44 



RISTVET 



sediment production and bioerosion of the reef flat. This 

 environment extends from the reef flat to the islands. 



Atoll Islands 



The present islands of Enewetak represent wave and 

 eoiian deposits of excess sediment production from the 

 reef stabilized in part by the formation of beachrock. 

 Islands are present on the reef except on the northwest 

 transitional reef. The islands are all approximately 3 to 

 4 m in elevation above the lowest low water. Two basic 

 island shapes exist for Enewetak Atoll: (1) long linear 

 islands that parallel the reef h-ont, such as Runit, 

 Enewetak, and Bokoluo and (2) the triangle-shaped islands 

 with the base on the lagoon side parallel to the reef front 

 and the point facing the seaward reef, such as Engebi 

 (Enjebi), Aomon, and Louj. The origin of these two island 

 shapes is not understood. The islands are covered with 

 vegetation and have fairly well-developed soil profiles. 



The origin of beachrock has been the subject of several 

 investigations at Enewetak and other carbonate beaches in 

 the world. Beachrock at Enewetak is present on 30 to 

 40% of all beaches. The formation of beachrock appears 

 to be a fairly recent phenomenon with significant formation 

 continuing today. 



The author has collected samples of beachrock at 

 Enewetak encapsulating World War 11 shell casings and 

 cables from the nuclear testing pjeriod. The origin of 

 beachrock was first investigated in the Marshalls by Emery 

 et al. (1954), who looked at interstitial water chemistry 

 and concluded that evaporation and heating of interstitial 

 seawater resulted in carbonate precipitation. Schmalz 

 (1971) studied the interstitial water of beach sediments on 

 the lagoon side of Bijire Island in 1967. He concluded that 

 precipitation of the dominant acicular aragonite and minor 

 micritic magnesian calcite cements in the interstices of the 

 carbonate sand was caused by the mixing of seawater with 

 the brackish meteoric water in the thin Gyben-Herzberg 

 lens. A succession of studies on the origin of beachrock 

 cements followed Schmalz (1971). Commonly invoked 

 processes for the precipitation of beachrock cements 

 include evaporation of seawater, mixed fresh-saline 

 waters, and vague types of biological involvement (Manor, 

 1978). Current models show that degassing carbon dioxide 

 from beach groundwaters appears to be the primary 

 phenomenon that forms beachrock (Manor, 1978). 



Atoll Lagoon 



The bathymetry of the lagoon was mapped in detail by 

 the U. S. Navy in 1944. Nearly 180,000 soundings were 

 made, and the results were contoured (Emery et al., 1954, 

 chart 5). The lagoon bathymetry is somewhat irregular due 

 to the presence of numerous coral knolls (patch and pinna- 

 cle reefs). The lagoon consists of four major bathymetric 

 features: (1) lagoon terrace; (2) lagoon basin; (3) coral 

 knolls; and (4) the reef openings. 



The lagoon bathymetry shows a terrace between 15 

 and 22 m depth (Emery et al., 1954). The terrace borders 



all edges of the lagoon except the northwest and southern 

 margins, where it is absent. The width is variable with 3 

 km being the greatest attained. The lagoon terrace is dot- 

 ted with numerous patch reefs. The slopes from the 

 islands to the terrace are gentle, averaging <2.5°. An 

 even gentler slope, averaging 1.25°, separates the terrace 

 from deep basin (Emery et al., 1954). 



The main lagoon basin is a relatively flat area with 

 slopes of 0.10°. The greatest depths are nearly 65 m in 

 the northwestern half of the lagoon. The mean depth of 

 the basin is approximately 55 m (Emery et al., 1954). 



Within the lagoon are a large number of individueJ 

 coral knolls or patch and pinnacle reefs. Emery et al. 

 (1954) reported the presence of 2293 individual coral 

 knolls. About 10% of the knolls rise to within 8 m of the 

 surface. Most have tops between 30 and 36 m depth. The 

 distribution of the coral knolls within the lagoon apf)ears to 

 be random. Seismic reflection profiles from EASl and 

 PEACE through knolls suggest that they are predominately 

 Molocene features. Nearly half of the knolls are formed 

 over what is interpreted to be preexisting eroded Pleisto- 

 cene patch or pinnacle reefs, whereas the other half of the 

 lagoonal coral knolls do not appear to have an antecedent 

 structure beneath them (Tremba, 1985; Grow et al., 

 1986). 



The bottom sediments of the Enewetak Lagoon were 

 first characterized by Emery et al. (1954) and most 

 recently by T. W. Menry and B. R. Wardlaw (personal 

 communication). Emery et al. (1954) found that the sedi- 

 ments consist of the following chief components: Halimeda 

 sand, coral sand and gravel, foraminifera sand, mollusc 

 shell sand and gravel, and fine debris. Fine debris was 

 defined as all grains <0.25 mm in diameter. Emery et al. 

 (1954) show the terrace to be dominated by fine debris 

 and the basin by Halimeda and foraminifera sand. Menry 

 and Wardlaw (1985) show a similar distribution but reiport 

 much more mud-sized (<0.062 mm) carbonate sediment 

 on the terraces and in the basin than Emery et al. (1954). 



McMurtry et al. (1985) have investigated the magni- 

 tude of bioturbation of the lagoonal sediments off Runit 

 Island and found that the burrowing shrimp of the family 

 Callianassidae nearly completely mix and redistribute the 

 surface sediments to a subbottom depth of at least 1.5 m. 



SUBSURFACE GEOLOGY AND 

 GEOPHYSICS 



The subsurface geology of Enewetak Atoll consists of 

 an approximately 1370 m thick carbonate sediment 

 caprock overlying the summit of a basaltic volcano that 

 rises 5000 m above the floor of the ocean (Ladd et al., 

 1953). Most o* the drill hole data for the interpretation of 

 the subsurface geology of Enewetak are derived from 

 drilling on islands or the reef flat. The PEACE program 

 has added data to 490 m subbottom depth on the north- 

 ern lagoon terrace and northwestern shallow lagoon. The 

 subsurface geology, as deduced from the analysis of the 

 borehole samples and seismic profiles, is very similar to 



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