Abstract

Cyclic units of late Pennsylvanian age on the Eastern Shelf of the Midland Basin were deposited under conditions of repeated transgression and regression, involving major changes in water depth. Carbonates and terrigenous clastics were deposited within different depth zones, but the locations of these zones migrated across the shelf in response to changes in sea level. Cycles are lithologically varied, but those with lithologic symmetry between transgressive and regressive intervals (regardless of thickness symmetry) provide the clearest indication of depositional environments, and can be used as a standard to interpret nearly all cyclic units. The lithologic patterns formed fit a model of deposition under regularly fluctuating sea levels on a ramp-topped shelf (distally-steepened ramp), rather that delta control on a platform shelf. Sea level curves for the late Pennsylvanian and Permian can be generated from these sequences, and used for basin-wide correlation. Sea level control also provides a superior means of understanding depositional patterns and sequences on the Eastern Shelf.