Contemporaneous Reconstruction and Cementation of Living Lagoonal Patch Reef, Enewetak Atoll, Marshall Islands: ABSTRACT

A patch reef on the sheltered side of the lagoon at Enewetak, a Pacific oceanic atoll, was sampled from the living coral-algae veneer near the water surface to the lagoon floor at -7 m water depth. The reef was exposed by selective blasting, and samples were studied by slabbed sections, thin sections, and the scanning electron microscope. The reef is repeatedly and pervasively cemented by crystal aggregates of aragonite and magnesium calcite and by microcrystalline magnesium calcite, and does not show marked preferential control by host mineralogy. Cements (1) fill skeletal cavities, (2) fill borings in both skeletal elements and other early cement, and (3) bind, as mud-sized magnesium calcite, fine-grained bioclastic debris that accumulates in voids of all sizes and origins. The distribution of both cements and cavities is extremely variable, as is cavity size and geometry. Drilled plugs, from the reef core, had porosities of 12.7 to 36.5% and permeabilities of 135 to 32,843 md. Externally, the reef appears to be a porous boundstone; internally, it changes to wackestone and packstone. Contemporaneous and repeated penetration by micro- and macro-borers continually obliterate original boundstone framework, replacing it with cemented cavities and debris; conversion to wackestone and packstone continues as long as the reef is exposed to seawater. The diagenetic process results in transformation of the reef from delicate framework to compact limestone, from porous elements to dense rock, and from a large proportion of skeletal aragonite to apparently inorganic aragonite and crystalline and microcrystalline magnesium calcite. It also accounts for the difficulty of identifying the framework core of some fossil reefs. Despite the effects of pervasive boring, submarine cements and cemented debris maintain and increase the strength of the reef, which has withstood storms, typhoons, and nuclear blasts. End_of_Article - Last_Page 800------------