ABSTRACT

A northwest-southeast linear zone, here named the Frio River line, about 60 miles (100 km) southwest of San Antonio separates two areas of contrasting structural and stratigraphic history. To the northeast, structures include the uplifted Llano area, the Balcones and Luling zones of normal faulting, and a narrow graben which closely outlines the landward edge of Jurassic salt. All of these features disappear southwest of the Frio River line. To the southwest are northwest-southeast low-amplitude folds of the Rio Grande foldbelt, which are similar in orientation and age to Laramide folding in northeastern Mexico.

Northeast of the line, Jurassic salt was deposited over broad areas; to the southwest, seismic data suggests an erratic distribution of salt. Lower Cretaceous subsidence was marked southwest of the line, and Gulfward sliding predominated northeast of the line. Upper Cretaceous alkalic volcanics are restricted to a belt northeast of the line, and are most abundant where that belt intersects the line. Even Tertiary growth-fault styles show distinct changes across this line, probably due to a greater thickness of Upper Cretaceous shale in the south.

Fragmentary evidence indicates that the line may have a pre-Late Jurassic origin, possibly as the northeastern boundary of a Mesozoic strike-slip system, or as the southeastward continuation of the Devils River Uplift.

Oil and gas plays in Cretaceous rocks are different north and south of the Frio River line; the Northern Gulf is dominated by traps related to normal fault systems, while Southwest Texas contains mostly fold-related and cross-fault traps and stratigraphic traps.