GEOCHRONOLOGY OF THE SIERRA DE SANTA ROSA AND IMPLICATIONS OF AN EARLY JURASSIC RETROARC BASIN

The El Antimonio Group consists of the Antimonio, Río Asunción, and Sierra de Santa Rosa Formations, which are present in the vicinity of Caborca in Sonora, Mexico. Here we report on a large sample (n=2,334) of detrital zircon U-Pb ages for the upper most component - the Sierra de Santa Rosa Formation that evidence suggests is a retroarc basin deposited from the Hettangian to the early Toarcian. The El Antimonio Group is highly fossiliferous, representing 4.5km-thick open marine to fluvial sedimentary strata. It was previously proposed to have been the evolving shallow shelf of a forearc basin on the southern margin of Laurentia that migrated southeast to its present position by left-lateral displacement on the Caborca block during the Late Jurassic. The Sierra de Santa Rosa Formation is the upper component of the group with a previously interpreted Jurassic age ranging between the Hettangian and the Pliensbachian, as determined primarily from relative dating techniques of bivalves and ammonites. Two small detrital zircon U-Pb populations have also been previously analyzed, one in the Sierra de Santa Rosa mountains and the other near El Antimonio. These prior U-Pb ages did not significantly narrow the uncertainty in the age of the formation. Here we describe the large population detrital zircon U-Pb dating methods used to determine the absolute depositional ages of each member of the Sierra de Santa Rosa Formation. The youngest zircons vary across the members in a manner consistent with the occurrence of several magmatic episodes, either increasing in intensity or decreasing in distance from the depositional location. This information is then contrasted to the older zircon populations which are remarkably similar to the deposits previously reported from the Colorado Plateau. We present the evidence that indicates that sediments of the Sierra de Santa Rosa Formation were deposited proximal to cratonic Laurentia during the filled stage of a retroarc foreland system.