EWELAK ALULL
EN
RADIOLOGICAL CLEANUP OF
12
om Reef
a
-
Ocean Lever
cean Botton
s
STAGE 1:
Coral reef begins
volcanic
athe border of a
island.
VOLCANLC ISLAND
Crustal Layer
Coral Reef
Lagoon
Ocean Level
N
OceanLevel
Coral reef grows
STAGE 2:
as the
upward and inward
subsides
original island
el rises.
and/or the ocean lev
Ocean Botto
Tate
ent =
-*
77
Crustal Layer
Exposed Limestone
Island
Ocean Level
Ocean Level
STAGE 3:
Ocean level sub-
rises
sides and/or island
one
al limest
exposing the cor
rystaliza—
to leaching, rec
tion and erosion.
ORIGINAL ISLAND
Layer
ao “Grustal
Biken Is.
1
Lojwa Is.
Ocean Level
an Leve
“se
Small {stands form
STAGE 4:
umes a5
and coral growth res
/or
ocean Ievel rises and
Gcean Bottom
Ocean Bottop
eeie
‘cenied
ORIGINAL ISLAND
mor
l Layer
ae Crusta
ab pe
a
exposed, lateral growth is promoted. Erosion of the coral and cementation
of the resulting sediments also affect the formation and geology of the
atoll. Enewetak Atoll has been forming for at least 43 million years,
resulting in a 4,500-foot stratification of reef-derived carbonate deposits.
Several drilling programs have been conducted to determine the
subsurface composition and deposition of Enewetak Atoll. The Atomic
Energy Commission (AEC) and Los AlamosScientific Laboratorydrilled
~w
-7
ro ee
--* 7 7)
ides.
original {sland subs
nal
After USGS Professio
and
Papers 260-5 (Riatt)
and others)
260-BB (Schlanger
LL.
ION OF ENE WETAK ATO
FIGURE 1-17. EVOLUT
33 holes less than 200 feet deep during 1950-51. The U.S. Geological
Survey (USGS) drilled three deep holes, two to the basalt (volcanic rock
base), during 1951-52. An additional 174 shallow core holes were drilled
in support of Defense Nuclear Agency (DNA) programs’ to understand
the near subsurface geology (less than 300-foot depth) ofthe atoll in 197273,
Based on results of the USGS and DNA drilling programs, the
subsurface geology of the atoll is found to be both laterally and vertically
variable. In general, the ocean-side reef consists of well cemented
limestone, whereas the backreef and lagoon sediments consist of
uncemented coralline sands and gravels derived from the ocean reef
organisms and the many patch and pinnacle reefs in the lagoon. Holes
drilled near the ocean reef edge penetrated predominately moderate to
well cemented sediments, whereas holes near the lagoon penctrated
predominately uncemented to poorly cemented sediments. This
correlation between surface and subsurface distribution of rock types is
indicative of little lateral shifting of the reef and associated deposited
environment during the past few million years.
A generalized geologic profile beneath the islands is as follows:
unconsolidated coralline sands and gravels between the island surface and
the intertidal zone: within the intertidal zone, a layer of well cemented
coralline beachrock from a few inches to 8 to 10 feet thick is found. Recent
coralline sands and gravels exist between the beachrock and 45-foot depth,
whereas an alternating sequence of cemented and uncemented coralline
sands and gravels exist to 600 feet.2 Between 600 and 1,000 feet the
sediments again are composed of uncemented coralline sands and gravels,
and between |,000 and 1,200 feet cemented coralline sands and gravels are
encountered. Beneath 1,200 feet and to the top of the basalt, the sediments
are predominately uncemented coralline sands and gravels with occasional
cemented layers.
CLIMATE
Enewetak’s climate is of the tropical marine type with temperatures
ranging from 7I°F to 94°F and humidity in the 73 to 80 percent range.