--> Calcite Concretions in a Miocene Mixed System: Geochemical, Sedimentologic and Stratigraphic Characterization

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Calcite Concretions in a Miocene Mixed System: Geochemical, Sedimentologic and Stratigraphic Characterization

Abstract

The late Miocene mixed system in the Cibao Basin of the Dominican Republic contains calcite-cemented concretions that formed near the seafloor very early in the post-depositional history. Concretions occur within the inner to middle shelf siliciclastic-rich sediments. Siliciclastic and skeletal grains are cemented by primary, low-magnesium calcite marine cement (10–30 micron crystals), whereas aragonite or high-magnesium calcite cements dominate in tropical carbonates. Stable C and O isotope values of concretion calcite cement (-10 to -30‰ carbon: -2 to -5‰ oxygen) are consistent with formation in pore water where alkalinity was generated from oxidation of organic matter by sulfate ions in the zone of bacterial sulfate reduction. Oxidation of organic matter was relatively slow, and precipitation of marine low-Mg calcite was likely diffusion controlled. A mechanism for Mg2+ uptake was likely active in some form to limit the inhibitory effect of Mg2+ and allow low-Mg calcite precipitation. One possible mechanism for decreasing the Mg/Ca ratio may be the absorption of Mg2+ by clay minerals or other small grains. The clay mineral chlorite, common in the Cibao Basin and in many siliciclastic marine sediments, may be able to "fix" Mg2+ by absorption or ion exchange. Most concretions form along cycle boundaries, perhaps when deposition rate is low or during periods of non-deposition. Concretions also form as isolated bodies within the 1–2 m thick depositional cycles. Pervasive calcite cementation can form nearly continuous thin beds (5–20 cm thick) that can have anomalously low porosity (flow baffles) and high acoustic velocities relative to adjacent sediments at this early stage of burial.