Walter T. Haenggi

2007 Tradewinds Drive, Missouri City, TX 77459-2331

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Abstract

The Chihuahua trough is a right-lateral pull-apart basin that began to form ~159 to ~156 Ma (Oxfordian) during a period of relative counterclockwise rotation of the North American plate. Jurassic seas were well established by latest Oxfordian time and there was little change in basin configuration throughout the remainder of Late Jurassic, Neocomian and Aptian time. Elements of a broad zone of intersecting pre-existing northwest-trending and north-trending lineaments, along the southwest border of the North American craton, provide the fabric for development of the pull-apart basin between the Diablo and Aldama platforms. During Tithonian and Neocomian time sedimentation eventually outpaced tectonic subsidence and, as an ensuing “regressive” event commenced, the eastern area of the Chihuahua trough was the locus of extensive evaporite (including halite) deposition. Near the end of Aptian time, during deposition of the Cuchillo and equivalent formations, faulting along the margins of the Chihuahua trough ceased and the seas began to transgress onto adjacent platform areas. By middle Albian time seas had advanced onto previously emergent areas and the Chihuahua trough became a site of shallow-water carbonate deposition that prevailed, with minor interruptions, until early Cenomanian time. The Ojinaga Formation (early Cenomanian -Santonian?) records a marine clastic influx into the Chihuahua trough, coeval with Upper Cretaceous clastic wedges in the Western Interior Cretaceous Seaway of the United States. Retreat of the Cretaceous sea is reflected in the transition from marine to non-marine beds in the Santonian San Carlos Formation and overlying non-marine El Picacho Formation.

During the Laramide orogeny (84 to 43 Ma) the Chihuahua trough was inverted to form the Chihuahua tectonic belt. Laramide deformation is the result of left-lateral transpressional tectonics involving renewed movement along the pre-existing fabric that controlled the location of the Jurassic-Aptian basin. In the evaporite basin portion of the trough (eastern area) reactivation of basin-boundary-faults as Laramide reverse faults, with possible left-lateral components of motion, accompanied by development of gentle “ancestral” folds, was followed by amplification of folds in post-evaporite rocks caused by flow of evaporites toward the crests of anticlines. As deformation progressed, structural development involved thrust faulting (principally toward the Diablo Platform) and diapiric injection of evaporites along the margins of the evaporite basin. In the northwestern area of the trough, structure reflects northeast-southwestoriented compression and includes relatively minor southwest-directed thrusting toward and onto the Jurassic Aldama platform. Paleozoic formations are involved in the thrusts and all thrusting can be interpreted as a consequence of faulted basement rather than regional-scale décollement.

Post-Laramide tectonic activity includes a continuation of evaporite tectonism, scattered igneous intrusion, minor volcanism, gravity tectonics and late Oligocene-Miocene to Quaternary block faulting. In the eastern area of the Chihuahua trough, erosion, after formation of Laramide structure and before emplacement of Oligocene volcanic rocks, created a topography that was similar to that of the present day. During this interval, gravity-induced flaps and detached flaps developed on flanks of several large anticlines. Collapse structures, related to evaporite solution, have deformed Tertiary and Cretaceous formations in areas of diapiric intrusion along tear fault zones. Tertiary normal faulting occurred after realignment of the regional stress system from east-northeast compression to east-northeast extension ca.31 Ma. Initial faulting in Chihuahua is probably coeval with inception of block faulting in Trans-Pecos Texas (about 24 Ma). Seismic data in the northwestern area of the trough shows that a large part of the area has been affected by Miocene normal faults that are probably coeval with some of the faulting described in the Rio Grande rift.

Extensive Neogene and some Quaternary faulting has affected the area, along the Rio Grande, between El Paso and the Big Bend. This area is the continuation of a postulated intracontinental transform along the southern edge of the Colorado Plateau and has been the locus of faulting related to right transtension during the past 24 m.y. Many of the Cenozoic fault trends of this area are along elements of the pre-existing structural fabric that influenced the development and location of the Chihuahua trough.

Keywords: Chihuahua through, stratigraphy, tectonics, Mesozoic, Cenozoic.