THE CHIHUAHUA SEGMENT OF THE TRANS-PECOS VOLCANIC PROVINCE

Large amplitude folds associated with the Chihuahua tectonic belt have effectively blocked westward distribution of volcanic rocks originating from the Trans-Pecos Volcanic Field (TPVF). However, a relatively small area of eastern Chihuahua adjacent to Big Bend State Park (BBSP), near the town of Manuel Benavides (MB), contains volcanic exposures that are continuous with those in Texas. First-stage mapping of the MB area was completed in 1981 by three Masters students from the University of Texas at Austin (Chuchla, 1981; Gregory, 1981; Immitt, 1981), in which a composite volcanic section approximately 1100 meters in thickness was documented. The lower one-third of the section is heterogeneous and dominated by volcaniclastic sedimentary rocks and thick basaltic lavas above a basal conglomerate. Near the top a thin 33.5 Ma ignimbrite is a distinctive marker bed. This lithologic package is similar in sequence and stratigraphic level to that of the Chisos Group, although no specific correlation is implied. Above this package voluminous rhyodacitic lavas were emplaced from at least three centers within the MB area; these occupy approximately the same stratigraphic level as the Tule Mountain trachyandesite in Texas. A thin and discontinuous section of sedimentary rocks and tuffs including ca. 33.0 Ma ignimbrites is present above the rhyodacite lavas. Younger units include the 30.8 Ma San Carlos tuff (Big Hill “intrusive”) and 28.0 Ma Santana tuff, both of which can be traced physically across the Rio Grande into BBSP. Units related to the younger Bofecillos volcanic center, which is dominant in BBSP, apparently were not emplaced in Chihuahua. Extension-related basalts of the Closed Canyon Formation (Henry, 1988) are found in the MB area within the Rio Grande graben. Highly precise Ar-Ar sanidine ages for the major units in the MB area provide a basis to examine stratigraphic correlations with units of similar in age and lithology in Texas.

Caldera sources for the San Carlos and Santana tuff units certainly are located within the MB area, although exact location of the caldera boundaries is problematic. A partial circular feature, prominent on satellite images, has been proposed. However, this solution requires bending some “classic caldera rules”.