| Summary |
The Upper Mississippian Little Valley Limestone is a natural gas producing unit of mixed carbonate and siliciclastic composition transitional between largely terrigenous units below and carbonate units above. It was deposited during the inundation of the Virginia-Carolina delta complex. The Little Valley Limestone is approximately 220 meters thick in the study area, which is located in the Greendale Syncline in Washington County, Virginia. Petrographic point count data, geophysical logs and detailed hand sample characterization were used in both environmental and diagenetic determinations. Seven microfacies are recognized: (1) lime mudstone, (2) lime wackestone, (3) lime packstone, (4) lime grainstone, (5) calcareous muddy sandstone, (6) calcareous mudrock and (7) secondary gypsum. These microfacies were deposited during a marine transgression and represent three distinct environments. They are, in ascending stratigraphic order: (1) restricted, high intertidal mudflat to bordering supratidal sabkha containing algal-laminated evaporites (gypsum and anhydrite) and dolomitic shales; (2) low intertidal to shallow subtidal, siliiciclastic shoal containing interbedded calcareous-sandstones and raudrocks with a sparce biota; and (3) open marine, subtidal, with shelf lagoons characterized by lime- mudstones, wackestones, packstones and grainstones with an abundant stenohaline fauna. Diagenetic alteration of this unit consists of micritization, calcite and aragonite stabilization, microspar development, syntaxial overgrowths, and bladed and blocky spar cement formation, framboidal pyritization, dolomitization, silicification, secondary gypsum formation, stylolitization and vein and microfault formation. |
