COEVAL BASINS WITHIN EARLY MESOZOIC CORDILLERAN RETROARC FORELAND SYSTEM OF SONORA, MEXICO

Several coeval Mesozoic basins in Sonora and Arizona lie within a regional retroarc foreland system related to development of the Cordilleran magmatic arc along the western margin of North America. Lithofacies successions vary greatly within each basin fill, but ages of fossils and detrital zircons within the basins record contemporaneous sedimentation persisting for >90 m.y., spanning most of Triassic and Jurassic time. Correlative Lower Jurassic Sinemurian (199-190 Ma) strata are present in three basins here referred to informally as Rancho Placeritos (El Gameño Formation), Sierra del Álamo (Sierra de Santa Rosa Formation), and Sierra de San Javier (Santa Clara Formation). Each basin has a unique assemblage of fluvial, deltaic, and marine deposits.

Sinemurian ammonites and bivalves are found in the El Gameño and Sierra de Santa Rosa Formations, Early to Middle Jurassic tropical flora are present in the El Gameño Formation, and Late Triassic to Early Jurassic tropical flora are present in the Santa Clara Formation. Late Triassic and Early Jurassic detrital zircons were collected in sandstone and siltstone beds in all three basins.

In the El Gameño Formation, Paul Smith identified ammonites, Chris McRoberts identified bivalves, Gary Webster identified crinoids, Sid Ash identified plants, Wayne Premo and Chris Holm-Denoma dated zircons.