Abstract

Surface and subsurface structural geologic investigation has defined several major structural trends in parts of Edwards, Kinney, and Val Verde Counties, Texas. Seven rock units of the Cretaceous Comanche Series and two rock units of the Cretaceous Gulf Series crop out in the study area, part of the southwest Edwards Plateau physiographic province. Surface strata have a regional dip of ½ to 2° SW, interrupted locally by zones of faulting and/or folding that generally overlie larger scale late Paleozoic structural features. The east-west-trending Carta Valley fault zone, which crosses central Val Verde and southwest Edwards Counties, is marked by a pattern of en echelon folds and faults that bound a series of graben blocks. The deformational style of this zone suggests left-lateral wrench faulting in basement rocks. Two other surface fault zones that displace Cretaceous rocks are less well developed but also appear to overlie basement faults. A distinctive monocline trends northwest-southeast from east Val Verde through northwest Kinney Counties, approaching parallelism with the Balcones fault zone In the southeastern part. This flexure, the Sycamore Creek monocline, has exhibited positive tendencies since middle Cretaceous time. A poorly expressed southwest-plunging anticline marks the crest of the Devils River uplift in southeast Val Verde County.

Parts of three major late Paleozoic tectonic features are present in the subsurface—the southeast Val Verde basin, the Devils River uplift, and the north edge of the Ouachita thrust belt. A foreland sequence of lower to middle Paleozoic carbonate shelf deposits, overlain by thick upper Paleozoic clastic sedimentary rocks, fills the Val Verde basin. The Devils River uplift, the elevated southern margin of the Paleozoic Texas craton, marks the southern limit of the Permian Basin depositional province. Weakly metamorphosed deep-water sediments of the Ouachita interior zone are thrust northward over a Cambrian-Ordovician sequence on the uplift.

Subsurface mapping shows that a major down-to-the-north late Paleozoic fault, which separates the Val Verde basin from the Devils River uplift, underlies the surface Carta Valley fault zone. Transcurrent movement may have occurred along this fault during the Ouachita orogeny as a result of plate convergence. The axis of the Sycamore Creek monocline also lies above basement faults along which relatively minor up-to-the-north displacement occurred during Cretaceous and Cenozoic time, a reversal of major late Paleozoic movement.

In general, thorough analysis of subtly expressed surface structural features has proved a valuable guide to mapping and interpreting the more pronounced Paleozoic structures beneath the pre-Cretaceous unconformity.

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