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Geologic Reconnaissance of Natural Fore-Reef Slope and a Large Submarine Rockfall Exposure, Enewetak Atoll

Robert B. Halley, Richard A. Slater

In 1958 a submarine rockfall exposed a cross section through the reef and fore-reef deposits along the northwestern margin of Enewetak Atoll, Marshall Islands. Removal of more than 108 MT of rock left a cirque-shaped submarine scarp 220 m high, extending back 190 m into the modern reef, and 1,000 m along the reef trend. The scarp exposed older, steeply dipping beds below 220 m along which the rockfall detached. We sampled this exposure and the natural fore-reef slope surrounding it in 1984 and 1985 using a manned submersible.

The natural slope in this area is characterized by three zones: (1) the reef plate, crest, and near fore reef that extends from sea level to -16 m, with a slope of less than 10°, (2) the bypass slope that extends from -16 to -275 m, with slopes of 55° decreasing to 35° near the base, and (3) a debris slope of less than 35° below -275 m. Vertical walls, grooves, and chutes, common on other fore-reef slopes, are sparse on the northwestern slope of Enewetak.

The scarp exposes three stratigraphic units that are differentiated by surficial appearance: (1) a near-vertical wall from the reef crest to 76 m that appears rubbly, has occasional debris-covered ledges, and is composed mainly of coral; (2) a vertical to overhanging wall from -76 m to -220 m that is massive and fractured, and has smooth, blocky surfaces; and (3) inclined bedding below -220 m along which the slump block has fractured, exposing a dip slope of hard, dense, white limestone and dolomite that extends below -400 m.

Caves occur in all three units. Open cement-lined fractures and voids layered with cements are most common in the middle unit, which now lies within the thermocline. Along the sides of the scarp are exposed fore-reef boulder beds dipping at 30° toward the open sea; the steeper (55°) dipping natural surface truncates these beds, which gives evidence of the erosional nature of the bypass slope.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.