Sandstones of the Permian Post Oak Formation are texturally immature feldspathic litharenites to lithic arkoses derived from the adjacent Wichita Mountains. Sandstones were deposited in alluvial fan-alluvial plain environments under semiarid conditions. Possible residual deposits, occurring near basement outcrops, are massively bedded and poorly sorted and show evidence of significant feldspar dissolution. Other Post Oak sandstones exhibit a complex diagenetic history. Pore-filling clays are in many cases authigenic as shown by petrographic criteria. Diagenetic minerals were precipitated sequentially in the following order: kaolinite and hematite, carbonate, and Fe-rich illite. Barite and chlorite occur locally. The sequence of cements is interpreted to be the result of variation in the chemistry of formation waters related to changes in depth of burial with time.