Folds in the Big Bend Region were created during the Ouachita deformation and during Laramide deformation. The Ouachita folds are best displayed in the Solitario or just north of the national park along the road to Marathon. Laramide folds are found at Persimmon Gap, throughout the Sierra del Carmen Range, at Terlingua monocline, and in Mesa de Anguila.
The folds at Persimmon Gap and Dog Canyon were caused by the same Laramide-age thrust fault. At Persimmon Gap the exposed anticline is above the thrust fault whereas at Dog Canyon the exposed syncline is below the thrust fault. Actually, all along the fault there was an anticline above and a syncline below the fault. However, at Persimmon Gap the lower one is buried and at Dog Canyon the upper one was removed by erosion.
Interestingly, the Paleozoic rocks in the core of the upper anticline were folded during the late Paleozoic. Then during the Laramide deformation, they were folded again. As a result, they are greatly deformed and barely recognizable.
Diagram from Geological Society of America Centennial Field Guide-South-Central Section, 1988, by Peter R. Tauvers and William R. Muehlberger
In this hill near the entrance station at Persimmon Gap is a Laramide thrust fault that has an overturned anticline in its upper plate. Cretaceous rocks comprise most of the fold with Early Paleozoic rocks cropping out in the core of the fold.
Exposed in Dog Canyon are the vertical to overturned beds of an overturned syncline beneath the same thrust fault exposed at Persimmon Gap.
Terlingua Monocline and Quicksilver District
The Terlingua Monocline appears to result from folding of sedimentary strata over the southern edge of a large, uplifted basement block during the Laramide orogeny. The monocline trends N72W throughout much of its length, although there are significant local departures from this trend.
Numerous cinnabar deposits (cinnabar is the ore from which we get mercury) occur along the monocline in northeast-trending fault zones. Many of these northeast-trending faults show horizontal or sub-horizontal slickenlines suggesting strike-slip movement. Studies of stylolites by Erdlac and Erdlac, 1994, provide strong evidence for regional compression in this area. This area is also cut by a number of northwest-trending normal faults.
View to southeast from highest point on the Terlingua monocline. On the left the monocline is in the Cretaceous Santa Elena limestone. The non-resistant unit in the shadows is the Del Rio clay and the resistant strata on the right are the Buda limestone. The volcanic Chisos Mountains in Big Bend National Park are on the skyline.
Small folds in the Del Rio shale in the synclinal bend of the Terlingua monocline. These folds are disharmonic with the underlying, massive Santa Elena limestone.
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