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Early diagenesis of shallow-water periplatform carbonate sediments, leeward margin, Great Bahama Bank (Ocean Drilling Program Leg 166)

¹ Ocean Drilling Program and Department of Geology and Geophysics, Texas A&M University, 1000 Discovery Drive, College Station, Texas 77845, USA
² Department of Oceanography, Texas A&M University, College Station, Texas 77845, USA
³ Department of Earth Sciences, Oxford University, Oxford QX1 3PR, UK

Mineralogic, petrographic, and geochemical analyses of sediments recovered from two Leg 166 Ocean Drilling Program cores on the western slope of Great Bahama Bank (308 m and 437 m water depth) are used to characterize early marine diagenesis of these shallow-water, periplatform carbonates. The most pronounced diagenetic products are well-lithified intervals found almost exclusively in glacial lowstand deposits and interpreted to have formed at or near the seafloor (i.e., hardgrounds). Hardground cements are composed of high-Mg calcite (14 mol% MgCO₃), and exhibit textures typically associated with seafloor cementation. Geochemically, hardgrounds are characterized by increased ¹⁸O and Mg contents and decreased ¹³C, Sr, and Na contents relative to their less lithified counterparts.

Despite being deposited in shallow waters that are supersaturated with the common carbonate minerals, it is clear that these sediments are also undergoing shallow subsurface diagenesis. Calculation of saturation states shows that pore waters become undersaturated with aragonite within the upper 10 m at both sites. Dissolution, and likely recrystallization, of metastable carbonates is manifested by increases in interstitial water Sr and Sr/Ca profiles with depth. We infer that the reduction in mineral saturation states and subsequent dissolution are being driven by the oxidation of organic matter in this Fe-poor carbonate system. Precipitation of burial diagenetic phases is indicated by the down-core appearance of dolomite and corresponding decrease in interstitial water Mg, and the presence of low-Mg calcite cements observed in scanning electron microscope photomicrographs.

Key Words: Bahamas • carbonates • diagenesis • hardground • Ocean Drilling Program • periplatform

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