Abstract: Structural Geology of Southwest Edwards Plateau Region, Texas

Robert E. Webster, Calvin F. Miller, Donald F. Reaser

A detailed structural analysis of a 1,036-sq mi (2,680 sq km) area in Edwards, Kinney, and Val Verde Counties delineated several large-scale structural trends. Rock-stratigraphic units mapped include the Lower Cretaceous McKnight, Fort Terrett, Devils River, Salmon Peak, and Fort Lancaster formations and the Upper Cretaceous Del Rio, Buda, Eagle Ford, and Austin formations. These strata generally dip from 0.5 to 2° southwest.

The most prominent structural feature in the study area is an east-west-trending fault zone, herein named the "Carta Valley fault zone." This zone consists of a series of grabens bounded by en echelon, nearly vertical faults with throws up to 300 ft (92 m). Subsurface correlation on the Ellenburger dolomite indicates that the fault zone overlies a large, high-angle fault of late Paleozoic age. This basement fault, which marks the southern boundary of the deep Val Verde basin in the area, is downthrown to the north and has a displacement of approximately 10,000 ft (3,048 m). The alignment of surface faults and associated drag folds strongly suggests left-lateral strike-slip movement in the basement. Other structural features in the area include (1) a less pronounced northeast-trending fault zone that overlies a zone of basement dislocation, (2) a prominent northwest-trending monocline with associated minor faults which appears to define the western limit of the Balcones fault zone, and (3) several gentle anticlinal folds.

Folds and fault blocks in the area of investigation represent potential hydrocarbon traps that largely are untested.

AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma