Diagenesis of Cenozoic Limestones on Enewetak Atoll (Dolomite, Neomorphism, Radiaxial).

Abstract

Cores from Enewetak Atoll afford an excellent opportunity to study diagenetic alteration in carbonates from the modern sediment-water interface to burial depths of over 1,300 m. Intensity of diagenetic alteration in Cenozoic carbonates on Enewetak is largely a function of time which strata have been in diagenetic environments with active water circulation. In Pleistocene strata deposited in back-reef environments, diagenesis is dominated by meteoric processes including dissolution of aragonite, calcite cementation, and alteration of aragonite and high-magnesian calcite (HMC) to low-magnesian calcite (LMC). Petrographic and stable isotope data suggest that aragonite neomorphism occurred by a process of partial delicate intrafabric dissolution of aragonite followed by calcite infilling. Meteoric alteration in Pleistocene strata is intense both in intervals immediately below subaerial exposure surfaces and in intervals affected by meteoric-phreatic environments for significant amounts of time. Cenozoic carbonates (including some Pleistocene strata), away from exposure surfaces and out of ancient meteoric-phreatic zones, generally exhibit little meteoric alteration. Marine diagenesis on Enewetak is most intense in permeable strata near the seaward margin of the atoll. In Lower Miocene strata of the Enewetak F-1 well (375 - 850 m), extensive aragonite dissolution and radiaxial calcite cementation occurred in relatively deep marine waters circulating through the atoll. These radiaxial calcites apparently precipitated directly from marine water without any subsequent neomorphism. Dolomite is common in deeper parts of Upper Eocene strata on Enewetak Atoll (1,150 - 1,380 m deep). Petrography, stable isotope analyses, and strontium isotope analyses consistently indicate dolomitization probably occurred in deep marine waters between the Middle Miocene and the present. The products of marine diagenesis within Enewetak Atoll vary according to depth and carbonate saturation. In shallow marine waters above the HMC and aragonite saturation depths, diagenesis consists primarily of HMC and aragonite cementation. In actively circulating marine waters between the aragonite and calcite saturation depths (300 - 1,000 m), aragonite dissolution and radiaxial calcite cementation are pervasive. Below the calcite saturation depth (1,000 m), calcite dissolution and dolomitization have apparently occurred in marine waters.