Cover design by Nancy Polend, 1991
Sediment Facies of 

Enewetak Atoll Lagoon

By BRUCE R. WARDLAW, THOMAS W. HENRY, and WAYNE E. MARTIN

GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL, 

REPUBLIC OF THE MARSHALL ISLANDS

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1513-B

Prepared in cooperation with 

the Defense Nuclear Agency

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

MANUEL LUJAN, Jr., Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

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

d does not imj" 

Government

descriptive purposes only and does not imply endorsement by the 

u.o.

Library of Congress Cataloging in Publication Data

Wardlaw, Bruce R.

Sediment facies of Enewetak Atoll lagoon / by Bruce R. Wardlaw, Thomas W. Henry, and Wayne E. Martin.

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

Survey professional paper ; 1513-B) 

Includes bibliographical references (p. ). 

Supt. of Docs, no.: I 19.16:1513-B 

1. Marine sediments Marshall Islands Enewetak Atoll. I. Henry, Thomas W. II. Martin, Wayne E. III. Title.

IV. Series. V. Series: U.S. Geological Survey professional paper ; 1513-B. 

GC399.W37 1991

551.46'08349-dc20 90-3379

CIP

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

Federal Center, Box 25425, Denver, CO 80225
CONTENTS

Abstract............................................................................ Bl

Introduction....................................................................... 1

Topography and Physiography of Lagoon ......................... 1

Lagoon Water and Current Systems................................ 3

Previous Benthic Sampling and Ecologic Studies................ 5

Storm Effects on the Lagoon Environments...................... 8

PEACE Program Data Base and Procedures .......................... 9

Field Procedures.......................................................... 11

Laboratory Procedures.................................................. 13

Sediment Facies of Enewetak Lagoon.................................... 17

Distribution of Sediment Facies ...................................... 18

Granule-Coarse Sand Facies ..................................... 18

Granule-Sand-Mud Facies ........................................ 18

Sediment Facies of Enewetak Lagoon Continued 

Distribution of Sediment Facies Continued

Sand Facies......................................................... B18

Muddy Facies ...................................................... 20

Sediment Facies of Submarine Nuclear Craters ..................... 20

Mixed-1..................................................................... 21

Mixed-2..................................................................... 23

Mixed-3..................................................................... 23

Sorting and Subdivision of Facies........................................ 23

Modification of Sediment Facies by Nuclear Testing............... 25

Summary........................................................................ 27

References Cited.............................................................. 30

ILLUSTRATIONS

Page

FIGURE 1. Index and geographic maps of Enewetak Atoll, Republic of the Marshall Islands........................................................ B2

2. Photographs of Boston whaler and drill ship M/V Knut Constructor used during PEACE Program............................... 3

3. Simplified bathymetric map of Enewetak lagoon ................................................................................................... 5

4. Maps showing current systems in Enewetak lagoon............................................................................................... 6

5. Isopleth maps of Enewetak lagoon showing weight percentages of principal grain constituents of bottom sediments ........ 7

6. Map of Enewetak lagoon showing location of benthic samples taken during Marine Phase of PEACE Program ............. 9

7. Drawing of lead-weighted, snap-jaw grab (bottom) sampler used to collect most benthic samples during PEACE

Program................................................................................................................................................... 12

8. Maps of KOA and OAK craters showing borehole sites and general bathymetric contours............................................ 12

9. Grain-size scale used for the sedimentologic samples.............................................................................................. 13

10, 11. Tertiary plots of facies of:

10. Benthic sediments from Enewetak lagoon and OAK and KOA crater areas................................................... 17

11. All Enewetak benthic samples............................................................................................................... 18

12-14. Maps of:

12. Enewetak lagoon showing distribution of sediment facies ........................................................................... 19

13. Enewetak Atoll showing location of nuclear-detonation sites....................................................................... 21

14. KOA and OAK craters showing distribution of sediment facies ................................................................... 22

15, 16. Isopleth maps of Enewetak lagoon showing:

15. Distribution of fine debris ..................................................................................................................... 25

16. Weight-percent of mud, clay, and low-magnesium calcite, and distribution of elevated radionuclide

concentration................................................................................................................................ 26

17, 18. Histograms showing weight-percent of grain-size fractions of benthic samples from:

17. Enewetak lagoon................................................................................................................................. 32

18. KOA and OAK crater areas................................................................................................................... 52

in
IV CONTENTS

TABLES

	Page

TABLE 1. Location of benthic samples taken during PEACE Program on Enewetak Atoll............................................................... BIO

2. Sedimentologic analyses of benthic samples from Enewetak lagoon................................................................................ 14

3. Sediment facies and grain-size limits for benthic samples from Enewetak lagoon taken during PEACE Program................... 18

4. Diameters and sorting coefficients of sediment facies and subdivisions of facies based on sorting ........................................ 24

5. Results of X-ray diffraction analyses of benthic samples from Enewetak lagoon............................................................... 27

6. Summary of samples and sediment facies from Enewetak lagoon................................................................................... 28
GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL, 

REPUBLIC OF THE MARSHALL ISLANDS

SEDIMENT FACIES OF ENEWETAK ATOLL LAGOON

By BRUCE R. WARDLAW, THOMAS W. HENRY, and WAYNE E. MARTIN

ABSTRACT

Eight sediment fades were distinguished for benthic (bottomsurface) samples from Enewetak Atoll lagoon on the basis of grain size: 

granule, granule-sand, sand-granule, granule-sand-mud, sand, muddy 

sand, sand-mud, and mud. These fades are represented as nonoverlapping fields on a tertiary plot of weight-percent of three constituents 

(mud, sand, and granule). Four of the facies are subdivided by relative 

degree of sorting. Facies recognized for the lagoon as a whole generally 

apply to nuclear craters in the lagoon, with the addition of mixed (or 

rubble) facies; this mixed facies includes fine-grained sediments that 

are ascribed to lagoon facies and, in addition, cobble- and larger sized 

rubble from crater-related slumps, margin collapse, and sand-piping. 

From the scant information on sediment distribution prior to nuclear 

testing on Enewetak, modification of the distribution due to testing is 

shown to include a general increase in fine-grained sediment throughout the lagoon and a marked increase in clay-sized carbonate near sites 

of larger near-surface tests.

INTRODUCTION

Two sets of benthic (bottom-surface) samples were 

taken from the lagoon on Enewetak Atoll, Republic of 

the Marshall Islands, during the PEACE Program 

(1984-1985) (fig. 1). One set of 117 samples was collected 

from aboard a small whaler (fig. 2A) during the Marine 

Phase. Another set of 49 samples was taken from aboard 

the North Sea class M/V Knut Constructor (fig. 25) in 

the vicinity of KOA and OAK craters, in the northern 

part of Enewetak lagoon, during the subsequent Drilling 

Phase. These samples were collected to (1) familiarize 

project geologists with the distribution of sediment types 

and facies within Enewetak lagoon, (2) increase understanding of the distribution of modern microfaunas in the 

lagoon, and (3) supplement studies of the sea-floor features both within and near OAK and KOA craters. The 

benthic sample studies aided both evaluation of the 

stratigraphic sequence penetrated during the Drilling

Manuscript approved for publication February 27, 1990.

Phase and interpretation of the litho- and biostratigraphic framework used in analysis of OAK and KOA.

TOPOGRAPHY AND PHYSIOGRAPHY OF LAGOON

Enewetak Atoll is circumscribed by a barrier reef and 

a chain of about 40 low, small sand islands bordering an 

acorn-shaped lagoon (fig. 1). The islands are concentrated on the northeastern, eastern (windward), and 

southern sides of the atoll. The lagoon is about 40 

kilometers (km) (25 miles (mi)) long by 32 km (20 mi) 

wide and is elongated northwest-southeast. The total 

areas of the islands and the lagoon are about 6.7 square 

kilometers (km2) (2.6 square miles (mi2)) and 932 km2 

(360 mi2), respectively. The barrier reef is cut by three 

passages or channels that lead from the open ocean into 

the lagoon.

The detailed bathymetry of the Enewetak lagoon was 

mapped primarily by the U.S. Navy from the U.S.S. 

Bowditch in 1944. Contour maps made from more than 

180,000 soundings are presented in Emery, Tracey, and 

Ladd (1954, chart 5) and are greatly simplified as figure 

3. The lagoon is characterized by four major bathymetric 

features (Emery and others, 1954): (1) lagoon terrace, (2) 

lagoon basin, (3) coral "knolls" or pinnacle reefs, and (4) 

reef channels or passages.

The lagoon terrace forms the rim of the lagoon everywhere except on the northwestern side (near OAK 

crater), where the rim is a relatively steep precipice from 

the reef tract to the deep lagoon floor, and on the 

southern side, where the rim is interrupted by the South 

Passage and the East Channel (fig. 3). The terrace 

attains a maximum width of about 3 km (1.9 mi) on the 

western side, north of Biken (LEROY) Island (fig. 1). 

Water depth on the terrace varies from 15 to 22 meters 

(m) (49 to 72 feet (ft)); the most frequently recorded 

depth is about 18 m (60 ft) hence the term 10-fathom 

terrace (Emery and others, 1954). From the islands or

Bl
B2 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

162° 162°10' 162°20' 162°30'

S Boken (IRENE) 

+ Bokaidrik (HELEN)

Teiteiripucchi (GENE) 

Elugelab (FLORA) 

Bokinwotme (EDNA)

Louj (DAISY) 

Kirunu (CLARA)

Bokombako (BELLE) 

Bokoluo (ALICE)

a

Enjebi (JANET) 

Mijikadrek (KATE) 

Kidrinen (LUCY)

Bokenelab (MARY) 

Elle (NANCY) 

Aej (OLIVE)

Lujor (PEARL)

Eleleron (RUBY) 

Aomon (SALLY) 

Bijile (TILDA) 

Lojwa (URSULA) 

Alembel (VERA) 

Billae (WILMA)

|VY ,+ 160000

GRID 

(FT)

PREVAILING WIND

Biken(LEROY)-

SOUTHWEST 

PASSAGE

r j-cM^.^TAuT* HAWAIIAN \ '*. *ENEWETAK |SLANDS

7^^

N 4 ,

Kidrenen (KEITH) 

Ribewon (JAMES) 

Boken (IRWIN)

Mut (HENRY) 

Ikuren (GLENN)

SOUTH PASSAGE

+ 120000

Runit (YVONNE)

"M" (ZONA)

Jinedrol (ALVIN) 

+ 80000

Anaij (BRUCE) 

Jimini (CLYDE) 

Japtan (DAVID) 

EAST (DEEP) CHANNEL

Medren (ELMER)

+ 40000

5 NAUTICAL MILES 10,000

ilnnl

30,000 50,000 FEET

Enewetak (FRED)

10 KILOMETERS

FIGURE 1. Enewetak Atoll, Republic of the Marshall Islands, with locations of KOA, OAK, and MIKE craters, major channels (passages), 

principal islands (native names followed by military site names assigned during nuclear testing in parentheses), and coral pinnacles MACK 

and OSCAR. General location of Enewetak Atoll shown on inset map. (From Henry, 1988, fig. 12.)

reef tract, the terrace dips generally lagoonward, sloping 

at an average of about 2.5 degrees. The terrace surface is 

pockmarked with scattered, irregular depressions probably related to Pleistocene karst features (sinkholes). 

These depressions rarely are deeper than 3 to 4 m (about 

12 ft) below the general terrace surface. The inner slope 

of the lagoon terrace, which separates the terrace from 

the lagoon basin, is less than 1.5 degrees. Patch reefs and

small coral pinnacle reefs ("coral knolls") are common on 

the terrace.

The lagoon basin is an elliptical, gently undulating, 

nearly flat floored feature, with slopes averaging about 

0.1 degree. According to Emery, Tracey, and Ladd 

(1954), the basin has a mean depth of about 55 m (180 ft) 

and attains a maximum depth of 62 m (200 ft) in its 

northwestern half. Enewetak lagoon is relatively deep
SEDIMENT FACIES OF ENEWETAK ATOLL LAGOON B3

FIGURE 2A. Sample-collecting crew and 21-foot Boston whaler used in August and early September 1985 during Marine Phase of PEACE

Program (photograph courtesy of E.A. Shinn, USGS).

compared with lagoons of other Pacific atolls (Wiens, 

1962). The floor of the lagoon basin is marked by several 

thousand coral pinnacle and patch reefs of varying sizes, 

about 10 percent of which rise abruptly from the floor to 

within a few meters of the water surface and several of 

which are awash at lowest tides (for example, OSCAR 

and MACK, fig. 1).

The three passages or channels that breach the barrier 

reef provide major access from the open-marine environment into the lagoon (figs. 1, 3). The East (Deep) 

Channel, between the islands of Japtan (DAVID) and 

Medren (ELMER), is the main marine connection on the 

windward side of the atoll. This channel is 1.5 km (0.9 mi) 

wide and 55 m (180 ft) deep. The South Passage is 9.5 km

(5.9 mi) wide and 12 to 20 m (40 to 65 ft) deep, and the 

poorly defined Southwest Passage, between Kidrenen 

(KEITH) and Biken (LEROY) Islands, is a network of 

small passages 2 to 4 m (6.5 to 13 ft) deep.

LAGOON WATER AND CURRENT SYSTEMS

Windward and leeward cross-reef currents, channel 

currents, and tidal flow are major factors affecting the 

exchange of water between the lagoon and the surrounding open ocean (fig. 4A) (Atkinson and others, 1981; 

Atkinson, 1987). Enewetak lagoon is reasonably isolated 

from the general westward flow of the North Equatorial 

Current. However, the surf continually pounds the wind-
B4 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

FIGURE 2B. Portside aerial view of 237-foot drill ship M/V Knut Constructor photographed in OAK crater in early June 1985; ship is outfitted 

with a Failing-2000 rotary drill rig, located midship (photograph by Lt. Col. R.F. Couch, Jr., DNA/USAF; from Henry, 1988, fig. 13).

ward side, and, because the barrier-reef flats are shallow, water constantly spills over the reef into the lagoon. 

Most of the water coming into the lagoon enters over the 

windward reefs in this manner. Water cyclically flows 

into the lagoon through both the East (Deep) Channel 

and the Southwest Passage. Water flows out through the 

South Passage, the Southwest Passage, and the leeward 

reef. Because the amount of flow over the leeward reef 

from the lagoon is relatively small, most of the water 

ultimately moves south, exiting through the South Passage. Atkinson (1987, p. 68) estimated an average residence time of about 1 month for water in the lagoon; 

residence time of water entering the northern part of the 

lagoon is roughly 4 months.

Three current levels exist within the central lagoon 

surface, mid-depth, and deep. The wind-driven surface 

currents (fig. 4£), 5 to 15 m (16 to 50 ft) thick and 

averaging 10 m (32 ft) thick, primarily facilitate internal 

circulation rather than "flushing" of lagoon waters. 

Surface-current velocities are slow, averaging about 10 

centimeters per second (cm/s) (4 inches per second (in/s)); 

drift is generally downwind, to the southwest. The

mid-depth currents (10 to 30 m (32 to 98 ft) deep) 

generally flow northeastward counter to the surface flow 

at velocities of 2 to 4 cm/s (0.8 to 1.6 in/s). The deep 

currents, generally 30 to 50 m (98 to 165 ft) deep, flow 

southward at rates of only 0.5 to 1.5 cm/s (0.2 to 0.6 in/s). 

Around the periphery of the lagoon, this general threelayer current system is modified or eliminated by tidal 

and cross-reef currents. The magnitude of the cross-reef 

currents varies seasonally. As shown in figure 4C, the 

windward cross-reef currents follow the configuration of 

the atoll along the northeastern side of the lagoon 

(flowing generally northwestward), and in the southeast 

between Medren (ELMER) and Enewetak (FRED) 

Islands (fig. 1) (southwestward flow). Between these two 

areas and centered on the East (Deep) Channel is a 

region where the currents also follow the lagoon margin 

but reverse in response to the tides (fig. 4C).

The water column in the lagoon is nearly isohaline and 

isothermal; salinity averages 34.4 parts per thousand 

(±0.20 ppt), and temperature averages 27 to 29 °C. Both 

values are only slightly higher than for surface waters of 

the North Equatorial Current. Concentrations of inor-
SEDIMENT FACIES OF ENEWETAK ATOLL LAGOON B5

ganic nutrients and dissolved gases show no vertical 

stratification within the lagoon waters. Thus, despite the 

three-layer circulation system, the waters exiting the 

lagoon are well mixed.

PREVIOUS BENTHIC SAMPLING AND 

ECOLOGIC STUDIES

The first benthic samples were collected from 

Enewetak lagoon and surrounding areas in 1946 during 

Operation CROSSROADS (Emery and others, 1954), 

prior to the period of nuclear testing (1948-1958). This 

suite of 365 samples was taken mainly from aboard the 

U.S.S. Blish, whose primary mission was gathering 

supplementary data for detailed bathymetric maps of the 

atoll. Most of these samples were taken with a small 

underway sampler ("scoopfish") along selected 

bathymetric-traverse lines (fig. 4Z>); fewer than 5 percent were collected with a dredge. The distribution and 

average weight percentages of the sedimentary components were described by Emery, Tracey, and Ladd 

(1954, p. 97-100; see figs. 5A-5D of this report). The 

principal grain components, in decreasing order of importance, were Halimeda (fig. 5C), coral, foraminifers (fig. 

55), mollusk shells and debris (fig. 5A), and other 

fine-grained biotic and bioclastic material (fig. 5Z>), 

defined as particles less than 0.25 millimeter (mm) in 

diameter (that is, fine sand and finer). This mix is similar 

to that of benthic samples taken from other Pacific atolls. 

According to J.I. Tracey (oral commun., 1986), the 1946 

Enewetak lagoon samples were biased against the finegrained components because of the tendency of the 

sampler to not close completely upon impact with the 

bottom and the consequent "streaming" of the finer 

materials out of the sampler as it was raised from the sea 

floor through the water column.

A number of the 1946 samples were included in a study 

of the Recent foraminifer faunas of the Marshall Islands 

by Cushman, Todd, and Post (1954). Some of the same 

samples were reexamined for the PEACE Program for 

both foraminifers and ostracodes by Cronin, Brouwers, 

and others (1986).

Additional surface and near-surface samples were 

taken from the atoll following nuclear testing on 

Enewetak, principally to determine the distribution of 

radionuclides in the sediments of the lagoon and islands 

to assess their potential long-term effects on the food 

chain and their consequent impact on relocated human 

populations. The most extensive of these programs was 

the Enewetak Radiological Survey of 1972. Identified in 

the report of the radiological survey (Nelson and 

Noshkin, 1973) were two areas of significant contamination (fig. 16D), a large area encompassing most of the 

northwestern part of the lagoon (downcurrent from 

where most of the nuclear testing was done) and a

162°

1T401

1T201

162"10I 162°20' 162°30'

OAK

160000

120000

80000

40000

NAUTICAL MILES FEET KILOMETERS

FIGURE 3. Simplified bathymetric map of Enewetak lagoon. Sounding contours in fathoms (1 fathom equals 6 feet). (From Folger, 

Hampson, and others, 1986, fig. 1, generalized and simplified after 

Emery and others, 1954, chart 5.)

smaller area lagoonward from the island of Runit 

(YVONNE), on the eastern side of the atoll (see also 

Noshkin, 1980). More recently, McMurtry, Schneider, 

and others (1985) studied the vertical distribution of 

fallout radionuclides in shallow sediment cores less 

than 244 cm (8 ft) in length-taken off Runit (YVONNE) 

Island near the HOLLY/MAGNOLIA/LINDEN/ 

SEQUOIA composite crater. Subsequent studies of 

redistribution of radionuclides in the near-surface lagoon 

sediments by burrowing benthic organisms (bioturbation) were conducted by Suchanek and Colin (1986) and 

Suchanek, Colin, and others (1986).

The subtidal environments and biotic communities of 

Enewetak lagoon, with certain exceptions, are generally 

poorly known. This is particularly true of the deeper 

parts of the lagoon. Colin (1986, 1987) conducted a very 

general study of the subtidal lagoon environments based 

primarily on about 2,000 photographs taken with a 

submarine camera system. Supplementary information 

came from submersible dives in the deeper areas and 

scuba reconnaissances around the lagoon margin. Colin 

divided the subtidal area into (1) a lagoon-margin environment and (2) a deep-lagoon environment (the part 

exceeding 30 m (98 ft) in depth). The deep-lagoon environment is completely within the realm of the slowmoving, deep-current system. These environments were 

subdivided into a number of communities on the basis of 

substrate types and dominant organisms. For both environments, two general bottom types were recognized 

hard and soft substrates. Conclusions about the nature of
B6

162°10'

GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

162°20' 162°30' 162° 162°10' 162°20' 162°30'

SOUTHWEST 

PASSAGE

NAUTICAL MILES

SOUTH PASSAGE

0 10,000 30,000 50,000 

FEET

Medren (ELMER) 

Enewetak (FRED)

KILOMETERS

162°10' 162°20' 162°30'

°40'

ir20'

NORTHEAST 

TRADES

SOUTHWEST 

PASSAGE

NAUTICAL MILES

SOUTH PASSAGE

0 10.000 30,000 50,000 

FEET KILOMETERS

FIGURE 4. Current systems in Enewetak lagoon. A, Major currents 

and passages; area influenced by windward cross-reef currents 

shown with horizontal hachures, area influenced by leeward crossreef currents with vertical hachures; influx and outflux of water 

during tidal cycles shown with arrows (modified from Atkinson, 

1987, fig. 2). B, Lagoon surface currents; arrows represent smoothed 

drogue trajectories over varying lengths of time (modified from

the substrate and corresponding percentages were based 

on point counts from the photographic data.

The width of Colin's (1986, 1987) lagoon-margin environment low tide to 30 m (100 ft) depth varies from 

only a few hundred meters in the south (for example, 

near Enewetak (FRED) Island) to more than 1 km (0.6

IVY/

GRID 

(FT)

+120000

SOUTHWEST 

PASSAGE

EAST

(DEEP)

CHANNEL

+ 40000

NAUTICAL MILES

SOUTH PASSAGE

0 10,000 30.000 50,000 ''"FEET'' , KILOMETERS

162° 162°10' 162°20'

11-40'

160000

120000

80000

SOUTHWEST 

PASSAGE

0

SOUTH PASSAGE

0 10,000 30,000 50,000 0

NAUTICAL MILES FEET KILOMETERS

Atkinson, 1987, fig. 4). C, General current patterns in lagoon; arrows 

indicate current direction, stippled areas are influenced by nonreversing cross-reef currents, cross-hachured areas are regions of 

reversing currents; circular pattern indicates convergence (modified 

from Atkinson, 1987, fig. 9). D, 1946 sample locations (modified from 

Emery and others, 1954, fig. 43).

mi) in the north and northwest (for example, near Engebi 

(JANET) and Biken (LEROY) Islands). This environment is highly variable and consists of soft- and hardbottom communities, including numerous patch reefs and 

coral knolls. The types and distribution of the communities depend on their location (on the leeward or wind-
SEDIMENT FACIES OF ENEWETAK ATOLL LAGOON B7

162°

irso1

162°20' 162° 162°Ur 162°20' 162°30'

SOUTHWEST 

PASSAGE

40000

SOUTH PASSAGE

0 10,000 30,000 50,000 0 5

NAUTICAL MILES FEET KILOMETERS

SOUTHWEST 

PASSAGE

SOUTH PASSAGE

0 10,000 30,000 50,000 0 5

NAUTICAL MILES FEET KILOMETERS

162°

11 040'

H°30'

162°10' 162°20' 162°30' 1620 10' 162°20' 162130'

SOUTHWEST 

PASSAGE

40000

NAUTICAL MILES

SOUTH PASSAGE

0 10,000 30,000 1 50,000 

FEET KILOMETERS

SOUTHWEST 

PASSAGE

0

NAUTICAL MILES

SOUTH PASSAGE

0 10,000 30,000 50,000 

FEET KILOMETERS

FIGURE 5. Isopleth maps of Enewetak lagoon showing weight percentages of principal grain constituents of bottom sediments (data from 1946 

(pretesting) survey; modified from Emery and others, 1954, fig. 43). A, Mollusk shells and debris. B, Foraminifera in benthic sediments. 

C, Halimeda debris and other green algae grains and flakes in benthic sediments. D, Fine debris (that is, fine sand and finer) in 

benthic sediments.

ward side of the atoll), the position and proximity of 

sheltering sand islands, and the community's proximity 

to the passages to the open ocean. The lagoon-margin 

environment in the vicinity of the South Passage has a 

particularly high concentration of mollusk shells and 

debris and Halimeda (figs. 5A, 5C).

Within the lagoon-margin environment, patch reefs 

are very well developed and (in some places) almost 

continuous on the western (leeward) side of the atoll,

where the lagoon terrace is broad. Here, because of 

waves generated by the prevailing winds blowing across 

the lagoon and breaking on the lagoonward side of the 

main reef flat itself, a number of features (including 

spurs and grooves) are developed on the lagoonward side 

that resemble those found on the seaward side of the 

barrier-reef flats on the windward side of the atoll.

Colin (1986, 1987) estimated that about 85 percent of 

the deep-lagoon floor consists of soft substrates; the rest
B8 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

of the bottom is hard or a mixture of hard and soft. The 

hard-substrate communities are principally (1) pinnacle 

and patch reefs of very high diversity and of varying size 

and relief that are scattered throughout the basin and (2) 

so-called "coral pavements." The soft-substrate communities form four commonly intergrading types: (1) opensand lagoon floor without a visible algal mat, (2) sand 

floor with a visible surficial microalgal mat ("algal film"), 

(3) sand floor populated by calcareous, green macroalgae 

(for example, Halimeda "flats" or "meadows"), and (4) 

sand floor populated by large numbers of unattached, 

solitary fungiid corals. (Note that the term "sand floor" is 

a misnomer; in a sedimentologic sense, these generally 

are not sand bottoms.)

The first soft-substrate "community" is selfexplanatory. The microalgal mats (community 2) generally are composed of a diatom and blue-green algal film at 

the sediment-water interface. The algal-flat community 

(community 3), areas of high diversity and productivity, 

generally is made up of moundlike features composed of 

various kinds of calcareous algae and, commonly, stony 

corals. These mounds commonly project as much as 

several meters above the surrounding area because of 

the higher rate of production and accumulation of coarser 

grained skeletal material than the surrounding communities and because of the entrapment (baffling effect) by 

the algae of finer grained (muddy) particles being transported in suspension. Emery, Tracey, and Ladd (1954, p. 

98-99) reported that in the 1946 study the highest 

percentage of Halimeda debris was in samples (fig. 4B) 

from the deep-lagoon environment; in places, Halimeda 

debris made up as much as about 80 percent of the 

sample. The fourth community (the fungiid coral community) is uncommon and seemingly is restricted to areas of 

the lagoon 50 m (about 160 ft) or more deep.

All four communities generally are highly bioturbated 

and studded with conical, callianassid shrimp mounds. 

Emery, Tracey, and Ladd (1954; fig. 5B, this paper) 

reported samples from the middle part of the lagoon to be 

particularly rich in smaller benthic foraminifers. This 

foraminifer-rich facies is in the deepest part of the 

deep-lagoon environment in waters dark enough that 

Halimeda cannot flourish and overwhelm the accumulation of foraminifer tests in the benthic sediments.

STORM EFFECTS ON THE LAGOON ENVIRONMENTS

Under normal conditions, sediment that is generated 

in the lagoon subtidal environments, or that is swept 

across the reef flats on the windward side and thus 

enters the lagoon, generally stays in the lagoon. These 

sediments are subject to reworking by cross-reef currents and waves around the shallower margins of the

lagoon. Particles that enter the realm of the deep-lagoon 

floor (the part of the lagoon deeper than 30 m (98 ft)) by 

settling through the water column, or that are generated 

in situ by growth and death of calcareous organisms, 

generally stay on the lagoon floor or are reworked into 

the upper part of the sediment column by infaunal 

benthic organisms, such as the callianassids. Because of 

the shape of the deep-lagoon environment and the nature 

of the deep-current system, the possibility of this material being transported out of this environment into 

shallower parts of the lagoon or, indeed, out of the lagoon 

itself is slim.

Typhoons, tropical storms, and other tropical disturbances create wave and current conditions that disrupt 

normal sedimentary processes in shallow-water marine 

environments. Generally, in the western Marshall 

Islands, these disturbances occur during the wet season, 

particularly July through October (see Merrill and Duce, 

1987, for data on the tropical storms and disturbances 

affecting Enewetak). The oceanward side of the normally 

leeward part of the atoll is particularly vulnerable to 

these storms. From the shallow subtidal part of the reef 

tract and reef flat on the side of the islands facing the 

storm, material can be broken up by the storm waves and 

piled up on the seaward side of the islands, forming 

so-called boulder ramparts consisting mainly of coral 

heads. Smaller carbonate particles can be driven across 

the islands by the storm waves and deposited on the 

stormward side of the lagoon. Some of the very fine 

suspended material ultimately settles in the deep-lagoon 

environment after the storm passes.

The impact of such storms on subtidal environments in 

the lagoon can be equally devastating, particularly on 

atolls (such as Enewetak) that have channels or passages 

large enough to admit ocean swells generated by these 

storms into the lagoon. The broad South Passage 

between Enewetak (FRED) and Ikuren (GLENN) 

Islands is deep enough to allow ocean swells from the 

southeast to southwest to enter the lagoon. According to 

Colin (1987), the swells are refracted at the channel so 

that wave trains moving from the west and southwest 

can travel across the lagoon and impinge on the opposite 

lagoon shoreline. To quote Colin (1987, p. 124): "These 

long period swells have no direct effect on the deep 

lagoon communities. However, when they reach the 

lagoon shore of windward islands or patch reefs, they can 

turn these shallow-water communities into churning 

maelstroms of breaking waves." Although it is probably 

true that these waves have no direct effect on the 

deep-lagoon communities, the shallower parts of the 

pinnacle reefs originating from the deep-lagoon bottom 

are affected, and some of the disrupted material is shed 

as talus into the deep-lagoon environment in the vicinity 

of the pinnacle reefs.
SEDIMENT FACIES OF ENEWETAK ATOLL LAGOON B9

162° 162°10' 162°20' 162°30'

OAK

g 

IVY + 160000

GRID 7 

(FT)

+ 120000

80000

SOUTHWEST 

PASSAGE

EAST

(DEEP)

CHANNEL

+ 40000

SOUTH PASSAGE

NAUTICAL MILES

0 10,000

liiiilnnl______L

30,000

I_____I

50,000 i

10

I

FEET KILOMETERS

FIGURE 6. Location of benthic samples taken in Enewetak lagoon during Marine Phase of PEACE Program (see table 1 for IVY-grid coordinates

of samples).

Material churned up from the eastern and northeastern parts of the shallow lagoon margin by the 

impingement of these wave trains on the shores can 

also be exported from the lagoon by being swept across 

the islands and (or) reef flats. Colin (1987) also noted that 

the downslope movement of rubble on the oceanward 

parts of the reef during these storms is volumetrically 

significant.

PEACE PROGRAM DATA BASE AND 

PROCEDURES

The first set of 117 benthic samples taken during the 

PEACE Program was collected in September 1984, 

during the Marine Phase (fig. 6, table 1). The second set 

of 49 benthic samples was taken in the proximity of KO A 

and OAK from February through July 1985, during the 

Drilling Phase (table 1).
BIO GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

TABLE 1.--Location of benthic samples taken

Program on Enewetak Atoll

[IVY-grid coordinates in feet north and east. Sample 1

Sample

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

samples 110-113 were not collected]

during PEACE

was lost in transit;

IVY-grid coordinates

116,563 N.

116,250 N.

116,255 N.

120,169 N.

120,349 N.

116,864 N.

115,594 N.

113,923 N.

111,869 N.

109,779 N.

107,672 N.

105,389 N.

103,059 N.

100,911 N.

099,550 N.

104,000 N.

105,600 N.

105,000 N.

105,400 N.

105,600 N.

124,700 N.

125,800 N.

126,500 N.

127,400 N.

128,400 N.

123,700 N.

123,433 N.

125,083 N.

125,929 N.

125,423 N.

125,436 N.

125,649 N.

126,928 N.

135,333 N.

135,164 N.

135, 119 N.

135,052 N. x

134,957 N.

134,474 N.

134,750 N.

089,900 N.

090,339 N. 1

090,143 N.

090,125 N. 1

090,278 N. 1

090,111 N.

090,256 N. 1

089,847 N.

090,269 N. 1

090,114 N. 1

090,051 N.

081,528 N. 1

080,083 N.

104,893 N.

103,787 N.

102,496 N.

025,625 E.

026,250 E.

027,500 E.

034,022 E.

033,675 E.

028,860 E.

033,997 E.

039,437 E.

045,446 E.

051,012 E.

056,308 E.

062,053 E.

068,016 E.

072,994 E.

076,310 E.

087,200 E.

096,400 E.

105,700 E.

114,700 E.

123,500 E.

117,700 E.

117,700 E.

108,400 E.

102,400 E.

094,100 E.

079,571 E.

075,155 E.

070,078 E.

059,865 E.

053,733 E.

047,996 E.

042,008 E.

038,264 E.

048,432 E.

056,891 E.

062,314 E.

068,260 E.

074,334 E.

081,830 E.

088,045 E.

027,665 E.

040,438 E.

049,953 E.

060,321 E.

070,766 E.

079,886 E.

090,418 E.

100,030 E.

110,628 E.

120,257 E.

130,701 E.

131,360 E.

120,271 E.

023,570 E.

034,358 E.

043,504 E.

TABLE 1 . Location of benthic samples taken during PEACE

Program on Enewetak A toll Continued

[IVY-grid coordinates in feet north and east. Sample 1

Sample

58

59

60

61

62

63

64

65

66

67

68

69

70

71

72

73

74

75

76

77

78

79

80

81

82

83

84

85

86

87

88

89

90

91

92

93

94

95

96

97

98

99

100

101

102

103

104

105

106

107

108

109

114

115

116

117

samples 110-113 were not collected]

was lost in transit;

IVY-grid coordinates

102,488 N.

120,760 N.

116,225 N.

116,539 N.

116,104 N.

115,837 N.

116,078 N.

115,972 N.

143,198 N.

142,035 N.

075,024 N.

075,108 N.

074,938 N.

075,486 N.

075,273 N.

075,377 N.

075,000 N.

075,000 N.

071,333 N. 2

072,200 N. 2

059,907 N.

060,263 N.

059,850 N.

060,084 N.

059,976 N. 1

059,899 N.

060,122 N.

060,078 N.

058,730 N.

059,790 N.

056,721 N.

046,609 N.

044,985 N.

045,356 N.

032,115 N.

045,006 N.

045,034 N.

045,159 N.

045,553 N.

147,500 N.

151,300 N.

153,125 N.

151,563 N.

135,625 N.

106,250 N.

086,563 N.

076,563 N.

062,188 N.

048,120 N.

035,156 N.

037,500 N.

037,344 N.

072,033 N.

031,250 N.

034,688 N.

041,250 N.

053,802 E.

050,924 E.

060,082 E.

071,146 E.

079,923 E.

090,031 E.

100,618 E.

100,092 E.

077,635 E.

068,681 E.

110,040 E.

100,323 E.

090,070 E.

081,692 E.

070,622 E.

060,633 E.

050,359 E.

040,038 E.

031,200 E.

031,817 E.

050,165 E.

060,919 E.

070,209 E.

080,327 E.

090,360 E.

100,169 E.

100,004 E.

119,780 E.

130,262 E.

129,627 E.

129,518 E.

125,333 E.

110,053 E.

100,708 E.

095,191 E.

090,014 E.

080,008 E.

070,432 E.

060,673 E.

082,450 E.

082,188 E.

082,188 E.

080,625 E.

096,875 E.

124,000 E.

135,000 E.

130,625 E.

139,688 E.

130,000 E.

123,438 E.

124,531 E.

124,375 E.

030,667 E.

082,500 E.

069,688 E.

059,063 E.
SEDIMENT FACIES OF ENEWETAK ATOLL LAGOON Bll

TABLE I. Location of benthic samples taken during PEACE 

Program on Enewetak Atoll Continued

[IVY-grid coordinates in feet north and east. Sample 1 was lost in transit; 

samples 110-113 were not collected]

Sample IVY-grid coordinates

118

119

120

121

122

123

KBZ-4S

KBZ-4B

KBZ-4C

KCT-5S

KCT-5B

KDT-6S

KDT-6B

KET-7S

KET-7B

OAM-3C

OBZ-4B

OCT-5S

OCT-5B

ODT-6S

ODT-6B

OET-7S

OET-7B

OFT-8S

OFT-8B

OGT-9S

OHT-10B

OIT-11S

OIT-11B

OJT-12S

OJT-12B

OKT-13S

OKT-13B

OLT-14S

OLT-14B

OMT-15S

OMT-15B

ONT-16S

ONT-16B

OOR-17S

OOR-17B

OPZ-18S

OPZ-18B

OQT-19S

OQT-19B

ORT-20S

ORT-20B

OSR-21S

OSR-21B

OSM-22S

OTG-23S

OTG-23B

OUT-24S

OUT-24B

RXNMASS

056,250 N. 

072,656 N. 

096,875 N. 

140,625 N. 

112,188 N. 

075,313 N. 

149,290 N. 

149,382 N. 

149,350 N. 

148,758 N. 

148,828 N. 

148,129 N. 

148,240 N. 

147,985 N. 

148,049 N. 

127,818 N. 

124,983 N. 

125,434 N. 

124,467 N. 

126,109 N. 

126,138 N. 

125,898 N. 

125,943 N. 

125,768 N. 

125,805 N. 

125,723 N. 

123,996 N. 

124,214 N. 

124,216 N. 

124,031 N. 

124,044 N. 

124,327 N. 

124,364 N. 

123,561 N. 

123,550 N. 

123,956 N. 

123,962 N. 

124,204 N. 

124,198 N. 

119,843 N. 

119,817 N. 

124,787 N. 

124,765 N. 

123,631 N. 

123,640 N. 

123,281 N. 

123,294 N. 

120,304 N. 

120,308 N. 

120,282 N. 

124,204 N. 

124,180 N. 

125,482 N. 

125,538 N. 

121,824 N.

047,344 E. 

034,219 E. 

019,375 E. 

057,188 E. 

117,500 E. 

139,375 E. 

071,000 E. 

071,208 E. 

071,113 E. 

070,971 E. 

071,189 E. 

071,018 E. 

071,240 E. 

071,023 E. 

071,247 E. 

039,450 E. 

036,211 E. 

036,428 E. 

036,652 E. 

037,248 E. 

037,472 E. 

036,973 E. 

037,195 E. 

036,759 E. 

036,982 E. 

036,685 E. 

037,295 E. 

036,924 E. 

037,150 E. 

037,401 E. 

037,627 E. 

036,756 E. 

036,979 E. 

038,334 E. 

038,573 E. 

037,941 E. 

038,167 E. 

037,639 E. 

037,865 E. 

032,771 E. 

032,996 E. 

036,255 E. 

036,480 E. 

035,419 E. 

035,644 E. 

035,213 E. 

035,439 E. 

033,060 E. 

033,297 E. 

033,003 E. 

035,771 E. 

035,997 E. 

035,552 E. 

035,331 E. 

033,363 E.

'"Average" location of multiple-grab sample. 

2Location estimated from lagoon hydrographic map.

FIELD PROCEDURES

Sampling in Enewetak lagoon during the Marine 

Phase was done using a modified grid (fig. 6) from aboard 

a 21-ft Boston whaler (fig. 2A) equipped with a Motorola 

Falcon-IV Miniranger navigation computer. The number 

of sampling stations was limited by severe time constraints. A navigation specialist from Meridian Ocean 

Systems was aboard the whaler when the samples were 

collected. Thus, the network of five transponder stations 

established around the atoll for the program (see Folger, 

Hampson, and others, 1986, p. A2, and Henry and 

others, 1986, p. 32-39, for discussion) was used effectively, and the position recorded is highly accurate (±3 m 

(10 ft)). The whaler's location was recorded when the 

sampler struck the bottom. Although the heavy sampler 

dropped rapidly and almost vertically, minor allowance 

must be made for some drift, particularly on windy days 

in the deeper parts of the lagoon.

Most samples were taken with a 16-kilogram (kg) 

(35-pound (lb)), lead-weighted, snap-jaw, grab sampler 

(fig. 7). Generally, the jaws of the heavily sprung sampler closed cleanly, minimizing the loss of finer grained 

components as the sampler was retrieved. When the 

sampler did not close cleanly, the residual sample was 

discarded and another one was taken in the same area. 

Multiple samples were taken at many stations to obtain 

an acceptable volume of material for analysis, particularly in areas of predominantly hard bottoms or abundant 

fungiid corals. Thus, samples from such environments 

predominantly represent the soft-sediment component of 

the substrate. The location of multiple samples is given in 

table 1 as an "average" of the Miniranger locations for 

each grab. In shallower water, where the grab sampler 

was not effective, samples were obtained by diving from 

the surface and scooping the sediment into an approximately 2-liter (L) (2 quart (qt)) metal can. Samples taken 

with the grab-sampler generally weighed about 1 kg (2.2 

lb); those taken with the metal can weighed about twice 

as much.

Onsite, no attempt was made to correlate the Marine 

Phase sample locations with depths or with bottom 

types. The depth of each sample was estimated from 

available bathymetric maps (Defense Mapping Agency 

charts).

All but three of the samples collected during the 

Drilling Phase were taken from the starboard bow and 

starboard stern of the Knut Constructor (fig. 25), at 

most borehole sites using the same grab sampler (table 

1). Two additional grab sampler samples were taken 

from starboard midship onboard. One additional sediment sample was scooped from the top of the reaction 

mass, after it was retrieved from the lagoon bottom at 

the end of the program. The borehole locations on KOA
B12 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

1

o J

FEET

FIGURE 7. Lead-weighted, snap-jaw grab (bottom) sampler used to 

collect most benthic samples during PEACE Program.

152,000 N

64,000 E 78,000 E

150,000 N

145,000 N

140,000 N

130,000 N

31,500 E 45,500 E

125,000 N

120,000 N -

B

Apparent 

crater limit

OAK CRATER

FIGURE 8. Borehole sites (depicted by letters) and general bathymetric contours (in feet) in (A) KOA and (B) OAK craters (see fig. 1 for 

location of craters).

and OAK craters are shown in figures 8A and 85, 

respectively. Samples taken from aboard the drill ship 

were located trigonometrically from the borehole site 

(determined by the shipboard navigational computer), 

the heading of the ship, and the distance from the 

moonpool to the sampling station. The depths are 

approximately those of the borehole or represent leadline soundings made from the ship at the sample station. 

For both sample sets, the material was scooped and 

(or) washed, with a spatula and squeeze bottle, from the 

sampler into sealable plastic bags and identified with the 

appropriate station. Marine Phase samples were transported to the University of Hawaii/DOE Mid-Pacific 

Research Laboratory on Enewetak (FRED) Island. 

Excess sea water was decanted from the samples after
SEDIMENT FACIES OF ENEWETAK ATOLL LAGOON B13

the sediment was allowed to settle. From 50 to 75 

milliliters (mL) of 5-percent methanol and a few drops of 

buffered formaldehyde were added to each sample. The 

bags were then resealed, and inserted into larger plastic 

bags and sealed for shipment. Drilling Phase samples 

were sealed, labeled, and inserted into larger sealed bags 

for packing and shipment from the Knut Constructor.

LABORATORY PROCEDURES

The sediment samples were processed in U.S. Geological Survey (USGS) laboratories and in the Division of 

Sedimentology laboratory, Department of Paleobiology, 

at the Natural History Museum (Smithsonian Institution), Washington, D.C. The benthic samples were 

washed free of preservatives, if present, and then dried 

at low temperature (70 °C). After equilibration to standard conditions (room temperature and humidity), samples were weighed on a top-loading balance. Each sample 

was then split into paleontologic and sedimentologic 

fractions with a standard sediment splitter. Fragments 

(generally coral) too large for the splitter were weighed 

separately and retained with excess sedimentologic fractions as archival material.

The paleontologic split was transferred to USGS laboratories in Reston, Va., where selected samples were 

processed and picked for microfauna (benthic foraminifers and ostracodes) for study of the ecologic distribution 

of modern Enewetak lagoon microfaunas (Cronin and 

others, 1986, p. 36-37).

Twenty-three of the sedimentologic samples were split 

further and analyzed for total organic content (10 samples) and (or) for mineralogy (22 samples) by detailed 

X-ray analysis at the U.S. Air Force Weapons Laboratory, Kirtland Air Force Base, N. Mex. (Ristvet and 

Tremba, 1986b, and Tremba and Ristvet, 1986, respectively).

From the remaining fraction of the 23 samples just 

described and from all the other sedimentologic samples, 

100- to 200-gram (g) (3.5- to 7-ounce (oz)) portions were 

weighed and cleaned of organics by oxidation with a 

30-percent hydrogen-peroxide solution. From each, an 

approximately 50-g (1.8-oz) split was taken for grain-size 

analysis. These samples were wet-sieved to separate the 

silt and clay fraction from the sand and small-granule 

fraction by flooding the sample with less than a liter of 

0.03 percent "Calgon" water and washing it through a 

4-phi- (no. 230-) mesh brass screen. The sample retained 

on the screen was dried, weighed, transferred into a set 

of nested sieves, and placed in a sediment shaker for 15 

minutes. Sieve sizes were selected for the major 

sediment-size categories, which include -1, 0, 1, 2, 3, and 

4 phi (fig. 9). Material retained on each sieve screen then

mm 0

16 -r -*

2

1 -

1/2 -

1/4 -

I/O 

1 /1 C

1 /OCLC

- -1

- 0

- 1

- 2

H ^*J

h 4

Granule and small pebble

Very coarse sand

Coarse sand

Medium sand

Fine sand

Very fine sand

Silt

Clay

FIGURE 9. Grain-size scale (Wentworth scale) used for the sedimentologic samples.

was weighed, and the size fractions recombined and 

archived.

Pipette analysis in 1-L graduated cylinders was done 

on the material in the bottom pan after wet sieving, using 

the method described by Folk (1974). Pipette samples 

were dried, allowed to equilibrate to standard room 

conditions, and weighed on an analytic balance.

The ratio of granule:sand:mud was calculated for each 

sample (table 2) in the manner described by Folk (1974) 

(granule, -4 to -1 phi; sand, -1 to 4 phi; mud, >4 phi). For 

samples containing 65 weight-percent or greater sand, 

the sand size was differentiated into very coarse and 

coarse, medium, and fine and very fine fractions (see fig. 

9). Histograms of the size fractions for each sample are 

given at the end of the text (figs. 17, 18).

Noncarbonate fractions of the Enewetak lagoon samples make up very small percentages of the total sediment. Nonorganic insoluble residues are negligible in 

PEACE Program borehole samples (Ristvet and 

Tremba, 1986a), including subbottom samples from the 

Holocene section. Although the lagoon-bottom samples 

were not analyzed for insoluble residues, certainly a 

paucity of noncarbonate components is expected. On the 

other hand, the organic content of the modern lagoon 

benthic sediments averages 3.3 percent by weight, and 

the Holocene core samples only slightly less (3.0 percent) 

(Ristvet and Tremba, 1986b). Note that the organic 

fraction was not measured in the present study, and that 

the percentage values given in the sedimentologic table 

(table 2) reflect only the total weight of the carbonate 

material.
B14 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

TABLE 2. Sedimentologic analyses of benthic samples from Enewetak lagoon

[Analyses are ratios, in weight-percent, as follows: Entire sample, granule:sand:mud; Sand portion of sample: very coarse and coarse:medium:fine and very fine. Some 

ratios do not add to 100 owing to rounding. See text for determination of depths, given in meters and feet below Holmes and Narver datum (0.5 feet below 

approximate mean low water spring) for all samples except those marked with "+," which were taken above that datum. Samples marked with "x" were split for 

mineralogic (X-ray diffraction) analysis (Tremba and Ristvet, 1986); those marked with "o" were analyzed for organic content (Ristvet and Tremba, 1986b). "S" 

indicates stern sample, "B" bow sample, and "C" midship sample; "RXNMASS" is from reaction mass. Location of samples 1-123 shown in fig. 6; coordinates of 

all samples given in table 1]

Sample Sediment 

facies

Entire 

sample

Sand portion 

of sample 1

Depth

Meters Feet

Lagoon samples taken during Marine Phase

1 

2

3

4

5

6

7 

8

9xo

10

11

12

13

14 

15 

16 x

17xo 

18

19

20

21

22

23

24

25

26 x

27

28

29 x

30xo

31 

32 x

33

34

35

36

37xo

38 x

39 x

40 

41

42

43 x

44

45 

46 

47

48

49

Granule-sand

Granule-sand

Sand

Sand-granule 

Granule-sand-mud

Sand-granule 

Granule-sand-mud

Mud

Mud

Sand

Sand

Sand-mud 

Sand 

Granule 

Sand-granule 

Sand-granule 

Muddy sand

Sand-mud

Sand

Sand

Sand

Granule

Sand-mud

Granule-sand-mud

Muddy sand 

Sand-mud

Sand-mud

Sand-mud

Muddy sand

Muddy sand 

Mud

Granule-sand-mud

Sand

Sand

Mud

Mud

Mud

Mud

Muddy sand 

Granule

Sand

Mud

Sand-mud

Muddy sand 

Muddy sand 

Sand-mud

Granule

Muddy sand

(Sample lost in transit) 

44.3:54.0: 1.7

35.1:63.1: 1.7

2.0:96.2: 1.8

15.3:82.3: 2.4 

25.9:52.4:21.7

17.5:80.7: 1.8 

23.3:61.6:15.1

2.5:45.0:52.5

7.2:35.0:57.8

4.1:92.7: 3.2

8.5:88.8: 2.7

1.0:63.7:35.3 

8.7:89.3: 2.0

82.7:15.7: 1.6

18.4:74.5: 7.1

11.0:87.0: 2.0 

6.8:80.3:12.9

2.5:70.5:27.0

4.1:94.3: 1.6

0.1:98.8: 1.1

5.1:93.7: 1.2

70.7:28.1: 1.2

1.0:71.2:27.8

25.2:61.0:13.8

3.5:81.2:15.3 

7.8:60.0:32.2

2.2:75.5:22.3

2.5:63.9:33.6

8.4:78.3:13.3

8.7:79.2:12.1 

0.8:36.0:63.2

32.7:50.6:16.7

3.3:92.0: 4.7

3.3:95.3: 1.4

18.1:41.5:40.4

3.6:53.2:43.2

6.1: 8.2:85.7

0.1: 9.1:90.8

9.2:79.6:11.2 

50.3:37.1:12.6

7.3:91.4: 1.3

2.3:46.5:51.2

5.6:71.5:22.9

11.1:75.9:13.0 

8.6:74.5:16.9 

3.4:72.7:23.9

54.8:40.6: 4.6

3.7:86.1:10.2

32.4:60.5:17.2

52.4:34.9:12.7

34.3:51.5:14.2

42.7:28.3:29.0

54.7:28.4:16.9

43.3:30.7:26.0

71.0:15.4:13.6 

54.9:26.8:18.3 

45.1:20.9:34.0

39.8:22.4:37.7

52.9:32.3:14.8

31.2:56.8:12.0

51.1:40.3: 8.6

25.4:21.7:52.9

23.8:32.3:43.9

46.6:21.2:32.2

59.3:18.9:21.9

50.4:22.2:27.4

22.3:20.3:57.5

24.3:23.8:52.0

32.3:24.2:43.6

69.1:23.0: 7.9

52.7:20.2:27.1

52.6:21.1:26.4 

48.5:21.6:29.9 

47.0:21.9:31.0

57.8:19.5:22.7

+0.6

0.6

0.9

22.0 

9.1

4.6 

32.9

49.4

56.7

48.5

58.5

56.7 

51.2 

36.6 

58.5 

29.3 

31.1

54.9

18.3

5.5

3.4

1.8

18.0

22.0

43.9

40.2

53.0

53.0

53.0

53.0 

53.0

42.1

15.2

6.4

40.2

45.7

36.6

34.8

38.4 

29.3

3.7

40.2

18.3

51.2 

43.9 

54.9

54.9

38.4

+2

2

3

72 

30

15

108

162

186

159

192

186 

168 

120 

192 

96 

102

180

60

18

11

6

59

72

144

132

174

174

174

174 

174

138

50

21

132

150

120

114

126 

96

12

132

60

168

144 

180

180

126
SEDIMENT FACIES OF ENEWETAK ATOLL LAGOON B15

TABLE 2. Sedimentologic analyses of benthic samples from Enewetak lagoon Continued

Sample Sediment 

fades

Entire 

sample

Sand portion 

of sample 1

Depth

Meters Feet

Lagoon samples taken during Marine Phase Continued

50

51 

52

53 

54 

55

56 o

57

58 x 

59 

60

61

62

63 x 

64

65xo

66

67

68

69

70

71 

72 

73

74 x

75

76

77

78xo

79

80 

81

82 

83x 

84 

85

86

87

88 

89xo

90

91 x

92

93 

94

95

96 

97

98

99

100 

101

102

103

Granule

Sand-granule 

Sand

Muddy sand 

Sand-granule 

Sand-mud

Mud

Mud

Muddy sand 

Sand-granule 

Sand-mud

Sand-granule

Sand

Sand-granule 

Sand-mud

Sand-mud

Mud

Sand-mud

Sand

Sand-granule 

Granule

Muddy sand 

Sand-granule 

Sand-granule

Granule-sand-mud

Sand-mud

Granule-sand

Granule-sand

Sand

Granule-sand-mud

Sand-granule 

Granule-sand-mud

Sand 

Sand-granule 

Sand-granule 

Sand

Sand-granule 

Sand

Sand-granule 

Sand-mud

Sand

Sand

Sand

Sand-granule 

Sand

Granule-sand-mud

Sand-granule 

Sand

Granule

Granule

Sand-granule 

Sand

Granule-sand

Sand-granule

47.9:40.1:12.0

10.8:80.4: 8.8 

3.9:88.0: 8.1

0.2:83.9:15.9 

15.7:80.7: 3.6 

3.4:74.3:22.3

0.5:59.8:39.7

2.9:51.8:45.3

7.1:83.6: 9.3 

10.5:85.6: 3.9 

1.5:65.2:33.3

22.9:73.4: 3.7

6.4:89.7: 3.9

12.1:82.6: 5.3 

2.0:78.5:19.5

2.1:72.7:25.2

0.0:42.9:57.1

0.2:77.0:22.8

8.8:90.6: 0.6

17.5:78.7: 3.8 

54.9:41.5: 3.6

7.5:81.1:11.4 

14.2:77.3: 8.5 

14.9:79.2: 5.9

18.7:68.2:13.1

13.0:57.6:29.4

41.7:57.0: 1.3

45.3:53.6: 1.1

7.0:85.3: 7.7

21.6:67.6:10.8

19.3:73.5: 7.2 

21.1:67.2:11.7

7.1:87.0: 5.9 

15.8:77.8: 6.4

18.4:72.9: 8.8 

8.5:89.4: 2.1

20.6:77.2: 2.2 

4.5:93.9: 1.6

14.5:84.3: 1.5 

2.2:69.9:27.9

6.6:90.8: 2.6

4.8:91.3: 3.9

1.3:97.2: 1.5

14.5:78.6: 6.9 

7.0:89.9: 3.1

21.6:67.5:10.9

23.5:71.6: 4.9 

0.9:98.6: 0.5

54.2:44.5: 1.3

56.9:41.4: 1.7

16.3:82.3: 1.4 

9.0:90.1: 0.9

34.7:64.6: 0.7

24.5:73.5: 2.0

48.8:25.2:26.1 

42.2:16.0:41.9

22.5:29.1:48.4 

55.8:26.4:17.8 

9.5: 7.9:82.7

62.3:19.2:18.5 

63.2:22.0:14.9 

45.4:20.2:34.4

67.6:21.2:11.2

43.6:27.6:28.8

65.5:18.1:16.4 

43.9:22.9:33.2

18.4:21.0:60.6

14.4:13.5:72.1

77.2:13.3: 9.5

53.1:20.5:26.4

71.1:14.5:14.4 

57.5:22.3:20.2 

51.8:24.1:24.1

43.3:24.6:32.0

34.1:29.4:36.6

49.4:24.5:26.1

45.9:25.5:28.6 

39.1:21.1:39.8

51.8:23.4:24.8 

53.3:21.7:25.0 

42.5:24.2:33.3 

38.1:31.0:30.9

47.1:44.7: 8.2 

26.6:54.3:19.1

53.2:39.2: 7.6 

13.9:21.0:65.1

49.3:27.6:23.1

38.1:34.7:27.2

24.2:36.0:39.8

39.5:28.2:32.2 

40.6:33.0:26.4

43.1:23.7:33.2

59.8:20.6:19.6 

8.6:20.2:71.2

64.9:31.0: 4.1 

84.0:15.0: 1.0

83.9:10.8: 5.3

18.3

43.9 

13.4

38.4 

9.0 

6.1

38.4

38.4

58.5 

42.1 

45.7

36.6

54.9

54.9 

34.8

36.6

29.3

38.4

43.9

49.4 

43.9

58.5 

51.2 

51.2

38.4

40.2

+ 1.5

+0.3

36.6

40.2

38.4 

51.2

43.9 

43.9 

51.2 

40.2

18.3 

36.6

32.9 

42.1

45.7

42.1

31.1

38.4 

34.8

34.8

29.3

1.8

1.2

0.3

0.9 

3.0

3.0

4.9

60

144 

44

126 

29 

20

126

126

192 

138 

150

120

180

180 

114

120

96

126

144

162 

144

192 

168 

168

126

132

+5

+ 1

120

132

126 

168

144 

144 

168 

132

60 

120

108 

138

150

138

102

126 

114

114

96 

6

4

1

3 

10

10

16
B16 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

TABLE 2. Sedimentologic analyses of benthic samples from Enewetak lagoon Continued

Sample Sediment

facies

Entire Sand portion

sample of sample 1

Depth

Meters Feet

Lagoon samples taken during Marine Phase Continued

104 x

105

106

107

108

109

114

115

116

117

118xo

119

120

121

122

123xo

Sand

Sand-granule

Sand

Sand

Granule-sand

Sand

Granule

Sand

Sand-granule

Sand-granule

Sand-granule

Sand-granule

Sand

Sand

Sand-granule

Sand

0.2:92.1: 7.7 6.3:16.7:77.0

27.7:71.2: 1.1 76.8:18.6: 4.6

1.2:97.0: 1.8 69.7:26.1:4.3

0.0:98.8: 1.2 1.2:67.0:31.8

31.9:67.0: 1.1 83.4:14.8: 1.8

7.2:91.9: 0.9 65.6:32.6: 1.8

(Samples 110-113 not collected)

57.5:40.5: 2.0

0.7:98.1: 1.2 7.5:81.7:10.8

19.6:78.4: 2.0 46.8:33.3:19.9

16.7:81.6: 1.7 50.9:42.6: 6.5

13.6:85.3: 1.1 67.7:29.9: 2.4

23.3:74.6: 2.1 35.4:27.5:37.1

5.6:92.0: 2.4 63.3:27.1: 9.6

0.1:97.6: 2.3 3.8:54.0:42.2

13.8:84.5: 1.7 83.6:12.4: 3.9

9.8:89.1: 1.1 57.4:40.9: 1.7

38.4

9.1

3.0

3.0

3.0

4.3

1.2

3.0

3.7

4.3

32.9

9.1

3.7

3.7

4.6

4.3

126

30

10

10

10

14

4

10

12

14

108

30

12

12

15

14

Crater samples taken during Drilling Phase

KBZ-4S

KBZ-4B

KBZ-4C1

KBZ-4C2

KCT-5S

KCT-5B

KDT-6S

KDT-6B

KET-7S

KET-7B

OAM-3C

OBZ-4B

OCT-5S

OCT-5B

ODT-6S

ODT-6B

OET-7S

OET-7B

OFT-8S

OFT-8B

OGT-9S

OHT-10B

OIT-11S

OIT-11B

OJT-12S

OJT-12B

OKT-13S

OKT-13B

OLT-14S

OLT-14B

OMT-15S

OMT-15B

ONT-16S

ONT-16B

OOR-17S

Mud

Mud

Mud

Mud

Muddy sand

Muddy sand

Granule

Sand-mud

Sand

Sand-mud

Muddy sand

Sand-mud

Sand-mud

Granule-sand-mud

Muddy sand

Sand

Sand

Sand

Muddy sand

Sand

Muddy sand

Sand-mud

Sand-mud

Sand-mud

Sand-mud

Granule-sand-mud

Sand-mud

Sand-mud

Granule-sand

Sand

Sand-mud

Granule

Sand-mud

Granule

Granule-sand-mud

0.0:34.7:65.3

0.0:25.1:74.9

0.8:19.8:79.4

0.1:16.5:83.5

1.5:88.7: 9.9 32.9:31.9:35.2

0.0:87.3:12.7 9.7:28.7:61.6

60.5:34.2: 5.3

0.1:74.6:25.3 13.3:17.9:68.8

0.4:90.6: 9.0 9.7:21.3:69.0

0.3:73.0:26.8 5.2:11.9:82.9

11.8:75.0:13.2 17.8:11.8:70.4

0.0:62.5:37.5

0.2:66.8:33.0

31.9:60.6: 7.5

0.9:89.0:10.1 27.0:31.1:41.9

0.6:94.2: 5.3 37.4:44.0:18.5

0.7:93.7: 5.6 25.2:38.8:36.0

0.1:91.8: 8.1 8.3:33.3:58.5

0.0:84.2:15.8 14.5:39.0:46.5

0.3:94.1: 5.6 19.6:35.2:45.1

0.2:79.4:20.5 7.6:28.0:64.4

0.1:68.8:31.2 6.8:19.3:73.9

0.5:68.5:31.1 12.6:20.2:67.1

0.0:64.5:35.4

0.0:71.4:28.6 10.3:20.6:69.1

34.0:54.4:11.6

0.0:62.5:37.5

0.4:80.1:19.5 15.0:32.3:52.7

42.6:52.3: 5.1

1.7:88.8: 9.4 33.8:40.1:26.1

0.5:62.6:37.0

65.6:31.0: 3.6

8.3:53.4:38.2

90.7: 6.2: 3.1

29.9:62.3: 7.8

33.2

33.2

33.2

33.2

30.2

30.2

17.1

17.1

15.5

15.5

32.9

60.7

50.0

50.0

26.5

26.5

32.6

32.6

39.9

39.9

41.2

41.8

47.3

47.3

43.9

43.9

50.3

50.3

42.7

42.7

33.8

33.8

41.2

41.2

16.8

109

109

109

109

99

99

56

56

51

51

108

199

164

164

87

87

107

107

131

131

135

137

155

155

144

144

165

165

140

140

111

111

135

135

55
SEDIMENT FACIES OF ENEWETAK ATOLL LAGOON B17

TABLE 2. Sedimentologic analyses of benthic samples from Enewetak lagoon Continued

Sample Sediment 

fades

Entire 

sample

Sand portion 

of sample1

Depth

Meters Feet

Crater samples taken during Drilling Phase Continued

OOR-17B

OPZ-18S

OPZ-18B

OQT-19S

OQT-19B

ORT-20S

ORT-20B

OSR-21S

OSR-21B

OSM-22S

OTG-23S

OTG-23B

OUT-24S

OUT-24B

RXNMASS

Granule-sand-mud

Sand-mud

Mud

Muddy sand

Sand-mud

Muddy sand

Muddy sand

Granule-sand-mud

Granule-sand-mud

Sand-mud

Sand-mud

Mud

Sand

Sand

Granule-sand-mud

28.5:54.7:16.8

0.1:63.9:36.0

0.1:43.5:56.4

3.9:82.0:14.1

0.1:78.2:21.7

0.2:90.0: 9.9

8.7:79.9:11.4

33.9:49.3:16.8

29.1:46.7:24.2

14.8:54.7:30.5

0.3:67.3:32.4

0.0:54.6:45.4

0.0:95.6: 4.4

0.0:93.7: 6.3

24.0:58.5:17.5

25.4:27.2:47.3

15.8:22.8:61.4

25.9:33.1:40.9

28.6:29.2:42.3

8.3:19.8:71.9

21.7:33.2:45.1

13.1:31.5:55.4

16.8

61.6

61.6

36.0

36.0

30.8

30.8

25.6

25.6

23.2

50.0

50.0

44.8

44.8

?

55

202

202

118

118

101

101

84

84

76

164

164

147

147

?

1Sand portion of original samples containing 65 weight-percent or greater sand.

SEDIMENT FACIES OF ENEWETAK LAGOON

Eight sediment facies can be distinguished for the 

samples from Enewetak lagoon using a tertiary plot of 

mud, sand, and granules (figs. 10, 11). These facies are 

defined in table 3.

Halimeda plates, small corals, coral fragments, and 

mollusk (mainly gastropod and pelecypod) shells and 

fragments make up most of the granule-size (and coarser) 

fraction, and Halimeda plates also make up the bulk of 

the very coarse sand fraction. Foraminifers and skeletal

elements constitute most of the remaining sand fraction. 

Sand- to granule-sized fragments of calcareous red algae 

are common only in the sediments near the reef flat and 

sand islands on the windward side of the atoll and are 

absent from the other lagoon samples. Ostracodes, also 

sand sized and important biostratigraphically, make up 

far less than 5 percent of most samples taken from the 

lagoon (E.M. Brouwers, oral commun., 1987). The silt 

and clay (mud) fraction is dominated by broken and 

biodegraded skeletal grains or biogenic grains, including 

tunicate spicules.

GRANULE B GRANULE

SAND MUD SAND MUD

FIGURE 10. Plot of benthic samples from (A) Enewetak lagoon and (B) OAK and KOA crater areas on tertiary diagrams of weight-percent 

granule (and coarser), sand, and mud (silt plus clay). Sediment facies are enclosed by dashed lines: 1, granule; 2, granule-sand; 

3, sand-granule; 4, granule-sand-mud; 5, sand; 6, muddy sand; 7, sand-mud; 8, mud. (Grain-size limits described in table 3.)
B18 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

TABLE 3. Sediment fades and grain-size limits for benthic samples 

from Enewetak lagoon taken during PEACE Program (see fig. 10)

[The term "granule," as used here, is any particle granule sized or larger (that is, 

diameter 2 millimeters or greater)]

Fades

C

0

M

P

0

s

I

T 

E

1. Granule

2. Granule-sand

3. Sand-granule

4. Granule-sand-mud

5. Sand

M

U

D

D

Y

6. Muddy sand

7. Sand-mud

8. Mud

Grain-size limits

Greater than 47 percent granules, less

than 13 percent mud, 6-45 percent

sand

32-45 percent granules, less than 6

percent mud, 52-67 percent sand

11-28 percent granules, less than 9

percent mud, 71-87 percent sand

19-34 percent granules, 8-24 percent

mud, 47-68 percent sand

Less than 10 percent granules, less

than 9 percent mud, greater than 85

percent sand

Less than 12 percent granules, 9-17

percent mud, 75-90 percent sand

Less than 15 percent granules, 20-38

percent mud, 53-80 percent sand

Less than 19 percent granules, greater

than 39 percent mud, 8-60 percent

sand

DISTRIBUTION OF SEDIMENT FACIES

The distribution of sediment facies in Enewetak lagoon 

is shown in figure 12. The granule, granule-sand, and 

sand-granule facies (facies 1-3, table 3) intergrade over 

short distances on the floor of the lagoon and on the 

islands. Thus, for mapping purposes, these facies are 

treated as one composite facies, termed the "granulecoarse sand facies."

GRANULE-COARSE SAND FACIES

The composite granule-coarse sand facies forms a band 

around the edge of the lagoon around most of the reef 

tract (fig. 12). Even coarser grained facies (that is, facies 

containing substantial quantities of granules, pebbles, 

cobbles, and boulders) dominate the oceanward side of 

the islands, particularly on the windward side of the atoll 

(see Emery and others, 1954, p. 91-95, and plates 

referenced therein). These commonly include large coral 

heads and blocks of cemented reef plate or beach rock 

ripped up during storms. These coarser grained facies 

are not separated from the composite granule-coarse 

sand facies in this report and thus occupy the islands, 

reef plates, and parts of the lagoon-terrace areas of the 

atoll.

The composite granule-coarse sand facies also dominates the sediments in the vicinity of the coral knolls or 

patch reefs; the majority of large knolls and reefs that 

represent local distribution of this facies are shown in 

figure 12. A large area in the south-central part of the

GRANULE

SAND MUD

FIGURE 11. Fields defined for facies of all Enewetak benthic samples. 

1, granule; 2, granule-sand; 3, sand-granule; 4, granule-sand-mud; 

5, sand; 6, muddy sand; 7, sand-mud; 8, mud.

lagoon (fig. 12) that contains many large coral knolls that 

profusely shed sediment also is dominated by this facies.

GRANULE-SAND-MUD FACIES

The granule-sand-mud facies (facies 4, table 3) is 

mapped in three general areas (fig. 12). The largest area 

is in the southwest lagoonward from the composite 

granule-coarse sand facies in the vicinity of the Southwest Passage. The second area is lagoonward from OAK 

crater in the northwestern part of the atoll, and the third 

is in the northeastern part of the lagoon.

SAND FACIES

The sand facies (facies 5, table 3) forms a very narrow 

belt around the lagoonward edge of much of the atoll and 

includes some of the lagoon beaches (fig. 12) on the 

eastern and southern margins. These coarse to very 

coarse sands are generally foraminifer rich (Emery and 

others, 1954, p. 99; fig. 55, this report). The foraminifers 

in the subfacies on the northeastern, eastern, and southern sides of the lagoon are dominated by the large, 

heavy-walled benthic forms Calcarina, Amphistogina, 

and, to a lesser extent, Marginopora. The lagoon-beach 

sands, which contain as much as 65 percent of these taxa, 

and the shallow subtidal lagoon sediments adjacent to 

these beaches from Enewetak (FRED) to Runit 

(YVONNE) Islands are tinted grayish orange because of 

the superabundance of these foraminifers.

Within the sand facies, a small band just inside the 

lagoon extending from the reef tract on the northwestern 

side of the atoll westward and southward from OAK 

crater is dominated by well-sorted medium sand characterized by large ripple marks. This subfacies, the largest 

subtidal concentration of well-sorted sediment on the
SEDIMENT FACIES OF ENEWETAK ATOLL LAGOON B19

162°10' 162°20'

irw

irso1

EXPLANATION

Mud

Muddy sand 

Granule - coarse sand 

^rl Granule - sand - mud 

Sand

Sand - mud 

Coral knolls 

Crater

0 10,000 30,000 FEET

10 KILOMETERS

FIGURE 12. Distribution of sediment facies in Enewetak lagoon. (See table 3; facies 1-3 in table 3 have been combined to make up granule-coarse

sand facies.)

atoll, occurs in water generally less than 7.6 m (25 ft) 

deep on the broad terrace that rims this part of the 

lagoon. Referred to as the "sand slope" in Halley, Slater,

and others (1986, their figs. 4A-4B), this area is within 

easy reach of waves generated by the fetch of the winds 

blowing across the lagoon and is dominated by the
B20 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

leeward cross-reef currents. Here, few coarse-grained 

components are generated locally or transported into the 

area, and fine-grained components (very fine grained 

sand and silt) are constantly being winnowed and transported out. This "sand slope" subfacies is downcurrent 

from the part of the lagoon extending from Enjebi 

(JANET) to Bokoluo (ALICE) Islands (fig. l)-that has 

the highest productivity and concentration of phyto- and 

zooplankton in the lagoon (Colin, 1987, p. 92). This part 

of the lagoon is the only area in which phytoplankton 

blooms have been reported.

The sand facies also occurs peripherally to some large 

coral knolls and knoll clusters (fig1 . 12).

MUDDY FACIES

Mud-rich sediments (muddy sand, sand-mud, and mud 

facies (facies 6-8, table 3)) dominate the bottom in the 

northern half of the lagoon (fig. 12). A true mud facies 

(facies 8, table 3) occurs in only one area, in the northwest, extending from near Enjebi (JANET) toward 

Biken (LEROY) Islands (fig. 1).

Undoubtedly, some of the fine-grained constituents 

were produced naturally and incorporated in the modern 

lagoon sediments. The source of the rest of the muddy 

sediments is related to the cratering effects of the 

near-surface nuclear bursts and is discussed in a succeeding section.

SEDIMENT FACIES OF 

SUBMARINE NUCLEAR CRATERS

From 1948 through 1958, 43 nuclear tests were conducted on and in the vicinity of Enewetak Atoll. Most of 

the detonations occurred on the islands and on barges in 

the lagoon on the northern and northeastern sides of the 

atoll (fig. 13). Most of the near-surface bursts, ranging in 

yield from fractions of a kiloton to the 10.4-megaton (Mt) 

MIKE, created craters. A substantial volume of material 

was ejected physically from these sites during the 

bursts. However, in these water-saturated test beds, 

ejection does not account for either the final size and 

configuration of the crater or the total volume of material 

displaced into the lagoon.

Only a handful of these tests created substantial 

craters that potentially modified the distribution of sediments in the lagoon. These include OAK, KOA, MIKE 

(which includes several large tests), BUTTERNUT, and 

the composite crater area of YELLOWWOOD (fig. 13).

The craters produced by OAK and KOA thermonuclear devices (8.9 and 1.4 Mt, respectively) were selected 

for study for the PEACE Program by the Department of 

Defense (DOD). The physiographic features of these 

craters and environs were mapped in detail by the USGS 

during the program. The reader is referred to Folger, 

Robb, and others (1986) for detailed discussions of the

sidescan sonar mosaics of OAK and KOA craters and to 

Halley, Slater, and others (1986) and Slater, Roddy, and 

others (1986) for details of the bottom features of the 

craters as observed and mapped from a submersible 

during both phases of the program. A summary of the 

features, cratering processes and dynamics, and timing 

of various stages of crater development is given in 

Wardlaw and Henry (1986) and Wardlaw (1987). MIKE 

crater was transected by the submersible Delta during 

the program, but it was not mapped with sidescan sonar, 

drilled, or sampled benthically.

Benthic samples for sediment analysis of OAK and 

KOA craters are sparse (bow and stern samples from 

most borehole sites, table 2). However, the benthic 

sample information combined with observations from the 

submersible and seafloor image interpretation can be 

used to provide a general sediment facies breakdown of 

the crater seafloor.

Both OAK and KOA craters are strongly negative 

topographic features. The center of the crater floor of 

OAK is approximately 61 m (200 ft) below the tidal 

datum; KOA is 37 m (120 ft) below that datum. The inner 

bowls (crater floors) of KOA and OAK are essentially flat 

bottomed features. The inner bowl of KOA is an oval 

feature measuring 244 by 425 m (800 by 1,400 ft), and 

that of OAK is circular with a diameter of 402 m (1,320 

ft). Characteristically, surface and near-surface sediments from within the craters of KOA and OAK are 

markedly enriched in mud-sized grains (silt and clay), in 

contrast to the rest of the lagoon (contrast figs. 10A and 

105). The submersible traverses demonstrated that the 

floor of both craters is very fine grained, extensively 

bioturbated, and, like that of the deep-lagoon floor, 

covered with callianassid mounds 15 to 30 cm (0.5 to 1 ft) 

high (Halley and others, 1986; Slater and others, 1986). 

Bottom sediments of the inner bowl of KOA belong to the 

mud facies, whereas those of OAK are assignable to the 

sand-mud facies (fig. 14).

The inner bowls are surrounded by a series of steplike 

features (scarps and slopes) and terraces, described by 

Folger, Robb, and others (1986) and Halley, Slater, and 

others (1986). From the muddy facies of the inner bowl of 

both craters, the benthic sediments on these features 

generally coarsen outward to sandy facies, confirming 

the observations made from the submersible (see Halley 

and others, 1986, figs. 5 and 12). This transition appears 

to be much more abrupt in KOA crater. In OAK, a 

muddy sand facies can be recognized in the northeast and 

southwest "wings" of the crater (fig. 14). Coarser 

grained sediments are found in three general environments in the area of the slopes and terraces in OAK and 

KOA; these are referred to as "mixed facies" because the 

coarser material appears mixed in with finer, "normal" 

lagoon facies.
SEDIMENT FACIES OF ENEWETAK ATOLL LAGOON B21

162° 162°10' 162°20' 162°30'

H°30'

SEMINOLE

fZEBRA 

J DOG 

/\ CACTUS ILACROSS

BLACKFOOT 

FIG

QUINCE 

.OSAGE

r*

YELLOWWOOD

TOBACCO

WALNUT

ELDER

DOGWOOD

OLIVE

PINE

+

HOLLY

MAGNOLIA

LINDEN

SEQUOIA

BUTTERNUT 

ROSE

B

I

IVY +160000 

GRID / 

(FT)

+ 120000

jb

Runit (YVONNE)

ERIE

+ 80000

SOUTHWEST 

PASSAGE

EAST (DEEP) CHANNEL

+ 40000

WAHOO

5 NAUTICAL MILES 10,000

lllllll

30,000 50,000 FEET

Enewetak (FRED)

10 KILOMETERS

FIGURE 13. Nuclear-detonation sites on Enewetak Atoll. Nuclear shots with same or approximately same ground-zero location are grouped 

together with brackets. (Data from Defense Nuclear Agency, 1981, fig. 1.53, and Freisen, 1982, table 1.3.)

MIXED-1

Heterogeneous agglomerations (slump deposits) consisting of blocks and boulders of reef plate, coral heads,

beachrock, and cobble- and pebble-sized materials mixed 

with sand and occurring in a patchy distribution within 

sand are particularly common on the reefward side of 

OAK and the islandward side of KOA (fig. 14).
B22 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

162°1T30" 70000 162°12'

11°40'30"

150000

KOA

M2

35000 162°06' 162°06'30'

11 036'30"

125000

OAK

M2

1000 METERS

_I

1000 2000

T 

3000 FEET

FIGURE 14. Distribution of sediment fades in KOA and OAK craters. Fades are as follows: 5, sand; 

6, muddy sand; 7, sand-mud; 8, mud; Ml, mixed fades 1 (slumps); M2, mixed facies 2 (debris); and 

M3, mixed facies 3 (sand volcano swarms).
SEDIMENT FACIES OF ENEWETAK ATOLL LAGOON B23

MIXED-2

Debris deposits occur on the lagoon (southeastern) 

side of OAK crater and toward MIKE crater in KOA. 

On the lagoon side of OAK, the "debris blanket" of 

Folger, Robb, and others (1986) is a patchwork of areas 

characterized as sand-mud facies and various granuledominated (granule-and-coarser) facies. On the MIKE 

side of KOA, crater granule-sized and coarser sediments 

were observed on apparent debris mounds by the submersible. Unfortunately, these mounds were not sampled.

MIXED-3

Samples from the slopes of "sediment hills" or "sand 

volcanoes," which occur in clusters or "swarms" on the 

terraces, at least of OAK (see Halley and others, 1986, p. 

F8; Wardlaw, 1987, fig. 14) consist of coarse sand and 

granule-, pebble-, and (sparse) cobble-sized material 

stained moderate brown. The swarms of sand volcanoes 

occur within the sand, muddy sand, and sand-mud facies 

of OAK. Paleontologic analyses (Brouwers, Cronin, and 

Gibson, 1986; Cronin and others, 1986; Wardlaw and 

Henry, 1986; Cronin and Gibson, 1987; Wardlaw, 1987) 

confirm that much of this material was transported to the 

surface from stratigraphic intervals far below the excavation craters themselves. Similar clusters of "sand 

boils" or "sand volcanoes" are seen in photographs of 

KOA and MIKE taken soon after these detonations 

(Ristvet and others, 1978, figs. 7.24, .31, .32, .34). Many 

of these features in the MIKE and KOA area were 

obliterated by subsequent sedimentary and geologic 

processes (particularly slumping).

KOA is covered by a more extensive sand facies than 

OAK, which has a more widespread muddy facies. This 

agrees with the general observation of Halley, Slater, 

and others (1986) that KOA sediment is slightly coarser 

than OAK sediment. However, sediment analysis clearly 

shows that the central bowl of KOA is covered by finer 

grained sediment than the central bowl of OAK. The 

absence of the mixed-1 facies along the northeastern rim 

of KOA probably indicates active sand transport to the 

crater and deposition obscuring any probable underlying 

slump deposits. In general, the windward cross-reef 

currents are stronger than the leeward cross-reef currents. The windward cross-reef currents bring sediment 

from the fore-reef and reef-flat environments into the 

lagoon. Net water (and sediment) movement by the 

weaker leeward cross-reef currents is greater from the 

lagoon oceanward than from the ocean lagoonward. 

Consequently, the crater floor of KOA currently receives 

both more sediment and coarser grained sediment swept

from the fore-reef and reef-flat environments and moved 

from the northeast into the crater.

SORTING AND SUBDIVISION OF FACIES

The crater benthic sediments generally are finer than 

the lagoon benthic sediments but conform to the general 

facies described for the benthic samples except for the 

mixed deposits described above, which were established 

largely by seafloor observations. Therefore, the crater 

benthic samples are ascribed to lagoon facies and subfacies on the basis of sorting (table 4). Each sample was 

plotted on a cumulative curve, and the median diameter 

(second quartile, Q2) an^ sorting coefficient were determined. The sorting coefficient (S0 , Trask, 1932) is the 

square root of the ratio of the first quartile (Qx , the 25 

percent value) to the third quartile (Q3 , the 75 percent 

value):

These values are summarized into facies in table 4 and 

are listed for each sample in table 6.

Analysis of median diameter and sorting coefficient 

allows close comparison of crater and lagoon benthic 

samples. It also provides additional information about 

each facies and a clearcut subdivision of four of the eight 

facies.

1. Granule facies. The few crater samples assigned to 

the granule facies appear to have a larger median 

diameter than the lagoon samples; however, the 

sorting is extremely variable, from well sorted to 

unsorted in both suites of samples. No subfacies 

were differentiated on the basis of sorting characteristics.

2. Granule-sand facies. All samples assigned to the 

granule-sand facies are well to moderately sorted 

(S0= 1.62-2.18), and the suite is not subdivided on 

the basis of sorting characteristics.

3. Sand-granule facies. The sand-granule facies does not 

include any crater samples and is divided on the 

basis of sorting characteristics into two nonoverlapping subfacies: (a) moderately well to moderately 

sorted (S0= 1.58-2.09) and (b) poorly sorted 

CS0 =2.18-2.92).

4. Granule-sand-mud facies. The granule-sand-mud 

facies is generally poorly sorted to unsorted and is 

not subdivided on the basis of sorting characteristics.

5. Sand facies. The sand facies is divided into four 

subfacies on the basis of sorting characteristics: (a) 

well sorted (S0= 1.24-1.44), (b) transitional from 

well to moderately sorted (S0= 1.51-1.72), (c) moderately to poorly sorted (S0= 1.81-2.24), and (d) 

very poorly sorted GS0 =3.02).
B24 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

TABLE 4. Diameters and sorting coefficients of sediment fades and subdivisions of fades based on sorting

 ,. .... Number of Sediment facies samples

1. Granule

Lagoon

Crater

Total

2. Granule-sand

Lagoon

Crater

Total

3. Sand-granule

Lagoon

Crater

Moderately sorted

Poorly sorted

4. Granule-sand-mud

Lagoon

Crater

Total

5. Sand

Lagoon

Crater

Total

Well sorted

Lagoon

Crater

Transitional

Lagoon

Crater

Total

Moderately to poorly sorted

Lagoon

Crater

Very poorly sorted

Lagoon

6. Muddy sand

Lagoon

Crater

Total

Moderately sorted

Lagoon

Crater

Poorly sorted

Lagoon

Crater

Total

7. Sand-mud

Lagoon

Crater

Total

Poorly to moderately and well sorted

Lagoon

Crater

Total

Poorly to unsorted

Lagoon

Crater

Total

8. Mud

Lagoon

Crater

Total

9

3

12

6

1

7

27

0

15

12

8

7

15

31

8

39

10

0

6

8

14

14

0

1

11

9

20

0

6

11

3

14

15

16

31

5

14

19

10

2

12

11

6

17

Average median 

diameter (Eange)

2.521

5.701

3.311

1.54

1.7

1.56

0.71

0.77

0.62

0.72

0.89

0.80

0.46

0.27

0.42

0.43

0.61

0.27

0.42

0.42

0.32

0.46

0.23

0.36

0.24

0.46

0.21

0.41

0.19

0.11

0.15

0.11

0.10

0.10

0.24

0.14

0.22

0.057

0.039

0.051

(1.90 -4.0)

(3.70 -9.5)

(1.90 -9.5)

(1.30 -1.75)

(1.30 -1.75)

(0.43 -1.15)

(0.46 -1.15)

(0.43 -0.93)

(0.43 -1.2)

(0.40 -1.3)

(0.40 -1.3)

(0.15 -1.02)

(0.16 -0.40)

(0.15 -1.02)

(0.20 -0.80)

(0.15 -1.02)

(0.16 -0.40)

(0.15 -1.02)

(0.20 -0.58)

(0.20 -0.78)

(0.13 -0.31)

(0.13 -0.78)

(0.15 -0.31)

(0.20 -0.78)

(0.13 -0.275)

(0.13 -0.78)

(0.083-0.37)

(0.075-0.18)

(0.075-0.37)

(0.083-0.13)

(0.075-0.18)

(0.075-0.18)

(0.160-0.37)

(0.092-0.18)

(0.092-0.37)

(0.015-0.098)

(0.019-0.070)

(0.015-0.098)

Average sorting 

coefficient (Eange)

2.181

3.531

2.521

1.91

1.89

1.90

2.17

1.89

2.51

3.41

4.23

3.79

1.73

1.64

1.71

1.36

1.54

1.64

1.60

1.97

3.02

2.49

2.04

2.29

1.88

2.49

2.35

2.46

3.49

2.19

2.82

2.10

1.94

1.98

4.19

3.94

4.15

4.38

2.46

3.70

(1.41^3.32)

(2.28^5.16)

(1.41^5.16)

(1.62-2.18)

(1.62-2.18)

(1.58-2.92)

(1.58-2.09)

(2.18-2.92)

(2.69-5.33)

(2.34-6.04)

(2.34-6.04)

(1.24-3.02)

(1.57-1.72)

(1.24-3.02)

(1.24-1.44)

(1.51-1.60)

(1.57-1.72)

(1.51-1.72)

(1.81-2.24)

(2.09-2.93)

(1.70-2.53)

(1.70-2.93)

(1.70-1.98)

(2.09-2.93)

(2.21-2.53)

(2.09-2.93)

(1.41^5.48)

(1.55-4.60)

(1.41^5.48)

(1.41-2.87)

(1.55-2.14)

(1.41-2.87)

(2.97-^5.48)

(3.27-4.60)

(2.97-5.48)

(2.27-7.24)

(1.88-2.90)

(1.88-7.24)

'Averages only approximate.
SEDIMENT FACIES OF ENEWETAK ATOLL LAGOON B25

162° 162°10' 162°20'

irso1

162°30' 162° 162°10' 162°20' 162°30'

160000

120000

SOUTHWEST 

PASSAGE

+ 80000

40000

NAUTICAL MILES

SOUTH PASSAGE

0 10,000 1 30,000 50,000 

' ' FEET1 ' ' KILOMETERS

1T401

1T301

H°20'

SOUTHWEST 

PASSAGE

SOUTH PASSAGE

0 10,000 30,000 50,000 0

NAUTICAL MILES FEET KILOMETERS

FIGURE 15. Isopleth maps showing distribution of fine debris (fine sand, silt, plus clay) in Enewetak lagoon: A, Data from PEACE Program 

survey (1984-1985); B, Data collected in 1946 and reported in Emery, Tracey, and Ladd (1954), replotted on PEACE Program sample grid. 

Shading indicates area with greater than 50 weight-percent.

6. Muddy sand fades. The muddy sand fades is divided 

into two subfades on the basis of sorting characteristics: (a) moderately sorted (S0= 1.70-1.98) and (b) 

poorly sorted (S0 =2.09-2.93).

7. Sand-mud facies. The sand-mud facies shows a wide 

range in sorting, from very well sorted to unsorted, 

and is divided into two subfacies: (a) poorly to 

moderately and well sorted (S0= 1.41-2.87), composed of mostly moderately sorted samples, and (b) 

poorly sorted to unsorted (-S0 =2.97-5.48), composed of mostly very poorly sorted samples.

8. Mud facies. The mud facies shows a wide range in 

sorting, from moderately sorted to unsorted, but 

the majority of samples are very poorly sorted and 

the facies is not subdivided on the basis of sorting 

characteristics.

MODIFICATION OF SEDIMENT FACIES BY 

NUCLEAR TESTING

Unfortunately, detailed sediment size-fraction analysis or sampling was not carried out before nuclear 

testing. Emery, Tracey, and Ladd (1954) did report the 

distribution of sediments less than 1/4 mm ("fine debris" 

of their report). We have constructed a similar diagram 

based on the PEACE Program samples and replotted the 

1946 data on our sample grid (figs. 15A, 155). Even 

though the 1946 sampling was very sparse in the northern part of the lagoon (fig. 4D) and was biased against 

fine-grained sediments, as discussed previously, a modification of the distribution of "fine debris" is apparent. 

The entire lagoon shows a general increase in fine 

sediment, best illustrated by the decrease in area of less 

than 25 weight-percent distribution. The northern 

lagoon shows a marked increase in area and percentage 

of fine sediments. The difference between the data sets 

in the distribution in the southeast and east is thought to 

be relatively minor and due largely to differences in the 

sampling grids and in contouring those points. The 

distribution of mud in the lagoon (fig. 16A) is largely 

responsible for the noted change in the northern part of 

the lagoon (compare figs. 15A and 16A). Some of the mud 

in the lagoon is naturally produced sediment (for example, whole tunicate spicules, holothurian sclerids) or 

bioclastic particles created from physical abrasion and 

(or) biotic breakdown ("bioerosion") of coarser grain 

particles (for example, fine silt-sized "chips" created by 

the boring sponge Cliona). However, some of the mud, 

probably a substantial volume, was produced by the 

direct and indirect effects of the nuclear tests.

Why is the mud concentrated in the northern part of 

the lagoon? One reason, also discussed previously, is that 

the deeper part of the lagoon is a natural trap for 

fine-grained sediments. Any material that enters the 

realm of the deep-current system (that is, the deeplagoon environment) is not likely to escape the area, even 

during storm conditions. Second, the direct effects of the
B26 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

162° 162°10' 162°20' 162°30'

11"so1

IVY.+ 160000 

GRID 

(FT)

OAK

SOUTHWEST 

PASSAGE

0

SOUTH PASSAGE

0 10,000 30,000 50,000 0 5 10

NAUTICAL MILES FEET KILOMETERS

162° 162°2D' 162°301

H°40'

SOUTHWEST 

PASSAGE

NAUTICAL MILES

SOUTH PASSAGE

0 10,000 | 30,000 | 50,000 ' ' "FEET ' '

KILOMETERS

162° 162°10' 162°20' 162°30' 162° 162°10' 162°201 162°30'

n°40'

B

KOA

IVY,+ '60000 

GRID 

(FT)

SOUTHWEST 

PASSAGE EAST (DEEP) 

CHANNEL

NAUTICAL MILES

SOUTH PASSAGE

0 10,000 | 30,000 | 50,000 

FEET

11°30

Biken (LERO'

SOUTHWEST 

PASSAGE

Enewetak (FRED)

NAUTICAL MILES

SOUTH PASSAGE

0 10,000 30,000 | 50,000 0

FIGURE 16. Isopleth maps showing distribution (in weight-percent) of (A) mud (silt plus clay; greater than 20 weight-percent shaded), (B) clay, 

and (C) low-magnesium calcite, and CD) distribution of elevated radionuclide concentration in Enewetak lagoon (shaded) (modified from 

McMurtry and others, 1985).

KOA and OAK detonations (and the numerous other 

nuclear bursts in the northern part of the atoll) caused a 

tremendous amount of existing finer grained material to 

be excavated and stirred up from the bottom and subbottom. Even more was created by shattering, breaking, 

or "blast pulverizing" of individual coarser grains and 

"framework failure" of the cemented subbottom strata. 

Third, numerous independent lines of evidence indicate

that liquefaction and piping of slurries consisting of 

water plus sediment from mud-rich (mostly silt), unlithified (uncemented) sediments played a role in adding mud 

to crater-fill sediments and crater environs (Henry and 

Wardlaw, 1986; Trulio, 1987; Wardlaw, 1987). Some of 

this piped material came from strata far below the 

excavation craters themselves. Aerial photographs 

taken by the DOD 8 and 10 hours after the OAK burst
SEDIMENT FACIES OF ENEWETAK ATOLL LAGOON B27

TABLE 5. Results of X-ray diffraction analyses of benthic samples 

from Enewetak lagoon

[In weight-percent. Calcite is low-magnesium calcite; Mg-calcite is highmagnesium calcite. Location of samples shown in fig. 6. Modified from Tremba 

and Ristvet, 1986]

Sample 

number

9

16

17

26

29

30

32

37

38

39

43

58

63

65

74

78

83

89

91

104

118

123

Sedimentary 

facies

Mud

Sand-granule

Sand-granule

Muddy sand

Sand-mud

Muddy sand

Mud

Mud

Mud

Mud

Mud

Muddy sand

Sand-granule

Sand-mud

Granule-sand-mud

Sand

Sand-granule

Sand-mud

Sand

Sand

Sand-granule

Sand

Calcite

15

8

2

6

8

8

16

10

23

27

8

7

8

9

7

2

10

7

3

3

3

7

Mg-calcite

15

17

5

12

24

12

21

13

19

18

15

15

20

16

11

6

10

8

4

22

7

25

Aragonite

70

75

93

82

68

80

63

77

58

55

77

78

72

75

82

92

80

85

93

75

90

68

show a large plume of turbid water containing suspended, finer grained particles (silt and clay) extending 

southwestward downcurrent from the crater (Ristvet 

and others, 1978, fig. 8.10). Similar current-driven 

plumes existed after the other high-yield bursts, particularly MIKE (B.L. Ristvet, oral commun., 1987).

That some of this mud-rich sediment is derived from 

nuclear detonations also is clearly substantiated by the 

following lines of evidence. (1) X-ray analysis shows that 

the normal lagoon sediments and Holocene sediments 

contain less than 10 weight-percent low-magnesium (lowMg) calcite (Tremba and Ristvet, 1986; table 5, fig. 16C). 

However, the mud fades in the northwestern part of the 

lagoon is anomalously high in low-Mg calcite, the common to predominant carbonate mineral in the diagenetically altered Pleistocene sequence beneath the Holocene 

lagoon sediments (Videtich and Tremba, 1978; Tremba 

and Ristvet, 1986). (2) Modern, naturally produced 

lagoon carbonate muds normally contain only a tiny 

fraction of true clay-sized material (that is, particles less 

than 1/256 mm). However, in the northern part of the 

atoll, the mud fades contains a significant percentage of 

true clay-sized carbonate (fig. 16B). (3) McMurtry and 

others (1985) concluded for the barge-based bursts off 

Runit (YVONNE) Island that radionuclides are concentrated in the clay-sized carbonate sediment. They refer 

to this material as "blast-pulverized." They and Noshkin

(1980) show an area of radioactive contamination in the 

northern lagoon extending from Runit on the east to 

Biken (LEROY) Island on the west (fig. 16D), although 

Noshkin did not attempt to associate concentration levels 

of transuranics in specific sedimentary components. The 

only sediments exhibiting levels of radiation above background outside the craters noted during the PEACE 

Program were from benthic samples from the mud facies 

near MIKE and KOA craters.

Clearly, the nuclear testing on Enewetak disrupted 

the benthic sedimentary patterns and facies. Most telling 

is the comparison of the isopleth map of weight-percent 

clay (fig. 16J5) and the distribution of elevated radionuclide concentration (fig. 16Z7). The distribution of values 

greater than 3 weight-percent clay in the northern part 

of the lagoon mimic the radionuclide distribution. 

McMurtry and others (1985) and Wardlaw (1987) have 

pointed out the affinity of radionuclides and clay-sized 

sediment. The very similar distributions of elevated clay 

content and radionuclide concentration suggest that the 

majority of clay was derived from nuclear testing. The 

insensitivity of pretest benthic sampling techniques for 

silt and clay recovery and analysis precludes further 

development of this line of reasoning.

The benthic samples from the OAK reference boreholes (OAM-3, OOR-17, OSR-21, table 6) apparently are 

enriched in mud and poorly sorted, suggesting that the 

surface sediments have been affected by the nuclear 

testing, as previously suggested by Wardlaw (1987).

SUMMARY

Benthic samples from Enewetak Atoll can be subdivided by sediment-size analysis into eight facies: granule, 

granule-sand, sand-granule, granule-sand-mud, sand, 

muddy sand, sand-mud, and mud. The granule, granulesand, and sand-granule facies intergrade laterally over 

short distances and were combined into a composite 

granule-coarse sand facies for mapping purposes. The 

composite granule-coarse sand facies dominates the reef 

tract, small areas adjacent to the coral knolls, and much 

of the south-central portion of the lagoon. The mud, 

sand-mud, and muddy sand facies dominate the northern 

part of the lagoon.

Samples from within the apparent craters of KOA and 

OAK are depleted in coarser grained components compared with the rest of the lagoon floor. Furthermore, the 

samples apparently are enriched in mud and fine sand.

Sorting analysis was used to subdivide four of the 

sediment facies. The sand-granule facies was divided into 

moderately well to moderately sorted and poorly sorted 

subfacies. The sand facies was divided into well sorted, 

transitional from well to moderately sorted, moderately 

to poorly sorted, and very poorly sorted subfacies. The
B28 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

TABLE 6. Summary of samples and sediment fades from Enewetak

lagoon

[Analysis is ratio, in weight-percent, of granule:sand:mud; ratios may not add to 

100 owing to rounding. Median diameter is in millimeters. Location of lagoon 

samples shown in fig. 6; coordinates of all samples given in table 1]

TABLE 6. Summary of samples and sediment fades from Enewetak 

lagoon Continued

[Analysis is ratio, in weight-percent, of granule:sand:mud; ratios may not add to 

100 owing to rounding. Median diameter is in millimeters. Location of lagoon 

samples shown in fig. 6; coordinates of all samples given in table 1]

Sample Ratio Median 

diameter

Sorting 

coefficient

1. Granule facies

Lagoon samples 

15

23

41

48 

50 

70 

98 

99 

114 

Crater samples 

ONT-16B 

OMT-15B 

KDT-6S

83:16:02

71:28:01

50:37:13

55:41:05 

48:40:12 

55:42:04 

54:45:01 

57:41:02 

58:41:02

91:06:03 

66:31:04 

61:34:05

4.0

3.5

2.0

2.2 

1.9 

2.1 

2.2 

2.5 

2.25

9.5 

3.9 

3.7

-1.73

-2.43

-2.76

-1.79 

-3.32 

-1.41 

-2.20 

-2.36 

-1.61

-2.28 

-3.16 

-5.16

2. Granule-sand facies

Lagoon samples 

2

3

76

77

102

108

Crater sample 

OLT-14S

44:54:02

35:63:02

42:57:01

45:54:01

35:65:01

32:67:01

43:52:05

1.7

1.4

1.6

1.75

1.5

1.3

1.7

2.18

1.88

1.94

2.03

1.62

1.78

1.89

3. Sand-granule

Moderately sorted 

Lagoon samples 

5

7

16

17

59

61

63

86

88

100 

103

105

117

118

122

No crater samples 

Poorly sorted 

Lagoon samples 

51 

54

69

72

73

80

83

84

15:82:02

18:81:02

18:75:07

11:87:02

11:86:04

23:73:04

12:83:05

21:77:02

15:84:02

16:82:01 

25:74:02

28:71:01 

17:82:02

14:85:01

14:85:02

11:80:09 

16:81:04

18:79:04

14:77:09

15:79:06

19:74:07

16:78:06

18:73:09

0.60

0.46

0.97

0.64

0.78

0.97

0.84

0.60

0.64

0.70 

1.15

0.98 

0.59

0.80

0.88

0.52

0.70

0.74

0.66

0.63

0.57

0.66

0.48

1.93

1.83

2.09

1.95

2.01

2.02

2.08

2.07

1.90

1.74 

1.71

1.93 

1.80

1.76

1.58

2.41 

2.18

2.52

2.41

2.45

2.70

2.49

2.92

Sample

3.

93 

96

116

119

No crater samples

Ratio Median 

diameter

Sorting 

coefficient

Sand-granule Continued

15:79:07 

24:72:05

20:78:02

23:75:02

0.43 

0.93

0.63

0.50

2.44 

2.40

2.34

2.83

4. Granule-sand-mud

Lagoon samples 

6 

8 

25 

33 

74 

79 

81 

95 

Crater samples 

OCT-5B

OJT-12B

OOR-17B

OOR-17S

OSR-21S

OSR-21B

RXNMASS

26:52:22 

23:62:15 

25:61:14 

33:51:17 

19:68:13 

22:68:11 

21:67:12 

22:68:11

32:61:08

34:54:12

29:55:17

30:62:08

34:49:17

29:47:24

24:59:18

0.70 

0.70 

1.10 

1.20 

0.46 

0.62 

0.43 

0.52

0.76

1.0

0.9

1.2

1.3

0.7

0.4

5.33 

3.40

2.83 

2.69 

3.22 

2.85 

3.58 

3.36

3.72

4.20

4.29

2.34

4.24

6.04

4.76

5. Sand

Well sorted 

Lagoon samples 

4 

21

87

97

101

106

107

115

121

123

No crater samples 

Transitional

Lagoon samples 

22

42

68

104 

109 

120 

Crater samples 

KET-7S

ODT-6B

OET-7S

OET-7B

OFT-8B

OLT-14B

02:96:02 

00:99:01

05:94:02

01:99:01

09:90:01

01:97:02

00:99:01

01:98:01

00:98:02

10:89:01

05:94:01

07:92:01

09:91:01

00:92:08 

07:92:01 

06:92:02

00:91:09

01:94:05

01:94:06

00:92:08

00:94:06

02:89:09

0.40 

0.39

0.375

0.20

0.80

0.62

0.30

0.35

0.27

0.58

0.52

0.68

1.02

0.15 

0.66 

0.65

0.16

0.40

0.31

0.20

0.25

0.35

1.31 

1.37

1.39

1.29

1.44

1.37

1.31

1.24

1.43

1.48

1.51

1.51

1.58

1.52 

1.51 

1.60

1.59

1.57

1.64

1.64

1.69

1.66
SEDIMENT FACIES OF ENEWETAK ATOLL LAGOON B29

TABLE 6. Summary of samples and sediment fades from Enewetak 

lagoon Continued

[Analysis is ratio, in weight-percent, of granule:sand:mud; ratios may not add to 

100 owing to rounding. Median diameter is in millimeters. Location of lagoon 

samples shown in fig. 6; coordinates of all samples given in table 1]

TABLE 6. Summary of samples and sediment fades from Enewetak 

lagoon Continued

[Analysis is ratio, in weight-percent, of granule:sand:mud; ratios may not add to 

100 owing to rounding. Median diameter is in millimeters. Location of lagoon 

samples shown in fig. 6; coordinates of all samples given in table 1]

Sample Ratio Median 

diameter

Sorting 

coefficient

5. Sand Continued

OUT-24S

OUT-24B

00:96:04

00:94:06

0.27

0.22

1.72

1.63

Moderately to poorly sorted

Lagoon samples

11

12

14

20

34

35

62

78

82

85

90

91

92

94

Very poorly sorted

Lagoon sample

52

04:93:03

09:89:03

09:89:02

04:94:02

03:92:05

03:95:01

06:90:04

07:85:08

07:87:06

09:89:02

07:91:03

05:91:04

01:97:02

07:90:03

04:88:08

0.41

0.58

0.48

0.55

0.20

0.245

0.44

0.34

0.53

0.41

0.53

0.40

0.30

0.44

0.32

2.05

1.85

2.00

1.81

2.00

1.89

2.09

2.23

2.24

1.97

1.97

1.81

1.81

1.90

3.02

6. Muddy sand

Moderately sorted

No lagoon samples

Crater samples. 81

KCT-5S

KCT-5B

ODT-6S

OFT-8S

OGT-9S

ORT-20S

Poorly sorted

Lagoon samples

10lo

26

30

31

40

45

46

49

53

58

71

Crater samples

OAM-3C

OQT-19S

ORT-20B

02:89:10

00:87:13

01:89:10

00:84:16

00:80:21

00:90:10

Arr.QA.-i o Ul .oU.lo

04:81:15

08:78:13

09:79:12

09:80:11

11:76:13

09:75:17

04:86:10

00:84:16

07:84:09

08:81:11

12:75:13

04:82:14

09:80:11

0.31

0.175

0.27

0.255

0.15

0.27

O QQ .00

0.24

0.57

0.48

0.29

0.52

0.42

0.54

0.20

0.66

0.78

0.13

0.225

0.275

1.98

1.81

1.96

1.70

1.89

1.96

£. 9 771 1

2.09

2.66

2.60

2.43

2.83

2.93

2.41

2.21

2.28

2.20

2.53

2.31

2.21

7. Sand-mud

Poorly to moderately and well sorted

Lagoon samples

24

55

01:71:28

03:74:22

0.12

0.083

2.87

1.41

Sample Ratio Median 

diameter

Sorting 

coefficient

7. Sand-mud Continued

65

67

89

Crater samples

KDT-6B

KET-7B

OBZ-4B

OCT-5S

OHT-10B

OIT-11S

OIT-11B

OJT-12S

OKT-13S

OKT-13B

OMT-15S

OPZ-18S

OQT-19B

OTG-23S

Poorly to unsorted

Lagoon samples

13

19

27

28

29

44

47

60

64

75

Crater samples

ONT-16S

OSM-22S

02:73:25

00:77:23

02:70:28

00:75:25

00:73:27

00:63:38

00:67:33

00:69:31

01:69:31

00:65:35

00:71:29

00:63:38

00:80:20

01:63:37

00:64:36

00:78:22

00:67:32

01:64:35

03:71:27

08:60:32

02:76:22

03:64:34

06:72:23

03:73:24

02:65:33

02:79:20

13:58:29

08:53:38

15:55:31

0.13

0.10

0.10

0.11

0.10

0.09

0.105

0.10

0.10

0.09

0.10

0.075

0.18

0.084

0.08

0.135

0.096

0.20

0.20

0.16

0.28

0.22

0.37

0.28

0.17

0.29

0.20

0.092

0.18

2.22

1.74

2.24

1.91

1.69

1.98

1.96

1.99

2.14

2.10

1.96

1.55

2.04

2.03

1.70

2.07

2.04

5.48

3.75

4.07

3.33

5.48

3.73

3.56

5.14

2.97

4.39

3.27

4.60

8. Mud

Lagoon samples

9

10

32

36

37

38

39

43

56

57

66

Crater samples

KBZ-4B

KBZ-4S

KBZ-4C1

KBZ-4C2

OPZ-18B

OTG-23B

03:45:53

07:35:58

01:36:63

18:42:40

04:53:43

06:08:86

00:09:91

02:47:51

01:60:40

03:52:45

00:43:57

00:25:75

00:35:65

01:20:79

00:17:84

00:44:56

00:55:45

0.052

0.038

0.033

0.098

0.096

0.017

0.015

0.06

0.089

0.089

0.044

0.03

0.041

0.021

0.019

0.052

0.07

5.82

5.46

3.95

7.24

4.70

2.34

2.27

3.94

3.70

5.54

3.23

2.34

2.56

2.90

2.49

2.56

1.88
B30 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

muddy sand facies was divided into moderately sorted 

and poorly sorted subfacies. The sand-mud facies was 

divided into poorly to moderately and well sorted and 

poorly sorted to unsorted subfacies.

The nuclear testing on Enewetak Atoll disrupted the 

natural sediment patterns of the lagoon and contributed 

a large volume of fine sediment (particularly clay-sized 

carbonate) to the lagoon.

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event, in Henry, T.W., and Wardlaw, B.R., eds., Pacific 

Enewetak Atoll Crater Exploration (PEACE) Program, 

Enewetak Atoll, Republic of the Marshall Islands; Part 4: Analysis 

of borehole gravity survey and other geologic and bathymetric 

studies: U.S. Geological Survey Open-File Report 87-665, chap. 7, 

126 p., 43 figs., 2 tables, 2 app.

Videtich, P.E., and Tremba, E.L., 1978, Interpretation of the origin 

and diagenesis of Pleistocene chalk, Eniwetok Atoll, Marshall 

Islands: Journal of Sedimentary Petrology, v. 48, no. 1, p. 313-330, 

8 figs.

Wardlaw, B.R., 1987, Integration of material-property units, gravimetry, and additional studies of OAK and KOA craters, in Henry, 

T.W., and Wardlaw, B.R., eds., Pacific Enewetak Atoll Crater 

Exploration (PEACE) Program, Enewetak Atoll, Republic of the 

Marshall Islands; Part 4: Analysis of borehole gravity surveys and 

other geologic and bathymetric studies in vicinity of OAK and 

KOA craters: U.S. Geological Survey Open-File Report 87-665, 

chap. 7, 66 p., 33 figs., 9 tables.

Wardlaw, B.R., and Henry, T.W., 1986, Geologic interpretation of 

OAK and KOA craters, in Henry, T. W., and Wardlaw, B.R., eds., 

Pacific Enewetak Atoll Crater Exploration (PEACE) Program, 

Enewetak Atoll, Republic of the Marshall Islands; Part 3: Stratigraphic analysis and other geologic and geophysical studies in 

vicinity of KOA and OAK craters: U.S. Geological Survey OpenFile Report 86-555, chap. 14, 39 p., 21 figs., 2 tables.

Wiens, H.J., 1962, Atoll environment and ecology: New Haven, Conn., 

Yale University Press, 532 p.
B32 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

100 

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IND. % 50 

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IND. % 50 

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SAMPLE # 2

-1

SAMPLE # 4

-1

SAMPLE # 6

1 7

100 

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IND. % 50 

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100 

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IND. % 50 

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SAMPLE

SAMPLE # 5

28 29

-1

100 

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60

IND. % 50 

40 

30

SAMPLE # 7

-1

FIGURE 17. Histograms showing weight-percent of grain-size fractions (see fig. 9) of benthic samples from 

Enewetak lagoon. Percentages are rounded. IND., individual.
SEDIMENT FACIES OF ENEWETAK ATOLL LAGOON B33

100

SAMPLE # 8

348

FIGURE 17. Continued.

-1
B34 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

100 

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70 

60

IND. % 50 

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IND. % 50 

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SAMPLE # 14 SAMPLE # 15

100 

90 

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60

IND. °/o 50 

40 

30 

20 

10 

00

27

SAMPLE # 16

SAMPLE # 18

18 18

10

FIGURE 17. Continued.
SEDIMENT FACIES OF ENEWETAK ATOLL LAGOON B35

100

90 

80 

70 

60

IND. % 50 

40 

30 

20 

10 

00

100 

90 

80 

70 

60

IND. % 50 

40 

30 

20 

10 

00

100 

90 

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IND. % 50 

40 

30 

20 

10 

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SAMPLE <# 20 SAMPLE # 21

-1

SAMPLE # 22

36 38

-1

SAMPLE <# 24

12

-1

FIGURE 17. Continued.
B36 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

100 

90 

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70 

60

IND. % 50 

40 

30 

20 

10 

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IND. % 50 

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IND. % 50 

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SAMPLE # 26

SAMPLE # 28

17 18

SAMPLE # 30

24 22

11 10

100 

90 

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60

IND. % 50 

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IND. % 50 

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IND. % 50 

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SAMPLE # 27

SAMPLE # 29

17

SAMPLE # 31

-1

FIGURE 17. Continued.
SEDIMENT FACIES OF ENEWETAK ATOLL LAGOON B37

100

90 

80 

70 

60

IND. % 50 

40 

30 

20 

10 

00

100 

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IND. % 50 

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100 

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IND. % 50 

40 

30 

20

SAMPLE <# 32 SAMPLE 33

-1

SAMPLE 34

-1

SAMPLE <# 36

18

-1 -1

FIGURE 17. Continued.
B38 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

SAMPLE # 38 SAMPLE # 39

-1

FIGURE 17. Continued.
SEDIMENT FACIES OF ENEWETAK ATOLL LAGOON B39

SAMPLE 44

-1

100 

90 

80 

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60

IND. % 50 

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30 

20 

10

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IND. % 50 

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IND. % 50 

40 

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20 

10 

00

FIGURE 17. Continued.

SAMPLE 45

11

-1

SAMPLE <# 47

16 18

49

24 25

-1
B40 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

100

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IND. % 50

SAMPLE 50 SAMPLE # 51

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IND. % 50 

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30

SAMPLE 52

18 19

54

23 22

FIGURE 17. Continued.
SEDIMENT FACIES OF ENEWETAK ATOLL LAGOON B41

100 

90 

80 

70 

60

IND. % 50 

40 

30 

20 

10 

00

100 

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60

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IND. % 50 

40 

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SAMPLE <# 56 57

-1

SAMPLE 58

-1

SAMPLE <# 60

27

15 15 13

-1 -1

FIGURE 17. Continued.
B42 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OP ENEWETAK ATOLL

100 

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60

IND. % 50 

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SAMPLE # 62

18

SAMPLE 64

16

-1

SAMPLE # 66

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IND. % 50 

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SAMPLE 63

-1

SAMPLE # 65

21 23 21

-1

SAMPLE # 67

-1

FIGURE 17. Continued.
SEDIMENT FACIES OF ENEWETAK ATOLL LAGOON B43

SAMPLE 68

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IND. % 50 

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SAMPLE 69

-1

SAMPLE 71

73

-1 -1

FIGURE 17. Continued.
B44 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

100 

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IND. % 50 

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SAMPLE # 74

15 17

12 9 11

-1

SAMPLE # 76

-1

SAMPLE 78

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IND. % 50 

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SAMPLE 75

13 11

-1

SAMPLE 77

45

-1

SAMPLE # 79

-1

FIGURE 17. Continued.
SEDIMENT FACIES OF ENEWETAK ATOLL LAGOON B45

100 

90 

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70 

60

IND. % 50 

40 

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20 

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00

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30

SAMPLE <# 80

SAMPLE <# 82

22 23

-1

SAMPLE <# 84

10

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IND. % 50 

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SAMPLE 81

13 13 14 14 13

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SAMPLE <# 83

-1

SAMPLE 85

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FIGURE 17. Continued.
B46 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

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SAMPLE 86 SAMPLE 87

SAMPLE 88

20

14

-1

SAMPLE 90

26 25

-1 -1

FIGURE 17. Continued.
SEDIMENT FACIES OF ENEWETAK ATOLL LAGOON B47

100

90 

80 

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60

IND. % 50 

40 

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SAMPLE # 92 SAMPLE 93

SAMPLE # 94

SAMPLE 96

23 25

15

FIGURE 17. Continued.
B48 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

SAMPLE <# 98 SAMPLE 99

-1 -1

FIGURE 17. Continued.
SEDIMENT FACIES OF ENEWETAK ATOLL LAGOON B49

100 

90 

80 

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60

IND. % 50 

40 

30 

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00

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SAMPLE 104 SAMPLE # 105

-1

SAMPLE 106

-1

SAMPLE 108

30

26

-1

FIGURE 17. Continued.
B50 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

SAMPLE # 114

100 

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-1

SAMPLE # 117

17

119

-1 -1

FIGURE 17. Continued.
SEDIMENT FACIES OF ENEWETAK ATOLL LAGOON B51

SAMPLE 120 SAMPLE 121

-1 348

FIGURE 17. Continued.

-1
B52 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

SAMPLE # KBZ-4 B

SAMPLE # KBZ-4

SHIP CENTER (2)

SAMPLE # KBZ-4

SHIP CENTER (1)

00

FIGURE 18. Histograms showing weight-percent of grain-size fractions (see fig. 9) of benthic samples from 

KOA and OAK crater areas. Percentages are rounded. IND., individual.
SEDIMENT FACIES OF ENEWETAK ATOLL LAGOON B53

100 

90 

80 

70 

60

IND. % 50 

40 

30 

20 

10 

00

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60

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00

SAMPLE <# KDT-6 B SAMPLE <# KDT-6 S

-1

SAMPLE KET-7 B

-1

SAMPLE # OAM-3

2

0

4 8 

FIGURE 18. Continued.

SAMPLE <# KET-7 S

00 -1
B54 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

SAMPLE # OCT-5 B

100 

90 

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60

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00

100 

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60

IND. % 50 

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10

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60

IND. % 50 

40 

30 

20 

10 

00

SAMPLE OCT-5 S

29

-1

SAMPLE QDT-6 S

-1

SAMPLE QET-7 S

FIGURE 18. Continued.
SEDIMENT FACIES OF ENEWETAK ATOLL LAGOON B55

100

90 

80 

70 

60

IND. % 50 

40 

30 

20 

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00

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60

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IND. % 50 

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20 

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00

SAMPLE # OFT-8 B

SAMPLE # OGT-9 S

17

SAMPLE OIT-11 B

31 31

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00

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00

SAMPLE OFT-8 S

13

SAMPLE # OHT-10 B

30

SAMPLE # OIT-11 S

29 27

FIGURE 18. Continued.
B56 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

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SAMPLE OJT-12 B

SAMPLE OKT-13 B

26 26

15

-1

SAMPLE

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4 8

FIGURE 18. Continued.

OLT-14 B

SAMPLE OJT-12 S

-1

SAMPLE OKT-13 S

SAMPLE OLT-14 S

-1
SEDIMENT FACIES OF ENEWETAK ATOLL LAGOON B57

SAMPLE <# OMT-15 B

SAMPLE <# OOR-17 B

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60

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SAMPLE <# OMT-15 S

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60

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30 

20 

10 

00

SAMPLE <# ONT-16 S

SAMPLE <# OOR-17 S

-1 -1012348

FIGURE 18. Continued.
B58 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

OPZ-18 B

SAMPLE # ORT-20 B

100 

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60

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00

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IND. % 50 

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10

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60

IND. % 50 

40 

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20 

10 

00

SAMPLE OPZ-18 S

SAMPLE # OQT-19 S

20 19

11

-1

SAMPLE <# ORT-20 S

FIGURE 18. Continued.
100 

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60

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IND. % 50 

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20 

10 

00

SEDIMENT FACIES OF ENEWETAK ATOLL LAGOON

100 .

B59

SAMPLE # OSR-21 B

19

SAMPLE # REACTION 

MASS

17

14

-1 0

SAMPLE OTG-23 B

39 40

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60

IND. % 50 

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10 

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SAMPLE ORS-21 S

12

SAMPLE # OSM-22 S

(ANCHOR #33)

15

-1

SAMPLE OTG-23 S

FIGURE 18. Continued.
B60 GEOLOGIC AND GEOPHYSICAL INVESTIGATIONS OF ENEWETAK ATOLL

100 

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00

SAMPLE OUT-24 B

100 

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60

IND. % 50 

40 

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20 

10 

00

SAMPLE # OUT-24 S

-1 1012348

FIGURE 18. Continued.