Structural Geology of the Sierra de la Gavia, Coahuila, Mexico

Abstract

Sierra de la Gavia, in east-central Coahuila, Mexico, is one of a number of northwesterly aligned, mountain-forming anticlinal structures which constitute part of the Coahuila Marginal Folded province of northeastern Mexico. This province, an uplifted segment of the southwestern limb of the Rio Grande Embayment, lies north and east of the folds of the Sierra Madre Oriental. It includes several geologic elements: the Parras and Sabinas Basins, the paleogeographic Coahuila and Tamaulipas Peninsulas, and fold belts east of the Coahuila Platform and north of the Tamaulipas Arch. The fold, roughly 60 miles in length, rises abruptly out of bolson plains which border its northeast and southwest flanks. Lower-Upper Cretaceous-Quaternary rocks exposed within the Sierra include several thousand feet of detrital and chemico-detrital sedimentary rocks. Lower Cretaceous formations (Barril Viejo, Patula, La Mula, Taraises-Cupido, La Pena, Aurora-Cuesta del Cura), which are more than 7000 feet thick, form the major part of the mountain mass. Upper Cretaceous rocks (Indidura) and alluvial debris, together more than 500 feet thick, constitute minor mappable units. Younger Cretaceous units (Parras, Difunta), largely obscured by alluvium, form flanking outcrops, and generally occupy synclinal areas between Gavia and adjacent structural highs. In gross aspect, the axis of the structure is doubly inclined, striking generally northwest-southeast and plunging in both these directions. Areal mapping, however, discloses the presence of five separate areas of closure. Inclinations of the axial plane vary markedly along strike. Large-scale drag folding modifies both northeast and southwest limbs of the structure in certain areas. Minor en echelon normal (?) faults, thought to be shear fractures, occur at the structural culmination of the fold. It is suggested that Sierra de la Gavia and similarly aligned structures in the Coahuila Marginal Folded province owe their origin initially to movement of recurrent nature occurring along a suggested wrench fault of regional magnitude—a fault expressed at the surface in the abrupt change in strike and character of the frontal folds of the Sierra Madre Oriental between Monterrey, Nuevo Leon, and Torreon, Coahuila. Local northeast overturning on the southwest flank of Gavia, near its northwest end, is thought to be a surface reflection of the buttressing mass of the northeast edge of the Coahuila Peninsula.