The Triassic Zacatecas Formation in central Mexico: Paleotectonic, paleogeographic, and paleobiogeographic implications

Middle to Late Triassic turbidite sequences are exposed in the states of Zacatecas and San Luís Potosí in central Mexico. These strata, assigned mostly to the Zacatecas Formation, accumulated in continental slope, toe-of-slope, and basin-plain environments along the passive continental margin of western Pangea. Strata of the Zacatecas Formation are age equivalent to rocks of the Antimonio Formation and Barranca Group in Sonora, the La Boca Formation in Tamaulipas and Nuevo León, and unnamed strata in Baja California. Based on their age, the Zacatecas turbidites correlate with a drop in sea level during the Permian-Triassic assembly of Pangea. The Triassic paleogeographic setting of Mexico is complex and poorly understood, because only dispersed Triassic outcrops exist across Mexico. However, the biogeographic affinities of the faunas from the Zacatecas Formation in central Mexico with those from equivalent strata in Baja California and Sonora suggest that these three regions were connected through the eastern Pacific, and that the Atlantic Ocean did not exist during the Ladinian-Carnian. The Zacatecas sequences underwent three periods of compressive deformation: one during their obduction onto the continental margin at some time during the latest Triassic-earliest Jurassic (?); a second during the Middle to Late Jurassic (Oxfordian) (?), apparently related to transpression; and a third during the Late Cretaceous to Tertiary Laramide orogeny.