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Abstract: Tectonic History of the Ouachita Interior Zone: New Observations from the Sierra del Carmen, Northeastern Coahuila, Mexico

Danielle L. Carpenter

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, was first studied by P. T. Flawn. R. E. Denison obtained early Permian metamorphic ages from this locality, but the protolith age of the Sierra del Carmen rocks is undetermined. I have started a new study aimed at determining 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 some canyons to the north. The rocks are thinly interlayered mica schists, marbles and minor quartzites with prevalent quartz veining. In general the rocks to the south are more pelitic and, as the Rio Grande is approached to the north, the outcrop is dominated by marbles. At least 4,500^prime of Lower Cretaceous carbonates with a basal red pebble/cobble conglomerate were deposited unconformably on this basement. These massive carbonates have been deformed along the front of the escarpment and they are overturned in places. The compression is most likely Laramide.

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

Dating of these multiple deformations will be attempted through a variety of geochemical and geochronologic techniques. Correlations of rocks exposed in Mexico with those rocks encountered in wells in central and west Texas will also be attempted through microstructural and geochemical analyses. An important question to be answered is the relationship of the structural history at the Sierra del Carmen to that of the tectonic history of the entire Ouachita orogenic belt.

AAPG Search and Discovery Article #90955©1995 GCAGS 45th Annual Meeting and Gulf Section SEPM, Baton Rouge, Louisiana