Abstract

The Coahuila marginal folded province of Coahuila and western Nuevo Leon, northeastern Mexico, is characterized tectonically by numerous, widely spaced, northwesterly aligned, breached folds which form mountains. In some of these, exposed masses of gypsum-anhydrite are intrusive into overlying Late Jurassic and Early Cretaceous strata. Wide spacing and a general northwesterly alignment of these folds contrast with the closely spaced folds of the Sierra Madre Oriental which are abruptly westward at Monterrey, Nuevo Leon. In normal stratigraphic position the gypsum and anhydrite underlie the Zuloago limestone (=Smackover limestone of U.S.) of Sabinasian (Late Jurassic) age. Wells drilled on anticlinal folds (Sierra de Papagayos in coastal plain E. of Monterrey, Nuevo Leon, and Sierra del Fraile in folded province NW. of that city) have encountered salt of unknown structural and stratigraphic relationships below the gypsum-anhydrite. Diapirism of the gypsum-anhydrite is clearly demonstrable in the main arroyo of Potrero Chico - the valley formed by breaching of the northern fold of the Sierra del Fraile. Here the gypsum has flowed up through and around sharply folded and fractured Zuloaga limestone and La Casita formation (=Cotton Valley group of U.S.). The evaporites probably intrude the Taraises formation (=Hosston formation of U.S.) of Coahuilan (Early Cretaceous) age, but direct evidence of this is not available. The gypsum in Arroyo Potrero Chico resembles metamorphic rock, being banded, oriented along flowage lines, and including elongated masses and blocks of the formations through which it passed. In thin sections, individual grains of gypsum are elongated and somewhat rectangular, with distinct physical and optical orientation. Wavy extinction, indicating strain, is common. Anhydrite is present in minor amounts in all sections examined. Although the role of evaporites in crustal deformation is well documented in many parts of the world, the problem of motivation of the various structures in the Coahuila marginal folded province has not yet been resolved. The authors suggest: 1) tectonic (compressional) forces provided the primary energy for upward movement of the salts; 2) the folds were positioned and possibly shaped by thicker than normal evaporitic materials; and 3) gravity or density (geostatic) movements of the evaporites were secondary in the area.

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