and Miocene age pose a 

problem in that these minerals are rare. to lacking in 

the under lying volcanic rocks (Stark, 1963) ~ 

Sample 

Ii 8-L ••.•.•..•••. 

Fk 4-L .•.•••.••• 

2 ___________ _ 

a ___________ _ 4 ___________ _ 

5 ___________ _ 6 ___________ _ 

7 ___________ _ s ___________ _ 9 ___________ _ 

10 __________ ·_ 

11. •.••..•••• Hj 1-L __________ _ 

2 ___________ _ 

3 ___________ _ 4 ___________ _ 

5 ___________ _ 6. __________ _ 

8 7------------ ___________ _ 9 ___________ _ 

10._ --------- 11 __________ _ 

Ek 7-L _________ _ 

2 ___________ _ 

3 ___________ _ 4 ___________ _ 

TABLE 11.-Samples studied 

I 

Slides I studied 

Location 

Alutom Formation 

1 One-half mi southwest of 

Talofofo V1llage. 

3 Naval Ammunition Depot; roadcut at main 

gate. 3 ___ .. do •••.• __________ •• __ ._ 2 ••••• do ____________________ _ 

1 ..... do ••• ------------------ 2 __ ._.do _________________ ----

2 __ ••• do .. ------------------- 2 ••••• do .. ------------------- 1 __ ._.do ..••• __________ • ____ _ 3 ••••• do ____________________ _ 

2 ~----do •••. ·----------------- 3 _____ do ____________________ _ 

3. Just"South ·or Witek Road, 

·northeast of Naval Ammunition- Depot, Loop Road. 

1 ••••• do •••.. ---------------- 3 __ _._._do.~--~----------------

1 1 .•.•. _____ do do~---·--"--------------- ____________________ _ 

1 ___ •• do ••••• __ •• ----. __ ----. 4 ____ .do ______ ._.:.----•• --••• 

1 ~ ~~~~~~~~~~-~~~~=~~~~~~~=~~~~~ _____ do ____________________ _ 

1 _____ do.~-------------------

3 100 yards east of bridge on north bank of Ayuga 

River, Santa Rita. 

2 ~ =====~~===~~==-============== ••••• do ..• ------------------

Description 

Fragment from Bolanos 

Pyroclastic Member of 

Umatac Formation. 

Fragments from limestonebearing breccia of Alu- tom Formation. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Fragments from limestonebearing breccia of Alu- tom Formation. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Bedded limestone of Alu- tom Formation. 

Do. 

Do. 

Do. 

Maemong Limestone Member of the Umatac Formation 

Fd 3-L .... ------

Fd 4-L _________ _ 

Fd 5-L _________ _ 

Ed 6-L _________ _ 

Ed-7-L .....•••••• 

2 ___________ _ 

Ed 5-L ......... . 

Ed 4-1. ......... 7 • 

Ed 3-L _________ _ 

It __________ _ 

3. Geus River, 330 feet above sea level. 1 Downstream 250 ft from 

Fd 3-1. 3 Downstream 50 ft from 

Fd 4-1. 2 Uppermost limestone bed at Geus Dam. 2 upstream 200 rt from 

Geus Dam. 2 Upstream 100 ft from 

Geus Dam. 2 Downstream 50 ft from 

Geus Dam. 4 Downstream 300 ft from 

Geus Dam. 2 Downstream 400 ft from 

Geus Dam. · 1 _____ do .. -------------------

Gray-brown globigerinid mudstone. 

Foraminiferal-algal breccia. 

Gray-brown globigerinid 

mudstone. 

Coral breccia. 

Limestone cobble from bed 

of coarse breccia. 

Tuffaceous foraminiferalalgal microcoquinite. Limestone from lapilli con- glomerate. 

Tuffaceous coral breccia. 

Limestone from bed of 

pyroclastic conglomerate. Tuffaceous, foraminiferalalgal breccia. 
PETROLOGY OF THE LIMESTONES OF GUAM D47 

TABLE 11.-Samples stndied-Continued 

Sample Description 

I 

Slides I studied 

Location 

Maemong Limestone Member of the Umatac Formation-Continued 

DC 9-1a .••••••••• ~ 

Ee 6-1. •••••.•.... 

Ee 7-L-----------

Ee 8-1. ••.•••.•••• 

Ef 2-1.-----------

2 •••••••••••• 

Db 11-L •• -------

2----------

a _________ _ 

3 

2 

2,000 ft northeast of 

Umatac. 

South fork Umatac River, 

150 ft altitude. 

South fork Umatac River, 

200 ft altitude. 

South fork Umatac River, 

250ft altitude. 

North fork Umatac River, 

350 ft altitude. 2 ••••. do ___________ ---- __ ----

White globigerinid mud- stone. 

Gray globigerinid mud- stone. 

Foraminiferal-algal microcoquinite. 

Foraminiferal-algal coqui- nite. 

Foraminiferal-algal breccia. 

Gray globlgerinld mud- stone. 10,000 ft east of Facpl Banded globlgerinid mudPoint, above Lamlam stone. Road. 

3 ••••• do_____________________ Foraminlleral microbreccia. 3 ••••• do ____________________ _ Do. 

Erosional remnants or Maemong Limestone Member In the Fena basin area 

HI3-L ••••••••••• 

2------------

3 •••• -------. 4 ___________ _ 

5 •••••••••••• 

6 •••••••••••• 

8 •••••••••••• Hi4-L ••••••••••• 

2 •••••••••••• 

3 •••••••••••• 

4 •••••••••••• 5.----------- Hl5-l. •••••••.••. 

2 •••••••••••• 

3 •••••••••••• 

5 •••••••••••• HI 7-1 •••••••••••• 

2 •••••••••••• 

3 •••••••••••• 

2 West side of large outlier 

north of Talofofo River, 

south or Witek Road. 

2 ••••• do ....••• __ -----------_ 2 ••••• do ____________________ _ 3 _____ do ____________________ _ 1 _____ do ____________________ _ 1 ••••• do ____________________ _ 1 ••••• do ____________________ _ 

2 North side of large outlier 

north or Talofofo River, 

south or Witek Road. 1 ••••• do ____________________ _ 1 __ ••• do _____ ••• ____ •• __ •••• _ 

1 ••••• do .•.•.•••••••••••••••• 1 ----.do ________ ------------- 3 Top of bill, 1.5 mi due east or Fcna Dam. north or Maagas River. 2 ••••• do _________________ • __ _ 1 ••••• do ____________________ _ 3 ••••• do ____________________ _ 

3 Base of Macmong limestone, south side of 

Mahlac River. 

3 ••••• do ..•.••••••••• : •••••.•. 3 ••••• do ..•• -~--·~··-:-·-~------

White crystalline coralalgal incrustatc lime- stone and Halimeda and 

foraminiferal-algal para- coquinites. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

White crystalline foraminiferal-algal paracoquinites. 

Do. 

Do. 

Do. 

Do. 

Pink to white, dense, 

foraminiferal-algal para- coquinltes. Do. 

Do. 

Do. 

White to red, globigcrinid mudstone. 

Do. 

Do. 

Boulders from the Bolanos Pyroclastic Me;,ber of the Umatac Formation 

011-1.-----------

II 6-4 ••••••••••••• 

18 __________ _ 32 __________ _ 

Fi5-1 ••••••••.••• 

Oi2-L 2.----------- __________ _ 

a ___________ _ 

Gi4-L •......... 

2 •••••••••••• 

3.----------- OJ 1-L ••••••••••• 

2 ___________ _ 

OJ 6-L •••••••••• 

2.-----------

3 •••••••••••• 

Cut in bank opposite Fena Screen· House. 

Talofofo Village Road ncar CAA transmitter. ••••. do •...•••• ______ ------- -•••• do ___________ .------ __ _ 

Bonya Limestone 

Bunker 1-AT-13, Naval 

Ammunition Depot. • _ ••• do ____________________ _ 

Quarry, 600 ft north of 

Lost River Pumping Station. __ ••• do •.•••••••. __ .•••••••• From Gi 2 locality, 50-60 · ft above Bolanos Pyro- clastic Member. 

2 ••••• do ______ --------------- 1 ___ •. do ••..•• ----- _________ _ 

1 Bottom of stream bed east of Bunker 2-AT, Naval 

Ammunition Depot. .••.• do ____________________ _ 

Basal part of Bonya Lime- stone at Bunker 2-AT, 

Naval Ammunition Depot. ••••• do ____ ._ •• __ • _____ ••• __ __ ••• do •..••••••••••••• ____ _ 

White, foraminiferal para- coquinites and micro 

paracoquinites. 

Do. 

Do. 

Do. 

Buff-white bedded foraminiferal-algal coquinite. Do. 

Pink, tan, and white bedded flat-lying foraminiferal coquinite. Do. 

Foraminiferal coquinite carrying cobbles and 

boulders of Maemong Limestone Member. 

Do. 

Do. 

Buff fm:aminlferal coqui- nite. 

Do. 

Gray-white dense breccia 

carrying mollusks, algae, and coral. · 

Do. 

Do. 

TABLE 11.-Sampl~s studied-:-Continued 

Sample 

I 

Slides I studied 

Location 

lh 5-1.---·-------

3.----------- 5.----------- 8.-----------

Ib 10-L ----------

2.----------

Ib 14-L----------

Jj 1-L ••••..•••••• 

Jj 6-2-------------

Jj 5-3 ____________ _ Jj 9-L_ __________ _ 

Rr 13-L 2.------------ _________ _ 

Ts 2-L-----------

Ut 6-L----------

Rr 19-L _________ _ 

Rr 20-L •.•••••••• 

Eh 3-L ----------

2.---------- 3.---------- 4.- ______ . __ _ 

5.---------- 6.---------- 7----------- 8.----------

Dl 2-L -----------

2 .. ----------

Ej 3-L -·---------

2 ___________ _ 

3.----------- 4.-----------

Ej 4-L----------

Db 12-L ---------

Db 13-L ________ _ 

Gj8-L __________ _ 

Jl 2-2-------------

Jl 3-2------"·-----

Ts 16-L----------

2 __________ _ 

3 __________ _ 4 __________ _ 

5 __________ _ 6 __________ _ 

7-----------

g 8 __________ 

.•• -------_ 

10 _________ _ 11. ________ _ 

12 _________ _ 13 _________ _ 

Bonya Limestone-Continued 

South bank of Talofofo 

River, J mi above its 

confluence with Ugum River. 1 ••.•. do ____________________ _ 1 ••..• do ____________________ _ 

1 __ .•• do .•. ____ •..••••• ____ •. 3 North bank of Talofofo 

River, across valley from 

Ih 5locality. 3 ____ .do ___________ ••• __ • ___ _ 

2 North bank of Talofofo 

River, one-fourth mi 

northwest of Ib 10 locality. 

One mi west of mouth of 

Togcba River, in channel. 

60 ft above river channel. 

1.2 mi west of mouth of 

Togcha River. _._ •• do ____ •• _ •• _ ••. _ ... _.-- Mouth of Togcba River 

gorgei one-half mi north 

of Ta ofofo Village. 1 ____ .do. ________ •••• --••..•. 2 In small coastal reentrant 2,200 ft southwest of 

farmhouse at Lujuna Point. 

2 Sink hole on terrace, 600 

ft north of beach at Jaum Point. 

Cobble (xenoclast) in Jan- urn Formation, 2,500 ft 

southwest of Anao Point. 

Lujuna Point, 50 ft above sea level. 

2 Lujuna Point, 110ft above sea level. 

Alif"an Limestone 

Base of Mount Almagosa, 1,000 ft northeast of road 

fork leading to Chepeck and Dobo Springs. 1 _____ do._------------------

2 _____ do._------------------

1 _____ do._------------------

3 _____ do._------------------

2 ----~do._------------------

2 --~--dO •• ----------------- 2 _____ do._------------------

2 ·Base of limestone, 6,000 ft 

2 -~:~~~~~ _o!_~~~~-~~~~~--- 4 .. Low on west slope of 

··Mount AUf an between ·. :·its peak and village of · Agat. 3 _____ do. _____ .--------- ___ _ 

2 _____ do. __ ----------------- 3. · •.••. do ... ----------------- 2 · Bassi bed in the Naval 

Ammunition Depot 

quarry. 2 Highest point on Mount 

Lamlam. · 

2 ____ .do ___ ------------- __ .• 2 Bunker2-AT, Naval Ammunition Depot . 6 1.5 mi west of Yona ______ _ 

2 One mi west of Yona at limestone-volcanic con- tact. 2,500 ft east southeast of M~unt Santa Rosa. 

2 _____ do._------------------

1 _____ do._------------------

. 2 ----~do._------------------ 2 ...•. do._------------------ . 2 _· ____ do._------------------

2 _____ do __ ------------------ 5 -"·--do._------------------ 3 _____ do._------------------

.1 .': ... do._----------·------- 2 _____ do._------------------

3 _____ do._------------------

3 ••••• do._------------·------

Description · 

White, buff, brown, and 

black manganiferous co- qulnite and breccia. 

Do. 

Do. 

Do. 

Buff bedded foraminiferal 

microcoquinite, dipping 15° E. · Do. 

Foraminiferal-algal cciqut- n1te. 

Foraminiferal-algal mlcroparacoquinlte. 

Gray to buff argillaceous foraminiferal paracoqul- nlte. 

Do. 

Thick-bedded argillaceous foraminiferal mlcropara- coquinlte. Do. 

Dense white bedded foraminlleral mudstone. 

Do. 

Black manganiferous ·foraminiferal limestone. 

Dense white foramlnlferal 

mudstone. 

Dense white bedded foraminiferal mudstone. 

Dense red detrital fossll1f- erous mudstone. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Pink slabby fossiliferous 

coquinoid limestone. 

Do. 

Dense red coral-molluscan 

coquinoid limestone. 

Do. 

Do. 

Do. 

Dense red mudstone. 

Dense red-mottled fossilll- erous coquinoid lime- stone. Do. 

Buff-white argillaceous microparacoquinite. 

White foraminiferal microparacoquinite. 

Do. 

Pink, white, and buff, 

miniferal microparaco- quinites and breccias. 

Do. 

Do. 

Do. 

Do. 

Do . Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 
D48 GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS 

TABLE 11.-Samples studied-Continued 

Sample 

T~ 7-L-----------

3 2-------~---- ___________ _ 

4 ___________ _ 

Ru 8-3. _ ---------

Ts 9-L.·----------

Nr 1-7a __________ _ 

Ov 7-L ----------

2.----------

~- ----------

Ov 4-L ----------

Ov 5-L -------- .. 

Pv 9-L----------· 

Pv 10-2 __________ _ 

St 8-L. ----------

Bv 5-L-------.----

Rr 2-1------------

Rr 3-L-----------

Rr 5-L-----------

Rr 8-L-----------

Rr 9-L-----------

Rr 23-1. _________ _ 

Rr 14-1. _________ _ 

Ts 5-4 ___________ _ 

4a __________ _ 

6 ________ ----

7 ___________ _ 9 ___________ _ 

Uu 1-3 __ ---------

Po 1-1. __________ _ 

Rx 18-L---------

Rx 19-L ________ _ 

Rx 20-L--------- Rx 21-L ________ _ 

Sx 1-L __________ _ 

Sx 2-L __________ _ Tw 7-L _________ _ Kr 2-l_ __________ _ 

2 ___________ _ 

3 ___________ _ 

Kr 3-L. _________ _ 

Description 

I Slides I studied 

Location 

Alifan Limestone-Continued 

2,500 ft northeast of Mount 

Santa Rosa. 

1 _____ do._------------------ 2 _____ do._--------------"--- 3 _____ do.---·----------------

3 4,200 ft west of Salisbury Junction. 

2,450 ft east southeast of 

Mount Santa Rosa. 

Barrigada Limestone 

Deepest level, Harmon 

Quarry. 

NCS Finaguyen Transformer well, 94 ft. 

NCS Finaguyen Transformer well, 404 ft. 

4 NCS Finaguyen Transformer well, 543 ft. 

1,000 ft northeast of NCS 

Finaguyen. 2 1,000 ft north of NCS 

Finaguyen Gate. 

2, 750ft southwest of Potts 

Junction. 

2 1,870 ft southeast of Potts 

Junction. 

Foot of north slope of 

Mount Santa Rosa. 

3,000 ft northeast of en- trance to Andersen Air 

Force Base. 

Janum Formation 

2 400 ft north of house at Lujuna Point. 

3 _____ do. __ ----------------- 2 100 ft northeast of house at Lujuna Point. 

Ori trail to Lujuna Point, 

150ft altitude. 

3 On trail to Lujuna'Point, 200ft altitude. 

On trail to Lujuna Point, 

175 ft altitude. 

2 2,200 ft southwest of 

Lujuna Point. 

2 Uppermost bed at type section, Catalina Point. 2 10ft below Ts 5-4 ________ _ 

30ft below Ts 5-4 ________ _ 

3 50ft below Ts 5-4 ________ _ 3 70ft below Ts 5-4 ________ _ 

Anao Point. _____________ _ 

Mariana Limestone 

On cliff rim, Tarague Beach. _____ do._------------------

_ ____ do. __ ----------------- ----.do. __ ------- _________ _ _ ____ do._------------------

Tarague Beach, mean tide 

level. 

2 Tarague embayment _____ _ 1 Tarague cliff rim _________ _ 

2 Ypao Point, core-depth 15-20 ft. 

Ypao Point, core-depth 30-35 ft. 

2 Ypao Point, core-depth 45-50 ft. 

Ypao Point, core-depth 35 

ft. 

Pink-orange argillaceous limestone, incrustate in 

part, dolomitic in part 

(7-4). 

Do. 

Do. 

Do. 

Pale orange-pink argilla- ceous mudstone. 

Massive dense pink-yelloworange foraminiferal microparacoqulnite. 

Foraminiferal 

quinite. Do. 

Coral breccia. 

Algal mudstone. 

microcoWhite soft to hard foraminiferal mudstone. 

White dense foraminiferal 

mudstone. 

Massive cream-colored fo- . raminiferal mudstone. 

Do. 

White foraminiferal microparacoquinite. 

White foraminiferal mud- stone. 

Friable yellow-gray foraminiferal microparacoqui- nite. 

Do. 

Indurated orange-pink tuffaceous microparaco- quinite. Do. 

Do. 

Do. 

Friable light-pin:K foraminiferal microparacoqui- nite. · Friable brown tuffaceous 

microparacoquhiite. Brown tuffaceous microparacoquinite. ; Friable white foraminiferal 

microparacoquinite. Do. 1 

Dense white foraminiferal 

microparacoquinite. 

Friable orange-pink tuff- aceous micropa"racoqui- nite. 

Massive incrustate lime- stone. 

Coraliferous hicrustate 

limestone. 

Do. 

Algal incrustate lhnestone. 

Coral-algal incrustate lime- stone. ' 

Do. 

Do. 

Do. 

Coral breccia. 

Do. 

Do. 

Do. 

TABLE 11.-Samples studied-Continued 

I 

Slides I studied Sample Location · Description 

Mariana Limeston-Contlnued 

Po 1-2 ___________ _ 

Rx 7-L----------

Tw 8-L---------- Jj 6-l__ __________ _ 

Pr 3-3. _ ----------

Ij 2-L------------

Qu 4-L ----------

2.----------

Jh 1-L __________ _ 

2 ___________ _ 

Uu 3-L. ---------

Rr 16-L .. -------- Rx 17-1_ _________ _ 

Sw 2-4.----------

Cliff rim at Taguan Point. 

500 ft south of Tarague cliff rim. 

Foot of Tarague cliff. ____ _ 6,000 ft north of Talofofo 

village. 

Foot of Barrigada Hill, 

north side. 

West side of Camp Witek .. 4,600ftnortb ofMataguac bill. _____ do._------------------

2 1,500 ft east of Talofofo 

village. 2 _____ do. __ ----------------- 3 Baseoftypesection, Lafac 

Point. 2 Lujuna Point ____________ _ 

1 Tarague embayment _____ _ 3 2,600 ft east-southeast of 

Tarague well No.4. 

Buff Halimeda paracoqu~­

nite. 

Buff Halimeda and molluscan paracoquinite. Do. 

Molluscan microcoqui- noid limestone. 

White Turitella-bearing co~_noid limestone. 

Orange detrital coquinoid limestone. 

Buff pelecypod-bearing coquinoid limestone. 

White coral-foraminiferal 

breccia. 

Do. 

White friable foraminiferal 

microcoquinlte. White foraminiferal breccia. 

Do. 

Phosphatic limestone. 

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.1 
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Hutton, J. T., and Stephens, C. G., 1956, The paleopedology of 

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. lau Islands, Intelligence Division, Office of the Engineer, 

General Headquarters, Far East Command, 99 p. 

Johnson, J. H., 1954, An introduction to the study of rock 

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p. 362-389. 

Kulp, J. L., Turekian, K., and Boyd, D. W., 1952, Strontium 

content of limestone and fossils: Geol. Soc. America Bull., 

v. 63, p. 701-716. 

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.D50 GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS 

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A Page Acropora ____________ -" _______________________ 28,32 

Agana Argillaceous Member of Mariana 

Limestone, description___________ 34 

heavy minerals __________________ .. ___ .. ____ . 46 

Algae, use as indicator of limestone facies_____ 20 

Algal incrustate limestone, defined __________ _ 

Alifan Limestone, carbonate minerals _______ .... 8 

description_______________________________ 28 

dolomite__________________________________ 15 

heavy minerals _______________________ ~--- 46 

insoluble-residue content__~- ________ .... c .. _ 9 

microstylolites_________ ______ __ __ __ __ _ _ __ _ 13 

Nordstrandite deposits___________________ 14 

Alunite _______________________________________ 38,44 

Alutom Formation, description_______________ 21 

silicification ___________________________ .. __ 12 

Amphibole ____________ 1, 9, 40, 41, 42, 245, 46, pis. 8, 21 

Amphistegina ____ _____________ 20, 21, 33,34, pis. 15,17 

Analcime __________________________________ -"_ 40 

Apatite ______________________________ 1, 9, 38, 40, 41,42 

Aragonite .......... --------------------~ ·1, 8, 11, pis .. 9,17 

shells, stability and recrystallization _____ ·_ 11 

Archeolithothamnium ______ c __ )_~·------------~- pl. 2 

Articulate coralline algae, use as indicator of 

limestone facies_"_________________ 20 

Augite ________________________ 1, 40, 41, 42, 46, pl. 21 

B 

Barrigada Limestone, description_____________ 30 

Basalt, chemical analyses_____________________ 13 

Basin facies, Alutom Formation______________ 23 

Maemong Limestone Member of Umatac Formation _______________ c________ 25 

Beta-cristobalite _________________ ---~ ----- 1, 9, 25,40 

See also Opaline silica. · Biplanispira __________________ ,.:.: ____ " ____ __ __ _ 22 

Bonya Limestone, description-----"---------- 26 manganese minerals __________________ c __ ~ 12 

Breccias, Bonya Limestone___________________ 27 

defined __________________ ------~-------___ 7 

Mariana Limestone~_____________________ 33 

c 

Calcareous algae, replacement________________ 11 

use as indicator of limestone facies________ 20 

Calcarina ______________________________ c 6, 34, pl. l7 

Calcite ______________ 1, 8, 10, 12, 15, pls. 4, 7, 10, 17, 18 

chemically deposited .. ____ c_______________ 12 

crystals, Alifan Limestone________________ 15 

Calcitic dolomite, defined____________________ 8 

Calcium C\1fbonate___________________________ 8 

Carbonate minerals ___________ . _______ c________ 8 

Carpenteria ________________________ 5, 21, 33, pl. 15 

Cementation, Alifan Limestone______________ 16 

plfosphate pellets_________________________ 18 

Chabazite____________________________________ 40 

Chemical composition________________________ ·8 

Chlorite _______________ ---c_~- _____________ 1, 9, 38,43 

Classiftcation,limestone______________________ 5 

limestone facies__________________________ 18 

Clay aggregates .. _____________ ...... c .... 1, 9,38, 40, 41,42 

Clay grades, mineralogy______________________ 98 

Clay minerals ___ -- ____________ .. _-- __________ _ 

INDEX 

[Major headings are in italic] 

Page 

Clinoptilolite .... ____ .... c .... _____________________ .. 42, 45 

Compaction, relation to formation of microstylolites _________ .. ___ ------_______ 13 

Coquinite, defined____________________________ 6 

Coquinoid limestone, defined_. _______________ _ 

Coralfragments, recrystallization ___ .. ___ .. __ .__ 11 

use as indicators oflimestone facies_______ 21 

Coralline algae, resistance to replacement_____ 12 

Crickmay, G. W., quoted ___________________ " 10 

Cristobalite ____________________ ------~----- _ 1, 9, 38 

Crustose coralline algae, use as· indicator of 

· limestone facies_----------------- 20 

Cycloclypeus .... _ '20; 21, 27, 31, 32, 34, 35, pis. 13, 15; 16, 17 

t: D 

DaSycladacean algae; use as indicator of lime~ 

· · '. stone faci!)s _________ "--~---------- 20 

Dinnin, J. 1., analysL-------------------"---- 9 

Dolomite, Alifan Limestonec ___________ cc .. 15, pl. 5 

bolbinitic limestone, defined_________________ 8 · 

~ Dolomitization_·-------------~-'--~----"------- 8,16 

·Dunnington, H. V., quoted ____ -------------- 13 

E 

Environmental reconstruction, Alifan Limestone ______________ ------------~-_ 30 

Aiutom Formation ___ ----'---------------- . 23 

Barrigada Limestone ___ ·------------------ 31 

Bonya Limestone________________________ · 28 

Janilm Formation _______________________ ~ 32_ 

Maemong Limestone Member of Umatac 

Formation _____ ~------------------ sa 

Mahlac. Member of the Alutom Formation ___ ---------~-- _______ -------- 23 

Mariana Limes.tone .. __ ---------~--------- 34 

Eocene (b) limestones, description____________ 21 

silicification .. _______________ -------------- 12 

F 

::~:::.ad~~~d~---_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-:_-_ _-_~~· P\~ 

Favia ____________ ------------------------·--~-- 32. 

Feldspar _______________ 1,9,38,40,41,42~43,45, pl.16 

Fieldwork ______________________ c ~ --~ ____ ---- _ 2 

Foraminifera, preservation ____ --~------------ 11 

use as indicator of limestone facies ___ "-·-~- 20 

Forereef faCies, .Alutom Formation, description .. ____________ -----------------· 

Bonya Limestone .. _ c _ ----------c--------- Maemong Limestone Member of Umatac Formation ____ c· ______ " ___________ _ 

Mariana Limestone __ ----·--------------~ 

Forereef transition facies, Bonya Limestone .. _ 

Maemong Limestone Member. of Uuiatac 

22 

26 

24 

34 

27 

Formation~---"- ___ c ____ --"--~"--- S4 

Fossil ghosts ........ --------"--------~~"---- 15, 17, pl. 5 

Fossils, facies indicators .... _______ c____________ 19 

susceptibility to recrystallization_________ 10 

Fractures in limestone. _________ .:,____________ 11 

Fra:e;~~J:~;~~:t-o~~~~~~~~-~~~-~~~-~~~~~~~~~~ mudstone .. _--"---- _________ : ______ -------

paracoquinite ______________ • ____ - __ - _____ _ 

particulate limestone, defined~------------

7 

7 

7 

G Page 

Gibbsite ____________________________ 1, 9, 37, 40, 43,44 

Globiqerina ________________________________ c _ _ 21 

Glob:::~:~:if~~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~-~ ~ 21, ~~ tumida ____ _______ .. ____________ ------------ 25 

Goethite _________ ------------------- 1, 9, 38, 40, 43,44 

Goniolithon .. _ ---------- ____ ------------------- 20 

Gypsina ____________________________ 5, 27, pis. 13, 15 

H 

Halimeda_____________________________________ 1, 6, 

7, 8, 9, 10, 11, 15, 20, 21, 22, 23, 24, 31, 33, 

34, 35, 36, pis. 7, 10, 14, 17, 18, 19. 

Halloysite ____________________________ 1, 38, 39, 40,43 

1 • Hatch, F. H., and Rastall, R. H., quoted____ 5 

Heavy minerals ________ ---------------------- 9, 40 

Hematite _________ --c- ________ -------·- _. 1, 9, 38, 40,44 

Heteroste(jina ___ --------------------- 2, 5, 20, 35, pl. 7 

Heillandite .... ------------- ____________________ 40,42 

Hinnotrema __ ___ ~ _______________________ .. ____ .. 5, 21 

Hornblende ________ ~--------- 1, 9, 40, 41, 42; 46, pl. 21 

HyPersthene:----~--"----- 1, 9, 40, 41, 42, 45, 46, pl. 21 

Ilmenite~-----"--"·"·---- __________ -------- 1, 9, 40,41 

Inci:ustate, deftiled __________ c:.______________ 5 

Incrustate limestone, defined ...... -~------------ 5 

Insoluble residue, Mahlac Member of Alutom 

Formationc ______ . ____ c ____ · __ c----- 23 

petrography _________ c .. ______ -~___________ 97 

qui:mtity ___ .. c .... : _______ : ________ _::________ 38 

Irvllig, Earl, quoted .. ~------------------------ 17 

J 

Janum Formation, description________________ 91 

nordstrandite ________________ -----c----.. -- 15 

K 

Kaolinite ________________ · ___________ --- 1, 9, 38, 40,43 

L 

Lagoonfacies, Alifan Limestone______________ 29 

Alutom Formation________________________ U 

Maemong Limestone Member of Umatac 

Formation ...... c---.-----------"--~-- · 24 

Mariana Limestone~----------------•---- 33 

Lepidocyclina,_"--~------~-----------~- 27, pis. 12,13 

Limestorie facies, terminology and classification __ ------·"--------------------- 18 

recognition_-~---------"------------------ 19 

Limestone formations, description..-----~----- 21 

Lithiophorite ..... ---~---------------- ___ i, 9,12, 38,40 

Lithologic reversal, Alifan Limestone_________ 16 

Lithophyllum_ -~- ____ ----------- ______ "______ _ 5 Lithotliamnium. ____ __________ :.:______________ 5, 20 

M 

M6emong .. Limestone Member of Umatac 

Formation, description___________ 23 

mineralogy of sand grade_--"------------- 40 

present-day analogue.-------------------- !85 

sand grade, mineralogic relationships_____ 46 

D51 
D52 

Page 

Magnesium carbonate __________________ c_____ 8 

Magnetite ___________ 1,9,38,40, 41,45,46, pis. 8, 16,21 

Mahlac Member of the Alutom Formation, 

description_---------------------- 23 

heavy minerals.-------------------------- 46 

insoluble-residue content _____________ c.--- 9 

Manganese--------------------------- 1,26,37, Pl. 13 

minerals in Bonya Limestone_____________ 12 

Manganiferous limestone, defined ___ --------- 8 

Marginopora __________________ -----------"---- 34 

Mariana Limestone, description-------------- 32 

Halimeda segments.---------------------- 11 

phosphate qeposits_______________________ 16 

Matrix, recrystallizatio~----- _____ . _ -- ___ .--- _ 10 

Metasomatic limestone, defined.------------- 8 

Mica.~---- ----C-----------------------. 1, 9, 38, 40,43 

Microparacoquinite, defined__________________ 7 

Microstylolites, Alifan Limestone __________ 13, pl. 3 

Mineralogical analysis, insoluble residues____ 37 

~ineralogy, clay and silt grades____________ 38 generaL ______________ ---_________________ 8 

sand grades __ ---------------------------- 40 

Minerals, carbonate. ___ --.--._---.. c--•• ----- 8 

general relationships ___ ------------------ 43 

noncarbonate ____ ----- ________ --- _ _ _ _ _ _ __ _ 9 

Miocene (e) limestone, silicification___________ 12 

Mollusks, use as indicators of limestone facies. 21 

Montmorillonite. __ --------------~- 1, 9, 38, 40, 43,44 

Mosaic-fossil relationshiP-----------~--------- 10 

Mud, crystallization and recrystallization___ 10 

defined_--------- ___ .--- ___ -------. __ -.--_ 

Mudstone, defined. ______ ._.--- ____ ---.... --. Myers, E. H., quoted _____________ c__________ 20 

N Noncarbonate minerals ______________________ _ 

Nordstrandite ___ 1, 9,14,15, 37, 38, 39, 40, 43, 44, pl. 4 

0 

Off-reef deep-water facies, Bonya Limestone._ 27 

Off-reef shallow-water facies, Alifan Limestone.---------------------------- 29 

Alutom Formation._--------------------- 22 

Opaline silica •• -------------- 1, 9, 38, 40, 41, 42, pl. 21 

. See also Beta-cristo balite. 

p 

Palau phosphate pellets, compared to. Guam 

pellets ____ ------------------------ 17 

INDEX 

Page 

Paracoquinite, defined________________________ 7 

Particulate, defined ___ ----------------------- Particulate limestone, defined________________ 6 

Pellatispira_---- _--------------.------------.- 22 

Petrography, insoluble residues_______________ 37 

Mariana Limestone.~-------------------- 16 Pettijohn, F. J., quoted ___________________ 5, 6, 7,18 

Phillips, H. F., analyst_______________________ 9 

Phosphate deposits, Mariana Limestone______ 16 

pellets .. ---------------------------- 17, 18, pl. 6 

Phosphatic limestone, defined________________ 8 

Plagioclase feldspar-------------- 1, 9, 38, 40, 41, pl. 8 

Pocillopora. _ ---------- __ -------- __ ----------- 32 

Pore space, coquinite.------------------------ 7 Porites ________ -- ______ -_ .

7 

__ ------ ___ -- ______ - 28 

Porolithon. ----------------------------------- 33 

Porosity, AUfan Limestone.------------------ 29 

Bonya Limestone ___ --------------------- 27 

Maemong Limestone Member of Umatac 

Formation ________ ----- _______ ---_ 24 

Pyrolusite ______________________________ 1, 9; 12, 38 

Pyroxene ____________________________ 40, 41, pl. 8 

Q 

Quartz _________________ 1, 9, 38, 40, 41, 4S;-43, 45, pl. 3 

R 

Rastall, R. H., and Hatch, F. H., quoted.... 5 

Recrystallization, Alifan Limestone._________ 29 

Bonya Limestone________________________ 26 

dolomite, Alifan Limestone______________ 15 

Mariana Limestone______________________ 33 

original matrix and primary skeletal ma- teriaL___________________________ 10 

sequence_________________________________ 10.' 

Reef complex, defined________________________ 19 

Reef-wall facies, Alifan Limestone____________ 28 

Alutom Formation_______________________ 22 

Maemong Limestone Member of U matac Formation. ____________ -- ____ "·___ 24 

Mariana Limestone._____________________ 32 

Residual soilS, mineralogy____________________ 43 

·Rock descriptions, Alifan Limestone__________ . 28 

Alutom Formation."c·------------------- 22 

Barrigada Limestone_--_----------------- 30 

Bonya Limestone~-------------------·--- 26 

Janum Formation________________________ 31 

Rock dP..scriptions-Continued Page 

Maemong Limestone Member of Umatac 

Formation________________________ 24 

Mahlac Member of Alutom Formation___ 23 

Mariana Limestone~--------------------- 32 

Rotalia ...... --------~------- 15, 21, 28, 29, 30, pl. 14 

beccariL _____ ----- ---------------------~- 30 

8 

Sand grades, mineralogic relationships________ 44 

mineralogy ______ c .• --------·----~--------- 40 

Scott, P. W., analyst_________________________ 9 

Separation of limestone samples______________ 37 

Siliceous limestoPe, defined __________________ _ 

Silicification .. -~--- ________ -------------______ 12 

Silt grades, mineralogy_______________________ 38 

Sphene ________________________________ 1, 9, 40, 41,42 

Strontium carbonatiL.----------------------- 8 

Structure, incrustate limestone________________ 5 

Subaerial erosion_____________________________ 13 

T 

Taylor, W. R., quoted_______________________ 20 

Texture, particulate limestone________________ '6 

Thin sections, Alifan Limestone ___________ 13, 15,29 

fractures ___________ ., __ ------------------_ 11 

incrustate limestone. __ -------------------

Maemong Limestone Member of Umatac 

Formation_________________________ 23 

Mahlac Member of Alutom Formation___ 23 

manganiferous limestones_________________ 12 

Mariana Limestone ______________________ . 33 

mudstone________________________________ 7 

recrystallization sequence----------------- 10 Trochus _ _ _ _ __ _ _ __ _ _ __ _ __ __ _ ___ _ __ _ _ ___ _ ___ _ __ 33 

Turbo. __________________________ --------_____ 33 

Turritella filiola .... _. ---- __________________ --- 34,35 

v Volcanic contamination ________________ 24, 26, 28,31 

Volcanic material, alteration__________________ 9 

identified _________________ ----------______ 10 

Volcanism _________________________________ 23, 25,32 

w 

· ... Woodhouseite-like mineraL __ ... ______ 1, 9, 38, 40, 44 

z 

Zeolites _______________________ 1, 9, 38, 40, 41, 4£, 43; 45 

Zircon ... --- __________ ---------·------ __ 1,·9, 40, 41,42 
.. . ·. ~ 

" 

PLATES l-21 
PLATE 1 

A, B. Section through a mollusk shell, sample Fk 4-5, X 20. 

A .. Plane-polarized light. The shell section is filled with a coarsely crystalline 

mosaic of clear calcite that still shows strong traces of an original detritusfine-grained matrix relationship much like the matrix now surrounding 

the shell. This crystalline fill is in sharp oontact with the inner edge of 

the shell wall. The shell wall is cut by fine cracks, generally radial or 

zigzag with respect to the wall surface. It retains a light-yellow color 

and, according to the color pattern, apparently part of its original shell 

structure, a laminar inner layer and a prismatic outer layer. 

B. Crossed nicols. The present crystalline structure overides the color pattern and 

apparent traces of original structure described above. The radial and 

zigzag fractures seen in plain light are the borders of optically discontinuous calcite crystals in the shell mosaic. Although the outer shell 

wall is finely pitted, recrystallization has not proceeded outwards into 

the matrix. 

C. Sample Hj 1-1, X 8. A two-stage sequence of void filling. The branching vein ends in 

small fractures along which minor movement has taken place. In completely 

particulate rock this type of vein suggests postlithification solution along fracture 

lines. Following the widening of the fractures, a deposit of finely fibrous calcite 

partially filled the openings. After this chemical deposition, the remainder of the 

cavities were filled with carbonate mud detritus. The detrital fill is vaguely banded. 

In some veins the mud fill has begun to recrystallize into clear calcite. Completion 

of this recrystallization would make the rock appear to have been subject to two 

periods of chemical deposition rather than to one chemical and one detrital period. 

D. Sample Hj 1-5, X 25. Limestone in which a fracture system is well developed. These 

fine fractures are completely sealed by crystalline calcite. The cracks swell and 

pinch out but maintain their angular relationships. Minor offsetting of a fraction 

of a millimeter is common. These fractures are restricted on Guam to limestones 

of Eocene (b) age. 
GEOLOGICAL SUHVEY PROFESSIONAL PAPER 403-11 PLATE I 

A B 

C D 

SECONDARY FEATURES IN LIMESTONE FROM THE ALUTOM FORMATION 
PLATE 2 

A, B. Silica-filled cavity in sample Ek 7-4, X 25. 

A. Plane-polarized light. The roughly triangular white area might be taken for 

a void except for the fine fractures that cross it. The clear calcite lining 

of the former opening stands out in high relief above the silica filling. 

Not shown in the photomicrograph are fractures that lead into the cavity 

from the lower and upper right hand corners of the triangle. 

B. Crossed nicols. The silica filling and the calcite lining show contrasting crystal 

forms. The silica has taken the form of fibrous sheaves that show undulatory extinction. These extend inward from the calcite lining, perpendicular to the walls of the original cavity. In places the silica is in the form 

· of a fine-grained mosaic of anhedral crystals. The clear calcite lining 

shows up as a uniformly thick rim of optically discontinuous crystals. 

C, D. Metasomatic replacement by silica in sample Ek 7-4, X 25. 

C. Plane-polarized light. The white areas show the silicified parts of the rock. 

The dark surrounding area is unaffected limestone; the gray indistinct 

areas within the clear silica are occupied by incompletely altered limestone. The contact between the unaltered limestone and that completely 

replaced by silica is sharp and definite. The texture of the limestone has 

been subdued but not effaced by the silica replacement. Where the 

contact between replaced limestone and the original rock passes through 

an easily recognized fossil, organic structures can be definitely followed 

across the contact. This feature is shown in the upper left corner of the 

photomicrograph. The curving row of evenly spaced dark ellipses are 

sporangia in a fragment of Archaeolithithamnium, which straddles the 

silica-limestone contact. This line of sporangia can be followed from 

unaltered algae into fibrous silica. 

D. Crossed nicols. The habit of the silica in the replaced part of the rock is 

roughly the same as that in the void filling described above. In this 

type of silicification, fossil fragments are more clearly replaced than is 

the matrix material, particularly the fine-grained mud. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 403-D PLATE 2 

A B 

c D 

SILICIFIED LIMESTONE FROM THE ALUTOM FORMATION 
PLATE 3 

A. Sample Eh 3-5, X 15. A band of bifurcating and intersecting microstylolite seams cross 

the thin section from left to right. These seams have amplitudes of fro.m 0.1 to 

0.25 mm. The microstylolites are closely spaced; in one section, 14 seams were 

counted in a distance of 10 mm. Each seam is rendered distinct by an included 

film of reddish-brown clay. Interpenetration of fossils is common along each seam. 

The lithology of this specimen is identical to that of sample Eh 3-2 (pl. 14D). 

B. Sample Di 2-1, X 160. Detail view of a mollusk shell-matrix contact along a microstylolite seam. The clay concentration shows up well as the dark band between the 

solid shell on the left and the porous recrystallized fine-grained matrix on the right. 

C. Sample Di 2-1, X 90. Detail view of a mollusk shell having a microstylolite contact 

on the right and a normal contact on the left. 

D. Sample Gj 8-1, X 100. A euhedral crystal of quartz typical of the Alifan Limestone in 

central Guam. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 403-D PLATE 3 

A 

B 

c D 

MICROSTYLOLITES AND QUARTZ IN THE ALIFAN LIMESTONE 
PLATE 4 

A, B. Sample Ej 4-1, X 90. Recrystallized carbonate mud. 

A. Plane-polarized light. The otherwise featureless microcrystalline matrix 

is cut by a system of fine dark serpentine boundary lines that separate 

areas of differing relief. 

B. Crossed nicols. The rock is revealed to be made up of an intricate intergrowth of calcite in the form of a mosaic of single crystals. This type 

of recrystallization has only been seen in the lagoonal mudstone of the 

Alifan Limestone. 

C, D. Sample Dh 12-1, X 50. Nordstrandite cavity filling. 

C. Plane-polarized light. N ordstrandite occupies light area in the center of the 

photomicrograph. This area was evidently a solution cavity within 

which was deposited a thin layer of calcite. Fractures, now calcitehealed for the most part, lead away from the former cavity. 

D. Crossed nicols. Nordstrandite shows up as bladed crystals radiating from 

a point in the original cavity. Some nordstrandite has been deposited 

within incompletely filled fractures. The nordstrandite is evidently a 

late deposit that followed postdepositional lithification, fracturing, 

subaerial solution, and calcite deposition. 

. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 403- J) PLATE 4 

A B 

c D 

RECRYSTALLIZED MUD AND NORDSTRANDlTE IN THE ALIFAN LIMESTONE 
PLATE 5 

A. Sample Ts 16-8, X 20. The original structure of a coral fragment is preserved only as 

fine dust lines that define a ghost. The original interseptal openings are now 

lined with small euhedral crystals of dolomite. Both the coral ghost and the dolomite linings are invested in a clear calcite mosaic, fndividual crystals of which fill 

in several interseptal openings. The Foraminifera test to the lower left is enclosed 

within a single water-clear calcite crystal. 

B, C. Sample Ts 16-8, X 140. Detail view of the dolomite crystals lining interseptal 

openings. Many of the rhombs show irregular centers within an euhedral shell. 

The clear structureless areas are late-stage calcite fillings. 

D. Sample Tt 7-4, X 170. Detail view of dolomite crystals and unaltered areas. These 

dark patches are rimmed with dolomite but contain none. 

E. Sample Tt 7-4, X 80. Dolomite crystals within a fine-grained calcite matrix. Some 

textural control over the distribution of the dolomite is shown by the relatively 

unaltered dark patches. 

~·;;...·-
GEOLOGICAL SURVEY PROFESSIONAL 1' APER 40~-ll PLATE 5 

A B c 

E 

DOLOMITE IN THE ALIFAN LIMESTONE 
PLATE 6 

A, B. Sample Sw 2-4, Guam, X 20. 

A. Plane-polarized light. Well-rounded phosphatic pellets of two kinds, micro~ 

. ~~·crystalline and amorphous. The internal structure of the first type is 

~hown by the pellet in the upper left corner. The darker central part 

j~ .the result of one period of deposition, the lighter peripheral part the 

result of a later period. The light-colored tabular shell fragments lie 

roughly" parallel to the circumference. The pellet in the lower right 

corner shows a light interior and a darker exterior. Roughly concentric 

banding in several shades of brown can be seen around the large shell 

fragments in the pellet to the upper right. Some of these shell fragments 

are original aragonite. 

B. Crossed nicols. The three ellipsoidal pellets in the center are amorphous. 

The lowermost of these has a center of amorphous phosphate upon which 

has accumulated a later layer of particulate phosphate and shell debris. 

The center pellet shows incomplete alteration. Relict structures of shell 

fragments can be seen in the amorphous pellets. The initial calcite 

cement shows up as a light band between pellets and the later interstitial 

phosphate deposits. 

C, D. Sample from Anguar, X 20. 

C. Plane-polarized light. Two large pellets, one built around a fragment of 

mollusk shell, are in a calcite matrix. These pellets have altered to 

amorphous phosphate, but the internal structures can still be seen. 

The spherical pellet shows concentric banding and relict structures of 

tabular shell fragments. Clustered about the large pellets are smaller 

spherical bodies of phosphate. 

D. Crossed nicoh:L The large shell fragment in the ellipsoidal pellet is replaced 

by · aniorphous phosphate. The finely recrystallized lime mud matrix 

shows up as gray patches against the bright crystalline interstitial 

fillings of clear calcite. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 403-D PLATE 6 

A B 

c D 

PHOSPHATIC LIMESTONE FROM GUAM AND FROM ANGUAR, PALAU ISLANDS 
PLATE 7 

A. Sample Fk 4-2, X 8. Algal incrustate limestone from the reef-:-wall facies. Long sinuous 

dark bands of encrusting algae coat and bind organic debris. These bands also 

extend, unbroken, through the fine-grained particulate matrix. Much of this 

matrix has recrystallized into mosaic calcite. The calcareous algae and the encrusting Foraminifera retain their original structures, whereas the coral, Halimeda, 

and molluscan remains are all highly recrystallized. Alternations of the algal 

bands and layers of detritus indicate that, within the photic zone, periods of detrital 

deposition alternated with nondepositional periods that allowed algal growth to 

proceed. Pockets of volcanic material, such as the area to the upper left of the 

gastropod section, indicate subaerial erosion of volcanic terrain or volcanic activity. 

(See pl. lOA for a Maemong Limestone Member equivalent and pl. 19A for an 

equivalent limestone from the Heterostegina zone of southern Louisiana.) 

B. Sample Fk 4-5, X 8. Molluscan-algal paracoquinite from the lagoon facies. Disarticulated valves and fragments of mollusk shells are conspicuous in this limestone. All the shell material has recrystallized into coarse mosaic calcite that is 

not obvious in plain light but that shows up well between crossed nicols (pl. IA, 

B). The shells retain strong traces of their original fibrous structure, in the form 

of yellow streaks and bands that cross the crystal boundaries, despite the fact 

that these crystals are not in optical continuity. These shells also maintain, in 

many places, sharp boundaries against the matrix. 

C. Sample Fk 4-9, X 8. Foraminiferal-algal microparacoquinite from the lagoon facies. 

The limestone contains numerous well-preserved tests of miliolid, uniserial, biserial, 

and coiled small Foraminifera in addition to algal rods. These are all imbedded 

in fine-grained anhedral mosaic calcite. Traces of the original mud matrix have 

been largely effaced by postdepositional alteration. The interior spaces in the 

Foraminifera tests are filled with calcite texturally identical to that in the surrounding matrix. The calcite suggests an original mud fill for the tests, probably the 

result of infiltration. The abundance of miliolids and other small benthonic forms 

is indicative of a backreef shoal-water environment. (See pl. IOD for a Maemong 

Limestone Member equivalent and pl. 20B for an equivalent limestone of Oligocene 

age from Iraq.) 

D. Sample Fk 4-1, X 8. Foraminiferal-algal paracoquinite from the shallow-water off-reef 

facies. Tests of Fabiana are common in foraminiferal-algal debris. This material 

is imbedded in a fine-grained dark mud matrix, some of which has recrystallized 

into patches of clear calcite. These patches are limited, in some places, by bounding 

algae that have resisted alteration. The rock is completely particulate and much 

of the fossil material is in fragmental condition. The abundance of disjointed rods 

of articulate coralline algae indicates a fairly shallow source area for much of the 

detritus. Numerous large fossil fragments floating in the mud matrix are suggestive 

of rapid transport and deposition of coarse detritus into a quiet water environment. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 403-D PLATE 7 

A B 

c D 

LIMESTONE FROM THE ALUTOM FORMATION 
PLATE 8 

A. Sample Ek 7-2, X 16. Foraminiferal-algal, volcanically contaminated coquinite from 

the forereef facies. Broken tests of large Foraminifera and debris of calcareous 

algae dominate. Both articulate and encrusting algae are common, but the encrusting type is all fragmental and serves no binding function. The intact framework is 

cemented by coarse-grained clear calcite, now porous as the result of subaerial solution. The larg~ Foraminifera show a tendency towards preferred orientation 

parallel to the bedding. The texture of the rock and the abundance of large Foraminifera suggests deposition on a forereef slope. (See pl. liB, C for a Maemong 

Limestone Member equivalent and pls. 19 and 20 for similar limestone from 

southern Louisiana and Iraq.) 

B. Sample Ek 7-3, X 20. Foraminiferal-algal microcoquinite from the forereef facies. 

This sample was taken a few feet, stratigraphically, from sample Ek 7-2. Its fossil 

composition is almost identical to the coarser grained Ek 7-2, except for a trace of 

well-preserved globigerinid Foraminifera indicative of deeper water deposition. 

The elongate particles show a preferred orientation parallel to the bedding, and the 

texture indicates considerable sorting and transport of material before final deposition. 

C. Sample Ek 7-1, X 25. Foraminiferal mudstone from the basin facies containing wellpreserved tests of globigerinid Foraminifera. A trace of coiled tests of small Foraminifera is also present. These fossils are imbedded in a chalky matrix that appears 

dark brown in transmitted light and brilliant white under reflected light. Volcanic 

material includes angular fragments of magnetite, plagioclase, and some green and 

brown ferromagnesian minerals, probably pyroxene and amphibole. This sample 

was collected in the same sequence of beds as were Ek 7-2 and Ek 7-3. (See pl. 

liE for the Maemong Limestone Member equivalent and pls. 19 and 20 for similar 

limestones from southern Louisiana and Iraq.) 

D. Sample Ii 8-1, X 8. Foraminiferal paracoquinite, composed largely of tests of Biplanispira, from the forereef facies. Much of the remainder of the rock is made up of 

well-rounded pebbles of limestone (lower right corner of the photomicrograph). 

These particles are in a clear calcite matrix that shows exceptionally sharp contact 

with the enclosed detritus. This mosaic textures is secondary after an original 

mud matrix. Traces of the original matrix can be seen as dark patches. The 

recrystallization proceeds from the detrital grain outward into the mud. In the 

initial stages, each detrital grain shows a halo of clear calcite. (See pl. 10C for this 

type of recrystallization in the Maemong Limestone Member.) 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 403-D PLATE 8 

A B 

c D 

LIMESTONE FROM THE ALUTOM FORMATION 
PLATE 9 

A. Sample Fk 4-7, X 3. A roughly spherical growth of encrusting algae. Approximately 

35 to 40 layers are present although no one layer completely covers the entire mass. 

The growth appears to have begun on the dumbbell-shaped mass of encrusting 

algae and detritus in the foreground of the photomicrograph. Between many 

layers of the algae are lesser amounts of a thin-walled encrusting Foraminifera. 

The entire nodule is enclosed in a paracoquinite matrix similar to sample Fk 4-1 

(pl. 7D). . 

B. Sample Fk 4-7, X 25. Detail view of the algal nodule described above. 

C. Sample Hj 1-4, X 46. Thin-walled encrusting Foraminifera that serves to bind organic 

particles together. This form resembles twisted skeins of netting that wind through 

the rock. Under low magnification this organic deposit can be easily overlooked 

and mistaken for a fine-grained particulate matrix. 

D. Sample Fk 4-6, X 170. Detritus-filled interseptal spaces in recrystallized coral. The 

dark areas running vertically in the photomicrograph are masses of fine-grained 

detritus that have filled the original openings in the corallum. The aragonite 

skeleton of the original coral has been replaced by fine-grained anhedral mosaic 

calcite. This mosaic does not extend into the interseptal spaces; within these 

the matrix is microcrystalline. Part of the wall area of the interseptal voids is 

coated with dark tubular wormlike structures that appear to be an organic encrustation. Within these detrital fills, some of the dark flecks resemble algal 

particles. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 403- L> PLATE 9 

A B 

c D 

ENCRUSTING ALGAE, ENCRUSTING FORAMINIFERA, AND RECRYSTALLIZED 

CORAL FROM THE ALUTOM FORMATION 
PLATE 10 

A. Sample Hi 3-4, X 8. Coral-algal incrustate limestone from the reef-wall facies. Laminae of encrusting algae and Foraminifera are characteristic. Fragments of nodular 

encrusting algae (dark circular pieces) and Halimeda are present. Tlie recrystallized 

coral shows. dark mud-filled interseptal spaces that produce a jigsaw puzzle effect. 

These larger fossil elements are in a particulate matrix of fine-grained organic debris 

and mud. Much of this mud matrix has recrystallized into fine-grained calcite. 

(See pls. 7 A, 14A, 18B, and 19A for equivalent facies from the Alutom Formation, 

the Alifan Limestone, the Mariana Limestone, and southern Louisiana.) 

B. Sample Hi 3-2, X 8. Halimeda paracoquinite from the lagoon facies. This sample 

was taken from within a few .feet of the reef-wall limestone described above. It 

probably represents a local accumulation of Halimeda segments near the reef wall. 

Halimeda and subrounded fragments of encrusting algae dominate in a finely 

crystalline matrix of gray dusty calcite. The Halimeda segments have been completely recrystallized to calcite; only dust-line ghosts of segments remain. 

C. Sample Hi 4-5, X 12. Foraminiferal-algal paracoquinite showing recrystallization of an 

original carbonate mud matrix. The four irregular areas in the center of the figure 

are unrecrystallized remnants of an original matrix. The two patches in the lower 

left corner are separated by a large Foraminifera test around which have grown 

scalenohedrons of dusty .calcite. These crystals are arrayed perpendicular to the 

outer surface of the test and are encroaching on the mud. The patch in right center is surrounded by a dark line that is the base of growth of the crystals now encroaching on the remaining mud. Evidently the original carbonate mud has been 

largely destroyed by recrystallization that proceeded outward from the fossil debris. 

The bright pore spaces· are, in part, the results of the solution of rods of algae. The 

large void in the upper left corner appears to be the result of removal of the residual 

mud matrix. 

D. Sample Gi 1-1, X 16. Miliolid- and peneroplid-rich microparacoquinite from the lagoon 

facies. Abundant tests of delicate benthonic Foraminifera are in a calcite matrix. 

This matrix appears to have recrystallized from an original mud. Rounded fragments of encrusting Foraminifera and segments of algae represent elements that 

may have undergone transport. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 403-D PLATE 10 

A B 

c D 

SAMPLES FROM THE REEF-WALL AND LAGOON FACIES OF THE MAEMONG LIMESTONE MEMBER OF THE UMATAC FORMATION 
PLATE 11 

A. Sample Ed 6-1, X 8. Poorly sorted coral breccia in which coral fragments, pieces of 

foraminiferal-algal encrustations from pre-extant limestones, discoidal Foraminifera 

tests, Halimeda, and other algal debris are abundant. The encrusting algae and 

Foraminifera perform no binding function; these are present only as particulate 

elements. This unsorted debris is in a fine-grained matrix. 

B. Sample Ef 2-1, X 6. Foraminiferal-algal breccia from the forereef facies. Worn and 

broken tests of very large discoidal Foraminifera dominate this poorly sorted limestone; fragments of coralline algae are subordinate. The discoidal tests show a 

preferred orientation parallel to the largest test in the photomicrograph. The fragmentation of such large tests suggests vigorous transport. 

C. Sample Ee 7-1, X 12. Algal-foraminiferal microcoquinite of the forereef facies. This 

limestone shows definite effects of mechanical sorting. In addition to clastic detritus, 

such as the abundant coralline algae derived from relatively shallow water sources, 

the rock carries some planktonic tests of globigerinid Foraminifera. 

D. Sample Dh 11-3, X 8. Foraminiferal microbreccia from the forereef transition facies. 

Lenses and strings of benthonic detritus are scattered through a fine-grained mud 

matrix in which float numerous well-preserved tests of globigerinid and globorotalid 

Foraminifera. 

E. Sample Df 9-1a, X 20. Globigerinid-globorotalid mudstone from the basinal facies. 

Abundant tests of plantonic Foraminifera float in a relatively pure mud matrix; a 

small percentage of fine-grained volcanic material is present. No trace of reef-derived debris is seen in these well-bedded limestones. 
GEOLOGICAL SURVEY PROFESSTONAL PAPER 403- 0 PLATE 11 

A B 

C D E 

SAMPLES FROM THE FOREREEF AND BASIN FACIES OF THE MAEMONG LIMESTON E MEMBER OF THE UMATAC FORMATION 
PLATE 12 

A. Sample Fi 5-1, X 8. · Foraminiferal-algal coquinite from the forereef facies. Worn 

and broken tests of small and large thick-walled Foraminifera, mainly amphisteginids 

and subordinate amounts of Lepidocyclina and operculinids are characteristic, 

together with fragments of incrusting and articulate types. Echinoidal debris 

is common. These fossil elements are closely packed in a matrix of fine-grained 

fossil debris and a little dark mud. Primary porosity in the form of open cells in 

Foraminifera tests is largely preserved; solution of the matrix has produced some 

secondary porosity. 

B. Sample Ih 10-1, X 8. Foraminiferal-algal microcoquinite from the forereef facies. 

Small thick-walled tests of amphisteginids and algal debris dominate. The fossil 

elements show the effect of wear and breakage. Rounded, completely weathered 

grains of fine-grained volcanic rocks are present; these are in the same size grades 

as the fossil material and probably underwent transport along with the fossils. 

(See pls. 8B and 11 C for similar limestones from the Alutom Formation and the 

Maemong Limestone Member.) 

C. Sample Jj 9-1, X 14. Foraminiferal-algal microparacoquinite from the forereef transition facies. In this facies, tests of globigerinid Foraminifera are common in contrast 

to the forereef facies in which these planktonic tests are lacking. These tests and 

sorted algal and foraminiferal debris are in a mud matrix. In this limestone, the 

matrix is dominant over the fossil material in contract to the coarser grained limestones described above, in which the fossil elements form an intact framework. 

D. Sample Jj 9-2, X .14. Foraminiferal paracoquinite of the off-reef deep-water facies. 

Benthonic Foraminifera of great diameter relative to thier thickness are common. 

These fossils locally show a high degree of alinement in preferred orientation. These 

tests and finer debris of Foraminifera and algae are in a porous matrix of partly 

recrystallized mud. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 403-D PLATE 12 

A B 

c D 

SAMPLES OF THE BONY A LIMESTONE 
PLATE 13 

A. Sample Rr 13-1, X 8. Foraminiferal mudstone of the off-reef deep-water facies. Broken tests of Cycloclypeus, whole tests of amphisteginids, and rare Lepidocyclina 

are loosely packed and float in a mud matrix. Rare fragments of pelecypod shells 

are present. Common, but not shown in the photomicrograph, are nodular masses 

of encrusting Foraminifera, probably Gypsina. The position of many broken 

fragments of Cycloclypeus tests indicates that these thin fossils broke after burial. 

Throughout the thin section thes.e tests show a tendency towards parallel alinement. 

The lack of reef detritus suggests the absence of a nearby upslope reef wall. This 

limestone is interpreted as having accumulated on a submarine bank too deep for 

reef growth and too shallow for deposition of planktonic Foraminifera in any 

abundance. 

B. Sample Gi 4-2, X 8. Rounded pebble of the Maemong Limestone Member (dark 

area to the right) in a matrix of Bonya Limestone. These included pebbles are a 

common feature of the Bonya Limestone. They are probably derived from the 

Bolanos Pyroclastic Member as fragments, rather than directly from outcrops of 

the Maemong Limestone Member. 

C. Sample Ih 5-3, X 25. Manganese oxides in the Bonya Limestone. Black and brown 

manganese oxides have partly replaced the fine-grained matrix of this limestone. 

Thin films of these oxides have also been deposited along cell walls and in perforations of Foraminifera tests. 

D. Sample Ih 5-3, X 25. Advanced stage of replacement by manganese oxides. The 

echinoid-spine has been infiltered by black metallic oxides. The black area below 

and to the right of the spine has been massively replaced. This deposit cuts across 

grain boundaries. The ragged edge shown by the Foraminifera tests below the 

echinoid spine is the result of replacement. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 403-D PLATE 13 

A B 

c D 

SAMPLES OF THE BONYA LIMESTONE 
PLATE 14 

A. Sample Tt 7-3, X 6. Algal incrustate limestone from the reef-wall facies. Thick interlaminations- of. encrusting algae and encrusting Foraminifera are cut by fractures 

along which s·eparat.ion of the rock framework has taken place. The texture of this 

limestone is directly comparable to incrustate rocks from the Alutom Formation 

(pl. 7 A);. the Maemong Limestone Member (pl. lOA); the Mariana Limestone (pl. 

18B). Alterations of algae and Foraminifera bind detritus. Some of the fractures 

show lateral separations, in the plane of the thin section, as much as several millimeters in width. The separated laminae show sharp edges that can be matched 

across the fractures. These fractures are filled by sand-size foraminiferal debris. 

Where the fracture filling is mud, some recrystallization has converted the finegrained clastic material into clear anhedral mosaic calcite. 

B. Sample Ts 16-11, X 12. Foraminiferal microparacoquinite from the off-reef shallowwater facies dominated by tests of Rotalia. These tests and a few rounded fragments 

of encrusting algae and encrusting Foraminifera are packed in a matrix of fine sand 

and dark mud that has undergone recrystallization. The white area in the upper 

part of the photomicrograph had its matrix entirely recrystallized. (See pl. 10C 

for an example of this in the Maemong Limestone Member.) 

C. Sample Dh 13-1, X 8. Molluscan coquinoid limestone from the lagoon facies. Com-. 

pletely recrystallized shells of gastropods and pelecypods, Halimeda segments, and 

coral fragments are in a sand and mud matrix. Also present are well-preserved 

tests of miliolid Foraminifera. The shell-matrix contacts are vague and transitional; 

the entire thin section appears faded and washed-out owing to intense recrystallization. The original constituents have not been subjected to winnowing action by 

currents; the rock is poorly sorted. 

D. Sample Eh 3-2, X 10. Foraminiferal microcoquinite. Sand-size fragments of articulate and encrusting algae, encrusting Foraminifera, and shell debris are in a matrix 

of clear microcrystalline calcite. Also present are well-preserved tests of large 

Rotalia and small miliolid tests. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 403-D PLATE 14 

A B 

c D 

SAMPLES OF THE ALIFAN LIMESTONE 
PLATE 14 

A. Sample Tt 7-3, X 6. Algal incrustate limestone from the reef-wall facies. Thick interlaminations- of. encrusting algae and encrusting Foraminifera are cut by fractures 

along which s·eparat.ion of the rock framework has taken place. The texture of this 

limestone is directly comparable to incrustate rocks from the Alutom Formation 

(pl. 7 A);. the Maemong Limestone Member (pl. lOA); the Mariana Limestone (pl. 

18B). Alterations of algae and Foraminifera bind detritus. Some of the fractures 

show lateral separations, in the plane of the thin section, as much as several millimeters in width. The separated laminae show sharp edges that can be matched 

across the fractures. These fractures are filled by sand-size foraminiferal debris. 

Where the fracture filling is mud, some recrystallization has converted the finegrained clastic material into clear anhedral mosaic calcite. 

B. Sample Ts 16-11, X 12. Foraminiferal microparacoquinite from the off-reef shallowwater facies dominated by tests of Rotalia. These tests and a few rounded fragments 

of encrusting algae and encrusting Foraminifera are packed in a matrix of fine sand 

and dark mud that has undergone recrystallization. The white area in the upper 

part of the photomicrograph had its matrix entirely recrystallized. (See pl. 10C 

for an example of this in the Maemong Limestone Member.) 

C. Sample Dh 13-1, X 8. Molluscan coquinoid limestone from the lagoon facies. Com-. 

pletely recrystallized shells of gastropods and pelecypods, Halimeda segments, and 

coral fragments are in a sand and mud matrix. Also present are well-preserved 

tests of miliolid Foraminifera. The shell-matrix contacts are vague and transitional; 

the entire thin section appears faded and washed-out owing to intense recrystallization. The original constituents have not been subjected to winnowing action by 

currents; the rock is poorly sorted. 

D. Sample Eh 3-2, X 10. Foraminiferal microcoquinite. Sand-size fragments of articulate and encrusting algae, encrusting Foraminifera, and shell debris are in a matrix 

of clear microcrystalline calcite. Also present are well-preserved tests of large 

Rotalia and small miliolid tests. 
PLATE 15 

A. Samples Pv 10-2, X 8. Foraminiferal mudstone of the off-reef deep-water facies containing tests of Cycloclypeus, Amphistegina, and Operculinoides. Almost all these 

large tests show considerable wear and breakage; an encrusting type, Gypsina, is 

present only in fragments and does not bind detritus. These tests are in a recrystallized mud matrix. A trace of well-preserved to worn tests of miliolids and globigerinid Foraminifera is present. The porosity is low and is largely secondary owing 

to subaerial solution of the original mud matrix. The globigerinid tests are little 

altered and show pseudo-uniaxial crosses in polarized light. The abundance of 

Cycloclypeus coupled with the lack of reef detritus precludes a reef or forereef origin. 

The limestone was probably deposited on a submarine bank in water several hundreds 

of feet deep. 

B. Sample Nr 1-7a, X 25. Friable microcoquinite. Most of the specimen is an aggregate 

of subangular to rounded grains of unidentifiable debris, probably of organic origin. 

The detritus shows a high degree of sorting that cannot be ascribed to inherent size 

of the material. The particles are loosely cemented in a matrix of fine-grained clear 

anhedral calcite. 

C. Sample St 8-1, X 12. Porous foraminiferal microparacoquinite. Debris of amphisteginids, operculinids, and encrusting Gypsina and Carpenteria is in a matrix 

similar to (but somewhat porous due to solution) that of sample Pv 10-2 (pl. 15A). 

Whole Cycloclypeus tests are lacking; fragments are rare. 
PLATE 16 

A. Sample Ts 5-7, X 20. Microparacoquinite from the basin facies, composed mainly of 

tightly packed whole and fragmental tests of globigerinid Foraminifera. Test 

fragments show a preferred orientation parallel to fine laminae, several millimeters 

thick, produced by streaks and lenses of fine-grained debris. The tests, and a small 

percentage of volcanic material, are in a mud matrix. Many cells in the large unbroken tests are empty, but the majority are filled with matrixlike material. Volcanic 

material is present as clay and a few single crystals of magnetite, feldspar, and 

ferromagnesian minerals. The high content of broken tests appears to be due to 

crushing and flattening. The faunal as·semblage indicates a depth of deposition of 

100 to 1,500 fathoms, probably towards the shallow end of this range. 

B. Sample Ts 5-9, X 8. Foraminiferal microparacoquinite from the basin facies. Tests 

of amphisteginid and globigerinid Foraminifera and a trace of Cycloclypeus test 

fragments are loosely packed in a mud matrix. The amphisteginid tests show 

preferred orientation parallel to the bedding. The worn amphisteginid and broken 

Cycloclypeus tests indicate some transport of material. The high content of amphisteginids in this sample indicates a depth of deposition possi6ly as shallow as 40 

to 100 fathoms; the abundant globigerinids suggest the lack of any barrier to the 

circulation of oceanic currents. 

C. Sample Ts 5-12, X 27. Argillaceous foraminifer_al microparacoquinite from the basin 

facies. Abundant well-preserved tests of globigerinids float in a mud matrix that 

also encloses abundant volcanic material in the form of deeply weathered single 

crystals and sand-size fragments of fine-grained crystalline rocks. 

D. Sample Rr 9-1, X 8. Pebbles of the Bonya Limestone in the Janum Formation. The 

pebbles are white pure limestone whereas the enclosing Janum Formation sediments 

are pink to red and contain disseminated volcanic clay. 
PLATE 17 

A. Sample Jj 6-1, X 20. Molluscan microcoquinoid limestone from the lagoon facies. 

Completely recrystallized small gastropod shells and disarticulated valves of pelecypods float in a fine-grained detrital matrix containing algal and foraminiferal 

debris. A trace of well-preserved miliolids and coiled Foraminifera tests, possibly 

a life assemblage, is present. Recrystallization has reduced the shells to anhedral 

mosaic calcite. The primary porosity of the rock was undoubtedly low, but solution of shell fragments and patches of the matrix has resulted in secondary porosity. 

B. Sample Uu 3-1, X 25. Foraminiferal microcoquinite from the forereef facies. This 

porous friable limestone is made up almost entirely of worn tests of Calcarina, 

Amphistegina, and Marginopora, that are loosely packed with a trace of echinoid 

spines. Each test is coated with a thin layer of clear granular calcite that serves 

as a weak cement; tests can be easily rubbed from the rock. The rock shows a high 

degree of biologic sorting owing to the inherent size of the tests. 

C. Sample Po 1-2, X 17. Halimeda paracoquinite from the lagoon facies. Disjointed 

segments of the green alga, Halimeda, ranging in width from 2 to 8 mm, make up 

much of the rock. Traces of articulate and encrusting algae, tests of shallow-water 

Foraminifera, and fragments of molluscan remains constitute the remainder of the 

fossils. The state of preservation of the Halimeda and their relationship to the 

matrix varies within a single thin section. The area covered by the photomicrograph shows Halimeda segments, originally aragonite, that are now fine-grained 

mosaic calcite within which the original organic structures are well preserved as 

darker gray areas. These segments, and the other fossil elements, are in ·a crystalline 

calcite matrix, the result of the recrystallization of an original mud matrix. In 

other parts of the thin section, calcite-coated segments are in a matrix of banded 

fibrous aragonite and late-stage granular calcite, which is the result of carbonate deposition within what must have been an open-work gravel of Halimeda. Some segments 

retain a light brown color and are still composed of cryptocrystalline aragonite; 

many have been completely removed by solution and leave Halimeda-shaped voids 

in the matrix. 

D. Sample Rr 16-1, X 17. Foraminiferal breccia from the forereef facies containing worn 

and broken tests of· Cycloclypeus. These large tests and lesser amounts of all the 

fossil groups considered are randomly oriented in a sand-size matrix that contains 

angular debris of the same fossil types. The rock shows high porosity, mainly 

secondary, due to solution. The fossil content and texture of this limestone resemble 

that of material dredged from the outer slopes of Bikini Atoll at depths ranging from 

580 to 800 feet. 
PLATE 18 

A. Sariipl~-: R~ l:~~l,:::k._i.o. ·Recrystallized corallum. The originally aragonitic- corallum, 

· and an interseptal _carbonate filling, have recrystallized into medium-grained anhedral mosaic calcite:,- ~e outline of the original skeletal material is preserved 

in a jigsaw-Shaped pattern visible as cloudy darker calcite against the clear lighter 

calcite of the interseptal filling. There is some tendency for the calcite within the 

cloudy areas to remain as a distinct crystal, optically discontinuous, separated from 

t~e crystals in the· filling. However, most of the crystal boundaries cross the 

corallum-interseptal filling boundaries. Between crossed nicols the coralluminterseptal filling boundaries are partially obscured by the large continuous crystals. The limestone has virtually no porosity at present, in contrast to what 

originally was an extremely porous coral structure. 

B. Sample Tw 7-1, X 3. Coral-algal incrustate limestone from the reef-wall facies. Thick 

laminar masses of encrusting algae coat and bind recrystallized mud-filled coral. 

Caught within the algal laminae are tests of M arginopora, Halimeda segments, and 

other reef debris. A few lighter colored laminae of encrusting Foraminifera contribute to the cementing mass. Halimeda segments are abundant in the detritus 

surrounding the coral-algal mass. Anhedral, clear, mosaic calcite fills much of the 

original void space. Some of these void spaces were partially filled by fine 

detritus; the upper surfaces of these deposits are parallel throughout the slide as 

well as parallel to the upper surface of the algal colony. This parallism is indicative 

of mud fillin,g of 'tbe. reef limestone during reef growth. 

C. Sample Kr 2-3, ')( :f ·.Coral breccia from the lagoon facies. Poorly sorted large fragments of algal- and foraminiferal-encrusted coral, echinoid spines and plates, 

Halimeda segments,,peiecypod shells, and tests of smaller discoidal Foraminifera are 

in a porous riledil.lm~gr.ained matrix. 
,· 

. "/. 

PLATE 19 

A. Coral-algal ihctustate li~estone from reef-wall facies, X 8. This sample is identical to 

Hi 3-4 from the ·.Maemong Limestone Member (pl. lOA). Fossil elements include 

mud-filled algal-encrusted coral, disjointed Halimeda, alga detritus, and tests of 

Foraminifera. Much of the detritus between the coral is in a fine-grained mud 

matrix. Cores from this facies show coral heads in growth position (Forman and 

Schlanger, 1957). 

B. Coral breccia from the forereef facies, X 8. This limestone has an equivalent in sample 

Ed 6-1 from the Maemong Limestone Member (pl. llA). Many of the fossil elements in this material are also common to rocks from the reef-wall facies. However, 

all the components· are detrital in the forereef facies. Cores from this facies show 

characteristic bedding. 

C. Foraminiferal-algal microparacoquinite from the off-reef facies, X 8. Tests of large 

Heterostegina and some miogypsinids dominate; fragments of coralline algae are also 

present. This limestone is equivalent to that represented by sample Hi 4-5 from 

the Maemong Limestone Member (pl. 10C). 

D. Foraminiferal mudstone· from the forereef transition facies, X 8, containing tests of large 

benthonic lepidocyclinids and small planktonic globigerinids that float in a matrix of 

fine-grained detritus::and mud. The discoidal tests of the large Foraminifera show 

a high degte.e.<?f.parallelism. This material is comparable to rock from the bedded 

forereef transition' facies of the Maemong Limestone Member (pl. llD). 
PLATE 20 

The limestones of Oligocene age from the forereef facies at Kirkuk, Iraq, are comparable 

to (1) samples Ek 7-2 (pl. SA) and Ek 7-3 (pl. SB) from bedded limestones of Eocene (b) 

age in the Alutom Formation, (2) samples Ef 2-1 and Ee 7-1 from bedded sections of the 

Maemong Limestone Member (pl. liB, C), and (3) bedded sections of the Bonya Limestone 

(pl. 12B). 

A. Limestone from the reef-wall facies, X 8. This limestone is considered equivalent to 

rock from southern Louisiana (pl. 19A), the Maemong Limestone Member (pl. lOA), 

and limestone from the Alutom Formation (pl. 7 A). 

B. Foraminiferal microparacoquinite from the lagoon facies, X 20. This fine-grained 

miliolid-rich rock is directly comparable to material from the Maemong Limestone 

Member (pl. IOD) and limestone from the Alutom Formation (pl. 7C). 

C. Foraminiferal microbreccia from the forereef facies, X 8. 

D. Foraminiferal microbrecci!t from the forereef facies, X 8. 
PLATE 15 

A. Samples Pv 10-2, X 8. Foraminiferal mudstone of the off-reef deep-water facies containing tests of Cycloclypeus, Amphistegina, and Operculinoides. Almost all these 

large tests show considerable wear and breakage; an encrusting type, Gypsina, is 

present only in fragments and does not bind detritus. These tests are in a recrystallized mud matrix. A trace of well-preserved to worn tests of miliolids and globigerinid Foraminifera is present. The porosity is low and is largely secondary owing 

to subaerial solution of the original mud matrix. The globigerinid tests are little 

altered and show pseudo-uniaxial crosses in polarized light. The abundance of 

Cycloclypeus coupled with the lack of reef detritus precludes a reef or forereef origin. 

The limestone was probably deposited on a submarine bank in water several hundreds 

of feet deep. 

B. Sample Nr 1-7a, X 25. Friable microcoquinite. Most of the specimen is an aggregate 

of subangular to rounded grains of unidentifiable debris, probably of organic origin. 

The detritus shows a high degree of sorting that cannot be ascribed to inherent size 

of the material. The particles are loosely cemented in a matrix of fine-grained clear 

anhedral calcite. 

C. Sample St 8-1, X 12. Porous foraminiferal microparacoquinite. Debris of amphisteginids, operculinids, and encrusting Gypsina and Carpenteria is in a matrix 

similar to (but somewhat porous due to solution) that of sample Pv 10-2 (pl. 15A). 

Whole Cycloclypeus tests are lacking; fragments are rare. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 403-D PLATE 15 

B 

A 

c 

SAMPLES OF THE BARRIGADA LIMESTONE 
PLATE 16 

A. Sample Ts 5-7, X 20. Microparacoquinite from the basin facies, composed mainly of 

tightly packed whole and fragmental tests of globigerinid Foraminifera. Test 

fragments show a preferred orientation parallel to fine laminae, several millimeters 

thick, produced by streaks and lenses of fine-grained debris. The tests, and a small 

percentage of volcanic material, are in a mud matrix. Many cells in the large unbroken tests are empty, but the majority are filled with matrixlike material. Volcanic 

material is present as clay and a few single crystals of magnetite, feldspar, and 

ferromagnesian minerals. The high content of broken tests appears to be due to 

crushing and flattening. The faunal as·semblage indicates a depth of deposition of 

100 to 1,500 fathoms, probably towards the shallow end of this range. 

B. Sample Ts 5-9, X 8. Foraminiferal microparacoquinite from the basin facies. Tests 

of amphisteginid and globigerinid Foraminifera and a trace of Cycloclypeus test 

fragments are loosely packed in a mud matrix. The amphisteginid tests show 

preferred orientation parallel to the bedding. The worn amphisteginid and broken 

Cycloclypeus tests indicate some transport of material. The high content of amphisteginids in this sample indicates a depth of deposition possi6ly as shallow as 40 

to 100 fathoms; the abundant globigerinids suggest the lack of any barrier to the 

circulation of oceanic currents. 

C. Sample Ts 5-12, X 27. Argillaceous foraminifer_al microparacoquinite from the basin 

facies. Abundant well-preserved tests of globigerinids float in a mud matrix that 

also encloses abundant volcanic material in the form of deeply weathered single 

crystals and sand-size fragments of fine-grained crystalline rocks. 

D. Sample Rr 9-1, X 8. Pebbles of the Bonya Limestone in the Janum Formation. The 

pebbles are white pure limestone whereas the enclosing Janum Formation sediments 

are pink to red and contain disseminated volcanic clay. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 403-D PLATE 16 

A B 

c D 

SAMPLES OF THE JANUM FORMATION 
PLATE 17 

A. Sample Jj 6-1, X 20. Molluscan microcoquinoid limestone from the lagoon facies. 

Completely recrystallized small gastropod shells and disarticulated valves of pelecypods float in a fine-grained detrital matrix containing algal and foraminiferal 

debris. A trace of well-preserved miliolids and coiled Foraminifera tests, possibly 

a life assemblage, is present. Recrystallization has reduced the shells to anhedral 

mosaic calcite. The primary porosity of the rock was undoubtedly low, but solution of shell fragments and patches of the matrix has resulted in secondary porosity. 

B. Sample Uu 3-1, X 25. Foraminiferal microcoquinite from the forereef facies. This 

porous friable limestone is made up almost entirely of worn tests of Calcarina, 

Amphistegina, and Marginopora, that are loosely packed with a trace of echinoid 

spines. Each test is coated with a thin layer of clear granular calcite that serves 

as a weak cement; tests can be easily rubbed from the rock. The rock shows a high 

degree of biologic sorting owing to the inherent size of the tests. 

C. Sample Po 1-2, X 17. Halimeda paracoquinite from the lagoon facies. Disjointed 

segments of the green alga, Halimeda, ranging in width from 2 to 8 mm, make up 

much of the rock. Traces of articulate and encrusting algae, tests of shallow-water 

Foraminifera, and fragments of molluscan remains constitute the remainder of the 

fossils. The state of preservation of the Halimeda and their relationship to the 

matrix varies within a single thin section. The area covered by the photomicrograph shows Halimeda segments, originally aragonite, that are now fine-grained 

mosaic calcite within which the original organic structures are well preserved as 

darker gray areas. These segments, and the other fossil elements, are in ·a crystalline 

calcite matrix, the result of the recrystallization of an original mud matrix. In 

other parts of the thin section, calcite-coated segments are in a matrix of banded 

fibrous aragonite and late-stage granular calcite, which is the result of carbonate deposition within what must have been an open-work gravel of Halimeda. Some segments 

retain a light brown color and are still composed of cryptocrystalline aragonite; 

many have been completely removed by solution and leave Halimeda-shaped voids 

in the matrix. 

D. Sample Rr 16-1, X 17. Foraminiferal breccia from the forereef facies containing worn 

and broken tests of· Cycloclypeus. These large tests and lesser amounts of all the 

fossil groups considered are randomly oriented in a sand-size matrix that contains 

angular debris of the same fossil types. The rock shows high porosity, mainly 

secondary, due to solution. The fossil content and texture of this limestone resemble 

that of material dredged from the outer slopes of Bikini Atoll at depths ranging from 

580 to 800 feet. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 403-D PLATE 17 

A B 

c D 

SAMPLES OF THE MARIANA LIMESTONE 
PLATE 18 

A. Sariipl~-: R~ l:~~l,:::k._i.o. ·Recrystallized corallum. The originally aragonitic- corallum, 

· and an interseptal _carbonate filling, have recrystallized into medium-grained anhedral mosaic calcite:,- ~e outline of the original skeletal material is preserved 

in a jigsaw-Shaped pattern visible as cloudy darker calcite against the clear lighter 

calcite of the interseptal filling. There is some tendency for the calcite within the 

cloudy areas to remain as a distinct crystal, optically discontinuous, separated from 

t~e crystals in the· filling. However, most of the crystal boundaries cross the 

corallum-interseptal filling boundaries. Between crossed nicols the coralluminterseptal filling boundaries are partially obscured by the large continuous crystals. The limestone has virtually no porosity at present, in contrast to what 

originally was an extremely porous coral structure. 

B. Sample Tw 7-1, X 3. Coral-algal incrustate limestone from the reef-wall facies. Thick 

laminar masses of encrusting algae coat and bind recrystallized mud-filled coral. 

Caught within the algal laminae are tests of M arginopora, Halimeda segments, and 

other reef debris. A few lighter colored laminae of encrusting Foraminifera contribute to the cementing mass. Halimeda segments are abundant in the detritus 

surrounding the coral-algal mass. Anhedral, clear, mosaic calcite fills much of the 

original void space. Some of these void spaces were partially filled by fine 

detritus; the upper surfaces of these deposits are parallel throughout the slide as 

well as parallel to the upper surface of the algal colony. This parallism is indicative 

of mud fillin,g of 'tbe. reef limestone during reef growth. 

C. Sample Kr 2-3, ')( :f ·.Coral breccia from the lagoon facies. Poorly sorted large fragments of algal- and foraminiferal-encrusted coral, echinoid spines and plates, 

Halimeda segments,,peiecypod shells, and tests of smaller discoidal Foraminifera are 

in a porous riledil.lm~gr.ained matrix. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 403-D PLATE 18 

A B 

c 

SAMPLES OF THE MARIANA LIMESTONE 
,· 

. "/. 

PLATE 19 

A. Coral-algal ihctustate li~estone from reef-wall facies, X 8. This sample is identical to 

Hi 3-4 from the ·.Maemong Limestone Member (pl. lOA). Fossil elements include 

mud-filled algal-encrusted coral, disjointed Halimeda, alga detritus, and tests of 

Foraminifera. Much of the detritus between the coral is in a fine-grained mud 

matrix. Cores from this facies show coral heads in growth position (Forman and 

Schlanger, 1957). 

B. Coral breccia from the forereef facies, X 8. This limestone has an equivalent in sample 

Ed 6-1 from the Maemong Limestone Member (pl. llA). Many of the fossil elements in this material are also common to rocks from the reef-wall facies. However, 

all the components· are detrital in the forereef facies. Cores from this facies show 

characteristic bedding. 

C. Foraminiferal-algal microparacoquinite from the off-reef facies, X 8. Tests of large 

Heterostegina and some miogypsinids dominate; fragments of coralline algae are also 

present. This limestone is equivalent to that represented by sample Hi 4-5 from 

the Maemong Limestone Member (pl. 10C). 

D. Foraminiferal mudstone· from the forereef transition facies, X 8, containing tests of large 

benthonic lepidocyclinids and small planktonic globigerinids that float in a matrix of 

fine-grained detritus::and mud. The discoidal tests of the large Foraminifera show 

a high degte.e.<?f.parallelism. This material is comparable to rock from the bedded 

forereef transition' facies of the Maemong Limestone Member (pl. llD). 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 403-D PLATE 19 

A B 

c D 

LIMESTONE FROM SOUTHERN LOUISIANA 
PLATE 20 

The limestones of Oligocene age from the forereef facies at Kirkuk, Iraq, are comparable 

to (1) samples Ek 7-2 (pl. SA) and Ek 7-3 (pl. SB) from bedded limestones of Eocene (b) 

age in the Alutom Formation, (2) samples Ef 2-1 and Ee 7-1 from bedded sections of the 

Maemong Limestone Member (pl. liB, C), and (3) bedded sections of the Bonya Limestone 

(pl. 12B). 

A. Limestone from the reef-wall facies, X 8. This limestone is considered equivalent to 

rock from southern Louisiana (pl. 19A), the Maemong Limestone Member (pl. lOA), 

and limestone from the Alutom Formation (pl. 7 A). 

B. Foraminiferal microparacoquinite from the lagoon facies, X 20. This fine-grained 

miliolid-rich rock is directly comparable to material from the Maemong Limestone 

Member (pl. IOD) and limestone from the Alutom Formation (pl. 7C). 

C. Foraminiferal microbreccia from the forereef facies, X 8. 

D. Foraminiferal microbrecci!t from the forereef facies, X 8. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 403-D PLATE 20 

A B 

c D 

LIMESTONE OF OLIGOCENE AGE FROM KIRKUK, IRAQ 
PLATE 21 

A. Heavy minerals, in the 0.25- to 0.10- mm-size grade, of the Janum Formation from sample 

Ts 5-4, X 33. Augite and amphibole (table 9). 

B. Heavy minerals, in the 0.25- to 0.10- mm-size grade, of the Maemong Limestone Member 

from sample Ed 7-2 X 32. Augite, hypersthene, and amphibole (table 9). 

C. Heavy minerals, in the 0.25- to 0.10- mm-size grade, of the Mahlac Member from sample 

Gj 11-1, X 33. Brown-green amphibole, basaltic hornblende (table 9). 

D. Heavy minerals in the 0.25- to 0.10- mm-size grade, of the limestones in the Alutom 

Formation from sample Ek 7, X 34. Brown-green amphibole, augite (table 9). 

E. Magnetite octahedra in the Janum Formation from sample Rr 23-1, X 32. 

F. Foraminifera test fillings of opaline silica (beta-cristobalite) in the Maemong Limestone 

Member, X 32. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 403-D PLATE 21 

A B 

c D 

E F 

INSOLUBLE RESIDUES FROM SELECTED GUAM LIMESTONE 

U.S. GOVERNMENT PRINTING OFFICE; 1963 ~90-219