Abstract

The Melchor (San Pedro) Ocampo region lies on the boundary of Coahuila and Zacatecas in the central part of a broad belt of eastward-trending folds characterized by strong asymmetry. In contrast to the region farther north, the Jurassic limestone is extensively developed and forms the highest parts of the mountain ranges; the Cretaceous rocks form the foothills. About 100 square miles adjacent to Melchor Ocampo was studied intensively as a foundation for reconnaissance studies in adjoining areas.

During the Upper Jurassic and Lower Cretaceous epochs the distribution of marine facies of deposition was controlled by erosion from the Coahuila Peninsula to the north. The seaward margins of the clastic facies and the limestone reef facies approximately coincide with the southern side of the Sierras de Parras and Jimulco. Facies of Upper Cretaceous marine sedimentation were controlled by rising land masses in central Mexico, as shown by increasing thickness of clastics toward the south.

Folding in northern Zacatecas involves both the Jurassic and the Cretaceous beds and apparently occurred at the same time as that farther north. The Jurassic formations become progressively more elevated toward the south and in general are as intensely folded as the Lower Cretaceous formations of the Sierras de Parras and Jimulco in Coahuila.

The greater height of the ranges bordering the site of the Coahuila Peninsula (e.g., Sierras de Parras, Jimulco, Rosario) than that of the ranges farther south, is ascribed (1) to the presence of the thick rudistid-bearing Aurora limestone, and (2) perhaps to less intense orogeny with consequent less effective erosion.

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