Early Jurassic stratigraphy and ammonites at Cerro Pozos de Serna, Sonora,Mexico, and their tectonic significance

Jurassic strata exposed in the Cerro Pozos de Serna of northwestern Sonora, Mexico, are assigned on a lithostratigraphic basis to the Sierra de Santa Rosa Formation. These strata produce an Early Jurassic (latest Sinemurian) assemblage of ammonites that includes Coroniceras cf. C. pseudolylra ERBEN, Microderoceras cf. M. bispinatum (GEYER) and cf. Oxynoticeras palomense ER BEN. Identification of the Sierra de Santa Rosa Formation in the Cerro Pozo s de Serna supports the conclusion that this unit was broadly deposited in an autochthonous, volcanic-are setting across northwestern Sonora during th e Early Jurassic.