Cambrian Stratigraphy of San José de Gracia, Sonora, Mexico: El Gavilán Formation, a new lithostratigraphic unit of middle Cambrian open shelf environment

An approximately 600 m interval of Cambrian strata is exposed near the town of San Jos de Gracia, Sonora, Mexico. This succession is divided into four formations, one of which is proposed here as a new formal stratigraphic unit: El Gaviln Formation, following the rules of the North American Commission on Stratigraphic Nomenclature. The new nomenclature reflects substantial lithologic differences, particularly in the middle Cambrian strata in comparison with other Cambrian rocks exposed in other parts of the state. The Proveedora Formation consists of bioturbated, fine-grained quartz arenite with ichnofossils; the Buelna Formation consists of interbedded limestone and sandstone, with abundant trilobites of either the ?Bristolia mohavensis or ?Bristolia insolens Zone (Series 2?, Stage 4?); the Cerro Prieto Formation is a highly recrystallized oolitic limestone with fragments of ?Amecephalus arrojosensis at the base, suggesting the A. arrojosensis Biozone (Series 2?; Stage 4?); and the El Gaviln Formation consists of interbedded limestone and intensely fractured red shale, with abundant fauna of the upper part of the Mexicella mexicana Zone, Albertella highlandensis Subzone, Ptychagnostus praecurrens Zone (open shelf) (Series 3?, Stage 5?). The Cambrian rocks of Sonora were deposited in the occidental part of the North American craton during the early and middle Cambrian, when Laurentia must have been constituted by terrestrial masses near the paleoequator, aligned in an east-west direction, providing the right conditions for an abundant invertebrate marine fauna.