SUBSIDENCE EVOLUTION OF THE MIDLAND BASIN CONSTRAINED BY ROBUST CHRONOSTRATIGRAPHY, AND IMPLICATIONS FOR PENNSYLVANIAN-PERMIAN BOUNDARY SUBSIDENCE STYLES ACROSS THE GREATER ANCESTRAL ROCKY MOUNTAINS

Subsidence patterns of ancestral Rocky Mountain (ARM) basins are often qualitative or poorly constrained with biostratigraphic data. Here, we present a robust chronostratigraphic evolution of the Midland Basin that is constrained by over 3000 biostratigraphic reports of fusulinid taxa. The position of the shelf-edge break was geometrically reconstructed along the Eastern Shelf to assess the roles that subsidence, eustasy and sediment supply played on shelf stacking patterns. These results demonstrate that shelf-edge trajectories were: 1) retrogradational and aggradational in the Pennsylvanian; 2) strongly progradational in the early Permian; and 3) aggradational in the early Leonardian. Constraints on sediment supply variation and eustatic variation indicates that the dominate control on these shelf-edge patterns was subsidence. Thus, the Midland Basin experienced a pronounced lull in subsidence in the early Permian.

Elsewhere across the ARM, some basins (e.g., Paradox and Anadarko basins) record a change in the style of subsidence near the Pennsylvanian-Permian boundary. Pennsylvanian strata in those basins accumulated from accommodation created through slip on a basin-bounding fault. Conversely, early Permian strata accumulated in those basins and atop the adjacent Precambrian-cored uplifts (strata accumulation approached 1 km in thickness on uplifts) suggesting that regional rather than localized subsidence affected a basin and uplift yoked pair. The Midland Basin appears to experience a similar pattern, but slightly younger or more protracted, to those other basins. The discrepancy could be the result of poor age control for the other ARM basins or dynamic migration of this subsidence pattern. Regardless, the results are not consistent with long-lived erosional beveling model for the demise of the ARM.