Boletín de la Sociedad Geológica Mexicana Volumen 69, núm. 3, 2017, p. 711 ‒ 738

The Triassic/Jurassic boundary and the Jurassic stratigraphy and biostratigraphy of northern Sonora, northwest Mexico

Carlos M. González-León^1,*, George D. Stanley, Jr.², Timothy F. Lawton³, József Pálfy^4,5, Montana S. Hodges²

¹ Universidad Nacional Autónoma de México, Instituto de Geología, Estación Regional del Noroeste, Apartado Postal 1039, Hermosillo, Sonora, México, 83000.
² University of Montana Paleontology Center, 32 Campus Drive, Missoula, Montana, 59812, USA.
³ Universidad Nacional Autónoma de México, Centro de Geociencias, Blvd. Juriquilla 3001, Juriquilla, Querétaro, Qro., México, 76230.
⁴ Department of Physical and Applied Geology, Eötvös Loránd University, Pázmány Péter sétány 1/C, Budapest, H-1117 Hungary.
⁵ MTA-MTM-ELTE Research Group for Paleontology, POB 137, Budapest, H-1431 Hungary.

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Abstract

The Triassic and Jurassic geology of northern Sonora encompasses important events that are linked to the late Paleozoic history of the region. The fossiliferous El Antimonio Group in the Sierra del Álamo includes the upper Permian-Triassic Antimonio, and Río Asunción formations and the Hettangian-Sinemurian Sierra de Santa Rosa Formation. These formations consist of upward-fining sequences from I to XIV that represent fluvial to shallow and deep marine environments of deposition. The Triassic/Jurassic boundary in this region is a hiatus represented by a disconformity between sequence IX of the Río Asunción formations and sequence X of the Sierra de Santa Rosa Formation. The shallow to deep marine succession of the Sierra de Santa Rosa composes the upper part of the Sierra de Santa Rosa Formation that ranges in age from late Sinemurian to early Pliensbachian. Ages of the Permian to Triassic plutonic rocks of northwesternmost Sonora, the Mojave Desert and the Jurassic continental margin Nazas arc that crossed through northern Sonora, are also well recorded by igneous clasts and detrital zircon grains that have been dated from the El Antimonio Group and other Jurassic formations of this region. The upper Oxfordian-lower Tithonian Cucurpe Formation in north-central Sonora recorded the onset of continental extension and incursion of marine waters from the Gulf of Mexico into northwestern Mexico, once activity of the Jurassic magmatic arc ended.

Keywords: Triassic-Jurassic, El Antimonio Group, Sierra de Santa Rosa Formation, Cucurpe Formation, biostratigraphy.