Abstract

Sierra del Cuervo is a north-northwest–trending mountain range ∼30 km northeast of Chihuahua City, Chihuahua, Mexico. Rocks exposed in the sierra range from late Precambrian to Quaternary. It has long been recognized that Sierra del Cuervo contains one of the most lithologically different Paleozoic outcrops in Chihuahua, and the recent discovery of Precambrian rocks in the south-central part of the range distinguishes Sierra del Cuervo as one of only two known Precambrian outcrops in the state.

Mapping and structural analysis in the south-central part of Sierra del Cuervo reveal that at least five major periods of deformation, ranging from Precambrian to late Tertiary, are represented. The most dramatic structures are a series of thrust faults and associated folds. Thrusting involved both Precambrian basement and Paleozoic sedimentary rocks, with tectonic transport toward the east-southeast. Stratigraphic relationships in Sierra del Cuervo restrict the age of this deformation only to post-Early Permian and pre-Cretaceous, but regional considerations suggest that it occurred during the middle Permian.

The origin of these late Paleozoic structures has previously been related to the westward extension of the Ouachita frontal zone; however, their orientation is oblique to the postulated trace of the frontal zone and suggests a much more complex relationship with the Ouachita system. Paleocurrents and compositional variations in sandstones of the Rara Formation indicate the presence of a basement-involved uplift in west-central Chihuahua during the Late Pennsylvanian and Early Permian. This apparent unroofing of a western basement-cored block, followed by east-southeast–directed thrusting, is inconsistent with the tectonic style of the Ouachita frontal zone and suggests that the late Paleozoic structures in Sierra del Cuervo are related to Ancestral Rocky Mountain deformation. Additional evidence for this southern extension of the Ancestral Rocky Mountains is found in other parts of northern and central Chihuahua and indicates that most of the region was in the Ouachita foreland during the late Paleozoic.

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