GE0LCX3Y AND GEOHYDROLOGY 



43 



flat, and back reef flat. Figure 2 also displays the relation- 

 ship of the reef to the islands and lagoon. Each of the five 

 zones has unique biologic and geologic characteristics. 

 Each of these zonations provides a model for what is seen 

 in the subsurface. However, as will be seen in subsequent 

 sections of this chapter, the Pleistocene subsurface appears 

 to consist dominantly of subtidal deposits, whereas the 

 modern reef flat consists of predominantly intertidal 

 environments. 



Tracey and Ladd (1974) and Buddemeier et al. (1975) 

 present evidence that the broad intertidal, rocky platform 

 of the modem windward reef flat consists of lithified sub- 

 tidal sediments implying a previous higher-than-present 

 Holocene sea level. The modern windward reef is an ero- 

 sional platform develo[)ed after a growth of the Holocene 

 reef to a higher sea level. Hence, there is the possibility 

 that the modern Enewetak windward reef flat is not a good 

 model to use to interpret former aggradational reef 

 environments seen in the subsurface. 



The windward fore reef consists of an area 30 to 50 m 

 wide, sloping gently seaward at 10° to 15° and covered 

 with coral and Halimeda sp. These gentle slopes do not 

 exist on the leeward reef where the fore reef has 40° to 

 60° slopes. The same biological communities exist on the 

 leeward fore reef as on the windward side (Colin et di., 

 1986). The fore reef extends to a depth of 30 m where 

 the slope rapidly steepens, and the presence of stony 

 corals and Halimeda declines drastically. At fairly regular 

 intervals along the slop>e, there are nearly straight grooves 

 perpendicular to the reef face. These grooves are from 

 2 to 3 m wide and 8 to 15 m long and are separated by 

 spurs 5 to 10 m or more wide. The spurs are composed 

 of living encrusting coralline algae (Emery et al., 1954). 

 The origin of the grooves and spurs has been suggested by 

 Munk and Sargent (1954) to dissipate the wave energy 

 against the reef front. These grooves often extend into the 

 algal ridge, especially on the transitional and leeward 

 edges. The fore reef appears to be a site of active reef 

 building with the sediments being cemented by biologic 

 binding and penecontemporaneous marine cementation. 



The algal ridge is primarily composed of encrusting red 

 algae, primarily Porolithon. The algal ridge may actually 

 grow above the reef flat elevation to as much as 0.3 to 

 0.6 m above the lowest low water due to wave action 

 keeping the living algae wet during low water. The algal 

 ridge with its biological and marine cementation provides 

 the framework for the preservation of the back reef and 

 lagoonal sediments from the erosion of ocean waves 

 (Emery et al. 1954). Algal ridges occur on both the 

 lagoon and ocean sides on the leeward reef. Both of these 

 leeward algal ridges are poorly developed and do not rise 

 much above the lowest low water. 



On the inner side of the algal ridge, there is a belt of 

 rich coral growth from 50 to 150 m wide. Stony corals 

 cover more than 50% of the reef surface. Shallow pools 

 contain most of the coral. The remainder of the zone is a 

 pavement of encrusting red algae. The growth forms of the 

 coral colonies are low or encrusting to withstand the wave 



action and low tides. Corals are predominantly Acropora. 

 Pocilhpora, and Montipora. 



Again the coral-algal zone through biological and 

 marine cementation provides well-cemented sediments for 

 incorporation into the subsurface. 



The windward reef flat at Enewetak is a fairly level 

 rock surface that may be divided into two rather distinct 

 parts: (1) a barren rock surface that appears to be the ero- 

 sional surface of an older reef and (2) a rock substrate with 

 a thin veneer of organisms, primarily the articulate red 

 alga, Jania, giving the surface an appearance of being 

 covered by a mat which Smith and Kinsey (1976) dubbed 

 the "algal-turf." 



Tracey and Udd (1974) and Buddemeier et al. (1975) 

 present evidence to suppxsrt a higher-than-present sea level 

 between 4000 to 2200 ybp. This higher sea level may 

 have been 1 m or more greater than the present. The ero- 

 sional nature of the present reef flat is postulated to be 

 due to the lowering of sea level to near its present datum 

 around 2000 ybp. Tracey and Ladd (1974) support their 

 hypothesis with age dates of planed coral heads in the 

 present windward reef flat seaward of Runit and Aomon 

 Islands. Additional evidence is provided by Buddemeier et 

 al. (1975), who through age dating show that much of the 

 windward reef flat seaward of Aomon Island is composed 

 of cemented subtidal deposits now present in an intertidal 

 zone, the result of a recently lowered sea level. Additional 

 evidence for a higher-than-present Holocene sea level 

 around 4000 ybp for other Pacific islands may be found in 

 Curray et al. (1970) and Chappell and Veeh (1978). 



Despite its apparent erosional character, the present 

 windward Enewetak reef flat is a highly productive reef 

 environment in terms of the mass of carbonate sediments 

 produced (Smith and Harrison, 1977). The algal-ridge, 

 coral-algal zone, and the reef flat compose what is termed 

 the "reef plate" (Henny et al., 1974). The reef plate con- 

 sists of well-cemented rock resulting from penecontem- 

 poraneous biologic and marine cementation. 



During PACE and EXPOE, several holes were drilled 

 on the reef plate seaward of Aomon and Runit Islands. 

 These holes, in addition to the outcrops exposed in quar- 

 ries on the Enewetak, Medren, Runit, and Engebi (Enjcbi) 

 reef flats and the outcrops exposed in the LaCrosse 

 nuclear crater on the Runit reef flat, show that the Holo- 

 cene reef plate is a lagoonward prograding wedge of well- 

 cemented sediments overlying unconsolidated subtidal car- 

 bonate sands and gravels. The seaward edge of the wedge 

 begins approximately at the reef plate /coral-algal zone 

 boundary. Within the shallow Quaternary subsurface, sedi- 

 ments beneath the coral-algal zone appear to be continu- 

 ously well cemented with depth. Beneath the reef flat, the 

 thickness of the wedge tapwrs from 3 to 4 m at the center 

 of the reef flat to <1 m at the back reef /reef flat bound- 

 ary (Ristvet et al., 1977). 



The back reef is characterized by small to large solitetfy 

 coral heads of Pontes and Heliopora in a rocky to sandy 

 substrate. Little or no marine cementation app>ears to be 

 occurring, and the sands and silts have their origin from 



