PENNSYLVANIAN PLACID SHALE: TRANSITIONAL MEMBER OF THE BRAD FORMATION

The Pennsylvanian Canyon Group is found exposed throughout the northeast of Central Texas. This group contains four formations, including the Graford, the Winchell, the Brad, and the Caddo Creek Formations. At the study area in Ranger, Eastland County, Texas, is found exposed the Placid shale and Ranger limestone members of the Brad formation, where the Placid Shale directly underlies the Ranger Limestone. The Ranger site was chosen due to the lack of study of this unit, especially in the northern part of Central Texas. The Placid Shale at this location is composed of interbedded and interlaminated muds and quartz arenite sands, with top-set bedding structures preserving onset of deltaic sequences throughout the outcrop. Other sedimentary structures which support the interpretation as a transitional environment include flaser laminations, wavy laminations, tool marks, crossbedding, channel fills, and burrows. No evidence for marine deposition was found. The contact with the overlying Ranger Limestone caps the unit, and signifies the onset of marine limestone deposition, with abundant marine fossils and lime supporting this interpretation.