so 



RISTVET 



sisted of several closely spaced unconformities similar to 

 the Pleistocene section described by Ristvet et al. (1977). 

 Preliminary results from the PEACE program show the 

 presence of at least four subaerial surfaces between 310 

 and 350 m subbottom depths. This suggests that the Mid- 

 dle Miocene may have had episodic continental glaciation 

 conditions similar to those well documented for the 

 Pleistocene/Late Pliocene epKjchs. At least two of these 

 unconformities show karstic features suggesting relatively 

 long periods of subaerial exposure (B. R. Wardlaw, per- 

 sonal communication). 



Resubmergence of the atoll occurred in Tertiary / time 

 with the deposition of shallow-water sediments. From 210 

 to 252 m the sediments represent very organic-rich, nor- 

 mal lagoonal, or shallow-water deposits. Preliminary 

 PEACE data suggest that the sea level did not fall during 

 this depositional interval. Lignitic material is scattered 

 throughout the interval. Leopold (1969) reported a polli- 

 niferous interval from the early deep holes from 205 to 

 270 m. This interval is interpreted as being a time when 

 the atoll had rather large islands and large mangrove 

 swamps developed (B. R. Wardlaw, personal communica- 

 tion). At a depth of more than 210 m, the sediments indi- 

 cate normal shallow-water deposition and a return to the 

 small island configuration. 



Schlanger (1963) describes the presence of a major 

 solution unconformity at a 90 m depth. Preliminary 

 PEACE data show this to be the top of the Pliocene 

 (T. M. Cronin, personal communication). A second 

 subaerial exposure surface is recognized approximately 

 15 m below the 90 m solution unconformity. 



The PEACE drilling program has provided nearly con- 

 tinuous sampling of the upper 490 m of the Enewetak sub- 

 surface near its northern and northwestern lagoonal edges. 

 Unfortunately, results of this recent drilling program are 

 still forthcoming. Preliminary results of the PEACE drilling 

 confirm the general interpretations made by Schlanger 

 (1963); however, they provide a significant increase in the 

 detailed understanding of the post-Lower Miocene strati- 

 graphic section unavailable to Schlanger (1963) due to the 

 poor sample recovery of the 1951 and 1952 drilling. It is 

 anticipated that the PEACE results will lead to redefinition 

 of the biostratigraphy, based both on foraminifera and 

 ostracods of the post-Eocene of Enewetak and the Pacific 

 in general. A detailed understanding of the Enewetak sea 

 level history will also be forthcoming as well as additioneil 

 insight into the processes of the diagenesis of carbonate 

 sediments. 



The Quaternary subsurface of Enewetak is well-defined 

 by the data obtained during the EXPOE drilling and is now 

 further supplemented by the PEACE drilling. Five major 

 unconformities were recognized by Ristvet et al. (1974, 

 1977); Goter (1979); and Szabo et al. (1985). Figure 7 

 presents an ocean reef to lagoon cross section across 

 Engebi (Enjebi) Island constructed from the logs of EXPOE 

 drill holes (Couch et al., 1975) and supplemented by data 

 from the geologic rcdescriptions of several of these holes 

 for the PEACE program (B. R. Wardlaw, personal com- 



munication). Each of the five unconformities represents 

 paleosubaerial exposure surface and is marked by the pres- 

 ence of paleosols (terra rosa type), soil base features (lam- 

 inated crusts, rhizoconcrctions, etc.), and/or prominent 

 changes in the mineralogical and chemical compwsition and 

 cementation of the sediments. These five unconformities 

 arc within the upper zone of leached and altered sediments 

 described by Schlanger (1963). Because of the excellent 

 core recovery during the EXPOE drilling, the identification 

 of these Quaternary unconformities was easily made. 

 Szabo et al. (1985) have dated three of the first four litho- 

 somes using a variety of radiochemical techniques. These 

 ages are shown in Fig. 7, 



The first unconformity at approximately 10 m depth is 

 the Holoceiie/Pleistocene contact. Radiocarbon dates indi- 

 cate that the Holocene sea transgressed the emergent plat- 

 form reef by about 8000 ybp. The reef grew rapidly 

 upward (about 5 to 10 mm yr~') until approximately 

 6500 ybp. Following 6500 ybp, vertical growth slowed to 

 about 0.5 mm yr~' prompting lateral development of the 

 reef (Szabo et al., 1985; Tracey and Ladd, 1974). As pre- 

 viously discussed, sea level may have been nearly 1 m 

 higher than present between 4000 and 2200 ybp. Current 

 relative sea level rise at Enewetak may be near that of the 

 long-term subsidence rate of 0.02 to 0.04 mm yr~' (Bud- 

 demeier et al., 1975). Smith and Kinsey (1976) estimate 

 that the present Enewetak reef has potential for upward 

 growth of approximately 1 mm yr~^ The difference in 

 growth potential versus modem relative sea level rise may 

 explain why the reef plate is prograding lagoonward as 

 noted by Ristvet et al. (1977) for the windward reef off 

 Runit and Aomon. 



Pleistocene rocks in the lithosome directly below the 

 first unconformity are dated at 131,000 ± 3000 ybp by 

 Szabo et al. (1985) and 100,000 to 120,000 ybp by 

 Thurber et al. (1965). There is also a significant change in 

 the mineralogical and chemical composition of the sedi- 

 ments below this first unconformity versus the Holocene 

 sediments above. Ristvet et al. (1974) document the near 

 total loss of high-magnesian calcite below this layer and 

 significant decreases in the whole rock trace element con- 

 centrations of Mg, Fe, and Mn. Calcitic vadose and 

 phreatic carbonate cements are first encountered in this 

 lithosome. 



The development of the unconformities and the associ- 

 ated diagenesis of the underlying carbonate sediments are 

 the result of relative sea level changes during the past. 

 Periods of worldwide continental glaciations cause a lower- 

 ing of sea level. At Enewetak during the Quaternary, this 

 may have been in excess of 100 m (Walcott, 1972) during 

 each major glacial advance. During these periods of sea 

 level lowstands, the Enewetak Lagoon is above sea level 

 and the atoll becomes a large, high carbonate island, 

 resulting in severe changes to both the hydrologic regime 

 and sediment production of the atoll. Because the reefs 

 are subaerially exposed, only minor reef growth occurs as 

 a fringing reef on the outer slopes of the atoll-island. The 

 atoll-island undergoes subaerieJ erosion and soil develop)- 



