THIN- AND THICK-SKINNED TECTONICS IN CENTRAL COAHUILA, NORTHERN MEXICO: STRUCTURAL AND STRATIGRAPHIC CONSIDERATIONS

South of the Cuatrocienegas valley in central Coahuila State, Mexico, Mesozoic strata exhibit complex fold structures both in map view and cross-section. In map view fold axis trends vary from the regional WNW-ESE to N-S and NNE-SSW. In cross-section, fold geometry is either 1) clearly related to low-angle thrust faults with fault bend and fault propagation folds with top to the NNE kinematics or 2) related to subvertical structures with southerly or southwesterly vergence.

The distribution of sedimentary facies is also complex and reflects vertical block movements during sedimentation. The largest and best documented block is the Coahuila block in the south of the study area, with a regional unconformity separating permo-triassic rocks of the Delicias arc from Early Cretaceous platform carbonate rocks. This unconformity clearly documents the exposure of the island of Coahuila during the early Mesozoic. To the north of the block a sequence of clastic sedimentary rocks of Jurassic age is exposed, overlain by the Early Cretaceous San Marcos Formation, a sequence of conglomerates up to 1 km thick. These units are absent on the Coahuila block and grade northwards to marine facies. Platform and reef facies carbonate rocks of Aptian and younger age cover both the Coahuila block and the area to the north. The Coahuila block strata are locally interbedded with gypsum. A regional marine transgression is represented by the Indidura Formation shale of Upper Cretaceous age.

The San Marcos Formation clastic wedge and its truncation against the northern edge of the Coahuila block have been cited as evidence for the San Marcos fault, a steeply north-dipping structure active since the Jurassic and through the Eocene.

The structural and stratigraphic data suggest that the San Marcos fault acted as a normal fault during the Jurassic and Early Cretaceous. The Laramide orogeny developed a fold and thrust belt with low angle detachments and classical ramp-flat geometry and associated folds with tectonic transport toward the NE. Toward the end of the Laramide orogeny, the San Marcos fault was reactivated as a high-angle reverse fault forming drape folds with a southerly or southwesterly vergence. Furthermore, E-W facies variations and the N-S-trending folds are associated with N-S-trending basement faults which have not been previously described.