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New Insight Into the Shark Bay Microbial System, Part 2: Carbonate Factory


Our recent comprehensive mapping program in the Hamelin Pool embayment of Shark Bay has revealed extensive microbial precipitation that results in accretion of stromatolites and carbonate sediment production. Microbial precipitates are abundant in many stromatolites, in some cases forming a framework structure. Irregular micritic grains and peloids, the most common grain types in the sediment of Hamelin Pool, appear to be derived by the erosion of microbial mats on the Pool margins. Square kilometers of gelatinous microbial mat colonizes the subtidal platform on the southwest margin of the Hamelin Pool, often forming low relief microbial buildups. Gelatinous mats are characterized by thin, brittle layers of micrite. As the gelatinous mats are eroded and/or decay, these layers are broken down into irregular micritic grains. Irregular micritic grains produced within gelatinous microbial mats are an important component of the depositional environment, comprising an average of about 40%, and in some cases as much as 67%, of the total sediment. Furthermore, erosion of extensive fields of weakly lithified pustular mats appear to be a prolific source of peloidal grains. Hamelin Pool is thus recognized as a major microbial carbonate factory, with constructive processes leading to stromatolite formation, and erosion and degradation of microbial mats leading to massive production of sand-sized micritic grains. Local production of sand-sized micritic grains within the Hamelin system has major, direct applicability to the understanding of microbial deposits in the geologic record in terms of sedimentary budget, accumulation rates, distribution, etc. Indeed, microbial production of sand-sized micritic grains may be important in the formation of sedimentary deposits in the Pre-Salt of Brazil, as well as other subtidal micritic and peloidal limestones.