Polyphase Map-scale Laramide Folds in the Southeastern Marathon Uplift, Texas

Joseph I. Satterfield, Ryan C. Sonntag, and Henry F. Schreiner III
Angelo State University, San Angelo, TX

A detailed, 1:10,000-scale geologic map of a thirty km2 region within the Slaughter Ranch, West Texas, shows two Laramide deformation phases overprinting an Ouachita phase of folding. The Slaughter Ranch, approximately 50 km northeast of Persimmon Gap and Big Bend National Park, lies within the eastern margin of Laramide deformation and includes the southeasternmost Paleozoic exposures of the Marathon uplift. Paleozoic rocks exposed include the Caballos Novaculite and Tesnus Formation. Cretaceous map units overlying a well-exposed angular unconformity include a basal Glen Rose siliciclastic member, Glen Rose Limestone, Maxon Sandstone, Telephone Canyon Fm., Del Carmen Limestone, Sue Peaks Fm., and Santa Elena Limestone. Cretaceous units fit Big Bend formation descriptions by Maxwell and others (1967), except for the unusually thick Maxon Sandstone (169 m thickness). The first deformation event (D1) produced outcrop- and map-scale folds and a penetrative axial-planar foliation within Paleozoic rocks during the late Paleozoic Ouachita orogeny. D1 axial plane orientations average N84E 20SE. The second deformation event (D2) formed map-scale folds within Cretaceous and older rocks during the Laramide orogeny. A D2 anticline, locally overturned and adjacent to an overturned syncline, extends outside the map area for at least 10 kilometers. D2 axial planes strike on average N48W. The third deformation event (D3) formed outcrop- and map-scale folds; some are overturned. A mapscale D3 anticline appears to overprint the lengthy D2 anticline. D3 axial planes are oriented N34W 69SW. Relative timing relations between D2 and D3 folds are poorly documented at present and could be reversed. D3 folds correlate with the single phase of Laramide shortening widespread in Sierra del Carmen and found throughout the Big Bend region. Laramide folds in the map area could be fault propagation folds above one or more blind thrust faults defining the northeastern flank of a northwest-trending basement uplift that includes the Marathon uplift and the El Burro-Peyotes uplift in northern Mexico. This uplift is shown on a recently published tectonic map of the Big Bend region (Satterfield and Ashmore, 2009, www.borderlandstudies.net).

AAPG Search and Discovery Article #90089©2009 AAPG Southwest Section Meeting, Midland, Texas, April 26-29, 2009