PETROGRAPHIC ANALYSIS OF SANDSTONES IN THE TRIASSIC WADESBORO SUBBASIN, DEEP RIVER BASIN, NORTH CAROLINA

The Wadesboro subbasin (Upper Triassic) is the southernmost member of the Deep River Basin, Newark Supergroup, and is exposed for ~75km trending northeast-southwest from the North Carolina/South Carolina border to near Candor, NC. At its widest extent the basin is ~17km and is composed of conglomerates, mudstones, shales, siltstones, and sandstones dipping an average of 15-20° to the southeast. The sediments of the Wadesboro subbasin have been deposited in an asymmetric rift basin with a major border fault system on the eastern margin and smaller subparallel normal faults common on the western margin. The central portion of the subbasin is dominated by axial meandering stream deposits and their associated overbank deposits. Lacustrine facies, i.e., organic rich shales, are laterally and vertically restricted suggesting they were deposited in shallow, short-lived floodplain ponds in contrast to basin scale lakes identified in other Newark Supergroup basins, e.g., Dan River/Danville, Newark, and others. Basin margin facies are alluvial fan deposits composed of both matrix supported and clast supported conglomerates on the western margin while no conglomerates are exposed on the eastern border fault system. Previous studies of fine grained sedimentary rocks in the Wadesboro subbasin, i.e., siltstone and mudstone, suggest a drying climate during synrift deposition that is reflected in a decrease in the abundance of residual minerals (i.e., kaolinite, illite, hematite, and goethite) through time. A petrographic analysis of sandstones in the basin can be used to determine: 1) the relative sediment transport distance from particle roundness and sphericity; 2) mineralogy that may reflect changes in climate and/or sediment provenance; and 3) rock fabric, texture, and pore morphology and distribution can be used to examine temporal changes in diagenesis within the basin. Sandstones were selected from across the central portion of the basin, perpendicular to basin trend, to identify temporal and spatial variations in sandstone characteristics which will aid the interpretation of the geologic history and depositional environments of the Wadesboro subbasin.