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After deposition as a coastal marine deposit, the Silurian Keefer Sandstone underwent an elaborate diagenetic sequence. In general, the processes were: quartz cementation, calcite cementation, dolomitization, and finally dedolomitization. Diagenesis in this formation, as interpreted from petrographic study, was complex because the composition of pore fluids changed throughout its postdepositional history. Quartz overgrowths precipitated early, most likely from marine connate brines. The second generation of cement, poikilotopic calcite, precipitated when meteoric-derived phreatic water percolated through the sand during a major regression of the sea. Clay minerals in the matrix favored dolomitization of calcite cement, fossils, and calcite mud by serving as sites of nucle tion or catalysts and perhaps by providing magnesium. Later, at a moderate burial depth, saline subsurface brines dedolomitized isolated dolomite crystals.
Primary porosity in the Keefer (Big Six) of southwestern West Virginia was virtually occluded by these early cements. Most microscopically visible porosity is secondary and attributed to the volume reduction of carbonate material with dolomitization of local calcite. Several Silurian-Devonian sandstones are petrographically similar to the Keefer, and its postdepositional history, as interpreted here, may serve as an example of diagenesis in other dolomitic and calcareous quartzarenites of the Appalachian basin.
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