PALEOTHERMOMETRY AND THERMOCHRONOLOGY OF CARBONIFEROUS STRATA, CENTRAL APPALACHIAN BASIN, SOUTHERN WEST VIRGINIA: BURIAL AND GEOMORPHIC EVOLUTION OF THE APPALACHIAN PLATEAU

Multiple thermometric and thermochronologic techniques were used to constrain the burial and exhumation histories of Carboniferous sandstones in the central Appalachian basin. Core-collected sandstones from New River and Glenshaw formations (Lower and Upper Pennsylvanian, respectively) were recovered from 100 to 400 meters present depth. Fluid inclusion microthermometry, vitrinite reflectance, and apatite fission track dating document a comprehensive thermal history of these sandstones, from deposition through maximum burial to early exhumation. Fluid inclusions in quartz cement and vitrinite reflectance data from surrounding organic-rich intervals were studied from the core samples, whereas apatite fission track dates were compiled from published sources. Radiogenic helium dating ((U-Th)/He) will be used to constrain the final stages of exhumation.

Fluid inclusion and vitrinite reflectance data indicate maximum burial depths exceeded 4 kilometers (approximately 145 °C) during the Permian (estimated burial rate: 100 m/m.y.) for lower Pennsylvanian sandstones. Published apatite fission track dates indicate slow exhumation (~13 m/m.y. to a depth of 3 km) from the Permian to mid-Mesozoic. Radiogenic helium ages are currently being analyzed and will constrain the time of cooling below a closure temperature of ~ 65°C (typical of slow cooling rates). This will provide control on the exhumation from ~ 2-3 kilometers depth.

Results from this work establish a framework that aids interpretation of sandstone diagenesis, because diagenetic processes are controlled by temperature, pressure, and residence time within certain depth/thermal windows. This study also addresses outstanding questions concerning the geomorphic evolution of the Appalachian orogen using (U-Th)/He techniques. Reconstructing the burial and exhumation histories using the thermometric and thermochronologic techniques described above, is possible only because coarse-grained detrital rocks, especially sandstone, are associated with all of the components analyzed, including quartz cement, organic matter (e.g., coal and black shale) and apatite.