A succession of Triassic to Jurassic strata occurs in the vicinity of Caborca, Mexico, where the Antimonio, Rio Asuncion, and Sierra de Santa Rosa Formations contain a nearly continuous marine section deposited in previously reported shallow basin environments. The Sierra de Santa Rosa Formation is known to be Early Jurassic, but with an 18 m.y. uncertainty in age. Here we establish the ages of the three members of the Sierra de Santa Rosa Formation as early Sinemurian to middle Toarcian. Detrital zircon U-Pb geochronology from a set of 2334 grains along with ammonite zonation is used to establish depositional ages and to support the interpretation of the formation as being deposited in a retroarc basin. We find 2 zircon populations of 199 Ma and 192 Ma that mark the onset of Early Jurassic magmatic activity in the formation. Older Neoproterozoic and Early Devonian populations not attributable to local sources imply a robust Early Jurassic exchange with southwestern Laurentian eolianites. Here we also establish the ages of three fossiliferous units containing solitary and colonial corals in the Sierra de Santa Rosa Formation. The end-Triassic mass extinction decimated coral reefs worldwide and reports of Early Jurassic corals have been particularly rare in North America. The ages indicate an earlier regional recovery for North American corals than previously proposed; this has implications for understanding postextinction reef recovery.
Detrital zircon U-Pb geochronology of the Sierra de Santa Rosa Formation, Sonora, Mexico, and implications for an Early Jurassic retroarc basin
Montana S. Hodges, Carlos M. González-León, George D. Stanley, Jr., Christopher L. Hodges; Detrital zircon U-Pb geochronology of the Sierra de Santa Rosa Formation, Sonora, Mexico, and implications for an Early Jurassic retroarc basin. Lithosphere 2017 doi: 10.1130/L633.1
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