GEOLOGY AND GEOHYDROLOGY 



39 



of Enewetak Atoll, was the site of the first sampling well 

 drilled on an atoll (David ct a!., 1904). This well, drilled 

 from a ship in the lagoon, penetrated 337 m of carbonate 

 sediments, demonstrating the great thickness of atoll reef 

 sediments. 



A major review of the theories of atoll formation was 

 written by Davis (1928). Davis carefully evaluated the data 

 and hypotheses; his evaluation supported Darwin's (1842) 

 subsidence theory as correct and rebutted alternate 

 theories. Although Davis (1928) rejected Daly's (1915) the- 

 ory that glacial period sea level histories resulted in atoll 

 formation, he did enter them as an important new element 

 to consider in evaluating the geologic history of atolls. 



Before 1946, atolls of the Marshall Islands provided lit- 

 tle evidence for the aforementioned theories. During the 

 period of 1918 to 1944, the Marshalls were under the 

 control of Japan; although Japanese scientists conducted 

 studies on the atolls, much of the resulting data are not 

 readily available (Emery et al., 1954). Stearns (1945) made 

 some general comments on possible battle damage to the 

 reef of Enewetak Atoll following the American occupation 

 in 1944. 



In summary, at the end of 1945, only a small body of 

 data existed on atoll geology. The general locations and 

 morphologies were described, and the types of reef- 

 building organisms and their environmental requirements 

 were known in a general sense. Conclusive evidence on 

 atoll formation had not been found, and sparse data 

 existed on subjects such as atoll foundations, ages, lagoon 

 and outer slojDe sediments, reef zonations and productivi- 

 ties, and ecology. 



1946 to 1964 Period 



A period of intense scientific study on the northern 

 Marshall Islands began in 1946 to establish baselines from 

 which damage could be assessed from the U. S. Nuclear 

 Weapons Testing Program. Bikini Atoll was chosen to be 

 the site of the first nuclear weapons effects tests conducted 

 by the United States. Operation Crossroads, consisting of 

 the detonation of two atomic bombs over and under naval 

 ships in Bikini Lagoon, was conducted in 1946. Two 

 expeditions to Bikini were made in 1946 and 1947 to 

 study the atoll environment. The 1946 effort included gen- 

 eral surficial geologic studies of the rcjf, lagoon floor and 

 outer slopes, and a seismic refraction study of the subsur- 

 face structure of the atoll (Emery et al., 1954). The 1947 

 studies yielded much geologic information on the subsur- 

 face through the drilling of three holes on Bikini Island and 

 reef, with one hole penetrating 775 m of carbonate sedi- 

 ments (Ladd et al., 1948). In 1950 additional seismic 

 refraction studies were completed at Bikini and the adja- 

 cent Sylvania Guyot and the southern part of Kwajalein 

 Lagoon (Dobrin and Perkins, 1954; Raitt, 1954). An 

 aeromagnetic survey of Bikini Atoll (Keller, 1954) was also 

 completed. 



Nuclear testing began at Enewetak Atoll in 1948 with 

 three events of Operation Sandstone. Shortly thereafter. 



the USGS began a series of geological and scientific inves- 

 tigations again to establish baselines to measure the effects 

 of the nuclear detonations. In 1950 four shallow holes 

 were drilled by the AEC in the reef on the seaward side of 

 Engebi (Enjebi) Island to locate a suitable rock quarry 

 (Ladd and Schlanger, 1960). In 1951 the AEC drilled 17 

 shallow holes on six different islands for soils engineering 

 studies related to the construction of structures for the 

 nuclear testing. The AEC also drilled three deep holes on 

 the atoll in 1951 and 1952 under the technical guidance 

 of the USGS: K-IB was drilled to 390 m on Engebi 

 (Enjebi), F-1 was drilled to 1411 m on Elugelab, and E-1 

 was drilled to 1287 m on Medren Island. Both F-1 and 

 E-1 reached volcanic basement with 5 m of olivine basalt 

 being recovered from E-1. The confirmation of a basaltic 

 foundation beneath Enewetak Atoll substantiated Darwin's 

 subsidence theory of atoll formation (Ladd et al., 1953). 

 Drill holes K-IB and F-1 were subsequently destroyed 

 during nuclear tests, but the E-1 hole is still open to at 

 least 609 m (Daniels et al., 1984). 



After 1952 field study of the geology of the northern 

 Marshall Islands was reduced significantly, although the 

 nuclear testing continued through August 1958. The con- 

 tinued availability of Enewetak for future field studies was 

 ensured by AEC's establishment in 1954 of the Enewetak 

 Marine Biology Laboratory, now known as the Mid-Pacific 

 Research Laboratory. The AEC completed some additional 

 shallow drilling in 1953 and 1956 for soils engineering 

 (Pratt and Cooper, 1968), but no more drilling for geologic 

 study was completed until 1971. However, the vast 

 amount of field data and samples yielded in the 1948 to 

 1952 efforts were studied and evaluated through 1964. 



Formal presentation of the completed studies was com- 

 piled in the USGS Professional Papers 260 Series com- 

 pleted in 1964. This 28-paper series comprises the most 

 comprehensive single body of geologic, geophysical, and 

 oceanographic data ever assembled on a group of atolls. 

 Much of the rest of this chapter will draw heavily on the 

 data presented in these papers. 



A major pap)er by Emery et al. (1954) is a comprehen- 

 sive study of the surface geology of Bikini, Enewetak, and 

 nearby atolls. It also presents data on the sediments of the 

 lagoons, reefs and islands, reef morphologies and 

 lithologies, and coral zonations of different reefs as well as 

 many other topics. Also presented in the paper are the 

 lithologic sections for the deep holes drilled on Bikini. Sub- 

 surface zones of calcitic limestones are described which are 

 overlain and underlain by aragonitic sediments. These lime- 

 stones are postulated to represent times of subaerial exp)0- 

 sure of the atoll (Ladd et al., 1948; Emery et al., 1954). 



Another paper in the 260 series by Munk and Sargent 

 (1954) describes the variation in the spur and groove 

 structure of the Bikini reefs and relates them to distribu- 

 tion and direction of wave energy. This relationship 

 demonstrates that these are not relict Pleistocene erosional 

 forms. Wells (1954) defined ecological zones of windward 

 reefs in the northern Marshalls on the basis of dominant 

 coral faunas and compared these zonations with reefs else- 



