A Stratigraphic Datum, Cisco Group (Upper Pennsylvanian), Brazos and Trinity Valleys, North-Central Texas

The Blach Ranch and Breckenridge limestone members of the Thrifty formation are the most persistent mappable rock units in the Cisco group of the Brazos and Trinity River valleys. These limestone beds occur above the complex post-Bunger deposits of the Graham formation and below an equally complex section of Harpersville rocks. The Blach Ranch limestone, which was mapped from southwestern Young County to north-central Jack County where it pinches out in the clastic Trinity Valley section, serves as a stratigraphic datum to tie the Cisco section in the Trinity Valley to the better-known Brazos Valley rocks. The Blach Ranch limestone exhibits three distinctive facies in Young and Jack counties; earlier workers correlated the Blach Ranch limestone of Jack County with the Breckenridge limestone of Young County because of similar appearance. Sandstone channel deposits cut out the Blach Ranch limestone locally near the Jack-Young County line, increasing the problem of correlating between the Brazos and Trinity Valleys. The Breckenridge limestone, the top of Plummer and Moore’s Thrifty formation, pinches out eastward in north-central Young County; a thin lenticular limestone occurs near the Breckenridge stratigraphic level in eastern Young and western Jack counties. Along the outcrop two depositional patterns are apparent: (1) relatively uniform rock units trending north-northeast through Stephens County to central Young County and (2) non-uniform rock units displaying abrupt facies changes east-northeast from central Young County to north-central Jack County. Rocks of the first pattern were deposited under relatively uniform conditions, while the second pattern reflects decreasing marine conditions with a source area to the northeast, based on facies and faunal changes, channeling, and other features.