Cover design by Nancy Polend, 1991
Calcareous Nannofossils and Planktic 

Foraminifers from Enewetak Atoll, 

Western Pacific Ocean

By LAUREL M. BYBELL and RICHARD Z. POORE

GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL, 

REPUBLIC OF THE MARSHALL ISLANDS

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1513-C

Prepared in cooperation with the 

Defense Nuclear Agency

Calcareous nannofossils and planktic 

foraminiferal data from seven Enewetak Atoll 

wells and boreholes were combined with a local 

benthic biostratigraphy based on ostracodes and 

benthic foraminifers to produce a composite 

biostratigraphic sequence

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

Bybell, Laurel M.

Calcareous nannofossils and planktic foraminifers from Enewetak Atoll, western Pacific Ocean / by Laurel M. Bybell and Richard

Z. Poore. 

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

Survey professional paper ; 1513-C) 

Includes bibliographical references 

Supt. of Docs. no. : I 19. 16: 1513-C

1. Nannofossils Marshall Islands Enewetak Atoll.

2. Foraminifera, Fossil Marshall Islands Enewetak Atoll.

3. Paleontology Miocene. 4. Paleontology Pliocene.

5. Paleontology Marshall Islands Enewetak Atoll. 6. Pacific Enewetak Atoll Crater Exploration Program. I. Poore, Richard

Z. II. Title. III. Series. IV. Series: U.S. Geological Survey professional paper ; 1513-C. 

QE719.B93 1992

560-dc20 91-10890

CIP

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

Federal Center, Box 25425, Denver, CO 80225
CONTENTS

Abstract.........................................................................

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

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

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

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

Microfossil Zonations and Datums ......................................

Calcareous Nannofossils..............................................

Planktic Foraminifers.................................................

Microfossil Occurrences and Zone Assignments for Individual 

Boreholes...........................................................

OOR-17...................................................................

OBZ-4.....................................................................

OAR-2 and 2A..........................................................

KBZ-4.....................................................................

KAR-1 ....................................................................

XEN-3 ....................................................................

Page

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5

5

6

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6

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13

Microfossil Occurrences and Zone Assignments for Individual 

Boreholes Continued

E-l........................................................................

Composite Sequence........................................................

Zone MM ................................................................

Lower Zone LL .......................................................

Upper Zone LL........................................................

Lower Zone KK.......................................................

Upper Zone KK and Zone JJ......................................

Zones II, HH, and GG...............................................

Zone EE/FF............................................................

Zone DD.................................................................

Zones CC, BB, and AA .............................................

Radiometric Dating.........................................................

Conclusions....................................................................

List of Species Found in Boreholes at Enewetak..................

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

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16 

16 

16

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17

17

17

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19

20

21

ILLUSTRATIONS

FIGURE 1. Map showing location of Enewetak Atoll and boreholes discussed in this paper .....................................................

2-8. Charts showing:

2. Correlation of planktic foraminiferal and calcareous nannofossil zones to epochs .........................................

3. Calcareous nannofossil occurrences in boreholes KBZ-4 and OOR-17........................................................

4. Planktic foraminiferal occurrences in boreholes KAR-1 and OOR-17........................................................

5. Planktic foraminiferal occurrences in boreholes OAR-2 and 2A, KBZ-4, OBZ-4, and XEN-3.......................

6. Calcareous nannofossil occurrences in boreholes OAR-2 and 2A...............................................................

7. Calcareous nannofossil occurrences in boreholes KAR-1 and XEN-3........................................................

8. Combined calcareous nannofossil and planktic foraminiferal age data for boreholes KBZ-4, KAR-1, OOR-17, 

OAR-2 and 2A, and XEN-3 and final age assignment for each Enewetak benthic zone .........................

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in

GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL, REPUBLIC OF THE

MARSHALL ISLANDS

CALCAREOUS NANNOFOSSILS AND PLANKTIC FORAMINIFERS FROM 

ENEWETAK ATOLL, WESTERN PACIFIC OCEAN

By LAUREL M. BYBELL and RICHARD Z. POORE

ABSTRACT

Calcareous nannofossils and planktic foraminifers were examined 

from five boreholes that were sampled as part of the Pacific Enewetak 

Atoll Crater Exploration (PEACE) Program and from two previously 

drilled wells in the Enewetak area. Both calcareous nannofossils and 

planktic foraminifers were sparse and poorly preserved in these 

samples, but by combining all available age data from these two groups 

with a local benthic biostratigraphy based on ostracodes and benthic 

foraminifers, it was possible to obtain reliable ages for many of these 

sediments. The oldest sediments in the PEACE Program samples that 

were dated with planktic microfossils are middle Miocene in age 

(calcareous nannofossil Zones NN 5-7). Late Miocene Zones NN 9 and 

NN 11 also are definitely represented. There is a thick early Pliocene 

section at Enewetak, which cannot be dated any more accurately than 

somewhere within Zones NN 12-15, and a thin, poorly constrained late 

Pliocene section. Zone NN 19, which spans the Pliocene-Pleistocene 

boundary, is present at Enewetak, and it is overlain by middle to late 

Pleistocene sediments of undifferentiated Zones NN 20-21.

INTRODUCTION

PURPOSE AND SCOPE

Boring of the carbonate sequence at the northern end 

of Enewetak Atoll, Republic of the Marshall Islands, was 

conducted in 1985, as part of the Pacific Enewetak Atoll 

Crater Exploration (PEACE) Program (fig. 1). The 

overall goal of the program was to characterize physical 

effects of large-scale nuclear blasts, which were conducted in the early 1950's, on the sediments of the atoll. 

Most boreholes were drilled to several hundred feet 

subbottom, although a few were deeper than 1,000 ft, 

and one was drilled to 1,600 ft subbottom. Samples from 

PEACE Program cores and cores taken during earlier 

investigations of Enewetak Atoll were examined for 

microfossils to provide a refined correlation for the upper

Manuscript approved for publication February 5, 1991.

sedimentary sequence at Enewetak. The shallow-water 

reef, back-reef, and lagoonal sequence sampled at 

Enewetak commonly yielded abundant benthic microfossils (ostracodes, larger foraminifers, and smaller benthic 

foraminifers). These benthic microfossils, primarily 

ostracodes and benthic foraminifers, were used to 

develop a local biostratigraphy that proved useful for 

correlating and interpreting subsurface units sampled 

during the PEACE Program (Cronin and others, 1986; 

Wardlaw and Henry, 1986, this volume).

In contrast, calcareous nannofossils and planktic 

foraminifers occur sporadically and in low abundance at 

Enewetak and were not useful for detailed correlations 

among cores. However, some samples did yield stratigraphically diagnostic assemblages, and these samples 

made dating intervals within several Enewetak boreholes possible.

In this report we document the occurrences of stratigraphically diagnostic planktic microfossils in samples 

from Enewetak (generally referred to as core) and 

outline our rationale for incorporating all available diagnostic planktic assemblages into a composite sequence 

that was used to date the Enewetak benthic zonation.

ACKNOWLEDGMENTS

We wish to thank Nancy Goodwin, Linda B. Gosnell, 

Joseph E. Hazel, Jr., and Craig Montgomery for preparing the numerous samples, and we thank Jean M. SelfTrail for drafting the figures.

We appreciate the thoughtful reviews of Thomas M. 

Cronin, Bruce R. Wardlaw, and Harry J. Dowsett.

MATERIALS AND METHODS

A total of 32 boreholes were drilled at Enewetak as 

part of the PEACE Program. Many samples from every

Cl
C2

162°

GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL 

162°10' 162°20' 162°30'

KBZ-4

OAR-2 

OBZ-4 

OOR-17

40°

20°

IVY / 160000 GRID 7 

(FT)

+ 120000

+ 80000

SOUTHWEST 

PASSAGE

INDEX MAP 

160°E 180° 160°W 140° 120° 100°

EAST (DEEP; CHANNEL 

E-1

+ 40000

. t Ma

 I L

" .. h

rshall

.lands

awaii

V N.A.

\

X

SOUTH PASSAGE

5 NAUTICAL MILES 0 10,000 30,000 i

50,000 FEET 0 10 KILOMETERS

FIGURE 1. Location of Enewetak Atoll and position of the seven boreholes discussed in this paper.

borehole were examined, but stratigraphically diagnostic 

planktic assemblages were found only in five cores: 

KAR-1, OAR-2 and 2A, OOR-17, KBZ-4, and OBZ-4 

(fig. 1). The first three boreholes are reference holes that 

were drilled into undisturbed sediments away from areas 

affected by large-scale nuclear blasts. Cores KBZ-4 and 

OBZ-4 are moderately deep boreholes that were drilled 

in the center of two large blast-induced craters. In 

addition to the PEACE boreholes, XEN-3, a core which 

was taken on Engebi Island during the Exploration

Program on Enewetak (EXPOE) in 1973-74, yielded 

diagnostic planktic assemblages. Planktic microfossils 

also were examined from one deep interval in the 

Enewetak E-1 hole, which was drilled in 1952 as part of 

an Atomic Energy Commission test program of the Los 

Alamos Scientific Laboratory.

Information on the PEACE and EXPOE cores, including location, details of coring, and lithologic descriptions, 

is in Wardlaw and Henry (1986, this volume). A description of the E-1 hole is in Schlanger (1963). Processing
CALCAREOUS NANNOFOSSILS AND PLANKTIC FORAMINIFERS FROM ENEWETAK ATOLL C3

techniques for planktic microfossil samples are outlined 

in Cronin and others (1986).

MICROFOSSIL ZONATIONS AND DATUMS

In general, planktic microfossils are sparse, poorly 

preserved, or absent in most samples that we examined 

from the Enewetak cores. For example, our most productive calcareous nannofossil samples yielded one or 

two specimens per field of view at 500 x magnification, 

and most taxa present were stratigraphically longranging and solution-resistant species. When found, 

most planktic foraminiferal assemblages contained 

sparse, poorly preserved specimens of long-ranging species of Globigerinoides, such as G. sacculifer, G. ruber, 

or G. obliquus. Therefore, routinely using a single published calcareous nannofossil or planktic foraminiferal 

zonation such as Martini (1971) to interpret the assemblages was impossible. Instead, we used a variety of 

published and unpublished sources to assemble a group 

of biostratigraphic events (or biostratigraphic datums) 

that we found useful during this study.

These biostratigraphic events must be interpreted 

conservatively in the Enewetak boreholes because the 

sporadic occurrence of planktic microfossils makes 

observing an actual FAD (first appearance datum) or 

LAD (last appearance datum) highly unlikely. Thus, the 

observed lower and upper limits of taxa in Enewetak 

cores are used to set limits on the biostratigraphic 

assignment of samples; they are not interpreted as actual 

FAD's and LAD's.

We have prepared two lists of biostratigraphic events 

that proved to be useful in dating Enewetak samples, one 

for calcareous nannofossils and one for planktic foraminifers. Asterisks indicate taxa that have been used to 

formally define zone boundaries in the calcareous nannofossil zonations of Martini (1971) and Bukry (1973, 1975) 

or in the planktic foraminiferal zonation of Blow (1969).

CALCAREOUS NANNOFOSSILS

LAD *Pseudoemiliania lacunosa top Zone NN 19 

Pleistocene (Martini, 1971) 

LAD Calcidiscus macintyrei lower part of Zone NN

19 early Pleistocene (Gartner, 1977a) 

FAD Gephyrocapsa large species near base of Zone

NN 19 latest Pliocene (Berggren and others, 1985) 

LAD Sphenolithus abies complex or genus Sphenolithus near or at top of Zone NN 15 early Pliocene

(Martini, 1971; Bukry, 1973; Perch-Nielsen, 1985) 

LAD *Reticulofenestra pseudoumbilica top Zone NN

15-early Pliocene (Martini, 1971; Bukry, 1973) 

FAD Pseudoemiliania lacunosa near top of Zone NN

15 early Pliocene (Perch-Nielsen, 1985)

LAD * Discoaster quinqueramus top Zone NN 11 late 

Miocene (Martini, 1971; Bukry, 1973; Perch-Nielsen, 

1985)

LAD Discoaster berggrenii upper part of Zone NN 

11-late Miocene (Bukry, 1973; Perch-Nielsen, 1985)

LAD Discoaster neohamatus sporadic occurrence up 

into mid-Zone NN 11 late Miocene (Martini, 1971; 

Bukry, 1973; Perch-Nielsen, 1985)

FAD *Discoaster berggrenii base Zone NN 11 late 

Miocene (Bukry, 1973; Perch-Nielsen, 1985)

FAD Discoaster quinqueramus base Zone NN 

11-late Miocene (Martini, 1971; Bukry, 1973; PerchNielsen, 1985)

LAD Discoaster calcaris upper Zone NN 10 late Miocene (Martini, 1971; Perch-Nielsen, 1985)

LAD Discoaster prepentaradiatus upper Zone NN 

10-late Miocene (Bukry, 1973; Perch-Nielsen, 1985)

LAD *Discoaster hamatus top Zone NN 9 late Miocene (Martini, 1971; Bukry, 1973; Perch-Nielsen, 1985)

FAD Discoaster prepentaradiatus Zone NN 9 late 

Miocene (Bukry, 1973; Perch-Nielsen, 1985)

FAD Discoaster neohamatus sporadic occurrence in 

lower Zone NN 9, consistent from upper Zone NN 

9-late Miocene (Martini, 1971; Bukry, 1973; PerchNielsen, 1985)

FAD Discoaster pentaradiatus lower Zone NN 9, but 

possibly as high as Zone NN 11 late Miocene (Martini, 1971; Perch-Nielsen, 1985)

FAD * Discoaster hamatus base Zone NN 9 late Miocene (Martini, 1971; Bukry, 1973; Perch-Nielsen, 1985)

FAD Discoaster calcaris within Zone NN 8 middle to 

late Miocene (Martini, 1971; Perch-Nielsen, 1985)

FAD Discoaster brouweri upper Zone NN 5, possibly 

Zone NN 8 middle Miocene (Martini, 1971)

FAD Discoaster moorei within Zone NN 5 middle 

Miocene (Bukry, 1973; Perch-Nielsen, 1985)

FAD Discoaster variabilis sporadic occurrence within 

Zone NN 4 early Miocene, consistent from base of 

Zone NN 5 middle Miocene (Martini, 1971; Bukry, 

1973; Perch-Nielsen, 1985)

The use and interpretation of stratigraphic ranges of 

some taxa require additional explanation. Discoaster 

variabilis has been reported to occur sporadically in 

Zone NN 4 and then to occur consistently from Zone NN 

5 up to Zone NN 16. Because calcareous nannofossils, 

particularly the discoasters, are never very common in 

the Enewetak sediments, we assumed that any occurrence of D. variabilis in Zone NN 4 was unlikely to be 

observed. Therefore, when this species is present in 

Enewetak samples, we assumed that the sample was no 

lower than Zone NN 5. We do not consider the FAD of 

D. brouweri to be a very reliable datum because it 

appears to occur sporadically below Zone NN 11. For
C4 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

example, Martini (1971) indicated that the base of this 

species was in the top of Zone NN 5, while Bukry (1973), 

Gartner (1977b), and Perch-Nielsen (1985) placed its 

base no lower than Zone NN 8. The FAD of Discoaster 

pentaradiatus also is somewhat questionable. Martini 

(1971) and Perch-Nielsen (1985) indicated a FAD for this 

species within lower Zone NN 9, Gartner (1977a) placed 

it in lower Zone NN 10, and Bukry (1973) placed it in 

lower Zone NN 11. We find a Zone NN 9 position for the 

FAD of D. pentaradiatus to be consistent with other 

species' ranges observed within the PEACE cores.

Berggren and others (1985) (fig. 2) placed the MiocenePliocene boundary in the lower part of Zone NN 12 at the 

base of the Gilbert Chron (above the LAD of D. quinqueramus). The closest calcareous nannofossil event in 

our material to the Miocene-Pliocene boundary is the 

LAD of D. quinqueramus, which Berggren and others 

(1985) placed at 5.6 Ma. For this paper, because of the 

poor and sporadic occurrences of fossil datums, we 

consider sediments above the LAD of Discoaster quinqueramus (Zone NN 12 or younger) to be Pliocene.

Reticulofenestra pseudoumbilica is never an easy species to identify with a light microscope, and we have 

confined usage of this species to larger specimens of the 

genus Reticulofenestra having a well-developed central 

area. Sphenolithus abies and S. neoabies, the two youngest representatives of this genus, both become extinct at 

or near the top of Zone NN 15. The genus Sphenolithus 

is more prevalent in the PEACE samples than R. 

pseudoumbilica, and so the LAD of Sphenolithus often 

was used to approximate the top of Zone NN 15.

Species of the genus Gephyrocapsa are virtually 

impossible to differentiate without the use of a scanning 

electron microscope (SEM). For this study, we divided 

all the species in this genus into Gephyrocapsa large 

species and Gephyrocapsa small species. Gephyrocapsa 

oceanica and G. caribbeanica are grouped together as 

Gephyrocapsa large species because we cannot consistently distinguish between these two species in our 

Enewetak material. Another reason for combining these 

two species is the continued disagreement about the 

amount of variation within each of these species and 

about their actual stratigraphic ranges. Berggren and 

others (1985) placed the FAD of Gephyrocapsa caribbeanica at 1.74 Ma and the FAD of Gephyrocapsa oceanica 

at 1.68 Ma, both near the Pliocene-Pleistocene boundary 

at 1.6 Ma and above the base of Zone NN 19 at 1.9 Ma. 

Rio (1982) also placed the FAD of G. oceanica near the 

Pliocene-Pleistocene boundary. Okada and Bukry (1980), 

however, placed the FAD of G. oceanica much higher in 

the Pleistocene (mid-Zone NN 19). We follow the usage 

of Berggren and others (1985) and place the FAD of 

Gephyrocapsa large species near the base of Zone NN 

19.

SCALE GEOCHRONOLOGIC MAIN 

1 _ 

2 _ 

3

4_ 

5

6_ 

7 _ 

8- 

9_ 

10

11 - 

12 _

13 _ 

14 _ 

15 _ 

16 _ 

17 _

PLANKTIC FORAMINIFERAL 

ZONES 

(BLOW, 1969)

N23

N22

N21

N 19

NN 18

N 17

N 16

N 15

N 14

N 13

N 12

N 11

N 10

N9

N8

N7

NANNOFOSSIL CALCAREOUS 

ZONES 

(MARTINI, 1971)

NN21

NN20

NN 19

NN 18

NN 17

NN 16

NN 15

NN 14

NN 13

NN 12

NN 11

NN10

NN9

NN8

NN7

NN6

NN5

NN4

LJJ 

O 

<

PLEISTOCENE

LJJ 

Z 

LJJ 

0

o

_J 

a.

MIOCENE

LLJ

5

EARLY

LJJ

§

MIDDLE

EARLY 

(PART)

FIGURE 2. Correlation of planktic foraminiferal and calcareous nannofossil zones to epochs, based on Berggren and others, 1985.

Although large species of Gephyrocapsa are known 

only from the Pleistocene, sporadic occurrences of small 

species of the genus Gephyrocapsa have been reported 

well into the Pliocene in well-preserved pelagic 

sequences (Rio, 1982; Perch-Nielsen, 1985; Berggren and
CALCAREOUS NANNOFOSSILS AND PLANKTIC FORAMINIFERS FROM ENEWETAK ATOLL C5

others, 1985; Driever, 1988). A recent study of calcareous nannofossils in Panama by Bybell (unpub. data, 1990) 

indicates that the FAD of Gephyrocapsa small species 

may occur in upper Zone NN 16 or lower Zone NN 17 in 

Panama. In the Enewetak samples, however, the FAD 

of Gephyrocapsa small species often is only slightly 

below or at the same depth as the FAD of Gephyrocapsa 

large species (near the base of Zone NN 19). The absence 

of Gephyrocapsa small species well below the FAD of 

Gephyrocapsa large species may indicate that upper 

Zone NN 16, Zone NN 17, and Zone NN 18 are missing 

at Enewetak.

The LAD of Discoaster brouweri at 1.9 Ma (Berggren 

and others, 1985) marks the top of Zone NN 18, and this 

horizon often is used to approximate the PliocenePleistocene boundary, which Berggren and others (1985) 

placed at 1.6 Ma. However, D. brouweri occurs only 

sporadically in Enewetak material and was not a useful 

horizon here. The LAD of Calcidiscus macintyrei, a 

more commonly occurring species at Enewetak, is at 1.45 

Ma (within the early Pleistocene, according to Berggren 

and others, 1985). For this paper, if Gephyrocapsa large 

species (FAD at 1.74 Ma or 1.68 Ma) and Calcidiscus 

macintyrei (LAD at 1.45 Ma) are both present in a 

sample, the age is assumed to be lower Zone NN 19 

(Pliocene). If a sample is above the LAD of C. macintyrei, the age is assumed to be upper Zone NN 19 

(Pleistocene).

Pseudoemiliania lacunosa (its LAD marks the top of 

Zone NN 19) is rarely observed in most of the PEACE 

samples (probably due to the difficulty in identifying this 

species when it is poorly preserved), and Emiliania 

huxleyi (whose FAD marks the base of Zone NN 21) was 

never observed. This absence made effectively separating Zone NN 19 from Zone NN 20 difficult and separating Zone NN 20 from Zone NN 21 on the basis of 

calcareous nannofossils impossible.

PLANKTIC FORAMINIFERS

LAD Globigerinoides obliquus base Zone N 22 Pliocene (Blow, 1969; Kennett and Srinivasan, 1983)

LAD Globoquadrina altispira base N 21 Pliocene 

(Blow, 1969)

LAD Sphaeroidinellopsis spp. base N 21 Pliocene 

(Blow, 1969)

LAD Globigerina nepenthes N 19 Pliocene (Blow, 

1969)

FAD Globorotalia ungulata base N 19 Pliocene 

(Blow, 1969)

FAD Globigerinoides conglobatus N 18 Pliocene 

(Blow, 1969; Kennett and Srinivasan, 1983)

LAD Globoquadrina dehiscens N 18 Pliocene (Kennett and Srinivasan, 1983)

FAD Pulleniatinaprimalis N 17 late Miocene (Blow, 

1969)

FAD *Neogloboquadrina acostaensis base N 16 late 

Miocene (Blow, 1969)

FAD * Globigerina nepenthes base N 14 middle Miocene (Blow, 1969)

FAD Globorotalia menardii top N 13 middle Miocene 

(Blow, 1969)

LAD Globigerinoides subquadratus top N 13 middle 

Miocene (Blow, 1969)

FAD *0rbulina spp. base N 9 middle Miocene (Blow, 

1969)

Globoquadrina altispira, Sphaeroidinellopsis spp., 

Globigerinoides conglobatus, and Globoquadrina dehiscens were useful in delineating the Pliocene interval of 

the Enewetak samples. Most compilations (for example, 

Blow, 1969; Kennett and Srinivasan, 1983; Bolli and 

Saunders, 1985) show that the LAD's of Sphaeroidinellopsis spp. and Globoquadrina altispira fall near the 

base of Zone N 21 at a Pliocene level dated at about 2.9 

Ma (Dowsett, 1989). The zonation outlined by Blow 

(1969) included Zone N 20 in the Pliocene. Subsequent 

work has shown that Zone N 20 cannot be identified with 

confidence because the taxon whose first occurrence 

defines the base of Zone N 20 (Neogloboquadrina acostaensis pseudopima (Blow)) is not recognized by most 

workers. As a result, Zone N 20 either has not been used 

or it has been combined with Zone N 19 (for example, 

Berggren and others, 1985; Kennett and Srinivasan, 

1983). The first occurrence of Globorotalia tosaensis 

Takayanagi and Saito s.l. defines the base of Zone N 21. 

However, G. tosaensis has a restricted geographic distribution and has limited use for routine identification of 

Zone N 21. Thus, we use the last occurrences of Globoquadrina altispira and Sphaeroidinellopsis spp. to 

approximate the base of Zone N 21.

The base of the Pliocene is now considered to coincide 

with the base of Zone N 18 (Berggren and others, 1985). 

We approximated the base of the Pliocene with the last 

occurrence of Globoquadrina dehiscens (LAD at the top 

of Zone N 18) and the first occurrence of Globigerinoides 

conglobatus (FAD at base of Zone N 18) (Kennett and 

Srinivasan, 1983; Bolli and Saunders, 1985). We consider 

that this convention is adequate for this study because 

the duration of Zone N 18 is very short (about 100,000

yr).

MICROFOSSIL OCCURRENCES AND ZONE 

ASSIGNMENTS FOR INDIVIDUAL BOREHOLES

The stratigraphic significance of calcareous nannofossil 

and planktic foraminiferal occurrences and their relation-
C6 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

ship to the local benthic zonation are discussed by 

individual fossil group for each core, and the discussion of 

each core begins with the deepest sediments at the 

bottom of each core and ends with the shallowest sediments at the top of each core. Therefore, the first 

appearance of a species or FAD within an individual 

core, as described in this paper, is necessarily deeper in 

a core than its last appearance or LAD.

OOR-17

CALCAREOUS NANNOFOSSILS

Calcareous nannofossil occurrences in samples from 

borehole OOR-17 are listed on figure 3. The lower part of 

the core from 1,068 ft to 948 ft is barren of calcareous 

nannofossils, and the interval from 941 ft to 825 ft does 

not contain diagnostic calcareous nannofossils. The presence of Discoaster pentaradiatus at 813 ft indicates an 

assignment no older than Zone NN 9 or a late Miocene 

age. The occurrence of Discoaster prepentaradiatus at 

777 ft indicates assignment to either Zone NN 9 or NN 10 

(late Miocene). Therefore, the interval from 813 ft to 777 

ft is within Zones NN 9 and (or) NN 10. The occurrence 

of D. pentaradiatus below the occurrence of D. prepentaradiatus (FAD in Zone NN 9) in this core indicates 

that the FAD of D. pentaradiatus is more accurately 

placed in Zone NN 9 than in Zone NN 11. The last 

occurrence of the genus Sphenolithus at 339 ft is used to 

approximate the top of Zone NN 15 in the core. Specimens in this sample could be identified only as sphenoliths, and presumably they belong to the species Sphenolithus abies. Regardless of the species assignment, the 

339-ft level can be no younger than Zone NN 15 or 

mid-Pliocene, on the basis of last occurrence of this 

genus. The occurrence of Pseudoemiliania lacunosa and 

the absence of the genus Sphenolithus at 292 ft indicate 

that this depth is in Zone NN 16 or younger (late 

Pliocene). The first occurrence of large specimens of 

Gephyrocapsa at 137 ft indicates that this depth is 

definitely no older than Zone NN 19 or latest Pliocene to 

Pleistocene. The FAD of Gephyrocapsa small species is 

at 194 ft, which may indicate that Zone NN 19 extends 

down to this depth; the stratigraphic placement for the 

FAD of Gephyrocapsa small species is discussed in the 

Composite Sequence section. The presence of Pseudoemiliania lacunosa at 132 ft indicates that this depth is no 

younger than Zone NN 19.

PLANKTIC FORAMINIFERS

Occurrences of planktic foraminifers in samples from 

borehole OOR-17 are shown on figure 4. The occurrence 

of Orbulina universa, together with Globigerinoides

subquadratus, in the sample from 930 ft indicates that 

the assemblage is within the interval from Zones N 9 to 

N 13 or the middle Miocene. Samples from 866 ft to 292 

ft are difficult to attribute to a zone with any confidence. 

The presence of Globigerina nepenthes in samples at 722 

ft and 642 ft indicates placement somewhere in the 

interval from Zones N 14 to N 19 or the middle Miocene 

to Pliocene. The occurrence of a form close to Neogloboquadrina acostaensis in the sample from 614 ft indicates 

that this sample is no older than Zone N 16 (late 

Miocene), but it may be as young as Zone N 19 (Pliocene) 

because Globigerina nepenthes is present. Pulleniatina 

primalis first evolves in the upper part of Zone N 17, and 

its occurrence in samples from 511 ft and 331 ft indicates 

that the samples are no older than Zone N 17 or the late 

Miocene.

The overlap of Globigerinoides conglobatus and 

Sphaeroidinellopsis spp. in samples from 292 ft and 270 

ft indicates that these assemblages are within Zones N 19 

to the lower part of Zone N 21 (Pliocene). Samples from 

101 ft and 67 ft, which contain Globigerinoides tenellus 

and red Globigerina rubescens (at 67 ft), are within the 

interval of the Pleistocene Zones N 22 and N 23.

OBZ-4

CALCAREOUS NANNOFOSSILS

Seven samples between 1,543 and 924 ft were examined for calcareous nannofossils, and none of these samples yielded any identifiable species. No additional samples were examined from this core.

PLANKTIC FORAMINIFERS

Occurrences of planktic foraminifers in samples from 

borehole OBZ^l are shown on figure 5. The sample at 808 

ft from this core contains the planktic foraminifers Orbulina universa and Globigerinoides subquadratus, which 

are indicative of middle Miocene Zones N 9 to N 13 

(calcareous nannofossil Zones NN 5-NN 7 and within 

benthic zone lower LL). The other samples examined 

from this core were barren or yielded extremely poor 

planktic foraminiferal assemblages.

OAR-2 AND 2A

CALCAREOUS NANNOFOSSILS

Calcareous nannofossil occurrences for boreholes 

OAR-2 and OAR-2A are listed on figure 6. The interval 

between 836 ft and 749 ft has no age diagnostic calcareous nannofossils. Discoaster variabilis, which occurs at 

726 ft, indicates an age assignment no older than Zone
CALCAREOUS NANNOFOSSILS AND PLANKTIC FORAMINIFERS FROM ENEWETAK ATOLL C7

NN 5 or middle Miocene. The occurrence of Discoaster 

pentaradiatus at 663 ft indicates an assignment no older 

than Zone NN 9 or late Miocene for this depth. Discoaster berggrenii, which occurs at 659 ft, restricts this 

sample to Zone NN 11 (late Miocene). The presence of 

Reticulofenestra pseudoumbilica at 247 ft and Sphenolithus abies at 245 ft indicates that these samples are no 

younger than Zone NN 15 or early Pliocene. The presence of Pseudoemiliania lacunosa and Calcidiscus 

macintyrei and the absence of the genus Sphenolithus at 

141 ft indicate that the age of this sample is between 

Zone NN 16 and lower Zone NN 19 (late Pliocene). 

Calcareous nannofossils were not examined in these 

cores above 141 ft.

PLANKTIC FORAMINIFERS

Planktic foraminiferal taxa recorded in samples from 

boreholes OAR-2 and OAR-2A are listed on figure 5. 

The occurrence of Globigerina nepenthes or Globorotalia 

menardii, along with Globoquadrina dehiscens in assemblages from samples at 760 ft and 726 ft, indicates 

assignment within the interval from Zone N 14 to Zone N 

17 or middle to late Miocene. We did not routinely 

separate Globigerinoides extremus from G. obliquus 

during this study because of the poor preservation and 

limited populations available. However, good representatives of Globigerinoides extremus, which first occurs in 

Zone N 17, are present in the sample at 569 ft. Thus, the 

interval from 569 ft up to the first occurrence of Globigerinoides conglobatus at 290 ft is no older than Zone N 

17, and it may represent Zone N 18 or N 19. The overlap 

of Globigerinoides conglobatus with Globigerina nepenthes in the sample at 290 ft indicates placement in Zone N 

19 (Pliocene). The presence of Sphaeroidinellopsis spp. 

and Globoquadrina altispira, without Globigerina 

nepenthes, in samples from 268 ft and 245 ft indicates 

that these samples are in the interval of Zone N 19 to the 

lower part of Zone 21 (Pliocene). Assemblages from the 

top two samples examined for these cores at 195 ft and 90 

ft indicate that the age may be from Zone N 19 to Zone 

N 23 (Pliocene to Pleistocene).

KBZ-4

CALCAREOUS NANNOFOSSILS

Calcareous nannofossil occurrences for borehole 

KBZ^l are listed in figure 3. The bottom portion of this 

hole between 1,045 and 880 ft cannot be dated by using 

calcareous nannofossils. The presence of Discoaster variabilis at 876 ft indicates an age of probably no older than 

Zone NN 5 or middle Miocene. This age is confirmed by

the presence of both Discoaster brouweri (FAD in NN 5 

or younger) and Discoaster moorei (FAD in Zone NN 5) 

in the sample at 864 ft.

The next diagnostic species occurs at 777 ft, where 

Discoaster hamatus restricts this depth to Zone NN 9 

(late Miocene). The presence of several other discoasters 

at this depth is consistent with a placement within Zone 

NN 9: Discoaster neohamatus, Discoaster brouweri, 

Discoaster challengeri, and a possible Discoaster bollii. 

Discoaster signus also occurs at this depth; this species 

has been reported only from Zones NN 5 and NN 6 

(Bukry, 1973; Perch-Nielsen, 1985), but its presence 

here, along with the other discoasters that definitely 

indicate a younger age, suggests that D. signus has a 

longer range than previously reported. The presence of 

Discoaster quinqueramus at 756 ft indicates an age of 

Zone NN 11 (late Miocene). Therefore, the interval 

between 777 ft and 756 ft is confined to Zones NN 9-11. 

Discoaster berggrenii occurs at 751 ft, and D. neohamatus is present at 751 and 746 ft. The highest occurrence 

of D. quinqueramus is at 575 ft, and so the interval from 

756 and 575 ft is within Zone NN 11. The next significant 

horizon is the last occurrence of the genus Sphenolithus 

at 490 ft, whose presence means that this sample is no 

younger than Zone NN 15. Between 490 ft and 206 ft, 

this core is essentially barren of calcareous nannofossils, 

and there are no diagnostic species in the few samples 

that do contain calcareous nannofossils. At 206 ft is the 

FAD of Gephyrocapsa small species and Gephyrocapsa 

large species; these species indicate an age no older than 

Zone NN 19. Pseudoemiliania lacunosa occurs at 162 ft, 

and so the interval from 206 to 162 ft is restricted to Zone 

NN 19. Calcidiscus macintyrei last occurs at 187 ft, and 

so the interval from 206 to 187 ft can be placed in the 

lowest part of Zone NN 19 (Gartner, 1977b), the latest 

Pliocene. No diagnostic calcareous nannofossils occur 

above 162 ft.

This borehole was drilled inside one of the nuclear 

blast craters, and there is a zone of obvious mixing above 

142 ft (see fig. 8). Below 142 ft, there also is evidence of 

upward piping of sediments; this piping can be recognized by the presence of older macrofossil material near 

the surface of this core. However, the calcareous nannofossil occurrences in borehole KBZ^l below 142 ft are 

consistent with occurrences in the other PEACE boreholes, and we see no evidence for displacement of calcareous nannofossils in these samples. Thus, these data 

have been incorporated into our evaluation of the 

Enewetak boreholes.

PLANKTIC FORAMINIFERS

Planktic foraminifers recorded in samples from borehole KBZ^l are listed on figure 5. The occurrence of
C8 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

BOREHOLE

CALCAREOUS NANNOFOSSIL ZONES 

(MARTINI, 1971)

"^Trr: - ___ SAMPLE DEPTH (FEET) TAXA - ^ ^_^^

BARREN 

Calcidiscus leptoporus 

Calcidiscus macintyrei 

Calcidiscus spp. 

Coccolithus pelagicus

Discoaster aulakos 

Discoaster berggrenii 

Discoaster sp. cf. D. bollii 

Discoaster brouweri 

Discoaster challengeri

Discoaster exilis 

Discoaster hamatus 

Discoaster moorei 

Discoaster neohamatus 

Discoaster nephados

Discoaster pansus 

Discoaster pentaradiatus 

Discoaster quinqueramus 

Discoaster signus 

Discoaster variabilis

Discoaster woodringi 

Discolithina spp. 

Gephyrocapsa large spp. 

Gephyrocapsa small spp. 

Helicosphaera carterii

Helicosphaera spp. 

Pseudoemiliania lacunosa 

Reticulofenestra pseudoumbilica 

Reticulofenestra large spp. 

Reticulofenestra small spp.

Rhabdosphaera procera 

Scyphosphaera pulcherrima 

Sphenolithus abies 

Sphenolithus spp. 

placoliths

Abundance 

Preservation

BOREHOLE

CALCAREOUS NANNOFOSSIL ZONES 

(MARTINI, 1971)

~~ ___ SAMPLE DEPTH (FEET) 

TAXA - -~^-^^____

BARREN 

Calcidiscus leptoporus 

Calcidiscus macintyrei 

Calcidiscus spp. 

Ceratolithus spp.

Coccolithus pelagicus 

Discoaster brouweri 

Discoaster challengeri 

Discoaster pentaradiatus 

Discoaster prepentaradiatus

Discoaster subsurculus 

Discolithina spp. 

Gephyrocapsa large spp. 

Gephyrocapsa small spp. 

Helicosphaera carterii

Pseudoemiliania lacunosa 

Reticulofenestra pseudoumbilica 

Reticulofenestra small spp. 

Reticulofenestra spp. 

Sphenolithus abies

Sphenolithus spp. 

placoliths 

Abundance 

Preservation

KBZ-4

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FIGURE 3. Calcareous nannofossil occurrences in boreholes KBZ-4 and OOR-17. C on the chart indicates probable contamination, 

abundance, C=common or 1-10 specimens per field of view at 500 x magnification, F=frequent or 1 specimen per 1-10 fields of 

G=good preservation, F=fair preservation, P=poor preservation, and T=terrible preservation.
CALCAREOUS NANNOFOSSILS AND PLANKTIC FORAMINIFERS FROM ENEWETAK ATOLL C9

KBZ-4

NN

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BOREHOLE

CALCAREOUS NANNOFOSSIL ZONES 

(MARTINI, 1971)

SAMPLE DEPTH_______ -~~~

BARREN 

Co. leptoporus 

Ca. macintyrei 

Ca. spp. 

Cc. pelagicus

D. aulakos 

D. berggrenii 

D. sp. cf. D. bolli 

D. brouweri 

D. challenger!

D. exilis 

D. hamatus 

D. moore 

D. neohamatus 

D. nephados

D. pansus 

D. pentaradiatus 

D. quinqueramus 

D. sign us 

D. variabilis

D. woodringi 

D. spp. 

G. large spp. 

G. small spp. 

H. carterii

H. spp. 

P. lacunosa 

R. psuedoumbilica 

R. large spp. 

R. small spp.

Rb. procera 

S. pulcherrima 

S. abies 

S. spp. 

placoliths

Abundance 

Preservation

OOR-17

NN 

9-1E

(D c*

4

4

NN9-19

I

BB|

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i

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BOREHOLE

CALCAREOUS NANNOFOSSIL ZONES 

(MARTINI, 1971)

SAMPLE DEPTH (FEET)_____- ~"

^________ -~~~~~ TAXA

BARREN 

Ca. leptoporus 

Ca. macintyrei 

Ca. spp. 

Cer. spp.

Cc. pelagicus 

D. brouweri 

D. challenger! 

D. pentaradiatus 

D. prepentaradiatus

D. suBsurculus 

D. spp. 

G. large spp. 

G. small spp. 

H. carterii

P. lacunosa 

R. pseudoumbilica 

R. small spp. 

R. spp. 

S. abies

S. spp. 

placoliths 

Abundance 

Preservation

a question mark indicates a possible occurrence, and B indicates a sample barren of calcareous nannofossils. For 

view at 500 x magnification, and R=rare or 1 specimen per 10-100 fields of view at 500 x magnification. For preservation,
CIO GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

LU 

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N 22-N 23

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887

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270

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N17-N 21

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574

589

614

642

722

N 16-N 19

N 14-N 19

866

930 N 9-N 13

FIGURE 4. Planktic foraminiferal occurrences in boreholes KAR-1 and OOR-17. C indicates probable contamination, and a question mark indicates a possible occurrence.

Globigerinoides subquadratus in the sample from 864 ft 

indicates that this sample is within or below Zone N 13 or 

middle Miocene or older. The occurrence of Globoqiiadrina dehiscens with Globorotalia menardii at 777 ft

indicates that this sample is within the interval from 

Zone N 14 to Zone N 17 or middle to late Miocene. Our 

interpretation excludes the unlikely possibility that this 

sample represents the short basal Pliocene Zone N 18.
CALCAREOUS NANNOFOSSILS AND PLANKTIC FORAMINIFERS FROM ENEWETAK ATOLL Cll

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FIGURE 5.-Planktic foraminiferal occurrences in boreholes OAR-2 and 2A, KBZ-4, OBZ-4, and XEN^3. 

A question mark indicates a possible occurrence, and a down arrow indicates that the zonal assignment 

may be older but not younger.

The isolated occurrence of Pulleniatina obliquiloculata 

in the sample from 333 ft appears to be in situ because it 

is poorly preserved and encrusted with secondary calcite 

overgrowths, as are several other unidentified specimens recovered from the sample. This condition matches

the recrystallized nature of the sediments in this interval. Because Pulleniatina obliquiloculata first occurs in 

Zone N 19, the sample at 333 and the sample at 226 ft, 

which contains Globigerina nepenthes and Globigerinoides conglobatus, are assigned to the Pliocene Zone N
C12 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

BOREHOLE

CALCAREOUS NANNOFOSSIL ZONES 

(MARTINI, 1971)

~~~" - -____SAMPLE DEPTH (FEET) 

TAXA ^~~~~~--------___________^

BARREN

Calcidiscus leptoporus 

Calcidiscus macintyrei

Calcidiscus spp.

Coccolithus eopelagicus

Coccolithus pelagicus

Discoaster berggrenii

Discoaster brouweri

Discoaster deflandrei

Discoaster pansus

Discoaster pentaradiatus

Discoaster variabilis

Helicosphaera carterii 

Pseudoemiliania lacunosa

Reticulofenestra spp.

Reticulofenestra pseudoumbilica

Reticulofenestra umbilica

Rhabdosphaera spp.

Scyphosphaera intermedia

Sphenolithus abies

Sphenolithus spp.

placoliths

Abundance

Preservation

OAR-2 and 2A

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FIGURE 6. Calcareous nannofossil occurrences in boreholes OAR-2 and 2A. See figure 3 for an 

explanation of abundance, preservation, and symbols used on the figure.

19. Assemblages found between 777 ft and 333 ft can be 

assigned only to the broad interval from Zone N 14 to 

Zone N 19 or late middle Miocene to Pliocene.

KAR-l

CALCAREOUS NANNOFOSSILS

Occurrences of calcareous nannofossils from borehole 

KAR-l are shown on figure 7. Much of the lower part of 

KAR-l between 1,146 ft and 870 ft is barren of calcareous nannofossils. However, the presence of the species 

Discoaster brouweri, along with D. variabilis, at 927 ft 

indicates that this level in the core is no older than NN 5 

or middle Miocene. The presence of Discoaster pentaradiatus, which indicates an age no older than Zone NN 9, 

and Discoaster calcaris, which indicates an age of Zones 

NN 8-10, at 852 ft constrains this sample within Zone 

NN 9 or NN 10 (late Miocene). The occurrence of the 

species Discoaster moorei at 817 ft is anomalous; this 

species is reported to occur only in Zones NN 5-7

(Perch-Nielsen, 1985). Its presence at this depth indicates either reworking or the extension of this species' 

range into Zone NN 9. Discoaster quinqueramus, which 

is restricted to Zone NN 11, occurs at 741 ft; Discoaster 

berggrenii, which also is confined to Zone NN 11, occurs 

at 626 ft. Thus, the interval between 741 ft and 626 ft is 

placed in Zone NN 11 or the late Miocene. Sphenolithus 

abies, which has its LAD near the top of Zone NN 15, 

last occurs at 448 ft, and so the age at this depth is no 

younger than Zone NN 15 or Pliocene. One Sphenolithus 

specimen found in the sample at 197 ft is assumed to be 

due to contamination because not only is it out of place 

stratigraphically, but also it was the only identifiable 

calcareous nannofossil in the entire sample. Large specimens of the genus Gephyrocapsa first occur at 118 ft; 

this occurrence indicates that this sample is no older than 

Zone NN 19, although Gephyrocapsa small species first 

occurs at 152 ft. Occurrences of Pseudoemiliania 

lacunosa at 118 ft, 100 ft, and 95 ft indicate that these 

samples are no younger than Zone NN 19. There are 

questionable occurrences of Calcidiscus macintyrei at
CALCAREOUS NANNOFOSSILS AND PLANKTIC FORAMINIFERS FROM ENEWETAK ATOLL C13

100 and 70 ft. These two occurrences have been attributed to contamination, probably as reworking from 

below, because correlative material elsewhere at 

Enewetak has calcareous nannofossil, as well as radiocarbon and uranium-series, dates that place these sediments within the Pleistocene, thus above the last occurrence of C. macintyrei (see Composite Sequence and 

Radiometric Dating sections). Scattered throughout this 

core are several other examples of obvious contamination 

due to reworking. These are marked with a C in the 

occurrence chart on figure 7 and include a few Eocene 

forms and two occurrences of Discoaster deflandrei.

PLANKTIC FORAMINIFERS

Planktic foraminifers identified from KAR-1 are 

shown on figure 4. Assemblages from the lower part of 

the core are not particularly diagnostic, although the 

bottom two samples at 893 ft and 887 ft are Miocene, on 

the basis of the presence of Globoquadrina dehiscens.

Several assemblages from the upper part of the borehole are diagnostic of the Pliocene. For example, 

Globorotalia ungulata occurs with Globigerina nepenthes in the sample from 557 ft. Globigerina nepenthes 

also occurs in the sample from 552 ft, and Sphaeroidinellopsis occurs in the sample from 283 ft. Thus, the 

samples from 557 ft and 552 ft are assigned to the 

Pliocene Zone N 19, and the sample from 283 ft is within 

the Pliocene interval of Zone N 19 to Zone N 21. One 

relatively well-preserved specimen of Pulleniatina 

obliquiloculata was found in a split from the sample at 

866 ft. The remainder of this sample was examined 

extensively, and the only other planktic foraminifer 

found was a poorly preserved "Globorotalia." Thus, we 

consider the specimen of Pulleniatina obliquiloculata 

from this sample to represent sampling or laboratory 

contamination.

XEN-3 

CALCAREOUS NANNOFOSSILS

Calcareous nannofossil occurrences for borehole 

XEN-3 are listed in figure 7. There are no diagnostic 

calcareous nannofossils in the lower part of this borehole 

from 297 ft to 250 ft. The presence of Gephyrocapsa large 

species at 248 ft indicates an age no older than Zone NN 

19. Calcidiscus macintyrei, which does not occur above 

the lower part of Zone NN 19, is found up to 148 ft, and 

so the interval between 248 ft and 148 ft is within lower 

Zone NN 19 (latest Pliocene in age). The interval 

between 144 ft and the top of the core, which is above the 

last occurrence of C. macintyrei, is most probably within 

the Pleistocene on the basis of calcareous nannofossils,

but it cannot be further subdivided due to the absence of 

other diagnostic Pleistocene species.

PLANKTIC FORAMINIFERS

Planktic foraminifers identified in core XEN-3 are 

listed in figure 5. The sample from 262 ft, which contains 

Globorotalia plesiotumida, Sphaeroidinellopsis, and 

Globorotalia ungulata, is Pliocene in age. The specimen 

questionably assigned to Sphaeroidinellopsis in the sample at 245 ft is too poorly preserved to be used to assign 

a Pliocene age to that sample.

E-l

One core sample (2,003-2,028 ft) was examined from 

the E-l borehole. The sample contains the calcareous 

nannofossils Coccolithus eopelagicus, C. pelagicus, Pedinocyclus larvalis, Reticulofenestra floridana, and 

Sphenolithus sp., most of which occur in both the Oligocene and Miocene. However, P. larvalis does not appear 

to occur later than late Oligocene (Perch-Nielsen, 1985). 

Bybell (unpub. data, 1982) indicates that its occurrence is 

as high as either Zone NP 24 or NP 25. The E-l sample 

also contains the planktic foraminifers Globorotalia 

kugleri, Globigerina ciperoensis, G. bulloides, Streptochilus sp., and a questionable G. angulisuturalis. The 

presence of G. kugleri may place the sample either in the 

latest Oligocene or earliest Miocene (Bolli and Saunders, 

1985) or within the earliest Miocene (Berggren and 

others, 1985). If Pedinocyclus larvalis does not occur 

later than the Oligocene, it indicates that this sample is 

confined to the late Oligocene. However, neither Dictyococcites bisectus, Helicosphaera recta, nor Zygrhablithus bijugatus (species that do not extend above the 

late Oligocene) is present in this sample. Therefore, the 

E-l sample has been dated as either latest Oligocene or 

earliest Miocene.

COMPOSITE SEQUENCE

Once individual boreholes were dated as closely as 

possible by using calcareous nannofossils and planktic 

foraminifers, the data for each borehole were plotted 

against the local benthic zones of Cronin and others 

(1986), as modified by Cronin (written commun., 1990) 

(fig. 8). Cronin (written commun., 1990) gave each of the 

zones and subzones ostracode faunal names. The following is a list of each new ostracode zone and its corresponding old alphabetic designation from Cronin and 

others (1986):
C14 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

BOREHOLE

CALCAREOUS NANNOFOSSIL ZONES 

(MARTINI, 1971)

TAXA~~ - SAMPLE DEPTH (FEET)

BARREN 

Braarudosphaera bigelowi 

Calcidiscus leptoporus 

Calcidiscus macintyrei 

Calcidiscus spp.

Ceratolithus sp. cf. C. acutus 

Ceratolithus spp. 

Cocco/ithus pelagicus 

Cruciplacolithus spp. 

Discoaster berggrenii

Discoaster brouweri 

Discoaster ca /can's 

Discoaster challenger! 

Discoaster deflandrei 

Discoaster moorei

Discoaster pansus 

Discoaster pentaradiatus 

Discoaster quinqueramus 

Discoaster surculus 

Discoaster uariabilis

Discoaster spp. 

Gephyrocapsa large spp. 

Gephyrocapsa small spp. 

Helicosphaera carterii 

Helicosphaera lophota

Pseudoemiliania lacunosa 

Reticulofenestra pseudoumbilica 

Reticulofenestra small spp. 

Reticulofenestra spp. 

Reticulofenestra umbilica

Scyphosphaera sp. cf. S. apsteinii 

Spheno/ithus abies 

Spheno/ithus spp. 

placoliths 

Abundance 

Preservation

BOREHOLE

CALCAREOUS NANNOFOSSIL ZONES 

(MARTINI, 1971)

T737~ ' -__ SAMPLE DEPTH (FEET)

BARREN 

Ca/cidiscus leptoporus 

Calcidiscus macintyrei 

Ca/cidiscus spp. 

Cocco/ithus pelagicus

Discolithina spp. 

Gephyrocapsa large spp. 

Gephyrocapsa small spp. 

Helicosphaera carterii 

Pseudoemiliania lacunosa

Reticulofenestra large spp. 

Rettculofenestra small spp. 

placoliths 

Abundance 

Preservation

KAR-1

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CALCAREOUS NANNOFOSSILS AND PLANKTIC FORAMINIFERS FROM ENEWETAK ATOLL C15

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CALCAREOUS NANNOFOSSIL 

ZONES (MARTINI, 1971)

SAMPLE DEPTH ____ - "^ 

ftETl^~--~~~ TAXA

BARREN 

Ca. leptoporus 

Ca. macintyrei 

Ca. spp. 

Cc. pelagicus

Dl. spp. 

G. large spp. 

G. small spp. 

H. carterii 

P. lacunosa

R. large spp. 

R. small spp. 

placoliths 

Abundance 

Preservation

BB

i

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P PP

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ZONES (MARTINI, 1971)

SAMPLE DEPTH___ - ~~ 

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BARREN 

B. bigelowi 

Ca. leptoporus 

Ca. macintyrei 

Ca. spp.

Cer. sp. cf. C. acutus 

Cer. spp. 

Cc. pelagicus 

Cr. spp. 

D. berggrenii

D. brouweri 

D. ca /can's 

D. challenger! 

D. deflandrei 

D. moorei

D. pansus 

D. pentaradiatus 

D. quinqueramus 

D. surculus 

D. variabilis

D. spp. 

G. large spp. 

G. small spp. 

H. carterii 

H. lophota

P. lacunosa 

R. pseudoumbilica 

R. small spp. 

R. spp. 

R. umbilica

S. sp. cf. S. apsteinii 

S. abies 

S. spp. 

placoliths 

Abundance 

Preservation

FIGURE 7. Calcareous nannofossil occurrences in boreholes KAR-1 

and XEN-3. See figure 3 for an explanation of abundance, preservation, and other symbols used on the figure.
C16 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

Zone AA Neonesidea schulzi Zone

Zone BB/CC Cletocythereis marshallensis Zone, 

Bairdoppilata Subzone

Zone DD Cletocythereis marshallensis,

Paracytheridea tschoppi Subzone

Zone EE/FF Loxoconcha-Xesotoleberis Zone

Zone GG Radimella-Hermanites Zone

Zone HH Bosasella Zone

Zone II "Procythereis" teeteri Zone

Zone JJ Miocyprideis Zone

Zone KK Stigmatocythere spinosa Zone

Upper Zone LL "Capricornia," Neomonoceratina Zone

Middle Zone LL Pokornyella pseudojaponica Zone

Lower Zone LL Tenedocythere setigera Zone

Zone MM tPokornyella sp. b Zone

Calcareous nannofossil and planktic foraminiferal zonal 

assignments are plotted on separate columns for each 

borehole (fig. 8). For ease in comparison and interpretation, planktic foraminiferal zonal assignments also are 

converted to calcareous nannofossil zonal equivalents by 

using the calibrations of Berggren and others (1985). By 

using the local benthic zones as a biostratigraphic framework, we were able to compare the scattered calcareous 

nannofossil and planktic foraminiferal age data from 

borehole to borehole and to calibrate the local benthic 

zonation to internationally recognized planktic zonations 

for the PEACE boreholes (fig. 8).

ZONE MM

Zone MM was used by Cronin and others (1986) to 

include all the strata from the bottom of the PEACE 

boreholes to the base of Zone LL. No planktic foraminiferal calibrations are available for Zone MM. The only 

diagnostic calcareous nannofossil found in Zone MM is 

Discoaster variabilis, which occurs at 876 ft in KBZ^i. 

This species indicates an age no older than Zone NN 5 or 

middle Miocene. The boundary between Zones MM and 

LL in this core is equivocal, and the occurrence of D. 

variabilis in KBZ^i may actually fall within lower Zone 

LL.

LOWER ZONE LL 

(NANNOFOSSIL ZONES NN 5-7, MIDDLE MIOCENE)

Zone LL was subdivided into lower Zone LL, middle 

Zone LL, and upper Zone LL, on the basis of ostracodes. 

Middle Zone LL cannot be dated on the basis of calcareous nannofossils or planktic foraminifers and is not 

discussed in this paper. In borehole OOR-17, lower Zone 

LL is constrained to the interval of Zones N 9-13 (NN 

5-7), on the basis of the occurrence of the planktic

foraminifers Orbulina universa and Globigerinoides subquadratus at 930 ft. Borehole OBZ^l also has these same 

two species in the lower Zone LL at 808 ft. In KBZ^i, 

lower Zone LL is constrained to the middle Miocene by 

the occurrence of G. subquadratus at 864 ft (ranges no 

higher than Zone N 13 or NN 5-7) and the occurrence of 

Discoaster variabilis at 876 ft (ranges no lower than NN 

5). Therefore, lower Zone LL occurs within calcareous 

nannofossil Zones NN 5-7.

UPPER ZONE LL 

(NANNOFOSSIL ZONE NN 9-LATE MIOCENE)

In KAR-1, the co-occurrence of Discoaster pentaradiatus and D. calcaris at 852 ft dates upper Zone LL as 

Zones NN 9-10. Furthermore, the presence of Discoaster hamatus (confined to Zone NN 9) in the overlying Zone KK further restricts the assignment of upper 

LL to Zone NN 9. This assignment is supported by 

planktic foraminifers from upper Zone LL in OAR-2 and 

2A.

LOWER ZONE KK 

(NANNOFOSSIL ZONE NN 9-LATE MIOCENE)

In OOR-17, lower Zone KK falls within Zones NN 9 

and 10, on the basis of the occurrences of Discoaster 

pentaradiatus (FAD in Zone NN 9) at 813 ft and 

Discoaster prepentaradiatus (LAD in Zone NN 10) at 

777 ft. In KBZ^, the sample at 777 ft is definitely in 

Zone NN 9, on the basis of the occurrence of Discoaster 

hamatus (FAD and LAD in Zone NN 9). The lower part 

of KK is confined to Zones NN 9-11 in KAR-1, on the 

basis of the occurrences of Discoaster pentaradiatus 

(FAD in Zone NN 9) at 852 ft and Discoaster quinqueramus (LAD in Zone NN 11) at 741 ft. By combining 

these data, the lower part of Zone KK is within Zone NN 

9.

UPPER ZONE KK AND ZONE JJ 

(NANNOFOSSIL ZONE NN 11-LATE MIOCENE)

Upper Zone KK and Zone JJ are constrained to Zone 

NN 11 in KBZ^i by the presence of Discoaster quinqueramus (FAD and LAD in Zone NN 11) between 756 ft and 

575 ft. In KAR-1, D. quinqueramus occurs at 741 ft in 

Zone KK, and Discoaster berggrenii occurs at 626 ft 

(Zone JJ); these occurrences confine this interval to Zone 

NN 11. Moreover, Zone JJ is assigned to Zone NN 11 in 

OAR-2 and 2A on the basis of the presence of D. 

berggrenii at 659 ft. Therefore, upper Zone KK and Zone 

JJ are assigned to Zone NN 11 or the late Miocene.
CALCAREOUS NANNOFOSSILS AND PLANKTIC FORAMINIFERS FROM ENEWETAK ATOLL C17

ZONES II, HH, AND GG 

(NANNOFOSSIL ZONES NN 12-15-EARLY PLIOCENE)

The middle part of Zone II in KAR-1 contains the 

planktic foraminifers Globorotalia ungulata and Globigerina nepenthes at 557 ft; these species occur together 

only in Pliocene Zone N 19 (Zones NN 12-15). Because 

such a thick sequence of Zone NN 11 occurs within KK 

and JJ, and because D. quinqueramus and D. berggrenii 

do not occur in Zone II, Zone II probably is confined to 

Zone NN 12 or younger (Pliocene). Thus, all of Zone II 

most likely is Pliocene in age, and the Miocene-Pliocene 

boundary in the PEACE boreholes is placed at the JJ-II 

boundary.

Sediments from Zone HH are highly recrystallized, 

and planktic microfossils are very sparse. However, an 

assemblage from Zone HH in KBZ^l is referable to Zone 

N 19 (Zones NN 12-15). Assemblages in Zone GG from 

OAR-2 and 2A indicate that Zone GG is still within 

Pliocene Zones N 19-21 (Zones NN 12-16), on the basis 

of both planktic foraminifers and calcareous nannofossils. 

Note that we assume that the highest LAD's of Reticulofenestra pseudoumbilica and Sphenolithus found in 

Enewetak cores are probably the closest to their actual 

extinction levels. Therefore, we judge that the top of 

Zone NN 15 occurs near the top of Zone GG and that 

Zones II, HH, and GG correlate with Zones NN 12-15.

ZONE EE/FF

(PREDOMINANTLY LOWER NANNOFOSSIL ZONE NN 

19-LATE PLIOCENE)

The occurrence of a few specimens of Sphaeroidinellopsis spp. in samples from the lowest interval assigned 

to Zone EE/FF in KBZ-4, OOR-17, and XEN-3 suggests that the lower part of EE/FF may be within the 

Pliocene Zone N 19 (Zones NN 12-16). This interpretation is supported by the presence of Pseudoemiliania 

lacunosa (FAD in uppermost Zone NN 15) and the 

absence of Sphenolithus species (LAD at the top of Zone 

NN 15) at 292 ft (base of Zone EE/FF) in OOR-17. 

However, most occurrences of planktic microfossils 

above the base of Zone EE/FF indicate assignment to 

Zone NN 19. In borehole OOR-17, Gephyrocapsa small 

species first occurs at 194 ft, and Gephyrocapsa large 

species first occurs at 137 ft; both of these horizons are 

within the upper part of Zone EE/FF. Both Gephyrocapsa large species and Gephyrocapsa small species first 

occur at 248 ft in XEN-3; this occurrence is 42 ft above 

the base of Zone EE/FF. In KBZ-4, Gephyrocapsa small 

species and Gephyrocapsa large species both first occur 

at 206 ft; this occurrence is 29 ft above the base of Zone 

EE/FF. Assuming that the first appearance of Gephyrocapsa large species occurs near the base of Zone NN 19, 

then all of Zone EE/FF above the first Gephyrocapsa

large species can be no older than Zone NN 19. Calcidiscus macintyrei (LAD in the lower part of Zone NN 19) 

does not occur above 187 ft in KBZ^l; this occurrence is 

in the middle part of Zone EE/FF. Calcidiscus macintyrei has a last definite occurrence at 183 ft in KAR-1, 

and this depth is near the middle of Zone EE/FF. The 

presence of Calcidiscus macintyrei in EE/FF further 

restricts the age to lower Zone NN 19. The absence of C. 

macintyrei in the uppermost part of Zone EE/FF indicates that this portion of EE/FF is above the extinction 

of C. macintyrei and thus within the Pleistocene, according to Berggren and others (1985).

ZONE DD 

(UPPER NANNOFOSSIL ZONE NN 19-PLEISTOCENE)

Zone DD cannot be dated directly by planktic microfossils, except for one occurrence of Pseudoemiliania 

lacunosa (LAD at top of Zone NN 19) in KAR-1 at 118 

ft (1 ft below the top of Zone DD). Available radiometric 

determinations suggest that Zone DD is referable to 

uppermost Zone NN 19 or NN 20 (see Radiometric 

Dating section).

ZONES CC, BB, AND AA 

(NANNOFOSSIL ZONES NN 19-21-PLEISTOCENE)

The occurrence of Pseudoemiliania lacunosa in 

KAR-1 at 100 and 95 ft places the lower portion of Zone 

CC within Zone NN 19. The remainder of Zones CC, BB, 

and AA cannot be subdivided by using calcareous nannofossils or planktic foraminifers. They are no older than 

Zone NN 19, on the basis of the presence of the genus 

Gephyrocapsa, although these zones are probably confined to Zones NN 20 and 21, on the basis of radiometric 

dating (see following section). However, this assignment 

cannot be made on the basis of calcareous nannofossils. 

The paucity of Pseudoemiliania lacunosa specimens 

(LAD at top of Zone NN 19) throughout the PEACE 

boreholes indicates that the absence of P. lacunosa in 

this interval is not a significant biostratigraphic indicator.

RADIOMETRIC DATING

Previous shallow drilling carried out during the Pacific 

Atoll Cratering Experiment (PACE) (Henny and others, 

1974) and EXPOE (Couch and others, 1975) recovered a 

suite of samples and cores from a number of islands in the 

northern part of the Enewetak Atoll. Study of this 

material, including XEN-3, allowed recognition and correlation of six stratigraphic intervals within the upper 

230 ft of the atoll sequence that can be correlated within
CIS GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

0.

<

V)

0 -i 

100 - 

200 - 

300 - 

400 - 

500 - 

600-

700 -

800-

!7VIU

 mnn

KBZ-4

FEET

142

235 

260 

270

400 

450

570

700

795

830

855 

870

MIXED

EE/FF

GG

77

HH

??

II

JJ

KK

UPPER

MIDLL

MM

*

FEET ZONE FEET

46

117 

132

 162 NN 19or J/ 

 187 lower NN 19 or ^ 

 206 NN 19 or <f 

 226 N19(NN 12-16) 

241

279 

 333 N19(NN 12-16)

 490 NN 15 or ^ 482

 575 NN11 

605

700

 756 NN11 

 777 NN9 

N 14-17 8oo 

(NN 7-12)

864 N 13orJ/ 856 

v (NN 7 orJ^, ) 

\ 904 

876 NN 5 or 'f

952

CAR-1 OOR-17

FEET ZONE EEET

AA

BB/CC

DD

EE/FF

GG

HH

II

JJ

KK

UPPER 

LL

MIDLL

LOWER 

LL

MM

71 

 95 NN 19 or J/ 

 118 NN19or<f 111

137 

 183 lower NN 19 or J/

(NN 12-18) 

350

 448 NN15or J/

497

 552 N 19 (NN 12-16)

^^ 557 N 19 581 

(NN 12-16)

 626 NN 11

700 

 741 NN 11

813

840 

 852 NN9-10

^-887 N ISorJ^ (NN 11 orJ^, ) 

 893 N 18 or X 906

(NN11 orJ^) 

^ 927 NN 5 or -f 969

AA

BB/CC

DD 

EE/FF

GG

HH

II

JJ

KK

UPPER LL

MIDLL

LOWER 

LL

MM

FEET ZONE 

67 N 22-23

(NN 19-21) 

 101 N 22-23

(NN 19-21) 

^^ 137 NN19or 't

, 270 N 19-lower21 

/ (NN 12-16)

/ / 292 N 19-lower 21

\^ (NN 12-16) 

NN 16-19

\^ 331 N 17 or f A 

\ (NN 11 orT ) 

X 339 NN ISorJ^

 511 N 17 or T. 

(NN 11 or 'f )

/ 614 N 16-19 

/ (NN8-16)

 642 N 14-19 

(NN 7-16)

 722 N 14-19 

(NN 7-16)

 777 NN9-10

 813 NN9or 'f

 930 N9-13 

(NN 5-7)

FIGURE 8. Combined calcareous nannofossil and planktic 

foraminiferal age dates for boreholes KBZ-4, KAR-1, 

OOR-17, OAR-2 and 2A, and XEN-3 and final age assignment for each Enewetak benthic zone. Calcareous nannofossil and planktic foraminiferal zone assignments for diagnostic samples in each core are shown. Footages are 

subbottom. In the case of planktic foraminiferal zones, 

equivalent calcareous nannofossil zones as determined by

the correlation chart of Berggren and others (1985) are given 

in parentheses. Zonation assignments for Enewetak benthic 

zones have been integrated into a composite, showing the 

overall relationship of calcareous nannofossil zones to the 

Enewetak benthic zonation and resulting age assignments. 

An up arrow indicates that the zonal assignment may be 

younger. A down arrow indicates that the zonal assignment 

may be older.

and between islands (Couch and others, 1975). Radiocarbon and uranium-series dates are available from two 

cores, PAR-15 and PAR-16, from Aranit Island (Szabo 

and others, 1985). The uppermost interval or interval 1

was dated at 8,000 yr by radiocarbon. Uranium series 

dating of intervals 2 and 4 yielded ages of 131,000 ± 3,000 

yr and 454,000 ± 100,000 yr, respectively. No dates were 

obtained from intervals 3, 5, or 6.
CALCAREOUS NANNOFOSSILS AND PLANKTIC FORAMINIFERS FROM ENEWETAK ATOLL C19

i

LU

Q

aQ.

5

V)

0 -

100 -

200 -

300-

400 -

500 -

600 -

700 -

800-J

OAR-2/2A

FEE! EEEI

XEN-3

26

91

242

300

443

572

680

725

739

AA

BB/CC 

DO

EE/FF

GG

HH

JJ

KK

UPPER 

LL

ZONE FEET

49 

90 N 19-23 (NN 12-21)

141 NN16-lower19 127 

141

195 N 19-23 (NN 12-21)

245 N 19-21 NN 12-16 

247 NN 15orX ) 

268 N 19-21 (NN 12-16) 

290

290 N 19 (NN 12-16)

AA

BB

CC

EE/FF

GG

FEET ZONE

Composite 

Sequence

148 lower NN 19 or

248 NN 19 or " 

262 N 18-21 

(NN 12-18)

569 NN

N 17-19 (NN 11-16)

659 NN11 

663 NN 9 or "f

726 NN 5 or "t

N 14-17 (NN 7-12) 

760 N 14-17 (NN 7-12)

LJ

4.

LJ

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LJJ

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MIDLL

LOWER 

LL

MM

NN20- 

21

NN 19

NN16?

NN 12- 

15

NN 11

NN9

NN8-9?

NN5-7

?

FIGURE 8. Continued

Although we have not examined samples from 

PAR-15 and PAR-16 for calcareous nannofossils or 

planktic foraminifers, the relation between the dated 

stratigraphic intervals in these boreholes and the benthic 

zonation identified in XEN-3 from Engebi Island is 

reasonably clear. Interval 1 of Szabo and others (1985) 

represents sediments deposited during the Holocene and 

can be identified readily by the presence of bright red 

specimens of the encrusting foraminifer Homotrema and 

a pronounced solution surface at its base in core samples 

from the islands and the lagoon. Interval 1 contains Zone 

AA benthic microfossils in XEN-3 and all other boreholes that were examined during project PEACE.

Following the stratigraphic correlation of Couch and 

others (1975), interval 2 of Szabo and others (1985) 

correlates with Zone BB and perhaps part of Zone CC in

XEN-3, and interval 4 correlates with Zone CC and 

possible Zone DD. Thus, radiocarbon and uranium-series 

dates of 454,000 yr or less for Zones AA-DD corroborate 

a Pleistocene assignment for these sediments.

CONCLUSIONS

As a result of this study on Enewetak PEACE borehole samples, a composite biostratigraphic sequence was 

established (fig. 8) that integrates all calcareous nannofossil and planktic foraminiferal data into the benthic 

zonation of Cronin and others (1986) and Cronin (written 

commun., 1990). Although individual cores contain a 

limited number of reliable, planktic calibration points, 

the composite data for Enewetak benthic Zones LL-AA
C20 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

can be correlated with the international planktic zonations and the time scale of Berggren and others (1985).

Our correlations indicate that lower Zone LL is 

middle Miocene in age and upper Zones LL-JJ are late 

Miocene. Upper Zone LL and the lower part of Zone KK 

correlate with calcareous nannofossil Zone NN 9, and the 

upper part of Zone KK and Zone JJ correlate with 

calcareous nannofossil Zone NN 11. The position of the 

middle-late Miocene boundary is poorly controlled but 

occurs within lower to middle Zone LL. The MiocenePliocene boundary is at or near the boundary between 

Zones JJ and II. Zones II-GG are early Pliocene (calcareous nannofossil Zones NN 12-15). Part of Zone EE/FF 

may be referable to late Pliocene Zone NN 16, but most 

of Zone EE/FF correlates with nannofossil Zone NN 19. 

The Pliocene-Pleistocene boundary is within Zone 

EE/FF. Zones DD-AA are assigned to the Pleistocene 

on the basis of both paleontologic and radiometric data. 

Zones DD and CC most likely represent sediments 

deposited during two separate late Pleistocene interglacials (marine isotope stages 9, 11, or 13; Szabo and 

others, 1985). Zone BB represents sediments deposited 

during the last interglacial period, whereas Zone A A 

represents the Holocene.

The late Cenozoic sedimentary record at Enewetak 

reflects the complex interaction of a fluctuating sea level 

and a subsiding atoll. Although our stratigraphic resolution is not sufficient to discern individual, highfrequency, sea-level events older than the latest Pleistocene, we can identify intervals where the record 

suggests changes in the frequency and amplitude of 

sea-level fluctuations. The relatively thin and discontinuous late Pliocene and Pleistocene section preserved at 

Enewetak represents an interval of generally lower and 

unstable sea level associated with late Neogene glacialinterglacial cycles. The relatively thick early-to-middle 

Pliocene section, which has few obvious disconformities, 

indicates an interval of high and relatively stable sea 

level for parts of this interval. Late Miocene Zone NN 11 

also is well represented in the Enewetak section; its 

presence suggests that part of the late Miocene was also 

an interval of relatively high and stable sea level.

LIST OF SPECIES FOUND IN BOREHOLES AT 

ENEWETAK

CALCAREOUS NANNOFOSSILS

Braarudosphaera bigelowii (Gran & Braarud, 1935)

Deflandre, 1947 

Calcidiscus leptoporus (Murray & Blackman, 1898) Loeblich & Tappan, 1978 

Calcidiscus macintyrei (Bukry & Bramlette, 1969) Loeblich & Tappan, 1978

Ceratolithus acutus Gartner & Bukry, 1974 

Coccolithus eopelagicus (Bramlette & Riedel, 1954)

Bramlette & Sullivan, 1961

Coccolithus pelagicus (Wallich, 1877) Schiller, 1930 

Dictyococcites bisectus (Hay, Mohler, & Wade, 1966)

Bukry & Percival, 1971 

Discoaster aulakos Gartner, 1967 

Discoaster berggrenii Bukry, 1971 

Discoaster bollii Martini & Bramlette, 1963 

Discoaster brouweri Tan, 1927 emend. Bramlette &

Riedel, 1954

Discoaster calcaris Gartner, 1967 

Discoaster challengeri Bramlette & Riedel, 1954 

Discoaster deflandrei Bramlette & Riedel, 1954 

Discoaster exilis Martini & Bramlette, 1963 

Discoaster hamatus Martini & Bramlette, 1963 

Discoaster moorei Bukry, 1971 

Discoaster neohamatus Bukry & Bramlette 1969 

Discoaster nephados Hay in Hay and others (1967) 

Discoaster pansus (Bukry & Percival, 1971) Bukry, 1973 

Discoaster pentaradiatus Tan, 1927 emend. Bramlette &

Riedel, 1954

Discoaster prepentaradiatus Bukry & Percival, 1971 

Discoaster quinqueramus Gartner, 1969 

Discoaster signus Bukry, 1971 

Discoaster subsurculus Gartner, 1967 

Discoaster surculus Martini & Bramlette, 1963 

Discoaster variabilis Martini & Bramlette, 1963 

Discoaster woodringi Bramlette & Riedel, 1954 

Helicosphaera carteri (Wallich, 1877) Kamptner, 1954 

Helicosphaera lophota (Bramlette & Sullivan, 1961)

Locker, 1973

Helicosphaera recta (Haq, 1966) Jafar & Martini, 1975 

Pseudoemiliania lacunosa (Kamptner, 1963) Gartner,

1969

Reticulofenestra pseudoumbilica (Gartner, 1967) Gartner, 1969 

Reticulofenestra umbilica (Levin, 1965) Martini & Ritzkowski, 1968

Rhabdosphaera procera Martini, 1969 

Schphosphaera apsteinii Lohmann, 1902 

Schphosphaera intermedia Deflandre, 1942 

Schphosphaera pulcherrima Deflandre, 1942 

Sphenolithus abies Deflandre in Deflandre & Fert (1954) 

Zygrhablithus bijugatus (Deflandre in Deflandre &

Fert (1954)) Deflandre, 1959

PLANKTIC FORAMINIFERS

Globigerina angulisuturalis Bolli 

Globigerina bulloides d'Orbigny 

Globigerina ciperoensis Bolli 

Globigerina decoraperta Takayanagi and Saito 

Globigerina nepenthes Todd
CALCAREOUS NANNOFOSSILS AND PLANKTIC FORAMINIFERS FROM ENEWETAK ATOLL C21

Globigerina rubescens Hofker

Globigerina woodi Jenkins

Globigerinella aequilateralis (Brady)

Globigerinita glutinata (Egger)

Globigerinoides conglobatus (Brady)

Globigerinoides extremus Bolli and Bermudez

Globigerinoides obliquus Bolli (s.l.)

Globigerinoides ruber (d'Orbigny)

Globigerinoides sacculifer (Brady)

Globigerinoides subquadratus Bronnimann

Globigerinoides tenellus Parker

Globoquadrina altispira (Cushman and Jarvis)

Globoquadrina dehiscens (Chapman, Parr, and Collins)

Globorotalia kugleri Bolli

Globorotalia menardii (Parker, Jones, and Brady)

Globorotalia miocenica Palmer

Globorotalia plesiotumida Blow and Banner

Globorotalia tumida (Brady)

Globorotalia ungulata Bermudez

Neogloboquadrina acostaensis (Blow)

Neogloboquadrina humerosa (Takayanagi and Saito)

Orbulina universa d'Orbigny

Pulleniatina obliquiloculata (Parker and Jones)

Pulleniatina primalis Banner and Blow

Sphaeroidinellopsis spp.

Streptochilus spp.

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19. Assemblages found between 777 ft and 333 ft can be assigned 

only to the broad interval from Zone N 14 to Zone N 19 or late 

middle Miocene to Pliocene.