--> ABSTRACT: Evolution of Reef and Atoll Margin Carbonates, Upper Eocene Through Lower Miocene, Enewetak, Marshall Islands, by Arthur H. Saller and Seymour O. Schlanger; #91030 (2010)

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Evolution of Reef and Atoll Margin Carbonates, Upper Eocene Through Lower Miocene, Enewetak, Marshall Islands

Arthur H. Saller, Seymour O. Schlanger

Two wells drilled along the margin of Enewetak Atoll penetrated approximately 1,000 m of upper Eocene, Oligocene, and lower Miocene carbonates. Strontium isotope stratigraphy indicates relatively continuous deposition of carbonate from 40 Ma to 20 Ma. Depositional environments show a gradual basinward progradation of facies with slope carbonates passing upward into fore-reef, reef, back-reef, and lagoonal carbonates. Slope strata contain wackestones and packstones with submarine-cemented lithoclasts, coral, coralline algae fragments, benthic rotaline forams, planktonic forams, and echinoderm fragments. Fore-reef strata are dominantly packstones and boundstones containing large pieces of coral, abundant benthic forams, coralline algae fragments, stromatoporoids(?), and min r planktonic forams. Reef and near-reef sediments include coralgal boundstones and grainstones with abundant benthic forams. Halimeda and miliolid forams are common in lagoonward parts of the back reef. Sponge borings, geopetal structures, and fractures are common in reef and fore-reef strata. Lagoonal strata are wackestones and packstones with common mollusks, coral, coralline algae, and benthic forams (rotaline and miliolid). Diagenesis has extensively altered strata near the atoll margin. Aragonite dissolution and calcite cements (radiaxial and cloudy prismatic) are abundant in fore-reef, reef, and some back-reef strata. Petrographic and geochemical data indicate aragonite dissolution and calcite cementation in seawater at burial depths of 100 to 300 m. Dolomite occurs in slope and de ply buried reefal carbonates. Most dolomitization occurred at burial depths of more than 1,000 m in cool marine waters circulating through the atoll. Lagoonal strata are not significantly altered by marine diagenesis and still contain abundant primary aragonite and magnesium calcite.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.