RECONSTRUCTING MIDDLE PENNSYLVANIAN SEDIMENT PATHWAYS ACROSS A TECTONICALLY ACTIVE BASIN MARGIN: A SUBSURFACE STUDY OF “GRANITE WASH” CLAST COMPOSITION AND COARSE-GRAINED LITHOFACIES IN WASHITA AND BECKHAM COUNTIES, ANADARKO BASIN, OKLAHOMA

Abstract

During the late Paleozoic assembly of Pangea, regional compression and transpression resulted in uplift and unroofing of rift-basin fill and basement of the Oklahoma Wichita Igneous Province (WIP), rapid basin subsidence, and accumulation of greater than 5,400 meters of Pennsylvanian sediments deposited in the deep Anadarko basin. Although Pennsylvanian “Granite Wash” sediments and thickness trends have been intensely studied for reservoir characterization, less attention has been given to the source-to-sink relationships and the variations in source catchments and sediment delivery along the southern margin of the Anadarko Basin. Additionally, published literature which documents evolution of clast composition and thereby, unroofing of Pennsylvanian uplifts, is sparse. This study addresses these knowledge gaps by comparing coarse-grained lithofacies, clast size and composition, and thickness trends within Atokan, Desmoinesian, and Missourian deposits in Beckham County and Washita County, OK, to make inferences regarding middle Pennsylvanian tectonic unroofing, sediment routing, and paleogeography. Our results identified temporal and spatial variation in depositional mechanisms, paleoenvironments, catchment configuration, and source area lithologic diversity, suggesting that deposition along the Wichita Uplift is best represented by multiple depositional models and that source-to-sink configurations evolved throughout the Pennsylvanian. As compared to previously proposed models, this study shows that a single depositional model does not adequately explain the depositional variation observed within “Granite Wash” deposits in both Beckham County and Washita County, through the Atokan to Missourian time interval. In Beckham County, Atokan-aged deposits are interpreted as sandy fan-deltas where the adjacent shelf is narrow or absent. By the Desmoinesian and Missourian, the depositional system transitions to smaller subaqueous deltas derived from localized catchments, characterized by ephemeral hyperpycnal flows and reduced mud content. In contrast, Atokan deposits in Washita County represent muddy fan-deltas likely sourced from a mud-rich shelf. During the Desmoinesian and Missourian, Washita County hosted a large subaqueous delta system fed by sustained hyperpycnal flows from a broad regional catchment which included the nearby wave-influenced marine shelf with an active carbonate factory. Clast modal analysis revealed “Granite Wash” conglomerates contain sedimentary, volcanic, and plutonic clasts, with variations in relative proportions through the Atokan to Missourian. Atokan conglomerates are dominated by volcanic lithologies associated with Cambrian rift tectonics, whereas the increasing proportion of plutonic clasts in Desmoinesian and Missourian conglomerates from both counties reflect further erosion and unroofing of the WIP basement along the Wichita-Amarillo Uplift. However, Washita County contains a wider range of mafic and felsic igneous clast lithologies as compared to Beckham County, especially within Missourian deposits. This suggests source catchments of the Washita system contained more diverse igneous WIP lithologies, and potentially increased uplift and unroofing of WIP basement in Washita County as compared to Beckham County, Oklahoma. The results of this study refine our understanding of the middle Pennsylvanian regional paleogeography and sediment routing across the transpressional southern basin margin of the Anadarko Basin, while documenting topographic variations in the unroofing of the Wichita Igneous Provence basement.