Possible Cretaceous Growth-faulting and a Laramide Detachment in Northern Sierra del Carmen, Big Bend region, Texas
A detailed, 1:12,000-scale geologic map of a 35 km2 area in northern Sierra Del Carmen (SDC) within Big Bend National Park and Black Gap Wildlife Management Area constrains the locations of potential Cretaceous growth faults and shows an extensive bedding-parallel detachment surface at the Buda Limestone – Boquillas Formation contact. The map area is located within the easternmost Laramide orogen and easternmost Basin and Range province. It is also located within the eastern margin of the Chihuahua trough, which subsided from Late Triassic through Late Cretaceous (Gray and Page, 2008). The map area has not been mapped in detail before: St John (1966; 1:62,500) and Moustafa (1988; 1:48,000) published less detailed maps of much of Sierra del Carmen. Eight Cretaceous formations crop out in the map area: Glen Rose Limestone, Telephone Canyon Formation, Del Carmen Limestone, Sue Peaks Formation, Santa Elena Limestone, Del Rio Clay, Buda Limestone and Boquillas Formation. Tertiary mafic sills intrude the Boquillas Formation. Cretaceous units were first deformed by possible Chihuahua trough syn-depositional faults. Two phases of Laramide folds followed. Tight Laramide outcrop-scale folds and steep dips are common in the thin-bedded Boquillas Formation while outcrop-scale folds and steep dips are absent in the underlying thick-bedded upper Buda Limestone, requiring a detachment fault at the Buda-Boquillas contact. Basin and Range high-angle faults reactivated older structures. This mapping project is significant because: a) possible Cretaceous growth faults would be the first documented in eastern Big Bend region, b) thickness variations in Cretaceous units can document fault timing and offset amounts, c) a Buda-Boquillas detachment changes significantly how structures are mapped beneath areas of Boquillas exposure.
AAPG Datapages/Search and Discovery Article #90214 © 2015 Southwest Section AAPG Annual Convention, Wichita Falls, Texas, April 11-14, 2015