ABSTRACT

Sierra Baluartes and Sierra de Pajaros Azules are in the Sabinas basin of Coahuila, Mexico. These two areas differ in structural character. Sierra de Pajaros Azules trends northwest-southeast and is an elongated, breached, asymmetric anticline. The Sierra Baluartes area, on the northwesterly plunging nose of Pajaros Azules, is an oval-shaped mass of diapiric gypsum flanked on the north and east by a discontinuous ridge of nearly vertical Aurora Limestone. Exotic blocks of various sizes and lithologies are incorporated within the gypsum. Normal faults radiate outward from the diapir. Doming and possibly diapirism, occurred in the Baluartes area during the Neocomian, Aptian and Albian. Subaerial exposure may have existed during the Neocomian.

Sierra Baluartes and Sierra de Pajaros Azules likewise differ lithologically. A comparison of the Cupido and Aurora limestones in the two areas illustrates their differing Early Cretaceous depositional and tectonic histories. In the Pajaros Azules area, a 6,000 ft. section of the complete Taraises, Cupido, La Pena and Aurora Formations crops out. The units here are low-energy, marine carbonate and terrigenous muds characteristic of this part of the Sabinas basin. The Cupido is predominantly miliolid, pellet wackestones and packstones which represent shallow, semi-restricted, intertidal-subtidal, marine conditions. The Aurora is a calcisphere, globigerinid and tintinnid rich, pellet wackestone which represents "deeper-water", open-marine conditions. The Cupido and Aurora here indicate low-energy depositional environments. In comparison, the Cupido Limestone in the Baluartes area is predominantly thickly-bedded grainstones rich in lithoclasts (eroded Taraises) and intraclasts. A few intervals are boulder conglomerates. The Cupido sediments represent high-energy, shallow-marine conditions. The Aurora is predominantly oncolite wackestones and packstones indicative of "higher-energy" conditions than those of the Pajaros Azules area. The Cupido and Aurora in this area were deposited over a submarine topographic high.

In the Baluartes area salt tectonics caused domal uplifting, possibly with minor diapirism, in the Early Cretaceous. This doming affected the thickness and lithology of the sediments. Laramide tectonics deformed both areas, forming the Sierra de Pajaros Azules anticline by decollement on the upper Jurassic evaporites, and causing diapirism and associated faulting in the Baluartes area.

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