Pennsylvanian Subsurface Geology of Sutton and Schleicher Counties, Texas

Abstract

Pennsylvanian and Lower Permian deposition in the Sutton-Schleicher area was controlled largely by the Concho arch on the east, the Midland basin on the west and the Val Verde basin on the south. The oldest Pennsylvanian rocks are limestones and dark shales of Bend age which are confined to depressions of tectonic and physiographic origin and are thickest in grabens belonging to the Fort Chadbourne fault system. Lower Strawn limestones continued to fill the depressions, gradually spreading out until by middle Strawn time the highest features had been covered, though local movement along the Fort Chadbourne faults continued throughout the Pennsylvanian period. Four distinct sedimentary facies of the upper Strawn are recognized, beginning on the east with: (1) bedded shelf imestones intercalated with gray and red shales, (2) a back-reef black shale facies, (3) a barrier-type reef, and (4) the Midland basin black shale facies on the west. The nature and origin of the reefs are discussed and the conclusion reached that though probably true reefs, not all requirements in the strictest definition of the term can be demonstrated. The terms unda, clino, and fondo are used to describe the sedimentary environments associated with the starved Midland basin during the late Strawn, Canyon, and Cisco epochs. Bedded limestones of the undaform, synonymous with the Concho platform, become biostromal and thicken at the top of the clinoform before they thin and disappear abruptly on the slope. Canyon and Cisco dark shales and sandstones in the starved Midland basin are as little as one-third as thick as their platform equivalents, but in the Wolfcamp the condition is reversed, with thicker deposition in the basin.