52 



RISTVET 



merit from colian sources. Such subaerial exposure to 

 meteoric waters results in the development of an extensive 

 Gyben-Herzberg lens within the island which is conducive 

 to the alteration and cementation of the sediments. 



During atoll emergence, several processes acting alone 

 or in various combinations can produce significant modifi- 

 cations in those carbonate sediments exposed to meteoric 

 waters. Most of these processes arc dependent on the 

 initial dissolution of carbonate minerals into an aqueous 

 phase. Subsequent precipitation of calcium carbonate may 

 be caused by changes in carbon dioxide pressure, tempera- 

 ture, evaporation, mixing of waters of differing ionic 

 strength, and other mechanisms (Bathhurst, 1971). Precipi- 

 tation appears to be highly variable in both space and 

 time. It may be contempKsraneous with dissolution or may 

 involve transport over large distances. 



The model prop>osed for the diagenesis of Enewetak 

 sediments is similar to that profxssed for other carbonate 

 sequences (Thorstenson et al., 1972). Meteoric waters 

 passing through a soil approach equilibrium with the 

 ambient CO2 pressure which is normally significantly 

 higher than atmospheric CO2. These high CO2 waters 

 promote dissolution of the metastable aragonite and 

 magnesian calcite mineralogy of recent carbonate sedi- 

 ments and approach equilibrium solubility. The saturated 

 waters at a later stage encounter an environment of lower 

 CO2, causing degassing of CO2 and the subsequent precipi- 

 tation of calcite. The process of CO2 control on 

 solution-precipitation of carbonates occurs within both the 

 vadose and phraetic zones. At standard pressures and tem- 

 peratures, the loss of high-magnesian calcite to calcite 

 generally precedes the solution of aragonite and the con- 

 current development of moldic porosity before the precipi- 

 tation of calcite. 



As may be seen in Fig. 7, several [>eriods of atoll 

 emergence have been followed by submergence during the 

 Quaternary. For the Quaternary, it appears that following 

 each sea level rise, the new depositional environment 

 parallels that below the unconformity and buries it with 

 new sediments as the platform subsides. The processes 

 involved in subaerial diagenesis of the sediments during 

 each f)eriod of emergence are multiple upon the older 

 lithosome below each unconformity. In other words, for 

 any depth within the meteoric vadose and phraetic regime, 

 there is a potential for the solution reprecipitation process 

 to occur as many times as there are subaerial exposures 

 above that depth. This multiple diagenesis results in pro- 

 gressive increases in cementation and mineral stability with 

 increasing depth for at least the Quaternary section of the 

 Enewetak subsurface. 



The Quaternary subsurface of Engebi (Enjebi) (Fig. 7) 

 consists of a complex mosaic of depositional lithofacies, 

 which have subsequently been affected by diagenetic 

 processes. In general, cementation increases with depth 

 and towards the reef within each stratigraphic unit. This 

 lateral change in cementation and, as shown by Ristvet et 

 al. (1974), corresponding changes in the rates of mineral 

 stabilization and trace element petrochemistry may be in 



part due to (1) the occurrence of marine cements in those 

 sediments near the reef flat versus those deposited lagoon- 

 ward and (2) to diagenetic processes affecting the sedi- 

 ments as a function of the paleohydrologic regime and the 

 paleochemistry of the meteoric lens (Ristvet et al., 1977). 



Shallow seismic refraction surveys were conducted on 

 windward, leeward, and transitional islands during EXPOE 

 and yielded consistent profiles for the Quaternary 

 Enewetak subsurface (Ristvet et al., 1977). As shown on 

 Fig. 7, four distinct velocity intervals exist. The velocity in 

 the unsaturated island sediments, Vq, is 330 to 600 m 

 s~'; \Ji is the velocity in saturated, unconsolidated Holo- 

 cene sediments and is typically about 1600 m s~^ The 

 velocity in poorly to moderately cemented Pleistocene sedi- 

 ments, V2, is typically 2500 m s ^ The V1/V2 interface 

 corresponds to the first unconformity. The higher velocities 

 of well-cemented sediments which occur on the reefward 

 side of the island and at depths below 60 m as inferred 

 from lithologic descriptions of drill holes are represented 

 by V3 (Ristvet et al., 1977). 



The unconformities recognized by the drilling on the 

 atoll edges may also be followed into the lagoon on seismic 

 reflection profiles obtained during EASI and PEACE 

 (Ristvet et al., 1980; Tremba et al., 1982; Tremba, 1985; 

 Grow et al., 1986). Figure 8 is the interpretation of a 

 seismic reflection record which is a lagoonward extension 

 of the Engebi (Enjebi) reef to lagoon geologic cross section 

 shown in Fig. 7. The seismic profile is perpendicular to the 

 reef front and crosses the lagoonal terrace into the lagoon 

 basin. In Fig. 8, the first reflector/refractor corresponds to 

 the Holocene/Pleistocene unconformity at 15 m subbottom 

 depth. The reflector at 66 m subbottom depth seems to 

 correspond to a Pleistocene unconformity seen in the 

 Engebi (Enjebi) drill holes. From the PEACE drilling, it is 

 apparent that the deeper reflectors between 150 and 

 330 m correspond to lithologic changes and do not neces- 

 sarily represent unconformities. The 330 m reflector does 

 represent the top of a series of closely spaced reflectors 

 corresponding to the Middle Miocene unconformities recog- 

 nized in the PEACE boreholes. Of interest is that parallel- 

 ism of the reflectors to the present bathymetry. This 

 feature of the seismic records was noted atoll-wide for 

 reflectors above the Middle Miocene unconformities helping 

 to confirm the hypothesis that the present-day reef 

 environments have shown little lateral migration since the 

 Middle Miocene. 



GEOHYDROLOGY 



Studies of the hydrology of Enewetak Atoll were ini- 

 tiated in 1972 to evaluate possible environmental effects of 

 the proposed PACE high explosive craters on the ground- 

 water resources of the islands (Koopman, 1973). Addi- 

 tional studies sponsored by the DOE have been conducted 

 as part of a program to determine the physical, chemical, 

 and biological mechanisms controlling the distribution and 

 transport of radionuclides in the atoll environment (cf. Bud- 

 demeier and Holladay, 1977; Wheatcraft and Buddemeier, 



