Geology of the Grosvenor quadrangle, Brown and Coleman Counties, Texas

Abstract

The Grosvenor quadrangle is underlain by Pennsylvanian and lower Permian sedimentary rocks that dip gently to the west-northwest. Isolated patches of Lower Cretaceous strata, dipping slightly to the southeast, overlap the Pennsylvanian and Permian rocks. Quaternary alluvium and terrace deposits cover older rocks along stream valleys. The Pennsylvanian rocks include the Canyon and Cisco groups; the Wichita group represents the Permian system. The position of the systemic boundary is believed to be in the Waldrip shale member of the Pueblo formation near the base of the Wichita group but is drawn at the base of the Waldrip for practical purposes. The Pennsylvanian and Permian rocks consist mainly of gray shale but also contain limestone, siltstone, sandstone, conglomerate, and red shale. Most of the sandstone and conglomerate occupy broad shallow channels cut during Pennsylvanian and early Permian time. Shale and siltstone also fill parts of some channels. The Canyon group contains thicker limestone units and a larger proportion of limestone than do the Cisco and Wichita groups. Only one channel-fill deposit was found in the Canyon group within the quadrangle, although two other local channels have been reported in nearby areas. The Cisco and Wichita groups contain many channel deposits. These are especially numerous in the lower part of the Wichita group. Three or more of these channel deposits are superimposed on one another and can be differentiated only by detailed study of their relationship to limestone beds. Within some of these main channels are smaller channels that seem to be the result of local cut and fill. The Cretaceous rocks of the quadrangle consist of conglomerate, sandstone, siltstone, and shale. Sandstone is the most abundant and in many parts of the area has weathered to a blanket of loose sand. Quarternary deposits consist of alluvium and scattered patches of terrace gravel along the main streams. The alluvium typically forms low-level terraces that have been incised recently and contain abandoned meanders and ox2 PENNSYLVANIA!*, PERMIAN STRATIGRAPHY, NORTH-CENTRAL TEXAS bow lakes. Remnants of higher terraces, covered with gravel, are several tens of feet above the present stream levels. Large amounts of oil and gas have been produced in the Grosvenor quadrangle. Most of the oil and gas produced have come from lenticular sandstone beds in the Canyon group and the underlying Strawn group, but similar beds in the Cisco group produce oil west and north of the quadrangle. These oilproducing sandstones in the Cisco group resemble channel-fill deposits described in this report and appear to be their subsurface equivalents.