Stratigraphy of the Blach Ranch-Crystal Falls section (Upper Pennsylvanian), northern Stephens County, Texas

Three mappable members in the upper part of the Thrifty formation (Blach Ranch limestone, unnamed shale, and Breckenridge limestone), two in the lower part of the overlying Harpersville formation (Quinn clay and Crystal Falls limestone), and six minor lentils, all in the outcropping Cisco group, northern Stephens County, Texas, have been mapped and are described. The Blach Ranch, Breckenridge, and Crystal Falls limestone members, which are important regional stratigraphic markers in surface Cisco rocks of the Brazos Valley, have been redescribed at type localities in the area. Plummer and Moore's original Thrifty and Harpersville formations are used, but future revision of the classification, based on additional detailed mapping in the Brazos Valley, is recommended. Cisco is recognized as a group (rock unit), that includes the Graham, Thrifty, and Harpersville formations, rather than as a series (time-rock unit). The rocks (about 100 feet thick) were deposited under cyclic, rapidly changing shallow marine to nonmarine conditions. Thin fossiliferous persistent limestone beds mark maximum but shallow marine transgression; they are separated by clay and shale containing sandstone beds and channels that mark maximum regression for this section in the area. Each limestone member is cut by one or more channel sandstones that originated in the overlying clay and shale members. Minor local calcareous units, which are commonly fossiliferous, near-shore deposits, pinch out laterally and/or change composition abruptly.