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Deposition and Diagenesis of Marine Oncoids: Implications For Development of Carbonate Porosity
Microbially mediated carbonates form in numerous environments and produce a range of lithologic fabrics and textures. While the environmental and biological factors controlling microbial carbonate precipitation are becoming more established, the influence of precursor microbial textures on diagenetic processes is less understood. Here we describe microbial oncolites from Maré, New Caledonia, to assess their formation, textures, and subsequent diagenetic alteration. Calcified filaments and sheets are interpreted as microbial-mat constituents (bacterial sheaths and EPS sheets) within a constructive micritic cortex of uniform crystal size (5–10 μm) and shape formed around a nucleus grain. Thin-section analyses show trapped and bound grain inclusions and irregular laminations within the cortex, also consistent with a microbial origin. The combination of a dense micrite cortex, subrounded shape, and irregular laminations are consistent with oncoid formation under periodic wave agitation in a back-reef setting. The Maré oncolites also display an unexpected pseudomorphic stabilization of the nucleus grains and cortex, likely due to the original mineralogy and low permeability of the cortex micrite. To our knowledge, this is the first example of microbial coatings leading to pseudomorphic stabilization of aragonite and high-Mg calcite skeletal grains. This textural preservation lies in stark contrast to the extensive dissolution and moldic porosity found outside the oncolite beds and common to many tropical shallow-water carbonates. These findings have implications for development of meteoric porosity, the interpretation of δ13C values in marine carbonates, and potential controls on fluid flow.
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