Deposition of Enewetak Atoll Reef, Middle Pleistocene to Holocene: ABSTRACT

Abstract

Cyclic deposition and diagenesis of a large atoll reef, associated with major eustatic sea-level fluctuation, is documented in six cores from the northeast (windward) reef of Enewetak Atoll. To 250 ft (76 m) depth, the reef section comprises six primary depositional packages that represent periods of reef growth during Pleistocene and Holocene high sea-level stands. These depositional packages are separated by unconformities, former subaerial exposure surfaces, which are demarcated by paleosols. During emergent periods at low sea-level stands, the reef was subjected to meteoric-water diagenesis. However, the depositional textures generally are well preserved. Diagenetic textures, most indicative of meteoric-vadose diagenesis, usually have only partly replaced original tex ures. An oceanward shift of reef environments through time is apparent in a large-scale view of the cores. Reef crests in succeedingly younger depositional packages apparently are offset oceanward. Younger marginal lagoonal deposits unconformably overlie older backreef deposits that unconformably overlie older reef-crest deposits. Within depositional packages, influence of rising sea level on facies development is evident. Subaerial surfaces flooded by rising sea level were rapidly colonized by both solitary corals and patch reefs of diverse composition. Rapid upgrowth of oceanward reef crests led to moderation of wave energy in backreef areas and great increase in deposition of sediment which locally buried and killed patch reefs. Shoaling of backreef areas by sediment aggradation to a tabilized sea level resulted in renewed development of patch reefs and deposition of coarser sediment. End_of_Article - Last_Page 713------------