Subsurface Geology 

of Eniwetok Atoll

GEOLOGICAL SURVEY PROFESSIONAL PAPER 260-BB
Subsurface Geology 

of Eniwetok Atoll

By SEYMOUR O. SCHLANGER

tPith sections on CARBONATE MINERALOGY 

By DONALD L. GRAF and JULIAN R. GOLDSMITH

PETROGRAPHY OF THE BASALT BENEATH THE LIMESTONES

By GORDON A. MACDONALD

DATING OF CARBONATE ROCKS BY IONIUM-URANIUM 

RATIOS

By WILLIAM M. SACKETT and HERBERT A. POTRATZ

BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

GEOLOGICAL SURVEY PROFESSIONAL PAPER 260-BB

A study of limestone and dolomite recovered 

during deep drilling on Eniwetok

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1963
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEW ART L. UDALL, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

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

Washington 25, D.C.
CONTENTS

Abstract.. _ ______________-__---__--_--.-__-_____-_-_

Introduction. _____-____-___--_-_--____-_-_____--___

Acknowledgments _____-____----___--_-__-----___

Leached zones and unconformities ____________________

Paleoecology _ _ _____________________________________

Comparison of Eniwetok drill holes with Bikini drill holesLower parts of Eniwetok drill holes _______________

Geologic history ____________________________________

Deposition and truncation of Eocene sediments-.---

Deposition and truncation of lower Miocene sediments _-_________---____----__----____----_-_

Upper Tertiary to Recent sediments ___ ___-___-__

Analyses _ ________________________________________

Chemical analyses. _____________________________

Mineralogical analyses..... _ _____________________

Spectrographic analyses. ________________________

Dolomitization _____________________________________

Stratigraphic distribution. _-______-_----_-___-___

Comparative petrography of dolomite from Pacific

Formation of dolomite in coralline algae ___________

Future work ___________________________________

Conclusions ___-___----__-_---__-----_____--____

Detailed descriptions of cores and cuttings _____________

Drill hole E-l-__-_-_ — — — _ — _-__--- — — ----__-

Drill hole F-l_______-__-___-___---____-________

Petrography of the basalt beneath the limestone, by 

Gordon A. Macdonald____-_______ --______-____

Page 

991 

993

993

994

998

999

999

1001

1001

1002

1002

1003

1003

1004

1006

1007

1008

1009

1009

1010

1011

1011

1012

1019

1038

Petrography of the basalt beneath the limestone, by

Gordon A. Macdonald—Continued Page 

Megascopic features___----_-_---_-____________ 1039

Microscopic features ____________________________ 1042

General description.________________________ 1042

Alteration products.________________________ 1044

Veins____-_-__-__--_----__--_.___________ 1045

Chemical composition ___________________________ 1047

Consanguinity of the rock-__----_-______________ 1048

Carbonate mineralogy, by Donald L. Graf and Julian R.

Goldsmith.__________________________________ 1048

X-ray diffraction analysis of carbonate mineral mixtures. __ ____--_--_---___-_--____-__________-_ 1049

Calcite_----------_________________________ 1052

Dolomite--------------_--------------__---_--- 1052

Aragonite_ --___-----_------_-_-______________ 1053

Dating of carbonate rocks by ionium-uranium ratios, by

William M. Sackett and Herbert A. Potratz_ _ _________ 1053

Previous work_______--_-__---_--__-___-___-____ 1054

Isotopic thorium content of ocean water___________ 1055

Ionium-uranium ratios in marine limestones______-_ 1057

Uranium content of marine limestones_____-__- 1057

Ionium-uranium ratios in cuttings from

Eniwetok Atoll drill holes __________________ 1058

Ionium-uranium versus radiocarbon and other ages.. 1062 

Other possible methods for dating marine-deposited

calcium carbonates__________________________ 1063

Description of materials studied___---_______--___ 1064

References-_ _______________________________________ 1065

ILLUSTRATIONS

[Plate 281 in pocket; plates 282-288 follow references]

PLATE 281. Diagrammatic history of Eniwetok Atoll. 

282-285. Dolomitized limestones—

282. From hole F-l, Eniwetok Atoll.

283. Tertiary b age from hole E-l, Eniwetok Atoll.

284. From Funafuti Atoll.

285. From Funafuti Atoll and Kita-daito-jima. 

286-288. Photomicrographs of limestones from cores—

286. F-l-4 and F-l-5.

287. F-l-5, F-l-6, and F-l-7.

288. F-l-9, F-l-10, F-l-11, and F-l-12.
IV CONTENTS

FIGURE 306, 307. Maps of— Pase

306. Eniwetok Atoll showing location of holes E-l, F-l, and K-1B________________________________ 992

307. Western Pacific Ocean showing location of Kita-daito-jima, Funafuti, Bikini, and Eniwetok______ 993

308. Drill-hole section showing recrystallized and partly dolomitized rock below Eniwetok and Bikini Atolls__-_ 995

309. Diagrammatic section showing development of a solution unconformity_____________________________ 996

310. Paleoecology logs of drill holes E-l and F-l on Eniwetok and drill hole 2A-B on Bikini- *____________-_ 1000

311. Diagram of MgCOa analyses of samples from drill holes on Funafuti and Kita-daito-jima_______________ 1004

312. Polished section of basalt core from drill hole E-l________________________________________________ 1042

313, 314. Photomicrographs of thin sections of basalt—

313. Fresh basalt from drill hole E-l_____________________.._______.-_____________ 1042

314. Altered basalt from drill hole E-l________________________________________________ 1043

315. Integrated intensities of selected powder X-ray diffraction maxima obtained from mixtures of calcite,

aragonite, and dolomite______________________-___--_______-___---_-_-___-_------_-_--------_ 1049

316. Uranium, actinouranium, and thorium decay series_______________________________________________ 1054

317. Uranium content and ratio of ionium-uranium activities of cuttings from drill holes E-l and F-l ________ 1059

318. Theoretical growth of Th230 and Pa231 from initially pure uranium and change in ratio of activities of Pa231

and Th230 with time___________________________________-----_________-____ 1063

TABLES

TABLE 1. Chemical analyses of limestone and dolomite samples from Eniwetok_______________________________________ 1003

2. Calcite, aragonite, and dolomite content of representative samples______-_-------------_-_----_--_---------- 1005

3. Quantitative spectrographic analyses (in weight percent) of limestones from Eniwetok._______________________ 1006

4. Megascopic description of cores from hole E-l, Parry Island, Eniwetok-____________________________________ 1039

5. Chemical analyses of basalt from Eniwetok and other similar rocks_______-_-_____________-_-__-_-_-______-_ 1047

6. Abundance of trace elements in samples of the basaltic cores from Eniwetok-________________________________ 1048

7. Carbonate mineral composition of selected samples.____-_-_-_-_---_____-----_-_-_-_-____--------_-_-_-_-- 1049

8. Calculated concentrations of thorium isotopes in ocean waters-____-_____---__--_____-___-------_-_______-_ 1055

9. Thorium alpha activity in Pacific Ocean water___________________________________________________________ 1056

10. Uranium content versus crystal structure of marine-deposited calcium carbonates ____________________________ 1057

11. Uranium content of calcium carbonate deposited by some marine organisms.________________________________ 1057

12. Ratio of ionium to uranium in cuttings from drill hole F-l_______--__-____---___________-----_----__------ 1059

13. Ratio of ionium to uranium in cuttings from drill hole E-l____________-_-_-------_--_--_-_-_-----------__- 1059

14. Comparison of ionium-uranium ratios and indicated ages________________________________________________ 1060

15. Ionium-uranium ratios in Recent materials.________________-____-_______-_-____---_____-__-_-____--___-_ 1060

16. Ionium-uranium ratios in cores taken from drill holes on Eniwetok Atoll____-____-__-_______---_______-___-_ 1061

17. Uranium content versus depth for cuttings_____________________________________________________________ 1061

18. Thorium232 versus uranium in some calcium carbonates__________________________________________________ 1061

19. Ionium-uranium ages for some marine shells.____________________________________________________________ 1062

20. Ionium-uranium versus radiocarbon ages for Eniwetok Atoll calcium carbonates_____________________________ 1062
BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

SUBSURFACE GEOLOGY OF ENIWETOK ATOLL

By SEYMOUR O. SCHLANGEB

ABSTRACT

Two deep holes, drilled on opposite sides of Eniwetok Atoll, 

reached the basement below the cap of Recent to Eocene limestone at depths of 4,610 and 4,158 feet. The subsurface geology 

of Eniwetok Atoll, as deduced from these borings, is related to 

that of Bikini and Funafuti Atolls and to the emerged island 

of Kita-daito-jima.

Below both Eniwetok and Bikini Atolls, intervals characterized by fossil molds and solution channels alternate vertically 

with intervals marked by the presence of primary skeletal 

aragonite. The leached intervals are directly and sharply overlain by unaltered unconsolidated sediments. The tops of the 

leached zones are termed "solution unconformities." These 

unconformities were formed during periods when the atolls were 

emergent, and they can be precisely correlated between Eniwetok 

Atoll and Bikini Atoll, 210 miles away. Below both atolls there 

are well-defined unconformities at approximately 300 and 1,100 

feet below sea level; at Eniwetok, another is present at 2,780 

feet below sea level. The unconformities generally coincide with 

faunal breaks, which indicate that periods of nondeposition 

interrupted the formation of the 4,600-foot-thick section below 

Eniwetok Atoll and at least the upper 2,556 feet of the limestone 

column below Bikini Atoll.

The post-Eocene limestones beneath Eniwetok are of shallowwater, mostly lagoonal, origin. The correlative sections at 

Bikini are also of shallow-water origin. The coincidence of 

solution unconformities at both atolls and the common origin 

of the limestones indicate that both atolls followed an identical 

course of development from Miocene time to the present. At 

Eniwetok the Eocene section in hole F-l is of fore-reef, outerslope origin, whereas correlative deposits in hole E-l are of 

shallower water origin—no comparable section at Bikini was 

reached by drilling. The earliest reef at Eniwetok grew below 

what is now the southeast side of the atoll; through the Tertiary 

the atoll gradually developed in a northwesterly direction until 

it reached its present configuration. The atoll was emergent in 

early Miocene, in late Miocene, and again in Pleistocene time. 

The thickness of the leached intervals indicates emergence of as 

much as 700 feet.

Chemical analyses show that the highest MgCO3 content is 

38.8 percent, 7 percent less than the theoretical MgCO3 content 

of pure dolomite, although X-ray analyses show more than 98 

percent dolomite. Dolomite from both Funafuti and Kitadaito-jima also show excess calcium. X-ray studies by Graf and 

Goldsmith show that the excess calcium is due to the presence of 

calcium ions in some magnesium positions in the dolomite 

structure. Mineralogical analyses indicate that nowhere in these 

atolls do aragonite and dolomite coexist, a situation that suggests

that either the removal of aragonite by solution or its replacement 

by calcite is a prerequisite to dolomitization. Spectrographic 

analyses show these limestones and dolomites to be very low in 

magnesium and iron; the strontium content drops as recrystallization and dolomitization proceed.

A review of the stratigraphic distribution and a study of the 

comparative petrography of dolomite from Eniwetok, Funafuti, 

and Kita-daito-jima indicate that even within the restricted 

conditions that prevail during deposition and diagenesis of atoll 

limestones, dolomite can apparently form in at least three ways:

1. By development of dolomite from metastable, high-magnesian 

calcite originally precipitated by coralline algae.

2. By reaction of sea water with low-magnesian calcite.

3. By direct precipitation of dolomite in vugs in the limestone.

Cores and cottings from drill holes E-l and F-l on Eniwetok 

Atoll are described in detail.

Alkali olivine basalt containing analcime was cored between 

depths of 4,208 and 4,222 feet in drill hole E-l on Parry Island, 

Eniwetok Atoll. Petrographic study shows that the rock consists of olivine, plagioclase feldspar, titanian augite, magnetite, 

ilmenite, alkali feldspar, and analcime. Parts of the core are 

nearly fresh, but much of it is intensely altered to saponite and 

chlorite. The alteration is related to numerous calcite veins that 

cut the basalt. Chlorite, zeolite, and cellophane veins also are 

present.

The rock is similar to alkali olivine basalts and basanites among 

the late lavas from the volcanoes of the State of Hawaii and 

other islands of the central Pacific Ocean. Its extreme denseness 

is not evidence of extrusion in deep water, as some subaerial 

lavas of Hawaii are equally dense. All the core appears to 

belong to the dense part of a single flow. The alteration of the 

lava resembles that effected by hydrothermal solutions elsewhere, 

but it and the vein formation are believed to have been caused by 

epigene solutions that dissolved calcium carbonate from overlying limestones at a time when the limestones were emergent and 

the basalt platform beneath them stood near sea level.

Th230/U238 ratios in ocean water and marine limestones were determined to ascertain the conditions under which this ratio may 

serve as a measure of geologic age. Marine limestones contain 

uranium in amounts that range from 0.10 to 5.0 ppm (parts per 

million). The highest uranium values are associated with aragonitic limestones, whereas calcitic materials are almost invariably 

low in uranium. All newly deposited marine limestones are, 

however, virtually free of Th230 .

If a calcium carbonate deposit is formed that (1) contains a 

measurable amount of uranium, (2) has very much less than 

the equilibrium amount of Thm, and (3) does not gain or lose

991
992 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

uranium or lose the Th230, that grows into the calcium carbonate 

from the radioactive decay of uranium, then the time at which 

the calcium carbonate was deposited can be determined from the 

Th230/U238 ratio within the experimental error of the analyses for 

materials from 1,000 to 300,000 years old.

In all marine carbonates analyzed, conditions (1) and (2) seem 

to be satisfied. In some of these materials, however, condition (3) 

has apparently been violated, and a lack of agreement between 

C14 ages and ages determined from Th230/U238 ratios is observed in

which C14 ages are generally younger. Two explanations are suggested. In some marine carbonates, conversion of aragonite to 

calcite has occurred through a solution-redeposition process in 

which C14 was gained and uranium was lost. Other samples of 

older limestones have been contaminated with younger material 

that increases the C14 content.

Apparent ages have been calculated from the Th230/U238 ratios 

in samples that were obtained from drill hole E-1 on Parry Island 

The ratios indicate that deposition of all material lying above

162°00'

———I———

162°30' 

——I——

INSERTS-APPROXIMATE

ENIWETOK ATOLL

PARRY ISLAND

STATE OF HAWAII 

ENIWETOK

180°

INDEX MAP

FIGUEE 306.—Map of Eniwetok Atoll showing location of holes E-1, F-l, and K-IB.
SUBSURFACE GEOLOGY OF ENIWETOK ATOLL 993

the 60-foot depth has occurred during the last 12,000 years. 

Samples taken at greater depths gave the following apparent 

ages: 70 to 80 feet, 38,000 years; 90 to 100 feet, 106,000 years; 

100 to 110 feet, 160,000 years; 120 to 130 feet, 280,000 years. 

Two additional radiochemical procedures for dating recent marine limestones are suggested.

INTRODUCTION

In 1952, two deep holes drilled on Eniwetok Atoll 

(fig. 306) reached the volcanic basement below the 

limestone cap. Hole F-l struck hard basement rock 

at 4,610 feet; in hole E-l, basalt cuttings were recovered at 4,158 feet, and solid basalt cores were taken 

from 4,208 to 4,222 feet. This report contains a detailed petrographic description of the cores and cuttings 

recovered from these drill holes, and these are the 

bases for interpretations of the geologic history of the 

atoll. Comparisons are drawn between the histories 

of Bikini and Eniwetok Atolls, and the problem of 

dolomitization in Pacific atolls is discussed with reference to both Funafuti Atoll and Kita-daito-jima (fig. 

307).

This report is but one chapter of a long series on the 

geology of Bikini and nearby atolls published as U.S. 

Geological Survey Professional Paper 260. It supplements and expands data published in chapter Y (Ladd 

and Schlanger, 1960), which covers the drilling operations on Eniwetok Atoll and contains logs of deep holes 

F-l, E-l, and K-lB, as well as those of many shallower 

holes. The surface and marine geology of Eniwetok 

Atoll is covered in chapter A (Emery, Tracey, and Ladd, 

1954). Essential to an understanding of the present

report is a, knowledge of the subsurface geology of 

Bikini Atoll as discussed by Tracey in chapter A. The 

stratigraphic divisions used in this report are based on 

foraminiferal zones defined by Cole (1957) in chapter 

V and by Todd and Low (1960) in chapter X.

ACKNOWLEDGMENTS

In the study of the Eniwetok cores and cuttings, the 

findings of many of the workers who contributed to U.S. 

Geological Survey Professional Paper 260 were used as 

essential background material. The chapters on larger 

Foraminifera by W. Storrs Cole, smaller Foraminifera 

by Ruth Todd and Doris Low, and subsurface geology 

of Bikini by J. I. Tracey, Jr. (in chapter A) were particularly valuable. Special thanks, regrettably posthumous, are due the members of the Coral Reef Committee of the Royal Society, particularly G. J. Hinde and 

J. W. Judd; the classic report of C. G. Cullis (1904) on 

the Atoll of Funafuti stands as a model for atoll research. Discussions on the dolomite problem with 

Donald L. Graf, Illinois State Geological Survey, and 

Julian R. Goldsmith, Department of Geology, University of Chicago, were of great help.

The materials described in this report were recovered 

during deep drilling on Eniwetok that was supported by 

the Armed Forces Special Weapons Project and carried 

out for the U.S. Atomic Energy Commission and the 

Los Alamos Scientific Laboratory in cooperation with 

the Office of Naval Research.

The work by William M. Sackett and Herbert A. 

Potratz, Department of Chemistry, Washington Uni160* 160" 140*

*KITA-DAITO-JIMA

o STATE OF HAWAII

MARIANA

\ M J c ENIWETOK BIKINI 

n* 0 , **

/]/ MARSHALL 5 PALAU

CAROLINE IS

PA C I F I C

-crSOLOMON^ S^FUNAFUTI

OCEAN

20*-

NEW 

CALEDONIA

160* 180° 160' 120*

FIGURE 307.—Map of the western Pacific Ocean showing location of Kita-daito-jima, Funafuti, Bikini, and Eniwetok.
994 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

versity, St. Louis, Mo., on uranium and thorium was 

supported by the Aluminum Company of America and 

the Van Blarcum Fund of Washington University 

through fellowships, and by the U.S. Atomic Energy 

Commission under contract number AT (11-1)-581. 

The Los Alamos Scientific Laboratory furnished equipment. Meyer Rubin and W. S. Broecker of the Geological Survey obtained radiocarbon dates on several 

marine carbonates. E. D. Goldberg, of Scripps Institution of Oceanography, Thor V. N. Karlstrom, of the 

Geological Survey, J. C. Brice, R. P. Bullen, and C. R. 

McGimsey supplied some samples.

LEACHED ZONES AND UNCONFORMITIES

Below both Eniwetok and Bikini Atolls, zones characterized by fossil molds and solution channels alternate 

vertically with zones marked by the presence of primary 

skeletal aragonite. 1 Figure 308 is a plot of the five 

deep holes—K-lB, F-l, and E^l on Eniwetok and 2A 

and 2B on Bikini—and shows that there is a remarkable 

similarity in the vertical extent pf these zones.

Leached zones overlain by unaltered unconsolidated 

sediments and separated from them by sharp boundaries are found in all five of the holes studied. The 

upper surfaces of the leached zones are here termed 

"solution unconformities." These solution unconformities, leached zones, and unaltered zones can be correlated even though the holes are widely separated. Drill 

hole K-lB is on Engebi, F-l is on Elugelab, and E-l is 

on Parry Island, Eniwetok Atoll (fig. 1). Drill holes 

2A and 2B are 180 feet apart on Bikini island, Bikini 

Atoll, 210 miles east of Eniwetok Atoll (Emery, Tracey, 

andLadd, 1954, fig. 33).

Ladd, Tracey, and Lill (1948, p. 52) suggested that 

the leached zones below Bikini were developed under 

subaerial conditions and thus recorded periods of emergence of parts of the marine sediments of the atoll. The 

unaltered zones were believed to represent rocks that 

were never emergent (Emery, Tracey, and Ladd, 1954, 

p. 2, 132-133, 244, 247).

Rocks in which primary aragonite either has been 

leached out or still persists are shown on figure 308. 

The limestones (calcite) are dense, well-lithified rocks 

that show molds of originally aragonitic shells and 

skeletons of mollusks, corals, and Halimeda. In places 

the aragonite has been completely leached out, and the 

molds are filled or lined with fibrous and drusy calcite 

deposited from solution. The original mud 2 and sand 

matrix has been largely recrystallized to calcite, and

1 All depths given are measured from an assumed drilling platform elevation of 0 

feet. Platforms actually were at the following approximate elevations (above 

MLWS): K-lB, 4 feet; F-l, 17 feet; E-l, 19 feet; 2A, 17 feet; and 2B. 20 feet.

2 The term "mud" is here used to include all fine sediments—clay size, silt size, and 

fine sand. Clay minerals do not occur in the Eniwetok section; all clay-size material 

is pure calcium carbonate.

irregular channels and vugs are common. These voids 

truncate fossils and are commonly lined with laminar 

deposits of calcite. The overall effect is one of removal 

of aragonite and addition of calcite.

The limestones with unlithified sediments differ strikingly from the leached limestones. These unaltered 

sediments are generally unconsolidated and are made up 

of fossil debris that includes abundant corals and mollusks with aragonite skeletons and shells. Originally 

calcitic fossils such as Foraminifera and coralline algae 

are mixed with the corals and mollusks, but the sediments have not been cemented by the addition of 

calcite.

The leached rocks grade downward into unaltered 

ones through transition zones (shown on fig. 308 by 

saw-toothed boundaries) as much as 200 feet thick. 

These transition zones are composed of lithified limestones made up of partly replaced and recrystallized 

aragonitic fossils in a calcite matrix. In many places 

within the transition zones, aragonitic fossils are filled 

and cemented by calcite, and this indicates the addition 

of calcite without solution of aragonite. Aragonite 

fossils in these zones commonly show a tendency to 

disintegrate into loose aragonite powder.

Detailed descriptions of the lithology of the above 

rocks on Emwetok are given on pages 1011-1038, and 

the rocks on Bikini are described in chapter A (Emery, 

Tracey, and Ladd, 1954, p. 80-94, 214-259). Above 

a depth of 75 feet in K-lB, 70 feet in F-l, 80 feet in E-l, 

and 105 feet in 2A, the sediments are unconsolidated 

and unaltered, except for a few layers of cemented 

beach rock at or near present sea level. Below these 

depths in all four holes (drilling records for the upper 

part of 2B are too poor for interpretation), however, 

partly recrystallized and leached limestones are found; 

thus, poorly developed solution unconformities are 

probably present at the depths indicated. Figure 308 

shows the patchy and discontinuous nature of the 

alteration.

A lower, better developed unconformity shows up at 

302 feet in K-lB, 330 feet in F-l, 300 feet in E-l, 29.4 

feet in 2A, and 290 feet in 2B; this will be referred to as 

the 300-foot unconformity. At these depths, moderately to well-consolidated limestones containing prominent molds of coral and mollusks were encountered; 

coarsely crystalline calcite fills many cavities in this 

rock. Below these levels, leached and cemented limestone persists for 200 to 400 feet, as shown on figure 308, 

but the interval varies in thickness from one hole to 

another. All five holes penetrate a transition zone in 

and below which the sediments take on a brown color 

that may be due to the presence of unbleached organic 

matter. Below the transition zone, the lowermost part 

of the unaltered zone under the 300-foot uncomformity
DEPTH, IN FEET

K-1B

ENIWETOK 

F-l

SUBSURFACE GEOLOGY OF ENIWETOK ATOLL

BIKINI

995

E-l 2-A 2-B

1000-

2000-

3000-

4000-

Top of Tertiary g (Cole)

Top of Tertiary e (Cole)

2780ft

Top of Tertiary b (Cole) ,

Total depth 2556 ft

EXPLANATION

Length of bar indicates roughly the percentage

Limestone (calcite) with solution 

features

Partly recrystallized and leached limestones

Limestone and unlithified sediments 

with primary aragonite Gradational zone

Total depth 4222 ft

Solution unconformity in hole 

Correlation between unconformities

Total depth 4630 ft

Foraminiferal zone

Numbers 1-15 along drill hole F-l, and 1-3 along 

drill hole E-l indicate cored intervals

FIGURE 308.—Drill-hole section showing recrystallized and partly dolomitized rock below Eniwetok and Bikini Atolls.
996 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

is characterized in all holes by limy mud, unsorted 

aragpnitic sand, and a striking abundance of shallowwater branching corals and perfectly preserved pelecypod and gastropod shells; some of the shells retain 

traces of their original color patterns.

At 1,080 feet in K-lB, 1,070 feet in E-l, 1,127 feet in 

2 A, and 1,120 feet in 2B, the drill passed abruptly from 

the brown unaltered sediments described above into 

hard, cemented limestone with well-developed molds; 

aragonite is lacking. A third unconformity, therefore, 

is placed at these levels and will be referred to as the 

1,100-foot unconformity. No cuttings were obtained 

from hole F-l between 970 and 1,040 feet, but the 

contrast in lithology between these two depths indicates 

that an unconformity is present somewhere in this 

interval.

On Eniwetok, the top of the Tertiary e, as picked by 

Cole (1957, p. 746) on the basis of larger Foraminifera, 

lies at the 1,100-foot unconformity; on Bikini this same 

fauna! horizon lies just below the unconformity. This 

is interpreted as showing a post-Tertiary e emergence 

of the atoll followed by removal by erosion of some

limestone of both Tertiary e and / ages at Eniwetok; 

at Bikini, erosion did not completely strip off all the 

limestone of Tertiary/ age.

Both holes E-l and 2B show transition zones below 

the 1,100-foot unconformity that are similar to those 

below the 300-foot unconformity; holes K-lB and 2A 

bottomed in the leached zone below the 1,100-foot 

unconformity. Below the 1,100-foot unconformity hole 

F-l is anomalous; all cores and cuttings recovered 

were crystalline calcitic limestone. Dolomite is present in many of the cores. (See fig. 308).

A fourth unconformity overlain by a thick section of 

aragonite sediments, was penetrated in hole E-l at 

2.780 feet. Cole (1957, p. 748), again on the basis of 

larger Foraminifera, picked the top of the Tertiary b at 

this depth. This coincidence of fossil horizon and unconformity suggests post-Eocene emergence and removal 

of Tertiary b limestone prior to deposition of Miocene 

limestones.

Figure 309 shows diagrammatically the development 

of a solution unconformity. During stage 1, sediments 

accumulated as the atoll foundation subsided or as sea

STAGE i

SEA LEVEL

Limestone cap

Volcanic basement

Sea level rising and (or) 

basement subsiding

STAGE 2

Recrystallized and leached limestone

SEA LEVEL

Sea level falling and (or) 

basement rising

STAGE 3

SEA LEVEL

Sea level rising

FIGURE 300.—Diagrammatic section showing development of a solution unconformity.
SUBSURFACE GEOLOGY OF ENIWETOK ATOLL 997

level rose eustatically, so that the aragonite-rich sediments were not exposed to subaerial conditions. In 

stage 2, emergence of part of the sedimentary column 

through uplift of the basement or a fall in sea level led 

to solution of aragonite and deposition of calcite. The 

recrystallized and leached rocks that resulted are illustrated in figure 309. Upon resubmergence in stage 

3, a new cap of aragonite-rich sediment was deposited 

on the hardened surface of the solution unconformity.

If an atoll some 20 miles in diameter is raised above 

the sea, a fresh-water lens would develop and would 

extend several hundred feet below sea level. This lens, 

called the Ghyben-Herzberg lens, floats upon the sea 

water and depresses it. The effect on the limestone 

immersed in this fresh-water lens is not known, but 

solution of the emergent section takes place probably 

at least down to the upper surface of the lens. On 

Guam and other high limestone islands, zones of caves 

are commonly well developed just above raised relict 

sea nips along limestone coasts. Carbonate carried 

down by rainwater may precipitate as calcite in the 

saturated fresh-water lens and cement aragonitic materials such as are found in the transition zones; 

thus, the transition zone between thoroughly recrystallized limestone and underlying unaltered sediment may 

mark the site of a former Ghyben-Herzberg lens.

There is little doubt that the strongly leached zones 

below the atolls represent intervals that were emergent 

at one time. Shells of land snails have been described 

(Ladd, 1958, p. 188-190) from the subsurface of Bikini, 

Eniwetok, and Funafuti that are of a type found today 

on high forested islands. Dense concentrations of the 

pollen and spores from land plants have also been recovered (Ladd, 1958, p. 194).

Presently emergent limestones on Guam (Schlanger, 

1963) show the characteristics of the limestone from 

the leached zones; fossil molds, solution channels, and 

calcite cement are also present.

On Guam, rare, isolated pockets of limestone contain 

aragonite in otherwise completely recrystallized masses 

that are miles in extent. One of these pockets is at an 

altitude of approximately 600 feet in limestone of probably early Pleistocene age and must have been emergent 

for some thousands of years. These relict aragonitic 

limestones are partly recrystallized, however, and the 

process of solution of aragonite has not progressed to 

completion; therefore, the occurrence of aragonite does 

not necessarily indicate a history of nonemergence. 

Nowhere in these emergent limestones on high islands 

were thick sections of unconsolidated aragonitic fossil 

debris found, such as exist in the subsurface of Eniwetok 

and Bikini.

Even a cursory search of recent literature on limestones shows a prevailing concept that aragonite is unstable in carbonate sediments. Pettijohn (1957, p. 387) 

states "Aragonite is an unstable form of calcite and is 

found therefore only in recent materials." Folk (1959, 

p. 32) states, "Aragonite shell material (most pelecypods, many gastropods) inverts to calcite with 

time* * *" Weller (1959, p. 299) states, "Aragonite 

is somewhat more dense than calcite and it is relatively 

unstable under natural sedimentary conditions and 

tends to invert to calcite."

The general tone of these remarks is that somehow 

aragonite inverts spontaneously to calcite, and that 

the postdepositional history of the sediment has little 

to do with the inversion. The distribution of primary 

skeletal aragonite in the subsurface of both Bikini and 

Eniwetok (fig. 308) shows that time alone is not the 

factor controlling the replacement of aragonite by calcite. Where the carbonate sediments have been exposed to subaerial conditions, the aragonite has been 

leached out and partly or wholly replaced by calcite; 

where the sediments have remained submerged in sea 

water since their deposition, aragonite persists. Further, the sediments in the aragonite-rich sections are 

not consolidated; the mollusk shells retain their original nacreous luster; and the sediment closely resembles 

Recent deposits. Evidently, aragonite under these atolls has shown no tendency to invert to calcite, at least 

over the span of Tertiary time.

Another implication of some recent writings on the 

aragonite-calcite relationship is that the aragonite simply goes through an inversion in which the orthorhombic aragonite structure rearranges itself to form the 

rhombohedral calcite structure. The writer has seen 

no compelling evidence for such a crystallographic inversion. Rather he believes that the process is one in 

which the aragonite is dissolved and replaced by calcite 

on a volume for volume basis. This solution-replacement idea was originally suggested by J. I. Tracey, Jr. 

(oral communication), who came to such a conclusion 

after his study of the Bikini drill samples.

In individual pieces of coral the results of this 

process can be clearly seen. The boundary between 

the original aragonite and the replacing calcite closely 

resembles a stylolitic seam and is occupied by a thin 

film of powdery aragonite. Under the action of solutions during subaerial exposure, the aragonite evidently 

breaks down into a powder and is then dissolved and 

replaced by the more coarsely crystalline calcite.

It should also be pointed out that if the primary 

aragonite inverted structurally, it would retain its content of strontium, which is high relative to the strontium 

content of calcite. Spectrographic analyses (table 3) 

show that corals that are now calcitic have lost much 

of their original strontium. This loss is understandable 

if the aragonite is first dissolved and then replaced by
998 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

calcite that has a low tolerance for the trapping of 

large cations during crystallization.

The sequential development of the unconformities 

described above is discussed on pages 1001-1003.

PALEOECOLOGY

The high percentage of recognizable fossil material 

in the subsurface limestones permits interpretations of 

the depth and site of deposition of the sediments with 

reference to the reef complex. Many of the corals, 

Foraminifera, algae, and mollusks are known to have 

fairly restricted habitats in the reef complex today, and 

therefore fossil assemblages are the main guide in 

making paleoecologic interpretations. As the ecology 

of these faunal and floral groups has been discussed by 

Finckh, 1904, p. 125-150; Tracey, Ladd, and Hoffmeister, 1948, p. 861-878; Ladd, Tracey, Wells, and Emery, 

1950, p. 410-425; Cloud, 1952, p. 2125-2149; Emery, 

Tracey, and Ladd, 1954, p. 79-80; and Forman and 

Schlanger, 1957, p. 611-627, it will not be repeated here. 

Paleoecological interpretations of the sediments in holes 

E-l and F-l are tabulated below. More detailed 

descriptions of the individual rocks are given on 

pages 1011-1038.

Drill hole E-l

Interval 

(ft) Environment and significant fossil groups

0-30 Near reef, lagoonal, and beach; rich in coral and 

worn and broken reef Foraminifera.

30-45 Lagoonal; made up of small Halimeda segments 

probably deposited in water 20 to 30 fathoms 

deep.

45-70 Shallow lagoonal; coarse, rounded coral debris, 

Foraminifera, and Halimeda.

70-140 Lagoonal; rich in reef Foraminifera and Halimeda.

140-300 Reef knoll or reef wall; abundant coral and encrusting algae. 

Solution unconformity

300-430 Shallow lagoonal; dominated by corals and mollusks.

430-590 Shallow water, probably lagoonal; corals, mollusks, 

and Halimeda.

590-1, 070 Shallow bank or lagoonal; well preserved brown 

fragments of delicate branching coral, pelecypods and gastropods. 

Solution unconformity

1, 070-2, 003 Shallow water; miliolid Foraminifera and coral.

2,003-2,028 Shallow water, lagoonal; rich in miliolids and 

mollusks.

2, 028-2, 290 Shallow water; rich in fragments of Lithophyllum. 

Lower part of the interval contains abundant 

larger Foraminifera and probably represents 

open, shoal-water deposits containing local forereef sediments.

2, 290-2, 410 Shallow water, locally lagoonal; abundant Lithophyllum and mollusks.

2,410-2,540 Lagoonal; in part laminated mudstones with 

mollusks.

Drill hole E-l—Continued

Interval 

(ffi Environment and significant fossil groups 

2, 540-2, 780 Shallow water, lagoonal; coral and molluscan remains are especially abundant near base of the 

section, as in the 590-1,070 foot interval. 

Solution unconformity

2. 780-3, 130 Fore reef or open shoal water; abundant benthonic 

smaller Foraminifera. Locally abundant dasycladacean algae are indicative of water only a 

few feet deep and occur with typically lagoonal 

faunal elements, indicating that local lagoonal 

conditions existed nearby.

3. 130-4, 078 No recovery.

4. 078-4, 100 Shallow lagoonal and reef wall; massive corals,

coralline algae, and mollusks. 

4, 100-4, 170 No recovery. 

4, 170-4, 190 Basalt. 

4, 190-4, 208 No recovery. 

4, 208-4, 222 (Total depth) Basalt.

Drill hole F-l

Interval 

(ft) Environment and significant fossil groups

0-55 Near reef, lagoonal, and beach deposits; worn and 

broken reef Foraminifera, coral, and Halimeda.

55-70 Fore reef; abundant segments of large Halimeda.

70-110 Shallow water; Halimeda, coral, and molluscan 

debris.

110-170 Lagoonal; abundant coral algae and mollusks.

170-190 Lagoonal; miliolids and Halimeda.

190-330 Lagoonal; local concentrations of Halimeda indicate

deposition in 20 to 30 fathoms of water. 

Solution unconformity

330-600 Lagoonal; corals, mollusks, and miliolids.

600-970 Lagoonal or shoal banks; abundant fragments of 

delicate, branching coral and numerous gastropods and pelecypods. Towards the bottom of 

the interval the sediments take on a tan to 

brown color. Sedimentation conditions were 

like those in the 590- to 1,070-foot interval in 

hole E-l.

970-1, 040 No recovery. (Solution unconformity somewhere 

in this interval.)

1, 040-1, 230 Shallow water, probably lagoonal in part; Halimeda, mollusks, and smaller Foraminifera.

1, 230-1, 248 Shallow water, probably lagoonal in part; miliolids, 

algae, corals, and Halimeda.

1, 248-1, 718 No recovery.

1, 718-1, 740 Near reef, shallow water near top of interval, lagoonal toward bottom of the interval, quietwater conditions prevailed.

1, 740-1, 978 No recovery.

1. 978-2, 130 Reef wall and near reef; massive coral and algae.

2. 130-2, 662 No recovery.

2, 662-2, 687 Fore reef; coral debris and globigerinid Foraminifera. Coral probably represents material swept 

downslope from a nearby reef.

2. 687-3, 052 No recovery.

3. 052-3, 055 Fore reef; globigerinid Foraminifera with transported reef debris laid down in water not less 

than 100 fathoms deep.

3, 055-3, 350 No recovery.
SUBSURFACE GEOLOGY OF ENIWETOK ATOLL 999

Drill hole F-l—Continued

Interval 

(ft) Environment and significant fossil groups

3, 350-3, 353 Fore reef; globigerinid Foraminifera mixed with 

reef debris deposited in water not less than 100 

fathoms deep.

3, 353-3, 655 No recovery.

3, 655-3, 665 Fore reef globigerinids mixed with heavy contributions of shallow-water, reef-derived coral conglomerate.

3, 665-3, 963 No recovery.

3, 963-3, 988 Fore reef; larger Foraminifera and globigerinids 

with coral pebbles swept down from nearby reef.

3. 988-4, 107 No recovery.

4. 107-4, 222 Fore reef; planktonic Foraminifera with high concentrations of shallow-water sediments swept 

down from nearby reef.

4, 222-4, 316 No recovery.

4, 316-4, 341 Fore reef; rich in broken and worn larger Foraminifera and fragments of coralline algae and corals 

of shallow water origin.

4, 341-4, 406 No recovery.

4, 406-4, 431 Fore reef; planktonic Foraminifera and pebbles of 

shallow-water limestone.

4, 431-4, 500 No recovery.

4, 500-4, 525 Fore reef as above; local concentrations of shallowwater sediment from upslopei

4, 525-4, 528 No recovery.

4, 528-4, 553 Fore reef; rich in coarse debris of worn and broken 

larger Foraminifera and coralline algae.

4, 553-4, 630 No recovery.

COMPARISON OF ENIWETOK DRILL HOLES WITH 

BIKINI DRILL HOLES

The above logs show that the post-Eocene sediments 

below Eniwetok are all of shallow-water, lagoonal, 

shoal bank, or reef-wall origin. According to Emery, 

Tracey, and Ladd (1954, p. 90-91) "* * * the entire 

2,556-foot section of limestone at Bikini accumulated 

in a shallow-lagoonal, or near-reef environment." In 

hole F-l, at Eniwetok, shallow-water deposits extend 

to a depth of at least 2,130 feet and possibly to 2,600 

feet (no material was recovered between 2,130 and 

2,662 feet). In hole E-l, shallow-water deposits 

persist without interruption down to the solution 

unconformity at 2,780 feet.

Very high concentrations of rod-shaped fragments of 

Lithophyllum were noted between 2,070 and 2,401 feet 

at Bikini and between 2,028 and 2,410 feet in hole 

E-l at Eniwetok; all the fragments were in sediments 

of Tertiary e age. Between 2,288 and 2,327 feet at 

Bikini and between 2,200 and 2,300 feet in hole E-l, 

fragments of Lithophyllum make up 50 to 75 percent 

of the cuttings. At Bikini, the following species contribute to the sediments that are rich in algal remains 

(Johnson, 1954, p. 537):

Depth (ft) 

L. oblongum Johnson____________________ 2, 100-2, 500

L. kladosum Johnson____________________ 2, 300-2, 600

L. profundum Johnson_________________ 2, 200-2, 500

L. oblongum was not found at Eniwetok, and L. 

kladosum and L. profundum show the following ranges 

(Johnson, 1962):

F-l (ft) E-l (ft)

L. kladosum Johnson____ _ 1, 720-2, 000 2, 100-2, 680

L. profundum Johnson___ 1,978-2,003 2,690-2,700

Thus, at Eniwetok, L. kladosum is the only species 

that contributes to the abundance of fragments between 

2,200 and 2,300 feet in hole E-l. In hole F-l these 

algae occur higher in the section than they do in hole 

E-l. In hole F-l the algae are associated with reefwall and near-reef limestones, whereas in hole E-l 

the section is composed entirely of lagoonal or open 

shoal-bank sediments (fig. 310). It is probable that 

reef sediments at hole F-l were contemporaneous with 

off-reef sediments deposited in water 200 feet deeper 

at hole E-l.

Another striking lithologic similarity between the 

Eniwetok and Bikini sections is the high concentration 

of sticklike fragments of delicate branching coral and 

large pelecypods and gastropods that is found just 

above the 1,100-foot solution unconformity in all 

the holes studied. Figure 310 shows the distribution 

of sediments of this composition in holes E-l, F-l, 

2A and 2B. In all the holes these sediments are 

unaltered and have a characteristic brown color. 

In addition, in hole E-l there is a similar concentration 

of coral and mollusks directly above the solution 

unconformity at 2,780 feet. Evidently the first 

organisms to grow on the recrystallized surface of 

these unconformities after resubmergence were thicketlike colonies of delicate coral, around which lived a 

rich population of gastropods and pelecypods. Coral 

thickets such as these are found today only in shallow 

sheltered back-reef and lagoonal areas, where the 

energy of incoming waves is absorbed by the reef 

wall.

LOWER PARTS OF ENIWETOK DRILL HOLES

The section of hole F-l below core 6 (2,662-2,687 

feet) and that part of hole E-l below the solution 

unconformity at 2,780 feet have no counterpart at 

Bikini, as the deepest Bikini hole did not reach this 

depth. Except for core 6, all these sediments are of 

Tertiary b age (Cole, 1957, p. 749). In hole F-l, 

all these sediments are fore-reef, outer-slope deposits 

characterized by the presence of planktonic Foraminifera mixed with worn and broken debris composed
1000

DEPTH,IN FEET

BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

ENIWETOK

0

500-

1000-

1500-

2000-

2500-

3000-

F-l E-1

BIKINI 

2A-B

3500-

4000- E 9

2 E "* £

4500-

Coral-molluskrich zone Coral-molluskrich zone

jLith&phyllum-nch zone

Coral-mollusk- rich zone 

J Top of Tertiary b

Top of Tertiary g

Coral-molluskrich zone

Top of Tertiary e

}Lith&phyllum-r\cb zone

Total depth 2556 ft

EXPLANATION

Lagoon and undifferentiated No samples recovered

Open shoal

Core

Total depth 4222 ft

Numbers 1-15 along drill hole F-1, and 1-3 along 

drill hole E-1 indicate cored intervals

Total depth 4630 ft

FIQUEE 310.—Paleoecology logs of drill holes E-1 and F-1 on Eniwetok and drill hole 2A-B on Bikini.
SUBSURFACE GEOLOGY OF ENIWETOK ATOLL 1001

of shallow-water organisms such as corals, coralline 

algae, and larger Foraminifera. As shown on figure 

310, the three foraminiferal zones in these two holes 

are as foUows (Todd and Low, 1960, p. 812):

Peneroplis-Triloculina f-i (ft) E-l (ft) 

assemblage..____________ 4, 200-4, 400 2, 770-2, 940

Ornatamolina assemblage_ 4,420 2,940-2,960

Asterigerina rotula

assemblage.... — — — . 4, 500-4, 550 3, 010-3, 120

There is a 1,400-to 1,600-foot difference in elevation 

between the zones in the two holes. Todd and Low 

(1960, p. 813) state that "* * * in hole F-l the three 

above-mentioned assemblages are accompanied by 

minor amounts of planktonic specimens; in hole E-l, 

on the other hand, no planktonic specimens were 

found in the Eocene section and none of the accompanying benthonic species are of known deepwater 

types." The F-l section between 4,200 and 4,550 

feet therefore represents outer-slope deposits contemporaneous with near-reef, shallow-water deposits in 

hole E-l from 2,770 to 3,120 feet.

As shown on figure 310, the section of fore-reef deposits in hole F-l down to 4,200 feet has no counterpart 

in hole E-l. While this thick section, represented by 

cores 7 through 10, was accumulating in F-l, it may be 

assumed that reef or near-reef deposits were building 

up at E-l and that the shallow-water debris moved 

downslope from E-l to F-l. The solution unconformity at 2,780 feet in hole E-l coincides (within 10 

feet) with the uppermost of the three foraminiferal 

zones. The writer believes, therefore, that a section of 

sediment was removed at hole E-l during the development of the solution unconformity. This section 

probably corresponded to the section in hole F-l that 

is still preserved down to 4,200 feet. Thus, a truncated 

wedge of fore-reef sediment that tapers from a thickness 

of at least 1,000 feet at hole F-l to 0 feet at E-l now 

exists beneath Eniwetok.

The deepest sediments in hole E-l have no counterpart in hole F-l, as the deepest part of the F-l section 

correlates with the 3,120-foot level in hole E-l. The 

deepest limestones in hole E-l consist of massive coralrich reef-wall deposits that represent the earliest deposits on the volcanic foundation.

GEOLOGIC HISTORY

DEPOSITION AND TRUNCATION OF EOCENE 

SEDIMENTS

Seismic refraction studies at Eniwetok (Raitt, 1957, 

685-698) show that the upper surface of the volcanic 

foundation of the atoll slopes gently to the northwest,3

3 A detailed discussion of guyots and their relation to the foundations of atolls is 

given in Emery, Tracey, and Ladd (1954, p. 125-131).

and that it is shallower below Eniwetok and Parry 

Island than below Elugelab. This slope probably influenced the deposition of Eocene limestones, and thus 

also the striking differences in facies and thickness of 

correlative foraminiferal zones in the deepest parts of 

holes F-l and E-l (pi. 281).

The following discussion of the geologic history is 

highly interpretative. It is based entirely on records 

from two widely separated holes, and in these records 

there are large gaps where no limestone was recovered.

The oldest sediments recovered were reef-wall limestones found between 4,070 feet and 4,100 feet in hole 

E-l; the limestones in the unsampled interval between 

4,553 feet and 4,630 feet in hole F-l may contain beds 

that are time equivalent. If so, about 900 feet of 

sediments, part of which was reef-wall limestone, 

accumulated at the site of hole E-l while only 80 feet 

of sediments were deposited below the site of F-l. 

Plate 281A shows a schematic representation of conditions prior to the deposition of the Asterigerina rotula, 

zone. A thick reef grew on the higher part of the 

volcanic foundation while thin fore-reef deposits accumulated on the outer slopes. During this period the 

atoll foundation was slowly subsiding.

Deposition of the three foraminiferal zones shown on 

figure 310 followed. As discussed in the preceding 

section on paleoecology, these zones in hole F-l are 

fore-reef, outer-slope deposits characterized by planktonic Foraminifera, whereas the equivalent rocks in hole 

E-l lack planktonic species and were laid down in 

shallower water. The Miocene history of the atoll 

indicates that the shallow-water deposits migrated 

northwestward from the location of hole E-l until they 

extended entirely across the present site of the atoll. 

As the reef was expanded and moved northwestward, it 

left small local reefs to the southeast, in the vicinity of 

hole E-l, and shallow-water foraminiferal sands accumulated around them. At F-l the water was deeper, 

and planktonic species were more significant; however, 

much of the rock (cores 12, 13, 14, 15) is made up of 

reef and shallow-water debris that was transported 

downslope.

The thick fore-reef deposits, represented by cores 7 

through 10 from hole F-l, accumulated as the atoll 

foundation continued to subside slowly. The presence 

of abundant shallow-water skeletal debris in these forereef limestones shows that a reef grew upward nearby, 

in pace with subsidence. Plate 281(7 illustrates conditions prior to the formation of the solution unconformity 

at 2,780 feet in hole E-l, during deposition of the 

youngest Eopene and oldest Miocene limestones. The 

three foraminiferal zones were deeply buried at F-l, 

whereas at E-l an unknown thickness of posi-Peneroplis-zoue sediments was laid down.
1002 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

The scheme developed so far calls for considerable 

asymmetry in deposition. However, the differences 

in elevation between time-equivalent foraminiferal 

zones and the thick section of fore-reef deposits in hole 

F-l are most easily and logically explained in this way.

Sometime early in Tertiary e time the atoll was 

emergent and a solution unconformity developed 

(pi. 28lZ>), as explained in the preceding section. The 

truncation of the asymmetrical deposits resulted in 

the exposure of the Peneroplis zone at the site of hole 

E-l while the equivalent zone in hole F-l remained 

buried beneath approximately 1,000 feet of Eocene 

sediments and a hundred feet of basal Miocene fore-reef 

sediments. Definite Oligocene deposits have not been 

recognized -by Cole (1957) or Todd and Low (1960), 

although these would be expected if the deposition 

were continuous from Eocene to Miocene.

DEPOSITION AND TRUNCATION OF LOWER MIOCENE 

SEDIMENTS

The foundation of the atoll subsided after the early 

Tertiary e emergence, and a thick section of Miocene 

sediment was deposited. At the site of hole E-l, 

lagoonal sediments make up the lowest Miocene deposits; corals and mollusks dominated the first population. Therefore, a protecting reef probably grew to 

the southeast of hole E-l.

Environmental conditions varied somewhat through 

time. The laminated mollusk-rich mudstones from 

2, 410 to 2,540 feet represent quiet-water, lagoonal deposits, whereas the interval of rock from 2,028 to 2,290 

feet is rich in fragments of Lithophyllum and was probably 

laid down in more open but still shallow and protected 

water. The whole picture is one of a slowly subsiding 

platform on which sedimentation kept pace with subsidence. In hole F-l, the oldest postemergent Miocene 

rock is reef-wall limestone and probably represents an 

early reef that grew on the truncated fore-reef Miocene 

limestone represented by core 6. This part of the atoll 

must have grown vertically more rapidly than the southeastern part, because the Lithophyllum zones found at 

2,100-2,700 feet in hole E-l are at 1,720-2,000 feet in 

F-l (fig. 310); thus, while a reef existed at F-l topographically lower Lithophyllum-rich sands were being 

deposited at E-l; plate 281-E' shows the atoll at this 

stage.

Deposition of shallow-water sediments under conditions of slow foundation subsidence probably continued 

through Tertiary e and part of Tertiary / time, and 

the atoll probably had a configuration much like its present one. Subsidence may have been discontinuous, however. The recrystallized limestone in the 1,978- to 

2,130-foot interval in hole F-l (fig. 308) correlates with 

unaltered sediment in hole E-l. This suggests that an

emergence during Tertiary e time affected only the 

northwestern part of the atoll and resulted in solution 

and recrystallization of the reef and near-reef limestone 

(represented, by the 1,978- to 2,130-foot interval in 

hole F-l). The correlative Lithopyllum-rich sediments 

in E-l being topographically lower as deposited, were 

not exposed by the lowered sea level and hence were 

not subjected to subaerial solution.

The 1,100-foot unconformity indicates that a major 

emergence occurred after deposition of the Tertiary e 

sediments. At Eniwetok the unconformity coincides 

with the top of the Tertiary e, but at Bikini some Tertiary 

/ limestones are found below the same unconformity 

(fig. 308). This suggests that the emergence took place 

during Tertiary / time. At Bikini the Tertiary/ sediments were not removed; but at Eniwetok, solution 

removed all the Tertiary / and some of the Tertiary e 

limestone.

The thickness of the recrystallized zone in hole E-l 

indicates that at least 700 feet of Eniwetok Atoll was 

exposed above sea level during early Tertiary / time, 

whereas at Bikini the maximum altitude was probably 

from 200 to 300 feet.

UPPER TERTIARY TO RECENT SEDIMENTS

Resubmergence of both Eniwetok and Bikini took 

place in Tertiary/ time. On both atolls sediments rich in 

delicate branching corals and large mollusks formed the 

initial deposits and this indicates that shallow-water and 

lagoonal conditions prevailed and were maintained on 

the truncated platforms through the remainder of 

Miocene time and throughout the Pliocene and some 

Pleistocene time. Sediments that now occupy the interval between the 1,100-foot and 300-foot unconformities 

were deposited under conditions of slow subsidence.

Sometime in the Pleistocene, both Eruwetok and 

Bikini were again emergent and the 300-foot unconformity formed. The thickness of the recrystallized zones 

at both atolls indicates emergence of 300 to 400 feet.

Resubmergence again took place, and deposition of 

still more shallow-water sediments was renewed on the 

300-foot platform. Between this submergence and the 

present time, the atoll was subjected to at least one 

more period of emergence, which led to the patchy 

recrystallization found in sediments between approximately 75 and 200 feet in the Eniwetok drill holes and 

between 105 and 185 feet in hole 2 at Bikini. Above 

these levels at Eniwetok the sediments are unaltered.

The similarity in geologic history between Eniwetok 

and Bikini, at least from Tertiary e time on, is striking. 

The two atolls evidently were synchronously affected 

by repeated emergences and submergences. Probably 

the entire Marshall Islands area was so affected, but 

drilling on other atolls in this region will be needed to
SUBSURFACE GEOLOGY OF ENIWETOK ATOLL 1003

determine the extent of these solution unconformities. 

The post-Miocene emergences can be ascribed to 

eustatic changes in sea level connected with Pleistocene 

glaciation. Earlier emergence-submergence cycles may 

have been effected by subsidence and uplift of the basement. If this is so, the correlation between the 1,100- 

foot unconformities at Eniwetok and Bikini suggests 

that basement movements may have affected wide 

oceanic areas simultaneously.

ANALYSES

Most of the samples listed in tables 1-3 were chips of 

limestone taken from representative pieces of core. 

Descriptions of these limestones are given on pages 

1011-1038. A few samples were specially chosen, such 

as pure coral, to check on changes in chemistry and 

mineralogy due to recrystallization. These special 

samples are listed below.

Sample 

K-l-24

TABLE 1.—Chemical analyses, in weight percent, of limestone and 

dolomite samples from Eniwetok

[Samples were analyzed by methods similar to those described by Shapiro and Brannock (1956). P. L. T>. Elmore, S. D. Botts, H. H. Thomas, and M. D. Mack, 

analysts]

K-1B-16 

E-l-1-6 

E-l-2-5 

F-l-5-26

F-l-6-30 

F-l-6-9o

Pure coral, completely recrystallized and 

replaced by coarse, granular calcite.

Pure coral, partly replaced by calcite.

Pure coral, entirely unaltered.

Shell fragments from mollusk-rich mud.

Pure coral, completely replaced by calcite.

Pure coral, completely replaced by calcite.

Powder scraped from crystals lining vugs 

in core F-l-6.

Chemical, mineralogical, and spectrographic analyses of 

similar materials collected during the study of Bikini 

are given in Emery, Tracey, and Ladd (1954, p. 67, 

84-87).

CHEMICAL ANALYSES

Examination of the record of MgCO3 content of the 

samples analyzed (table 1) shows that none of the rocks 

contain the theoretical limit of MgCO3 found in pure 

dolomite, 45.7 percent by weight. Samples F-l-12-46, 

F-1-12-5Z&, and F-1-12-7Z all contained more than 98 

percent dolomite by X-ray diffraction analysis; but 

F-l-12-46 and F-1-12-5Z& each contained only 38 percent, and F-1-12-7Z contained only 38.8 MgCO3 . A 

similar lack of MgCO3 was noted in the Funafuti 

material. In connection with chemical analyses of the 

Funafuti borings, Judd (1904, p. 366) states:

* * * the sample in which the highest percentage of that substance [MgCOsl was detected, at 950 feet deep, contained 43 

percent. As a rule, however, the percentage of magnesium 

carbonate in this lower part of the bore-hole is about 40, thus 

falling far short by nearly 6 percent of the quantity required to 

form a complete dolomite.

At Kita-daito-jima, Ota (1939, pi. 1) showed that the 

highest MgCO3 content, 42.0 percent, occurred at a 

depth of 68 meters (223 feet); most of the dolomitized

Sample

K-lB-16____---_

E-l-1-4- _______

E-l-2-l_ _______

E-l-3-10___-___

E-l-3-42. ______

F-l-l-2_____.__

F-l-2-l-_______

F-l-2-9- _______

F-l-3-4_ _ ______

F-l-3-10___-_-_

F-l-3-20. ______

F-l-4-2_ _ ______

F-l-4-10_ ______

F-l-5-3- . ______

F-l-5-26- ------

F- 1-5-41 __ _____

F-l-6-1- _ ----__

F-l-6-9_ ___-_-_

F-l-7-3- _______

F-l-7-7- -------

F-l-8-1--------

F-l-9-2________

F-l-10-5_____._

F-l-ll-l__----_

F-l-ll-9_---_-_

F-l-11-18------

F-l-ll-29_--___

F-l-11-37---- -

F-l-12-(l-2-3)__ 

F-l-12-4*3 __ __.

F-l-12-46______

F-l-12-5<6__--._

F- 1-12-7* _ _ __

F- 1-12-76------

F-l-12-10___--_

F-l-12-12______

F-l-12-23__-__-

F-l-13-2_ -__.__

F-l-14-8-------

F-l-14-ll_----_

F- l-14-28_ __ _-_

F- 1-14-29. ___--

F-l-15-2- ______

F-l-15-13_---__

CaCOa

92.8

Q4 1

96.6

94. 2

Q1 A

94.4

97.8

97.4

96.7

96.4

95.9

96.4

95. 3

95. 5

96. 6

96.4

96.6

96.2

96.6

96.9

96.2

96.0

96. 2

95.6

95. 5

93.7

92.8

95.8

93. 7 

70. 0

61.4

61.4

60. 1

63.0

63.3

81.4

82.4

96.6

96.6

96. 5

96.0

97.2

96. 5

97.0

MgCO 3

2. 1

2.9

2.2

4.5

7.7

1. 1

1.0

1.0

2. 2

2. 5

2.8

2.4

3. 2

3. 1

2. 7

2.3

2.2

2.8

2.0

1.9

2. 1

2.0

2. 1

2.6

2.8

4. 5

5.3

2.6

4. 4 

28.4

38.0

38.0

38.8

36. 5

35.9

18.2

17.2

2. 1

2. 2

2. 2

2.8

2. 1

2.4

1. 9

Na2 O

0.60

.36

. 10

. 14

. 13

.75

. 13

.20

. 16

. 13

. 14

.21

. 11

. 12

. 12

. 14

. 12

. 11

.24

.26

.32

.34

.40

. 19

.36

.25

.31

.28

.30 

. 18

. 10

. 12

. 14

. 14

. 10

.24

.22

.30

.28

.26

.32

. 13

.28

. 19

PS 05

0.00

.00

.00

.01

.00

.02

.01

.01

.00

.00

.00

.00

.00

.00

.00

.00

.01

.01

.01

.00

.01

.04

.06

.08

. 10

.09

. 11

. 18

.06 

. 13

.20

.18

.04

.04

.03

.03

.04

.04

.02

.00

.06

.04

.06

.04

'SOs

0.54

.41

. 10

.08

.08

.31

.04

.06

.08

. 10

. 14

. 14

. 13

. 13

. 12

. 15

. 10

. 14

. 10

. 12

. 16

. 17

. 10

. 10

. 17

. 16

. 15

. 15

. 14 

. 10

. 13

. 11

.08

. 14

. 11

. 11

. 11

. 17

. 18

. 16

. 16

. 14

. 12

. 13

Sum

296

200

on

on

on

207

99

99

99

99

99

on

99

99

100

99

99

no

99

99

99

99

99

99

99

99

99

99

99 

100

100

100

99

100

100

100

100

99

99

99

99

100

99

99

1 Total sulfur as SOs and KaO was reported in all samples as less than 0.05 percent.

2 Samples contained appreciable organic matter.

3 Samples designated by t, b, and tb indicate those taken from the top, bottom, and 

from both the top and bottom of core pieces respectively. This was done to show how 

MgCOs content varies in the space of a few inches.

part of the boring contained between 37 and 40 percent 

MgCO3 (fig. 311). Thus most dolomite from these 

atolls lacks 6 to 8 percent MgCO3 .

The explanation of the MgCO3 deficiency lies in the 

fact that the dolomite has an excess of calcium ions in 

its structure. As noted in the following section on 

mineralogical analyses, the ^-spacing of the dolomite 

was larger than that in dolomite with the ideal MgCO3 

content, and this indicates an excess of calcium ions
1004 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

rO

-200

400

-600

-800

1000

-1200

L 1400

KITA-DAITO-JIMA

MgC0 3 

Wt percent

FUNAFUTI

MgC0 3 

Wt percent

O

•a

&

FIGURE 311.—Diagram of MgCOs analyses of samples from drill holes on Funafuti 

and Kita-daito-jima.

that are larger than magnesium ions. Goldsmith and 

Graf (1958a) investigated a large number of Tertiary 

dolomites, including material submitted by the writer 

from Eniwetok, Funafuti, and Kita-daito-jima and 

found that the Pacific atoll dolomites contain approximately 5 mol percent excess structural CaCO3 . Instead 

of 50 calcium and 50 magnesium ions per 100 cations, 

these dolomites contain approximately 55 calcium and 

45 magnesium ions per 100 cations. The mineral has 

a true dolomite structure of alternate calcium and magnesium cation layers separated by CO3 complex ion 

layers, rather than merely a magnesium-rich calcite 

structure in which the magnesium ions are randomly 

scattered through the cation layers.

The analyses also show that MgCO3 content cannot 

be used as a safe guide to dolomitized zones. Samples 

F-l-3-20, F-l-6-9, and F-l-8-1 have 2.8, 2.8, and 

2.1 percent MgCO3 , respectively, yet all contained dolomite. In checking cores from Kita-daito-jima, scattered euhedra of dolomite were noted in rock that 

analyzed 2.74 percent MgCO3 (Ota, 1939, pi. 1); core 

203A from Funafuti contained 4.83 percent MgCO3 but 

had abundant dolomite. Higher percentages of MgCO3 

may or may not mean that dolomite is present as a

distinct mineral phase. At Eniwetok, samples E-l-3- 

10 and E-l-3-42 contained 4.5 and 7.7 percent MgCO3 

and 5 and 14 percent dolomite, respectively, whereas 

core 15 from Funafuti (depth 15 feet) showed 16.4 

percent MgCO3 (Judd, 1904, p. 364) but no dolomite. 

Schmalz (1956, p. 185), who studied the Funafuti cores 

by X-ray diffraction, states, "The magnesium-rich horizon near the top of the core is due not to the presence 

of dolomite, but to magnesium in metastable solid 

solution in calcite." The core in question is actually 

a solid piece of coralline algae, so a high magnesium 

content is normal.

Samples K-lB-16, E-l-1-4, and F-l-1-2 show 2 

percent or more organic matter. These three are 

aragonite-rich limestones (table 2) and are from slightly 

recrystallized or nonrecrystallized intervals (fig. 308). 

K-lB-16 and F-l-1-2 are of Pliocene to Pleistocene 

age, but E-l-1-4 is of Tertiary e age. The presence 

of 3 percent organic matter in this older sample substantiates the idea that the aragonitic rocks above the 

solution unconformity at 2,780 feet in hole E-l were 

never subjected to subaerial leaching. The highest 

sulfur contents of all the samples analyzed were found 

in these three samples.

The Na2O content of all samples may be ascribed to 

salts from the sea water in which these cores soaked 

until their removal.

MINERALO QICAL ANALYSES

Aragonite, calcite, and dolomite percentages, given in 

table 2, were determined by X-ray diffraction techniques by Paul D. Blackman of the U.S. Geological 

Survey. X-ray patterns of standard mixtures of calcite 

and aragonite and of calcite and dolomite were used to 

determine the compositions of the samples. No minerals other than aragonite, calcite, and dolomite were 

present in sufficient quantity to be detected by the 

X-ray analyses. All percentages of calcite, aragonite, 

and dolomite are given with the assumption that other 

minerals constitute less than 1 percent of the total 

composition.

The X-ray patterns indicated a slight deficiency 

of magnesium in all the dolomite. This was shown 

by an increase in the ^-spacing of dolomite from 2.882A 

to approximately 2.9005A.

The calcite in all samples except K-l-24, E-l-3-10, 

E-l-3-42, F-l-1-2, F-l-2-1, F-l-2-9, F-l-5-41, 

F-l-6-1, and F-l-7-(6-7) contains small amounts—to 

approximately 4 mol percent—of MgCO3 substituting 

for CaCO3 . This was indicated by a decrease in the 

d-spacing of calcite from its normal 3.035A to as low 

as 3.025A.

The MgCO3 substitution can be accounted for in 

samples that were originally calcitic inasmuch as
SUBSURFACE GEOLOGY OF ENIWETOK ATOLL 1005

TABLE 2.—Calcite, aragonite, and dolomite content, in weight percent, of representative samples

[Paul D. Blackman, analyst]

Sample

K-l-24____ __ ___ ______

K-lB-16____-_.______-_-_

E-l-l-4____-_-___________

E-l-l-6__________________

E-l-2-l__________________

E-l-2-5__________-___-__-

E-l-3-10— --------------

E-l-3-42___ ______________

F-l-l-2______ _-. ___ __

F-l-2-l.._.__ ___ ________

F-l-2-9__ ___ ----- _ ___

F-l-3-4_____________-____

F-l-a-10____ — __ __ — — _

F-l-3-20— --------------

F-l-4-2- ______ __ _____

F-1-4HLO____ — — — — — __

F-l-5-3— _____-_--._--__

F-l-5-9___ — -__- — — — -_

F-l-5-26-.. ___________ __-

F-l-5-41______. __________

F-l-6-l______-_____-___--

F-l-6-9— ___.___-__-____

F-l-6-9a____-_-__-_-_____

F-l-6-23 _ ______________

F-l-6-30 _ ______ _ _ ___

F-l-7-3__________________

F-1-7-C6 7} l

F-1-8-1-- -----------_-_

F-l-9-2__ _______________

F-l-9-6___-__-_-_-_-_____

F-l-10-5___-_--____-_____

F-l-11-1— ______________

F-l-ll-9_-_---__________-

F-l-ll-18___-____-______-

F-l-11-29 _ -___--__.____

F-l-ll-37_____--___ _____

F-l-12-(l, 2, 3) !_ _________

F-l-12-4<______ __________

F-l-12-46________________

F-l-12-5tf>_______________

F-l-12-7* — — — — - — -__-_

F-l-12-76____-___ _ _ __

F-l-12-10____-_-_-_______

F-l-12-12.. _____________

F-l-12-23____ ____________

F-l-13-2_________________

F-l-14-8-____-__---____-

F- 1-14-11 _ _____________

F-l-14-28-____-_-___--__-

F-l-14-29.__ ____________

F-l-15-2_________________

F-l-15-13__________-____-

Calcite

100

14

55

^9

100

80

95

86

40

100

100

98 +

100

100

100

100

100

100

100

100

100

100

98 +

100

100

100

100-

98 +

100

100

100

98+

98

92

92

98+

89

35

^9

^-">

11

7

55

58

100

98+

98 +

98 +

100

100

100

Aragonite

0

86

45

98+

0

20

0

0

60

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

Dolomite

0

0

0

0

0

0

5

14

0

0

0

Trace.

0

2 0

0

0

0

0

0

0

0

0

» <2

0

0

0

0

Trace.

0

0

0

•^9

2

8

8

<^9

11

65

98+

98+

98+

89

93

45

42

2 0

Trace.

Trace.

Trace.

0

2 0

2 0

1 Composite sample.

2 Dolomite was identified in thin sections.

3 From vug lining in F-1-6-9.

many of the organisms that contribute carbonate 

to these limestones precipitate calcite skeletons that 

contain appreciable amounts of MgCO3 . Chave 

(1954a, p. 269-273) lists analyses of Foraminifera

and coralline algae that show as much as 15.2 percent 

and 29 percent MgCO3 by weight, respectively.

Some samples that lack MgCO3 substitution were 

probably composed of aragonite originally. Organisms 

such as coral and pelecypods and gastropods secrete 

aragonite skeletons that are low in magnesium. Ten 

Madreporian corals analyzed by Chave (1954a, p. 269) 

showed 0.59 percent or less MgCO3 ; aragonitic gastropods 

and pelecypods showed 0.27 percent or less MgCO3 . 

Sample K-l-24, a piece of completely recrystallized 

coral, now solid calcite, showed no MgCO3 substitution, 

which is a reflection of its having been originally 

aragonite. However, samples F-l-5-26 and F-l-6-30 

were reported to show MgCO3 substitution. These 

two samples were pure coral that had been completely 

replaced by calcite and that had calcite added in the 

interseptal spaces by deposition from solution. MgCO3 

substitution in these corals contrasts to the lack of 

substitution in K-l-24. Samples F-l-5-26 and F-l-6- 

30 are of Tertiary e age, whereas K-l-24 is of Pliocene 

to Pleistocene age. Perhaps the substitution in the 

older limestones is the result of long-term immersion 

in sea water during which magnesium from the sea 

water replaced calcium in the secondary calcite.

Some samples did not show MgCO3 substitution 

although according to their fossil contents they should 

have. In particular, sample F-l-5-41 is rich in laminar 

and fragmental coralline algae that normally contain 

MgCO3 , yet the X-ray analysis showed none. This 

lack may be interpreted as showing that the Mg ions 

in the original algal calcite have been removed, possibly 

by leaching when the interval from which this sample 

came was emergent. Samples E-l-3-10 and E-l-3-42 

are both rich in coral and algae and have been dolomitized. In these rocks the coralline algae in particular has been dolomitized, and in many places 

the relict algal calcite has been removed by postdolomite 

solution. The lack of MgCO3 substitution may be 

due to leaching of the Mg ions from the relict algal 

calcite. Another possibility is that the Mg ions 

originally scattered through the algal calcite are now 

concentrated in the dolomite, which leaves the remainder of the calcite free of magnesium.

Dolomite was identified in several samples, although 

it was not detected by the X-ray method. In some 

samples the dolomite was identified by staining thin 

sections with 2N copper nitrate, which produces a deep 

opaque blue in calcite but leaves the dolomite clear and 

transparent. In sample F-l-3-20, a few rhombs of 

dolomite were found in a vug lining (pi. 282A) by this 

method; X-ray analysis of a larger bulk sample cut at 

random from the core failed to reveal dolomite. In 

sample F-l-6-9, analysis of a bulk sample failed to 

show dolomite, but analysis of a powder scraped (F-l-6-
1006 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

9a) from a vug lining showed that the lining contained 

a small amount—less than 2 percent—of dolomite.—In 

thin sections from cores F-l-13-2, F-l-15-2, and F-l15-13, rare, small, scattered euhedral rhombs of dolomite make up perhaps 0.1 percent of the rock. The 

above information is given in detail to emphasize the 

small amounts of dolomite in some of the samples.

SPECTROGRAPHIC ANALYSES

Twenty-two samples were quantitatively analyzed 

to trace the behavior of minor elements such as strontium during recrystallization and dolomitization and to 

add data to that obtained from the Bikini drillings 

(Emery, Tracey, and Ladd, 1954, p. 86-87). Tracey 

stated,

1. Corals and detrital sand that show little or no alteration have 

relatively high percentages of strontium (0.6 to 0.9 percent in 

Myer's analyses).

2. Corals and detrital sand that are broken down to microgranular aragonite have high percentages of strontium, probably 

somewhat less than in unaltered material (0.6 percent).

3. Corals that are broken down and recrystallized to microgranular calcite have intermediate amounts of strontium (0.4 

percent).

4. Corals and detritus that are completely recrystallized to 

crystalline calcite have low percentages (0.3 to 0.1 percent).

Variation in the strontium content of Eniwetok samples (table 3) seems to be definitely related to the degree 

and kind of recrystallization, although the relationship 

is not precise. The least altered samples contain the

most strontium, as much as 0.4 percent; and the older, 

highly recrystallized limestones show low strontium 

content, less than 0.1 percent. In particular, the intensely dolomitized samples show the lowest content; 

the strontium percentage drops to 0.003 in sample F-l12-5^6, which is more than 98 percent dolomite.

Strontium content in the less altered limestones from 

Eniwetok does not correspond quantitatively to the 

Bikini results. Samples K-l-24 and K-l-16 each contain 0.2 percent strontium. Both were originally coral 

and wholly aragonitic; but K-l-24 is now completely 

recrystallized to calcite, and K-lB-16— which is 14 percent calcite—is only slightly recrystallized. Thus 

K-l-24 has more strontium and K-lB-16 less strontium than would be expected according to the Bikini 

results. In addition, the highest strontium content in 

the Eniwetok material, 0.4 percent, in samples E-l-1-6 

and E-l-1-4 corresponds to the strontium content in 

corals recrystallized to microgranular calcite at Bikini. 

Yet sample E-l-1-6 is a piece of pure, virtually unaltered coral that is now 98 percent aragonite, and E-l-1-4 

is a bulk sample of limestone that is now 45 percent 

aragonite.

Manganese shows a tendency to increase with depth 

in hole F-l but here the percentages are too low to be 

significant. Copper and lead show erratic distribution, 

and the other elements listed in table 3 show little or no 

significant variation with age, degree of recrystallization, or original lithology.

TABLE 3.—Quantitative spectrographic analyses, in weight percent, of limestones from Eniwetok

[Sodium carbonate and silica were used as matrix material. Sol Berman, analyst. M, element present in major amount (more than 5 percent). O, element not detected. 

Looked for but not found: Ag, Au, Hg, Ku, Rh, Pd, Os, Ir, Pt, Mo, W, Re, Ge, Sn, As Sb, Bi, Te, Zn, Cd, Tl, In, Co, Ni, Ga, Sc, Y, Yb, La, Zr, Hf, Th, Nb, Ta, U 

Be, and P]

Sample

K-1B-16. _______

K-l-24_________

E-l-l-4_________

E-l-l-6____.-_._

E-l-2-l_________

F-l-1-2. _ _____

F-l-4-2_________

F-l-5-26________

F-1-6-30-. _ _

F-l-7-(6-7)-_-_-

F-l-8-l_________

F-l-12-(123)___- 

F-l-12-4t__.--__

F-l-12-4b_ ______

F-l-12-5tb______

F-l-12-7t_____._

F-l-12-7b__.____

F-l-12-10__.____

F-l-12-12.______

F-l-12-23 _ „__

F-l-13-2___.__._

F-l-14-29_ ______

Cu

0. 0007

.003

0

0

.0002

.0002

0

0

.0002

0

0

0 

.0002

.0003

0

0

0

.0006

0

0

0

0

Pb

0.02

0

0

0

0

0

.06

0

0

.02

0

0 

0

0

0

.03

0

0

0

0

0

0

Mn

0. 0009

.0006

.0006

.0005

.0006

.0009

.001

.0007

.002

.002

.003

.003 

.004

.005

.004

.004

.004

.006

.003

.003

.004

.01

Fe

0.02

.004

.002

.01

.005

09

.03

.009

.003

.02

.02

.005 

.009

.02

.003

.008

.007

AQ

.005

.003

.02

.009

Cr

0.001

0

.002

0

.003

.001

.0008

.0006

.0004

. 0004

.0005

.0005 

.001

.0006

.0006

.001

.0006

.001

nftftfi

.0006

.0006

.0006

V

0.01

.007

.009

.005

.008

.007

.008

.007

.006

.01

.009

.008 

.006

.005

.008

.004

.008

.006

.009

.009

.01

.01

Al

0.008

.004

.002

no

.006

.04

.008

.002

.003

.007

.009

.004 

.005

.009

.002

.003

.003

.004

.001

.001

.002

.002

Ti

0.009

.01

.007

.007

.008

.008

.004

.007

.008

.006

.01

.003 

.003

.002

.008

.002

.01

.003

.004

.003

.004

.004

Mg

0. 5

. 2

.9

.08

. 5

.3

. 7

. 6

. 9

. 2

. 2

.5

M

M

M

M

M

M

M

M

.6

. 6

Sr

0.2

. 2

. 4

.4

. 1

. 2

.08

. 1

. 1

.05

.04

.02 

.02

.02

.003

.04

.006

.02

.02

.01

.04

.06

Ba

0. 2

.001

0

0

0

.001

0

0

0

0

0

0 

0

0

0

0

0

0

0

0

0

0

B

0.005

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SUBSURFACE GEOLOGY OF ENTWETOK ATOLL 1007

DOLOMITIZATION

The dolomite problem has long engaged the interest 

of geologists. Within this general problem, the discussion that follows deals only with the formation of 

dolomite on atolls and limestone islands in the tropical 

seas, particularly those in the western and southern 

Pacific. Atolls in this area have attracted students of 

dolomitization because of their uncomplicated geologic 

history and setting. The limestone columns below the 

atolls have not been affected by hydrothermal solutions 

or magnesium-rich ground waters nor have they been 

contaminated by extrabasinal sediments which could 

have disturbed the sea water-limestone system and thus 

contributed to the formation of dolomitic limestones. 

Further, the succession of faunal and floral communities 

that produced these limestones indicates that ecologic 

conditions, and hence oceanographic conditions, in the 

region of these atolls remained generally constant 

through Tertiary time to the Recent. The mineralogy 

and chemical composition of the skeletal material that 

formed the limestones are well known from studies of 

comparable living forms.

Thus the worker deals with limestones that have wellknown primary constituents, that were deposited in 

marine environments similar to those of today which 

have been well studied, and that have been subjected 

only to immersion in normal sea water and to periodic 

emergences above sea level. Yet even within this 

simple setting the mode of occurrence of dolomite varies 

from atoll to atoll. Following a study of the now classic Funafuti borings, Cullis (1904, p. 388) was led to 

remark:

While certain portions of a reef may contain a high percentage 

of magnesium carbonate, other parts of the same reef may be almost free from that salt, and the reason why some parts of the 

mass are almost completely dolomitized while other parts remain 

chemically unchanged, is by no means obvious.

The writer is in full agreement with this statement.

Through a brief survey of the vertical distribution 

and texture of dolomitized limestones from Eniwetok, 

Funafuti, and Kita-daito-jima, it is attempted here 

to outline various lines of evidence that must be 

considered in any theory of dolomitization applied to 

these atolls.

STBATIGBAPHIC DISTRIBUTION

Drilling at Funafuti (Judd, 1904), Kita-daito-jima 

(Ota, 1939), Bikini (Emery, Tracey, and Ladd, 1954), 

and Eniwetok, and surface studies on a number of limestone islands (Skeats, 1903; Schlanger, 1963) show that 

the stratigraphic distribution of dolomite follows no regular pattern through the Pacific Ocean area. Figure 

311 shows the MgCO3 content vertically through the 

Funafuti and Kita-daito-jima borings. At Funafuti

only the section of the hole below a depth of 637 feet is 

dolomitized; at Kita-daito-jima the reverse is true, 

although both dolomitized sections are Pliocene to 

Pleistocene in age. Further, at Kita-daito-jima the 

concentration of MgCO3 at about 425 and 600 feet is as 

high as in the completely dolomitized upper section. 

At Bikini, beach rock near the surface, rich in calcareous 

algae, contains about 8 percent of MgCO3 ; but at lower 

levels, through 2,556 feet of drilling that reached rocks 

of early Miocene age, no strata with a MgCO3 content 

higher than 3 to 4 percent were found, and no dolomite 

was identified.

At Eniwetok, dolomite is restricted to rocks of Eocene 

age, except for traces found in cores F-l-6 and F-l-3. 

Also, at Eniwetok there are nondolomitic sections such as those represented by cores F-l-7 and 

F-l-9 between dolomitized sections represented by 

cores F-l-6 and F-l-8. Studies of core F-l-12 show 

that within an interval of several feet the dolomite content ranges from 11 to 98 percent. In core F-l-12-4 

the dolomite content ranges from 65 percent to more 

than 98 percent within a distance of 4 inches.

From one point of view, the selection of Funafuti as 

a drilling site was unfortunate in that the neat concentration of dolomite at depth led to theories based on 

pressure as a critical factor in dolomitization. Reuling 

(1934, p. 35) proposed that dolomitization takes place 

at a depth of approximately 638 feet below the lagoon 

floor; this idea has persisted and still influences modern 

thought. Schmalz (1956, p. 186), on physicochemical 

grounds, suggested that

The chemical system, limestone plus sea water, lies in the 

stability field of calcite plus magnesium-ion above 640 feet; 

below this level, probably due to increased hydrostatic pressure, 

dolomite is the stable phase.

Fairbridge (1957, p. 148) expands on Reuling's ideas 

and holds that dolomitization

* * * could be brought about soon after burial at relatively 

shallow depth in the sediments at a pressure corresponding to 

about 100 fathoms (20 atmospheres).

Fairbridge (1957, p. 148, fig. 8) dismissed evidence 

from Bikini and Eniwetok by stating

Nondolomitized atolls, such as Bikini and Eniwetok may, for 

some reason or other, have failed to provide the necessary 

physico-chemical requirements, such as the reducing conditions 

(owing to freer circulation) or may have subsided too rapidly.

Inasmuch as the "necessary physicochemical requirements" for dolomitization are as yet not demonstrable, 

the rejection of evidence from Eniwetok and Bikini is 

not justified. The geologic history of Eniwetok and 

Bikini show that, far from subsiding rapidly, these 

atolls were subjected to several emergences. There 

would have been ample opportunity for dolomite to
1008 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

develop at Bikini and be more widespread at Eniwetok 

as various parts of the column passed, or paused at, 

the depth of 638 feet. Further, if pressure is a determining factor, then the undolomitized portion below 

Kita-daito-jima and the deep parts of the Bikini and 

Eniwetok subsurface are difficult to explain.

Skeats (1903, p. 124) was cognizant of the pressure 

hypothesis but said that field evidence was against 

the idea:

Recently it has been suggested that the introduction of magnesium into a coral limestone is only effected when the rock has 

been submerged for some time to a considerable depth corresponding to a particular pressure. This view is, however, not 

in harmony with the facts at Christmas Island, Mango, Vatu 

Vara, Ngillangillah, etc., where the highest rocks of the island 

are dolomitized, and the only movement of which there is 

evidence since their formation in shallow water is one of elevation.

Although pressure may be a factor in some occurrences of dolomitization, it is certainly not a limiting 

one. Petrographic evidence from Funafuti, as outlined 

below, indicates that even in this atoll dolomitization 

may be a shallow-water phenomenon.

The apparent random distribution of dolomite with 

respect to stratigraphic horizons indicates that there 

was no single period during which dolomitization was 

widespread. Also, throughout the atolls the faunal and 

floral successions show no breaks that could be ascribed 

to radical changes in oceanographic conditions. Therefore, one cannot invoke oceanographic controls over 

dolomitization, such as changes in temperature and 

salinity. Each atoll displays a unique pattern of distribution of dolomite, and the cause of the alteration 

must be sought within the atoll itself.

COMPARATIVE PETROGRAPHY OF DOLOMITE FROM 

PACIFIC ATOLLS

In addition to the confusing distribution pattern of 

dolomite, examination of thin sections shows that a 

wide variety of textures results from dolomitization.

In hole F-l at Eniwetok the dolomite largely takes 

the form of discrete rhombs scattered throughout the 

limestone (pi. 282). Plate 2825 shows a typical thin 

section from heavily dolomitized pieces of core F-l-12. 

In the final stage of dolomitization, the rock becomes 

a sugary mass of subhedral dolomite crystals. Only 

rare traces of original texture remain. In some places 

lens-shaped voids indicate that tests of larger Foraminifera resisted dolomitization and were later completely dissolved (pi. 282(7). All stages of dolomitization are seen. In some cores, such as F-l-14 and 

F-l-15, dolomite crystals are rare, and the rock shows 

no sign of alteration. Only one core piece, F-l-6-9a, 

showed dolomite as a vug lining. In this core subhedral rhombs of dolomite formed part of one lamina.

In hole E-l, much of the dolomite shows selective 

replacement of certain fossils. Plate 283 shows dolomitized coralline algae. Coralline algae apparently are 

most prone to replacement, and in many thin sections 

these are the only fossils dolomitized. In other thin 

sections mollusk shells are selectively replaced (pi. 

283E). As shown on plate 283Z> the interseptal mud 

in coral may be dolomitized, although the original 

corallum may have been largely dissolved.

The Funafuti dolomite shows quite different textures. 

Banded cavity fillings are common, and these show 

alternate layers of calcite and dolomite (pi. 284.4). 

Cullis (1904, p. 410), in describing core 205A (pi. 284tf) 

stated,

The dolomite layer [b in plate 284C] may represent part of the 

original deposit converted into dolomite, or, as seems more probable, it may have been directly deposited as such. The encrusting material, the introduction of which was apparently subsequent to the dolomitization of the rock, is separated from the 

boundaries of the original organisms by a layer of well-formed 

water-clear dolomite crystals.

Cullis noted that the number of alternate bands reached 

15 in some cores. Skeats (1903, p. 70) noted agatelike 

deposits in dolomitized limestones from Fiji with as 

many as six laminae of alternate calcite and dolomite. 

Such finely banded deposits are lacking at Eniwetok.

It is difficult to see how fine laminae of alternate 

calcite and dolomite could be formed at depth. A limestone at depth would be replaced, and fine calcite laminae would not persist. Banded deposits are more 

easily explained if one assumes cyclically changing 

conditions, and such cyclic changes would be more likely 

to take place in shallow water than in deep water.

Another feature of dolomitized limestone from both 

Funafuti and Kita-daito-jima is the resistance of coralline algae to textural effacement by encroaching dolomite (pi. 2845). In many sections from these two 

atolls, the coralline algae are the only recognizable fossils left in the rock. This is in sharp contrast to the 

condition of the algae in core E-l-3 from Eniwetok.

Further, most of the completely dolomitized rock at 

Funafuti and Kita-daito-jima still shows much of its 

original texture. Plate 285Z> shows a completely dolomitized foraminiferal limestone from Funafuti. The 

rock contains about 40 percent MgCO3 , but there is 

little textural evidence to indicate that it is dolomitic. 

No rhombs are present, and the dolomite is so fine 

grained that all details of original texture are preserved. 

Plate 284.D shows a low-magnesium rock, but it contains abundant dolomite (a).

In dolomitized coral from Eniwetok (pi. 283Z>), all 

the dolomite is in the form of discrete rhombs in the 

interseptal mud fillings. At Funafuti (pi. 285(7), no
SUBSURFACE GEOLOGY OF ENIWETOK ATOLL 1009

euhedral crystals are seen; the dolomite there lines coral 

structures.

Plate 285 shows dolomitized limestones from Funafuti 

and Kita-daito-jima. The Funafuti material is the 

"soft dolomite" of Cullis. Evidently the rock has 

undergone considerable solution since formation of the 

dolomite. The Kita-daito-jima material, on the other 

hand, shows perfectly preserved Halimeda; yet the 

coralline algae (see a in pi. 2855) have been removed by 

solution and are not replaced by coarsely crystalline 

dolomite as are algae in core E-l-3 at Eniwetok.

These textural differences show that the paragenesis 

of the dolomite varies from atoll to atoll and that each 

occurrence must be considered separately. Broad general theories on the formation of dolomite in these atolls 

must take textural differences into account.

FORMATION OF DOLOMITE IN CORALLINE ALGAE

The writer can add nothing to speculation on the 

formation of dolomite by direct precipitation, as seen 

in the Funafuti banded deposits (pi. 284(7), or by the 

all pervasive replacement by microcrystalline dolomite 

(pi. 285.4). However, the dolomite associated with 

coralline algae in core E-l-3 may have formed from 

the magnesium-rich calcite of the algae (Schlanger, 

1957).

The high magnesium content of calcitic algae has 

long been known. Recent work by Chave (1954a) 

shows that these algae, among which he lists Comttina, 

contain from 7 percent to almost 30 percent MgCO3 by 

weight. He also shows that the MgCO3 content is 

directly proportional to the temperature of the water 

in which the algae grew. Goldsmith, Graf, and 

Joensuu (1955) found magnesium in some algal material in concentrations greater than those indicated 

by X-ray study. From this they (p. 220) conclude 

that

«* * * fine}y divided or adsorbed Mg exists in some 

other form than in the calcite structure in at least 

some of the algal materials."

The differentation of high-magnesian calcite from 

dolomite, in a crystal-structure sense, is important in 

any discussion of dolomitization of algae. Chave (1952) 

believed that X-ray and chemical studies demonstrated 

a partial solid solution between the minerals calcite, 

CaCO3 , and dolomite, CaMg(CO3) 2 . However, Goldsmith, Graf, and Joensuu (1955, p. 212) stated that 

the use of the expression "solid solution between 

calcite and dolomite" by Chave (1952) is permissible 

compositionally but fails structurally to take into 

account the ordered nature of dolomite with respect 

to the calcium and magnesium ion positions. The dolomite structure involves the ordered substitution of magnesium ions into approximately 50 percent of the calcium

ion positions in the lattice. The magnesium and 

calcium ions occupy separate and alternate cation 

planes. Thus, the random addition of more magnesium ions from sea water to a magnesian calcite would 

not result in a true dolomite phase inasmuch as an 

ordering process must also take place.

Based on the above information and the well crystallized dolomite in the Eniwetok material, the writer 

suggests the following line of reasoning in an attempt to 

explain the dolomite crystals in the CoralHna:

1. There is no reason to assume that the magnesium 

ions in the algal solid solution are distributed in 

equal concentration throughout. There may be 

regions within the algal fragment in which the 

magnesium-ion concentration is in excess of the 

"average" for the fragment that is shown by a 

chemical analysis. Chave (1954a) showed that 

a considerable variation exists within a single 

algal colony, possibly because of seasonal temperature variation. Other factors may influence 

the metabolism of the algae and induce shortterm high uptake of magnesium. Some statistical 

fluctuation of cation distribution within a solid 

solution would also be expectable, and thus there 

would be a few centers within high magnesium 

regions where the calcium and magnesium ions 

are in the ordered condition which defines true 

dolomite structure.

2. Thus, the alga, as it grows, may contain scattered 

dolomite nuclei, perhaps only a few unit cells in 

size. These nuclei, at this stage, would be too 

small to be detected by X-rays.

3. Through time, the dolomite nuclei, evidently more 

stable than the surrounding calcium-magnesium 

solid solution under existing conditions, enlarge 

by diffusion of the magnesium ions in the structure, by the ordering of magnesium ions originally 

only adsorbed, and by the addition of magnesium 

ions from the sea water.

Chave (1954b) considered that the magnesium in 

solid solution within calcite is unstable and that 

eventually a new form should develop. He believed 

that as one of three possible processes by which equilibrium may be reached, the magnesian calcite could 

reorganize crystallographically into two stable phases: 

low-magnesian calcite and dolomite. He considered 

this type of dolomitization of little importance. In 

giving examples of fossil evidence of crystallographic 

reorganization, Chave (1954b) failed to make clear 

whether the dolomite phase is inferred from the MgCO3 

content or whether it is present in the true dolomite 

structural sense.

D. L. Graf and J. R. Goldsmith (p. 1098-1053) used 

single-crystal and powder-diffraction techniques to
1010 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

determine the structure and composition of the dolomite 

from Eniwetok submitted by the writer. They found 

that the dolomite deviated some 5 mol percent from 

the theoretically ideal 1:1 calcium-magnesium ratio 

in the direction of excess CaCO3 . The partial filling of 

some magnesium ion positions by calcium ions is 

indicated. Goldsmith also reported that the dolomite 

crystals contain very small calcite particles in random 

orientation.

The writer believes that the addition of large amounts 

of magnesium ions from the surrounding sea water need 

not be a requirement for dolomitization of coralline 

algae because (1) perhaps as much as 20 to 25 percent 

of the cation positions (not considering high values 

found by Chave, 1954a) in growing algae may be initially filled with magnesium ions; (2) ordered dolomite 

crystals can form with only 45 percent of the cation positions filled by magnesium ions; and (3) calcite particles may account for several percent of the volume of 

the crystal.

Graf and Goldsmith (1956) pointed out the difficulty 

of crystallizing dolomite at low temperatures and attribute this difficulty to the decrease of the probability, 

at low temperatures or during rapid crystallization, 

that ideal cation sorting into ordered positions can take 

place. They (p. 185) also pointed out that "Ultimately, an understanding of dolomite formation at 

earth-surface temperatures must include the mechanisms by which ordered dolomite nuclei are 

formed * * *."

The writer suggests that (1) the effect of time—the 

algae in the Eniwetok core are from 40 to 60 million 

years old—may be greater than generally assumed in 

low temperature environments, and that (2) the nuclei 

of dolomite may be inherent in the original algal precipitate as the result of environmental conditions, such 

as occasional water temperatures of more than 30° C, 

under which crystallographically local high magnesium-ion concentrations may build up. The dolomitized coralline algae figured and described in this report demonstrate the extent to which dolomite growth 

in high-magnesian calcite can proceed.

Petrographic evidence indicates that the final stage of 

dolomitization includes the complete obliteration of 

all organic remains and original textures. The amount 

of magnesium needed for this complete alteration precludes a solely algal source for all the dolomite. In 

massive dolomite, the sea water must act as the supplier of magnesium. Thus the high-magnesian algal 

calcite may merely serve as a trigger to initiate dolomite growth.

This hypothesis of dolomitization explains only a 

minor percentage of the dolomite in these atolls. Conditions under which the nonmagnesian calcite around 

the algae can be replaced by dolomite remain unknown.

FUTURE WORK

Any general theory on atoll dolomitization must take 

into account the seeming lack of influence of original 

texture, porosity, and fossil composition on the formation of dolomite. Examination of many thin sections 

of dolomitized limestone from atolls shows that original 

porosity does not correlate with intensity of dolomitization. Either dense muds or porous sands may be 

dolomitized. The original fossil composition likewise 

appears to have little influence on the dolomitization 

process. The tendency of algae to be early hosts to 

dolomite at Eniwetok contrasts with the persistence of 

the same fossils at Funafuti and Kita-daito-jima. The 

behavior of tests of large Foraminifera shows similar 

variation; at Eniwetok they resist dolomitization, 

whereas at Funafuti they are dolomitized in some cores 

and relict in others.

A second point is that nowhere in these atolls has 

dolomite been found in association with aragonite. If 

dolomitization takes -place at depth, then an aragoniterich sediment, such as most atoll sediments, might 

undergo dolomitizatibn. Therefore, some dolomite 

could be expected in the thick intervals of aragonitic 

sediments below Bikini and Eniwetok. As pointed out 

elsewhere, these aragonitic sediments do not appear to 

have been subjected to subaerial solution. The relationship between persistence of aragonite, lack of dolomite, 

and lack of subaerial solution indicates that the conversion of primary aragonite to secondary calcite is a possible prerequisite to dolomitization. At Eniwetok, 

core E-l-2, which correlates precisely with core F-l-11, 

is made up of aragonite and calcite, whereas core 

F-l-11 is dolomitic. The aragonitic core has not been 

subject to prolonged subaerial solution, but the dolomitic core has. Because both these cores are the same 

age, time alone cannot be a limiting factor in dolomitization.

The lack of coexisting dolomite and aragonite does 

suggest that rate of subsidence may be a factor in dolomitization. Skeats (1903, p. 125) considered this and 

stated:

Whatever the conditions for the introduction of magnesium carbonate into coral limestone may be, it seems probable, from 

the distribution of magesium carbonate in upraised coral islands, 

that the introduction takes place at or near the surface of the 

water, and that a limestone exposed to suitable conditions for a 

sufficiently long time will become dolomitic. * * * An ordinary 

non-magnesian limestone might result from a somewhat rapid 

subsidence or elevation, while a constant and slow subsidence or 

upheaval might result in the formation of a completely dolomitized island.
SUBSURFACE GEOLOGY OF ENIWETOK ATOLL 1011

In a subsiding atoll the aragonite-rich sediments 

pass downward into a region where physicochemical 

conditions are fairly constant in comparison to the 

upper water layers. Evidently the aragonite is stable 

enough to persist indefinitely without recrystallizing 

to calcite, much less to dolomite. However, if these 

sediments remain near the surface long, perhaps in 

the intertidal zone, or are moved in and out of sea 

water, they will lose all aragonite and become calcite. 

If conversion of aragonite to calcite is a prerequisite 

to dolomitization, then most dolomitization may well 

be a shallow-water phenomenon.

Any attempt to explain the formation of dolomite 

in these atolls must take the following into account:

1. The vertical distribution of dolomite in all drilled 

atolls. Undue emphasis on the pattern of dolomitization below Funafuti has led to the acceptance, 

by many geologists, of Reuling's pressure hypothesis which is obviously inapplicable to Kita-daito-jima and Emwetok.

2. The presence of dolomite as a distinct mineral 

phase in limestones that contain as little as 2 

to 3 percent MgCO3 . Leaching of originally 

magnesium-rich rocks could not account for these 

traces of dolomite. Further, the lack of dolomite 

in magnesium-rich limestones, such as those in 

the upper 50 feet of the Funafuti section, indicate 

that a high initial magnesium content does not 

necessarily foster dolomitization. The magnesium in these oft-discussed limestones below Funafuti is due to an abundance of algal calcite.

3. The differing susceptibility of any single fossil 

group to dolomitization. In hole E-l coralline 

algae are preferentially dolomitized, although at 

Funafuti and Kita-daito-jima these same fossils 

resist dolomitization.

4. The apparent lack of effect of primary porosity on 

dolomitization. Evidently, both dense finegrained and porous coarse-grained limestones are 

equally liable to dolomitization.

5. The lack of coexistent dolomite and aragonite, 

which suggests that atoll limestones may have to 

be leached and recrystallized to secondary calcite 

before they can be dolomitized. This view is in 

opposition to that held by some workers who 

believe that aragonitic limestones are more easily 

dolomitized than calcitic ones.

6. The lack of correlation between length of immersion 

in sea water and intensity of dolomitization 

suggests that mere prolonged soaking in magnesiumrich water does not by itself cause dolomitization.

7. The wide variety of secondary textures seen in 

limestone from these atolls suggests that emplacement of dolomite takes place in several 

ways with differing paragenetic sequences.

8. The fact that the dolomite from these atolls is 

structurally calcium rich.

9. The fact that most of the limestone from both 

atolls and high islands in the Pacific is not dolomitized. Therefore one must look at dolomitization as an "abnormal" rather than a "normal" 

diagenetic effect.

10. That the three-dimensional distribution of dolomite 

in the subsurface of any atoll is unknown. Future 

drilling may show that dolomite forms a sheath 

around the outside of the limestone column; 

perhaps it is present only in a central plug or is 

scattered randomly throughout. A knowledge of 

whatever pattern exists is vital to any general 

hypothesis of atoll dolomitization.

CONCLUSIONS

This discussion stresses the fact that there is actually 

more than one "dolomite problem." Even within the 

restricted conditions that prevail during deposition and 

diagenesis of atoll limestones, dolomite apparently may 

form in at least the following three ways:

1. From metastable high-magnesian calcite, as illustrated by the growth of dolomite crystals in 

coralline algae, which possibly may take place at 

any depth.

2. By reaction of sea water with low-magnesian calcite 

to form the massive coarsely crystalline dolomite 

in hole F-l at Eniwetok and most of the finegrained dolomite at Funafuti and Kita-daitojima.

3. By direct precipitation of dolomite, with alternate 

layers of calcite, in vugs as the chemistry of the 

interstitial water or the pressure and temperature 

vary cyclically, probably in shallow water.

DETAILED DESCRIPTIONS OF CORES AND CUTTINGS

The information given below supplements that of 

Ladd and Schlanger (1960, table 3). It includes detailed descriptions of thin sections from the 15 cored 

intervals of hole F-l and the 3 cored limestone intervals 

of hole E-l. Cuttings from those intervals that 

appear to have been exposed to subaerial solution, as 

shown by fossil molds and casts and the replacement 

of primary aragonite skeletal material by secondary 

calcite, are given particular attention. Paleoecological 

interpretations based on depth are made for each hole; 

the geologic implications of the solution intervals and 

faunal and floral changes are discussed in the body of 

the text.

The recovered cores were in pieces that ranged in

665106 O—62
1012 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

size from irregular lumps less than 1 inch across to 

perfect cylinders more than 1 foot long. Each piece 

from each core was numbered consecutively from the 

top of the core downward. Thus F-l-1-1 designates 

the uppermost piece from core run number 1 in hole

F-l. The aggregate length of these pieces is given as 

the recovered footage in the descriptions below.

Cuttings were bottled, prior to the writer's work, by 

10-foot intervals and later examined for evidence of 

lithologic and faunal breaks.

Interval (feet) Drilling method Recovery 

10-30 Rock bit _ _ _ _ Cuttings _.

30-45 Rock bit _ _ _ _ Cuttings

45-70 Rock bit _ _ _ _ Cuttings - _.

70-140 Rock bit _ _ _ . Cuttings

DRILL HOLE E-l

Remarks

Coarse, unconsolidated sand made up largely of worn and broken tests of Calcarina 

and Marginopora and subordinate worn and polished fragments of coral and coralline algae. Alcyonarian spicules and echinoid debris make up a trace in the samples. 

A small percentage of the cuttings are in the very fine sand and silt sizes and contain 

abundant delicate well-preserved miliolid Foraminifera. The sample, taken 

from 20 to 30 feet, contains pieces of several types of coral as much as 2 inches 

in length; some of these are well rounded, and all are unaltered. No recrystallization, lithification, or chemical deposits of calcite are present.

Interpretations.—The abundant reef-type Foraminifera, now worn and broken, 

indicate that many of these sediments were deposited in a shallow-water environment. Well-preserved miliolids may have been indigenous lagoon forms that are 

now largely masked by transported material. Thus the sediment may have been 

deposited in a lagoon very close to a reef.

Unconsolidated coarse cuttings made up almost entirely of unaltered Halimeda 

segments between 3 and 6 mm across. A minor part of the cuttings consists of 

irregular pieces of unaltered coral and polished coralline algae as much as 10 mm 

in length. Also present are tests of Calcarina, Marginopora, and Amphistegina, 

and a few echinoid spines. No recrystallization, lithification, or chemical deposits 

of calcite are seen.

Interpretations.—The overwhelming abundance of small Halimeda segments indicates 

deposition in a lagoon.

Heterogeneous mixture of coarse fossil debris that includes angular to worn rounded, 

and polished pieces of several types of coral as much as 1 inch in length, crusts 

and nodules of coralline algae, Halimeda segments, tests of Calcarina and Marginopora, and other sand-size fossil debris. Among the corals, delicate branches of 

Seriatopora are common. Well-rounded and worn pieces of purple "slate-pencil 

urchin" spines as much as 1 inch in length are present. A trace of unaltered 

molluscan shells is also present. No recrystallization, lithification, or chemical 

deposition of calcite took place.

Interpretations.—The rounding and polishing of much of the coarse material is similar 

to that seen in many beach deposits on atolls. The fossils were probably all 

derived either from a reef or the shallow water behind a reef and deposited, after 

considerable movement, in shallow water—possibly within a lagoon or other 

back-reef area—or on a beach.

Cuttings from 70 to 90 feet are largely unconsolidated fossil debris dominated by 

whole and broken segments of Halimeda as much as 4 mm across. Angular chips 

of massive coral and pieces of delicate branching types are abundant, as well as 

pieces of coralline algae. Approximately 5 percent of the cuttings are worn individual tests of reef-type Foraminifera such as Marginopora and Calcarina. 

Fragments of mollusk shells and small well-preserved gastropods are common. 

The only consolidated materials are clusters of Halimeda segments as much as ^ 

inch across that are cemented by thin films of calcite, and a few irregular chips of 

lithified foraminiferal sand. The loose fossils are all unaltered. From 90 to 140 

feet rock chips become more abundant, and the fossil debris is still unaltered. 

Most of the rock consists of weakly cemented aggregates of Foraminifera and algae. 

Halimeda and mollusks in the rock fragments remain unaltered. Below 100 feet 

many rock chips are made up of finely crystalline calcite that is secondary after 

original fine carbonate matrix.

Interpretations.—The appearance of the recrystallized rock shows that these sediments had been exposed to subaerial solution for a short time; the top of the recrystallized zone is placed between 80 and 90 feet. The fossil assemblage indicates 

lagoonal deposition.
SUBSURFACE GEOLOGY OF ENIWETOK ATOLL 1013

DRILL HOLE E-l—Continued

Interval (feet) Drilling method 

140-300 Rock bit-- __

430-590 Ro ck bit - _ - _ Cuttings

Recovery Remarks

Cuttings.--- The cuttings are coarse; most of the limestone recovered is in the form of angular 

rock chips and pieces of coral between % and % inch across. The rock chips are 

well-lithified aggregates of coral and foraminiferal sand but include small pieces of 

coralline algae that have evidently broken from larger pieces. Stained samples of 

cuttings'from 140 to 150 feet and 230 to 240 feet show that most of the original 

aragonite remains. Many rock chips, however, contain finely disseminated crystalline calcite that is evidently in the process of replacing original matrix. In some 

samples, such as cuttings from 220 to 230 feet, there are abundant angular chips 

of massive coral that are still aragonite and which appear to have come from a 

single large head.

Interpretations.—The most striking characteristic is the intimate mixing of recrystallized rock and unaltered fossils. This indicates that the section drilled was partly 

recrystallized and probably had been exposed to some solution. The high content 

of coral and encrusting algae suggests deposition of these limestones on or near a 

reef.

300-430 Rock bit. _ __ Cuttings. _ _ _ The cuttings are all angular rock chips as much as 1 inch in length of dense recrystallized limestone. Solution cavities due to the removal of fossils are abundant. 

Most of the rock is made up of coarsely granular secondary calcite, but it also 

includes large clusters of yellow sparry calcite. Corals, mollusks, and Foraminifera 

are the most abundant fossils. The fossil debris is poorly sorted, and most megafossils are fragments. In the lower part of the section mollusks are represented 

only by molds. Despite intense recrystallization, stained cuttings from 340 to 350 

feet and 410 to 420 feet show a few percent of primary aragonite. 

Interpretations.—From 140 to 300 feet the degree of recrystallization gradually 

increases, but there is a definite strong increase in rock alteration at 300 feet. 

Evidently this whole section has undergone some solution. Recrystallized zones 

begin at approximately 300 feet in holes E-l and F-l and indicate a solution 

unconformity. The abundant corals and mollusks in these cuttings suggest that 

the sediments may have been deposited in shallow water, possibly within a lagoon. 

A gradation from rock chips with fossil molds to abundant unaltered fossils is seen 

in this interval. From 430 to 540 feet the cuttings are all angular chips of highly 

recrystallized limestone. Many pieces show one or more sides coated with drusy 

calcite that was probably deposited on the walls of solution channels. Abundant 

clusters of euhedral yellow and colorless calcite crystals are present and some 

individual crystals are between }i and % inch long. Stained cuttings from 510 to 

520 feet show no aragonite. Few recognizable fossils are present, but coral was 

abundant, as evinced by molds. Some crusts of coralline algae are also present. 

From 540 to 590 feet fragments of partly altered to unaltered fossils that include 

coral, mollusks, and Halimeda become more abundant. Stained cuttings from 570 

to 580 feet are mostly primary aragonite. At 590 feet the cuttings are approximately 50 percent recrystallized rock chips and 50 percent unaltered fossil debris. 

Interpretations.—This interval covers the lower part of the recrystallized zone that 

started at 300 feet and demonstrates the gradual downward decrease in the effects 

of recrystallization and solution associated with subaerial exposure. The fossil 

assemblage indicates deposition in shallow water, possibly a lagoon.

Cuttings.--- - Cuttings are all characterized by an abundance of primary aragonite skeletal material; 

the interval is comparable to the 650- to 970-foot interval in hole F-l. There are 

variations in color and size of cuttings throughout the interval. From 590 through 

650 feet angular pieces of both branching and massive coral dominate the fauna, 

and these are as much as 1 inch in length. The interseptal openings are clear, and 

the coralla are formed of aragonite. Abundant fragments of large pelecypod 

shells still retain their nacreous luster. The finer fraction, which is largely coarse 

sand, contains mostly coral and mollusk fragments and lesser amounts of benthonic 

Foraminifera tests. In addition, a few chips of recrystallized limestone are present. 

Most of these rock chips are lithified aggregates of coral, mollusks, and Foraminifera 

in a fine-grained calcite matrix. Staining shows, however, that almost all the 

primary aragonite still persists. From 650 through 830 feet, cuttings are less 

than one-quarter inch in diameter, and coral and mollusks still predominate. 

Some of the pelecypod shell fragments indicate that the original shells were as 

long as 2 inches. Worn tests of Heterostegina and whole shells of gastropods are 

common in the 1- and 2-mm size fraction. Below 760 feet the cuttings have a 

brownish tan color similar to that seen in the 650- to 970-foot interval in hole F-l.

590-1, 070 Rock bit.
1014 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

DRILL HOLE E-l—Continued

Interval (feet) Drilling method Recovery Remarks

590-1, 070 Rock bit.-__ Cuttings___ Stained samples from 650 to 660 feet and 740 to 750 feet show that about 80 percent

of the cuttings are composed of original aragonite. From 830 to 900 feet the cuttings are again coarse and are composed mainly of fragments of several types of. 

branching coral; the finer fraction still contains abundant unaltered pelecypod 

fragments. Between 900 feet and 1,070 feet the cuttings become finer again but 

remain similar in color and fossil content to those above.

Interpretations.—Cuttings from above 670 feet probably came from rock that was 

partly recrystallized and represent the gradational zone below the solution unconformity at 300 feet. Below 650 feet the sediments have not been subjected to 

subaerial solution. The abundant branching corals and large mollusks indicate 

that these sediments were deposited in shallow sheltered waters, probably in a 

lagoon.

1,070-1,658 Rock bit.---_ Cuttings.-__- From 1,070 to 1,150 feel, cuttings show a brownish-tan color like that of the unaltered sediments above. At 1,070 feet, however/ the cuttings are largely angular 

chips of tan recrystallized limestone, that is permeated by coarsely granluar calcite 

mosaics. Many chips are from miliolid-rich limestones made up of tightly packed 

tests of Foraminifera, algae, and coral debris in a fine-grained matrix. Corals are 

completely recrystallized, and the interseptal spaces are filled with recrystallized 

mud. Stained rock chips from 1,090 feet show no aragonite. Clear calcite casts 

of small gastropods are present, and many chips have molds owing to the removal 

of aragonite fossils by solution. Much unaltered debris is present, but this is 

thought to be contamination from above. The cuttings from 1,170 to 1,190 feet 

contain, in addition to rock chips, many fresh-looking tests of Calcarina, Marginopora, and pieces of bright pink Homotrema; these are very like tests from Recent 

sediments and are apparently the result of contamination. Below 1,200 feet 

rock chips become more abundant and unaltered fossils less so. At 1,310 feet the 

cuttings are almost all angular flakes and chips of recrystallized white-yellow 

dense limestone.

Interpretations.—The top of a solution unconformity is placed at approximately 

1,070 feet in this hole because of the flood of recrystallized rock chips that appears 

at this depth. On the basis of larger Foraminifera, Cole (1957, p. 746) places the 

top of the Miocene e at 1,080 feet in this hole. Todd and Low (1960, p. 802), 

on the basis of smaller Foraminifera, place the Tertiary e boundary between 1,020 

and 1,100 feet. Thus the top of the recrystallized zone is a faunal boundary that 

suggests emergence following deposition of the Miocene e sediments. During later 

submergence, sediments containing a new post-Miocene e fauna were deposited, 

but the small size of the chips makes paleoecological interpretations difficult. 

However, the abundance of bentonic smaller Foraminifera and the presence of 

coral indicate a shallow-water deposit.

1,658-2,003 Rockbit____ Cuttings——— Cuttings from this interval are a mixture of partly recrystallized-rock chips and

fragments of slightly altered to unaltered fossils, largely pelecypods, gastropods, 

corals, and Foraminifera. In the upper part of the interval, tests of larger 

Foraminifera including Lepidocyclina, Spiroclypeus, and Heterostegina are abundant 

and make up the bulk of the 1- to 4-mm fraction. These tests are generally chipped 

and worn. Most of the rock chips are made up of sand-size fossils, largely 

Foraminifera, packed in a fine-grained matrix. Granular mosaics of sparry calcite 

are common in these chips. At 1,860 feet the number of included larger Foraminifera diminishes, and the cuttings contain abundant tests of smaller Foraminifera, including miliolids and amphisteginids. Stained cuttings from the 

1,776- to 1,805-foot interval show that approximately 50 percent of the material 

is aragonite, in the form of coral and molluscan debris. The lower part of the 

interval, below 1,860 feet, is characterised by an abundance of soft limestone 

chips that are made up of foraminiferal, coral, and molluscan debris. Although 

these rocks contain patches of secondary calcite, solution channels were not seen, 

and much of the limestone is aragonite. Mollusk shells in these limestones are 

generally chalky, but many still show a nacreous luster. Individual stained chips 

contain between 20 and 25 percent aragonite.

Interpretations.—This interval lies below intensely recrystallized limestone and shows 

the effect of solution decreasing downward; the chips from the upper part of the 

interval show more re crystallization than those from the lower part. The lower 

part was probably not subjected to more than slight exposure to subaerial conditions, if at all. The abundance of larger Foraminifera in the upper part is suggestive of fore-reef or open shoal water conditions. The abundant miliolids and
SUBSURFACE GEOLOGY OF ENIWETOK ATOLL 1015

Interval (feet) Drilling method

2,003-2,028 Core barrel.. _ _ •

Recovery

ft (core 1), 

13 pieces

DRILL HOLE E-l—Continued

Remarks

amphisteginids in the lower part also are suggestive of shallow-water deposits. 

Todd and Low (1960, p. 815), on the basis of smaller Foraminifera, interpret the 

sediments in this interval as of shallow-water origin.

Rocks recovered in this run are gray friable poorly sorted soft limestones rich in 

disarticulated pelecypod valves, mostly Cardium. These well-preserved valves 

are randomly oriented in a matrix of sand-size fossil debris and fine carbonate. 

Most of the valves are approximately one-fourth inch across and many are broken. 

No bedding is seen. Echinoid spines, rods of articulate coralline algae, and rounded 

fragments of coral are also present. Some core pieces are single fragments of 

massive Porites. Although only 13 pieces are numbered, there are many small 

pieces because the rock breaks easily and tends to disintegrate in water. No 

molds or solution channels are present. A bulk X-ray analysis of E-l-1-4, a 

pelecypod-rich piece, showed 55 percent calcite and 45 percent aragonite (table 

2). E-l-1-6, a piece of Porites, showed less than 2 percent calcite and more 

than 98 percent aragonite (table 2). Chemical and spectrographic analyses of these 

samples are given in tables 1 and 3, respectively. These rocks are unaltered and 

show no megascopic sign of calcite cement. Detailed descriptions of representative thin sections are given below.

E-l-1-1.—A thin section shows the matrix to be a packed aggregate of rounded 

carbonate grains that are between 0.05 and 0.1 mm in diameter. Set in this 

porous matrix are abundant tests of well-preserved benthonic smaller Foraminifera. 

Many of these are miliolid types. Most have empty tests and still show a brown 

color in transmitted light; they appear isotropic in plane polarized light. Pelecypod shells show their original fibrous and prismatic internal structure. Fragments of both articulate and encrusting coralline algae are common. Much of the 

fine-grained matrix is recognizable fossil debris; the brown color of many of these 

fragments suggests comminuted Foraminifera. Although the material is tightly 

packed, the rock lacks any kind of chemical calcite cement. No signs of recrystallization or solution are seen.

E-l-1-4-—As above, except that the fine matrix particles average approximately 

0.2 mm in diameter; many of these are rounded fragments of coralline algae.

E-l-1-6.—A section cut through a piece of solid Porites. The corallum is unaltered 

aragonite, and there is no sign of recrystallization of the microcrystalline skeletal 

material. There is only a trace of chemically deposited calcite, and this is found 

as thin linings in the interseptal spaces. This calcite may account for the calcite 

detected in the X-ray analysis. In transmitted light the septae are light brown 

with darker lines through their centers; in plane polarized light the aragonite 

shows even birefringence and no extinction, which indicates a microcrystalline 

aggregate. Traces of calcite show as microgranular aggregates.

E-l-1-9 and -11.—These sections show randomly oriented, scattered tests of thickwalled benthonic smaller Foraminifera and coralline algal debris in a fine-grained 

matrix of comminuted fossil debris. In reflected light the thin sections appear 

cloudy owing to irregular areas of white against a tan background. In transmitted 

light these white areas are opaque black. A few larger Foraminifera tests are 

present. The miliolid tests show their characteristic brown color.

Interpretations.—The high content of original aragonite and the lack of solution, 

recrystallization, and calcite cement indicate that this interval has not been subjected to subaerial solution. The fossil assemblage, which is rich in miliolid 

Foraminifera and pelecypods, is suggestive of a lagoonal deposit. The lack of 

sorting and bedding indicate deposition in quiet water. However, the disarticulation and breaking of most mollusk shells evidence some movement of sediment 

before final deposition.

All cuttings from this interval are sand and granule sizes. Toward the upper part 

the material is dominantly sand size, and toward the bottom the cuttings are as 

much as 4 mm in diameter. More than 90 percent of the cuttings are discrete 

fragments of fossil^, or whole fossils. Rod-shaped fragments of coralline algae are 

very abundant, 4 as are angular fragments of unaltered mollusk shells. Foraminifera 

make up much of the cuttings; the fine sand fraction contains abundant wellpreserved tests of delicate benthonic smaller types including miliolid, coiled, and 

serial forms. Angular fragments of unaltered coral and echinoid spines form only 

a minor part of the cuttings. The coarse cuttings from below 2,100 feet contain

4 Tracey, in Emery, Tracey, and Ladd (1954, p. 253), reports that rod-shaped pieces of Lithophyllum make up from 50 to 75 percent of the cuttings from 2,288 to 2,327.5 ft in 

hole 2B at Bikini. The algae found here at Eniwetok probably are the same type as those found at Bikini.

2, 028-2, 290 Rock bit _ _ _ _ Cuttings _. _ _
1016 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

Interval (feet) 

2, 028-2, 290

Drilling method Recovery 

Rockbit____ Cuttings __

2, 290-2, 410 Rock bit. _ _. Cuttings_

2, 410-2, 540 Rock bit. Cuttings

2, 540-2, 780 Rock bit _ _ . _ Cuttings

DRILL HOLE E-l—Continued

Remarks

numerous tests of larger Foraminifera as much as 4 mm in diameter. Also, the 

coralline algal debris is coarser. In the lower part of the interval particularly, many 

of the fossils have coatings of fine-grained white limestone. Rare chips of hard 

limestone are present, and a few pieces show extensive recrystallization. Stained 

cuttings from 2,110 to 2,120 feet and 2,230 to 2,240 feet show that much primary 

aragonite is present as molluscan and coral debris.

Interpretations.—The coating of white limestone on the fossils indicates that the drill 

penetrated soft, friable limestone made up of sand- and granule-size algal and molluscan debris and tests of Foraminifera packed in a fine-grained matrix. The 

general agreement in size between the various fossil fragments and whole tests 

suggests that the debris underwent some sorting before final deposition. The 

abundant algae suggest deposition in shallow sheltered water.

Cuttings are a mixture of angular light-tan rock chips and discrete fossil fragments. 

The chips range from % to % inch across. The proportion of rock chips to fossils 

varies throughout the interval, but the rock chips generally make up the bulk of 

the material. These chips are partly recrystallized chalky limestones composed 

of randomly oriented foraminiferal, algal, and molluscan debris. The mollusk 

shells are chalky but still aragonitic. These fossils are packed in a fine-grained 

matrix which locally has recrystallized to granular mosaics of calcite. Stained 

chips from 2,370 to 2,380 feet show that the rock contains 10 to 20 percent aragonite. 

Pelecypod shell fragments are aragonitic. The sand fraction of the cuttings is 

dominated by cylindrical, rod-shaped fragments of Lithophyllum, similar to those 

in the interval described above, and angular fragments of pelecypod shells. In 

some samples, such as that from 2,330 to 2,340 feet angular chips of well-preserved 

coral as much as one-half inch in length are common.

Interpretations.—This interval was probably soft limestone that yielded many fossil 

fragments on drilling. The high aragonitic content and lack of solution features 

indicate that this interval, like the one above, underwent little, if any, subaerial 

exposure. The abundance of mollusks and Lithophyllum and local concentrations 

of coral suggest a lagoonal environment, or at least shallow-water conditions.

Cuttings are largely coarse rock chips. In the upper part of the interval these chips 

are composed of light-tan porous and soft limestone made up of well-sorted mediumsand-size fossil debris dominated by fragments of mollusks, tests of benthonic 

smaller Foraminifera, and unidentifiable fossils. In the central part of the interval 

the rock chips are mixed with a small percentage of granule-size algal rods and 

angular fragments of pelecypods. A few of the rock chips contain unaltered shells 

of small pelecypods and gastropods. Most of the rock appears unaltered, but there 

are fine seams and patches of crystalline calcite scattered throughout the interval. 

From 2,440 to 2,450 feet the cuttings are flat chips of soft tan mudstone without 

recongizable fossils. Many of these chips show laminar structure similar to that 

seen in finely bedded shales and tend to part into flakes. Along some partings are 

films of flexible black carbonaceous (?) material. In transmitted light these show 

up as deep brown to red translucent structureless films; they probably are plant 

remains.

Interpretations.—The rock chips are well lithified but show no solution features; the 

presence of aragonitic shell material indicates no prolonged exposure to subaerial 

conditions. The laminated mudstones may have accumulated in the deeper quiet 

water of a lagoon. The drilling record notes that the mudstones and sandy limestones may be interbedded. These well-sorted sands may represent periodic 

deposits of sediment on a muddy lagoon bottom.

The upper 20 feet of cuttings are coarse sand size and are made up mainly of rodshaped fragments of algae along with a few tests of larger Foraminifera, molluscan 

debris, and angular chips of slightly recrystallized rock. Many of the algal rods 

are coated with fine-grained white limestone; others are coated with finely crystalline calcite. It appears as though these rods were broken from a partly recrystallized rock. Below 2,560 feet the cuttings are coarser; they are mixtures of 

tan rock fragments like those described from the interval above and angular 

fragments of unaltered coral and mollusks. A stained sample from 2,580 to 2,590 

feet shows that both rock chips and fossils are highly aragonitic. Most of the 

pelecypod remains retain their original porcelaneous appearance. In many 

samples, angular chips of pelecypod shells as much as one-half inch in length and 

one-fourth inch in thickness are prominent; these must have been broken from
SUBSURFACE GEOLOGY OF ENIWETOK ATOLL 1017

Interval (feet) Drilling method Recovery

2, 780-2, 802 Rock bit _ _ _ _ Cuttings

2, 802-2, 808 Core barrel. _ 6 ft (core 2),

9 pieces.

DRILL HOLE E-l—Continued

Remarks

large valves. Several genera of coral are represented and include both delicate 

branching and massive types. The coral is slightly chalky, but the structure is 

well preserved and the interseptal spaces are open. Proportions of the fossil types 

vary; some 10-foot intervals are rich in coral and molluscan remains, and some are 

high in rodlike algal debris.

Interpretations.—This interval is transitional from the one described above. The 

main change is the downward increase in content of unaltered coral and large 

pelecypod fragments. Evidently the drill penetrated a section of soft limestone 

that is only slightly if at all recrystallized, and it contains fossils that are easily 

broken out of their matrix. The lack of alteration of the corals and mollusks 

indicates little or no exposure to subaerial conditions. The high percentage of 

coral and molluscan remains suggests deposition under quiet-water, probably 

lagoonal, conditions.

Cuttings are much like those described above except for the appearance of angular 

chips of white dense crystalline limestone at this depth. These chips are rare 

in the 2,780- to 2,790-foot sample but become abundant below 2,790 feet. Through 

the upper 20 feet of this interval the cuttings take on a white color as opposed to 

the tan of the overlying sediments. The white rock chips are composed largely of 

foraminiferal limestone with some mollusk molds. Stained cuttings from 2,790 

to 2,800 feet show that the rock is all calcite except for a trace of aragonite. Some 

of the rock chips are made up of packed tests of miliolids and fragments of coralline 

algae cemented by acicular and granular calcite. These rocks strongly resemble 

those described from core run 2, taken directly below this interval.

Interpretations.—Cole (1957, p. 749) places the top of the Eocene at 2,780 feet on the 

basis of the first appearance of a diagnostic smaller Foraminifera, Pseudochrysalidina. Todd and Low (1960, p. 807) place the top of the Eocene at 2,770 feet. 

The writer accepts 2,780 feet as the top because of the appearance of a different 

lithology. Chips that appear at this depth show strong solution features, such as 

molds, which indicate that a solution unconformity separates the rocks of Eocene 

and Miocene age, just as the Miocene e sediments are separated from younger 

rocks by a similar but better developed unconformity. The faunal and lithologic 

resemblance of these cuttings to the cores below suggest that this interval represents 

shallow-water sediments like those found in the cores.

Core pieces are gray porous to dense irregularly cemented to friable limestones rich 

in molluscan fossils. Generally disarticulated pelecypod valves and gastropod 

shells, many of which are broken, are randomly oriented in a matrix of sand-sized 

foraminiferal and algal debris. The fossils have been partly dissolved, leaving 

molds, but much original shell material remains. The mollusks are as much as 

2 inches in length and include broken shells which may have been as long as 3 

inches. A striking feature is the irregularity of cementation; areas of the matrix, 

particularly around molds, are well-cemented by hard white calcite that fills the 

interstices between the sand-size fossils. Much of the matrix is friable porous 

sand. The boundaries between the dense white patches and the porous patches 

are fairly sharp; a complete transition takes place in a few millimeters. About 

half the rock is cemented, and the other half is friable; the two types are intimately 

mixed so that the pieces are coherent. There are a few irregular areas as much as 

one-half inch across where the opaque white calcite grades into clear yellow calcite. 

Other fossils seen megascopically include echinoid and algal debris. Dasycladacean 

algae are common as molds; these have diameters as large as 3 mm. In piece 

E-1-2-8 they are especially abundant; more than a dozen molds are seen on a 

sawed surface of about 10 square inches. Some of these algae are broken. Piece 

E-1-2-9 is especially rich in large gastropod molds; eight of these, all between 1 and 

2 inches in length, are in an irregular piece of core 4 inches long. Bulk X-ray 

analysis of E-l-2-1 showed 100 percent calcite, but picked molluscan fragments 

from E-l-2-5 showed 20 percent aragonite and 80 percent calcite. Chemical and 

spectographic analyses of E-l-2-1 are given in tables 1 and 3, respectively. Thin 

section descriptions are given below.

E-l-2-1, -3, and -8.—Sections show that the sand-size matrix contains abundant 

well-preserved tests of Triloculina, a miliolid Foraminifera. Tests of these, other 

benthonic smaller Foraminifera, and rounded fragments of coralline algae are 

closely packed. In places these are cemented only by thin films of granular 

calcite and correspond to the friable parts of the rock. Other areas show the inter-
1018 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

Interval (feet) 

2, 802-2, 808

Drilling method 

Core barreLRecovery 

6ft (core 2), 

9 pieces.

2, 808-3, 130 Rock Bit- _ _ _ Cuttings.

3. 130-4, 078

4. 078-4, 100

Rock bit____ 

Core barrel -_

No recovery.

13ft (core 3),

42 pieces.

DRILL HOLE E-l—Continued

Remarks

stices between fossils to be filled with finely granular mosaics of calcite that correspond to the indurated part of the rock. In places the granular mosaics contain 

patches of dark, cloudy original mud. Many of the miliolid tests retain a brown 

color as in core 1; most of the tests are filled with calcite similar in texture to the 

cement.

Interpretations.—Abundant pelecypods and gastropods, miliolid Foraminifera, and 

dasycladacean algae make up an assemblage characteristic of shallow-water, probably lagoonal, deposits. The algae, particularly, flourish in water only a few feet 

deep. However, these may have been carried downward into a lagoon several 

fathoms or more deep, as indicated by broken algal fragments. The large mollusks were probably indigenous, although breakage and disarticulation suggest 

movement before final deposition. The development of molds points to a period 

of subaerial solution. However, the persistence of aragonite suggests that the 

exposure was of comparatively short duration.

Cuttings from this interval are all sand- and granule-size angular chips of hard white 

limestone, although the upper few feet contain some unaltered fossils that are probably contamination from above the solution unconformity at 2,780 feet. Stained 

cuttings from 2,840 to 2,850 feet, 2,940 to 2,950 feet, 2,990 to 3,000 feet, and 3,020 

to 3,030 feet show these chips to be entirely calcite. There are clusters of clear 

yellow calcite crystals. The limestone is predominantly well cemented and recrystallized and is made up of abundant tests of benthonic smaller Foraminifera, 

including miliolids that are closely packed to floating in a fine-grained crystalline 

calcite matrix. Some chips are made of recrystallized coral.

Interpretations.—Todd and Low (1960, p. 812) found three foraminiferal zones in the 

2,770- to 3,120-foot intervals; they are as follows: a Peneroplis-Trioculina assemblage, 2,770-2,940 feet; a Ornatonomalina assemblage, 2,940-2,960 feet; and a 

Asterigina rotula assemblage, 3,010-3,120 feet. Todd and Low interpret these 

sediments as shallow-water deposits. The comparable zones in hole F-l range 

from 4,200 to 4,550 feet, and the sediments there are interpreted as outer slope 

equivalents of the 2,770- to 3,120-foot interval in E-l. The recrystallization and 

solution molds in this interval show that it is still within the strongly affected zone 

below the solution unconformity at 2,780 feet.

The lithology through this interval remains constant. The rocks are all white dense 

very hard and well-cemented, highly recrystallized dolomitic limestones containing 

numerous molds of massive corals as much as several inches in length and mollusks, 

dominantly disarticulated pelecypod valves. The larger fossils are randomly 

oriented and no bedding is evident. The bulk of the rock is made up of sand-size 

fossil debris that includes abundant articulate coralline algae in a fine hard mud 

matrix. Laminae of encrusting algae are common. In addition to the fossil 

molds there are irregular solution voids lined with drusy calcite and dolomite. 

Dolomite is easily seen megascopically by sparkles of light from the euhedral 

crystal faces. The dolomite is evenly distributed, as all pieces have some dolomite, 

but none of the rocks are as intensely dolomitized as some of the pieces in core 

12 from hole F-l. Bulk X-ray analyses of pieces E-1-3-10 and E-1-3-42 showed 

5 percent and 14 percent dolomite, respectively; the remainder of the rock was 

calcite. Chemical analyses of these samples are given in table 1. The following 

descriptions include features seen in sections from pieces E-l-3-1, -2, -5, -8, -10, 

-20, -23, -26, -31, -33, -36, -40, and -42. Almost all these pieces contain some 

coral. All the interseptal spaces are filled with semiopaque lithified mud. Plate 

283Z) shows the state of preservation of coral in this interval. The corallum itself 

has either been dissolved out or replaced by granular calcite. The mud filling is 

the host for numerous dolomite euhedra. Of interest is the fact that in no thin section was the originally aragonite corallum itself dolomitized. Mollusk shells are generally replaced by finely granular mosaics of dolomite that show sharp boundaries 

against the dark matrix (pi. 283.E). However, some valves still show traces of an 

original fibrous and prismatic internal structure and appear yellow in transmitted 

light. The fine-grained matrix appears to be rich in coralline algae, although 

intense recrystallization makes positive identification difficult. Pockets of packed 

rounded dark particles that average 0.04 mm in size are common between larger fos-
SUBSURFACE GEOLOGY OF ENIWETOK ATOLL 1019

Interval (feet) Drilling method

Interval (feet) 

20-55

Drilling method 

Rock bitDRILL HOLE E-l—Continued

Recovery Remarks

sils. Irregular solution channels are present but are completely filled by laminar 

deposits of fibrous calcite. Much of the dolomite is restricted to rod-shaped segments of articulate coralline algae (pi. 283C). These algal rods, where not too 

greatly recrystallized, are identifiable as Corallina. The rods appear in thin section 

as dark circles, ellipses, or rectangles according to the orientation of their long ax,es 

with respect to the plane of the thin section. Plate 283C shows a typical dolomitized 

algal rod, cut parallel to its long axis, that is 0.3 mm wide by 1.7 mm long. The 

original algal rod is almost completely replaced by clear dolomite across its width 

and is recognizable only by a small amount of algal material at each end. The 

dolomite crystals are definitely restricted to the algae, and their growth seems to 

be controlled by the shape of the rod. The area near the axis of the rod is occupied by a fine-grained mosaic of anhedral dolomite that grades outward into 

coarser crystals. Under crossed nicols the rhombic habit of some of these crystals 

is clearer due to separate extinction. In a section cut perpendicular to the long 

rod axis, the dolomite forms a rosette of subhedral crystals that radiate from a 

center of fine-grained anhedra (pi. 283B). Many algal rods contain separate, 

euhedral dolomite rhombs that apparently are not so closely controlled by the 

shape of the algal fragment. Plate 283B shows one such rod cut parallel to its 

long axis. In this view three dolomite rhombs are well developed. The crystal 

on the left has a cloudy, dark center that is typical of many of these euhedra. In 

any given thin section, some rods may be completely dolomitized, whereas a few 

millimeters away, a similar rod may bear no trace of dolomite. In general, however, most of the rods are partly replaced by dolomite. Thin sections may show 

circular, ellipsoidal, and rectangular areas of 'dolomite that have apparently obliterated the original algal rods and spread into the matrix. Dolomite is scattered 

through the matrix and in the mud fillings. Pa"rt or all of this dolomite may have 

originated in small algal fragments in this mud. In some slides, later solution has 

removed parts of the undolomitized algae and has left only clusters of dolomite 

rhombs within algae-shaped voids.

Interpretations.—The high content of large massive corals, mollusks, and abundant 

coralline algae suggest a shallow-water deposit. The poor sorting and the ubiquitous 

mud fillings, both within the fossils and as a rock matrix, are common attributes of 

reef and near-reef limestones. This interval was probably deposited on a reef 

or within a shallow lagoon. The degree of recrystallization shown by these limestones indicates at least one period of subaerial solution; the laminar calcite deposits 

show that chemical deposition of calcite was cyclic. Further, the solution of 

algal rods from around their dolomitized cores shows that some rock was removed 

following dolomitization.

DRILL HOLE F-l

Recovery Remarks

Cuttings. _ _ _ Unconsolidated sand, from 20 to 45 feet, dominated by worn tests of Calcarina and 

Baculogypsina. Tests of Marginopora and other benthonic shallow-water types 

are present. Shells of high-spired gastropods as much as 3 mm long are common. 

Broken fragments of unaltered Halimeda segments as much as 4 mm in length are 

also present. Fragments of red Homotrema and Tubipora speckle the cuttings, 

and rounded fragments of coral are common. Most of the material is between 

}{ and 2 mm in diameter. The fragments of larger organisms such as coral and 

Halimeda are roughly the same size as the whole tests of Foraminifera and gastropod 

shells. This size relationship indicates sorting; the abraded state of much of the 

material supports the interpretation of sorting. The sample from 45 to 55 feet is 

coarse and is dominated by fragments and whole segments of Halimeda as much 

as 6 mm across. Coarse Tubipora debris is common. Only a small percentage 

of the sample consists of foraminiferal tests. The entire interval is unaltered; 

primary aragonite is abundant, and no signs of recrystallization or cementation is 

seen.

Interpretations.—The abundant reef-type Foraminifera, now worn, indicate that 

these sediments were largely of shallow-water origin, and that they underwent 

some transport. They were finally deposited off the reef, either in a lagoon or a 

fore-reef area.
1020 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

Interval (feet) 

55-70

Drilling method 

Rock bit- _ _

70-110 Rockbit____ Cuttings. ___

110-170 Rock bit. _ _ _ Cuttings170-190 Core barrel.__ 1 foot (core 1),

3 pieces.

DRILL HOLE F-l—Continued

Recovery Remarks

Cuttings. ____ Samples are unconsolidated and are made up almost entirely of whole and broken 

unaltered Halimeda segments as much as 10 mm in length. There are two types 

of Halimeda present: large fan-shaped and twisted segments; and smaller ones in 

the form of three distinct tubes, in a plane, fused at one common juncture and 

radiating at an angle of from 60 to 90 degrees. These segments average approximately 5 mm long. A small percentage of Tubipora fragments and foraminiferal 

sand accompany the algal segments. No recrystallization or deposition of chemical 

calcite is seem.

Interpretations: The high content of Halimeda indicates deposition in a lagoon, 

possibly at depths of from 10 to 30 fathoms.

Cuttings are a mixture of discrete unaltered whole and broken fossils including 

Halimeda, Foraminifera, Tubipora, gastropods, and angular rock chips. The 

unaltered fossil material is similar to that described in the above interval. The 

rock chips include poorly lithified partly recrystallized foraminiferal sands and 

dense, well-cemented crystalline limestone. Many chips are aggregates of sparry 

calcite. Fragments of partly to wholly recrystallized coral are common. The 

upper 10 feet of the interval contains a high percentage of large Halimeda segments, similar to those in the 55- to 70-foot interval, and fewer rock chips than 

the rest of the run. Evidently the drill passed from wholly unconsolidated and 

unaltered material into a zone of lithified and recrystallized sediment in this 

interval. The drilling time record shows a sharp increase in drilling time per 10 

feet between 60 and 70 feet in hole F-l; this may indicate the top of the recrystallized zone.

Interpretations.—The mixture of unaltered fossils, which may be contamination from 

above, makes interpretation difficult; however, the common fragments of coral 

indicate a shallow-water deposit.

Coarse cuttings made of angular to subrounded rocks and individual fossil fragments, almost all of which are between % and ^ inch in diameter. Among the 

fossil fragments, angular chips of massive coral are common and several genera 

are represented. Delicate branching coral fragments are also present. Rods and 

fragments of branching coralline algae are common, and molluscan debris is prominent. A few Halimeda segments are present. Most of the rock chips are partly 

recrystallized, and fossils are obscured. However, much of the rock evidently 

was coralliferous. Small solution molds are common. Cuttings from the 110- 

to 120-foot interval were stained with Meigen's solution, and this revealed that 

most of the coral, molluscan, and Halimeda debris is unaltered aragonite. Many 

of the rock chips show abundant sand-size fragments of aragonite in a partly 

recrystallized calcite matrix.

Interpretations.—This interval underwent some subaerial solution, but much of the 

primary aragonite remains. The abundant coral, algal, and molluscan remains 

suggest deposition in a lagoonal environment.

F-l-1-1 and F-l-1-2.—White and tan porous, friable limestone made up of unsorted 

randomly oriented fragments of coral as much as 1 inch long, Halimeda segments 

one-half an inch long, pelecypod and gastropod debris, and irregular fragments of 

encrusting algae. These fossils are white, but the rock matrix has a large proportion of tan finely filamentous encrusting Foraminifera (Ladd and Schlanger, 1960, 

pi. 7 fig. 2). The rest of the matrix is porous, slightly lithified carbonate of spongy 

texture. Thin sections of F-l-1-1 show that the Halimeda segments are largely 

unaltered coffee-colored micro crystalline aggregates of aragonite. A few segments 

show originally open tubes now filled with clear microgranular calcite. The corals 

are also largely unaltered but fibrous aragonite. Some of the interseptal spaces 

are filled with dark carbonate mud rich in small spherulitic bodies identified as 

tunicate ascidian spicules. Minor parts of the coral have been replaced by finely 

granular calcite. Encrusting Foraminifera and mollusk shells both show a fibrous 

structure: the former, calcite; and the latter, aragonite. Encrusting algae show 

irregular, blotchy coloration; some parts are black and semiopaque, and others are 

light tan. The matrix is a fine open fretwork of unidentifiable carbonate. Parts 

of it are dominated by a network of subcircular links resembling a soap-bubble 

pattern; other parts appear to be relict after laminar encrustations. A bulk X-ray 

analysis of F-l-1-2 showed 60 percent aragonite and 40 percent calcite (table 2). 

Chemical spectrographic analyses are given in tables 1 and 3, respectively.
SUBSURFACE GEOLOGY OF ENIWETOK ATOLL 1021

Interval (feet) Drilling method Recovery

1 90-280 Rock bit._.. Cuttings....

280-330 Rock bit _ _ _. Cuttings

330-600 Rock bit-___ Cuttings.

DRILL HOLE F-l—Continued

Remarks

F-l-1-3.—As above, but better cemented by masses of laminar encrusting algae. 

This core shows a denser, more lithified matrix than other samples from this run.

Interpretations.—The fretted appearance of the matrix and the minor amounts of 

calcite in the Halimeda and the coral suggest that the material of this core has 

undergone only a minor amount of aragonite solution and calcite deposition; subaerial exposure for any significant time has not taken place. Cole (1957, p. 745) 

reports no diagnostic larger Foraminifera from this run. Todd and Low (1960, p. 

805; table 3) report Heterostegina, Homotrema, Calcarina, Marginopora, Amphistegina, Carpenteria, and two species of Triloculina from this interval. This foraminiferal assemblage, the abundant small Halimeda segments, the coral debris, the 

mollusk shells, and the lack of sorting suggest deposition in a lagoonal environment.

Very coarse cuttings made up largely of unaltered and porous angular chips of 

massive coral as long as 1 inch, broken and whole unaltered shells of both gastropods 

and pelecypods as long as three-fourths of an inch, and finger-shaped fragments 

of branching coral as long as 1}£ inches. Abundant angular rock fragments are 

present and include slightly recrystallized but still aragonitic foraminiferal sands 

and dense partly crystalline limestone. Proportions of various fossil types change 

throughout the interval. Cuttings from the 240- to 250-foot section are almost 

all angular chips of a single type of massive coral; apparently a single large coral 

head was broken by the drill. From 250 to 260 feet, fragments of branching 

coral are prominent; many of these are coated with white crustose coralline algaeOnly a small percentage of material finer than one-quarter inch in diameter is 

present. Worn tests of Amphistegina, fragments of pelecypod shells, and whole 

shells of gastropods make up most of this finer fraction. Cuttings from the 200- 

to 210-foot section were stained with Meigen's solution and showed that recrystallization, although evident, is not as strong as that shown by the 110- to 170-foot 

interval. Many rock chips show only slight lithification and little recrystallization; 

no solution molds were seen. Rock chips include Halimeda-rich limestone; the 

algal segments are still aragonite, but the rock matrix is fine-grained calcite.

Interpretations.—This interval has undergone only minor solution and recrystallization. Replacement of aragonite by calcite and chemical deposition of calcite is 

rare. Thus there seems to be some diminishing of the effects of subaerial exposure 

downward from the above interval. These sediments were probably deposited in 

a lagoon.

Cuttings are very similar in fossil content to those from the 190- to 280-foot interval. 

However, from 280 to 330 feet the cuttings have a higher percentage, approximately 

10 to 20 percent, of material between 2 and 4 mm in diameter. Most of this finer 

fraction is made up of worn tests of benthonic Foraminifera such as Amphistegina, 

angular fragments of unaltered mollusk shells, and coral and algal debris. Many 

partly recrystallized rock chips are present, but many of the aragonitic fossils, 

such as coral and Halimeda, are unaltered. Staining of cuttings from the 290- to 

300-foot and 320- to 330-foot intervals reveals abundant primary aragonite. 

From 310 feet to 330 feet angular chips of unaltered and massive coral are again 

abundant. This interval should probably be grouped with the 190- to 280-foot 

interval, but the change in cuttings size warranted the split.

Interpretations.—The degree of recrystallization and the fossil content indicate that 

the interval records a history of deposition under shallow-water conditions, probably 

lagoonal, followed by a slight exposure to solution and concomitant recrystallization.

Cuttings from this interval are made up almost entirely of angular chips of buffcolored well-cemented, recrystallized limestone. Thus, there is a marked change 

in degree of recrystallization and loss of primary aragonite in comparison to the 

above aragonitic intervals. Stained cuttings from 330 to 340 feet, 430 to 440 feet, 

520 to 530 feet, and 550 to 560 feet show that only a small percentage of the original 

aragonite remains; most of the coral has been recrystallized and replaced by 

secondary calcite. The angularity of most of the chips indicates that hard, dense 

rock was penetrated. In many chips the rock matrix is now a finely granular 

mosaic of anhedral calcite, and the original texture is obliterated. Many chips 

show solution cavities. A few chips still show original textures, such as foraminferal 

sands rich in small benthonic types. On a few chips, fragments of large pelecypod 

shells are evident; some of these are still partly aragonitic. A few unrecrystallized 

chips of massive coral are present, but these are similar to those in higher intervals 

and may be contamination. Clusters of sparry light-yellow calcite are common.
1022 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

Interval (feet) Drilling method Recovery 

330-600 Rock bit. . . . Cuttings. _ .

600-625 Core barrel... 2 ft (core 2), 

10 pieces.

625-650 Rock bit. _ _ _ Cuttings.

DRILL HOLE F-l—Continued

Remarks

Some of these crystal aggregates show a curving flat side in the form of an impression 

of a pelecypod shell; some shell material still clings to the impression. Thus 

chemical deposition of calcite in empty mollusk shells has evidently taken place. 

In some samples (each of a 10-foot interval), fine cuttings are abundant; these are 

made up of discrete tests of benthonic Foraminifera such as Amphistegina and 

Marginopora and small rock chips. Although Todd and Low (1960, p. 805) placed 

the Miocene-post-Miocene boundary at 560 to 570 feet, no change in lithology, 

degree of recrystallization, or fossil content was apparent in the cuttings. The 

most abundant fossils in this interval are corals, Foraminifera—dominantly shallowwater types—and mollusks; echinoid spines are scattered throughout and rare 

Halimeda are present.

Interpretations.—The degree of recrystallization and loss of primary aragonite 

indicates that this interval was subjected to prolonged subaerial solution. The 

marked increase in these characteristics at 330 feet evidences the top of a leached 

zone at that depth (fig. 308). The fossil assemblage suggests deposition under shallow-water, probably lagoonal, conditions.

F-l—2—1 through -10.—Core run is homogeneous buff well-indurated vuggy limestone. 

Original rock was highly coralliferous, as cores contain abundant randomly oriented 

molds of finger coral as much as 1 inch in diameter and 4 inches in length. Many 

smaller molds of delicate Seriatopora are present. Much coral shows thick encrustations of laminar algae. Molds of broken and whole gastropods and pelecypods as much as 1 inch in length are common. This fossil debris is embedded 

in a hard fine-grained matrix. Many original voids, such as the interior chambers 

of gastropods, are entirely filled with coarse granular calcite. Drusy coatings of 

light yellow calcite occur throughout the rock. Thin sections of F-l-2-1 show 

the matrix to be a dense sandy mud that contains abundant tests of Amphistegina 

with lesser amounts of benthonic Foraminifera, fragments of encrusting Foraminifera, rare echinoid spines, articulate coralline algae, and other unidentifiable 

organic debris. Coral is shown only by virtue of the persistence of mud-filled 

interseptal spaces, because the original aragonite corallum has been dissolved. Irregular patches of calcite mosaic spread through the original mud matrix and impart a grainy texture. The matrix of F-l-2-9, as seen in thin section, is an Amphistegina sand. These originally calcitic tests retain a fine fibrous structure that 

shows pseudonuiaxial crosses in plane polarized light; original aragonitic shell 

material is represented only by molds or granular secondary calcite. Bulk X-ray 

analyses of both F-l-2-1 and F-l-2-9 showed 100 percent calcite (table 2). 

Chemical analyses of these two samples are given in table 1.

Interpretations-.—The large amounts of drusy calcite and the recrystallization of the 

mud matrix coupled with the solution of all the aragonite fossils indicate that this 

interval underwent a prolonged period of subaerial solution and redeposition of 

carbonate. Cole (1957, p. 745) reported no diagnostic larger Foraminifera from 

this core run, but Todd and Low (1960, p. 805; table 3) reported several genera 

of miliolids, two species of Elphidium, and one genera each of Homctrema, Planorbulina, Calcarina, Marginopora, and Amphistegina. These smaller Foraminifera 

were found immediately above and below the cored interval in cuttings. The 

foraminiferal assemblage, the abundance of branching coral and mollusks, and the 

poor sorting indicate a lagoonal environment unaffected by strong currents. 

Cuttings are composed entirely of irregular fragments of soft moderately lithified, 

buff-colored limestone as much as 1 inch in length. Most of the pieces are rich 

in sand-size fossil remains. The rock chips contain disseminated irregularly 

distributed anhedral finely granular calcite. Clusters of sparry calcite are common. 

Some chips show pieces of pelecypod shells, and pieces of coral are present. 

Staining by Meigen's solution of cuttings from 630 to 640 feet reveals a few percent 

of primary aragonite.

Interpretations.—As in the cored interval directly above, the 625- to 650-foot interval 

appears to have undergone considerable solution of primary aragonite and chemical 

deposition of calcite. However, this 25-foot interval grades downward into little 

altered aragonitic sediments, which start at 650 feet (see description below).
SUBSURFACE GEOLOGY OF ENIWETOK ATOLL 1023

Interval (feet) 

650-970

Drilling method

Rock bit- . _ _

Recovery 

Cuttings.

970-1,040 

1, 040-1,230

Rock bit. _ _ _ 

Rock bit____

No recovery. 

Cuttings

DRILL HOLE F-l—Continued

Remarks

Cuttings from 650 feet to about 700 feet are transitional between the above described 

interval and the cuttings below 700 feet. This 50-foot section is made up of 

angular rock chips, like those found in the 630- to 650-foot interval, and large 

pieces of coral. The coral is unaltered, and several genera are represented, including massive types that are present only as angular chips, delicate branches of 

Seriatopora, and thick finger-shaped pieces as much as \l/z inches in length of 

other branching types. Some are encrusted with coralline algae. Unaltered 

angular fragments of both large and small mollusk shells are present but form 

only a minor part of the sediment. Approximately 80 percent of the material is 

identifiable as coral. The interseptal spaces of these are open, and little deposition 

of calcite has taken place. From 700 to 720 feet the percentage of rock chips 

diminishes rapidly, and below this depth the cuttings are coarse fragments of coral 

and mollusks. The content of molluscan debris varies throughout this part of 

the interval, but shells never make up more than 15 to 20 percent of the cuttings. 

Single pieces of finger-shaped coral fragments attain lengths of 2 inches and diameters of one-half inch. The color of the cuttings gradually darkens from a light 

gray to a brownish tan at 810 Teet and from there down remains constant. Chips 

of blue Heliopora are scattered throughout the section. No significant alteration 

of the fossils beyond darkening in color nor recrystallization or calcite deposition 

was noted. Many of the samples contain a small percentage of fine cuttings that 

range from fine sand to fragments one-quarter inch in diameter. Much of this 

is broken coral and molluscan debris, but individual well-preserved tests of 

benthonic Foraminifera are common. Also present are chips and flakes of soft 

black carbonaceous material; these are porous, and some look like burned pieces 

of wood. When split, the black chips show a brown color on fresh surfaces. The 

drilling time chart shows a uniform high rate of penetration through this whole 

interval.

Interpretations.—The lack of lithification, of chemical calcite, and of alteration of 

aragonite indicate that these sediments were never subjected to subaerial solution. 

The transition downward to unaltered material from the thoroughly recrystallized 

core (no. 2) taken between 600 and 625 feet shows that the effect of subaerial 

exposure diminishes gradually, rather than abruptly, below the exposed section. The very high content of coral, including the delicate branching types, 

indicates that this entire interval was deposited in shallow water, probably in a 

lagoon.

There are marked differences between cuttings from this interval and those of the 

650- to 970-foot interval. From 1,040 to 1,230 feet the cuttings are white and 

fine; most of the material is sand and granule size. Large pieces of coral are 

lacking. From 1,040 to 1,060 feet the cuttings contain approximately equal amounts 

of recrystallized dense limestone chips and unaltered fossils debris including abundant Halimeda fragments, small gastropods and pelecypods, miliolid and other 

bethonic Foraminifera, and coral. A stained sample from 1,040 to 1,050 feet 

showed abundant primary aragonite. From 1,080 feet downward the ratio of 

rock chips to unaltered fossils increases, and below 1,100 feet the cuttings are all 

rock chips. Many rock chips show abundant Foraminifera in a finely crystalline 

matrix. Aggregates of sparry calcite are common, as are drusy coatings of calcite. 

Most of the chips are angular, which indicates that solid limestone was penetrated 

by the drill.

Interpretations: Circulation was lost from 975 to 1,045 feet, and no cuttings were 

recovered in this 70-foot interval. Evidently there was a change in lithology from 

unconsolidated sediments composed of large coral fragments to sediments rich in 

Halimeda, small mollusks, and miliolid Foraminifera. Further, the abundant chips 

of recrystallized limestone in the 1,040- to 1,050-foot interval show that by this 

depth the drill had struck recrystallized rock. Therefore, there is a solution 

unconformity within the 975- to 1,040-foot interval. Cuttings from the 1,040- to 

1,230-foot interval are evidently contaminated by postunconformity unaltered 

sediments for some distance below 1,040 feet. It is possible that the unaltered 

sediments in the 1,040- to 1,060-foot interval were laid down on and filtered into 

a base of rock that had been previously uplifted above sea level and recrystallized. 

Limestones thus exposed characteristically develop a rough and porous upper surface. Therefore, drilling through the top of the recrystallized zone might recover
1024 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

Interval (feet) Drilling method Recovery 

1,040-1,230 Rock bit.______ Cuttings..__

1230-1248 Core barrel.. 11 ft (core 3), 

26 pieces

DRILL HOLE F-l—Continued

Remarks

younger sediments within the older rock. Upon resubmergence, the younger sediments, including those from the 630- to 970-foot interval have remained below 

sea level. The abundance of Halimeda, mollusks, and miliolid Foraminifera in 

the 1,040- to 1,050-foot interval indicates that these sediments deposited directly 

on the recrystaiiized base were lagoonal in origin. The site of deposition of the 

older rock is difficult to interpret because of the small size of chips. However, 

Todd and Low (1960, p. 815) believe that this section of hole F-l is of shallowwater origin.

The 26 pieces of core range in length from 2 to 8 inches. The most striking megascopic feature is the variation in porosity through the run. Pieces 1 through 4 

show only small unconnected vugs with a few coral molds. From piece 5 through 

piece 16 the pore spaces become more numerous, fretted, and interconnected. 

Many of these features are due to solution of corals, rotted impressions of which 

are still visible. From piece 17 through piece 26 the pores are again more regular 

openings as much as 1 inch across. They appear to be solution channels rather 

than molds. Representative pieces are described below.

F-l-3-3.—White dense slightly vuggy limestone. Sawed faces show abundant 

Halimeda segments and other fossil debris solidly set in a hard-mud matrix. Vugs 

show thin coatings of drusy calcite. A small part of the rock is finely porous owing to coral molds now represented by interseptal spaces partly filled with calcite. 

Sawed surfaces show a splotchy pattern of gray, white, and light buff because of 

differences in skeletal texture and in degree of crystallization of the matrix. Thin 

sections show a poorly sorted matrix of closely packed Halimeda segments; rare 

tests of miogypsinids; lepidocyclinids, common encrusting Foraminifera; articulate 

coralline algae; and unidentifiable fossil debris. The spaces between the fossil 

fragments is in part filled with "mud" and in part with microgranular calcite. 

Between the fossils and the final void-filling calcite are two fine laminar encrustations of calcite.

F-l-S-4-—As above, except at bottom of piece where most primary voids between 

the packed fossil debris are open. Here the fossils are cemented by a thin layer 

of calcite that laps over from one fragment to another and gives the rock a pelletlike texture. Thin sections show thick laminae of encrusting algae and completely 

recrystaiiized coral that shows foraminiferal encrustations in addition to debris 

similar to that in F-l-3-3. A bulk X-ray analysis (table 2) of this piece showed 

98 percent calcite and a trace of dolomite. No dolomite was identified in thin 

sections. A chemical analysis is given in table 1.

F-1-3-10.—Extremely porous and vuggy owing to many molds of coral and smaller 

fossils and incomplete filling of interstitial spaces by matrix. Thin sections show 

that the high porosity is in part due to the removal by solution of originally abundant 

Halimeda segments. Ghosts of Halimeda show up as dark fine outlines of original 

segments that are now partly packed by microgranular calcite and fringed by 

acicular calcite. The ragged and patchy appearance of the calcite fillings indicates 

further solution subsequent to initial solution of original aragonite and recrystallization. The acicular fringes of calcite on and in the Halimeda and other fossils 

are postsolution deposits. The matrix of this sample is a miliolid and corallinealgal fine sand in a partly recrystaiiized mud. The miliolids are well preserved and 

generally have fillings of granular calcite. Coral in these sections shows up as 

mud-filled interseptal spaces, now recrystaiiized into dark granular calcite, fringed 

by acicular and granular calcite. A bulk X-ray analysis showed 100 percent 

calcite; no dolomite was seen in the thin section. A chemical analysis is given 

in table 1.

F-l-3-18.—Large vugs, as much as 2 inches in length and irregular in outline, are 

well developed in this piece. Vugs are lined with botryoidal masses of laminated 

calcite. Three megascopically conspicuous layers, each a fraction of a millimeter 

thick are usual. Generally the layer nearest the matrix is clear, the central layer 

is white, and the final layer is clear light-yellow sparry calcite. The vugs ramify 

through the rock and are solution channels rather than fossil molds.

F-l-3-20.—Hand specimen as above. A thin section through a vug shows six distinct laminae of calcite (pi. 282A) that vary in thickness and degree of "dustiness." 

In plane polarized light the laminae are seen to cut across sheaflike calcite crystals 

that are oriented perpendicular to the bands; these crystals show optical continuity from the fossil base out through the sixth layer. Thus the bands merely 

represent interrupted crystal growth. The matrix of this piece, like F-1-3-10, has
SUBSURFACE GEOLOGY OF ENIWETOK ATOLL 1025

Interval (feet) Drilling method Recovery

1, 248-1, 718 

1, 718-1, 740

Rock bitCore barrel. _

No recovery.

10 ft. 6 in. 

(core 4), 

24 pieces.

DRILL HOLE F-l—Continued

Remark*

abundant miliolid tests, although fragments of encrusting algae are the most prominent constituent. Bulk X-ray analysis showed 100 percent calcite. However, a 

thin section etched and stained with copper nitrate revealed rare dolomite crystals 

in one of the vug linings. The dolomite is concentrated in the central layers of 

the set of calcite laminae. The dolomite has a tendency to form a band, but some 

of it is in the form of disconnected clusters of rhombs. The writer is unable to tell 

whether the dolomite is a primary precipitate, such as that described by Cullis 

(1904, p. 392-420) from Funafuti, or has developed by replacement of calcite. A 

chemical analysis of the specimen is given in table 1.

F-l-3-26.—This lowermost piece is like those above it but has fewer well-developed 

vugs and calcite fillings. No dolomite was seen in thin sections.

Interpretations.—The distribution of solution vugs through this interval indicates 

that these rocks have undergone prolonged subaerial solution. The repeatedly 

interrupted deposition of calcite in the vugs suggests that the rock was in a cyclically changing environment, possibly at or near sea level. The traces of dolomite 

within enclosing calcite laminae suggests that either primary dolomite deposition 

or conversion of calcite to dolomite took place. Whichever of these processes 

occurred, the dolomite formation was probably a submarine, or at least intertidal, 

phenomenon. The miliolid-rich matrix, the abundance of Halimeda, coralline 

algae, and the coral content point to deposition in a lagoon?! environment. The 

presence of Miogypsinoides dehaartii (Cole, 1957, p. 747) suggests a shallow-water 

deposit. On Guam, the writer (Schlanger, 1963) found that in limestones of 

comparable age this foraminifer was largely restricted to reef and shallow water 

limestones.

Cores taken in this run exhibit striking differences in texture and fossil content. 

Representative pieces are described below.

F-1-J+-2.—Tan porous well-cemented and sorted limestone composed of tests of 

Foraminifera as much as 4 mm across but averaging about 1 mm; a coarse-grained 

calcarenite. Thin sections show tbat the rock is almost completely an aggregate 

of Rotalia tests (pi. 286A) with subordinate amounts of articulate coralline algae, 

a few large test of Lepidocyclina and Heterostegina, and rare pebbles of rounded 

coral fragments; a few miliolids are present. This fossil debris is cemented by a 

single thin layer of granular to acicular calcite. The texture is strikingly similar 

to beach rock. The primary voids between fossils are open; mud matrix is completely lacking. The internal chambers of the Foraminifera are open but lightly 

lined with acicular calcite perpendicular to the chamber walls. The algal debris 

is the same size as the dominant Rotalia tests. A bulk X-ray analysis of this piece 

showed 100 percent calcite. Spectrographic and chemical analysis are given in 

tables 1 and 3, respectively.

F-l-4-8 through -2J+.—The foraminiferal limestone described above grades through 

piece 3 into dense coral-rich rock that makes up most of the interval. Instead of 

being finely porous, these lower pieces are dense owing to a large amount of lithified 

"mud" that forms the matrix; and large voids are numerous owing to the solution 

of abundant delicate branching corals, many of which appear to be in growth 

position. Some of these molds have been lined with drusy calcite since the solution 

of the coral. Irregular vugs are also common. Pieces 4 and 5 have parted along 

a steeply dipping fissure that is largely lined with a fine film of drusy calcite. Thin 

sections show the matrix of the rock to be a foraminiferal sand, as in piece 2, but 

with the addition of interstitial "mud" and thicker clear calcite. A thin section 

of piece 10 shows the matrix to be irregularly distributed; small patches as much as 

5 mm across lack a mud fill and are cemented only by calcite films. The mud 

itself has been partly recrystallized to a calcite mosaic. Corals are generally coated 

with algae. In a few places the coral has been crosscut by irregular solution voids; 

some of these voids in pieces 10 and 20 show extremely fine laminae of alternating 

clear and dusty calcite. Tests of Heterostegina and Lepidocyclina become more 

abundant in deeper cores, and the grain size shows a general increase to where much 

of the rock is made up of particles 2 mm or more in diameter. A thin section of 

piece 24 shows abundant randomly oriented tests of Lepidocyclina as much as 5 mm 

in diameter; these make up about 5 percent of the rock. As above, the interstices 

between the still abundant Rotalia tests are irregularly choked with recrystallized 

mud. Chemical and mineralogical analyses of piece 10 are given in tables 1 and 2, 

respectively.
1026 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

Interval (feet) 

1, 718-1, 740

Drilling method 

Core barrel__

Recovery 

10 ft. 6 in. 

(core 4), 

24 pieces.

1, 740-1, 978 

1, 978-2, 003

Rock bit____ 

Core barrel __

No recovery. 

20 ft 6 in.

(core 5), 41

pieces.

DRILL HOLE F-l—Continued

Remarks

Interpretations.—The solution voids and the la,minar calcite in the coral indicate that 

this interval, like that from which core 3 was taken, underwent a period of subaerial 

solution and cyclic calcite deposition. The sorting and lack of mud matrix in the 

upper pieces suggest that these foraminiferal sands were laid down under the 

influence of a strong current. The abundance of Rotalia tests leads the writer 

to compare these pieces to the equally Rotalia-nch limestone of Miocene age from 

Guam (Schlanger, 1963) which were closely associated with reef and shallow-water 

limestone. The delicate branching coral associated with the foraminiferal rock in 

the lower part of the interval suggests quiet-water conditions; the abundant mud 

matrix in these coralliferous cores corroborates this. Thus the lower part of the 

interval was probably deposited in shallow water that was generally quiet but 

which was influenced by currents that bore sorted debris into the possibly reefprotected area.

Cores from this interval are all very dense white limestones that, taken as a whole 

consist of a framework of large reef-type corals, between which is packed a formainiferal-algal sand in a fine matrix. Several cores are composed entirely of 

completely recrystallized massive coral, much of which still shows open interseptal 

spaces that give the rock a locally high porosity. Much of the coral is coated with 

thick winding laminae of encrusting algae that are strikingly well developed in 

this interval. Algal coatings also serve as a cement for a large part of the rock. 

Many foraminiferal tests and other fossil debris are enveloped by algal laminae. 

Gravel-size fossils, such as tests of larger Foraminifera, echinoid spines, mollusk 

shells, and algal debris, are randomly packed between algal nodules and coral; 

primary voids between this coarse debris are largely open, as the fossils are cemented 

by drusy calcite. Large sections of core consist of irregular pockets of well-sorted 

foraminiferal-algal sand; the majority of particles range fro'm 1 to 2 mm in diameter. 

Intergranular porosity in this material is low owing to the abundant fine matrix. 

Striking features of this interval are the well-developed solution channels that 

commonly pierce the core and show openings as much as 3 inches in diameter at 

the core surface. These ramify through the rock and are invariably lined by thin 

coatings of yellow drusy calcite; the surfaces of the vugs show a botryoidal structure. Bulk X-ray analyses of pieces F-l-5-3, F-l-5-9, F-l-5-26, and F-l-5-41 

all showed 100 percent calcite. Chemical analyses of F-l-5-3, F-l-5-26, and 

F-l-5-41 and a spectrographic analysis of F-l-5-26 are given in tables 1 and 3, 

respectively. Descriptions of selected thin sections are given below.

F-l-5-3.—Composed largely of subrounded fragments, mostly 1 to 2 mm in diameter, 

of both articulate and encrusting coralline algae, in a matrix of dark mud and 

fine-sand- to silt-size fossil debris. A small percentage of broken and whole tests 

of Lepidocyclina as much as 6 mm in diameter are present. However, one thin 

section is made up entirely of » single piece of recrystallized, massive coral. The 

algal-rich section shows numerous irregular voids lined with calcite. Some of 

these have the shape of pelecypod shells but may represent solution channels.

F-l-5-6.—This section was cut from a pocket of matrix between coral and algal 

deposits. It is dominated by closely packed whole tests of Rotalia; minor amounts 

of subrounded coralline algal debris, a few broken tests of Lepidocyclina and Heterostegina, and echinoid spines are present. The Rotalia tests, coralline algae, and 

spines are almost all between 1 and 2 mm in diameter; the fossils are well sorted 

and the rock has an intact framework. Many of the original intertest voids are 

filled with dark "mud." This matrix is partly recrystallized owing to the growth 

of rims of acicular calcite on the outer surfaces of the Foraminifera tests. In 

places these acicular rims have coalesced completely and replaced the dark "mud." 

The calcite overgrowths are in optical continuity with the original tests; in plane 

polarized light very irregular pseudouniaxial crosses extend from the test wall out 

through the overgrown rim. In some places the "mud" has recrystallized to a 

granular mosaic. In a few places a mud matrix is lacking, and the cement consists 

only of acicular calcite rims on fossils. In these areas interfossil voids are present. 

The chambers of the Foraminifera, however, are filled with granular calcite that is 

clearer and coarser than the recrystallized matrix. The writer thinks that these 

tests were empty of sediment when deposited and were later filled with a chemical 

deposit of calcite. The intact framework suggests that the tests were deposited 

as a coquina that was later filled by fine mud, possibly after some cementing calcite
SUBSURFACE GEOLOGY OF ENIWETOK ATOLL 1027

DRILL HOLE F-l—Continued

Interval (feet) Drilling method Recovery Remarks

was deposited; the mud fill has been partly recrystallized. This rock strongly 

resembles F-l-4-2.

F-l-5-9.—Sections from this piece are largely recrystallized coral that shows welldeveloped partly recrystallized dark sandy mud as interseptal fillings. The edges 

of the coral have encrustations of both Foraminifera and algae. One section 

(pi. 286.B) shows what appears to be a worm boring through a coral. The tube 

is filled with two types of deposits; a thin outer shell, adjacent to the coral, of fine 

sand and mud containing abundant angular fossil fragments, and a final filling of 

coarse sand made up largely of Rotalia tests and subrounded coralline algal debris 

in a mud matrix.

F-l-5-16.—Section cut through a coral and enclosing foraminiferal-algal sand. 

Half the section is well cemented closely packed tests of Rotalia and rounded 

fragments of coralline algae. This debris is very well sorted; almost all fragments 

are about 1 mm in diameter. Much of the algae is in the form of slightly elongated rods. These show a tendency toward preferred orientation (pi. 286C); the 

section illustrated is cut perpendicular to the core axis. The matrix consists of 

cementing laminae of alternate dark and clear calcite which culminate in a final 

pore-filling deposit of granular calcite. In some interfossil voids the cloudy appearance of the matrix suggests that some "mud" may have been present. The 

coral, which is completely recrystallized, shows irregular solution channels that 

are now lined with five layers of alternate clear and dark calcite. The sequence 

of laminae is identical in each channel; from the coral base out there is a thick 

layer of clear coarse calcite, a thin very dusty layer, a very thin clear layer, a 

second dusty layer, and a final clear layer. All the layers are in optical continuity. 

In addition to the definite laminae, there are several fine lines parallel to the laminae 

that indicate short hiatuses in crystal growth.

F-l-5-28.—Although not strictly representative, this section is used (pi. 286D) to 

illustrate the complex sequence of solution and recrystallization that affect both 

the corallum and the interseptal fillings of a coral. A description is given in the 

figure caption.

F-1-5-S5.—Poorly sorted conglomerate composed of whole and broken tests of 

randomly oriented Lepidocyclina, gravel-size coralline-algal debris, and rounded 

pebbles of coral in a matrix of sand-size foraminiferal-algal debris and dark mud 

(pi. 287A).

F-l-5-41.—This section shows an unusual association of larger Foraminifera, chiefly 

Lepidocyclina, in a matrix of laminar encrusting algae (pi. 2875). Individual 

tests are enveloped in a complex net of the algae, which show that these tests 

evidently once lay on an algal mat long enough to be cemented by further algal 

growth. The algae also cement fine fossil debris and mud. Lesser numbers of 

encrusting Foraminifera are present.

Interpretations.—As in cores 2, 3, and 4, solution, recrystallization, and chemical 

deposition of calcite have been operative in this interval and indicate a period of 

subaerial solution and movement of material through the rock. Limestones from 

core 5 bear a strong resemblance, with regard to fossil composition, texture, and 

postdepositional changes, t6 limestones of Miocene e age from Guam (Schlanger, 

1963). The Guam rocks are reef and fore-reef limestones that have been exposed 

to subaerial solution at least once in their history. The rounding of the algal 

debris, the good sorting, and the intact framework of the foraminiferal-algal sand 

between the coral indicate that currents played a part in deposition of much of the 

rock. The massive coral framework is indicative of reef or near-reef conditions. 

Some pieces of core, such as F-l-5-35, show poor sorting and a conglomeratic 

texture suggestive of rapid deposition or trapping of sediment by the coral. The 

algae-enmeshed Lepidocyclina of F-l-5-41 show rapid growth of these plants. 

Thus this interval was probably deposited in a shallow reef or near-reef environment influenced by strong currents.

2,003-2,130 Rock bit____ Cuttings---- Cuttings from the 2,000-to 2,020-foot interval are heterogeneous granule-size debris

containing fresh wood chips, flakes of iron, unaltered Halimeda fragments, gastropod shells, and tests of benthonic Foraminifera, as well as recrystallized rock 

chips. The two samples appear to be heavily contaminated because of caving. 

They are not considered representative of the interval and are disregarded.

Below 2,020 feet the cuttings are all very angular flakes and chips of dense creamcolored recrystallized limestone. The size of the cuttings varies, but the lithology
1028 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

Interval (feet) 

2, 003-2, 130

Drilling method Recovery 

Rock bit___ Cuttings____

2, 130-2, 662 

2, 662-2, 687

Rock bit____ 

Core barrel...

No recovery. 

16 ft 5 in.

(core 6), 30

pieces.

DRILL HOLE F-l—Continued

Remarks

appears uniform. From 2,020 feet through 2,050 feet most of the chips range from 

}i to /z l inch across; below 2,050 feet the cuttings become finer, and at 2,130 feet 

most are between }i and % inch across. By looking at the large chips while they 

are under water in a shallow dish, the original textures are clearly seen. Many 

chips are from massive completely recrystallized coral; interseptal spaces are filled 

with dense recrystallized mud. Other chips show laminar deposits of calcite lining 

old solution channels. A split of cuttings from 2,110 to 2,120 feet was stained with 

Meigen's solution, but no trace of aragonite was seen.

Interpretations.—These cuttings strongly resemble those described from the 1,978- to 

2,003-foot interval (core 5), and there is probably no significant change in sediment 

type or postdepositional history from the top of the cored interval to 2,130 feet.

The lithology of this interval of rock is fairly constant throughout. The rocks are 

white very dense and well-cemented slightly vuggy limestones rich in coral, encrusting algae, and tests of larger Foraminifera. Some of the coral shows open 

interseptal spaces and small pockets of larger Foraminifera tests that lack a matrix; 

these give the rock local porosity. Large irregular vugs lined with botryoidal 

layers of calcite are present but are not as well developed as those in core 5. Most 

of the rock has very low porosity owing to the well-lithified mud matrix and the 

closing of many small solution channels by chemical calcite deposits. In general 

the interval is conglomeratic and poorly sorted. Sawed faces show pebble size 

irregular fragments of algae-encrusted coral, encrusting algae, and echinoid spines 

and plates in a matrix of foraminiferal sand and mud. The bulk of the rock is 

coarse-sand- and gravel-size tests of Foraminifera and other fossil debris. Todd 

and Low (1960, p. 806) report that smaller Foraminifera from this core are abundant 

and varied; they include bolivinids, globigerinids, amphisteginids, and biserial and 

uniserial arenaceous benthonic types. Cole (1957, p. 748) reported Borelis, 

Heterostegina, and two species of Lepidocyclina from this interval. Bulk, X-ray 

analyses of samples F-1-6-1, F-l-6-9, F-l-6-23, and F-l-6-30 all showed 100 

percent calcite. Material scraped from a vug lining in piece F-l-6-9 was reported 

to contain less than 2 percent dolomite. A spectrographic analysis of F-l-6-30 

and chemical analyses of F-l-6-1 and F-l-6-9 are given in tables 1 and 3, respectively. Descriptions of representative thin sections are given below.

F-l-6-1.—Abundant broken and whole tests of Heterostegina packed in a poorly 

sorted matrix of partly recrystallized mud and sand. The sand-size fraction is 

rich in tests of smaller Foraminifera, which include very well preserved globigerinids 

and benthonic types, rounded fragments of coralline algae, pieces broken from tests 

of larger Foraminifera, fragments of encrusting Foraminifera, and unidentifiable 

fossil debris. Pebbles of recrystallized coral are present. Irregular solution channels that truncate fossils are almost entirely blocked with laminar deposits of clear 

and dark calcite. In one vug, at least nine distinct layers are visible. The biconvex tests of the larger Foraminifera show a high degree of parallelism. This 

type of orientation was also noticed in fore-reef limestones from Guam (Schlanger, 

1963).

F-l-6-9.—In this section, subparallel tests of Heterostegina dominate and are 

packed in a matrix similar to that described in F-l-6-1. Coral pebbles here are 

distinctly rounded. One solution channel (pi. 287C) evidently formed by solution 

of the matrix around the tests of larger Foraminifera. A few of the tests have 

been almost freed from the original matrix. Following the solution, the void was 

completely filled with chemical deposits of varying thickness, opacity, and grain 

size. It was material scraped from a void like this in F-l-6-9 that showed dolomite 

in the X-ray analysis. No dolomite was identified in thin section so it is not 

known whether any single lamina is dolomitic or whether the dolomite is scattered 

through the void filling.

F-l-6-23.—The most prominent fossils here are well-preserved tests of Lepidocyclina. These large tests are randomly oriented in a matrix of fine sand and mud 

made up of smaller Foraminifera that include some globigerinids, amphisteginids, 

and serial types; fragments of coralline algae are abundant, as are pieces of encrusting Foraminifera. Tests of Heterostegina, so abundant directly above, are 

lacking. Pebbles of mud-filled coral are common; the mud in these pebbles is 

similar to the matrix of the rock. As in the above pieces, irregular calcite-filled 

solution channels are present.
SUBSURFACE GEOLOGY OF ENIWETOK ATOLL 1029

Interval (feet) Drilling method Recovery

2. 687-3, 052

3. 052-3, 055

Rock bit- _ _ . 

Core barrel._

No recovery. 

3 ft (core 7),

many small

pieces.

DRILL HOLE F-l—Continued

Remarks

F-l-6-30.—The lower part of this piece is a solid mass of completely recrystallized 

coral that is recognizable in thin section only by virtue of fine dark dust lines 

(pi. 287D) which form a jigsaw pattern that represents the original interseptal 

spaces. The corallum has been dissolved, and the resultant space has been filled 

with fine granular calcite. Thin crooked partly open veins traverse the coral; 

these are lined with acicular calcite.

Interpretations,—As in cores 3 through 5, evidence for subaerial solution is abundant 

in this interval. The presence of relatively abundant tests of well-preserved 

globigerinids suggests that these limestones accumulated in water deeper than 

that associated with reef limestones. The rounded pebbles of coral and the broken 

condition of many of the larger Foraminifera are suggestive of transported material. These are features also found in fore-reef limestones of Miocene e age 

from Guam (Schlanger, 1963). Thus much material from shallow water was 

probably swept down into deeper, fore-reef areas before final deposition.

Cores from this interval are irregular pieces of chalky white friable limestone composed of sand-size fossil debris packed in a soft chalky matrix. Pieces from the 

upper part of the run, such as F-1-7-3, when wetted, and only after hard rubbing, 

break down into a coarse sand with some gravel-size pieces of coral. The pieces 

from the lower part of the run disintegrate easily, when wetted, into a gritty 

mixture of sand, gravel, and greasy white mud. No solution channels are present, 

and, on megascopic inspection, no chemical deposits of calcite cement were seen. 

On a broken surface, rounded sand-size grains, most of which are tests of smaller 

Foraminifera, project from the matrix; the rocks break around the fossils rather 

than through them. This core represents a distinct lithologic change from the 

well-cemented rocks recovered in core runs 3 through 6. Todd and Low (1960, 

p. 806) reported a rich assemblage of globigerinids with minor amounts of small 

benthonic types from this interval; Cole (1957, p. 749, table 4) reported only 

abundant tests of Heterostegiha saipanensis Cole. Bulk X-ray analysis of piece 

F-1-7-3 and a combined sample of pieces F-1-7-6 and F-1-7-7 showed 100 

percent calcite. Chemical and spectrographic analyses of these samples are given 

in tables 1 and 3, respectively. Description of thin sections follow.

F-l-7-8.—The slight degree of cementation shown by this piece facilitated the 

making of a good thin section. This shows sand-size fragments of coralline algae, 

worn and broken tests of larger Foraminifera, and well-preserved tests of globigerinids in a sandy mud matrix. Echinoid debris is also present. A small percent 

of the algal and foraminiferal particles reach a size of 4 to 6 mm. The mud matrix 

shows a slight degree of alteration to finely granular calcite. Some of the fossils 

that project into irregular pores are coated with thin films of granular and acicular 

calcite that form the cementing material (pi. 2&7E). No vugs filled or lined 

with laminar calcite deposits are present. This is in sharp contrast to such 

features seen in cores 3 through 6.

F-l-7-6 and -7.—This material broke down during cutting, and only small chips 

could be mounted for sectioning. Most of these pieces appear as masses of unrecrystallized dark mud containing scattered tests of larger Foraminifera and algal 

debris as well as tests of globigerinids. This mud is largely unlithified, as shown 

by the smearing out of many pieces and the rapid disintegration in water. A few 

pieces are made up of coral pebbles. This coral is comparatively well-preserved; 

the original corallum still shows as a light-brown finely fibrous structure. The 

interseptal spaces are open but lightly lined with clear granular calcite. The 

coral appears unaltered; however, staining of other coral pebbles with Meigen's 

solution shows the coralla to be calcite. Also the bulk X-ray analysis of this piece 

of core showed 100 percent calcite. Rounded and broken tests of Heterostegina 

are common. The chambers and edges of these have thin films of granular calcite.

Interpretations.—The unconsolidated nature of much of this material suggests that 

the rocks were not subjected to any prolonged period of subaerial solution. However, the change of the originally aragonite corals to calcite shows that some alteration took place. The appearance of these corals in thin section suggests that the 

original aragonite was dissolved prior to the deposition of calcite. These actions 

did not reach completion inasmuch as the original skeletal structure is still distinct. 

The abundant globigerinids in the cores indicate, according to Todd and Low 

(1960, p. 815), a depth of deposition of not less than 100 fathoms. The abundant 

rounded coralline-algae debris and the coral pebbles show that material from
1030 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

Interval (feet) 

3, 052-3, 055

3, 055-3, 350 

3, 350-3, 353

Drilling method 

Core barrel. _

Rock bit._ __ 

Core barrel _ _

Recovery 

3 ft (core 7),

many small

pieces. 

No recovery. 

3 inch (core

8) 1 piece.

3, 353-3, 655 

3, 655-3, 665

Rock bit_ 

Core barreL _

No recovery.

3 ft. 3 in. 

(core 9), 

10 pieces.

DRILL HOLE F-l—Continued

Remarks

shallow-water, possibly reef, environments was carried down into deeper water, 

where it mixed with planktonic tests that were also being deposited there.

One piece of core was recovered in this interval. The limestone is a white porous 

friable poorly sorted conglomerate. The single core contains an irregular piece 

of massive coral 2 inches in diameter. The rock has a chalky appearance and 

resembles pieces from the 170- to 190-foot interval of core 1. The dense lithified 

mud and solution channels common in cores from runs 3 through 6 are lacking 

here. Spaces between fossil fragments are largely open; drusy calcite cement is 

lacking. The bulk of the rock is coarse sand- and pebble-size foraminiferal and 

algal debris. In thin section the rock has a fretted appearance; much of the coral 

has been dissolved and replaced by a lacework of finely granular calcite (and 

dolomite?). In one slide a coral fragment still shows a trace of its original brown 

fibrous structure. The foraminiferal-algal debris, which includes many worn 

and broken tests of Heterostegina, is lightly cemented by very thin films of granular 

calcite. Patches of recrystallized dark mud partly fill some interstices between 

fossils. Well-preserved tests of globigerinids are common. The chambers of the 

Foraminifera and interseptal spaces in the coral are largely open but lined with 

granular calcite (and dolomite?). No dolomite could be positively identified in 

this thin section, but a bulk X-ray analysis of F-l-8-1 showed 98 percent calcite 

and a trace of dolomite. Chemical and spectrographic analyses are given in 

tables 1 and 3.

Interpretations.—The solution and replacement of corals in this interval and the 

minor calcite cement suggests a period of subaerial solution and chemical deposition 

of calcite. As in core 7, solution effects are minor compared to those seen in 

cores 3 through 6. Todd and Low (1960, p. 815) interpret the abundant globigerinids in this interval as indicating deposition at a depth of not less than 100 fathoms. 

The coarse coral and algal debris is probably shallow-water or reef-derived material 

that was swept down to greater depths, where it mixed with deeper sediments 

represented by the planktonic Foraminifera.

Cores from the upper part of this interval differ markedly from those recovered in 

the lower part with regard to texture, degree of cementation, and fossil content.

F-l-9-2 and -3.—These pieces are representative of the upper part of the interval 

and strongly resemble the piece recovered in core 8. They are white chalky friable 

porous foraminiferal-algal conglomerates that contain a few pieces of rounded coral 

as much as one-half inch across; most of the fossils are from 1 to 4 mm in diameter. 

The size of the fossil debris varies, and there is one lens in which the dominant 

size is approximately 4 to 6 mm. Surrounding this illdefined lens is rock composed 

of dominantly sand-size fossils. Solution channels and drusy calcite cement are 

lacking. Individual Foraminifera are easily picked out of the soft matrix. The 

rock disintegrates readily in running water. A bulk X-ray analysis of F-l-9-2 

showed 100 percent calcite. A chemical analysis is given in table 1. A thin 

section of F-l-9-2 shows the dominant fossils to be broken and worn tests of 

Heterostegina, rounded fragments of coralline algae, and well-preserved tests of 

globigerinids. Minor amounts of echinoid debris, fragments of mollusk shells, 

recrystallized coral, and tests of benthonic Foraminifera are present. Most of the 

interstices between the fossils are open and the cement is a fine layer of dusty 

granular calcite. In addition to the individual fossils, there are numerous subrounded pebbles of dense well-cemented recrystallized rock containing coral, laminar 

encrusting algae, and foraminiferal debris in a dense mud matrix.

F-l-9-6, 8 and -10.—These pieces are typical of the lower part of the interval. 

They are white dense but vuggy well-cemented poorly sorted conglomerates containing fragments of coral as much as 1 inch across and a single Pecten valve 2 

inches long. Molds of small fossils are abundant, and the coral has been replaced 

by calcite. The Pecten shell, made of calcite, appears to be relatively unaltered. 

Much of the cementing material is white drusy calcite; this material also lines 

some of the vugs and fossil molds. Thin sections of pieces F-l-9-6 and F-1-9-8 

show abundant molds of Halimeda segments, corals, mollusks, and even coralline 

algae (pi. 288^4). These molds are responsible for the fine porosity shown in these 

pieces. Undissolved and recognizable fossil remains include fragments of coralline 

algae, tests of smaller Foraminifera including globigerinids and benthonic types, 

and echinoid debris. The fossils and molds are randomly oriented in a recrystallized
SUBSURFACE GEOLOGY OF ENIWETOK ATOLL 1031

Interval (feet) Drilling method Recovery

3, 669-3, 963 

3, 963-3, 988

Rock bit_ 

Core barrel _ _

No recovery.

3 ft, (core 10)

11 pieces.

DRILL HOLE F-l—Continued

Remarks

mud matrix. In places the matrix has recrystallized to a clear mosaic of calciteAround many fossils the original mud has recrystallized into fibrous calcite that 

extends from the fossil edges perpendicularly into the mud. A bulk X-ray analysis 

of F-l-9-6 showed 100 percent calcite.

Interpretations.—The lithologic break in this interval resulted from a change in type of 

sedimentation. The globigerinid- and coral-bearing conglomerate in the lower 

part of the interval may have formed by mass slumping of material such as Halimeda 

and coral downslope from shallow water into a fine "mud" containing globigerinids 

in deeper water. The comparatively good sorting shown by the sediments in the 

upper part of the interval, the high percentage of broken and worn Heterostegina 

tests, and the lack of mud matrix suggest deposition by moving water. Both rock 

types were probably deposited seaward from a reef or shoal, as evidenced by the 

abundant well-preserved globigerinids. Todd and Low (1960, p. 815) interpreted 

these rocks as having been deposited in no less than 100 fathoms of water. The 

abundant fossil molds in the lower part indicate that this interval underwent some 

subaerial solution. The pebbles of rock seen in F-l-9-2 and F-l-9-3 indicate that 

some breaking up of previously lithified older sediment was going on during deposition of this interval.

F-l-10-1.—Four small pieces are included. White, friable to cemented, porous limestone composed of coarse-sand-size tests of Foraminifera and rod-shaped algal 

debris containing irregular fragments of encrusting algae and recrystallized coral. 

A few mollusk shells are present. In a few places the encrusting algae forms a 

cement for the smaller fossils. A thin section through the foraminiferal-algal sand 

shows it to be a coquina of well-preserved globigerinid tests, rounded fragments of 

coralline algae, broken and worn tests of both larger and smaller benthonic Foraminifera, and echinoid debris (pi. 2885). The globigerinid tests show up as pseudouniaxial crosses in plane-polarized light. The echinoid spines and plates show welldeveloped optically continuous overgrowths of clear clacite that extend completely 

across original interstices and form local cement. The rock has high porosity 

owing to lack of a mud matrix. The fossils are lightly cemented by thin films of 

dusty granular calcite.

F-l-10-3 through -6. These pieces are all white porous slightly friable limestone 

composed of tightly packed tests of both larger and smaller Foraminifera and 

coralline algae. The great bulk of the material ranges from 1 to 4 mm across; a 

small percentage of the algal fragments reach lengths of 6 to 8 mm. The biconvex 

lens-shaped tests of larger Foraminifera and the rod-shaped fragments of coralline 

algae show a preferred orientation such that their long axes tend to lie perpendicular 

to the core axis. On a sawed surface cut perpendicular to the core axis the elongate fragments show no preferred orientation. Thin sections of piece F-l-10-5 

show, in addition to the fossils mentioned above, abundant well-preserved globigerinids. The framework of the rock is intact, and a mud matrix is lacking. The 

fossils are cemented by thin films of granular calcite; primary voids are open. A 

bulk X-ray analysis of F-l-10-5 showed 100 percent calcite; a chemical analysis 

is given in table 1.

F-l-10-8 through -11. In these lower pieces, large fragments and, in places, cementing 

crusts of coralline algae reappear. A few molds of pebble-size coral fragnents are 

also present. The bulk of the rock is similar to pieces F-l-10-3 through -6. In 

addition to the algal cement, the sand-size foraminiferal-algal debris is well cemented 

although still porous. Locally however, patches of this sand have a dense, lithified 

mud matrix. Thin sections of piece F-l-10-11 show that in addition to the usual 

globigerinids and other fossils similar to those in the rest of the interval, completely 

recrystallized mud-filled coral fragments are common. The encrusting algae is 

evidently in place and binds a lot of slightly recrystallized mud. Irregular patches 

of the rock show a dark mud matrix. This piece is well cemented by thick intergrown rims of granular and acicular calcite.

Interpretations.—The coral molds in pieces from this interval indicate some exposure 

to subaerial solution. Cole (1957, p. 749, table 4) identified Asterocyclina, Operculina, Gypsina, and Pellatispira from this interval and concluded that the rocks 

were deposited in about 70 fathoms of water. On the basis of the planktonic foraminiferal assemblage Todd and Low (1960, p. 215) conclude that the depth 

of deposition was at least 100 fathoms. The degree of sorting, the general lack of
1032 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

Interval (feet) 

3, 963-3, 988

3. 988-4, 197

4. 197-4, 222

DRILL HOLE F-l—Continued

Drilling method Recovery Remarks

Core barrel-. 3ft. (core 10), fine matrix, and the numerous broken tests of larger Foraminifera all combine to

11 pieces. show that the sediment underwent considerable transport prior to final deposition.

The abundant rounded fragments of coralline algae and the coral pebbles were

probably shallow water contributions that were swept by currents into deeper

water.

Rock bit.___ No recovery.

Core barrel._ 11 ft., 2 in. Pieces from this interval are all white porous friable limestones that are composed 

(core 11), almost entirely of tests of both larger and smaller Foraminifera and fragments of 

37pieces. coralline algae; no coral or molluscan remains were seen megascopically. The 

fossils are closely packed and the interstices between them are largely open. The 

only lithologic variation, on a megascopic scale, is a gradual change in grain size 

from the top of the interval downward. This is due to an increase in abundance 

of test of larger Foraminifera that reach diameters of 8 mm. The lower pieces are 

composed largely of coarse-sand-size foraminiferal and algal debris that contains 

about 10 percent of larger tests. The upper pieces are almost entirely coarse-sandsize fossil debris. On freshly broken surfaces scattered dolomite crystals are recognized by virtue of sharp reflections of light from well-developed crystal faces. Bulk 

X-ray analyses of F-l-11-1, F-1-11-9, and F-l-11-37 showed less than 2 percent 

dolomite; F-l-11-18 and F-l-11-29 showed less than 8 percent. Chemical 

analyses of these same pieces are given in table 1. Cole (1957, p. 749, table 4) 

identified Asterocyclina, Fabiana, and Biplcmispira from this interval. Todd and 

Low (1960, p. 807; table 3) identified abundant Triloculina, Peneroplis, and 

globorotalids. Thin-section descriptions of representative pieces are given below. 

F-1-11-5.—The section shows an intact framework of sorted sand-size fossils dominated by rounded fragments of articulate coralline algae and a few fragments 

of encrusting coralline algae. Tests of Triloculina and other benthonic smaller 

Foraminifera are also abundant; the foraminiferal and algal debris make up at 

least 90 percent of the rock. A few broken and worn tests of larger Foraminifera 

and rare pieces of echinoid spines and plates are present. Almost all the voids 

between fossils are open, but there are rare patches of a microgranular calcite 

matrix. The coralline algae shows light-brown rims and black cores. Euhedral 

rhombs of dolomite are scattered throughout, and many of these are within or 

projecting from the rim of a piece of coralline algae. The fossils are cemented 

by thin coatings of acicular and granular calcite.

F-l-11-9.—This section is similar to F-1-11-5 except for the presence of a recrystallized mud matrix that fills about one-third of the interstices between fossils. 

Also much of the algal and foraminiferal debris appears to have been broken 

following deposition. Many fossils are cracked, and the adjoining pieces are 

slightly offset. There is a slight degree of interpenetration of fossils with the 

development of stylolitelike contacts. No thin sections from cores taken above 

this interval show such interpenetration and breakage.

F-l-11-18.—This piece is similar to F-1-11-5 and F-l-11-9 except for the presence of crude bedding caused by variation in grain size. The core piece is approximately 2 inches long. The upper inch and the lower half inch are made up of 

fossils approximately 1 mm in diameter. These layers are porous. The nearcentral half inch of the piece is finer grained and less porous. The thin section, 

cut perpendicular to the bedding, shows the finer layer to contain much mud and 

abundant small globigerinids (pi. 288c). The coarser layers are rich in Triloculina, 

which are rare to lacking in the finer layer. Further, elongate algal rods in the 

coarser layers tend to lie parallel to the bedding. Euhedral rhombs of dolomite 

are more abundant in this piece than in those taken above.

F-l-11-26.—Well-sorted porous limestone much like F-1-11-5 but showing abundant dolomite crystals. Most of the dolomite is in the form of scattered rhombs, 

but local patches of intergrown crystals form irregular mosaics. Some pieces of 

algae are almost entirely replaced by these mosaics. Tests of Foraminifera are 

almost entirely free of dolomite crystals except where these crystals encroach on 

the tests from adjacent algal fragments. However, a few tests do contain euhedral 

rhombs, and some algal fragments are free of dolomite.

F-l-11-30.—This piece differs from the above in that it contains abundant tests of 

Biplanispira and Asterocyclina. A few rounded pieces of recrystallized coral the 

same size as the larger Foraminifera tests are present. These tests show a preferred orientation parallel to the bedding. Dolomite is abundant and not partic-
SUBSURFACE GEOLOGY OF ENIWETOK ATOLL 1033

Interval (feet) Drilling method Recovery

4,222-4,316 Rock bit_..__ No recovery. 

4, 316-4, 341 Core barrel. . 8 ft. (core 12),

24 pieces.

DRILL HOLE F-l—Continued

Remarks

ularly associated with algal debris. Much of the dolomite is in a fretted microgranular matrix which looks like a recrystallized sandy mud. Patches of this 

matrix close many interfossil areas. Tests of the larger Foraminifera are generally 

free of dolomite.

F-1-11-S4-—The texture of this piece is basically similar to F-1-11-30 except that 

broken and worn oriented tests of larger Foraminifera are the dominant fossil 

(pi. 288D). Mud matrix is almost completely lacking, and the fossil debris is in 

close contact. The chambers of the Foraminifera are entirely open. Dolomite 

crystals form only a trace of this piece, and they are small and scattered. 

Interpretations.—The most striking characteristics of this interval are the vertical 

variations in grain size, matrix content, and obvious fossil orientation. These 

features give parts of the interval crude bedding. The rounding and sorting of 

the fossils and the high degree of breakage shown by the larger Foraminifera 

combine with the bedding to indicate deposition under the influence of currents 

of varying competence. Most of the material probably underwent some transport 

before burial. Association of abundant planktonic Foraminifera with the finegrained bed in F-l-11-18 suggests that this thin bed represents untransported 

material. On the basis of the smaller Foraminifera, Todd and Low (1960, p. 815) 

interpreted this interval as an outer-slope deposit. The abundant algal debris 

and the rare rounded coral fragments are shallow water contributions. The 

origin of the dolomite in this and other intervals is discussed in the body of the 

text.

The chief feature of this section is the high content and vertical variation of dolomite. 

Bulk X-ray analyses show a range from 11 to more than 98 percent dolomite. 

Analyses of pieces F-l-12-(l, 2, 3) composite, -4 top, -4 bottom, -5 top and bottom 

composite, -7 top, -7 bottom, -10, -12, and -23 are given in table 2. Chemical 

and spectrographic analyses of these same pieces are given in tables 1 and 3 

respectively. To study the distribution of dolomite better, 10 individual pieces 

were stained with copper nitrate, which coats calcite with a blue precipitate and 

does not affect the dolomite. Descriptions of individual pieces and thin sections 

are given below, and a brief discussion of relations between pieces follows.

F-1-12(1~2-S).—These are white porous friable limestones made up of tests of larger 

Foraminifera from 2 to 6 mms in diameter; coarse-sand-size debris of coralline 

algae, rare rounded pebbles of coral, rare echinoid spines, and megascopically unidentifiable fossil debris. Voids between fossils are largely open, although irregular 

patches and streaks in the rock have a fine matrix. Slight variations in grain size 

and preferred orientation of the biconvex discoidal tests of the Foraminifera impart a crude layering. The layers dip at about 22°. Clear euhedral rhombs of 

dolomite several tenths of a millimeter in diameter are scattered through the rock. 

Some of the dolomite forms irregular solid patches composed of clusters of crystals. 

A bulk X-ray analysis of a composite sample of pieces -1, -2, and -3 showed 11 

percent dolomite. Thin sections show most of the fossils to be worn, broken, 

and subangular. Much of the rock is cemented by a fretted and patchy matrix 

of recrystallized mud and granular calcite. Some of the fossils are slightly interpenetrating and show fractures and slight offsets indicative of breakage by compaction after deposition. The coral pebbles are barely recognizable owing to intense recrystallization, but dolomite crystals are abundant in the coralline algae 

and scattered through the matrix. The tests of larger Foraminifera are particularly resistant to dolomitization. Most dolomite crystals are from 0.1 to 0.5 mms 

in diameter, but there are rounded pebbles(?) of lithified mud that are hosts to 

very small dolomite crystals which float in this mud. Much of tbe fossil debris 

shows corroded edges where it projects into interstices, and many dolomite crystals 

show rounded and corroded faces in the same situation. Many voids cut across 

fossils, and this suggests that some solution and removal of matrix and fossil material took place after formation of the dolomite.

F-l-12-4.—This is a 5-inch piece of highly dolomitized limestone whose upper end 

is largely a well-indurated mixture of tan sugary dolomite and white chalky calcite. 

The relict calcite is present both as fine matrix packed between touching dolomite 

crystals and as scattered irregular patches up to 4 mm across. This part of the 

core is finely porous. Down from the upper end, the rock becomes more dolomitic
1034 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

DBILL HOLE F-l—Continued

Interval (feet) Drilling method Recovery Remarks

4, 316-4, 341 Core barrel__ 8ft. (core 12), as the white calcite thins and disappears toward the bottom. At the lower end

24 pieces. the core is a solid mass of tan sugary dolomite. Voids as much as 10 mm across 

are common. These are lined with perfect euhedral rhombs of dolomite. Many 

of these voids have the shape of larger Foraminifera and are oriented with their 

long axes parallel to the diameter of the core. The voids probably resulted from 

the solution of tests that lay in the bedding planes. Many of the dolomite crystals 

show white cloudy centers. Bulk X-ray analyses of a chip from the top of the piece 

showed 65 percent dolomite and 35 percent calcite; the bottom of the piece analysed 

at more than 98 percent dolomite and less than 2 percent calcite. A stained chip 

from the upper piece shows the dolomite to be irregularly concentrated; areas as 

much as 1 inch across are almost entirely clear, unstained dolomite. Irregular 

areas of calcite that stained blue reach lengths of one-half inch. A stained chip 

from the bottom of the piece showed no detectable calcite. A thin section of 

F—l-12-42 (top) shows a mosaic of dolomite rhombs that average about 0.3 mm 

in long dimension. In many places these are closely packed and largely subhedral 

to anhedral. Irregular voids bounded by crystal faces are common. Between 

many crystals are patches, bounded by crystal faces, of original rock. Some of 

these patches are relict mud matrix, others are relict pieces of fossils including 

coralline algae and larger Foraminifera. These are recognizable only by virtue of 

their internal structure; encroaching dolomite crystals have destroyed their outlines. This relict calcite forms the chalky spots seen megascopically. Most of 

the dolomite crystals show cloudy centers and good cleavage parallel to the crystal 

faces. Under high magnification (344X), using a petrographic microscope, the 

individual dolomite crystals show cloudy and patchy extinction owing to the 

presence of abundant shredlike bodies of calcite(?) that extinguish in various 

positions other than tha,t of the host crystal. These inclusions make up 3 or more 

percent of the dolomite crystal. A thin section of F-1-12-4& (bottom) shows a 

mosaic of anhedral even-grained dolomite crystals from 0.1 to 0.3 mm across. 

Most crystals show cloudy centers and clear edges, but there are vague streaks 

through the thin section where the entire dolomite mosaic is cloudy. A photomicrograph of F-l-12-10 (pi. 282c), which is similar to F-l-12-4, illustrates this 

texture. Relict calcite is present as small angular areas, dark in thin section, 

between dolomite crystals.

F-l-18-5.—This is a 5-inch core of massive tan sugary hard dolomite. Numerous 

disk-shaped voids less than 8 mm in diameter are present that result from solution 

of Foraminifera. A few larger voids as much as 1 by }(. inch in size are present. 

The long axes of these voids tend to parallel the diameters of the core. Clusters 

and strings of clear dolomite crystals lie in these voids. There are rare scattered 

flecks of chalk-white relict calcite. A bulk X-ray analysis of a composite sample 

composed of chips from both the upper and lower parts of the piece showed more 

than 98 percent dolomite and less than 2 percent calcite. The original texture of 

the rock has been completely destroyed except for the fossil-shaped solution voids 

that show preferred orientation. Thin sections of F-1-12-52 (top) show an evengained mosaic of anhedral dolomite with an average grain size of 0.2 mm. The 

inner halves of the grains are charged with inclusions that give the rock a dusty 

look. In general the rims of the crystals are clear. There are streaks and pods of 

very dusty anhedral dolomite; in the interstices between crystals there is relict 

calcite that looks like mud matrix or relict fossils (pi. 2825). The section contains 

many Foraminifera-shaped and minute irregular voids.

F-1-12-7.—This is an 8-inch-long piece of massive tan dolomite. The upper part is 

aphanitic dense dolomite that grades downward to sugary dolomite containing scattered flecks of chalky white calcite. Foraminiferal-shaped voids are scattered 

throughout and do not tend to lie in planes as do those in E-1-12-5. Irregular 

voids as much as l/i l inches by 1 inch in size are present; these show traces of coral 

impressions and result from solution of coral fragments. A conspicuous feature of 

this piece is a set of fine fractures that extends through the entire core and roughly 

parallel its axis. These fractures are paralleled by closely spaced hairline fractures 

that reach about one-quarter of an inch from the central fracture. En echelon 

fractures extend from the main fractures and make acute angles toward the top 

of the core of from 10° to 30°. The cracks are most distinct at the top of the piece 

and fade out toward the bottom. In a few places, small slivers that resemble 

structural horses and that are as much as one-half inch in length are enclosed 

where the main fracture splits in two and rejoins. No offsetting can be seen,
SUBSURFACE GEOLOGY OF ENIWETOK ATOLL 1035

DRILL HOLE F-l—Continued

Interval (feet) Drilling method Recovery Remarks

4,316-4,341 Core barrel.- 8ft. (core 12), but the fracturing suggests that some movement took place along these cracks,

24 pieces. probably after the rock had been dolomitized to its present dense well-lithified 

condition. Bulk X-ray analysis of the upper part showed more than 98 percent 

dolomite and a trace of calcite; the lower part contained 89 percent dolomite and 

11 percent calcite. Thin sections of F-1-12-72 show a very fine grained mosaic 

of anhedral dusty dolomite with an average grain size of 0.04 mm. Many interstices between dolomite grains are filled with dark relict calcite. Certain areas, 

less than 1 inch in width, are characterized by regularly spaced voids about 0.5 mm 

in diameter surrounded by coarser clearer dolomite (pi. 282Z)). The size and 

shape of these areas and the void distribution within them suggest that they were 

once occupied by coral fragments, now completely replaced. Many irregular 

patches of microgranular very dusty dolomite as much as several millimeters in 

diameter are present; these may be fossil "ghosts." Thin sections from the lower 

part of the core show numerous sand-size relict fossils, many of which are coralline 

algae and echinoid debris. The dolomite matrix is coarser than that in the upper 

part and averages approximately 0.2 mm in grain diameter.

F-l-12-8 and -9.—These two pieces of core, totaling 6 inches in length, are composed 

of white slightly porous slightly friable mixtures of sugary dolomite and chalky 

calcite. About 75 percent of the rock is dolomite in the form of evenly distributed 

crystals; the remainder is relict fossils, including tests of larger Foraminifera and 

coralline algae. The disclike tests have preferred orientation. Many tests have 

been dissolved and left platy voids. Thin sections of F-l-12-8 and -9 illustrate 

how the dolomite crystals have largely obliterated the original texture. However, 

enough parts of the tests and coralline algae persist to show that the rock was a 

coarse-grained mixture of packed Foraminifera and coralline algae; echinoid debris 

that resisted dolomitization is common. The original rock must have closely resembled core pieces 1 through 3.

F-1-12-10.—The upper part of this 3-inch piece is solid tan dolomite that is sugary 

in texture and contains abundant platy voids. The lower inch, however, contains 

abundant calcitized fossils, including larger Foraminifera, which parallel the voids 

in the upper part. A bulk X-ray analysis of the upper part showed 93 percent 

dolomite and 7 percent calcite. The lower part contains about 85 percent dolomite 

and 15 percent calcite. A thin section of F-1-12-10 shows a typical void in a 

matrix of solid dolomite (pi. 282C); no recognizable fossils remain except rare 

echinoid debris.

F-l-12-12.—This piece is very similar to pieces 8 and 9; however, it contains 45 

percent dolomite by X-ray diffraction method, and the original texture and fossil 

content is easily seen. Abundant tests of larger Foraminifera, coralline algae fragments and echinoid debris are closely packed and well oriented. Many of the 

interstices between fossils are open but, in places, original fine matrix is present. 

Some fossils have been slightly attacked by solution. The dolomite is evenly scattered throughout. It shows corroded edges that project into irregular voids and 

suggest some solution following dolomitization.

F-l-12-14.—In this piece, which is similar to F-l-12-12, the resistance of larger 

Foraminifera to dolomitization is evident (pi. 288£J). Growth of secondary dolomite has effaced original texture, and these tests remain unaffected except for the 

encroachment of automorphic dolomite around their edges. Solution of these tests 

would produce voids.

F-l-12-16.—This piece shows a variation in dolomite crystal size in a plane perpendicular to the crude bedding. Where the rock is composed of coarse fossil 

debris, such as larger Foraminifera, the dolomite crystals average about 0.2 mm 

in diameter and almost completely fill the interfossil spaces. In a crude bed of 

sand-size fossils packed in a dense mud matrix, the dolomite crystals are more 

widely scattered and average approximately 0.1 mm in diameter.

F-1-12-2S and -24.—These lower most pieces are dolomitic conglomerates made up 

of coral fragments, tests of larger Foraminifera, and corollain algae. Bedding is 

lacking. Piece 23 showed 42 percent dolomite by X-ray diffraction.

Interpretations.—Todd and Low (1960, p. 807) state that the fauna of core 12, like 

that of core 11, represents a transitional foraminiferal assemblage between a largely 

planktonic one in cores 7 through 10 and a largely benthonic one in cores 13 through 

15. Todd and Low interpreted this assemblage as indicating deposition on an 

outer slope subject to contamination by shallow-water sediments swept downward. 

665106 0—62———3
1036 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

Interval (feet) 

4, 316-4, 341

Drilling method 

Core barrel _ _

Recovery

8ft. (core 12), 

24 pieces.

4,341-4,406 

4,406-4,431

Rock bit_ 

Core barrel _ _

No recovery. 

1 foot (core 

13), 5 

pieces.

4, 431-4, 500 

4, 500-4, 525

Rock bit_____ No recovery. 

Core barrel. _ 8 ft (core 14), 

29 pieces.

DRILL HOLE F-l—Continued

Remarks

The writer agrees inasmuch as the bedding, the larger Foraminifera-coralline 

algae-coral fragment assemblage, and the wearing of the fossils are reminiscent of 

these same features in fore-reef limestones of both Miocene and Eocene age on 

Guam (Schlanger, 1963). The interpenetration of fossils and the offsets along 

fractures in the fossils are indicators of compaction following deposition. The 

lithology of the interval of rock was evidently fairly homogeneous prior to dolomitization; the rocks were largely foraminiferal-algal aggregates. The variation 

in intensity of dolomitization was probably not controlled, therefore, by initial 

differences in texture. Piece F—1-12-16 however, shows that original texture may 

have had some effect. Following dolomitization the rock was subjected to some 

solution, as evidenced by the numerous platy voids relict after Foraminifera tests. 

Plate 288.E shows how these tests resist dolomitization but do allow dolomite 

crystals to penetrate their edges. Upon solution the resultant voids appear to be 

lined by euhedral crystals much like a drusy calcite vug lining. If it were not for 

the few remaining tests, one might interpret the euhedral dolomite that lines the 

voids as having grown into open spaces.

¥-1-13-2.—Porous white slightly friable conglomerate composed of tests of larger 

Foraminifera from 4 to 8 mm in diameter, rounded fragments of coralline algae, 

irregular masses of encrusting algae and Foraminifera, and pebbles of dense rock 

set in a matrix of sand-size foraminiferal and algal debris. Interstices between 

fossils are largely open. The larger Foraminifera show a tendency to preferred 

orientation parallel to the diameters of the core. Most of the tests are chipped 

and worn. Some have been broken in place following burial. Bulk X-ray 

analysis of F-l-13-2 showed 100 percent calcite; however, etching with dilute 

hydrochloric acid and staining with copper nitrate revealed the presence of 

scattered rare dolomite crystals. These amount to perhaps 0.1 to 0.2 percent of 

the rock by volume and average 0.08 mm across. Large parts of the piece have 

no dolomite; the chip submitted for analysis may have been from one of these 

parts. The dolomite is not associated with any particular fossil group. Thin 

sections of F-l-13-2 and -3 reveal the presence of well-preserved tests of planktonic Foraminifera, rounded echinoid debris, and tests of small benthonic Foraminifera. Some of the rounded pebbles are seen to be pieces of fine-grained, 

recrystallized rock. Also present are irregular masses of encrusting algae and 

intergrown Foraminifera, whose convolutions contain mud and fossil debris finer 

than the coarse sediment of the surrounding rock. Contacts between the pebbles 

and algal-foraminiferal masses and the enclosing sediment are sharp.

F-l-18-4 and -5.—These pieces contain abundant larger Foraminifera and algal 

debris like the pieces described above, but these are confined to irregular pockets 

separated by denser rock that contains much fine well-lithified matrix. Molds of 

small coral fragments and mollusks are present. Staining of these pieces reveals 

very rare small crystals of dolomite.

Interpretations.—Cole (1957, p. 749, table 4 ) reports four species of Asterocyclina 

plus Operculinoid.es and Camerina from this interval. Todd and Low (1960, p. 815) 

identified many small benthonic types and some planktonics including globorotalids. 

Todd and Low interpreted this as an outer slope assemblage. In core 12 the texture 

of the rock and the fossil content allow comparison to similar features of fore-reef 

limestones from Guam (Schlanger, 1963). The rounded pebbles of "foreign" rock 

and the masses of encrusting algae and Foraminifera indicate that destruction of 

previously lithified rock was taking place during deposition of this interval. 

Perhaps a subaerially exposed area existed upslope at this time and contributed 

these fragments. The coral and mollusk molds in F-l-13-4 and -5 suggest some 

solution.

The upper 24 pieces of core, aggregating about 7 feet, are almost identical lithologically. They are porous well-cemented well-sorted tan foraminiferal-algal limestones. The pieces are massive; no bedding, solution voids, or fractures are present. 

Except for rare scattered echinoid and mollusk fragments, which attain lengths 

of several millimeters, the foraminiferal and algal debris is between % and /z l mm 

in diameter. Bulk X-ray analyses of F-l-14-8 and F-1-14-11 showed more than
SUBSURFACE GEOLOGY OF ENIWETOK ATOLL 1037

DRILL HOLE F-l—Continued

Interval (feet) Drilling method Recovery Remarks

98 percent calcite and a trace of dolomite. A stained piece of F-1-14-11 revealed 

rare scattered euhedra of dolomite. Pieces 25 through 28 are somewhat coarser 

grained owing to: (1) the presence of tests of larger Foraminifera as much as 

4 mm in diameter, fragments of various mollusk shells up to half an inch long, 

including Pecten and small gastropods, and fragments of coralline algae; and (2) 

a general increase in grain size of the bulk of the foraminiferal-algal debris to an 

average size of about half a millimeter. Rare crystals of dolomite are also present 

in these pieces. Piece 29 is radically different from the above pieces. It is wellcemented white limestone composed of thick encrusting algal laminae in which 

are enclosed coral molds and foraminiferal-algal debris packed in a lithified mud 

matrix. Bulk X-ray analysis of this piece showed 100 percent calcite, but a stained 

piece showed rare crystals of dolomite. Thin section descriptions are given 

below.

F-l -14~2.—The bulk of the rock is randomly oriented thick-walled benthomic 

smaller Foraminifera. The chambers of these are largely open, and in plane 

polarized light the tests show a fibrous structure. Next most abundant are fragments of coralline algae, both articulate and encrusting, these are rounded and the 

same size as the Foraminifera. Also present, in minor amounts, are fragments of 

mollusk shells and echinoid debris as much as 2 mm in length. The mollusk shells 

still show their original fibrous structure. There is no evidence for strong recrystallization and solution. In many places the dark coralline algae have been 

squeezed between and penetrated by the tests of Foraminifera and appear at first 

glance to be a dark mud matrix, but cellular structure can be seen. There is some 

true fine-grained matrix present, however, 'between the intact framework. No 

dolomite was seen in thin section.

F-l-14-17-—A very porous rock made up mostly (65 percent) of test of benthonic 

smaller Foraminifera. A small percentage is composed of fragments of coralline 

algae. The chambers of the tests are open and almost all the interstices between 

the fossils are open. These are cemented by thin films of granular and acicular 

calcite, and the chambers are lightly lined with the same material. Some of the 

fossils show postdepositional breakage. A few complete tests of Asterocyclina 

and many fragments of tests of Operculinoides(f) are present. The algal and 

operculiniod debris is the same size as the dominant tests of complete smaller 

Foraminifera, which indicates sorting. There are some ragged patches of microgranular calcite between tests that may be relict, recrystallized, and partly dissolved mud matrix. No dolomite was seen in thin section.

F-l-14-28.—This piece shows a less sorted aggregate of foraminiferal and algal 

debris within a well-lithified patchy mud matrix. Foraminifera tests range from 

thin-walled planktonic types 0.1 mm in diameter to benthonic larger types as 

much as 6 mm in diameter. Even though mud matrix is common, the chambers 

of these tests are open. Scarce dolomite euhedra are scattered throughout. 

F-l-14-29.—This thin section shows sinuous laminae of encrusting coralline algae 

enveloping coral fragments and finer debris. The coral has been completely dissolved, leaving open space where the originpl septae were and granular mosaics 

of calcite in place of originally mud-filled interseptal openings. Also enclosed by 

the algae is poorly sorted sandy mud that contains coralline algae and well-preserved miliolid Foraminifera. These tests retain a light-brown color in transmitted light. The original mud matrix has recrystallized into microgranular 

calcite.

Interpretations.—Both larger and smaller foraminiferal assemblages in the upper 28 

pieces indicate deposition in a fore-reef, outer slope environment. Good sorting 

and largely open interstices suggest deposition from moving water. Piece 29 

shows such a different lithology and fossil content that the writer regards it as a 

cored piece of a block that was swept downslope. Further, the bottom of piece 

29 shows a coating of foraminiferal-algal sand like the rest of the core run. The 

high content of laminar algae and coral suggest that this lowermost piece is of 

reef origin.

4, 525-4, 528 Rock bit- _ _ _ No recovery.
1038 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

Interval (feet) 

4, 528-4, 553

Drilling method 

Core barrel. _

Recovery 

2 ft 6 in. 

(core 15), 

13 pieces

1, 553-4, 619 

4, 619-4, 630

Rock bit____ No recovery. 

Core barrel. _ No recovery. 

Bottom of hole.

DRILL HOLE F-l—Continued

Remarks

Pieces recovered in this interval show lithologic similarities to those from core 14; 

two distinct rock types are present. Pieces 1 and 2 are foraminiferal-algal limestones, and pieces 3 through 13 are dominated by laminar algae. Bulk X-ray 

analyses of F-l-15-2 and -13 showed 100 percent calcite; however, copper nitratestained pieces and thin sections showed that rare dolomite persists throughout 

the interval, amounting to perhaps 0.1 percent of the rock by volume. Representative pieces and thin sections are described below.

F-l-15-1 and -2.—These are white porous slightly friable limestones made up 

largely of poorly oriented tests of Foraminifera and algal debris from % to 1 mm in 

diameter. Also present are angular to rounded fragments of encrusting algae, 

coral, and echinoid fragments as much as 6 mm in diameter. Staining with copper 

nitrate revealed rare euhedra of dolomite. Some of the fossils show postdepositional fracturing. A thin section of F-l-15-2 shows that many of the tests of 

benthonic larger and smaller Foraminifera are worn and broken, but tests of planktonic types are abundant and well preserved. Mollusk shell fragments are now 

calcite but retain their original fibrous structure. These fossils are packed— 

the rock has an intact framework—in an irregularly distributed mud matrix. 

In places the interstices between fossils are open. Dolomite is present both as 

widely scattered single crystals and as rare clusters of crystals. The thin section 

shows crude bedding due to subtle changes in grain size and rough orientation 

of lenticular fossils.

F-l-15-3 through -IS.—These are irregular pieces, some of which are roughly ellipsoidal and resemble rounded boulders. Lithologically they are identical with 

piece F-1-14-29. They are dominated by laminae of encrusting algae that encloses pockets of lithified poorly sorted sandy mud. Molds of coral and mollusk 

shells are common. Rare single crystals and clusters of dolomite are common.

Interpretations.—As in the case of core run 14, the rocks rich in laminar algae are 

interpreted as blocks from previously lithified and partly dissolved limestones that 

were swept down into deeper water sediments represented by the foraminiferalalgal sand rich in planktonic Foraminifera. Thus, during the deposition of the rock 

in intervals 14 and 15, there was probably a mass of reef limestone, either subaerially exposed during deposition of the rock in this interval or prior to it, that 

was contributing detritus to an outer slope upon which foraminiferal-algal sands 

were being deposited. The amount and distribution of dolomite in both the foraminiferal-algal sands and the reef rock suggest that dolomitization took place after 

the reef limestone was deposited on the outer slope.

PETROGRAPHY OF THE BASALT BENEATH THE 

LIMESTONE

By Gordon A. Macdonald

Hard basement rock was reached both in drill hole 

F-l, on Elugelab Island on the north-northwest side 

of Eniwetok Atoll, and in drill hole E-l, on Parry 

Island on the southeast side of the atoll. Although 

the basement rock in hole F-l is presumed to be 

basaltic, no samples were recovered. Basaltic rock 

was found in hole E-l, and good samples were obtained.

In drilling the 4,170- to 4,190-foot interval, a brief 

return circulating drilling fluid brought to the surface 

a few small chips of basalt. The corrected depth from 

which these cuttings originated is 4,154 feet (Ladd and 

others, 1953, p. 2266). No samples were obtained in 

the 4,190- to 4,208-foot, interval, but between 4,208 

feet and the bottom of the hole at 4,222 feet, 14 feet of 

basaltic core was obtained. The cuttings from 4,154

feet may have come from a boulder enclosed in the reef 

limestone, but there can be no question that the core 

obtained below 4,208 feet represents the volcanic base 

upon which the reef accumulated.

The basaltic core has been examined both megascopically and microscopically—62 thin sections were 

cut for the microscopic work. Through cores 4 and 5 

(4,208 to 4,216 feet), in which recovery was respectively 97 and 100 percent, one horizontal and one 

vertical section were cut approximately every 4 inches. 

Through core 6 (4,216 to 4,222 feet), one approximately horizontal section was cut every 6 inches. 

These sections were studied to ascertain the variations 

in mineral composition and texture from one part of the 

core to another and the relation of alteration in the 

basalt to veins of calcite that cut it.

Kefractive indices were determined in ordinary light 

by immersion in fluids of known index and are believed 

to have an accuracy of ±0.003.
SUBSURFACE GEOLOGY OF ENIWETOK ATOLL 1039

Because these are the only cores thus far obtained 

from a basaltic basement underlying an atoll, and 

because of the great difficulty and expense incurred in 

getting them, a more detailed megascopic description 

is given, in tabular form, than might otherwise be 

warranted. The description is summarized in the text. 

Microscopically, the rock is so uniform that a similar 

detailed description, piece by piece, is pointless.

Previous work and acknowledgments.—A preliminary 

study of the basaltic cores was made by Earl Ingerson 

(Ladd, and others, 1953, p. 2272-2276). For the most 

part, the present more detailed study confirms Ingerson's findings.

I wish to thank H. S. Ladd and Earl Ingerson for 

the privilege of studying the basaltic cores. The microscopic work was done in the laboratory of the Hawaii 

Institute of Geophysics. The chemical analysis was 

made by P. L. D. Elmore, I. H. Barlow, and S. D. 

Botts, of the U.S. Geological Survey, Washington, D.C., 

by rapid methods resembling those described by Shapiro 

and Brannock (1956).

MEGASCOPIC FEATURES

A detailed megascopic description of the cores is 

given in table 4. The nature of the materials produced 

by alteration is described more precisely on pages 1044- 

1045.

In measuring the amount of dip of the calcite veinlets, 

it was assumed that the hole is vertical. Thus, a dip 

of 20° means that the veinlet is inclined 70° to the long

TABLE 4.—Megascopic description of cores from hole E-1, Parry Island, Eniwetok

No.

4

Core

Depth (feet)

4, 208-4, 211

Length

feet

2

inches

11

Kecovery 

(percent)

97

Section of core

Number

4-1

_2

-3

-3A

-4

-5

-6

Length 

(inches)

2.3

1. 2

8. 75

0. 25

2. 3

6. 5

"*

5.8

Description

Kemarks

Dense dark-gray basalt with partly altered olivine phenocrysts.

Basalt is cut by two irregular compound calcite veinlets 2 to 8 mm

thick that dip approximately 20° and 65°. Joint surfaces also dip

approximately 20° and are covered with a film of green alteration

products.

Do.

Top: very dense dark-gray basalt with brownish-green to grayishgreen phenocrysts of olivine as much as 2 mm in length. Most of

the phenocrysts are partly altered to saponite, but some appear

fresh and glassy.

Bottom: same, but with some acicular olivine phenocrysts as much as

3 mm in length and 0.8 to 1 mm wide. More commonlv the phenocrysts are more nearly equidimensional and blocky in outline. A

few ovoid vesicles (amygdules) are as much as 5 mm in length and

filled with calcite.

Compound veinlet, apparently dipping about 20°, that consists

largely of calcite but includes thin irregular interleaves of altered

basalt.

Like section 4-3. Joints with green alteration material on their

surfaces dip from 0° to 30°. One calcite veinlet is 4 to 6 mm thick,

and one is 0.5 mm thick; both are very irregular; the smaller branches

from the larger in both directions, and the two and connected by

another small veinlet.

Small piece at the top contains a 4-mm veinlet of calcite. Veinlet is

irregular but dips approximately 20°. The joint surface along the

side of the vein is altered to saponite and chlorite. The main

piece of core, 6 inches long, is dense dark-gray basalt with many

phenocrysts of olivine as much as 2 mm in length; most are less than

1 mm. A few irregularly ovoid cavities as much as 4 mm in length

are filled with calcite. A very irregular veinlet of calcite, which is

about 0.3 mm thick and dips about 30°, cuts the middle of the piece

and dies out about two-thirds of the way through the core. The

bottom of the core segment is truncated by an altered joint surface

that dips about 50° and to which some fragments of calcite vein

still adhere.

Like section 4-3. Some tabular olivine phenocrysts appear very

acicular in cross section; they are as much as 2 mm in length and

only 0.3 mm wide. Sections includes some irregular spheroidal

masses of calcite as much as 1 mm in length. The top is truncated

by two intersecting joints that dip 45° and 60° in approximately

the same direction; both are coated with calcite. Other very thin

irregular calcite veinlets are less than 1 mm thick and occur in

groups partly parallel to the bigger veins, partly roughly horizontal,

and partly dipping about 90°.
1040 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

TABLE 4.—Megascopic description of cores from hole E—l, Parry Island, Eniwetok—Continued

No.

4

5

Core

Depth (feet)

4, 208-4, 211

4, 211-4, 216

Length

feet

2

5

inches

11

3

Recovery 

(percent)

97

100

Section of core

Number

-7

-8

5-1

-2

-3

-4

-5

-6

-7

-8

-9

-10

-11

-12

-13

-14

-15

Length 

(inches)

4. 4

3. 5

8

6

3. 5

3. 5

5-5

5

9

7

3

2. 5

4

2

1

2. 5

Description

Remarks

Like section 4-3. Includes two veinlets that strike approximately

30° from each other, average 1.5 mm thick, dip 45° and 30°, and

have anastomosing branches that tend to be approximately at right

angles to the larger veinlets. The veins and cross veins separate

the rock into rough polygons. These polygons are joint blocks,

not aa clinker or breccia from a block lava flow. Another irregular

veinlet 0.5 mm thick dips 30° but passes only part way through the

core. The bottom of the piece is truncated by a vein-coated joint

surface that dips 10°.

Like section 4-3. The bottom is truncated by a multiple vein that

is 1 cm thick and dips 25°. Thin irregular laminae of altered basalt

enclosed in calcite parallel vein walls. Other tiny veinlets less

than 0.5 mm thick dip at approximately 90° to the larger vein.

Near the top a veinlet 1 to 2 mm thick dips about 15° but strikes at

a slightly different azimuth from the one at the bottom.

Like section 4-3. Core section includes very irregular veinlets that

are from less than 1 mm to 2.5 mm thick and dip 30° to 60°; other

veinlets are approximately at right angles to them. Joint surfaces

adjacent to the veins are coated with serpentinous material. A

few irregular spheroidal openings as much as 1 cm in length are

filled with slightly greenish, argillaceous-appearing material.

Like section 4-3. A 5-mm multiple veinlet at the top dips 35°, and

a less regular veinlet 1 mm thick dips 25° in nearly the opposite

direction.

Like section 4-3. At the top, an 8-mm multiple veinlet dips 45°.

Another 1- to 2-mm veinlet dips 65°. A few irregular amygdules of

saponite and chlorite are as much as 6 mm across.

Like section 4-3. An irregular veinlet of calcite partly coats the top

of the piece. One fairly regular ovoid amygdule of saponite and

chlorite is about 5 mm long, and another very irregular mass of the

same material 3.5 cm long, dip 30°, approximately parallel to the

predominant veins.

Like section 4-3, very dense. Includes several irregular calcite

veinlets less than 1 mm thick, and a few irregular cavity fillings

of saponite and chlorite as much as 5 mm long; one cavity filling is

2.5 cm long.

Like section 4-3; abundance and size of phenocrysts is unchanged.

The section contains several irregular veinlets of calcite that are

from less than 1 mm to 8 mm thick and dip 15° to 65° and one irregular altered joint face that dips about 65°. The veins are

multiple.

Like section 4-3. Includes amygdules of saponite and chlorite as

much as 2.5 mm in length. One multiple veinlet, 1 cm thick, lies

on a joint surface that splits the piece diagonally at a dip of 70°.

Another 1- to 4-mm veinlet strikes at a slightly different angle and

dips 20°. Another strikes at a considerable angle to the first and

dips 50°, cutting the other veins. Other irregular veinlets less than

1 mm thick lie at various angles; one is nearly vertical.

Like section 4-3. A prominent multiple veinlet 5 to 10 mm thick

dips 20°. The rock along the veinlet is much altered to saponite

and chlorite. Other thin irregular veinlets cut the rock in all

directions. One cuts sharply through an amygdule.

Like section 4-3. An 8-mm multiple veinlet cuts the core with a dip

of 20°. Many other small irregular veinlets cut it at all angles.

Like section 4-3. Contains a few vesicle fillings of saponite and chlorite

as much as 2.5 mm in length.

Like section 4-3. Principal veinlets cross the core with dips of about

20°. Vesicles as much as 1 cm in length are flattened and roughly

almond shaped and filled with greenish alteration products. The

long direction of the amygdules and olivine phenocrysts are at

almost right angles to the veins, some of which cut sharply across

the amygdules.

Small fragments of rock, same as section 4-3.

Small chips of basalt and vein material.

Fragment of basalt like that of section 4-3.

Rock like that of section 4-3. The core section contains a few amygdules of saponite and chlorite and small calcite veinlets.
SUBSURFACE GEOLOGY OF ENIWETOK ATOLL 

TABLE 4.—Megascopic description of cores from hole E-l, Parry Island, Eniwetok—Continued

1041

No.

6

Core

Depth (feet)

4, 216-4, 222

Length

feet

5

inches

10

Recovery 

(percent)

97

Section of core

Number

6-1

-2

-3

-4

-5

-6

-7

-8

-9

-10

-11

-12

-13

-14

-15

Length 

(inches)

3

12

4

4

2. 5

2. 5

4

6. 5

5

2. 5

1

8

4

7

4

Description

Remarks

Fragments of dense dark-gray basalt, somewhat rounded by attrition

during drilling and apparently identical with section 4-3, with a

2- to 4-mm veinlet of calcite, which dips about 30°, at the top.

Like section 4-3, with many anastomosing veinlets of calcite. The

two largest veinlets, each averaging about 1 cm thick, dip 15° and

25° in nearly opposite directions. Near the top, two cavities about

3 cm long, one irregular and the other fairly regular, are filled with

marly material.

Like section 4—3 ; several fragments.

Like section 4-3, but so broken up that detailed description cannot

be made.

Small fragments of basalt, same as above.

Do.

Do.

Like section 4-3, with many ovoid vesicles filled with greenish saponite

and chlorite.

Like section 4-3, with a few small amygdules and several calcite

veinlets.

Several fragments of dense basalt.

A few very small fragments of basalt.

Dense basalt with some amygdules of saponite and chlorite and several calcite veins. A multiple vein 2 to 3 cm thick near the bottom

of the piece dips about 20°.

Small fragments of dense basalt.

Dense dark-gray basalt with a few thin veinlets of calcite.

Do.

direction of the core. The greatest inclination of the 

hole shown in the drilling record is only 1 °.

Throughout the core the rock is a dark-gray basalt 

with many phenocrysts of olivine. Some of the phenocrysts are as long as 3 mm, but most of them are less 

than 1 mm. Some appear fresh, but many are altered 

to saponite and chlorite.

A few vesicles are present, but for the most part the 

rock is very dense. Some of the vesicles are flattened 

and roughly almond shaped, but most are very irregular in shape and resemble the vesicles typical of aa 

lava flows (Macdonald, 1953 p. 179). Some are filled 

with greenish-brown saponite and chlorite, and others 

are filled with calcite. No aa clinker or block lava 

breccia are present.

Most of the olivine phenocrysts are nearly equidimensional, less than twice as long as they are wide. 

Some, however, are tabular parallel to the side pinacoid, 

and in cross sections perpendicular to that direction 

they appear highly acicular, the long dimension being 

as much as 8 times the width. Olivines of this sort 

were recognized in Hawaiian lavas long ago by Dana 

(1889, p. 445) and recently have been redescribed by 

Drever and Johnston (1957, p. 294-295).

The basalt core is cut by many veins of calcite (fig. 

312) that range from less than 1 mm to 2 cm in thickness. Assuming that the hole is vertical, most of the 

veins dip at an angle of 20° to 30°. However, because

the core is not oriented, it is not possible to give the 

direction of dip. It is also impossible to be sure that 

the dip is always in the same general direction; but in 

view of the prevalence of dips within a rather limited 

range, it is probable that the direction of dip in various 

parts of the core is fairly uniform. There is also a lesser 

tendency for the formation of veins at approximately 

right angles to the predominant direction—that is, with 

dips of 60° to 70° in roughly the opposite direction. 

There is no question that the veins formed along former 

joints in the lava. Not all the joints are filled with vein 

material, but all the joints, whether filled or not, are 

coated with greenish alteration products.

At several places in the core there was a distinct tendency for the olivine phenocrysts to be alined parallel 

to the predominant veinlets, and, at places in thin sections, eutaxitic structure is oriented similarly. This 

suggests that the joints that guided the predominant 

veins were flow planes in the molten lava along which at 

least incipient shear continued after the lava had congealed. The joints at right angles to them are tension 

fissures that resulted from the same tendency of continued flowage after consolidation of the lava. The 

first type is similar to platy joints parallel to flow planes 

that are very common in such andesitic rocks as the 

mugearites (Macdonald, 1949a, p. 54). However, at 

one place (core fragment 5-11) the long direction of
1042 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

FIGUKE 312.—Polished section of basalt core from drill hole E-1 on Parry Island' 

Eniwetok, showing calcite veins. The scale is in centimeters. From Ladd and 

others, 19S3, plate 1, figure 2.

phenocrysts and amygdules are nearly at right angles 

to the trend of the principal veinlets.

Most of the larger calcite veinlets are multiple and 

enclose laminae of altered basalt. The disposition and 

shape of the laminae indicate repeated shearing and vein 

filling., Many of the calcite veinlets contain enough 

saponite and chlorite to give them a distinct pale-green 

or brownish-green color.

The outstanding characteristic of the rock is its overall denseness. This denseness is not, however, diagnostic of the depth in the ocean at which the flow was 

extruded, and indeed, the flow need not have been 

submarine. To be sure, the denseness might be the 

result of the weight of overlying water preventing the 

formation of gas bubbles in the molten lava. However, 

similarly dense lava is found in the State of Hawaii in 

flows that were erupted sub aerially, particularly among 

the valley-filling flows belonging to the late groups 

(Honolulu and Koloa volcanic series). Scoriaceous 

character in a lava flow probably is good evidence that 

the flow was not extruded in very deep water, but 

denseness of the lava does not necessarily indicate the

converse.

The specimen chosen for chemical analysis (table 5), 

from the bottom of core fragment 4-3 at a depth of 

approximately 4,209 feet, is as fresh as any. It is 

described in detail below, and variations from it found 

in other sections are noted.

GENERAL DESCRIPTION

The analyzed specimen is an olivine basalt containing 

olivine phenocrysts as much as 3 mm in length. Strictly 

speaking, the texture is seriate rather than porphyritic; 

the large grains of olivine grade in size to small grains 

in the gronndmass. The latter is intersertal, with an 

average grain size of approximately 0.1 mm, and has a 

moderately well developed eutaxitic structure. The 

primary magmatic minerals of the groundmass are 

olivine, augite, plagioclase feldspar, opaque iron oxides, 

apatite, alkali feldspar, and analcime.

The large grains of olivine are irregular, with an 

embayed appearance (fig. 313), and have islands of 

groundmass material enclosed within them. This seem 

to be the result of irregular crystal growth, as described 

by Drever (1956, p. 32-35) and Drever and Johnston 

(1957), rather than of resorption in the magma. There 

is a slight zonal difference in composition within the 

large grains: the centers have an optic axial angle of 

approximately 90°, which corresponds to a composition 

of about Fo90 Fai0 ; and the outsides have an optic angle 

of (—) 80° to 85°, which corresponds to a composition 

of about Fo70 . There is also a small increase in the 

strength of birefringence in the outermost part of the 

grains. The small groundmass grains are similar in 

composition to the outsides of the large grains.

The olivine phenocrysts, and less commonly the 

groundmass of olivine, are partly altered to a yellowishMICROSCOPIC FEATURES

Except for variations in the degree of alteration, the 

basaltic rock of the cores is uniform from top to bottom.

FIGTJEE 313.—Photomicrograph of a thin section of fresh basalt from drill hole E-1, 

Eniwetok, showing one of the acicular phenocrysts of olivine. Another large olivine 

crystal lies just to the left of it. Note the embayed outlines of the olivine, resulting 

from irregular growth of the crystals, not resorption. The groundmass is labradorite, 

augite, olivine, and opaque oxides. Crossed nicols, X 35.
SUBSURFACE GEOLOGY OF ENIWETOK ATOLL 1043

FIGURE 314.—Photomicrograph of a thin section of altered basalt from drill hole E-1, 

Eniwetok, showing an olivine phenocryst completely altered to saponite. Ordinary light, X 35. From Ladd and others, 1963, plate 2, figure 1.

brown to golden-yellow or greenish-yellow weakly pleochroic fibrous saponite (fig. 314) which is described in 

detail later. The saponite is in turn altered in part to 

chlorite.

A slide cut from core segment 4-4 contains a single 

inclusion of dunite, 1 mm across, composed of nearly 

equant anhedral to subhedral grains of olivine as much 

as 0.1 mm across; most are less than 0.05 mm across. 

The olivine grains in the dunite are.partly altered to 

saponite and chlorite.

The monoclinic pyroxene in the basalt is augite, 

with (+)2V=60°±, and moderate dispersion with 

p>ju. The grains are subhedral and typically slightly 

elongated with the length generally about twice the 

width. They are very pale brown to slightly purplish 

brown in color and probably contain some titanium. 

Some grains show poorly developed hourglass zoning, 

and others show a small decrease in extinction angle in 

the outermost parts.

In some other slides, some of the augite is distinctly 

purplish, with strong dispersion, and is almost certainly 

titanian. In those slides the augite varies in color,

and probably in titanium content, from one part to 

another. Several instances were noted in which the 

augite in masses of groundmass material enclosed in 

large olivine crystals is distinctly more purplish than 

that in the surrounding rock. As suggested by Drever 

and Johnston (1957, p. 298), this seems to indicate an 

entrapment of volatile compounds, including titania, 

within the olivine crystal.

In a slide cut from the top of core fragment 5-8 a 

single composite phenocryst (glomerocryst) of augite 2 

mm long, composed of half a dozen grains 0.25 to 1.5 mm 

across, shows pronounced zonal structure and very 

strong dispersion.

Most of the plagioclase is in subhedral lath-shaped 

grains with pronounced albite twinning. A few grains 

also show Carlsbad twinning. The center of the grains 

is medium labradorite (/3= 1.563), and around the edge 

is a thin zone in which the composition changes abruptly to labradorite-andesine. A small amount of alkalic 

feldspar occurs in anhedral interstitial grains with a 

mean refractive index of about 1.524 and a maximum 

birefringence of about 0.007. It is probably sanidine, 

but it has not been possible to confirm this by other 

optical properties. The norm (table 5) contains 5 percent orthoclase and 44.7 percent plagioclase with average composition of Ab4 i.

Most of the opaque grains are blocky or nearly square 

in cross section and are probably magnetite, but others 

have lathlike outlines and probably are ilmenite. In 

a few other slides, distinct skeletal shapes of magnetite 

crystals have been observed.

Minute acicular crystals of apatite are enclosed in the 

other minerals, particularly in the feldspar. Brown 

crystals of spinel, probably picotite, are enclosed in 

olivine grains in a few slides, but none were found in the 

analyzed specimen. The spinel crystals generally are 

less than 0.05 mm in diameter, but, in a slide cut 

from core section 4-4, one of them is 0.2 mm across.

Small interstitial patches consist of single grains or 

mosaics of very weakly birefringent analcime with a 

refractive index of about 1.490. The mosaics consist 

of several variously oriented anhedral but generally 

nearly equant grains. Associated with the analcime in 

a few places are tiny flakes of very pale green chlorite, 

but most of the surrounding constituents are the magmatic minerals, augite, olivine, and plagioclase, and are 

wholly unaltered. The analcime appears to be a very 

late magmatic primary mineral. Similarly, in lavas of 

the Honolulu volcanic series on the island of Oahu 

(Winchell, 1947, p. 25) and the Koloa volcanic series on 

the island of Kauai, Hawaii, the analcime is almost 

certainly of late magmatic origin and occurs interstitially or in pegmatoid segregations. Surrounding 

minerals are not altered, and there is no evidence to
1044 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

indicate that the analcime is secondary in the sense of 

being an alteration product of earlier crystallized minerals.

The estimated mineral composition of the analyzed 

rock, in volume percent, is:

Olivine-_______________________________________ 20

Augite __ _____________________________________ 25

Plagioclase. _.___---_-__--_-----________________ 44

Opaque oxides____--------_-------______________ 7

Alkali feldspar._________________________________ 1

Analcime-_____________________________________ 2

Chlorite and saponite___________________________ 2

Both the analcime and the chlorite and saponite are 

irregular and patchy in distribution. In lower sections 

of the core analcime is more abundant than in the analyzed specimen and becomes as much as 5 percent of the 

rock. This may represent an enrichment in late-crystallizing constituents within the central part of a thick 

flow.

In a few other slides, notably from core fragment 4-4, 

a few small irregular interstices, probably originally 

vesicles, contain radiating and divergent length-slow 

colorless fibers with a very low refractive index, a birefringence about 0.008, and parallel extinction. They 

probably are natrolite. The masses of natrolite are 

enclosed in thin layers of pale green chlorite. A slide 

cut from the bottom of core section 5-6 contains one 

vesicle filled with spheroidal bodies about 0.05 millimeter across and composed of radiating fibers of stilbite, associated with a little calcite. The rock immediately around it, through a radius of 0.5 to 1 mm, is 

thoroughly chloritized; both olivine and plagioclase are 

almost completely destroyed, although augite remains 

unaffected. The natrolite, stilbite, calcite, and chlorite 

are of secondary origin.

The cuttings from a depth of 4,154 feet are dense 

black opaque basaltic glass containing microlites of 

labradorite and monoclinic pyroxene. They probably 

are fragments of quickly chilled explosive ejecta. They 

resemble some of the scoriaceous fragments dredged up 

at Bikini (Macdonald in Emery, Tracey, and Ladd, 

1954, p. 120-124).

ALTERATION PRODUCTS

The basalt is moderately to highly altered. The 

commonest alteration product is a brownish yellow to 

golden yellow fibrous material with moderately high 

birefringence. The fibers are length-slow, the mean 

refractive index is about 1.56, and the maxium birefringence is 0.025 to 0.030. The mineral was called 

"saponite-like" by Ingerson (Ladd, and others, 1953, 

p. 2275), and it agrees reasonably well in properties with 

one variety of saponite listed by Larsen and Berman 

(1934, p. 158). It also resembles rather closely the

yellow hydrothermal alteration product of olivine known 

as bowlingite, which is probably a member of the same 

group. The mineral grades imperceptibly into coarser 

material that bears a striking resemblance to pale 

biotite. This material forms micaceous grains as much 

as 0.2 mm long that are pleochroic from X= very pale 

brownish yellow to Z'= medium greenish brown or 

brownish green. The cleavage flakes have positive 

elongation and parallel extinction, with X sensibly normal to the cleavage. The optic axial angle ranges 

from sensibly 0° to about 20°, and the optic sign is 

negative. Birefringence is somewhat variable, as much 

as 0.040, with a= approximately 1.530 and ft and 7= 

approximately 1.570. The mineral resembles griffithite 

but has a lower birefringence and higher low index of 

refraction. In a slide cut from the top of core segment 

5-1, similar material is pleochroic from bright orangered to pale yellowish orange, and in a few other slides 

material of otherwise similar properties it is pleochroic 

from very pale yellow to clear green and resembles green 

biotite. The typical yellow fibrous alteration product 

also grades into a greenish-brown to brownish-green 

fibrous material with similar optical properties, and 

this in turn seems to grade toward typical chlorite.

All this material is conveniently included in the 

general term "saponite." Its variation in optical properties probably results from substitution of varying 

amounts of ferrous iron for magnesium.

Nearly as widespread as saponite is a typical chlorite 

that is pleochroic from very pale yellowish green to 

medium bluish green, has very low birefringence, and 

typically shows "ultra-blue" interference colors. Less 

widespread, but common, is a very bright grass-green 

to bluish-green chlorite resembling celadonite, with 

interference colors so weak that they are hidden by the 

very strong green body color. In some slides there 

appear to be intergradations between the two varieties 

of chlorite.

In all the slides the olivine is at least partly altered, 

but the degree of alteration ranges from slight, as in the 

analyzed specimen, to complete. Commonly the 

amount of alteration is obviously related to the proximity and size of calcite veins, with rock close to large 

veins showing a high degree of alteration. The precise pattern of alteration is irregular and differs considerably from grain to grain, even in the same thin 

section. In general, however, the sequence is: (1) 

formation of narrow borders of fibrous yellow-brown 

saponite and veinlets of similar material along fractures 

in the olivine; (2) alteration of the intervening olivine 

to fibrous saponite, with local development of large 

micaceous plates; (3) alteration of unaltered olivine or 

saponite to chlorite along fractures and in irregular 

patches; and (4) in extreme cases, further alteration of
SUBSURFACE GEOLOGY OF ENIWETOK ATOLL 1045

the material between the original saponite veinlets to 

chlorite or calcite, or both.

The first-formed veinlets of saponite are composed of 

moderately coarse fibers mostly oriented about normal 

to the edge of the olivine grain or to the fracture that 

guided the alteration. In the latter, the fibers form a 

double row which extends in both directions from a 

median line that marks the original fracture. Rarely, 

as in a slide cut from the bottom of core fragment 5-5, 

the fibers form a series of radiating groups on both sides 

of the medial fracture. Quite commonly, there is a 

film of hematite and limonite directly along the border 

of the fibers on the side away from the original fracture, 

and this apparently represents iron driven out as a 

miniature "iron front" during the alteration of olivine 

to-saponite.

Later alteration of olivine between the network of 

original veinlets produces an irregular mat of fibers in 

which the fibers generally show little or no order or 

arrangement. The shape of the original olivine grain 

and its pattern of fractures are perfectly preserved. 

Commonly, the structure of the saponite pseudomorph 

at this stage strongly resembles the net structure in 

ordinary metamorphic serpentine. Within the mat 

between the veinlets, larger micaceous grains may form 

without any particular orientation with regard to each 

other or to the original veinlets.

Some saponite pseudomorphs after olivine are cut by 

veinlets or contain irregular patches of chlorite. There 

appears to have been an alteration of saponite to 

chlorite. Other pseudomorphs consist wholly of chlorite except for borders of saponite and veinlets of 

saponite along the fractures of the original olivine. 

This also may represent alteration of earlier saponite 

to chlorite, or it may be the result of direct alteration 

of olivine to chlorite under somewhat changed conditions. Some support for the latter suggestion is the 

occurrence of occasional patches of chlorite enclosed in 

fresh olivine without saponite. Similarly, the areas 

between saponite veins in pseudomorphs may be occupied wholly by calcite or by a mixture of calcite and 

chlorite. In some areas, brown grains of limonite are 

mixed with the calcite, and rarely the borders of the 

pseudomorphs are limonite enclosing calcite, or calcite 

and chlorite. The chlorite in the pseudomorphs is 

generally the ordinary chlorite, but in some places it is 

partly the very bright green variety.

Several different types of alteration of olivine may be 

present even within a small part of a single slide.

In a few places very small pale flakes of chlorite are 

associated with interstitial analcime, but whether it 

was formed directly with the analcime in a deuteric 

stage or is of secondary origin is uncertain.

At a few places along calcite veins, minerals other 

than olivine have been attacked, with plagioclase 

being partly or even largely chloritized, and rarely 

also the augite. Even where both plagioclase and 

augite have been partly altered to chlorite, the magnetite appears completely unaffected.

A slide cut from the bottom of core fragment 5-8 

contains pseudomorphs of saponite, chlorite, and 

calcite after olivine, with which there is associated 

abundant brown limonite and red hematite. Several 

irregular openings are filled with radiating fibrous 

masses of chlorite and opaque to red hematite. The 

rock immediately surrounding these openings contains 

irregular patches of bright grass-green chlorite and 

bright purple titanaugite. The combination of these 

minerals with white laths of feldspar and black skeleton 

crystals of magnetite results in a rock with a spectacular appearance in thin section.

Alteration of the olivine to saponite and chlorite 

definitely is related to the same solutions that deposited 

the carbonate and formed the saponite and chlorite 

veins described below. The association with the carbonate veins seems to preclude the possibility that 

the alteration was brought about by hot water during 

and immediately following a submarine eruption. 

It is natural to think of the altering solutions as coming 

from, and deriving their dissolved carbonate from, 

the overlying reefs. This is to some extent confirmed 

by the formation along the same fissures of veins of 

cellophane, the phosphate for which almost certainly 

must have been derived from organic debris in the 

reefs.

Solutions attacking the olivine and altering it to 

saponite must have introduced a little aluminum 

and removed a little iron. Some of the iron was 

driven ahead of the advancing wave of alteration 

and was deposited at the inner edge of the saponite. 

The aluminum may have been derived by alteration 

of aluminous minerals, particularly plagioclase, in 

the adjacent rock. The resemblance of the saponite 

in these rocks to bowlingite may indicate that there 

actually is very little aluminum present in it.

VEINS

The carbonate veins (fig. 312) range from less than 

0.01 mm to more than 2 mm in thickness. In thin 

section most of them appear to be simple mosaics of 

anhedral grains of calcite. Some, however, show 

fairly well developed comb structure.

X-ray examination of a sample of carbonate from 

a vein in core 6-12 by Daphne R. Ross of the Geological 

Survey confirms the microscopic identification of the 

carbonate mineral as calcite; a partial chemical analysis
1046 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

by rapid methods run by P. L. D. Elmore and 

S. D. Botts of the Geological Survey shows the carbonate to contain 53.5 percent CaO and only 1.2 

percent MgO. The abundance of trace elements 

in the sample is given in table 6.

Veins in core segment 4-1 are typical. They are as 

much as 3 mm in thickness and anastomose rather 

irregularly through the rock. Bands of irregular granules of limonite lie within the calcite, and the center of 

one 3-mm vein is almost wholly limonite. The principal vein has borders of saponite and chlorite; the 

chlorite lies directly against the wallrock, and the saponite is between the chlorite and the calcite. The fibers 

of saponite are normal to the wall of the vein. The 

calcite is at least partly later than the saponite-chlorite 

border, because angular fragments of the border enclosed in the calcite are detached and slightly shifted 

from their original positions, and small calcite veinlets 

cut sharply across the border and pass out into the 

wallrock without the development of similar borders. 

Narrow bands of saponite and chlorite, 0.01 to 0.03 mm 

wide, lie within the calcite parallel to the vein wall 

and appear to mark former vein boundaries.

In several slides, other thin veins of saponite and 

chlorite lie within the wallrock a few tenths of a millimeter from the main vein and parallel to it, separated 

from the vein by partly altered wallrock. They seem 

to represent subsidiary breaks roughly parallel to the 

main one. Projecting angles on one side of the vein 

commonly match well the re-entrants in the other wall, 

which suggests that the opening of the vein fissures 

has been by simple distension rather than by shearing.

Although minerals in the wallrock adjacent to the 

veins have been altered by the vein-forming solutions, 

room for the veins seems to have been made by simple 

opening of the fracture rather than by replacement. 

Evidence points to repeated opening of fractures and 

vein deposition, with the chlorite and saponite veins 

being formed first, and then those of calcite.

Veinlets of saponite and (or) chlorite unassociated 

with calcite are in some slides. Some veinlets composed 

almost wholly of saponite and chlorite have a little 

calcite in the very center.

Some calcite veinlets have borders of red hematite or 

brown limonite granules. The granules generally are 

rounded and at some places show botryoidal forms 

and concentric layering. Botryoidal masses as much 

as 1 mm across are enclosed in the veins. Roundish 

masses of chlorite coated1- with iron oxide also are 

enclosed in some veins. A calcite veinlet in the top of 

core fragment 5-5 contains strongly pleochroic plates 

of saponite, as much as 0.2 mm in length, arranged 

along the middle part of the vein and oriented normal 

to the vein walls.

Along one edge of the same vein is a band of cellophane 0.1 to 0.3 mm thick. Across part of the slide 

the cellophane band leaves the edge of the calcite vein 

and extends within the vein. Along the edges of the 

cellophane, but within the calcite, are many roughly 

spherical to irregular bodies consisting of radiating fibers of yellowish-brown saponite. A veinlet only 0.1 

mm thick of cellophane without any associated calcite 

but with an irregular border of saponite diverges from 

the main vein and cuts the basalt wallrock. A calcite 

veinlet diverging in another direction is similarly bordered by saponite but has no cellophane. The collophane appears to be later than most or all the calcite. 

Core fragment 6-12 also contains a veinlet of calcite 

cut by a smaller veinlet of cellophane.

In the bottom of core fragment 6 -2 a veinlet of zeolite about 0.1 mm wide has been partly broken up by 

a later veinlet of calcite that follows the same general 

line of fracture. In part, the zeolite enclosed in the 

calcite has recrystallized to euhedral rhombic crystals 

of nearly square cross section. Its index of refraction 

is approximately 1.490, and its birefringence ranges 

from 0 to about 0.005. The zeolite is probably 

chabazite.

In the top of core fragment 4-7 also, a calcite vein 

is bordered by what appears to be a remnant of an older 

vein of zeolite that encloses very thin wavy septa of 

chlorite.

The sequence of vein formation, probably with some 

overlapping, appears to be: (1) formation of veins of 

chlorite and saponite; (2) formation of veins of zeolite; 

(3) formation of veins of calcite, commonly with iron 

oxide borders, and some with borders and associated 

veins of the very bright green chlorite; and (4) formation of veinlets of cellophane.

Evidence and conclusions bearing on the origin of the 

carbonate veins and the associated alteration of the 

basalt may be summarized as follows:

1. The carbonate veins contain no recognized .fossils, 

not even Foraminifera, and therefore probably 

were formed by precipitation from solution, not by 

infiltration of calcareous ooze from the overlying 

sea bottom.

2. The alteration of the basalt is closely related spacially 

to the carbonate veins and decreases away from 

the veins. It is, therefore, definitely related genetically to the vein-filling fissures as channel ways 

for the altering solutions and probably to the same 

solutions that deposited the carbonate veins.

3. No similar veining has been found in the State of 

Hawaii or, to my knowledge, in any other of the 

mid-Pacific volcanic islands. It is, therefore, not 

a feature typically related to Pacific volcanism at 

levels commonly visible—that is, above sea level.
SUBSURFACE GEOLOGY OF ENIWETOK ATOLL 1047

It may be concluded that it either resulted from 

solutions of nonvolcanic origin or formed below 

sea level, or both.

If the solutions were of nonvolcanic origin, were they 

of some other hydrothermal nature, or of epigene 

nature? The presence of cellophane veinlets in 

and closely associated with the carbonate veins 

suggests strongly that they were of epigene origin 

and derived their calcium carbonate and phosphate 

from the overlying reefs. If this is true, the vein 

formation must have taken place long enough after 

the solidification of the lava for the top of the volcanic pile to have been planed off by wave erosion 

and covered by an unknown thickness of calcareous reef.

The alteration of the basalt resembles hydrothermal 

alteration in other parts of the world, rather than 

alteration by ordinary weathering. This may, 

however, simply be the result of alteration under 

nonoxidizing conditions, beneath the water table 

or below sea level.

There is no evidence, however, other than the lack 

of resemblance of the alteration to ordinary weathering, that the alteration and vein formation did 

not take place above sea level.

If the solutions were of epigene origin, as is likely, 

some condition must have existed that caused 

them to migrate downward from the reefs into the 

basalt, carrying with them dissolved calcium carbonate. In a static condition of reefs saturated 

with sea water resting on a relatively impermeable 

basalt platform beneath sea level, it is unlikely that 

much such vertical migration would occur. However, a condition involving downward migration 

of solutions from the reefs into the basalt might

o

well result from an elevation of the reefs above sea 

level, such as is indicated by the solution unconformity at 2,780 feet in drill hole E-1 or the earlier 

elevation suggested by the absence of deposits of 

Oligocene age. Recent hydrologic investigations 

indicate that much of the ground water that sinks 

into an oceanic island does not move out laterally 

at the top of the Ghyben-Herzberg lens, but penetrates deeply into it before moving laterally. 

Thus meteoric water dissolving calcium carbonate 

from the elevated Eniwetok reefs might be expected to move downward into the basalt, provided the Ghyben-Herzberg lens extended deep 

enough into the island to reach the basalt. The 

thickness of the lens would depend on the abundance of rainfall and the permeability of the rocks 

above, and just below, sea level. 

Reef limestone is highly permeable, and if the lens 

was thick enough to reach the basalt, as is strongly

suggested by the evidence listed above, then, 

with low rainfall, the basalt platform at the base 

of the reef must have been above or very close to 

sea level in order to have been reached by a thin 

ground-water lens; or rainfall must have been 

high enough to produce a thick lens. However, 

high rainfall implies a high island, and this in 

turn would imply elevation of the island to a 

degree that would have brought the top of the 

basalt above, or close to, sea level. Thus, in 

either event the^basalt platform must have been 

close to or above sea level, and the top of the 

limestone must have stood at least several hundred 

feet above sea level, possibly more than 1,400 

feet (the thickness of the limestones between the 

basalt and the Miocene unconformity). 

9. The carbonate veins are not dolomitized, although 

the overlying rocks less than 100 feet above 

contain some dolomite. This may be because, 

once formed, the veins are much less permeable 

than the overlying reef limestone. 

In summary, the alteration of the basalt and formation of the veins appear to have been by epigene solutions that derived the carbonate and cellophane by 

solution of the overlying reefs at a time when the top 

of the basalt stood close to sea level and part or all 

the limestone stood well above sea level.

CHEMICAL COMPOSITION

The rock chosen for analysis is from the bottom of 

core segment 4-3, one of the least altered parts of the 

core. The chemical composition and norm are given 

in table 5, column 1. The rock is undersaturated with

TABLE 5.—Chemical analyses of basalt from Eniwetok and other 

similar rocks

[1. Analcime-bearing alkali olivine basalt, from bottom of core fragment 4-3, at a 

depth of approximately 4,209 feet in drill hole E-1, Parry Island, Eniwetok Atoll. 

P. L. D. Elmore, I. H. Barlow, and S. D. Botts, analysts. 2. Analcime basanite, 

Koloa volcanic series, Kauai Island, Hawaii. F. A. Gonyer, analyst. (Macdonald, 

Davis, and Cox, 1960.) 3. Average normal alkali basalt (average of 96 analyses). 

Nockolds, 1954, p. 1021, col. IX]

Si02 -------

A12 03

Fe2 0i_______-

FeO

MgO. __.--__

CaO__-______

Na2 0__-__-__

K2 0-________

H2 0+__--__-

H2 0-_______

Ti02 __. _._.__

P2 0B - — — _

MnO. _._._..

C02 -__.__.__

Total. _

i

44. 4 

14. 9 

2.3 

9. 4 

10. 7 

9. 4 

2.6 

.82

} 2. 4

1. 8 

. 48 

. 20 

. 22

100.0

2

45. 30 

11. 76 

3.98 

8. 80 

12. 88 

10. 72 

2.36 

. 72 

. 67 

.00 

2. 30 

. 31 

. 11 

. 00

1 100. 04

3

45. 78 

14. 64 

3. 16 

8. 73 

9. 39 

10. 74 

2. 63 

. 95 

. 76

2. 63 

. 39 

. 20

ab_

|

wo____

f<5

,/fO-.-.-

ol|fa--_-

mt__

il___-____

ap

Norms

1

5.0 

18. 3 

26. 4 

2.0 

7.0 

4. 4 

2. 1 

15. 6 

8. 3 

3. 2 

3. 5 

1. 3

2

3.9 

16. 8 

19. 7 

1. 7 

13. 2 

9. 4 

2.6 

16.0 

5. 0 

5. 8 

4. 4 

.7

3

6. 1 

18. 3 

24. 7 

2. 3 

10. 8 

7. 1 

2. 9 

11. 5 

5.0 

4. 6 

5.0 

1.0

i Includes 0.13 percent BaO.
1048 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

silica and contains a little normative nepheline. The 

content of trace elements is given in column 1 of table 6.

In chemical composition the rock closely resembles 

an analcime basanite from the Koloa volcanic series of 

the island of Kauai, Hawaii (table 5, column 2), except 

that it contains more alumina. It is also much like the 

average normal alkali basalt of Nockolds (table 5, 

column 3).

The average amount of modal analcime present is 

hardly enough to justify calling the rock an analcime 

basanite. Similar rocks in the Honolulu volcanic series 

on the island of Oahu, Hawaii, were called "linosaite" 

by Winchell (1947, p. 28), but the name "alkali basalt" 

now is in common use for these rocks. The most appropriate designation for the Eniwetok rock appears to 

be analcime-bearing alkali olivine basalt.

TABLE 6.—Abundance of trace elements in samples of the basaltic 

cores from Eniwetok

[The figures are reported in percent to the nearest number in the series 7, 3, 1.5,0.7, 

0.3,0.lfl, and so on. These numbers represent midpoints of group data on a geometric scale. The letter "M" indicates that the element is a major constituent, the 

abundance of which is reported elsewhere. Katherine V. Hazel, analyst. Looked 

for but not found: As, Au, B, Bi, Od, Ce, Cs, Dy, Er, Eu, Qd, Ge, Hf, Hg, Ho, In, 

Ir, Li, Nd, Os, Pd, Pr, Pt, Kb, Re, Eh, Ru, Sb, Sm, Ta, Tb, Te, Th, Tl, Tm, U, 

W, Yb, Zn]

Si_.__.

Al_..._

Fe_.._ 

Na_._. 

K_____

Ti...__

Mn___. B!--

_Dtt__ _ _ _

Be____

Co....

Cr__.._

Cu.._.

Basalt, core 4-3

M

M

M 

M 

M

M

M 

. 00003 

.015 

. 00015

.03

. 015

.015

Calcite veinlet, core 6-12

0 015

003

3 

03 

3

0

15 

00003 

0003 

0

0

0

.0007

Ga_-_

La_._.

Mo.._ 

Nb_-_

Ni.___

Pb.__.

Sc___. 

Sn_._. 

Sr__._ 

V__ —

¥_____

Zr___.

Basalt, core 4-3

0. 0007

0

0003 

0003 

03

07

0015 

003 

03 

015

0015

015

Calcite veinlet, core 6-12

0

.003

0 

0 

0

0

0 

0 

.015 

. 003

0

0

CONSANGUINITY OP THE BOCK

Alkali olivine basalts are widespread in the volcanic 

islands of the central Pacific Ocean. In the State of 

Hawaii some of them appear to be interbedded with 

tholeiitic basalts in the uppermost parts of the major 

volcanic shields. More typically, however, they are 

associated with picrite-basalts of ankaramite type, 

hawaiites, mugearites, and trachytes in later volcanic 

series that form thin cappings on the major shields 

(Macdonald, 1949b, p. 1566), or with nepheline basalts 

in volcanic series that were erupted much later, after a 

long period of erosion. In the titanium-bearing character of its augite, the Eniwetok rock resembles olivine 

basalts of the Honolulu volcanic series on the island of

Oahu, Hawaii (Winchell, 1947, p. 23, 25), the Kiekie 

volcanic series of Niihau Island (Macdonald, 1947, p. 

46), and the Koloa volcanic series of the island of Kauai. 

Analcime also is present in some of the lavas of the 

Honolulu and Koloa volcanic series, but not in earlier 

lavas in the Hawaiian group. These series were 

erupted as the last episode in the activity of the Hawaiian volcanoes, after a pause that permitted erosion of 

canyons several thousand feet deep into the flanks of 

the main volcanic mountains. By analogy, the 

Eniwetok rock probably represents a lava flow erupted 

very late in the history of the volcano that forms the 

volcanic pedestal of the island.

Specimens of volcanic rock dredged from the outer 

slopes of Bikini Atoll also appear to be partly, if not 

wholly, alkali olivine basalt (Macdonald in Emery, 

Tracey, and Ladd, 1954, p. 120-124).

CARBONATE MINERALOGY

By Donald L. Graf 5 and Julian R. Goldsmith 6

The results of powder X-ray diffraction and emission 

spectrographic analyses of 16 carbonate rock samples 

from cores at Eniwetok, Funafuti, and Kita-daito-jima 

are summarized in table 7. It should be emphasized 

that the natural samples studied were only small pieces 

of drill core, and that the values given in table 7 may 

not be valid even inches away.

The information upon which the determination of 

carbonate mineral abundances (table 7) is based is 

shown in figure 315.

X-RAY DIFFRACTION ANALYSIS OF CARBONATE 

MINERAL MIXTURES

The quantitative powder X-ray diffraction analysis 

of calcite, dolomite, and aragonite in mixtures presents 

unusual difficulties. The well-developed cleavage of 

the two rhombohedral carbonates is at the orientation 

of the uniquely strong {10T4} powder reflection for these 

materials. When, as in the present study, the strongest 

reflection is used to achieve maximum sensitivity, the 

elimination of preferred orientation is particularly important. On the other hand, a number of glide 

mechanisms are known for these minerals, the {0112}

5 Illinois State Geological Survey, Urbana.

6 Department of Geology, The University of Chicago.
SUBSURFACE GEOLOGY OF ENIWETOK ATOLL 1049

0

100

10

90

20 30 40 50 60 70 80 

80 70 60 50 40 30 20 

WEIGHT PERCENT (UPPER SCALE, CALCITE; LOWER SCALE, DOLOMITE AND ARAGONITE)

90

10

100

0

FIGURE 315.—The integrated intensities of selected powder X-ray diffraction maxima obtained from mixtures of calcite (Iceland spar, Taos, N. Mex.), aragonite (fresh-water

Unio shell), and dolomite (Upper Eocene, Levy County, Fla.).

A, Expectable range for integrated 

intensity of calcite (1014), based 

upon that of NaCl {200} and the 

data of Hanawalt, Rinn, and 

Frevel, 1938.

B, Expectable range for integrated intensity of aragonite

C, Expectable range for integrated intensity of dolomite {1014}, if 

dolomite contains 50 mole percent 

CaCOa.

D, Expectable range for integrated intensity of dolomite {1014},.if 

dolomite contains 55 mole percent 

CaCOs.

G, Aragonite crystals, Girgenti, Sicily.

H, Chalk, Dover, England, disaggregated but not further ground.

K, Dolomite, Kankakee, 111., ball milled 

for 8 hours.

0, Dolomite, Carey, Ohio, ball milled 

for 8 hours.

T, Iceland spar, Taos, N. Mex., after 

prolonged hand grinding in 

mortar.

U, Aragonite, fresh water Unio shell.

W, Dolomite, Gabbs, Nev., after prolonged hand grinding in mortar.

TABLE 7.—Carbonate mineral composition of selected samples

The minimum amounts of carbonates detectable by the X-ray diffraction method used are about 1 percent for calcite and dolomite and about 2 percent for aragonite. The 

quantities in parentheses were obtained by difference. The plus-minus ranges given for these mineralogical determinations indicate only the disagreement between values 

read from calcite and aragonite curves, or from calcite and dolomite curves, and are not necessarily equivalent to accuracy. The information on which the determination 

of carbonate mineral abundance is based is shown in figure 315]

Sample

Eniwetok: 

F-l-12-76_ _____________

F-l-12-23______________

F-l-ll-26_--______-___-

F-l-12-12____-____-_-_-

F-l-3-20-_________-____

F-l-ll-9____-_________-

F-l-12-24___-__________

Funafuti: 

15______.______________

491A--------_-_-------

4A-__---__-_--____--_-_

224A__--_-__-_--__--__-

668A___ — -__-_-- — _-_-_

Kita-daito-jima: 

595____--___-_____-____

691_--_----_-______-__-

557-------__---_-_--__-

737-____ — ,_-___-- — - —

Weight percent

Aragonite

(2) 

12±1

Calcite

2 

63±3

89 ±3 

53±3 

100

97 

(2)

88±1

66 ±1 

12±2

1 

76 ±2

96 ±1

Dolomite

(98) 

37±3 

11±3 

47±3

3

(2)

100 

100 

34±1

88±2

(99) 

24±2 

100 

4±1

Mol percent

MgCOs in 

calcite

3 

2 

0. 5 

2-3 

13. 5 

2 

(2)

'15;14

2

5-8

3-4 

i3;2

i4;3

CaCOa in 

dolomite

55. 2 

55. 6 

55. 2 

55. 2

55. 2 

55. 6

C)

54. 2 

56. 6 

56. 2

54. 2 

56.0 

55. 2 

55. 8

Weight percent

Mn in dolomite 

concentrate

0. 012 ±.004 

.004 ±.002 

. 004 ±. 002 

.005 ±.003

. 001- . 006

. 005 ±. 003 

. 005 ±. 003 

. 004 ±.002 

. 004 ±. 002

. 006 ±.003 

. 005 ±. 003 

. 005 ±. 003 

.005 ±.003

Fe in dolomite 

concentrate

0.004 ±.002 

. 004 ±.002 

.029 ±.006 

. 027 ±. 008

. 015±.008

. 001- . 004 

. 015 ±.005 

.009 ±.005 

. 005 ±. 003

.020 ±.006 

. 011±. 006 

.005 ±.003 

. 01-. 04

Maximum possible mol 

percent

MnCOsin 

dolomite

0. 026 

.01 

.01 

. 015

.01

3. 02

. 015 

. 015 

.01 

.01

.015 

.015 

.015 

.015

FeCOs in 

dolomite

0.01 

.01 

.058 

. 058

.038

3. 17

. 006 

.033 

.025 

.015

.043 

.028 

.015 

.065

1 Values obtained from back-reflection film measurements.

2 Only dolomite concentrate available.

3 Oiva I. Joensuu, spectrographer; these results were originally reported as weight percent MnO and weight percent FejOs. 

The other spectrographic determinations are by Juanita Witters.

4 Range of compositions between typical CaCOs-rich values and 1:1 molar CaCOs:MgCOs.
1050 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

twin glide for calcite occurring most readily, and prolonged grinding decreases measurable diffracted intensity markedly as progressively larger percentages of 

the crystal volume are occupied by disturbed regions 

resultant from differential movement. When mixtures of these minerals are ground together, the components incur structural damage at unequal rates, both 

because of the different energies required for glides in 

different minerals and because the more easily deformable component yields preferentially and protects the 

others. The problem is thus one of grinding fine 

enough to eliminate preferred orientation and other 

effects such as extinction and microabsorption while introducing no significant structural damage, or at least 

a constant and reproducible amount. Procedures such 

as that of Tennant and Berger (1957), involving long 

periods of ball milling, are presumed to introduce a 

reproducible amount of damage, but do not disclose the 

level of damage.

The experimental procedure used in the present study 

is applicable to smaller samples than would be required 

for ball milling and has permitted some monitoring of 

the grinding procedure influence. Initially, batches of 

Iceland spar from Taos, N. Mex., upper Eocene dolomite from Levy County, Fla., and aragonite from a 

fresh-water Unio shell were prepared by hand grinding 

small portions to smoothness in a small mullite mortar. 

The Florida dolomite is calcium rich, similar in composition and structure to the dolomites found in the Pacific 

core samples (Goldsmith and Graf, 1958a). A number 

of 2-gram calcite-aragonite and calcite-dolomite mixtures were then weighed into vials, mixed by the prolonged mechanical rotation of these vials through an 

eccentric orbit, and X-rayed without further grinding. 

A General Electric diffractometer and filtered copper 

radiation were used. A sample well 52 by 13 by 1.5 mm 

in size and a slit width of 3° were chosen in order to 

irradiate a relatively large mass of sample and thus to 

make weighing, sampling, and mixing requirements 

somewhat less stringent. No X-radiation penetrated 

to the bottom of the sample holder for the compounds 

studied. The proportional counter moved 1° in 20 per 

5 minutes, and the digital printer recorded every 20 

seconds (0.04° in 26). The results of 20 scans from_25° 

to 33°, including the dolomite {10T4}, calcite {10T4}, 

and aragonite {111} and (021} reflections, were plotted, 

and integrated intensities for those reflections were obtained by planimetering. Such data are suitable for 

constructing two-component standard curves, as employed by Klug and Alexander (1954).

The potential accuracy of such quantitative determinations of calcite, dolomite, and aragonite in mixtures 

is reduced somewhat by interference among the rather 

large number of reflections involved, although this

source of error proves to be much less important than 

those connected with grinding, as were discussed above. 

The integrated intensity for aragonite used in this study 

is actually the sum of those for the {ill} and {021} 

reflections, which are both quite strong and are close 

enough together that resolution is incomplete. For 

calcite-aragonite mixtures, it is necessary to subtract 

from the integrated aragonite intensity the contribution of (8 radiation from the very strong {10T4} reflection of calcite. The amount of /3 radiation passed by 

the filter, which was about 0.6 percent of the integrated 

intensity .of the a reflection, was determined for a 

calcite sample, and this percentage was then taken of 

the observed calcite {1014} radiation from calcite-aragonite mixtures. For a calcite-aragonite mixture that 

contains 5 weight percent aragonite, the /3 correction 

reaches 30 percent of the aragonite integrated intensity.

Similarly, graphical approximations based upon 

assumed symmetrical profiles for reflections in the 26 

region involved between 25° and 33° were made to 

resolve slight overlapping between {1014} of calcite and 

{1014} of dolomite, and between {1014} of dolomite 

{0006} of calcite. The weak {002} reflection of 

aragonite would interfere with {1014} of dolomite in 

dolomite-aragonite mixtures, but such mixtures were 

not encountered in this study. For calcite-dolomite 

mixtures in which both minerals are present in considerable amount so that miximum sensitivity is not 

of paramount importance, other reflections that do not 

interfere, such as {1123}, might be used to advantage. 

The intensity of the dolomite {1123} reflection, furthermore, would be relatively insensitive to ferrous iron 

substitution in ferroan dolomites, because this reflection 

owes its intensity primarily to contributions from 

oxygen atoms.

The integrated intensities derived from the mixtures 

are shown in figure 315 by the squares, triangles, and 

X's which do not have letter designations. The aragonite intensities fall accurately on a straight line that 

extends to the intensity for 100 percent aragonite (point 

C7), as should be the case because of the identical mass 

absorption coefficients of calcite and aragonite. The 

intensities for calcite in dolomite and dolomite in calcite 

fall, with somewhat greater scatter, on gently curving 

arcs. The curves actually shown in figure 315 passing 

through these two sets of points are theoretical ones. 

They were computed using mass absorption coefficients 

for copper radiation of 75.63 for calcite and 52.81 for 

dolomite containing 55 mol percent CaCO3 , and the 

relation (Klug and Alexander, 1954, p. 415)
SUBSURFACE GEOLOGY OF ENIWETOK ATOLL 1051

where /i is the integrated intensity of a selected reflection of component 1 in a two-component mixture, 

(/i)o is the integrated intensity of the reflection for 

a pure sample of component 1, Xl is the weight percent 

of component 1 in the mixture, and M* is the mass 

absorption coefficient of component 1. The experimental points for synthetic mixtures fit the curves 

satisfactorily.

The requirement of comparable textures in analysis 

by comparison with a standard makes necessary some 

appraisal of textural influences.

One independent check of the relative intensities of 

the calcite, dolomite, and aragonite reflections used 

in figure 315 may be obtained from the published 

data of Hanawalt, Rinn, and Frevel (1938) for the 

intensities of the calcite and dolomite {1014} reflections 

related to that of the NaCl {200} reflection, and 

from similar data of the Hanawalt group for aragonite 

that appears in the first set of ASTM powder diffraction 

cards. The relationship obtained from the data of 

these authors, who used the Debye-Scherrer method 

and molybdenum radiation, is

/NaCl{200) -/CalciteilOW} '/Dolomite! 1014) •/Aragonite(lll)+{021!

::150:125:100:92.

Samples of NaCl that have been hand ground to 

smoothness in a small mortar give a reasonably reproducible experimental value of

/NaCl 1200)

(see fig. 315), although this value can be reduced at 

least 25 percent by moderately prolonged grinding. 

If a ± 5 percent accuracy is assigned to the Hanawalt 

values, because they are given by those authors only 

to the nearest five units, the predicted ranges A, B, 

and C of figure 315 are obtained for the integrated 

intensities of, respectively, the calcite, aragonite, 

and dolomite reflections being used. Range D, for 

dolomite containing 55 mol percent CaCO3 instead of 

50 mol percent, was obtained by shifting range C 10 

percent of the distance toward range A. The difference between ranges C and D is an insignificant part 

of the total ranges. The predicted and observed 

intensities for calcite are in ideal agreement, and 

those for dolomite are quite satisfactory, especially 

in view of the structural and compositional irregularities shown by sedimentary dolomites (Goldsmith 

and Graf, 1958a). The Hanawalt aragonite value 

and that obtained from Unio shell material disagree 

by a factor of four. No explanation is apparent for 

this striking discrepancy; differences resulting from 

the use of molybdenum and copper radiations, respectively, for the two determinations are quite inadequate

to explain the anomaly. The integrated intensities 

for powdered aragonite crystals from Herrengrund, 

Hungary, and Girgenti, Sicily (G in fig. 315) agree 

well with that for the Unio shell material, which does 

suggest that the textures and resultant integrated 

intensities obtained by processing natural rocks would 

be comparable.

The ratio /caicite{ioi4!//Doiomite{ioi4), shown in figure 315, 

may be checked by a further computation. Using the 

variable parameters for calcite (Chessin and Post, 1958) 

and dolomite (Steinfink and Sans, 1959) accurately determined from single crystal studies, the scattering factors of Berghius and others (1955) for calcium, carbon, 

and oxygen, those of the same authors for Mg+2 but

modified at low values of —— in the direction of the A

neutral Mg atom, and appropriate Lorenz and polarization corrections, the calculated intensities for the two 

reflections give a ratio of 1.47. The inclusion of 

absorption factors (for diffractometer mounts, the 

reciprocals of the linear absorption coefficients, see 

Klug and Alexander, 1954, p. 397) lowers the ratio 

to 1.03. It is possible that the additional inclusion 

of temperature factors would increase the ratio. It 

should also be remembered that the Hanawalt group 

used filled capillaries of uniform diameter, a procedure 

that would give a spuriously low intensity for a material of low absorption if the capillary diameter were 

such that X-radiation just reached the sample center 

for more highly absorbing materials.

Continued hand grinding of several of the mixtures 

in a small mortar decreased calcite intensities markedly 

but had little effect upon those of dolomite and aragonite. Additional measurements were then made upon 

several calcite and dolomite samples in order to study 

the effect of grinding on the separate materials. Small 

parts of a sample of the Iceland spar from Taos, N. Mex. 

enough to fill the X-ray sample holder, were ground by 

hand in a small mortar for a total of 3 hours, and the 

integrated intensity indicated by point T in figure 315 

was obtained. Aragonite can be formed from calcite 

by prolonged grinding (Burns and Bredig, 1956; Jamieson and Goldsmith, 1960), but none was detected, 

after the moderate degree of grinding described, in 

this Iceland spar sample. A sample of Dover chalk, 

disaggregated but not further ground, gave an integrated intensity ("H" of figure 315) similar to that of 

the Iceland spar at T; this material may contain a 

small amount of poorly crystallized silica. Sedimentary dolomites from Kankakee, 111., and Carey, Ohio, 

ball milled for 8 hours, gave integrated intensities 

shown by points K and 0, respectively, in figure 315. 

Parts of large cleavage pieces of dolomite from Gabbs,
1052 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

Nev., whose CaCO3 and MgCO3 contents are within 0.5 

mol percent of the formula value (Goldsmith and Graf, 

1958b), were hand ground in the same manner as the 

Iceland spar sample, but intensity was reduced only to 

point W in figure 315.

More severe structural damage could be produced in 

these compounds by extended grinding with motordriven mortars (Bradley, Burst, and Graf, 1953), and 

there are unusual naturally occurring magnesian calcite 

and dolomite sediments (Alderman and Skinner, 1957) 

more poorly crystallized than any described here. On 

the whole, however, the materials examined and the 

grinding employed should be an indication of what to 

expect in attempting to determine proportions of these 

carbonates in powders using the diffractometer. The 

significant conclusion is that the observed integrated 

intensities for calcite will fall much farther below ideal 

values than those for aragonite and dolomite.

The Pacific samples are found to be very pure twophase carbonate mixtures, and it is therefore possible 

to require that values read from the appropriate two 

curves must total 100 percent, that is, they must lie 

vertically above each other in figure 315. Although 

this requirement is a more stringent test of the experimental data than the alternative of reading percentages 

from a plot of intensity ratios versus composition (see 

Tennant and Berger, 1957), it is not rigorous. Poorly 

ground preferentially oriented calcite associated with 

fine-grained poorly crystallized dolomite could give a 

highly concordant analysis spuriously high in calcite; 

well crystallized preferentially oriented dolomite that 

was protected during the protracted grinding and serious 

damaging of calcite could, at least in theory, give a 

highly concordant analysis spuriously low in calcite. 

If the mineralogical analysis of such a two-phase rock 

gives a total greater than 100 percent, one can keep 

regrinding and reanalyzing until the total drops to 100 

percent.

Initial examination of several of the Pacific core 

samples, however, using the calcite-in-dolomite and 

dolomite-in-calcite curves of figure 315 based upon 

standard mixtures, gave totals markedly below 100 

percent and suggest that the calcite was already diffracting with substandard efficiency. The empirical 

calcite-in-dolomite and calcite-in-aragonite curves that 

end at point T of figure 315 were then tried, in conjunction with the aragonite-in-calcite and dolomite-incalcite curves based upon standard mixtures, as 

representing a level of calcite structural quality that 

could be attained for all samples. It proved to be 

possible, using these curves and a single hand grinding, 

to get values agreeing to within ±2 weight percent for 

the Kita-daito-jima and Funafuti samples. The Eniwetok rocks, even after a second grinding, gave values

agreeing only to within ±3 weight percent; it is probable that they differ texturally from the other samples. 

In two samples where only 1 or 2 percent of one carbonate was indicated, and the integrated intensity for 

the other gave a value greater than 100 percent, the 

second value was obtained by difference. Even an 

error of 100 percent in the integrated intensity of the 

phase present in very small amount would still give 

results within ±2 weight percent of the true value, 

the standard generally sought here.

CALCITE

Precision determinations of the magnesium content 

of the calcite in these samples, using back reflections, 

are frequently not possible. The calcite, derived from 

skeletal remains that originally contained a wide range 

of magnesium concentrations, sometimes still shows a 

compositional range. In other samples, the amount of 

calcite remaining after dolomitization is so small that 

the back reflections are not recorded.

The procedure used for those samples containing 

both calcite and dolomite involves measuring the positions of maximum intensity for the {1014} reflections 

of calcite and dolomite by counting with the diffractometer at 0.01° intervals in 26 across the maxima. 

The difference between the dolomite {1014} spacing so 

obtained, and that computed from back-reflection measurements of the dolomite concentrate from the same 

rock, give a correction which is applied to the calcite 

{10T4} spacing. The procedure is less accurate when 

either of the two minerals is present in small amount 

and affords only a low gently sloping peak.

Back reflection film measurements were made for 

samples Eniwetok F-l-3-20 and Kita-daito-jima 557, 

which contain no dolomite, and for samples Funafuti 

15, Kita-daito-jima 691, and Kita-daito-jima 737 as a 

check on the diffractometer method described above.

Most of the calcite in these rocks contains a very low 

percentage of the magnesium in solid solution (table 7), 

which indicates that the magnesium has been lost extensively from inorganic skeletal remains. Where moderate amounts of magnesium are still present in the 

calcite (Funafuti 668A, Kita-daito-jima 595), a range 

of composition is present and presumably indicates 

either a diversity of composition in the original mixed 

skeletal debris or a variation in the rate of magnesium 

loss from different particle sizes, or both.

DOLOMITE

Dolomite concentrates were obtained from these 

rocks by leaching with cold dilute (about 2 percent) 

acetic acid. The values given in table 7 for mol percent CaCO3 in these dolomites are based upon measurements of the {4044} and {3148} back reflections on
SUBSURFACE GEOLOGY OF ENIWETOK ATOLL 1053

films taken with 114.59-mm diameter Straumanismount powder cameras, using manganese-filtered iron 

radiation. Changes of the d-spacings of these reflections with composition, for compositions near dolomite, 

are assumed to be linear (Goldsmith and Graf, 1958a). 

Spectrographic analyses of iron and maganese are also 

given in table 7, together with the corresponding mol 

percents of MnCO3 and FeCO3 that would result if the 

iron and manganese found were all in solid solution in 

the dolomite. The amounts of iron and manganese present are quite inadequate to produce significant changes 

in dolomite cell size, particularly in view of the fact that 

some of the iron is present as small particles of iron 

oxides that could not be completely removed during 

beneficiation.

All the dolomite fractions from these rocks contain 

CaCO3 in solid solution in excess of the 50 mol percent 

value of ideal dolomite. The excess CaCO3 is typically 

about 5 mol percent, as shown in table 7. No dolomite 

approaching the ideal molar ratio was observed.

The general characteristics of calcium-rich dolomites 

of this type are discussed by Goldsmith and Graf 

(1958a). In general, X-ray diffraction patterns of these 

materials have weakened order reflections, and reflections from basal planes that are broadened or diffuse 

relative to those reflections with little or no c-axis component. Heat treatment of the dolomite concentrate 

from Eniwetok sample F-l-12-24 clearly indicates 

that such a material which has excess CaCO3 and a defective c-axis succession in the structure, is metastable 

at earth-surface temperatures.

Several observations on the Pacific samples should be 

made in. addition to the general discussion by the above 

authors. X-ray powder patterns of the coarse and fine 

fractions of the dolomite crystals from sample F-l-12- 

24 showed significantly more occluded calcite in the 

coarse fraction. Little or no calcite could be detected 

in the powder patterns from the tiniest crystals, whereas 

the strongest reflection from calcite was of moderate 

intensity in the pattern made from the largest crystals. 

Several single-crystal X-ray photographs showed a 

weak Debye pattern of calcite superimposed on the 

single-crystal pattern of dolomite. This indicates that 

finely divided randomly oriented calcite was in these 

crystals. No compositional or structural difference in 

dolomite of the smallest and largest crystals could be 

detected. The calcite would thus seem to be relict from 

the original undolomitized rock and not an exsolution 

product from the calcium-rich dolomite.

Weissenberg photographs of several of the samples 

(Goldsmith and Graf, 1958a, plate 2Z>) show some 

interesting features. Moderately sharp reflections are 

apparent at positions appropriate for the calcium-rich 

dolomite and, in addition, all reflections show diffuse

tails that extend to the positions characteristic of ideal 

dolomite. In addition, the strongest reflections have 

weak-intensity maxima in the diffuse regions. The fact 

that reflections both normal and parallel to the c-axis 

(including order reflections) show this bimodal character indicates that these crystals could be considered 

multiphase in nature. One intrepretation of this observation is that the calcium-rich dolomite has begun 

the reorganization, akin to exsolution, necessary to 

achieve the ideal dolomite composition and structure. 

The intensity distributions observed in X-ray photographs of the Pacific materials should not be confused 

with those of similar calcium-rich dolomites from other 

localities (Goldsmith and Graf, 1958a, fig. 2c) that have 

bimodal intensities only for reflections with strong 

c-axis components. We have suggested that the latter 

materials may have a disturbed succession of basal 

cation planes.

It is now apparent that many dolomites, at least in the 

early stages of their formation, are calcium rich and 

structurally imperfect. It has not yet been demonstrated however, that it is possible for calcium-rich 

dolomites existing in natural sedimentary environments 

to reorganize to ideal dolomite, but the process has been 

observed experimentally at somewhat elevated temperatures (Graf and Goldsmith, 1956).

ARAGONITE

Aragonite has not been observed in those rocks that 

contain dolomite (table 7, and discussion of petrology). 

It is apparent that the metastable aragonite (Jamieson, 

1953; MacDonald, 1956; Clark, 1957) is destroyed by 

whatever diagenetic process is responsible for the forr 

mation of replacive dolomite. The tendency toward 

the development of an equilibrium assemblage is evinced 

by both changes in mineralogy. 7

DATING OF CARBONATE ROCKS BY IONIUM-URANIUM

RATIOS

By William M. Sackett and Herbert A. Potratz 8

The extent of disequilibrium between certain members of the three naturally occurring radioactive families 

in ocean water and in marine calcium carbonates has 

been investigated. The Th230/U238 ratios in these 

materials have been determined to ascertain the conditions under which this ratio may serve as a measure 

of geologic age.

The authors are indebted to the Aluminum Company 

of America and to the Van Blarcum fund of Washington University for support of this wor k through fellow7 As discussed by Schlanger in a preceding section it is entirely possible that all the 

aragonite is converted to calcite prior to dolomitization. If so, the formation of 

dolomite may have nothing to do with the lack of aragonite.

s Department of Chemistry, Washington University, St. Louis, Missouri.
1054 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

ships; to the U.S. Atomic Energy Commission for 

financial support under contract number AT (11-1)-581; 

to the Los Alamos Scientific Laboratory for samples 

and the loan of equipment; to Meyer Rubin and W. S. 

Broecker, U.S. Geological Survey, for readiocarbon 

dating of a number of the marine carbonates; to E. D. 

Goldberg and the Scripps Institution of Oceanography; 

to Thor N. V. Karlstrom, Harry S. Ladd, and J. I. 

Tracey, Jr., of the U.S. Geological Survey; and to J. C. 

Brice, R. P. Bullen, and C. R. McGimsey, who supplied 

the samples that formed the basis for this investigation. 

We are most grateful to Harry S. Ladd, S. O. Schlanger, 

and J. I. Tracey, Jr., for their helpful suggestions made 

during the course of the work and in the preparation of 

this report. Much of the material is taken from a dissertation presented by William M. Sackett to the Graduate Board of Washington University in partial fulfillment of the requirements for the degree of Doctor of 

Philosophy, June 1958.

PREVIOUS WORK

Barnes, Lang, and Potratz (1956), in work done at the 

Los Alamos Scientific Laboratory, found that coral limestone cuttings from the uppermost 200 feet of drill hole 

F-l on Elugelab Island in Eniwetok Atoll exhibited an 

irregular increase of ionium to uranium with increasing depth (fig. 316). The possibility of making 

the disequilibrium of ionium and uranium the basis of 

a method for determining geologic age was mentioned 

(Potratz, Barnes, and Lang, 1955; Barnes, Lang and 

Potratz, 1956).

The work here described is a continuation of that 

done by the Los Alamos group. The object has been 

to study in general the disequilibria in the three radioactive families of some naturally occurring materials, 

with the ionium-uranium disequilibrium and its use in 

dating calcium carbonates as the prime interest.

The main modes of decay for the three families are 

illustrated in figure 316.

For undisturbed samples 'of very old materials, a 

situation is reached in which the rate of formation of 

any daughter is equal to its rate of decay, a situation 

known as secular equilibrium. In nature, however, 

processes may occur which separate parent from daughter. Parent or daughter elements may, moreover, be 

brought into a material from external sources. Such 

natural processes are usually slow, and, consequently, 

disruption of equilibrium in the radioactive families 

will be noticeable only if the separation or enrichment 

processes involve nuclides with relatively long half-lives.

The disequilibria that one may expect to observe in 

the three naturally occurring radioactive families usually 

arise because of gain or loss of one or more of the 

following nuclides:

1. Uranium series

U238 (4.5X109yr), U234 (2.5X105yr), Th230 

(80,000 yr), Ra226 (1622 yr), Pb210 (22 yr), Po210 

(138 days)

2. Actinouranium series

U236 (7.1X108 yr), Pa231 (34,000 yr), Ac227 22 yr)

3. Thorium series

Th232 (1.4X1010 yr), Ra228 (6.7 yr), Th228 (1.9 yr).

In the foregoing listing the figures in parentheses are 

the half-lives of the nuclides (Friedlander and Kennedy, 

1955). The other radioactive daughters in the three 

families have half-lives of less than one month and 

should be generally in equilibrium with the long-lived 

parents listed above. The possibility of Rn222 loss 

should, however, be considered. This gaseous nuclide 

has a half-life of only 3.8 days. Uranium ores are 

nevertheless sometimes deficient in Rn222 and daughters 

because of continuous Rn222 loss. Because of the very 

short half-lives of Rn219 and Rn220, loss of these nuclides 

is probably negligible.

The disequilibrium situations in the naturally occurring radioactive families may provide clues to the geologic history and age of various materails. The dating 

of marine sediments by means of U238, Th230 , Ra226 

disequilibrium was one of the first applications. Early

. 238

234

234

230

226

222

218 n_214

Bi^ | Bi : 

214 p'b 21o"

235

231

Th 227

Ac 227

223

210

Pb Pb 206

223

219

215

,211

,211

207

U 207

Th 232 Th 228

228 

Ra"u Ra

Ac 

228 * 224

220

Po216 Po212

,212

208

FIGUBE 316.—Uranium, actinouranium, and thorium decay series.
SUBSURFACE GEOLOGY OF ENIWETOK ATOLL 1055

workers had found a large amount of radium activity 

in marine sediments (Joly, 1908). To explain this 

phenomenon, Pettersson in 1937 proposed that ionium 

(Th230) was coprecipitated on ferric hydroxide from the 

oceans, giving rise to ionium-supported radium on the 

ocean floor. Piggot and Urry (1939) a short time later 

found in successively deeper layers of cores of deep-sea 

sediments an increase of radium activity with depth 

until equilibration with ionium was attained. Below 

this point, the activity decreased exponentially with 

depth. Sedimentation rates were obtained from their 

work.

ISOTOPIC THORIUM CONTENT OP OCEAN WATER

The data complied by the Los Alamos group indicated 

that recently deposited limestones from Eniwetok 

contain several parts per million of uranium, but less 

than 1 percent of the equilibrium amount of ionium. 

When this work was begun in 1954, little was known 

about the ionium content of ocean water and it seemed 

that an experimental value was needed to answer the 

basic question: Does the calcium carbonate (aragonite 

and calcite) deposited by corals and other marine 

organisms contain no ionium because the organisms 

grow in an ionium-free environment, or do these organisms take up uranium and reject ionium?

Ocean water contains approximately 3X10~6 grams 

of uranium per liter (Nakanishi, 1951; Rona, Gilpatrick 

and Jeffy, 1956). This corresponds to an equilibrium 

ionium concentration of 5X1CT11 grams per liter. The 

lack of information concerning ionium concentration 

in ocean water is related to experimental difficulties 

associated with determining the nuclide at this extremely low concentration. The precipitation of ionium as 

proposed by Pettersson (1937) suggests that the actual 

concentration may be considerably lower than the 

equilibrium value.

Other groups have been interested in determining the 

ionium content of ocean water in order to understand 

more completely the factors involved in the ionium 

method of dating marine sediments.

In 1954 Holland and Kulp attempted to calculate the 

ionium concentration by considering the four main 

factors that regulate the concentration of radium, one 

of which is its growth from ionium. By assuming no 

change of radium concentration with time, by knowing 

the decay constant and approximate concentration of 

radium, and by estimating its deposition rate on 

sediments and its rate of influx from rivers, they were 

able to calculate a value for the ionium concentration 

of ocean water (table 8).

Koczy, Picciotto, Poulaert, and Wilgain (1957) using 

concentration of thorium isotopes by coprecipitation 

on ferric hydroxide, purification by ion exchange and

TABLE 8.—Calculated concentrations, in grams per liter, of thorium 

isotopes in ocean waters

Nuclide

Th230 ___

Th227 ._.

Th232___

Th228 __.

Deep ocean sample

i <3X10-13

<2X10-19

<5XlO-s

<7X10~18

2 <0. 6

percent

2 <15

percent

Surface sample

<2. 3X10~12 2 <5

percent

3. 8X10-" 2 27

percent

<4X10-7

2X10~17

1 Holland and Kulp (1954, p. 210) give the value 3.1 ± I.OXIQ-" grams per liter. 

Koczy and others (1957, p. 103) give limit 6X10-" grams per liter.

2 Percentages represent the fraction of secular equilibrium for Th^so and Th^? in 

the deep ocean and surface samples.

solvent extraction, and isotopic analysis by photographic emulsion techniques, obtained the first experimental values for the thorium isotopic content of ocean 

water. However, their water may not have been 

"typical" ocean water because it was collected in and 

around Scandinavian fjords (table 8).

The following work was done on two samples of 

Pacific Ocean water, one collected at the surface 

outside San Diego Bay in the summer of 1956 and the 

other at a depth of 3,500 meters from long 124°41.0/ W., 

lat 33°54.5' N., on March 25, 1957 (Sackett, Potratz, 

and Goldberg, 1958). The procedure included the 

collection of the thorium isotopes on ferric hydroxide, 

purification by ion exchange techniques, and isotopic 

thorium determination by observing the growth and 

decay of total thorium alpha activity (Sackett, 1958 9).

Experimental values are listed in table 9. The 

fractions listed as S-2 and D-2 were obtained by 

reworking the resin washings. The listed thorium 

activities have been corrected for counter background, 

yield of thorium, 51 percent counter geometry, and 

reagent blank.

The initial activities of Th227, Th228 , and Th232 +Th230 

in the surface-water sample are calculated from the 

growth and decay of the total alpha activity in the 

thorium that is separated from the sample. For the 

S-2 fraction of the surface water sample, the activities 

are:

ATh227=l.Q disintegrations per hour per liter 

AThm=l.$ disintegrations per hour per liter 

^4Th232+ylTh230 =6.1 disintegrations per hour per 

liter

Using these activities and the known half-lives of the 

isotopes, the concentrations of the individual thorium 

species in the surface sample are calculated. They are 

listed in table 8.

Also listed in table 8 are the results for the deep-water 

sample. The limits are arrived at by letting each 

thorium isotope have the maximum possible alpha 

activity listed (0.8 disintegrations per hour per liter).

9 Sackett, W. M., 1958, Ionium-uranium ratios in marine deposited calcium 

carbonates and related materials: Ph. D. thesis, Washington University.
1056 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

TABLE 9.— Thorium alpha activity in Pacific Ocean water

Sample designation

Surface fraction 

S-l_. .____.____._..._.__.

S-2

Deep ocean fraction 

D-l____ ..__.._.___-___.__

D-2_. _.._.. .__ ___-___-

Percent Th23« 

tracer recovered

10

40

28

45

Thorium alpha activity in disintegrations per hour per 

liter at time of 

separation

9 (±2)

9. 6 (±0. 5)

.3 (±0.3)

.4 (±0.4)

The activities of the individual thorium species in D-l 

and D-2 could not be determined because of the extremely low counting rates (about 0.1 count per minute).

The percentages listed in table 8 for Th230 and Th227 

represent the fraction of secular equilibrium for the 

two isotopes in the deep and the surface samples. They 

are based on a uranium concentration of 3.0X1CT6 

grams per liter (130 U238 disintegrations per hour per 

liter).

The 5 percent or less concentration of Th230 (table 8) 

found in the surface-water sample and the less than 1 

percent found in the deep-water sample show that the 

Th230 content was far below the amount required for 

secular equilibrium with the U238 present in the water 

samples.

The sea water around Eniwetok Atoll, where the Los 

Alamos group obtained the coral they analyzed, should 

be devoid of contamination from continental land 

masses because of its location in the middle Pacific. 

It is thus probable that the deep ocean sample more 

nearly represents the Eniwetok area sea water than 

does the surface sample, which was collected near the 

coast and which contained sand and other solid particles. Making this assumption would mean that the 

organisms on the reefs grew in an environment that 

was very poor in ionium. Table 8 lists a conservative 

less than 0.6 percent of the equilibrium quantity, but 

it is probably less than 0.1 percent. Thus these organisms get little if any ionium. The question of 

whether or not marine organisms reject ionium cannot 

be ascertained.

The results shown in table 8 bring to light several 

other points of interest.

The equilibrium Th227 activity corresponding to a 

uranium content of 3.0X10" 6 grams per liter is 6 

disintegrations per hour per liter and gives the results 

listed in the table of less than 15 percent and 27 percent 

of the equilibrium amount of Th227 in the deep ocean 

and surface samples, respectively. This indicates 

that not only Th230 but apparently Pa231 and (or) Ac227 

are precipitated with the sediments. It is probable 

that the limiting species is Pa231 , because it is so readily

coprecipitated in neutral and basic solutions. 10 The 

larger amount of Th227 in the surface shore sample is 

most likely due to the moderate solubility of Ac(OH) 3 , 

which may be either redissolved from the sediments or 

enriched in the shore waters from continental run-off 

water.

Development of techniques for determining very 

small amounts of protactinium might lead to a method 

for dating marine deposits.

If the Ra226 content of ocean water is taken as 1.3 X 

10~13 grams per liter (A-Ram= 17 disintegrations per hour 

per liter) (Faul, 1954, p. 116), then for the deep sample,

Thus the radium content is far in excess of the amount 

that can be supported by the ionium which is present. 

Koczy and others (1957) report a similar situation in 

their samples and suggest that the excess radium may 

arise from a redissolution of the radium that originated 

in the sediments.

Marine organisms growing in this environment may 

possibly reflect this excess radium in their carbonate 

skeletons, a condition that could give rise to reports of 

concentration of radium by various organisms. Nonsupported radium in marine limestones might be used 

in geologic dating if the amount that was present at 

the time of deposition could be estimated, possibly 

from the amount present in limestone now being deposited in the same locality as the material under 

consideration.

Koczy and coworkers also report Th228 in excess of 

the amount that can be supported by the Th232 present 

in several of their samples and again account for this 

phenomenon by suggesting either a redissolution of 

Ra228 from the near-shore sediments or by introduction 

from rivers. Because of the relatively short half-lives 

of Ra228 and Th228, this excess should be observed only 

near the shores or along the bottom.

Some preliminary work on river water has indicated 

that even at a pH = l, Th234 tracer is more than 90 

percent adsorbed on the suspended material. With 

this information, the following conclusions may be 

stated:

1. Significant quantities of thorium isotopes probably 

do not enter the ocean in solution, because thorium 

seems to be completely adsorbed on the river 

sediments, which in turn are probably largely deposited on the continental shelves. Thus, it would 

expected that the only Th230 and Pa231 (which is 

also easily coprecipitated) that are deposited on 

the main body of the ocean floor result from the 

decay of the U238 and U235 in solution in the ocean.

">ThepH of ocean water is between 7.5 and 8.4 (Sverdrup, Johnson, and Fleming, 1942, p. 194).
SUBSURFACE GEOLOGY OF ENIWETOK ATOLL 1057

As most of the Th232 is also deposited on the continental shelves, practically none of it or its 

daughters should be found in deep-sea sediments. 11

2. The more soluble, long-lived daughters—Ra226 in 

the uranium series, Ac227 in the actinouranium 

series, and Ra228 in the thorium series—may enter 

the oceans with the continental runoff waters or 

by a redissolution from the sediments.

3. The disequilibrium situations created by the excess 

of Ra226 , Ac227 , and Ra228 might be used to give 

time differentials in some marine processes.

IONIUM-URANIUM RATIOS IN MARINE LIMESTONES

If marine organisms deposit aragonite and calcite 

that (1) contains a measurable amount of uranium, 

(2) has very much less than the equilibrium amount of 

ionium (Th230), and (3) does not gain or lose uranium 

or lose the ionium that grows into these minerals from 

the radioactive decay of uranium, then the time at 

which the calcium carbonate was deposited can be 

determined from the ionium-uranium ratio within the 

experimental error of the analyses for materials from 

approximately 1,000 to 300,000 years old.

Should the calcium carbonate fulfill only condition 

(1), clues to the geologic history of the material might 

still be obtained from the ionium-uranium ratio.

URANIUM CONTENT OF MARINE LIMESTONES

If the disequilibrium of uranium and ionium is to be 

used in the geologic dating of limestones, the usefulness 

of the method will depend to a large degree on the 

reliability of the analyses. The best analyses and age 

estimates should be obtained for the materials with the 

highest uranium contents, as the amount of ionium is 

also dependent upon the amount of uranium.

From the results shown in tables 10 and 11, it can be 

seen that the uranium contents of the materials listed 

vary by a factor of 30 or more. The reason for this 

wide spread is of some concern, because much time may 

be saved by the selection of only those materials for 

age determination whose uranium contents are likely 

to be high:

Several factors may influence the uranium content 

of marine limestones. One is the concentration of 

uranium in the ocean waters from which the limestones 

were deposited. However, during the time span for 

which the ionium-uranium ratio might be used 

(300,000 years), the uranium concentration in the oceans 

has probably changed very little. Holland and Kulp 

(1954) estimated that the uranium concentration has 

changed less than 20 percent in 107 years. For very

TABLE 10.— Uranium content versus crystal structure of marinedeposited calcium carbonates

[Uranium content was determined by asodium fluoride-fluorimetric precedure (Price, 

Ferretti, and Schwartz, 1953; Orimaldi and Levine, 1954; Sackett, 1958). As a 

check on the validity of this procedure some samples were also analyzed by a neutron activation method (Moore, 1957; Sackett, 1958). N.D., none detected]

Sample

Mu-7-4__ —— _ __

Mu-7-13--_--____

Mu-7-23_..____ _

Mu-7-41__. ______

Ru-2-14__.__ _ _

En-6,-29______--_

F-l-1-1 _ _______

F-l-5-37_________

F-l-6-10____-____

F-l-10-5— -.__--

F-l-12-8____- __

F-l-14-24_.____-_

E-l-1-5— ____---

E-1-2-5- — ____--

E-l-3-7— __.___-

E-l, 1 0-20 »—____

E-l, 30-40 i_——— _

E-l, 40-45 1— ____

Major crystal 

constituent

Aragonite _ _ _ _

_.___do_________

___..do_________

__--_do__. ______

__-__do_______ __

__._,do_________

Aragonite and

calcite.

_____do_____ _ _

_____do__. ______

Dolomite and

calcite

Calcite- _______

Calcite __ _____

-_-__do_________

..—do -._-_-_-

Minor crystal 

constituent

N.D__________

N.D__________

N.D __ _ ... _

N.D...._ ____

N.D_____ _

N.D__________

N.D.__ __ ___

N.D__________

N.D__________

N.D__________

Aragonite____

> Calcite__ ___

Uranium 

content 

(ppm)

2. 1

2.7

2.4

2.0

2.5

2.2

3. 1

.34

.26

1Q

.46

.66

1.05

3. 1

. 17

.67

3.46

2.71

1 Indicates cuttings. The others are cores. Crystal structures were determined by 

X-ray diffraction. The X-ray photographs of the cuttings were made by Donald 

Graf.

old limestones, other factors, such as recrystallization, 

are more important in determining their present 

uranium content.

Another factor that seems to be related to the uranium content is the crystal structure of the calcium 

carbonate. Marine-deposited calcium carbonates may 

exist as either calcite (rhombohedral) or aragonite

TABLE 11.— Uranium content of calcium carbonate deposited by 

some marine organisms

Description

Reef coral (Porites australiensis 

Vaughan) . 

Clam (Tridacna):

shell.

shell.

(gastropod) .

(Porolithon 

craspedium) .

posit (Millepora) 

on coral listed 

next.

16 F-l cuttings 2 -__-

21 E-l cuttings 2 _-__

Sample No.

JIT-1______

JPEM-1___

TNVK-1-.

260A-67-6-

CKW-1____

CKW-2---.

Crystal structure

(Aragonite) J _

__ — _do__ — __ ._

calcite.

_._._do-_ ——— .

__-__ 1 do____-_-_

_____! do____-___

Uranium 

(ppm)

2. 4

. 1

. 32

.37

. 42

1. 83

3 3. 89

3 2. 34

11 Submarine volcanos, wind-blown and meteoric material, and possibly very finely 

divided colloidal material brought in by rivers may supply Th232 and other nuclides 

to the midocean areas.

1 Structures have not been determined experimentally.

2 The F-l uranium results were obtained by Los Alamos Scientific Laboratory. 

The cuttings contained coral, Foramtnifera, mollusk shells, echinoid spines, Halimeda segments, and so on. (Ladd and others, 1953).

3 Average.
1058 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

(orthorhombic). Bragg (1937) mentions that bivalent 

ions smaller than calcium tend to form carbonates 

with the calcite structure, and ions larger than calcium 

tend to form carbonates of the aragonite type. Strontium, for example, whose bivalent ion is larger than the 

bivalent ion of calcium, tends to form a carbonate with 

the aragonite structure. Thompson and Chow (1955) 

and Kulp, Turkeian, and Boyd (1952) have found 

that, in general, aragonite deposited by marine organisms contain three to four times more strontium than 

does organically deposited calcite. Thompson and 

Chow have determined that the atomic ratio of strontium to calcium in sea water is 8.90 X 10~3 and that 

many of the aragonite skeletons of marine organisms 

have very nearly this same ratio. Thus for these 

carbonates there seems to have been an isomorphous 

replacement of calcium by strontium according to the 

homogeneous distribution law. Uranium seems to 

exhibit the same behavior as strontium. Uranyl ion 

the stable bivalent cation of uranium in ocean water, is 

large, and its mineral carbonate, rutherfordine, crystallizes in the orthorhombic form (Christ, Clark, and 

Evans, 1955). Therefore, one would expect aragonite to contain more uranium than calcite, which is 

shown generally to be so for the data in tables 10 and 11. 

The concentration of calcium in ocean water is 0.40 

gram per liter (Rankama and Sahama, 1950, p. 287). 

Taking the concentration of uranium in ocean water as 

3.0 X 10~6 grams per liter, one may calculate the atomic 

ratio of uranium to calcium to be 1.2 X 10~6, which is 

about the ratio for a carbonate that contains 99 percent calcium carbonate and 2.7 ppm uranium. Most 

of the values for aragonite in table 10 are within ±20 

percent of this value.

Two coral specimens, JIT-1 and CKW-2 (table 11), 

and the F-l and E-l cuttings, which are mainly coral, 

all have a relatively high amount of uranium. The 

other aragonitic materials in table 11 have lower uranium contents. This evidence indicates that certain 

coral organisms may concentrate uranium to some 

extent or possibly cannot differentiate between calcium 

and uranium in ocean water, whereas other organisms 

may reject uranium.

The calcium required for the skeleton of marine 

organisms is believed to be obtained directly by adsorption from the sea water or indirectly by the ingestion of 

plant or animal food. The carbonate in both cases is 

thought to be derived from metabolic carbon dioxide 

(Robertson, 1941).

Uranium is present in ocean water in the sexivalent 

oxidation state, probably not as uranyl ion, but rather 

as the uranyl carbonate and double carbonate complex 

ions, whereas the calcium may be largely present as the 

dipositive ion. That is, the UO2+2/Ca+2 ratio is probably much less than the uranium-calcium ratio. Hence, 

if isomorphous replacement were the only factor that 

required consideration, the uranium-calcium ratio in 

calcium carbonate would probably be considerably less 

than the uranium-calcium ratio in ocean water. If an 

adsorption process is being used by an organism to 

obtain calcium, the different adsorption characteristics 

of the U(VI) and Ca(II) in ocean water should lead to a 

uranium-calcium ratio in the calcium carbonate which 

differs from the uranium-calcium ratio in ocean water.

Rao and Goldberg (1954) believe that specific enzyme 

systems in the gut may extract needed constituents that 

are adsorbed to the mucous. However, entry of ions 

directly into the mantle tissue from the adsorbing 

medium is sometimes indicated.

If the calcium is derived from the ingestion of plant 

and animal material, then the uranium-calcium ratio in 

the calcium carbonate produced by this mechanism 

might depend to a large extent upon the uraniumcalcium ratio in the material being ingested. Thus, 

unless the uranium serves some biological need and is 

concentrated by biological processes of the organism, 

it is not likely that the calcium carbonate produced 

would have a high uranium content.

In conclusion, high uranium values for marinedeposited calcium carbonates seem to be directly associated with aragonitic calcium carbonate. This may 

possibly result from the fact that some uranium specie 

can substitute for calcium in the aragonite structure. The possibility of uranium uptake by organisms 

which derive calcium by an absorptive process should 

also be considered.

IONIUM -UKANIUM KATIOS IN CUTTINGS FKOM ENIWETOK 

ATOLL DKILL HOLES

SixtBen samples of cuttings from the uppermost 190 

feet of the F-l hole were analyzed for ionium and uranium by the Los Alamos Scientific Laboratory (Barnes, 

and others, 1956). An irregular increase in the ioniumuranium ratio with depth was observed (fig. 317 and 

table 12).

Twenty-one samples of cuttings from the first 200 

feet of the E-l hole were analyzed by the writers, and 

again an increase in the ionium-uranium ratio was 

observed with increasing depth (fig. 317 and table 13).

We must recognize at the outset that probability of 

sampling error during the drilling operations is significant. 

The exact depth from which cuttings come cannot 

generally be determined with high accuracy, and the 

cuttings from a given interval may be contaminated by 

material from above. However, the similarity of the 

ionium-uranium ratios at the same depths over much of 

the first 200 feet in the two holes indicates an expected 

parallel development of the coral reefs of the atoll
SUBSURFACE GEOLOGY OF ENIWETOK ATOLL 1059

TABLE 12.—Ratio of ionium to uranium in cuttings from drill

hole F-l

[Data from Barnes, Lang, and Potratz, 1956. The uranium results are average 

values and the estimated standard deviation was ±5 percent. The uncertainty in 

the ionium values is the expected standard deviation, calculated from the number 

of counts taken]

Sample depth

(ft)

20-45___ _______

45-55--___-_-__

55-60__-_______

60-70__________

70-80__________

80-90__________

90-100_________

100-110_____.__

110-120_. ______

120-130_ _______

130-140. ____ ___

140-150. _______

150-160_ _______

160-170__.__.__

170-180

180-190_-_-_.__

Uranium

(ppm)

3.05

3. 67

4. 80

5.06

5. 35

4. 50

4. 14

3. 58

4.08

4. 10

4. 04

3. 72

3. 12

4.00

3.08

2.90

Ionium

(10-6 ppm)

<0. 2

<• 2

<• 1

1. 42(±0. 08)

2. 5( ±0. 2)

3. 8(±0. 1)

3. 5(±0. 2)

4. 3(±0. 2)

2. 7(±0. 1)

3. 1(±0. 2)

2. 9(±0. 1)

2. 6(±0. 1)

3. 4(±0. 2)

4. 3(±0. 1)

3. 7(±0. 1)

4. 4(±0. 2)

ArTh230XTh230

ATu238Xt]«8

<0. 04

<. 03

<.01

. 16(±0. 01)

. 27(±0. 03)

. 49 ( ±0. 03)

. 49(±0. 04)

. 70(±0. 05)

. 38(±0. 03)

. 44( ±0. 04)

. 42(±0. 03)

. 41 (±0.03)

. 64(±0. 05)

. 63(±0. 03)

. 70(±0. 04)

. 88(±0. 06)

E-l

F-l

FIGUBE 317.—Uranium content and ratio of ionium-uranium activities of cuttings 

from drill holes E-l and F-l.

and gives some assurance that the ratios are truly representative of the depths sampled.

A comparison of the ionium-uranium ratios for the 

samples from drill holes E-l and F-l is given in table 

14. The apparent ages were calculated for the E-l 

samples for the depths indicated because the rock in 

this range was highly compact (Ladd and others, 1953). 

It is assumed to be, in part, unaltered aragonitic 

material.

The data for the 200 feet of limestone in table 14 

indicate that a vertical growth of 60 feet has occurred during the last 12,000 years. Comparing this 

with the growth of about 100 feet during the previous 

250,000 years, one would assume that there were only

TABLE 13.—Ratio of ionium to uranium in cuttings from drill

hole E-l

[Los Alamos results (Potratz, Barnes and Lang, 1955), where given, are listed 

directly underneath the Eniwetok results. To determine ionium (Th230) in 

sample solutions, the total thorium was first concentrated by coprecipitation on 

ferric hydroxide. Pure carrier-free thorium was isolated from this concentrate 

by ion exchange techniques. The growth and decay of the total alpha activity 

in the carrier-free thorium which had been isolated was followed for a period of 

several months. From the data obtained the ionium content of the samples 

could be calculated. In making the calculation it was assumed that Th232 and 

Th228 activities in the original materials were equal. The beta particle emitter, 

Th234 , was used as a tracer to determine the chemical yield (Sackett, Potratz and 

Ooldberg, 1958; Sackett, 1958).

Through the kind cooperation of D. J. Henderson of the Argonne National 

Laboratory, alpha pulse analyses were obtained on the thorium that had been 

isolated from E-l samples 70-80, 110-120, 130-140 and 160-170. Only Th^o, Th228 

and Ra22* alpha activities were observed in the 4.5 to 5.8 MEV range that was 

covered. Although it appeared that activity had been lost in transporting two 

of the samples to and from Argonne, the fraction of nonionium activity could be 

calculated and applied to previous counting data. Generally the nonionium 

activity was less than 5 percent of the total activity]

10-20 ^. .......

20-30_____-_.__

30-40 »___-_____

35-40._.___.___

40-45 i____ — ___

45-50_-_---_-_-

50-60-_-_-_--_-

60-70_____-.-__

70-80_-_--__-__

80-90--_-___-_-

90-100__ __---__

100-110--_--_-_

110-120. _______

120-130_ ______ _

130-140. _ ______

140-150. _______

150-160_ ______ .

160-170. _______

170-180________

180-190_ _______

190-200_ _______

Uranium 

(ppm) (estimated

deviation ±5 percent)

0.67

.64 

1.82

2.03 

3.46

3.92 

3. 10

2.98 

2.71

2.73 

3.06

3.07 

2. 82

2.84 

3.31

3.33 

2.86

2.95 

2. 46

2.73 

3.21

2.94 

2.86

3. 14 

2.20

2.26 

1. 64

1. 68 

1. 98

1.91 

2. 48

2. 54 

2. 24

2. 18 

1.46

1. 44 

1.02

1.04 

1. 90

1.96 

1. 98

2. 13

0. 07(±0. 02)

. 12(±0. 03)

. 56(±0. 14)

.30(±0. 06)

. 30(±0. 02)

. 44(±0. 15)

<. 33(±0. 04)

. 34(±0. 03) 

<. 49(±0. 03)

.60(±0. 04)

1. 34(±0. 09)

2. 5(±0. 2)

2. 3(±0. 1)

3. 3(±0. 1)

3. 7(±0. 2)

3. 5(±0. 1) 

3. 4(±0. 2)

3. 2(±0. 1) 

2. 5(±0. 2)

2. 2(±0. 1) 

3. 2(±0. 2)

3. 0(±0. 2)

2. 7(±0. 1) 

3. 5(±0. 2)

2. 4(±0. 2)

1. 7(±0. 2)

3. 5(±0. 2)

3. 1(±0. 2)

3. 2(±0. 1)

-ZVThJ3l)XThJ30

Nv3M^jtu

0. 06(±0. 02)

.04(±0. 01)

.09(±0. 03)

.06(±0. 01)

.06(±0. 01) 

. 10(±0. 03)

<.06(±0. 01)

.06(±0. 01) 

<. 10(±0. 01)

. 11(±0.01)

. 28(±0. 03)

. 58(±0. 05)

. 48(±0. 03) 

. 60(±0. 04)

. 66(±0. 04) 

. 75(±0. 06)

. 65(±0. 04)

.91(±0. 07)

. 84(±0. 05) 

. 89(±0. 07)

. 77(±0. 04) 

. 93(±0. 07)

.70(±0. 06)

.63(±0. 04) 

.90(±0. 07)

. 97(±0. 08)

.99(±0. 09)

1. 08(±0. 09)

. 97(±0. 07)

. 89(±0. 05)

i These samples were obtained in solid form and the others as nitric acid solutions 

from Los Alamos.

665106 O—62-
1060

TABLE 14.—Comparison of ionium-uranium ratios and indicated

ages

[The age can be calculated from the following formula: age (years) = -2.66X105 log

BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

TABLE 15.—Ionium-uranium ratios in Recent materials

Sample depth (ft)

20-60__-----_--

60-70___----___

70-80.---------

80-90-___-____-

90-lOO_-_-_-_-_

100-110--__-__-

110-120_-------

120-130-_--____

130-140--- __---

140-150---___._

150-160____-_-_

160-170------.-

170-180. _______

180-190--------

190-200- _______

Ionium-uranium ratios

F-l

<0 1 

16 

27 

49 

49 

70 

38 

44 

42 

41 

64 

63 

70 

88

E-l

<0 1 

11 

28 

58 

60 

75 

91 

89 

93 

70 

90 

97 

99 

1 08 

92

Apparent ages for drill hole 

E-l ratios (years before 

present)

< 12, 000 

13, 000(±1000) 

38, 000 (±4000) 

100, 000(± 7000) 

106, 000(±12, 000) 

160, 000(±12, 000)

| 280, 000 (±20, 000)

a few select periods during the late Pleistocene when 

conditions' were appropriate for the vertical growth of 

the coral atoll. These times of growth possibly coincided with the interglacial periods.

Several explanations might be offered for the reversals in the ionium-uranium ratios observed in material 

from both the F-l and E-l drill holes. The deep cuttings may have been contaminated with cuttings from 

a higher level. The one reversal in the E-l hole that 

is not due to any experimental error is in the 140- to 150- 

foot interval. Ladd and others (1953) indicate that the 

drill hole was cased at 146 feet, and it is possible that 

the cuttings obtained on the resumption of drilling were 

contaminated with shallow cuttings that had fallen 

down during the casing operation. Contamination may 

be responsible for the reversals in the F-l hole, but 

this is very unlikely because there is such a large range 

with low readings. Other possible explanations for the 

reversals are the growth of young reef among the older 

cavernous unconsolidated material during periods of 

changing sea level or the migration of ionium or uranium 

into or from the calcium carbonate.

Using the Th230/U238 ratio as a measure, it appears 

that the material from the- first 60 feet of these holes 

was deposited since the last glacial stage, and that the 

material at the 200-foot level is approximately 300,000 

±100,000 years old.

Future analyses of short sections of a core taken from 

the first 200 feet of a coral reef may give detailed 

information about the times of sea level changes during 

the Pleistocene epoch.

Because all the marine limestones analyzed have a 

measurable amount of uranium, the first condition for 

age determination by the ionium-uranium ratio is 

generally fulfilled.

Description

Reef coral collected

live (Porites australiensis 

Vaughan) . 

Clam collected live 

(Tridacna). 

Modern shell (gastropod) . 

Algal deposit (Porolithon craspedium) .

Sample No.

JIT-1_._...

JPEM-1___ 

TNVK-1... 

260A-67-6__

Uranium (ppm)

2. 4(±0. 2)

0. 10 (±0.02) 

0. 32 (±0.03) 

0.37 (±0.04)

A-IbMO/AvTM

<0. 01

<0. 1 

<0. 01 

<0. 01

The very low ionium contents for the recently 

deposited E-l and F-l cuttings and the materials 

listed in table 15 indicate fulfillment of the second 

condition.

The third condition for the determination of the 

geologic age of limestones by use of the ionium-uranium 

ratio is that there should be no gain or loss of uranium, 

or of the ionium that grows from the radioactive decay 

of uranium. Unlike the first two conditions, it seems 

possible that nonadherence may be the rule rather than 

the exception.

By comparing the uranium and crystal structure 

results for the shallow cores with those of the deep 

cores, as given in table 16, it is seen that, in general, 

the shallow material has high uranium with the aragonite structure, and that the deep material has low 

uranium with either the calcite or dolomite structure.

Cuttings from a depth of 10 to 20 feet in hole E-l 

(table 13) may possibly represent material which has 

only recently been transformed. This sample has the 

calcite structure and contains only 0.67 ppm uranium, 

whereas material in the next 20 feet has the aragouite 

structure and a higher uranium content.

On the basis of the previous discussion concerning 

the relationship of uranium content to crystal structure, 

and from a consideration of the fact that samples E-l, 

10-20 feet; E-l, 40-45 feet; and F-l-1-1, 170-191 feet, 

contain much calcite, one might conclude that the rise 

and fall of uranium (table 17) with depth in holes F-l 

and E-l, result from variation in the amount of transformed material present. However, a similar variation 

of uranium content with depth appears in the series of 

Mu-7 core samples that are essentially pure aragonite. 

This distribution of uranium in the core may be purely 

coincidental, and the distribution of readings for the 

cuttings may indeed be related to the presence of varying amounts of transformed calcium carbonates.

Then the question that naturally arises is this—if 

there is a loss of uranium by the calcium carbonate, 

what happens to the ionium-uranium ratio? If the 

aragonite-calcite transition proceeds at a first order rate 

with a half-time much larger than the half-life of ionium,
SUBSURFACE GEOLOGY OF ENIWETOK ATOLL

TABLE 16.—Ionium-uranium ratios in cores taken from drill holes on Eniwetok Atoll

[Crystal types are: A, aragonite; C, calcite; D, dolomite. The standard deviation for uranium is ±5 percent in the 2 ppm range 

and ± 10 percent in the 0.5 ppm. range. The ionium values were calculated from the total thorium alpha count after TH227 and 

daughters had decayed. The Th232 content for a typical pair E-l-3-7 and Mu-7-41 determined by a neutron activation procedure 

(Potratz and Bonner, 1954; Sackett, 1958), was about 0.1 ppm, an amount which would not contribute appreciable alpha activity]

1061

Hole and core

Mu-7-4

Mu-7-13.- — — — — __

Mu-7-23— _____-__-

Mu-7-4 1

Ru— 2— 14

F-l-1-1 —— ---__-

F-l-5-37— — — — —

F-l-6-10— __-_---_-

F- 1-1 2-8

F-1-14-24-— _______

E-l-2-5—— — ---

E-1-3-7- — ---------

Depth (ft)

13-24 

34-46 

64. 5-69. 5 

90-97. 5 

47-52 

96-101

170-191 

1978-2003 

2662-2687 

3963-3988 

4316-4341 

4500-4525 

2003-2028 

2802-2808

4078-4100

Major crystal type

A 

A 

A 

A 

A 

A

A and C 

C 

C 

C 

C and D 

C 

A 

C

C

Uranium 

(ppm)

2. 1 

2.7 

2. 4 

2.0 

2. 5 

2. 2

3. 1 

0. 34 

0. 25 

0. 19 

0. 46 

0. 66 

1. 05 

3. 1

0. 17

ATh230

An238

0.067 (±0.01) 

.074(±0. 01) 

. 66 (±0.05) 

.71 (±0.05) 

. 73 (±0.05) 

.73 (±0.05) 

1.00 (±0.07) 

1.05 (±0.07) 

. 77 (±0.05) 

.74 (±0.05) 

. 91 (±0.07) 

. 98 (±0.07) 

1.00 (±0.07) 

1.04 (±0. 10) 

. 90 (±0. 10) 

1.00 (±0. 10) 

1.00 (±0. 10) 

. 96 (±0. 10) 

1.01 (±0. 10) 

.87 (±0.07) 

. 98 (±0. 07) 

1.08 (±0. 10)

Apparent ageX 1,000 

years

8. 4(±1. 2) 

132(±12) 

152(±11) 

>300 

160(±13) 

>300

>300 

>300 

>300 

>300 

>300 

>300 

>300 

>300

>300

TABLE 17. — Uranium content versus depth for cuttings TABLE 18. — Thorium232 versus uranium in soi

——————————————————————————————————— ... ... . — ... ——— r.n.rhnn.ni.ps

Depth (ft)

20-60._ 

60-110_. 

110-150. 

150-190.

Average uranium (ppm)

Drill hole F-l

3.7 

4. 5 

4. 0 

3.3

s Drill hole E-l

2.7 

3.0 

2. 1 

1. 6

[The Th"2 contents, other than those footnoted, were calculated from data obtained 

in following the growth and decay of the total alpha activity in the carrier-free 

thorium which had been isolated from the sample. Tb.232 and Th^s activities in 

the original sample were assumed to be equal. The values obtained by these last 

two methods may be in error by a factor of two]

the ratio will still give very nearly the correct age of the 

material. However, if the loss of uranium proceeds at 

a rate that is rapid compared to the rate of ionium decay 

and if such loss occurs in a material already containing 

a measurable quantity of ionium, then future determinations of the ionium-uranium ratio in the altered 

material will give enoneous ages the magnitude of the 

error varying with the amount of uranium which was 

lost. By assuming that ionium is retained, a correction 

might be applied by considering the uranium content of 

the surrounding unaltered material.

The thorium ion, with very nearly the same radius as 

that of the calcium ion, 1.10A0 and 1.06A0 , respectively 

(Moeller, 1952, p. 140), and with its additional bonding 

ability, is assumed to be largely retained when the calcium carbonate recrystallizes. Some evidence in table 

18 indicates that the large change in uranium concentration that apparently occurs with recrystallization 

is not accompanied by a corresponding change in thorium concentration. It is seen in the first few samples

Sample

E-l, 10-20---

E-l, 30-40- — -

E-l, 40-45---

E-l-3-7- __-_--

Mu-7-41_-__--

E-l, 20-30---

E-l, 35-40---

E-l, 45-50- — -

E-l, 50-60- — -

E-l, 60-70----

E-l, 70-80---

E-l, 80-90---

E-l, 1 00-110--

E-l, 110-120---

E-l, 120-130- -_

E-l, 130-140- --

E-l, 140-150--

E-l, 150-160- .

E-l, 160-170---

HP 1 1 7A 1 CA

E 1, 180-190.--

E

-i -i OA onn

A veraff 6

Th«2 (ppm)

0.04

. 06

.060 

. 07

1 . 010

» . 0084

. 02

. 07

<• 1

<• 1

. 006

2 . 06

AQ

. 03

2 . 05

. 009

2 . 06

no

. 07

2 . 04

AK

. 01

. 13

UM8 (ppm)

0. 67

3. 46

2. 71

. 17

2. 0

1. 82

3. 10

3. 06

2. 82

3. 31

2. 86

2. 46

2. 86

2. 20

1. 64

1. 98

2. 48

2. 24

1. 46

1. 02

1 QO

1 Q8

Crystal structure

calcite _ _ _

aragonite—

aragonite and

calcite.

aragonite

Not determined. 

--_-_do ------

--___do ------

--__do------

__--_do_------

__--do—---

_____do------

_-_--dO-------

-----do------

_-_-.do-------

_____do.------

___--do------

_.-__do------

--_._do_------

__-__do------

__---do_------

__-._do------

Th«2 U-i"

0. 060

. 017

. 026

. 059

. 0042

. 01

.02

<. 03

<. 03

. 002

.02

. 01

.01

.02

. 006

. 03

.04

. 03

. 03

. 05

. 005

.07

0. 02

1 Th232 content determined by neutron activation analysis. Estimated standard 

deviation is ±10 percent. 2 Th232 content determined from alpha pulse analysis data for Th 22 » and daughters, 

was assumed equal to ATh232 .
1062 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

TABLE 19.—Ionium-uranium ages for sortie marine shells

[A complete description of the marine shells is given on page 1064-1065. All samples appeared to be well consolidated and not appreciably altered. The shells were washed and 

scrubbed with distilled water before the analyses. For the first four samples, the total alpha activity at the time of separation was assumed to be entirely ionium. Therefore, the ionium-uranium ages may be on the high side. In sample TNVK-2, from the growth and decay of total thorium alpha activity, the activity of Th^28 was 

calculated to be 0.019 disintegrations per minute per gram. The upper limit, < 48,000 years, was obtained by assuming Th232 to be absent. The lower limit, > 33,000 

years, was obtained by assuming Th232 and Tbm activities in the sample to be equal]

Sample

V67-61_--_-______________

V67-70— ------_-____--__

V67-74_. _________________

Co-40A---------------__-

TNVK-2----- _ _ _

Uranium (ppm)

0. 11 (±. 01)

0. 14 (±.01)

0. 24 (±. 02)

0. 044 (±. 005)

0. 26 (±. 03)

Arhm dis/min/g

0. 002 (±.002)

0. 0035 (±. 0035)

0. 010 (±. 005)

0. 005 (±. 005)

0. 066 (±. 013)

0.047 (±.009)

ArhKo/AuKo

0. 025 (±. 025)

0. 035 (±. 035)

0. 056 (±. 028)

0.15 (±.15)

<0. 34

>0. 25

Apparent age (yr)

3,000 (±3,000)

4,000 (±4,000)

6,700 (±3,300)

18,000 (±18,000)

<48, 000

>33, 000

Age estimated by 

submitter (yr)

4,000

4, 000-5, 000

5,000

7, 000-9, 000

40, 000-60, 000

that the thorium-uranium ratio for the calcite material 

is about a factor of 3 greater than the ratio for the 

aragonitic material.

The ages given by the ionium-uranium ratios in the 

shallow core material help to substantiate the results 

obtained for the E-l and F-l drill holes. For the 

material from the Mu-7, En-6 and Ru-2 holes, very 

little if any transformation of aragonite to calcite has 

occurred, and thus there apparently has been little loss 

of uranium.

IONIUM-URANIUM VERSUS RADIOCARBON AND 

OTHER AGES

Through the cooperation of Wallace Broecker of the 

Lamont Geological Observatory and Meyer Rubin of 

the Geological Survey, four of the aforementioned 

samples of Eniwetok calcium carbonate have been 

dated by the C14 method. In addition, a number of 

marine shells, the ages of which had been estimated 

by other means, were dated by the ionium-uranium 

procedure. The results, shown in table 19, indicate 

relatively good agreement between the ionium-uranium 

age and the estimated age for these samples. The 

low uranium contents and the very low ionium activities 

in these relatively young materials are responsible for 

the high uncertainties in the ionium-uranium ages.

A comparison of the ionium-uranium and radiocarbon 

ages obtained on the four Eniwetok samples is given in 

table 20. There are wide disagreements here that 

cannot be attributed to experimental error.

It was proposed that the emergence of the coral 

atolls with lowered sea levels resulted in the solution 

of the calcium carbonate by fresh rainwater and a 

recrystallization upon evaporation of the water, a 

process that is greatly decelerated in the saturated 

ocean waters. This phenomenon suggests two mechanisms by which radiocarbon ages might be made 

smaller. First, the atmospheric carbon dioxide that 

is naturally present in rainwater could exchange with 

the carbon dioxide in the calcium carbonate when it 

temporarily enters solution. Secondly, the calcium 

carbonate in solution is deposited at a lower level, 

contaminating the surrounding older carbonate material.

It seems that the recrystallization process not only 

lowers the radiocarbon age but also raises the ioniumuranium age because of the loss of uranium.

Another process that apparently lowers the radiocarbon age is the filling of the primary cavities of the 

reef with young chemically precipitated or detrital 

calcium carbonate (Newell, 1955). If this secondary 

material contains a small amount of uranium and no 

ionium (such as the algal calcium carbonate in table 

11), the ionium-uranium age will be lowered by a much 

smaller percentage than will be the CM age. This can 

be illustrated by the following calculation.

If a calcium carbonate contains 2.0 disintegrations 

per minute per gram of uranium and 0.184 disintegration per minute per gram of ionium, then A^m/A-urn 

= 0.092, and it has an apparent age of 11,200 years. 

The C14 will have undergone two half-lives decay, and,

TABLE 20.—Ionium-uranium versus radiocarbon ages for Eniwetok Atoll calcium carbonates

Sample

Engebi Island, beach 

rock 13-3----______

Mu-7-4___.__________

Mu-7-13 ___ _______

E-l, 40-45._______.__

Laboratory 

No.

W-620

W-619

L-423B

L-423B

Uranium (ppm)

1.35 (±.07)

2.1 (±. 1)

9 7 ( _i_ i ^

9 7 ( -4- ]\

,4Th230

0.05 (±.01)

0. 11 (±.01)

1 Q7 ( -1- 07}

0. 44 (±. 15)

,4Th23%4u238

0. 05 (±.01)

0.07 (±.0)

0. 68 (±. 06)

0. 10 (±.03)

Apparent age (yr)

6,300 (4-1200)

8,400 (±1200)

132,000 (±10,000)

12, 200 (±4000)

C» age (yr)

1 2,700 (±250)

i 3,840 (±300)

2 23, 500 ( ± 1, 000)

2 4, 900 (±150)

1 Rubin, Meyer, and Alexander, Corrinne, 1960, Geological Survey radiocarbon dates V: Am. Jour. Sci. Radiocarbon Supp., v. 2, p. 1-57.

2 Olson, E. A., and Broecker, W. S., 1959, Lamont natural radiocarbon measurements V: Am. Jour. Sci. Radiocarbon Supp., v. 1, p. 1-18.
SUBSURFACE GEOLOGY OF ENIWETOK ATOLL 1063

assuming 1.82 disintegrations C14 per minute per gram 

for the original material, it has now decayed to 0.46 

disintegration per minute per gram.

If this material is diluted 25 percent with a young 

material that contains 1.82 disintegrations C14 per 

minute per gram, 0.2 disintegration uranium per 

minute per gram and no ionium, the resulting material 

will have AThm/Avm = 0.089, or an apparent age of 

10,800 years, as compared to 11,200 for the undiluted 

material. The ACM — 0.79 disintegration per minute 

per gram and gives an apparent age of 6,800 years. 

Thus the apparent ionium-uranium age has been 

lowered 3.5 percent and the apparent C14 age has been 

lowered 39 percent.

The ionium-uranium age will be lowered 25 percent, 

to 8,400 years, if the young material has the same 

uranium content as the older calcium carbonate.

The discrepancy between apparent ionium-uranium 

age and apparent C14 age will be even more pronounced 

if contamination by newly formed carbonate occurs in 

still older materials.

Samples E-l, 40-45 and Mu-7-4 have apparently 

not been subjected to the emergence and recrystallization effect, because they were deposited in recent times 

in a period of rising sea levels. Sample Mu-7-13 has 

probably not been recrystallized either, as it is very 

pure aragonite. Thus the discrepancy in ages must be 

attributed to the contamination factor. This is supported by the increasing difference between the ioniumuranium and C14 apparent ages with increasing age for 

the above three samples.

Unquestionably, the ionium-uranium ratio will give 

the age for a material that does not violate any one of 

the three previously mentioned conditions. It has been 

extremely difficult, however, to obtain samples in which 

these conditions are completely satisfied. It is apparent from the preceding discussion that C14 ages for the 

marine calcium carbonates cannot be correlated with 

ionium-uranium ages.

OTHER POSSIBLE METHODS FOR DATING MARINEDEPOSITED CALCIUM CARBONATES

The results obtained in analysis of sea water indicate 

that not only ionium but also Th227 in the U235 chain is 

present in the water in an amount that is very much 

less than the equilibrium quantity. It was suggested 

that Pa231 is the limiting specie in the U235 family because 

of its tendency to be coprecipitated. This apparent 

absence of Pa231 in sea water and also in newly deposited 

calcium carbonate can make two additional dating 

methods possible.

The first method is similar to the ionium-uranium 

method, except that it makes use of the disequilibrium 

between Pa231 and U235 . A modification of the protactinium procedure (Potratz and Bonner, 1954) could be 

used along with the fluorimetric method for the determination of uranium. Because the U235/U238 ratio is 

fixed and is very well known, the amount of U235 is 

obtained from the total uranium.

The protactinium-uranium method seems to offer 

certain advantages. There is only one naturally occurring alpha emitter of protactinium, and this eliminates 

the need of either an expensive pulse analyzer or 

periodic counting. Also, the protactinium will grow in 

at a faster rate than ionium, thus making the protactinium-uranium method more applicable than the 

ionium-uranium method for materials less than 10,000 

years old. The theoretical growth rates for the two 

methods are indicated in figure 318.

The second dating method made possible by the disequilibrium of Pa231 and U235 is a combination of the 

two previous methods. Because protactinium and 

ionium grow at different rates from their respective 

parents, the ratio of their activities changes with time, 

and if we begin with a material containing uranium but 

no Pa231 or Th230, a determination of the Pa231 /Th230 

ratio at subsequent times will tell us how much time

i.o

0.8

0.6

0.4

0.2

2.5

2.0-

1.5-

1.0-

0.5

Pa

231 Th

230

230 231 

Theoretical growth of Th and Pa

from initially pure uranium

Change in ratio of activities of Pa

230 

and Th with time

231

50 100 150 200 

TIME, IN THOUSANDS OF YEARS

250

FIGURE 318.—Theoretical growth of Th«» and Pa23* from initially pure uranium and 

change in ratio of activities of Pa"1 and Th»so with time.
1064 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

has elapsed since the Pa231 and Th230 growth began. 

As can be seen from figure 318, which shows the theoretical change in ratio with time, the largest difference in 

activities would be observed in the most recently deposited materials.

As Ac227 («*=22.0 years) and Th227 (#=18.6 days) 

are probably in equilibrium with Pa231 in calcium 

carbonates, a procedure that involves separation of 

thorium followed by alpha pulse analysis for Th230 

(4.68 MEV) and Th227 (5.7 and 6.0 MEV) would 

probably produce an age for the material.

The actual operation of these procedures would 

present difficulties because of the minute activities 

involved. These small activities are not only easily 

contaminated but must also be counted for extremely 

long periods to obtain good statistics.

DESCRIPTION OF MATERIALS STUDIED

1. Core samples received from H. S. Ladd, U.S. Geological Survey 

[For exact location of drill holes see figures 260 and 261 in Ladd and Schlanger, 1960]

Hole

Mu-7-_--_

Ru-2_____-

En-6-_-__-

13-3-------

F-l — _____

E-l___-_.__

Core No.

4

13 

23 

41

14

29

1 1-1

5-37 

6-10 

10-5 

12-8 

14-24 

1-5

2-5

3-7

Depth (ft)

13-24

48-51 

64. 5-69. 5 

90-97. 5

47-52

96-101

9-10. 5

170-191

1978-2003 

2662-2687 

3963-3988 

4316-4341 

4500-4525 

2003-2028

2802-2808 

4078-4100

Description

Pure coral core splits

from a drill hole in the 

northeast part of Mujinkarikku, Eniwetok. 

Altitude of drill hole, 

8.5 feet.

split. 

Engebi, pure coral core

split.

tion test hole near 

center of Engebi, Eniwetok. Sample is 

bedded rock similar to 

beach rock and a little 

below sea level. 

Elugelab, coral-algal

detritus. 

Recrystallized coral. 

Detrital limestone. 

Do. 

Dolomitic limestone. 

Detrital limestone.

split. 

Detrital limestone. 

Dolomitized coral.

1 First number is that of core run, second is number of core piece below top of core

2. Cuttings from drill hole E-l on Parry Island, Eniwetok, 

received from Los Alamos Scientific Laboratory (Ladd and 

others, 1953).

Depth (feet) Description 

10-30------- Uncemented sand made up largely of worn tests

of beach-type Foraminifera such as Calcarina 

and Marginopora, mixed with coral pebbles, 

mollusk shells, echinoid spines, and worn segments of Halimeda. Hard beach rock 18 to 

21 feet.

30-90- _ _ _ _ ^ _ Coarse, platy debris with numerous fresh and 

worn segments of Halimeda, some of which are 

cemented, and many small mollusks and 

Foraminifera. Larger fragments include worn 

corals and branching Lithothamnium and 

mollusks (Area, Cypracea).

90-145-_____ Worn coral and sand; some coral recrystallized 

to yellow calcite. Some mollusks well preserved; others occur as molds.

145-300----- Coral limestone; white friable limestone recrystallized in places to yellow calcite. Mollusks abundant, some preserved as molds.

3. Cuttings from drill hole F-l on Elugelab, Eniwetok (Ladd and 

others, 1953).

Depth (feet) Description

0-45_ _______ Uncemented sand made up largely of beach-type

Foraminifera such as Calcarina and Marginopora, mixed with fragments of larger forms, 

including corals (Tubipora) echinoid spines, 

mollusk shells, and segments of Halimeda. 

Hard beach rock from 12 feet 6 inches to 14 

feet 6 inches.

45-110------ Uncemented coarse, platy debris. Poorly sorted

mixture of Halimeda segments and worn coral, 

algae, mollusks, and Foraminifera.

110-190- _ _ _ - Worn, massive coral and delicate unworn branching coral. Large fragments of yellow calcite; 

some mollusks preserved as molds.

4. Sea-water samples from E. D. Goldberg, Scripps Institution of 

Oceanography.

Depth (feet) Description 

Sample ^4__-__-__ Surface water from outside San Diego Bay,

collected in the summer of 1956. 

Sample B_ _______ Water collected at 3,500 meters depth from

long 124°41.0' W., lat 33°54.5'. N. on

March 25, 1957.

5. Miscellaneous materials

Reef coral (Porites australiensis Vaughan), from lagoon channel 

reef off west side of Bogen, collected alive by J. I. Tracey, Jr., 

May 21, 1946. (Sample JIT-1, tables 11 and 15.)

Coral head (Favia) encrusted by hydrocoralline growth (Millepora). Sample was collected April 15, 1930, from Christmas 

Island by C. K. Wentworth. (Samples CKW-1 and CKW-2, 

table 11.)

Algal calcium carbonate (Porolithon craspedium), from Ourukaen 

island, Bikini, collected May 3, 1946. Sample submitted by 

J. I. Tracey, Jr. Chemical analyses of this sample are published by Emery, Tracey, and Ladd (1954, p. 67, sample 6). 

An X-ray analysis showed that the sample is calcite with a unit 

cell about 1.8 percent short. (Sample 260-A-67-6, tables 11 

and 15.)

Clam (Tridacna), from Lidilbut, collected alive by J. P. E. 

Morrison, June 1, 1946. (Sample JPEM-1, tables 11 and 15.)

Marine shells (gastropods), submitted by T. N. V. Karlstrom, 

U.S. Geological Survey. (Sample TNVK-1, tables 11 and 15; 

modern shells from Cook Inlet area, Alaska. Sample TNVK-2, 

table 19; shells from Bootlegger Cove, Alaska, estimated to be 

40,000 to 60,000 years old.)
SUBSURFACE GEOLOGY OF ENIWETOK ATOLL 1065

Shells from Sq. A-3 at depths of 6}£ to 7, 12 to 12>'2 , and 15 to 15^ 

feet with ages of approximately 4,000 to 5,000 years. These 

are three fragments of Busy con gouges from Indian shell midden, Bluffton site, Volusia County, Florida. (Samples V67- 

61, V67-70, V67-74, table 19.) (Bullen, R. P., 1955, The 

Florida Anthropologist, v. 8, p. 1.)

Shell material 7,000-9,000 years old. (Sample Co-40A, table 

19.) (McGimsey, C. R., 1956, Am. Antiquity, v. 22, p. 151.)

REFERENCES

Alderman, A. R., and Skinner, H. C. W., 1957, Dolomite sedimentation in the south-east of South Australia: Am. Jour. 

Sci., v. 255, p. 561-567.

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Goldsmith, J. R., and Graf, D. L., 1958b, Relation between 

lattice constants and composition of the Ca-Mg carbonates: 

Am. Mineralogist, v. 43, p. 84-101.

Goldsmith, J. R., Graf, D. L., and Joensuu, O. I., 1955, The 

occurrence of magnesian calcites in nature: Geochim. et 

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Graf, D. L., and Goldsmith, J. R., 1956, Some hydrothermal 

syntheses of dolomite and protodolomite: Jour. Geology, 

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Grimaldi, F. S., and Levine, H., 1954, The visual fluorometric 

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v. 45, p. 818-827.

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Atoll: U.S. Geol. Survey Prof. Paper 260-AA.

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Koczy, F. F., Picciotto, E., Poulaert, G., and Wilgain, S., 

1957, Mesure des isotopes du thorium dans 1'eau de mer: 

Geochim. et Cosmochim. Acta, v. 11, p. 103-129.

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content of limestones and fossils: Geol. Soc. America 

Bull., v. 63, p. 701-716.

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and Stephenson, H. K., 1953, Drilling on Eniwetok Atoll, 

Marshall Islands: Am. Assoc. Petroleum Geologists Bull., 

v. 37, p. 2257-2280.
1066 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

Ladd, H. S., and Schlanger, S. O., 1960, Drilling operations on

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———— 1949a, Petrography of the island of Hawaii: U.S. Geol. 

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———— 1949b, Hawaiian petrographic province: Geol. Soc. 

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tr1

to

CO

to 

to

CO 

CO
PLATE 282

A. Core F-l-3-20, X 12. Section across vug in limestone of Tertiary e age. Chemical analysis showed 2.8 percent MgCO3 for 

bulk sample, and a bulk X-ray analysis showed no dolomite. Staining of another thin section from this core showed that 

the clear layer (a) contains a few strings of dolomite crystals.

B. Core F^l-12-5t, X 20. Relict dark areas in almost completely dolomitized rock of Tertiary 6 age. A composite sample from 

the top and bottom of this core piece showed 38 percent MgCO3 ; X-ray analysis showed more than 98 percent dolomite. 

In this core piece, algae (a) are most resistant to the encroaching dolomite; most of the rock is a saccharoidal mass of subhedral dolomite grains. Dark areas are the only remains of the original coarse-grained foraminiferal-algal limestone.

C. Core F-l-12-10, X 30. Slightly calcitic dolomite of Tertiary 6 age. Chemical analysis showed 35.9 percent MgCO3 ; X-ray 

analysis showed 93 percent dolomite. Lens-shaped void in the center was formed by the removal by solution of a Foraminifera test after dolomitization of the original foraminiferal-algal limestone.

D. Core F-l-12-7£, X 8. Dolomite of Tertiary 6 age. Chemical analysis showed 38.8 percent MgCO3 ; X-ray analysis showed 

more than 98 percent dolomite, with only a trace of calcite. Some original texture is reflected in the patches of coarsegrained dolomite that have more and larger pore spaces than the surrounding fine-grained dolomite.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 260 PLATE 282

X 12 D X8

DOLOMITIZED LIMESTONES FROM HOLE F-l, ENIWETOK ATOLL
PLATE 283

A. Core E-l-3-9, X 30. Dolomitized coralline algae. Most of the dolomite in this section is restricted to rod-shaped segments 

of Corallina.

B. Core E-l-3-9, X 180. Closeup of single alg'al segment partly replaced by 3 euhedra of dolomite. The dark centers of the dolomite crystals appear white and cloudy in reflected light and are possibly calcite-rich central zones.

C. Core E-l-3-9, X 30. Algal segment almost completely replaced by a mosaic of dolomite. The dolomite is restricted to the 

algae and closely follows the outlines of the original segment.

D, Core E-l-3-10, X 20. Scattered dolomite crystals that occupy the mud-filled interseptal spaces in coral. All the original 

corallum has either been replaced by calcite or removed by solution. This sample showed an MgCO3 content of only 4.5 

percent; X-ray analysis showed 5 percent dolomite and 95 percent calcite.

E. Core E-1-3-40, X 10. Dolomite occupying a mollusk shell and coralline algae. Dolomite has faithfully followed the original 

shell of the mollusk.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 260 PLATE 283

A X30 B X 180

C X30 D X20

DOLOMITIZED LIMESTONES OF TERTIARY 6 AGE FROM HOLE E-1, ENIWETOK ATOLL

X1 °
PLATE 284

A. Funafuti core 341 A, X 30. Completely dolomitized limestone from a depth of 900 feet. This core contains about 40 percent 

MgC03 .

B. Funafuti core 634A, X 30. Completely dolomitized limestone from a depth of 1,070 feet. This core contains 39.4 percent MgCO3 . 

Note that although the rock is completely dolomitized, coralline algae (a) have not been texturally destroyed. This is in 

direct contrast to the state of algae in dolomitized cores from hole E-l at Eniwetok Atoll (pi. 283C).

C. Funafuti core 205A, X 20. Dolomite laminae in rock from a depth of about 830 feet. The MgCO3 content of this core is between 5 and 11 percent. According to Cullis (1904, p. 410, fig. 44) the layer of clear dolomite crystals (a) formed before 

depositions of the three bands of laminar deposits. Of these three laminae, the central one (b) is dolomite, the other two 

are calcite.

D. Funafuti core 203A, X 2.5. Dolomite in low-magnesium rock. MgCO3 content is only 4.83 percent but dolomite (a) 

is widespread.
PROFESSIONAL PAPER 260 PLATE 284

* -~-—-**^^~—t ^ '^i^m-MJ^•><*..

X30

B X30

C X20 Z) X2.5

DOLOMITIZED LIMESTONES FROM FUNAFUTI ATOLL
PLATE 285

A. Funafuti core 342, X 12. Porous dolomite from a depth of 659 feet. The rock contains about 40 percent MgCO3 . Most of 

the originally aragonite parts of the abundant Halimeda segments have been removed through solution. Those parts, such 

as tubes, normally occupied by calcite in fossil Halimeda are now fine-grained dolomite.

B. Kita-daito-jima core 551, X 12. Dolomite from a depth of 300 feet. The Halimeda in this rock, which contains 40.1 percent 

MgCO3 , are completely dolomitized but well preserved. Evidently dolomitization affected both the original aragonite— 

after it recrystallized to calcite—and the calcite filling of the Halimeda. Note that fragments of coralline algae (a) have 

been partly removed through solution whereas the Halimeda are entire.

C. Funafuti core 320, X 20. Dolomitized coral from a depth of 643 feet; the core analyzed at 31.9 percent MgCOs. Much of the 

dolomite is in the form of a film of anhedral crystals on ghosts of the original cofallum (a).

D. Funafuti core 578A, X 25. Dolomitized foraminiferal-algal limestone; this core contains about 40 percent MgCO3 . Although 

entirely dolomitized, the fossils are well preserved and show all fine skeletal details. Note contrast between this dolomite 

and the dolomite from core F-l-12-10 (pi. 282c); the latter was also originally a foraminiferal-algal limestone, but all original 

textures have been destroyed.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 260 PLATE 285

C X20 _ D X25

DOLOMITIZED LIMESTONES FROM FUNAFUTI AND KITA-DAITO-JIMA ATOLLS
PLATE 286

A. Core F-l-4-2, X 8. Porous well-cemented foraminiferal limestone. Cement is a single layer of acicular and granular calcite. 

B. Core F-l-5-9, X 8. Section through a large piece of recrystallized mud-filled coral. Packed debris of Foraminifera tests and algae

fill a cylindrical hole that is probably a worm burrowing.

C. Core F-l-5-16, X 8. Foraminiferal-algal sand that forms the matrix around corals in this core. 

D. Core F-l-5-23, X 8. Recrystallized coral showing intricate pattern developed in secondary calcite.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 260 PLATE 286

A X8

PHOTOMICROGRAPHS OF LIMESTONES FROM CORES F-l-4 AND F-l-5
PLATE 287

A. Core F-l-5-35, X 8. Conglomeratic limestone made up of tests of larger Foraminifera, coralline algae, and pebbles of coral.

B. Core F-l-5-41, X 8. Tests of larger Foraminifera in an unusual matrix of laminar encrusting algae and Foraminifera.

C. Core F-l-6-9, X 8. Solution channel into which project tests of large rForaminifera. Material scraped from filled channels such

as this showed a trace of dolomite on X-ray testing. 

D. Core F-l-6-30, X 12. Completely recrystallized coral. 

E. Core F-l-7-3, X 8. Poorly sorted foraminiferal-algal limestone with a cement of granular calcite.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 260 PLATE 287

A X8 B X8

xs D xi2 E xs 

PHOTOMICROGRAPHS OF LIMESTONES FROM CORES F-l-5, F-l-6, AND F-l-7
PLATE 288

A. Core F-1-9-8, X 8. Molds of Halimeda in completely recrystallized limestone. All aragonitic skeletal material has been dissolved and removed.

B. Core F-l-10-1, X 12. Well-sorted, porous, foraminiferal-algal limestone containing well-preserved tests of planktonic Foraminifera.

C. Core F-l-11-18, X 4. Bedding in foraminiferal-algal limestone (bedding planes run from lower left to upper right hand side of 

photo) deposited under outer slope conditions.

D. Core F-l-11-34, X 8. Porous coarse-grained foraminiferal limestone showing high degree of orientation of disc-shaped and biconvex tests of larger Foraminifera.

E. Core F-l-12-14, X 30. Unaltered tests of larger Foraminifera in a dolomitized matrix.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 260 PLATE

X 8 £, X 30

PHOTOMICROGRAPHS OF LIMESTONES FROM CORES F-l-9 F-l-10 F-l-11 AND F-l-12