In 2011 and 2012, I have worked on microbial oncolite deposits to better understanding how microbial processes influence rock textures with the characterization of cyanobacterial formed grains. These coated, concentric grains cap a large carbonate sequence on the atoll of Maré, New Caledonia. Formed during the late Pliocene (~2.8 Mya), these grains represent some of the final stages of atoll formation and marine deposition. By characterizing the chemical and physical attributes of these microbial carbonates, the goal of the project is to provide a better understanding of their formation and environment of origin. Physical and isotopic descriptions of the three layers of the coated grains have been collected and analyzed in order to formulate a model of formation and diagenetic alteration. This paper is published in JSR - Journal of Sedimentary Research in 2015.
ABSTRACT: 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 Mare ́, 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 mm) 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 Mare ́ 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 d13C values in marine carbonates, and potential controls on fluid flow.