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ABSTRACT: Mineralogy and Geochemistry of ^dgr13C-Depleted Carbonates in Offshore Trinidad

W. Fred Falls, D. F. Williams

^dgr13C records from the fine fraction (< 63 µm) of well cuttings provide an important tool for detecting stratigraphic intervals that derive a stable isotopic signal from authigenic carbonates. The purpose of this study is to evaluate the mineralogy and geochemistry of the authigenic carbonates responsible for a significantly depleted interval in a ^dgr13C fine-fraction record for the Pliocene-Pleistocene deposits of offshore Trinidad. To achieve this goal, the coarse fraction (> 63 µm) of the well cuttings are examined and analyzed. The ^dgr13C-depletion event in the fine-fraction record corresponds to a carbonate-cemented sand with extremely depleted ^dgr13C values of -25 to -58^pmil (PDB). Calcite with 11 to 4 mole % MgCO3 forms equant spherulitic crystals, which, when polished and viewed with reflected light, display several growth stages. Dolomite cement found in one sample does not have growth stages that are recognizable by similar means. Both types of cement have extremely depleted carbon isotope values and ^dgr13C values between 0.3 and 1.9^pmil (PDB). The stable isotope data suggest that the Trinidad shelf during the Pliocene-Pleistocene was a site of carbonate cementation either at the sediment/water interface or under shallow burial where pore-water conditions were strongly influenced by the oxidation of methane.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990