Map-scale folds in Big Brushy Canyon, Northern Sierra del Carmen, Big Bend Region, Texas
Detailed, 1:12,000-scale mapping and structural analysis of a 30 km2 area within northern Sierra del Carmen document two generations of Laramide folds cross-cut by Basin and Range high-angle faults, drag folds, and fault-propagation folds. The Big Brushy Canyon map area falls within the Black Gap Wildlife Management Area, Big Bend National Park, and the Shackelford Ranch. Geologic maps of Moustafa (1988; scale 1:48,000) and St. John (1966; scale 1:62,500) cover this area. Ferrill and others (2004) and Smart and others (2010) described an extensional fault-propagation fold in Big Brushy Canyon. Our mapping will be part of a geologic map of Northern Sierra Del Carmen to be submitted for publication in 2016. Cretaceous rock units exposed include the Del Carmen Limestone, Sue Peaks Formation, Santa Elena Limestone, Del Rio Clay, Buda Limestone, and Boquillas Formation. Tertiary mafic sills intrude the Boquillas Formation. First-phase Laramide folds contain axial planes at 332 85NE (average), fold axes at 327 45 (average), and average interlimb angles of 52°. Second-phase Laramide folds contain axial planes at 074 90 (average), fold axes at 254 8 (average), and average interlimb angles of 166°. Third-phase Basin and Range folds, caused by high-angle faults, contain axial planes at 322 64W (average), fold axes of 322 10 (average), and an average interlimb angle of 92°. Four other NW-striking map-scale monoclines have orientations similar to first-phase and third-phase folds and so could be Laramide fault-propagation folds above reverse faults, or Basin and Range fault-propagation folds above normal faults, or both. Folds containing small interlimb angles must be Laramide first-phase folds, while drag folds adjacent to Basin and Range faults must be third-phase folds.
AAPG Datapages/Search and Discovery Article #90214 © 2015 Southwest Section AAPG Annual Convention, Wichita Falls, Texas, April 11-14, 2015