STRATIGRAPHY AND PETROLOGIC EVOLUTION OF THE OLIGOCENE-MIOCENE COMONDU GROUP NEAR BAHIA CONCEPCION AND LORETO, BAJA CALIFORNIA SUR, MEXICO

The Comondú Group comprises extensive Upper Oligocene to Middle Miocene forearc basin and volcanic arc deposits in Baja California Sur, Mexico that formed immediately prior to plate boundary reorganization and rifting that opened the Gulf of California. This study contributes XRF whole rock data and zircon U-Pb ages within a ~20 x 100 km long belt from the Bahia Concepcion region to Loreto. Prior workers have previously divided the Comondú Group here into three informal units that reflect westward arc migration from the Sierra Madre Occidental (SMO) and petrologic evolution. Local aeolian quartz sandstone and arkose at the base of the sequence are of uncertain stratigraphic affinity but new detrital zircon U-Pb ages provide a maximum depositional age of ~25 Ma for these rocks and tie them into Comondú forearc basin history. Cretaceous and Proterozoic zircon are the dominant components within these sands. Associated rhyolitic tuff yield crystallization ages of ~24 Ma and are interpreted as distal forearc deposits derived from the SMO of mainland Mexico to the east. The lower clastic unit of the Comondú above this comprise ~300m of nonmarine fluvial sandstone and conglomerate that are dominated by basalt to basaltic andesite compositions; the mafic composition of these forearc clastics strongly contrast the more silicic composition of the SMO ignimbrite province. The approximately 750m thick middle Comondú unit is more andesitic in composition and contains andesite breccia deposited as proximal debris flows in the transition from forearc basin to volcanic arc, along with interbedded andesite lava flows. Minor stocks of biotite granodiorite porphyry intruded into forearc strata yield zircon U-Pb ages of 19.9 ± 0.73 and 16.3 ± 0.49 Ma. One of the stocks contains an abundance of Cretaceous zircon suggesting remobilization of the underlying Cretaceous batholith. Heat from the Comondú arc may have been a contributing factor that weakened the lithosphere and guided the position of rifting within the Gulf of California.