LATE CRETACEOUS FORELAND BASINS IN THE CABORCA AREA: A NEW APPROACH TO UNDERSTAND THE TECTONIC EVOLUTION OF NW SONORA, MEXICO

The Late Cretaceous tectonic evolution of NW Sonora has not been fully understood, mainly because of the lack of age dates of sedimentary sequences. Some were considered as Jurassic because they include volcanic rocks or have been metamorphosed. Using U-Pb ages from four volcaniclastic sequences we document the presence of the Upper Cretaceous in the región. These basins were not isochronous, as suggested by the nature of the sequences when referred to the “Cretaceous type section” in the Sierra El Chanate, north of which could be the orogenic front. Of great importance is the basement to these basins. The El Chanate Group was deposited upon the Lower Cretaceous Bisbee Group with no apparent angular unconformity. The two lower units of the El Chanate Group, the Pozo Duro and Anita Formations, have no Late Cretaceous zircons; and the Escalante Formation includes zircons as young as Campanian. In the Cerro Rajón the sequence is more than 3,000 m thick; the base of this unit lies upon the Cambrian and includes meter-sized, well rounded, quartz arenite boulders. Similar boulders are found in the Upper Cretaceous of the Cerros El Amol (East of Altar). In the Cerro Lista Blanca the Cambrian is overlain by a few meters thick limestone-pebble conglomerate followed by an andesitic breccia (Anita Formation?) less than 5 m thick and an andesite-rhyolite conglomerate, as in the Escalante Formation. Zircons are mainly of Campanian age. In the Sierra Santa Rosa and Sierra El Alamo, the Triassic is overlain by a volcano-sedimentary sequence. There is no clear cut unconformity between sequences. In Sierra Santa Rosa, youngest zircons are Campanian, no Early Cretaceous grains, and another peak in the Middle Jurassic. Paleozoic and Precambrian zircons are abundant. In the Sierra El Alamo, there are no zircons younger than Jurassic. The lithologies of these Late Cretaceous sequences strongly suggest that the Rajón sequence is older than the other three, based on the abundance of quartzite clasts, which make the basal unit of the El Chanate Group in the reference section. The Santa Rosa and the El Alamo units contain abundant andesitic and rhyolithic volcanic rocks, which could make them contemporaneous to the Anita Formation. These sequences were deposited in the foreland region of the Late Cretaceous orogenic system.