Arroyo Formation

The Arroyo Formation is a Permian rock unit in Texas that forms the basal part of the Clear Fork Group and extends from Concho County to Wilbarger County. Named in 1918 from exposures near Ballinger, it was eventually distinguished from older Wichita beds and the overlying Clear Fork sequence despite early uncertainty and naming disputes. Its character varies regionally: southern outcrops include terrestrial mudstones interrupted by marine limestone intervals, while northern outcrops are dominated by red clays, shales, and mudstones of a broad floodplain. The formation is especially important for its rich fossils, preserving diverse Early Permian amphibians, reptiles, synapsids, fish, and evidence of changing wet and dry conditions.

Texas · United States