Geology of the border region between Coahuila and Zacatecas, Mexico

The border region between Coahuila and Zacatecas is part of the mountainous country south of Parras in northeastern Mexico. It includes a thickness of about 2,600 meters of Jurassic and Cretaceous rocks that were deposited along the northern border of the Mexican geosyncline along the southern margin of the Coahuila Peninsula massif. During early Tertiary time these sediments were compressed into folds parallel to the borders of the massif. The majority of the anticlines in the area mapped is overturned to the north. After the compressive stage a tensional stage developed and a system of tensional faults was formed. Block faulting found place on a large scale. A suggestion by de Sitter that some longitudinal faults may be comparable to schistosity planes in microfolds is tested in the horizontal outcrop pattern of this area, and no indications are found which could contradict this hypothesis. It is suggested that this horizontal outcrop pattern should also vary with the relative competency of the rock formation. The stratigraphic column is divided into formations. The Jurassic includes the Zuloaga limestone of Oxfordian age and the equivalent La Caja and La Casita formations 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-lower Albian age, the Aurora limestone of middle Albian age, the Indidura formation of upper Cenomanian-Turonian age, the Caracol formation of Coniacian age, and the Parras shale of Santonian age. The La Caja formation contains a variable amount of phosphorites, the genesis of which is discussed. The conclusion is reached that there are indications that this deposit had a biochemical mode of origin rather than a purely chemical one as advocated by Kazakov.