Cover design by Nancy Polend, 1991
Larger Foraminifer Biostratigraphy of 

PEACE Boreholes, Enewetak Atoll, 

Western Pacific Ocean

By THOMAS G. GIBSON and RICHARD MARGERUM

GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL, 

REPUBLIC OF THE MARSHALL ISLANDS

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1513-D

Prepared in cooperation with the Defense Nuclear Agency

Larger foraminiferal and some smaller foraminiferal data from 

Enewetak Atoll boreholes were integrated into a local benthic 

foraminiferal and ostracode biostratigraphy for correlation with Pacific 

biostratigraphic sequences

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1991
DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, Jr., Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Any use of trade, product, or firm names in this publication is for 

and does not imj" 

.S. Government

descriptive purposes only and does not imply endorsement by the

U.f "

Library of Congress Cataloging in Publication Data

Gibson, Thomas G.

Larger foraminifer biostratigraphy of PEACE boreholes, Enewetak Atoll, western Pacific Ocean / by Thomas G. Gibson and

Richard Margerum. 

p. cm. (Geologic and geophysical investigations of Enewetak Atoll, Republic of the Marshall Islands : D) (U.S.Geological

Survey professional paper ; 1513-D) 

Includes bibliographical references. 

Supt. of Docs. no. : 119.16:1513-D 

1. Foraminifera, Fossil. 2. Paleontology Miocene. 3. Paleontology Pleistocene. 4. Paleontology Marshall Islands 

Enewetak Atoll. 5. Pacific Enewetak Atoll Crater Exploration Program. I. Margerum, Richard. II. Title. III. Series. IV.

Series: Geologic and geophysical investigations of Enewetak Atoll. Republic of the Marshall Islands ; ch. D. 

QE772.G43 1992

563'N.12-dc20 91-10889

CIP

For sale by the Books and Open-File Reports Section, U.S. Geological Survey, 

Federal Center, Box 25425, Denver, CO 80225
CONTENTS

Page

Abstract........................................................................... Dl

Introduction....................................................................

Purpose and Scope.....................................................

Acknowledgments......................................................

Materials and Methods .....................................................

Previous Foraminiferal Studies..........................................

Larger Foraminiferal Occurrences, Zonation, and Age

Assignments.............................................................

Smaller Foraminiferal Occurrences and Age Assignments .......

Rates of Accumulation.......................................................

List of Species Treated from PEACE and EXPOE Enewetak 

Subsurface Samples .................................................

Larger Foraminifers ...................................................

Smaller Benthic Foraminifers .......................................

References Cited..............................................................

Dll

12

13

13

13

13

ILLUSTRATIONS

Page

FIGURE 1. Map showing location of the five deepest PEACE boreholes, three EXPOE boreholes, and drill hole E-1 at Enewetak

Atoll....................................................................................................................................................... D2

2-8. Charts showing:

2. Thickness of strata penetrated and depth below sea level of the deepest PEACE borehole sites at Enewetak

Atoll.............................................................................................................................................. 3

3. Correlation of Tertiary Far East Letter Zones, planktonic foraminiferal zones, and European Stages ................. 4

4. Occurrences of larger foraminiferal species and Austrotrillina in borehole OBZ^l and placement in Enewetak

benthic zones and Tertiary Far East Letter Zones................................................................................ 6

5. Occurrences of larger foraminiferal species in boreholes OOR-17 and OAR-2 and placement in Enewetak benthic

zones and Tertiary Far East Letter Zones........................................................................................... 7

6. Occurrences of larger foraminiferal species in borehole KBZ^l and placement in Enewetak benthic zones and

Tertiary Far East Letter Zones......................................................................................................... 8

7. Occurrences of larger foraminiferal species in borehole KAR-1 and placement in Enewetak benthic zones and

Tertiary Far East Letter Zones......................................................................................................... 9

8. Stratigraphic ranges through Enewetak benthic zones of larger and selected smaller foraminifers from all PEACE

and three EXPOE boreholes at Enewetak Atoll................................................................................... 10

in

GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL, REPUBLIC OF THE

MARSHALL ISLANDS

LARGER FORAMINIFER BIOSTRATIGRAPHY OF PEACE BOREHOLES, 

ENEWETAK ATOLL, WESTERN PACIFIC OCEAN

By THOMAS G. GIBSON and RICHARD MARGERUM

ABSTRACT

Larger foraminiferal assemblages, including Lepidocyclina orientalis, Miogypsina thecideaeformis, Miogypsinoides dehaartii, Spiroclypeus margaritatus, Miogypsinoides ubaghsi, Heterostegina borneensis, and Lepidocyclina ephippoides, and a smaller foraminifer, 

Austrotrillina striata, are used to correlate upper Oligocene and lower 

Miocene strata in the Pacific Atoll Exploration Program (PEACE) 

boreholes at Enewetak Atoll, Republic of the Marshall Islands, western Pacific Ocean, with the Te and Tf zones of the previously established Tertiary Far East Letter Zonation. Correlation using these two 

benthic groups is critical because calcareous nannofossils and planktic 

foraminifers are absent in the lower Miocene strata. The deepest 

PEACE borehole, OBZ^l, has abundant diagnostic larger foraminifers 

throughout the upper Oligocene to middle Miocene strata. The three 

larger foraminiferal species found in the upper Miocene strata continue 

into the present-day assemblages. Three new subdivisions are proposed within the oldest local biostratigraphic Zone MM previously 

established for the Enewetak PEACE cores.

Biostratigraphic data from these boreholes delineate a thick (greater 

than 700 feet) sequence of upper Oligocene and lower Miocene strata 

corresponding to lower and upper Te zone. These strata document a 

major period of carbonate accumulation at Enewetak during the Late 

Oligocene and early Miocene (26 to 18 million years ago).

INTRODUCTION

PURPOSE AND SCOPE

The Pacific Enewetak Atoll Crater Exploration 

(PEACE) Program drilled 32 shallow- to moderatedepth boreholes in the northern and northwestern part of 

Enewetak Lagoon, Republic of the Marshall Islands, 

during 1985 from the drill ship M/V Knut Constructor 

(fig. 1). The PEACE boreholes were drilled and sampled 

with a combination of split spoon, hammer, and coring 

methods and had a generally good recovery of about 50

Manuscript approved for publication February 5, 1991.

percent in the intermixed hard and soft sediments 

(Henry and others, 1986). The deepest PEACE borehole, OBZ-4, penetrated to approximately 1,800 ft below 

sea level (bsl).

Earlier deep holes drilled at Bikini in the late 1940's 

and at Enewetak in the early 1950's (Emery and others, 

1954; Ladd and Schlanger, 1960) had only a few thin 

cored intervals, and the foraminiferal faunas were studied mostly from cuttings. These earlier drill hole samples 

indicated that about 4,200-4,600 ft of Eocene and 

younger carbonate strata overlie the volcanic basement 

rocks in Enewetak Atoll (Todd and Low, 1960).

The smaller benthic foraminiferal and ostracode 

assemblages from five PEACE boreholes and from three 

boreholes (XEN-3, XSA-1, and XRI-1, fig. 1) of the 

earlier Exploratory Program on Enewetak (Project 

EXPOE), drilled and sampled by a combination of split 

spoon and coring in 1973-74 (Couch and others, 1975), 

were used to construct a local biostratigraphic zonation 

for the middle Miocene to Holocene strata of the 

Enewetak holes (Cronin and others, 1986). However, 

smaller benthic species are difficult to use for longdistance biostratigraphic correlation over the Pacific. 

Most tropical areas of the Pacific have no long cores 

available, and outcrop sections provide only a fragmentary record. Thus, documenting first and last appearance 

datums of most benthic species is difficult.

The correlation of Enewetak deposits using intercontinental planktic zonations is difficult because the deposits commonly represent shallow, reef environments that 

yield sparse planktic components. For example, the 

study of calcareous nannofossils and planktic foraminifers from the PEACE and EXPOE boreholes contributes some tie points in the middle Miocene and younger 

strata with the intercontinental zonations (Bybell and 

Poore, this volume), but calcareous plankton generally 

are sparse. Further, calcareous plankton become sparse

Di
D2 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

162° 162°10' 162°20' 162°30'

§

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IVY J160000

KBZ-4

XSA-1

OAR-2 

OBZ-4 

OOR-17

GRID 

(FT)

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+ 80000

XRI-1

SOUTHWEST 

PASSAGE

INDEX MAP

160°E 180° 160°W 140° 120° 100° 40°

EAST (DEEP) CHANNEL

E-1

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SOUTH PASSAGE

5 NAUTICAL MILES 0 10,000

J hll.lMlll___________I

30,000 50,000 FEET 0

i____i____i i_

10 KILOMETERS

FIGURE 1. Location of the five deepest PEACE boreholes, three EXPOE boreholes, and drill hole E-1 at Enewetak Atoll.

and poorly preserved, presumably because of diagenetic 

effects, in the interval from 900 to 1,150 ft bsl in the 

PEACE boreholes; they are absent in all samples below 

1,150 ft bsl.

In these intervals where calcareous plankton are 

absent, both large and small benthic species must be 

used. Larger foraminiferal age assignments are particularly important in the upper Oligocene and lower Miocene strata, which occur deeper than 900 ft bsl in the 

PEACE boreholes. Well-preserved larger foraminifers

are present in most intervals in these older strata. Some 

smaller benthic species are present in the lower Miocene 

strata, and one of these, Austrotrillina striata, commonly has been incorporated into western Pacific biostratigraphic zonations (Adams, 1984; Chaproniere, 

1984).

This paper documents the occurrences of larger foraminiferal assemblages and a few critical smaller benthic 

species in the upper Oligocene to Holocene strata in five 

of the PEACE and three of the EXPOE boreholes. We
LARGER FORAMINIFER BIOSTRATIGRAPHY OF PEACE BOREHOLES, ENEWETAK ATOLL D3

BOREHOLE

KAR-1

KBZ-4

OAR -2

OBZ-4

OCR -17

THICKNESS OF STRATA 

PENETRATED (FEET)

1,146

1,046

886

1,605

1,091

DEPTH BELOW SEA LEVEL 

OF BOREHOLE SITE (FEET)

105

109

114

199

55

FIGURE 2. Thickness of strata penetrated and depth below sea level 

of the deepest PEACE borehole sites at Enewetak Atoll (data from 

Henry and others, 1986).

then correlate these occurrences in Enewetak with the 

known biostratigraphic distribution of larger foraminifers in the western Pacific to give some age assignment 

to the older portions of the PEACE boreholes.

ACKNOWLEDGMENTS

The large number of samples examined during this 

study involved a considerable amount of preparation by 

Elizabeth E. Hill and Craig Montgomery, and their help 

is greatly appreciated.

The authors are indebted to C.G. Adams for his advice 

on the identification of Austrotrillina species and the age 

significance of some larger foraminifers. Warren Blow of 

the Department of Paleobiology of the Natural History 

Museum of the Smithsonian Institution kindly facilitated 

the location and sampling of the E-l core from 

Enewetak. Laurel Bybell, George Chaproniere, Thomas 

Cronin, and Richard Poore made many helpful suggestions on the manuscript.

MATERIALS AND METHODS

More than 1,000 samples from all 32 PEACE boreholes 

and from 3 of the EXPOE boreholes were examined for 

benthic foraminifers. Diagnostic larger foraminifers 

were obtained only from the five deepest PEACE boreholes, KAR-1, KBZ^, OAR-3, OBZ^, and OOR-17 

(fig. 1). The thickness of the penetrated section and 

elevations of the PEACE boreholes in relation to the sea 

floor and sea level are given in figure 2.

All samples examined were composed of carbonate 

sediments of various degrees of induration. The samples 

were processed in U.S. Geological Survey laboratories in 

Reston, Va. They were soaked in water containing 

sodium carbonate and a calgon solution and agitated for 

several hours on a shaker; the more indurated samples 

then were treated with a 10-percent hydrogen peroxide 

solution to aid in their breakdown.

After soaking, the samples were washed through a set 

of sieves having screen sizes between 840 and 63 

microns. Smaller benthonics generally were examined 

from a number of sieve sizes ranging from 840 to 150 

microns. Larger foraminifers were collected largely from 

the 840- and 420-micron sieves.

Thin sections were used to identify the larger foraminifers; three to four horizontal and one to two vertical 

sections normally were made of each species per sample.

PREVIOUS FORAMINIFERAL STUDIES

Previous work on larger foraminiferal biostratigraphy 

in the western Marshall Islands was on drill hole samples 

from Bikini (Cole, 1954) and Enewetak (Cole, 1957). 

Companion studies of smaller foraminiferal biostratigraphy from the same drill holes were made by Todd and 

Post (1954) from Bikini and Todd and Low (1960) from 

Enewetak. These studies were based largely upon drill 

hole cuttings. Because these authors recognized considerable downhole contamination of foraminiferal specimens in the cuttings, their studies concentrated almost 

exclusively on the last appearance datums (LAD) of 

species; they did not attempt to give the first appearance 

datums (FAD) from the drill hole cuttings. Thus comparing the overall depth and age range of species between 

the previous studies and the present one is not possible. 

The coring, split spoon, and hammer sampling procedures used in the PEACE project drilling resulted in 

relatively precise depth information, particularly on the 

FAD of foraminiferal species and, thus, their total stratigraphic ranges.

The considerable similarity in the highest occurrence 

depths of many foraminiferal species between the older 

studies and the PEACE Project borehole samples is a 

tribute to the effort and diligence of both the foraminiferal workers and the drilling crew of the original drill 

holes.

We relate the larger foraminiferal stratigraphie ranges 

in the PEACE boreholes first to the Tertiary Far East 

Letter Classification of Van der Vlerk and Umbgrove 

(1927) and then to the planktic foraminiferal global 

zonation and the Cenozoic series and stages following the 

studies of Adams (1984) and Chaproniere (1984). Van der 

Vlerk and Umbgrove (1927) developed the Tertiary Far 

East Letter Classification (hereafter, informally 

referred to as the Tertiary Far East Letter Zonation 

(TFELZ)) in the Netherlands East Indies. Initially, the 

TFELZ was used for regional biostratigraphic work in 

the Indo-West Pacific, and there was little or no attempt 

at detailed correlation of these letter zones with the 

European Tertiary Stages. During the past 15 years, 

however, studies conducted on planktic foraminifers and
D4 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

calcareous nannofossils in the Pacific and Indian Oceans 

have resulted in generalized correlation of the TFELZ 

with intercontinental microfossil zones and European 

Stages (Haak and Postuma, 1975; Adams, 1984; and 

Chaproniere, 1984) (fig. 3).

The papers cited indicate that enough is presently 

known of larger foraminiferal biostratigraphy that it can 

be used to document the general age assignment of the 

older strata in the Enewetak PEACE boreholes, 

although more biostratigraphic tie points between the 

larger foraminiferal species first and last appearance 

datums in the Pacific and the intercontinental calcareous 

plankton zones are desirable to more precisely document 

the Oligocene and Miocene correlations.

Cole's two Marshall Island larger foraminiferal studies 

(1954; 1957) were published prior to the advent of the 

numerous Pacific planktic microfossil studies and, thus, 

before the general correlation of the TFELZ into the 

global planktic zonations and the European Stages. As a 

result of new correlations of the TFELZ, some changes 

are made here from the prior stage or series assignments 

made by Cole (1957) for the strata containing larger 

foraminiferal faunas in Enewetak. One particularly 

important change in age designation in the TFELZ is the 

correlation by Adams (1984) and Chaproniere (1984) of 

the upper part of Te (=Te 5) with an early Miocene age 

and the lower part of Te (=Te 1-4) with a late Oligocene 

age (fig. 3). Cole's older studies placed all of Te in the 

lower Miocene.

Enewetak strata containing diagnostic larger foraminifers are confined to the Te and Tf divisions of the 

TFELZ. Although the Te-Tf boundary was placed in 

planktic foraminiferal Zones N 7-8 by Haak and Postuma 

(1975), Adams (1984) and Chaproniere (1984) now place 

this boundary within Zone N 6 (fig. 3). The upper Te 

(=Te 5) -lower Te (=Te 1-4) boundary in the above three 

studies, as well as in Hashimoto and others (1977), 

corresponds with the Oligocene-Miocene boundary.

LARGER FORAMINIFERAL OCCURRENCES, 

ZONATION, AND AGE ASSIGNMENTS

Larger foraminifers occur in most sample intervals in 

all Enewetak PEACE boreholes. The 27 shallow-depth 

PEACE boreholes, however, penetrated only into the 

uppermost 100 to 300 ft of the sedimentary column; these 

shallower depths are composed of Pliocene and Pleistocene strata and contain only a few long-ranging larger 

foraminiferal species.

The five deepest PEACE boreholes, KAR-1, KBZ^, 

OAR-2, OBZ-4, and OOR-17, penetrated to depths 

greater than 500 ft. Additional genera and species of 

larger foraminifers appear in the middle Miocene and 

older strata found in the middle to lower parts of these 

boreholes. These include species of Heterostegina, LepiV) 

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FIGURE 3. Correlation of Tertiary Far East Letter Zones, planktic 

foraminiferal zones, and European Stages (from Adams, 1984, with 

some addition to and modification of nomenclature).
LARGER FORAMINIFER BIOSTRATIGRAPHY OF PEACE BOREHOLES, ENEWETAK ATOLL D5

docyclina, Miogypsina, Miogypsinoides, and Spiroclypeus, some of which have widespread biostratigraphic 

utility and are diagnostic of late Oligocene and early 

Miocene times. The larger foraminiferal assemblages at 

Enewetak contain some diagnostic biostratigraphic species of the Indo-Western Pacific area. However, the 

number of larger foraminiferal species at Enewetak is 

fairly low in comparison to some coeval assemblages 

found farther west in larger Indo-Western Pacific land 

masses.

Four boreholes contain only the upper part of the 

diagnostic larger foraminiferal sequence because of the 

shallower terminal depth of the holes. Diagnostic species 

are found in shorter intervals of about 200 ft in KAR-1 

and KBZ-4, about 140 ft in OOR-17, and just a few feet 

in OAR-2. However, diagnostic species occur through a 

728-ft interval in OBZ-4, the deepest PEACE borehole, 

and this hole provides the best developed sequence of 

larger foraminiferal assemblages and yields the oldest 

fossiliferous material sampled in the PEACE boreholes.

The diagnostic larger foraminiferal species that occur 

in the PEACE boreholes are discussed from the highest 

of the local Enewetak zones (AA) established by Cronin 

and others (1986) to the oldest (MM). The occurrences 

and abundances of larger foraminifers in the five deepest 

PEACE boreholes are given in figures 4-7. The stratigraphic ranges of the larger foraminifer species in the 

five deepest PEACE boreholes, as well as some smaller 

benthic species discussed below, in relation to the local 

Enewetak benthic zones are given in figure 8. The 

taxonomic concepts used in the present paper follow 

those of Cole (1957, 1975) unless otherwise mentioned. 

The correlation of the middle Miocene to Holocene 

Enewetak benthic zones with intercontinental planktic 

zonations is from the work of Bybell and Poore (this 

volume).

Zones AA-HH. These upper zones established in the 

PEACE boreholes are of middle Pliocene to Holocene 

age on the basis of the planktic foraminifers and calcareous nannofossils. They contain only three larger foraminiferal species, Heterostegina suborbicularis (not shown 

in fig. 8), Marginopora vertebralis, and Sorites marginalis. All three species have long time ranges, from the 

Miocene into the present day at Enewetak, and cannot be 

used to subdivide these strata. The FAD of both M. 

vertebralis and S. marginalis is in the upper part of Zone 

LL (fig. 8); this Enewetak zone is placed in Zone NN 9 of 

early late Miocene age by Bybell and Poore (this volume).

Zones II-LL. These zones are of middle Miocene to 

early Pliocene ages. Several other larger foraminiferal 

species occur in these zones in addition to the three 

species mentioned above. They include three species of 

Operculinoides, two species of Operculina, and a single

species of Alveolinella and Lepidocyclina. These species 

are of limited biostratigraphic use over the western 

Pacific area.

Alveolinella quoyi has a FAD in the late middle 

Miocene and extends into the present-day western 

Pacific faunas, according to Adams (1984). This species 

has a much more restricted stratigraphic range in the 

Enewetak subsurface section; its lowest occurrence is in 

the middle of Zone LL, and it is eliminated in the lower 

part of Zone II (fig. 8) (uppermost middle Miocene to 

lower Pliocene). It does not occur in higher horizons in 

any Enewetak PEACE samples. A similar restriction of 

this species to older stratigraphic levels also was found in 

previous drill hole studies at Enewetak (Cole, 1957) and 

at Bikini (Todd and Post, 1954). This species is absent in 

present-day Enewetak assemblages and in those from 

other Marshall Islands studied by Cushman, Todd, and 

Post (1954). Thus, although the earliest Enewetak occurrence of this species is similar to the age given by Adams 

(1984) for the western Pacific, it has an early local 

extinction in the Marshall Islands.

Operculina lucidisutura is found in Zone II downward 

into middle Zone LL (fig. 8), or in the lower Pliocene to 

upper or upper middle Miocene Series. Cole (1957) 

reported the highest occurrence of this species at 

Enewetak from strata that he placed in Tg. The uppermost occurrence of 0. lucidisutura in Zone II in the 

PEACE boreholes agrees with Cole's report; Zone II is 

assigned to the early Pliocene Epoch by Bybell and Poore 

(this volume); this assignment is where Adams (1984) 

places the base of Tg.

Operculina victoriensis is found sporadically in low 

abundances from lower Zone LL down into Zone MM 

(fig. 8), or in the middle Miocene to upper lower Miocene 

Series. Three species of Operculinoides occur in these 

zones. These three species are part of a grouping that 

exhibits considerable morphologic variation, particularly 

in the amount of test inflation. 0. rectilata is found from 

Zone II down into Zone KK (fig. 8), or in the lower 

Pliocene down into the upper Miocene Series. Cole (1957) 

gave an upper limit of Tg for this species (Pliocene 

according to Adams, 1984); this assignment is in agreement with the highest occurrence in Zone II found here. 

0. amplicuneata is found from Zone JJ down into middle 

Zone LL (fig. 8), or in the upper Miocene to possibly 

uppermost middle Miocene Series. Cole (1957) reported a 

highest occurrence of this species at Enewetak in strata 

that he assigned to Tg (Pliocene, according to Adams, 

1984); the occurrence here only in Miocene strata in 

KBZ-4 (fig. 6) and KAR-1 (fig. 7) suggests an upper 

limit in the upper part of Tf. The third species, 0. 

bikiniensis, occurs only in lower Zone LL strata and is 

discussed below.
D6 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

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borehole OEZ-4 and placement in Enewetak benthic zones and Tertiary Far 

East Letter Zones. A=greater than 12 specimens; F=9-12 specimens; 

C=5-8 specimens; E=l^l specimens, bsf, below sea floor.
LARGER FORAMINIFER BIOSTRATIGRAPHY OF PEACE BOREHOLES, ENEWETAK ATOLL D7

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0

DC

LEI EAST FAR 

DC

DC

LU

Tf

UPPER 

Te?

UPPER 

Te

7 

CC 

<

0

884

886

F R 

F R

MM1 UPPER 

Te?

FIGURE 5. Occurrences of larger foraminiferal species in 

boreholes OOR-17 and OAR-2 and placement in Enewetak 

benthic zones and Tertiary Far East Letter Zones. 

A=greater than 12 specimens; F=9-12 specimens; C=5-8 

specimens; R=l-4 specimens, bsf, below sea floor.

Lepidocyclina (Nephrolepidina) orientalis, the stratigraphically youngest ranging species of this genus in the 

subsurface of Enewetak, occurs only in the lowest 10 to 

15 ft of lower Zone LL (fig. 8) in boreholes OBZ-4, 

KBZ^, KAR-1, and OOR-17 (figs. 4-7). Cole (1957) also 

found this species to be confined to a 10-ft interval in the 

K-1B drill hole at Enewetak, which he considered to be 

the lowest part of Tf. Lower Zone LL of the PEACE 

boreholes is placed in Tf, but it is uncertain where in the 

middle Miocene this zone belongs.

Operculinoides bikiniensis co-occurs with L. orientalis in the lowermost part of Zone LL (fig. 8) in KBZ^l 

and KAR-1 (figs. 6 and 7); 0. bikiniensis is found only in 

these two PEACE coreholes, both located in KOA crater 

area, and only in samples that also contain L. orientalis. 

Cole (1957) considered that these two species were 

characteristic of the Tf zone; Adams (1970) also confined 

the range of L. orientalis to within Tf. Thus, the 

co-occurrence of these two species in lower Zone LL in 

the two PEACE boreholes suggests a placement of these 

strata within Tf, although it is uncertain in what portion 

of Tf they belong. Adams (1984) gave an age range of late 

early Miocene into the early Pliocene for Tf. A more

restrictive age range for the lower part of lower Zone LL 

strata to somewhere within the middle Miocene is suggested by the occurrence in these strata in several 

PEACE boreholes of planktic foraminifers and calcareous nannofossils that are characteristic of planktic foraminiferal Zones N 9 to N 13 (Bybell and Poore, this 

volume).

Zone MM. Zone MM was defined by Cronin and others 

(1986) to include all strata from the base of Zone LL to 

the bottom of the five deepest PEACE boreholes 

(KAR-1, KBZ^, OAR-2 and 2A, OOR-17, and OBZ^), 

which penetrated strata below Zone LL. No datable 

calcareous nannofossils or planktic foraminifers were 

recovered from definite Zone MM. Well-preserved 

smaller foraminifers and ostracodes are present in the 

upper part of Zone MM, where they were used to 

delineate this zone from Zone LL, but these groups are 

highly recrystallized and unidentifiable in the middle and 

lower parts of this zone. However, most parts of Zone 

MM contain abundant and well-preserved larger foraminiferal specimens.

The LL-MM zonal boundary is placed at 815 ft in 

OBZ^l and here coincides with unconformity number 7 of 

Wardlaw and Henry (1986). Unconformity 7 of Wardlaw 

and Henry (1986) is coincident with the LL-MM boundary in four of the five boreholes. The base of Zone LL is 

marked in the larger foraminifers by the FAD of L. 

(Nephrolepidina) orientalis and 0. bikiniensis] the top 

of Zone MM is marked by the LAD of Miogypsina 

(Lepidosemicyclina) thecideaeformis and Lepidocyclina 

(Nephrolepidina) sumatrensis (fig. 8). Zone MM can be 

divided into subzones MM1, MM2, and MM3 on the basis 

of the stratigraphic distribution of the larger foraminiferal assemblages. The new foraminiferal subdivisions of 

Zone MM and their characteristic larger foraminiferal 

assemblages are as follows.

Subzone MMl. This subzone is marked by the presence of Miogypsina (Lepidosemicyclina) thecideaeformis. The top of Subzone MMl is marked by the local 

extinction of M. (Lepidosemicyclina) thecideaeformis 

and Lepidocyclina (Nephrolepidina) sumatrensis and by 

the FAD of L. orientalis and 0. bikiniensis in the 

overlying lowermost part of lower Zone LL (fig. 8). The 

normal first appearance of M. (Lepidosemicyclina) thecideaeformis marks the base of the subzone, and the local 

extinction of Miogypsinoides (Miogypsinoides) dehaartii 

occurs at the top of the underlying Subzone MM2 (an 

exception to this pattern is found in one sample at 964 ft 

in OBZ^l, where M. thecideaeformis and M. dehaartii 

both occur). Cole (1957, p. 746) recognized a sequence of 

species occurrences similar to that found in the Bikini 

drill holes. Subzone MMl extends from 815 to 834 ft in
D8 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

BOREHOLE

rfr

 4

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SAMPLE DEPTH (FEET) bsf

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555

561

565

569

592

665

674

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756

760

765

777

780

812

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860

864

915

925

930

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980

996

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Operculina lucidisutura Operculinoides rectilata 

Operculinoides amplicuneata

F 

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Marginopora vertebralis 

Alveolinella quoyi Sorites marginalis

r

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A 

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Lepidocyclina orientalis Operculina victoriensis Operculinoides bikiniensis

1 

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Lepidocyclina sumatrensis thecideaeformis Miogypsina dehaartii Miogypsinoides 

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ENEWETAK BENTHIC ZONE

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LOWER 

LL

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MM2

TERTIARY FAREAST ZONES LETTER 

Tg

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Te?

UPPER 

Te

FIGURE 6. Occurrences of larger foraminiferal species in borehole KBZ-4 and 

placement in Enewetak benthic zones and Tertiary Far East Letter Zones. 

A=greater than 12 specimens; F=9-12 specimens; C=5-8 specimens; R=l-4 

specimens, bsf, below sea floor.

Both M. (Lepidosemicyclina) thecideaeformis and L. 

(Nephrolepidind) sumatrensis are known to range from 

upper Te into the lower part of Tf (Adams, 1970; Cole, 

1975; Chaproniere, 1984, found M. thecideaeformis 

extending as high as Zone N 9 in Tf strata in Australia); 

Subzone MM1 is tentatively placed in upper Te, however, as upper Te is the most common time of overlap of 

these two species and species commonly present in Tf, 

such as L. (Nephrolepidina) orientalis, are absent from 

the assemblages. Cycloclypeus transiens occurs in one 

sample in Subzone MM1 and in one sample in the 

uppermost part of Subzone MM2; Cole (1975) reported 

this species from upper Te and Tf in the western Pacific.

Subzone MM1 is present in the five deepest PEACE 

boreholes. The thickness of Subzone MM1 varies considerably in the different PEACE boreholes; much of this

variation may be an artifact of the absence of larger 

foraminiferal assemblages in some relatively thick intervals in the boreholes; this absence makes definitely 

placing the subzone boundaries difficult. Borehole 

OAR-2 reached into Subzone MM1 but did not penetrate 

into Subzone MM2, as the other four boreholes did.

Subzone MM2. This subzone is defined by the total 

range of M. dehaartii; it extends from 873 to 1,195 ft in 

OBZ^. The lower part of Subzone MM2 in OBZ^, from 

1,079 to 1,195 ft, is characterized by the co-occurrence of 

Spiroclypeus margaritatus, M. dehaartii, and L. sumatrensis.

Subzone MM2 is present in the four deepest PEACE 

boreholes, and it was recognized also by examination of 

the occurrences in Cole's (1957) data from Enewetak drill 

holes K-1B, E-l, and F-l. Borehole OBZ^ penetrated
LARGER FORAMINIFER BIOSTRATIGRAPHY OF PEACE BOREHOLES, ENEWETAK ATOLL D9

BOREHOLE

DC 

< 

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590

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Operculinoides rectilata lucidisutura Operculina 

Alveolinella quoyi

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vertebralis Marginopora 

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Operculinoides amplicuneata

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Operculinoides bikiniensis Lepidocyclina sumatrensis

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thecideaeformis Miogypsina 

Operculina victoriensis Miogypsinoides dehaartii

F 

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BENTHIC ENEWETAK ZONE

II

JJ

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UPPER 

LL

MIDDLE 

LL

LOWER 

LL

MM1

MM2

TERTIARY FAREAST LETTER ZONS

Tg

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Te?

UPPER 

Te

FIGURE 7. Occurrences of larger foraminiferal species in borehole KAR-1 and placement in Enewetak benthic zones and Tertiary Far East Letter Zones. A=greater 

than 12 specimens; F=9-12 specimens; C=5-8 specimens; R=l^l specimens, bsf, 

below sea floor.

through Subzone MM2 and into Subzone MM3; boreholes 

OOR-17, KBZ-4, and KAR-1 penetrated into Subzone 

MM2 but not deeply enough to reach the interval of S. 

margaritatus in the lower part of MM2.

The coincidence of the last appearance of both M. 

dehaartii and Austrotrillina striata (a smaller benthic 

species) at 873 ft in OEZ-4 suggests a possible hiatus at 

or slightly above this horizon. Unconformity number 8 of 

Wardlaw and Henry (1986) occurs at 866 ft in OBZ^l; 

this horizon may more accurately represent the last 

appearance of these two species because the next sample 

studied above 873 ft is from above this unconformity.

The concurrent ranges of M. dehaartii and L. sumatrensis from 873 to 964 ft in OBZ-4, and the presence of 

A. striata from 873 to 924 ft, suggest placement of upper 

Subzone MM2 into upper Te (Adams, 1970, 1984; 

Chaproniere, 1984). These authors assign upper Te from 

the Oligocene-Miocene boundary to the middle lower 

Miocene (from within planktic foraminiferal Zone N 4 to 

within Zone N 6).

The lower part of Subzone MM2 below 1,079 ft in 

OEZ-4 also is assigned to upper Te (lower Miocene). The 

co-occurrence here of M. dehaartii, L. sumatrensis, and 

S. margaritatus suggests that this part of Subzone MM2
D10 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

Enewetak 

benthic 

zonation 

Cronin and 

others, 1986

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middle LL

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SPECIES RANGES

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FIGURE 8. Stratigraphic ranges through Enewetak benthic zones of larger foraminifers 

and selected smaller foraminifers from all PEACE and three EXPOE boreholes at 

Enewetak Atoll (subdivision of zone MM is made in this paper).

probably also belongs in upper Te. The extinction of S. 

margaritatus is the only change noted within the lower 

part of Subzone MM2. Adams (1984) showed that, for the 

western Pacific area, the S. margaritatus group ranged 

from near the base of lower Te to the top of upper Te. 

Chaproniere (1984) reported that S. margaritatus 

occurred only in the uppermost part of lower Te in 

eastern Australia. Thus, the last occurrence of S. margaritatus may mark the lower Te-upper Te boundary at 

Enewetak.

Subzone MM3. This oldest subzone in the PEACE 

boreholes is recognized only in borehole OBZ-4, where 

the subzone extends from 1,211 ft to the lowest sample 

examined for foraminifers (1,543 ft). The Subzone MM2- 

MM3 boundary is marked by the first appearance of M. 

dehaartii at the base of subzone MM2 and by the last 

appearance of Miogypsinoides ubaghsi at the top of

Subzone MM3. L. (Nephrolepidina) sumatrensis occurs 

throughout subzone MM3, except in the bottom two 

samples. There are several other first and last larger 

foraminiferal appearances in this subzone. For example, 

Heterostegina borneensis is present only in the lower 

part of Subzone MM3, and Lepidocyclina (E.) ephippoides occurs only in the lowest sample examined in this 

subzone. S. margaritatus occurs from the lower part of 

Subzone MM2 down to 1,383 ft in the middle of Subzone 

MM3. M. ubaghsi is found only from the top of Subzone 

MM3 to a lowermost appearance at 1,300 ft.

This larger foraminiferal assemblage suggests assignment of Subzone MM3 to lower Te of the TFELZ (of late 

Oligocene age). Many of the Subzone MM3 species have 

lower Te ranges (Adams, 1984; Chaproniere, 1984). In 

addition, the highest appearance of M. ubaghsi and H.
LARGER FORAMINIFER BIOSTRATIGRAPHY OF PEACE BOREHOLES, ENEWETAK ATOLL Dll

borneensis was considered by Haak and Postuma (1975) 

to mark the top of lower Te, and both species occur in 

Subzone MM3.

SMALLER FORAMINIFERAL OCCURRENCES 

AND AGE ASSIGNMENTS

Diagenetic alteration of smaller benthic foraminiferal 

tests occurs in several stratigraphic intervals of the 

PEACE boreholes; this alteration presents problems in 

species identifications and thus in the determination of 

biostratigraphic ranges of some of these taxa. Alteration 

is most extensive in much of the lower strata (late 

Oligocene and early Miocene age), where essentially all 

smaller foraminiferal species are either so badly recrystallized as to be unidentifiable or have been completely 

removed by dissolution. Even in some intervals in the 

middle parts of the boreholes, particularly in Zone HH of 

Cronin and others (1986) (early Pliocene age), almost all 

smaller benthic species except for Amphistegina have 

been badly recrystallized and are generally unidentifiable.

Our examination of smaller benthic species distributions in the PEACE samples indicates that many species 

do have restricted biostratigraphic occurrences within 

Enewetak. Forty-nine smaller benthic foraminiferal species having short to moderately long local stratigraphic 

ranges were used to help subdivide the Holocene to 

Miocene strata into 13 biostratigraphic zones (Cronin and 

others, 1986). Although many of the Enewetak age 

ranges may be only of local significance, some species 

may have similar age ranges over much of the western 

Pacific area and are useful biostratigraphically. However, biostratigraphic ranges for most smaller benthic 

foraminiferal species found in the Enewetak samples are 

largely unknown for the western Pacific area. Many of 

the earlier species reports have broad or vague age 

ranges applied to them, and so the species are difficult to 

place into a refined biostratigraphic age.

A lack of biostratigraphic control for smaller benthic 

species is suggested by the earliest appearance of many 

smaller benthic species of the Pacific basin in the 

Enewetak subsurface stratigraphic sections (Gibson, 

1987). How many of these earliest appearances at 

Enewetak can be applied to the Pacific basin as FAD's is 

uncertain. Whether the large numbers of earliest occurrences at Enewetak are an artifact of the intensive 

sampling done in the numerous cores at Enewetak or 

whether this part of the western Pacific is the actual area 

of origination of many of these species still is uncertain. 

However, the biostratigraphic information presently 

available in the foraminiferal literature suggests that 

many of these species did originate in the western 

Pacific.

Adams (1984, fig. 5) gave, in addition to age ranges of 

larger foraminiferal species, the age ranges of several 

Indo-West Pacific smaller benthic foraminiferal species 

that he considered possibly useful biostratigraphically. 

Three of these taxa from Adams' study, Baculogypsina 

sphaerulata, Calcarina spengleri, and Austrotrillina 

striata, occur in the Enewetak samples. Their stratigraphic ranges at Enewetak and at other islands in the 

Marshalls are given here for comparison with their 

overall Pacific ranges.

Baculogypsina sphaerulata. Adams (1984) reported 

this species as ranging from the base of the Pleistocene 

Series (base of Calabrian age) into the present-day 

faunas in the Indo-Pacific area. Its stratigraphic range in 

Enewetak strata is considerably shorter, as it occurs 

there only in Zone CC of Cronin and others (1986). Zone 

CC is above the base of the Pleistocene Series, which is 

placed within Zone EE-FF at Enewetak (Bybell and 

Poore, this volume); thus, this species has a later FAD at 

Enewetak than in some other Pacific locations. It also 

exhibits an older LAD at Enewetak, as it is not found in 

the upper Pleistocene and Holocene strata in the cores 

(Zones AA and BB of Cronin and others, 1986). Todd and 

Post (1954) also reported a similar middle Pleistocene 

local extinction at Bikini. This species is not known from 

the living assemblages at Enewetak or throughout the 

entire Marshall Islands (Cushman, Todd, and Post, 

1954), even though it still is living in other parts of the 

western Pacific.

Calcarina spengleri. Adams (1984) recorded this 

species from basal Pleistocene strata, with questionable 

occurrences extending into the middle Pliocene, into the 

present-day fauna. This species has a similar stratigraphic distribution in the PEACE cores; C. spengleri is 

the overwhelmingly dominant species in the Holocene 

(Zone AA) foraminiferal assemblages from the shallow 

back-reef environments. This species is rare and sporadic in the older zones and extends to the middle to 

upper parts of Zone EE of Cronin and others (1986) 

(latest Pliocene to basal Pleistocene age, according to 

Bybell and Poore, this volume).

Austrotrillina striata. The smaller benthic genus 

Austrotrillina has been incorporated into western 

Pacific biostratigraphic zonations by Adams (1984) and 

Chaproniere (1984). Two species, A. striata and A. 

howchini, commonly are integrated into larger foraminiferal biostratigraphic zonations and used as characteristic species of the TFELZ in the western Pacific. A. 

striata occurs in middle and upper Oligocene through 

lower Miocene strata, or approximately upper Zone P 19 

to Zone N 6 (planktic foraminiferal zones of Blow, 1969), 

according to Adams (1984). Chaproniere (1984) gives an 

earlier extinction in Australia at the top of Zone N 4. 

This species is considered characteristic of uppermost Td
D12 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

through Te (both lower and upper Te) letter zones. A. 

howchini, which is characteristic of the lower part of Tf, 

occurs in upper lower and lowermost middle Miocene 

strata, most commonly in Zones N 7 and N 8, but ranges 

from Zone N 6 to within Zone N 9 (Chaproniere, 1981, 

1984; Adams, 1984). Only the older of these two species, 

A. striata, has been recognized in the PEACE boreholes.

Austrotrillina striata is found in only one of the 

PEACE boreholes, OBZ-4, in samples from depths of 

873 to 924 ft. Below 924 ft, smaller benthic foraminifera 

are unrecognizable because of recrystallization. This 

species is absent from equivalent intervals in boreholes 

KBZ-4 and OOR-17, presumably because of significant 

recrystallization of the microfaunas at these depths in the 

two boreholes. No planktic foraminifers or calcareous 

nannofossils were recovered from the interval containing 

A. striata in OBZ-4. The sample at 808 ft, which was 

placed within Zones N 9-13 (middle Miocene) on the basis 

of planktic foraminifers (Bybell and Poore, this volume), 

is 65 ft above the highest occurrence of A. striata in 

OBZ-4. Adams (1984) considered that the highest occurrences of A. striata are usually within Zone N 6.

The fairly close proximity in depth between presumably middle lower Miocene and middle Miocene strata in 

OBZ-4 suggests that sediment accumulation during 

some of late early and early middle Miocene Epoch may 

be missing here; if the middle Miocene strata are only 

from the higher part of this zonal range of N 9 to N 13, 

much of the middle Miocene may be missing at 

Enewetak. The possible absence of Zones N 7 and N 8 

age strata in OBZ-4 and in the other Marshall Island drill 

holes may explain the absence of A. howchini, which 

commonly occurs in these zones in other western Pacific 

areas (Adams, 1984). Ecological factors may provide an 

alternative explanation for this absence of A. howchini in 

the Marshall Islands, but this explanation seems unlikely 

because similar, shallow back-reef environments are 

characteristic of the lower to middle Miocene strata in 

the boreholes. The specimens reported from Enewetak 

and Bikini as A. howchini (Cole, 1954; Todd and Post, 

1954; Todd and Low, 1960) were examined subsequently 

by Adams (1968) and placed into A. striata.

Austrotrillina striata also is found at Enewetak in 

deep drill hole E-l (fig. 1) (Todd and Low, 1960), where 

it occurs abundantly in cuttings between 1,835 and 2,003 

ft and in core sample E-l-1-1 from 2,003-2,028 ft. On the 

basis of the presence of this genus, Cole (1957) placed 

this cored interval into the lower part of Te. Cole also 

found larger foraminiferal species typical of lower Te 

(Heterostegina borneensis and Lepidocyclina (Eulepidina) ephippioides) at similar depths (and therefore presumably a similar stratigraphic position) in another drill 

hole, F-l, at Enewetak; their presence supports this

assignment to lower Te. Sample E-l-1-1 contains 

Globorotalia kugleri (Bybell and Poore, this volume) and 

thus is of latest Oligocene or earliest Miocene age; the 

associated calcareous nannofossils suggest a possible 

Oligocene placement of these strata, which is in agreement with the lower Te assignment.

RATES OF ACCUMULATION

Rates of accumulation at Enewetak during the late 

Oligocene to Holocene are estimated from drill hole 

biostratigraphic data. Two data sources are available; 

middle Miocene to Holocene biostratigraphic data from 

middle and upper parts of the PEACE boreholes (Cronin 

and others, 1986; Bybell and Poore, this volume) and 

upper Oligocene and lower Miocene foraminiferal data 

from the lower part of the PEACE boreholes (this 

study).

The shallowest depth of species occurrences in the 

PEACE boreholes, as given in Cronin and others (1986), 

is referenced to the beginning point of drilling on the sea 

bottom at each site. PEACE borehole sites were all 

located below sea level; previous deep drill hole sites at 

Enewetak were on land. To compare biostratigraphic 

events from all these data sets, sample depths need to be 

standardized to depths below sea level (fig. 2).

Following normalization to depth below sea level, 

depth differences of considerably less than 100 ft generally are found for equivalent biostratigraphic horizons 

among the various Enewetak drilling sites. These depth 

differences are relatively small in relation to the considerable thickness of upper Oligocene and lower Miocene 

strata found at Enewetak and do not significantly affect 

the general thickness patterns. The depth differences in 

many cases may result more from the difficulty of finding 

species in sparse assemblages or the presence of nonfossiliferous intervals rather than from an actual difference 

in the LAD's. Differences in the depth of equivalent 

horizons in relation to sea level also may exist in the 

various drill holes because of erosional relief developed 

on the limestone surfaces on which they were deposited.

The placement of subzones MM1 and MM2 in the early 

Miocene demonstrates that a considerable sedimentary 

thickness accumulated during this time interval. Subzone MM2 is considered to have a maximum time extent 

from the beginning of the Miocene (Zone N 4) to the end 

of the time range of Austrotrillina striata (top of Zone N 

6), or about 7 m.y. (Haq and others, 1987). The accumulation of approximately 340 ft of sediment in borehole 

OBZ-4 during this subzone yields an accumulation rate of 

nearly 50 ft/m.y. during this part of the early Miocene. 

The addition of approximately 60 ft of lower Miocene 

subzone MM1 gives a thickness of about 400 ft for early
LARGER FORAMINIFER BIOSTRATIGRAPHY OF PEACE BOREHOLES, ENEWETAK ATOLL D13

Miocene sedimentation. This thickness compares with a 

thickness of about 1,000 ft for the middle Miocene to 

Holocene strata in OBZ-4 (to sea level); these strata 

represent a time span of as little as 11 m.y. or as many as 

15 m.y., depending upon where, in the Zone N 9 to Zone 

N 13 range, the sediments in the lower part of Zone LL 

belong. These age variations yield an accumulation rate 

of a minimum of 65 to a maximum of 90 ft/m.y.; the 

minimum rate is slightly greater than that calculated for 

the early Miocene, and the maximum rate is almost twice 

as great.

The top of Zone MM3, a zone herein considered to 

belong in lower Te and thus to be of late Oligocene age, 

is at a depth of 1,409 ft bsl in borehole OBZ-4. The 

lowermost sample dated in this borehole, at 1,742 ft bsl, 

still is placed in lower Te. This placement shows a 

minimum upper Oligocene thickness of 332 ft in OBZ-4. 

A late Oligocene sedimentation rate cannot be calculated 

from this borehole alone, however, because the exact 

placement in the late Oligocene for the lowermost strata 

is unknown; thus, the amount of time that these strata 

represent also is unknown.

Although the total thickness of upper Oligocene strata 

at Enewetak is uncertain, a minimum of slightly greater 

than 300 ft is present; the documentation herein of this 

thickness of upper Oligocene and approximately 400 ft of 

lower Miocene demonstrates that the late Oligocene and 

early Miocene are major times of accumulation at 

Enewetak. As the thickness of carbonate sediments in 

atolls appears to be limited largely by the rate of 

subsidence, the thickness suggests that these intervals 

were also times of significant subsidence.

Accumulation rates of 50 to 90 ft/m.y. are estimated in 

the present study for the lower Miocene and the middle 

Miocene to Holocene strata at Enewetak. These rates 

are similar to those proposed by Schlanger and others 

(1987), who calculated subsidence rates of 82 to 98 ft/m.y. 

for the Marshall Island area from the study of drowned 

atolls there.

LIST OF SPECIES TREATED FROM PEACE AND 

EXPOE ENEWETAK SUBSURFACE SAMPLES

LARGER FORAMINIFERS

Alveolinella quoii (d'Orbigny) 

Cycloclypeus transiens Tan 

Heterostegina borneensis van der Vlerk 

Heterostegina suborbicularis d'Orbigny 

Lepidocyclina (Eulepidind) ephippioides Jones and

Chapman

Lepidocyclina (Nephrolepidind) orientalis van der Vlerk 

Lepidocyclina (Nephrolepidind) sumatrensis (Brady)

Marginopera vertebralis Quoy and Gaimard 

Miogypsinoides dehaartii (van der Vlerk) 

Miogypsinoides (Lepidosemicyclind) thecideaeformis

(L. Rutten)

Miogypsinoides ubaghsi Tan 

Operculina lucidisutura Cole 

Operculina victoriensis Chapman and Parr 

Operculinoides amplicuneata Cole 

Operculinoides bikiniensis Cole 

Operculinoides rectilata Cole 

Sorites marginalis (Lamarck) 

Spiroclypeus margaritatus (Schlumberger)

SMALLER BENTHIC FORAMINIFERS

Austrotrillina striata Todd and Post 

Baculogypsina sphaerulata (Parker and Jones) 

Calcarina spengleri (Gmelin)

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