Journal of the Graduate Research Center


The strata of Pennsylvanian age in north-central Texas dip gently and uniformly to the northwest. They are divided, in ascending order, into the Strawn, Canyon, and Cisco Groups. A cover of Lower Cretaceous rocks on the Callahan Divide separates the two major Pennsylvanian outcrop areas in the Colorado and Brazos River valleys. The type area of the Canyon Group is in the Brazos region. The Canyon Group is distinguished by its thick bioclastic limestones and intervening shales, thin limestones, and lenticular sandstones. Deposition of the Canyon sediments took place under cyclic, alternating shallow marine to nonmarine conditions, and was continuous with only minor local disconformities. The Canyon strata are conformable with those of the underlying Strawn and overlying Cisco Groups. The Canyon Group was studied in the outcrop in its type area, located in the region of the common corner of Palo Pinto, Stephens, and Eastland Counties, Texas. Twenty-four mappable members of eight formations (in ascending order, Palo Pinto Limestone, Posideon Formation, Wolf Mountain Shale, Winchell Limestone, Placid Shale, Ranger Limestone, Colony Creek Shale, and Home Creek Limestone) have been mapped and are described. Of the members, three have been named (Fambro Sandstone and Wiles Limestone Members of the Posideon Formation, and the Staff Limestone Members of the Wolf Mountain Shale) and the remainder are informally designated by a rock type and geologic symbol. Five members of the Keechi Creek Shale of the Strawn Group, a small area of the Finis Shale of the Cisco Group, and numerous outliers of conglomerate of the Trinity Group (Cretaceous) have also been mapped and are described in the report. The Canyon is considered to be a group (rock unit) that includes all strata from the base of the Palo Pinto Limestone to the top of the Home Creek Limestone.

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