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Significance of Microbial Binding in Silurian Forereef Slope Deposits at Pipe Creek Jr. Quarry, Indiana


Silurian (Cayugan) forereef slope deposits exposed at Pipe Creek Jr. Quarry, Indiana are characterized by steeply dipping beds (35-45°) consisting of grainstone facies with abundant syndepositional abiotic marine cement. In many instances, the early stabilization of steep carbonate slopes has been previously attributed to this syndepositional abiotic marine cementation. However, recent studies propose that extensive microbial binding can be another significant factor. Microbial binding likely facilitates abiotic cementation by partially stabilizing the sediments, and together allows for the development and preservation of steep carbonate slopes. The combination of microbial and abiotic early cements may lead to an early reduction in primary porosity and permeability which may adversely affect subsequent reservoir development and preservation during burial. Comparison of interpreted depositional processes and slope geometry in the Silurian example to those described in subsurface examples including the supergiant Tengiz Field in Kazakhstan and the modern from Tongue of the Ocean in the Bahamas, shows similarity in terms of slope declivity, bed geometry, and the apparent presence of pervasive abiotic marine cements and microbial cements. The aim of this study is to quantify the contribution of microbial binding to the stabilization and potential reservoir modification in the Silurian forereef slope deposits exposed at Pipe Creek Jr. Quarry and compare with the subsurface Silurian reefs of the Michigan Basin.

Initial petrographic analysis reveals an abundance of syndepositional abiotic marine cements with varying morphologies as well as microfabrics indicative of early microbial binding such as asymmetric micritic crusts, trapping and binding structures and dense clotted micritic masses. Anticipated results with further analysis will provide insight into early reduction of porosity and permeability due to early abiotic marine cementation and microbial binding, identify if microbial binding precedes abiotic marine cements and provides a suitable substrate for later abiotic marine cementation, help to explain the early lithification and evolution of carbonate slopes, and further develop the fundamentals of sedimentology and diagenesis of Silurian (Niagaran) reefs in and around the Michigan Basin.