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Description
The Comondú Group of Baja California Sur, Mexico comprises extensive Oligocene to middle Miocene forearc basin and volcanic arc deposits that formed immediately prior to plate boundary reorganization and rifting that opened the Gulf of California. This study contributes new whole rock X-ray fluorescence geochemical data and zircon U-Pb ages within a 20 x 100 km long belt from Bahia Concepcion to Loreto. Prior workers have divided the Comondú Group here into three informal units that reflect westward arc migration from the Sierra Madre Occidental as well as the petrologic evolution of the region. Local aeolian 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 ~24-27 Ma for these rocks and tie them into Comondú forearc basin history. The predominant ages of the zircon within these sandstones are of early Tertiary, Cretaceous and Proterozoic age. Associated rhyolite tuff yield crystallization ages of ~23.9 Ma and are interpreted as distal forearc deposits derived from the Sierra Madre Occidental of mainland Mexico. The lower clastic unit of the Comondú Group comprises ~300 m of non-marine fluvial sandstone and conglomerate that are dominated by basalt to basaltic andesite compositions; the mafic composition of these forearc clastics strongly contrast the highly silicic composition of the Sierra Madre Occidental ignimbrite province to the east. The approximately 750m thick middle Comondú unit is more andesitic in composition and contains andesite breccia deposited as proximal debris flows along with interbedded andesite lava flows in the transitional facies from forearc basin deposition to volcanic arc. 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. A gabbroic stock on the Bahia Concepcion peninsula contains an abundance of Cretaceous zircon as do the basal pre-Comondú aeolian sands, suggesting regional 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.
