GEOLOGY AND GEOHYDROLOGY 



41 



et al., 1982; Trcmba, 1985) and participation in the 

 MPRL sponsored R/V Makali'i submersible dives in 1981. 

 The EASI seismic reflection profiles showed that shallow 

 unconformities recognized in the EXPOE drilling continued 

 across the lagoon paralleling the present day bathymetry. 

 Additional deeper reflectors at 150 and 245 m and a 

 series of reflectors between 320 and 365 m were noted 

 and compared to the unconformities described by 

 Schlanger (1963). It was hypothesized that the Middle 

 Miocene reflectors between 320 and 365 m may be a 

 representation of a series of closely spaced unconformities 

 much like the Pleistocene section described for Enewetak 

 and Bikini (Ristvet et al., 1974, 1977; Tracey and Ladd, 

 1974). Unfortunately, only deep drilling with high 

 core/sample recovery would resolve this issue. 



The PEACE program was a two-phase program with 

 the objective of understanding the surface and subsurface 

 morphologies of OAK and KOA nuclear craters. The first 

 phase of the PEACE field program was performed during 

 the summer of 1984 and included high resolution mul- 

 tichannel seismic reflection, bathymetric, side-scan sonar, 

 and submersible studies primarily of the two cratered areas 

 but included some studies of atoll-wide nature (Folger, 

 1986). The second phase of PEACE was conducted during 

 the summer of 1985 and consisted of overwater drilling 

 into and adjacent to the two nuclear craters. High core 

 recovery was obtained in drill holes as deep as 490 m 

 beneath the lagoon floor. The PEACE drilling data are in 

 the analysis phase. 



During this post- 1964 period, Enewetak was the site of 

 several geologic investigations sponsored by MPRL. Exam- 

 ples of these investigations include rates of calcification of 

 the windward reef (Smith and Harrison, 1977), studies of 

 Holocene sea level histories which suggest a higher than 

 present sea level 4000 to 2200 ybp (Tracey and Ladd, 

 1974; Buddemeier et al., 1975), and investigations of the 

 Quaternary history of the reef flat (Szabo et al., 1985). 

 Submersible studies of the outer slope have been con- 

 ducted by Colin et al. (1986) and Halley and Slater (1985) 

 to define the morphology of the outer reef slope. 



SURFACE GEOLOGY 



General 



Enewetak surficial geology is best divided on the basis 

 of depositional environments: the outer slope, the reef, the 

 islands, and the lagoon. 



Outer Slope 



The topography around Enewetak Atoll was first deter- 

 mined by 85 radial and five partially complete concentric 

 lines of soundings made by the USS Bowditch in 1944 

 (Emery et al., 1954). The profiles show a steep slope of 

 18° to 49° from the reef edge to 450 m depth changing 

 to a more gentle slope of 10° between 450 and 2000 m. 

 Sediments collected from a profile seaward of the South 



Channel showed a predominance of fine grain and 

 Halimeda debris to 1500 m depth (Emery et al., 1954). 



In 1981, 22 submersible dives were made on the outer 

 slope of the southern half of Enewetak to depths as great 

 as 360 m (Colin et al., 1986). The outer slope was found 

 to be quite steep, averaging about 60° between 90 and 

 360 m on the windward and transitional side and slightly 

 greater on the leeward side. No terraces or grooves were 

 noted below 30 m. Vertical grooves were noted on the lee- 

 ward side below 150 m depth. Talus accumulations were 

 noted below 150 m, with significant sediment slopes being 

 found seaward of the South Channel below 200 m depth. 

 Below 90 to 100 m depth, it appeared that no significant 

 reef framework was being constructed. Significant quanti- 

 ties of sediment are being transported down the face of 

 the outer slope on the windward side with little or none 

 being transjaorted on the leeward side. 



In 1984 and 1985, Halley and Slater (1985) investi- 

 gated the outer slope of the reef north of the MIKE 

 nuclear crater utilizing the research submersible R/V Delta. 

 Halley and Slater (1985) noted that the slope is character- 

 ized by three zones: (1) the reef plate, algal ridge and near 

 fore reef, from sea level to 16 m depth with less than a 

 10° slope; (2) the by-pass slop>e, from 16 to 275 m, with 

 slopes of 55° decreasing to 35° near the base; and (3) a 

 debris slop)e less than 35° below 272 m depth. 



Halley and Slater (1985) also examined an exp)osed 

 cross section through the reef and fore reef deposits within 

 a rockfall scarp created by the KOA nuclear detonation. 

 The slump scarp exposes three stratigraphic units that are 

 differentiated by the surficial apf)earance: (1) a near-vertical 

 wall from the reef crest to 76 m that appears rubbly and is 

 composed mainly of coral heads; (2) a vertical to overhang- 

 ing wall from 76 to 220 m that is massive and fractured, 

 producing smooth, blocky surfaces; and (3) inclined bed- 

 ding below 220 m along which the slump block has frac- 

 tured, exposing a dip slope of hard, dense white carbonate 

 rock that extends to below 400 m. Caves occur in all 

 three units. Fore reef boulder beds dipping seaward at 30° 

 are truncated by the current outer slope surface, thus 

 revealing the erosional nature of the bypass slop)e. 



Atoll Reefs 



The Enewetak reefs, like those described elsewhere in 

 the Marshalls and other localities in the world, show a 

 strong zonation in bands parallel to the front (Emery et al., 

 1954). These bands are defined by both coral and 

 coral-algal communities (Odum and Odum, 1955) and by 

 sediment deposition patterns (Emery et al., 1954). Differ- 

 ences in the zonation types are recognizable for the three 

 reef types: windward, leeward, and transitional. Most previ- 

 ous studies have concentrated on the zonation of the wind- 

 ward reef (Emery et al., 1954; Odum and Odum, 1955; 

 Wells, 1954); however, description of the leeward and 

 transitional reefs are presented by Emery et al. (1954). 



Figure 2 presents the zonation of the windward reef. 

 The zones consist of fore reef, algal ridge, coral-algal, reef 



