Pennsylvanian-permian chronostratigraphy of the eastern Midland Basin: Implications for basin filling evolution and paleogeography
Garcia, Jill N.
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The Midland Basin of the Permian Basin Province in West Texas has proven to be a lucrative source of petroleum and natural gas since the 1970’s. Unfortunately, large-scale sub-surface maps of the region, especially for the eastern shelf of the basin, are not readily available. Fusulinid biostratigraphy has provided an opportunity to construct chronostratigraphic surfaces of several Pennsylvanian-aged and Permian-aged units in the Midland Basin, including the Atoka, Strawn, Canyon, Cisco, Wolfcamp, and Leonard. A collection of fusulinid occurrence reports compiled by R.V. Hollingsworth catalog fusulinid last appearance datums of genera characteristic of the units above and represent the main data in the Permian Basin Archival of Biostratigraphic Tops (PABZT) project. These datums were used to create paleogeographic maps of the Midland Basin from the Late Pennsylvanian through early Permian. The shelf edge of the eastern Midland Basin is delineated using these maps, through geometric reconstruction of the eastern shelf. Along each cross section a topographic profile was created, which revealed the quantitative position of the shelf edge during each time interval. The shelf edge position produced through the PABZT data is then compared against maps depicting coeval time frames that were mined from existing Permian Basin literature. The distances were divided against one another, and produced a ratio that determined the accuracy of the constructed shelf edges versus the literature’s shelf edges. For example, an accuracy ratio of 1.0 indicates exact shelf positions locations, less or greater than that indicated how much distance existed between the positions. Comparison of accuracy ratios showed a high degree of similarity between the Canyon and Cisco paleogeographic maps and the BEG’s picks. Variance from north-to-south along the eastern shelf may be the result of the loss of chronostratigraphic significance when lithologically correlating thicker packages of strata in the south to thinner strata units in the north. Analysis of the chronostratigraphic surfaces also revealed the progradational and aggradational trends along the eastern shelf. The northern and southern regions displayed dissimilar histories of shelf movement. The maximum amount of progradation observed between the two regions was an event that began during the Virgilian through the Wolfcampian and exhibited 80 kilometers of landward shelf movement. One aggradation pattern that was correlated between both areas was the Atokan to Missourian/Virgilian event. This event has been correlated to both the northern and southern regions, and indicates a regional transgression event. The eastern shelf experienced local progradational and aggradational events of similar magnitudes, and two major events were recorded in northern and southern locations. Most likely the significant progradational episodes during the Virgilian to Wolfcampian were the result of sediment supply increasing from the Llano Uplift, and waning tectonic activity. This resulted in the reduction of available accommodation space, especially along the eastern shelf. The aggradational events seen during the Atokan to Missourian/Virgilian are likely driven by a higher rate of basin subsidence in the Pennsylvanian. These results suggest greatly reduced subsidence near the Pennsylvanian-Permian boundary, a relationship observed elsewhere in late Paleozoic basins across western equatorial Pangaea.