Abstract

An Edwards reef, exposed in a large quarry in Scurry County, Texas, exhibits the following biofacies, outward from the reef core: 1) caprinid - Chondrodonta zone (reef core), 2) Eoradiolites - Chondrodonta zone (upper forereef), 3) Toucasia - Eoradiolites - Chondrodonta zone (middle forereef), 4) Cladophyllia zone (lower forereef). Beyond the forereef and below the reef are tabular biostromes in which Eoradiolites, Toucasia, Chondrodonta and Cladophyllia are dominant. The Edwards reef was subaerially exposed soon after deposition, and stabilization of carbonate mineralogy, lithification, solution, of aragonite fossils and differential cementation occurred in a fresh water diagenetic environment. Microcavernous porosity developed in the upper Edwards from solution of aragonitic caprinid and corm shells of the reef core. Adjacent to large caprinid molds former body cavities of Eoradiolites are filled by coarse calcispar cement which initiated as overgrowths on large crystals of inverted (from aragonite) calcite of the inner shell layer. Primary porosity (body cavities of calcitic pelecypods) became occluded by calcite cement, while secondary porosity (molds of aragonitic fossils) was preserved. A sharp boundary, representing a paleo-groundwater table, separates the upper paleovadose zone of microcavernous porosity from a lower paleophreatic zone wherein all former voids, including caprinid molds, became occluded by calcispar cement. A heavy concentration of vadose internal sediment terminates at the palco-groundwater table. Coarse aggrading recrystallization of micrite matrix to pseudospar is well developed in the paleophreatic zone, but is lacking in the paleovadose zone. Differential porosity occlusion was largely controlled by vadose-phreatic groundwater dynamics.

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