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Transformation of Depositional Lime Mud to Microrhombic Calcite


Micropores are found in all limestone fabrics as intercrystalline pore space between microrhombic calcite crystals. Micropores are found within grains, in binding organism, and in mud-size carbonate fabrics. The focus in this study is on the origin of micropores in mud-sized carbonate. Most studies of micropores in mud-dominated carbonate fabrics focus on ancient examples and speculate on the origin using textural relationships and geochemical data. However, porous lime mud has a long and complicated diagenetic history starting with high porosity clay-size aragonite and calcite and ending with microrhombic calcite crystals with much reduced porosity. A critical step in this diagenetic history is the transformation of depositional carbonate mud, typically composed of micrometer to nanometer carbonate crystals, to larger calcite crystals. This study is focused on that critical transformation. The material used in this study is from the core taken from well Clino on the western slope of the Great Bahama Bank. Results of our study suggest that the initial phase of the transformation is a replacement process with dissolution of micrometer- to nanometer-sized aragonitic/calcitic precursor sediment and precipitation of 1- to 10-micron microspar crystals. No porosity is gained or lost during this process indicating that no carbonate is added to or subtracted from the system. Carbonate is added later in the form of porosity reducing cement.