A present trend in petroleum exploration is the search for buried shore lines. During Jurassic and Neocomian time the southern margin of the North American continent extended across northern Mexico and formed a peninsula extending southward across Coahuila. The basement rocks of this peninsula are to-day exposed in the deep canyons and along the pediments of some of the mountain ranges of Coahuila. The Mexican geosyncline lay adjacent to this peninsula on the east, west, and south. Much of the clastic sediment which accumulated in the geosyncline in Coahuila, and in eastern Chihuahua and Durango, was derived from this land mass.

In Aptian time the peninsula was peneplaned and the sea transgressed across it. Lagoons formed on the slowly subsiding land and gypsum deposits of great thickness and lateral extent accumulated in them. A gypsiferous facies in the Aptian-Albian series of Coahuila is closely limited to the area previously occupied by the peninsula.

The distribution of Upper Jurassic and Lower Cretaceous marine sediments, as well as certain similarities in the fossil faunas of these sediments in the Mexican geosyncline and in the Pacific geosyncline, strongly suggest a connection between the Atlantic and Pacific across northern Mexico during certain stages of the Mesozoic.

The Coahuila peninsula controlled the type of structures which developed during the Tertiary in the overlying sediments. There is a marked contrast between the relatively simple, open folding of Lower Cretaceous rocks above the peninsula, and the tight, asymmetrical folding and overthrusting in the area of the geosyncline on the east, west, and south. This relationship is similar to that observed elsewhere associated with foreland blocks.

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