Depositional Model for Upper Devonian Black Shale (Woodford Formation) in Permian Basin, West Texas and Southeastern New Mexico

John B. Comer

During the Late Devonian, west Texas and southeastern New Mexico were part of a low-relief region located along the western margin of North America in the arid tropics near 15° south latitude. Worldwide transgression combined with regional subsidence caused flooding of the craton, creating a broad epeiric sea in which sediment accumulated by slow settling of pelagic material and rapid episodic deposition of silt from turbidity currents. Very little freshwater runoff, coarse clastic debris, and terrigenous organic matter entered the basin because the adjacent lands were relatively low lying, barren, and dry. Density stratification developed as hypersaline brine formed in the arid climate and accumulated at the bottom of the water column. Biologic productivity in the u per part of the water column was high due to continuous influx of nutrient-rich water from a zone of coastal upwelling along the western margin of North America. High oxygen demand caused by organic decay and respiration caused water below the pycnocline to become anoxic, and high concentrations of organic matter were preserved in the sediment. Positive water balance persisted because evaporative losses and wind-driven surface water outflow required net inflow of deeper water from the open ocean. Marine black shale deposited in this setting during the Late Devonian is one of the most prolific petroleum source beds in the Permian basin.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.