The Sierra de Los Muertos area is part of the rugged mountainous country southeast of Saltillo in northeastern Mexico. It includes about 14,000 feet of Jurassic and Cretaceous rocks that were deposited along the eastern border of the Mexican geosyncline, a trough developed along the western and southern margins of the Coahuila platform. Early in the Tertiary these rocks were compressed into folds parallel to the borders of the platform. In the mapped area the folds are generally overturned to the north, but some are doubly recumbent.
The Jurassic includes the Zuloaga limestone of Oxfordian age and La Casita formation of Kimmeridgian-Portlandian age. The Cretaceous from the base upward includes the Taraises formation of lower Neocomian age, the Cupido limestone of upper Neocomian-lower Aptian age, the La Peña formation of upper Aptian age, the Aurora limestone of lower and middle Albian age, the Cuesta del Cura limestone of upper Albian age, the Indidura formation of Cenomanian-Turonian age, and the Parras shale of Coniacian-Santonian age.
The La Peña formation contains a large fauna of mollusks, brachiopods, and echinoids. Ammonites dominate the fauna and include such characteristic Aptian genera as Dufrenoya, Parahoplites, Acanthoplites, Hypacanthoplites, Cheloniceras, Colombiceras, Uhligella, and Pseudohaploceras. New genera include Burckhardtites and Megatyloceras. Nautiloids are represented by Paracymatoceras. A total of 42 species of cephalopods is described.