STRUCTURE AND DEFORMATION CHARACTERISTICS OF CRETACEOUS ROCKS AT PERSIMMON GAP, BIG BEND NATIONAL PARK, WEST TEXAS

Persimmon Gap is located in the northeastern portion of Big Bend National Park and at the northwestern end of the Sierra del Carmen trend. Outcrops located here reveal Ouachita and Laramide type structural events. The Ouachita events overturned and faulted the slightly metamorphosed Paleozoic sedimentary rocks. Unconformably overlying the Paleozoic rocks are Cretaceous formations that have been folded and thrust faulted during the Cretaceous Laramide events. Extension related to the formation of the Rio Grande Rift postdates these contractional events.

Persimmon Gap’s Cretaceous fold has a geometry that is very similar to an idealized fault propagation-fold. This type of fold develops concurrently with and immediately above a propagating fault. Bed orientations indicate that the Persimmon Gap fold plunges toward 331° at 03° with an interlimb angle of 44°. Calcite veins, minor faults, and fractures are common in the forelimb, but absent from the backlimb. Extensional, calcite-filled veins have mean orientations that imply extension along 145° 49°, and small-displacement faults extend bedding along 110°34°. Sample analysis of more resistant, heavily veined, limestone layers indicates local vein-accommodated extensions of 7, 10, and 18%. Small-displacement faults account for another 45% extension. The prevalence of extensional features within the forelimb of the Persimmon Gap fold indicates that the fold is not a simple single-step fault propagation fold, but one which has experienced forelimb extension and thinning as a result of limb-locking during folding. Forelimb deformation as a result of this extension, generated as much as 15% fracture porosity within the more resistant carbonate horizons. This porosity is absent from the fold’s backlimb.