Evolution of Permian Carbonate Shelf and Foreshelf Detrital Systems, Midland Basin, Texas: ABSTRACT

Abstract

A major phase of shelf progradation and consequent filling of the Midland basin began in the Early Permian. Prior to this time, shelf carbonate systems were mostly ineffective in generating coarse detritus in quantities sufficient to infill adjoining basins. Beginning in the Early Permian (Wolfcampian), however, the character of shelf and basin deposition was abruptly modified, in part because of faunal changes - proliferation of rock-forming biotic communities and the consequent potential for rapid shelf-marginal oversteepening - and the effects of sea level fluctuations and periods of shale influx. Shelf systems in the northern Midland basin and Eastern shelf evolved from ramps in the lowermost Wolfcamp to steep rimmed platforms by the lower Leonard. Shelf progradation into the Midland basin was cyclic, with extensive carbonate shelves having developed during sea level highstands and massive shale wedges deposited during alternate lowstands. Coarse megabreccia with markedly little sand dominates in the lower to middle Wolfcamp systems, despite the occurrence of ramps in this section. Such an anomaly can be explained by invoking widespread shelf wastage caused by the inherent instability and failure of the shale slopes on which these ramps were deposited. In certain middle Wolfcamp zones, thin shelf-ramp sections may be more » recognized only because any thick shelf-marginal buildups that may have been present have been displaced into adjoining basinal tracts. The evolution to rimmed-platform shelves by the lower Leonard strata, foreshelf debris occurs mostly as complexes of megabreccia wedges and aprons, anastomosing megabreccia, and sand-channel and lobe deposits derived from adjoining rimmed-platforms. Major variations exist in the gross geometries of foreshelf units and component reservoirs even laterally within rocks of the same age. « less