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.