Chapter 4 



Geologi; and Geohydrologi; of Enewetak Atoll 



BYRON L. RISTVET 



S-CUBED. A Division of Maxwell Laboratories, Inc. 

 Albuquerque, New Mexico 87198 



INTRODUCTION 



Enewetak Atoll is located at 162° east longitude and 

 11° north latitude in the Pacific Ocean. It is the north- 

 westernmost member of the western Ralik (Sunset) Chain 

 of the Marshall Islands. Enewetak Atoll is one of the larger 

 atolls; it is roughly elliptical in shape, with a north-south 

 length of 40 km and an east-west width of 32 km (Fig. 1). 

 The reef is cut by three passes. The Deep Channel on the 

 southeast side is only 1.5 km wide, but it has a depth of 

 55 m between Japtan and Medren Islands. The South 

 Channel is approximately 9.5 km wide but is only 10 to 

 20 m deep. The Southwest Passage is even shallower, 

 only 2 to 4 m in depth. Maximum tidal currents of nearly 

 1 m s~' in the Deep Channel and 0.5 m s~' in the South 

 Channel have been observed (Emery ct al., 1954). The 

 reef may be divided into four parts with distinct morpholo- 

 gies related to their positions relative to the prevailing 

 northeast trade winds. The parts are the windward reef on 

 the northeast, the leeward reef on the southwest, and the 

 two transitional reefs on the northwest and southeast 

 (Fig. 1). The reef encloses a lagoon of 920 km^ with a 

 maximum depth of 65 m. The lagoon has a relatively 

 smooth carbonate sediment bottom studded with hundreds 

 of coral pinnacle and patch reefs (Emery ct al., 1954). 

 Forty-two low-relief islands and islets composed of car- 

 bonate sands and gravels exist on the atoll with a total dry 

 land area of 6.7 km^ with the largest islands being about 1 

 km^ in area. 



Enewetak Atoll receives an average annual rainfall of 

 1470 mm, mostly during August to December. Rainfall is 

 highly variable with annual totals ranging from 605 to 

 2422 mm (Buddemeier, 1981). Tides are of the mixed 

 semidiurnal type with a maximum range of about 1.8 m. 



The purpose of this chapter is to summarize the vast 

 wealth of data on the geological aspects of Enewetak gath- 

 ered over the last 40 years. 



A SUMMARY OF GEOLOGIC 

 INVESTIGATIONS 



The history of investigations of atoll geology in general 

 and Enewetak Atoll in fiarticular may be divided into three 

 periods: pre-1946, 1946 to 1964, and post-1964. Th« 

 first period was one of discovery and initial exploration. 

 These early observations became the framework for many 

 hypotheses on the origin and evolution of atolls. Most of 

 the early studies focused on the surficial geologic features 

 and lacked the direct sampling of subsurface data to evalu- 

 ate the many hypotheses of the day. Beginning In 1946, 

 there was a significant increase in knowledge of atolls 

 resulting from a series of comprehensive scientific studies 

 of the northern atolls of the Marshall Islands, particularly 

 Bikini and Enewetak. These geologic investigations were 

 conducted by U. S. Geological Survey (USGS) scientists 

 for the U. S. Atomic Energy Commission (AEC) to estab- 

 lish baselines to assess effects from nuclear weapons test- 

 ing conducted at Enewetak and Bikini between 1946 and 

 1958. A vast amount of surface and subsurface geologic 

 data was gathered and analyzed, and the results were pub- 

 lished through 1964 (cf. Emery et al., 1954; Schlanger, 

 1963). From 1964 to the present, scientific studies have 

 been of two types: those which have continued to addrcs* 

 the problems conceptualized by earlier studies and those 

 which have addressed the effects of the nuclear weapons 

 testing at the two atolls. Enewetak Atoll continues to this 

 day as one of the sites of significant studies of atoll geol- 

 ogy, carbonate sedimentology, and organism/sedinrKnt 

 interrelationships. 



Prc-1946 Period 



The first geologic studies of the Marshall Islands were 

 conducted in 1816 and 1817 by Albert Chamisso. Cha- 

 misso, a naturalist with the Russian Von Kotzebue expedi- 

 tion to the northern and western Pacific Ocean, described 

 the reefs, islands, and lagoons of the eastern chain of the 

 Marshalls. For the rest of the 19**^ century the MesrshAls 

 were visited only by general surveying expcdftkxN (Em«T? 

 et al., 1954). 



37 



