40 



RISTVET 



where. The great organic productivity of atoll reefs versus 

 the surrounding oceans is demonstrated by Sargent and 

 Austin (1954). 



The subsurface geology and geophysics of Enewetak 

 Atoll somewhat dominates the 260 series. The penetration 

 of a basaltic basement by drilling and the aeromagnetic 

 and seismic refraction surveys indicated the presence of 

 volcanoes beneath Enewetak, Bikini, and Kwajalein Atolls. 

 Ladd and Schlangcr (1960) present the locations and drill- 

 ing data for the Enewetak drill holes. They conclude that 

 most of the near surface material to 60 m depth is uncon- 

 solidated, whereas deeper zones of recrystallized and 

 leached carbonate are postulated to represent fjeriods of 

 subaerial emergence of the atoll. Foraminifera were used 

 to establish a Tertiary biostratigraphy of the Enewetak sub- 

 surface and to document continuous shallow water condi- 

 tions in which the entire carbonate section had been de- 

 posited (Cole, 1957; Todd and Low, 1960). The oldest 

 carbonates were identified as Upper Eocene in age. 



The general subsurface geology of Enewetak was 

 defined by Schlanger (1963), who presents detailed litho- 

 logic logs of the Enewetak deep holes and provides an 

 interpretation of the geologic history. Schlanger (1963) 

 noted the presence of numerous "solution unconformities" 

 within the Enewetak geologic column. The term solution 

 was used because Schlanger felt these unconformities 

 represented karstic surfaces. 



The scientific programs in the northern Marshalls had a 

 stimulating effect on the academic interests in atolls. The 

 interest in the geology and biology of carbonate reefs is 

 still a dominant field of study. The geologic studies of this 

 period answered many of the basic questions about atoll 

 formation. Atolls rested on subsided volcanic foundations. 

 The compositions and dep>ositional environments of the 

 subsurface sediments were characterized and interpreted. 

 Specific zones of altered carbonates were identified and 

 interpreted to represent periods of atoll emergence and 

 given paleohydrologic meaning. Reef zonation and mor- 

 phology, as products of interacting biological aggradation 

 and mechanical and biological erosion, became better 

 understood. 



1964 to Present 



Geological studies conducted during this time in the 

 Marshall Islands have been centered on Enewetak Atoll. 

 Numerous studies primarily concerned with sediment/ 

 organism interrelationships, the distribution of radionu- 

 clides within the atoll sediments, and geohydrology have 

 been conducted under the auspices of the Mid-Pacific 

 Research Laboratory, which is sponsored by the Depart- 

 ment of Energy (DOE). The Defense Nuclear Agency has 

 sponsored four major field programs to understand the 

 craters resulting from the near-surface detonation of 

 nuclear weapons: the Pacific Cratering Experiment (PACE), 

 1971 to 1972; the Exploratory Program on Enewetak 

 (EXPOE), 1973 to 1974; the Enewetak Atoll Seismic 

 Investigation (EASI), 1980; and the Pacific-Enewetak Atoll 

 Craters Exploration (PEACE), 1984 to present. 



Until the early 1970s, studies of Enewetak geology 

 consisted of reviews or extensions of previous work. Gross 

 and Tracey (1966) used stable carbon and oxygen isotope 

 data to substantiate the hypothesis that the calcific lime- 

 stones in the subsurface were formed in fresh water 

 environments (Ladd et al., 1948; Ladd and Schlangcr, 

 1960; Schlanger, 1963). Thurber et al. (1965) performed 

 U/Th radiometric dating of corals of the Enewetak subsur- 

 face from the Quaternary [>eriod and revealed an absence 

 of corals dating between 6000 and 100,000 years before 

 present (ybp), indicating a significant hiatus in def)osition. 



At Enewetak, PACE was conducted to evaluate the 

 influence of the shallow subsurface geology on the dimen- 

 sions of nuclear explosion craters. It consisted of two 

 phases: (1) geological and geophysical investigations of the 

 shallow (<70 m) subsurface of the atoll and (2) a series of 

 high explosive cratering experiments. A federal court order 

 cancelled PACE before most of the high explosive crater- 

 ing experiments were conducted. However, much of the 

 first phase was completed, and nearly 250 shallow 

 boreholes were completed on seven islands, with 235 

 being drilled on Aomon or Runit Island (Henny et al., 

 1974). Most of the holes were soils engineering borings 

 which returned little or no sample. Sample recovery in the 

 cored boreholes was variable but was generally quite fxxjr. 

 A generalized four-layer engineering geology model for the 

 shallow subsurface at Aomon Island was developed by 

 Henny et al. (1974) using these limited samples and seis- 

 mic refraction survey data. 



The follow-on geologic and geophysical program to 

 PACE was EXPOE. The objective of the EXPOE program 

 was to develop a model of the near surface geology of the 

 atoll for the nuclear crater regions in the northern islands. 

 Forty-six cored boreholes and 13 water sampling wells 

 were completed on 11 islands on the windward, leeward, 

 and transitional sides of the atoll, and 250,000 lineal feet 

 of shallow seismic refraction surveys were completed from 

 1973 to 1974. The EXPOE program was notable for the 

 excellent sample recovery: recovery of 4-in. cores of both 

 consolidated and unconsolidated materials averaged over 

 80% (Couch et al., 1975). This recovery was far greater 

 than any previous drilling, especially in the pxjorly and 

 unconsolidated near surface sediments, and allowed for a 

 more detailed picture of the stratigraphy and petrology of 

 the upper 100 m of the Enewetak subsurface than gained 

 in previous studies (Ristvet et al., 1974, 1977). 



As will be discussed in greater detail later in this 

 chapter, the EXPOE findings indicate that the atoll pe- 

 riphery to at least 80 m depth consists of subordinate reef 

 and dominant back reef and marginal lagoon deposits of 

 the Holocene and Pleistocene ages. Five subaerial surfaces 

 were recognized in the Pleistocene section associated with 

 sea level drops during glacial periods (Ristvet et al., 1974, 

 1977). 



The EASI field program consisted of overwater high 

 resolution multichannel seismic reflection surveys of the 

 KOA and OAK nuclear craters and the undistu''bed lagoon 

 off of Engebi (Enjebi) Island (Ristvet et al., 19ti0; Tremba 



