ABSTRACT

The hinterland of the Late Paleozoic Ouachita orogen, or the Ouachita Interior Zone, has mainly been studied in wells drilled in central and west Texas. The only exposure, at the base of the Sierra del Carmen escarpment, previously yielded Early Permian metamorphic ages, but the protolith age of the Sierra del Carmen rocks is undetermined. A new study has been undertaken to determine the structural and tectonic history of this critical exposure.

The low-grade metamorphic rocks are exposed on the western front of the Sierra del Carmen escarpment almost continuously for approximately 4 km, with other minor outcrops in canyons to the north. Rocks are thinly interlayered mica schist, marble and minor quartzite with abundant quartz veins. In the southern part of the area, rocks are generally more pelitic, whereas further north rocks are dominated by marbles. At least 4,500 of Lower Cretaceous carbonates with a basal red pebble/cobble conglomerate unconformably overlie this metamorphic basement. The massive carbonates have been deformed along the front of the escarpment and are locally overturned. The contraction is most likely Laramide in age.

The metamorphic rocks have been isoclinally folded, and the dominant foliation is axial planar to the folds. In some places these isoclinal folds have ductilely refolded an earlier generation of folds. Porphyroblast internal trails also indicate an earlier deformational history. Subsequently, Laramide and Basin and Range brittle deformation overprinted the ductile structures and as a result the axial planar foliation is kinked, folded, and faulted.

This study will clarify the relationship of the structural history at the Sierra del Carmen to the tectonic history of the entire Ouachita orogenic belt.