46 



RISTVET 



the Hawaiian volcanoes and other islands of the central 

 Pacific Ocean. Kulp (1963) found the basalt to be Eocene 

 in age with a whole rock K/Ar radiometric date of 59 ± 

 2 million ybp and a pyroxene K/Ar date of 51 ±5 mil- 

 lion ybp. 



The shape of the volcano is characterized by the two 

 drill holes at the north and southeast edges of the atoll, 

 the seismic refraction profiles of Raitt (1957) and the 

 recent seismic reflection profiles of the PEACE program 

 (Grow et a!., 1986). Figure 4 displays the subsurface ve- 

 locity structure of the atoll from the surface to the upper 

 mantle as interpreted by Raitt. Although not depicted in 

 Fig. 4, the uppermost velocity layer is a thin (105 m 

 thick), low velocity (1920 m s~^) unit detected beneath 

 the reef northwest of Elugelab Island. The second and third 

 layers have apparent harmonic mean velocities of 2440 m 

 s~' and 3050 m s~\ respectively. Raitt (1957) suggests 

 that both velocities are characteristic of partly con- 

 solidated calcareous sediments. The fourth through sixth 

 layers comprise the volcanic basement underlying the car- 

 bonate caprock. Finally, the seventh layer has a seismic 

 velocity, 8.1 km s~\ characteristic of the upper mantle. 

 Grow et al. (1986) show, in seismic reflection profiles, the 

 top of the volcanics to be a relatively flat surface with only 

 minor topograpy. 



Carbonate Rocks 



Figure 5 displays Schlanger's (1963) generalized 

 interpretation of the subsurface of Enewetak Atoll based 

 on the three deep holes drilled in 1951 and 1952. Also 

 used for comparison is the interpretation of the subsurface 

 of Bikini (Emery et al., 1954) which shows a strong simi- 

 larity in the vertical extent of these zones for both atolls. 

 Schlanger (1963) recognized that beneath both Enewetak 

 and Bikini, there are zones characterized by the presence 

 of fossil molds and solution features (leached and altered 

 sediments) alternating vertically with zones containing pri- 

 mary skeletal aragonite (unaltered sediments) separated by 

 relatively sharp boundaries. Schlanger (1963) termed the 

 upper surfaces of the leached zones "solution unconformi- 

 ties," because they resemble karstic surfaces. Ladd et eil. 

 (1948) suggested that the leached zones at Bikini 

 represented periods of atoll emergence and exposure of 

 the marine sediments to subaerial conditions. The unal- 

 tered zones were believed to represent sediments that 

 were never emergent. 



Schlanger (1963) identified three major solution uncon- 

 formities in the subsurface of Enewetak at depths of 

 850 m (Early Miocene), 335 m (Middle Miocene), and 

 90 m (Pleistocene) below the surface (Fig. 5). Schlanger 

 (1963) also described an interval of partially leached and 

 altered sediments within the upper 90 m of the Enewetak 

 subsurface. However, due to limited sample recovery he 

 was unable to identify a solution unconformity within this 

 interval. He did conclude that at least one additional 

 p>eriod of atoll emergence had occurred during the Pleisto- 

 cene following the emergence related to the major solution 

 unconformity at 90 m depth. 



The high percentage of recognizable fossil material in 

 the three deep drill holes allowed Schlanger (1963) to 

 interpret the depositional environments of the sediments. 

 Figure 6 presents the interpreted paleoecology of holes 

 E-1 and F-1 at Enewetak and 2A-B at Bikini. The sec- 

 tion of Eocene fore reef deposits in hole F-1 between 

 1280 and 1385 m represents outer slop)e deposits contem- 

 fwraneous with near-reef, shallow-water deposits in hole 

 E-1 from 845 to 950 m depth. The section of fore reef 

 deposits in F-1 from 822 to 1280 m has no counterpart 

 in E-1. Schlanger (1963) interpreted this as evidence that 

 the earliest reef building at Enewetak began on the 

 southeast side of the atoll near Enewetak and Medren 

 Islands of today. Reef production and p>ossible erosion of 

 the southeast reef during the lower Miocene emergence 

 resulted in the wedge of fore reef sediments seen in F-1. 

 The seismic reflection studies of Grow et al. (1986) app>ear 

 to confirm a prograding reef front from southeast to 

 northwest starting in the presumed Eocene sediments and 

 continuing to the Middle Miocene unconformity. 



Post-Eocene carbonate sediments are all of shallow- 

 water origin as sampled in the three deep holes, the 

 EXPOE holes, and the PEACE holes. By the Middle 

 Miocene unconformity, the location of the reef tract 

 apF>ears to have been very close to the present position of 

 the modern day islands (B. R. Wardlaw, personal com- 

 munication). In the substantial time represented by the 

 upper 335 m of carbonate sediments, the reef tract has 

 only migrated seaward 200 to 300 m. Deposition of 

 shallow-water sediments under conditions of slow atoll 

 subsidence continued through the Middle Miocene (Cole, 

 1957). However, the presence of several disconformities 

 noted in the PEACE drilling from 335 to 490 m 

 (T. W. Henry and B. R. Wardlaw, personal communica- 

 tion) and Schlanger's (1963) reported presence of 

 recrystallized limestone from 603 to 650 m suggest some 

 periods of atoll emergence. 



Minor amounts of dolomitized limestone were noted 

 within the Eocene stratigraphic section in both F-1 and 

 E-1 and below the assumed Lower Miocene solution 

 unconformity in F-1 (Schlanger, 1963). The dolomite 

 appears as protodolomite replacing calcite. Schlanger 

 (1963) felt that its origin may have resulted from the 

 alteration of high magnesian coralline algae. Other 

 hypotheses have been proposed including dolomite forma- 

 tion in the mixing zone of meteoric groundwaters with sea- 

 water during atoll emergence and/or formation from 

 hyf)ersaline conditions in a restricted shallow-water environ- 

 ment penecontemp)oraneous with dep>osition. Sailer (1984) 

 presents new evidence using stable Sr isotope data that 

 the Enewetak dolomite precipitated from normal seawater 

 significantly following deposition at burial depths greater 

 than 900 m. 



The 335 m unconformity described by Schlanger 

 (1963) indicates a major emergence occurred after the 

 depKJsition of Middle Miocene sediments. Ristvet et eJ. 

 (1980) postulated, on the basis of the EASI seismic reflec- 

 tion profiles, that the 335 m solution unconformity con- 



