| Summary |
The Mississippian (Meramecian) Hillsdale Limestone consists of gray, medium-bedded to massive, cherty skeletal wackestone with minor interbedded lime mudstones, packstones, grainstones, and terrigenous siltones and shales. It was deposited during a transgressive onlapping of the underlying Little Valley Limestone, allowing for the development of a homoclinal carbonate ramp with a discontinuous offshore shoal sequence. The Hillsdale Limestone is approximately 90 meters thick in the study area, which is located in the Greendale Syncline in Washington County, Virginia. Petrographic point count data, measured outcrop sections, stable isotopic data, and trace element data were used for depositional and diagenetic interpretations. Four lithofacies are recognized: (1) shallow-subtidal siliciclastics, (2) lagoonal pelloidal wackestones and lime mudstones, (3) shoal ooid and echinoid grainstones, and (4) outer ramp skeletal packstones and wackestones. The outer ramp facies is the most abundant facies, accounting for 73.9% (204.5m) of the total combined stratigraphic thickness (276.9m). Skeletal wackestones are the most abundant lithology. A total of four shallowing-upward sequences can be recognized in the Hillsdale Limestone. Petrographic evidence indicates a complex diagenetic history. Early diagenetic events include: micritization and pyritization in stagnant marine environments in stagnant marine environments, and isopachous rim cementation and hematite formation in active marine environments. Localized meteoric-vadose environments resulted in limited formation of a meniscus cement. With encroachment of meteoric groundwater, the presence of a mixing zone resulted in dolomitization. With establishment of freshwater conditions, the meteoric-phreatic environment resulted in aragonite leaching, magnesium calcite stabilization, microspar formation, sytaxial cementation, blocky and drusy pore-filling cementation, and calcification. Compaction, sutured grains, stylolitization, and fracturing occurred throughout the burial of the Hillsdale Limestone. Textural relationships of pore-filling silica cements (within a rugose horn coral) indicate silicification occurred after dolomitization and before calcification. This indicates that this silicification occurred in the meteoric-phreatic environment. The diagenetic origin of chert nodules in the Hillsdale Limestone is indicated by the partial replacement of fossils within the chert nodules. Trace element and stable isotopic data of selected fossils indicate that brachiopods underwent only minor alteration in a partially closed system. Since brachiopods secret their shell in isotopic equilibrium with seawater, the good chemical preservation of the brachiopods allows for an estimate of the stable isotopic composition of Meramecian seawater. Comparison of the oxygen isotopic ratios of the brachiopod and silica cement indicates that silicification may have occurred in isotopically light (-6.5 SHOW) meteoric-phreatic waters. |
