Boletín de la Sociedad Geológica Mexicana Volumen 68, núm. 3, 2016, p. 429‒441 http://dx.doi.org/10.18268/BSGM2016v68n3a4

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

Francisco Javier Cuen-Romero¹, José Eduardo Valdez Holguín², Blanca Estela Buitrón³,Rogelio Monreal¹, Frederick Sundberg⁴,Alejandra Montijo González¹, Ismael Minjarez Sosa¹

1 Departamento de Geología, Universidad de Sonora, Blvd. Luis Encinas y Rosales, CP 83000, Hermosillo, Sonora, México.
2 Departamento de Investigaciones Científicas y Tecnológicas, Universidad de Sonora, Luis Donaldo Colosio s/n, entre Sahuaripa y Reforma, Col Centro. CP. 83000, Hermosillo, Sonora, México.
3 Departamento de Paleontología, Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, CP 04510, CDMX., México.
4 Show Low High School, 500 W. Old Linden Road, Show Low, AZ, USA.

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Abstract

An approximately 600 m interval of Cambrian strata is exposed near the town of San José de Gracia, Sonora, Mexico. This succession is divided into four formations, one of which is proposed here as a new formal stratigraphic unit: El Gavilán 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 Gavilán 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 praecurrensZone (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.

Keywords: Cambrian, Stratigraphy, Sonora, Mexico.