ABSTRACT The Santa Rosalía basin (Baja California Sur, México) contains a rich record of late Cenozoic volcanism, faulting, and sedimentation that provides a crucial constraint on the timing of marine flooding from the Pacific Ocean into the nascent Gulf of California oblique rift, yet the precise age of the basin is uncertain. Previous studies used reconnaissance paleomagnetic data and a 40Ar/39Ar age of 6.76 ± 0.90 Ma on the intrabasinal Cinta Colorada tuff to estimate a depositional age of ca. 7.2–6.3 Ma for the marine Boleo Formation and initial flooding of the central Gulf of California. Here, we present a large (n = 2091) detrital zircon U-Pb geochronology data set from the Boleo Formation that indicates a maximum depositional age of 6.35 ± 0.21 Ma for pumiceous sandstone at the base (below the basal limestone), a revised age of 5.86 ± 0.06 Ma for the Cinta Colorada tuff in the middle, and a maximum depositional age of 5.70 ± 0.21 Ma for the top. Detrital zircon age spectra suggest a local provenance for the Boleo Formation involving recycling from underlying Oligocene–Miocene strata in proximal source areas. Integration of detrital zircon ages with existing paleomagnetic data suggests that the lower ~30 m of the Boleo Formation accumulated during normal-polarity subchron C3An.1n (6.27–6.02 Ma), and the middle to upper Boleo Formation was deposited entirely during reverse-polarity chron C3r (6.02–5.24 Ma). We therefore reassign the depositional age span of the Boleo Formation to ca. 6.3–5.7 Ma. Although not preferred, a minimum-duration depositional model from ca. 6.1 to 5.8 Ma is also permissible if a consistently high sedimentation rate of ~0.4– 1.0 mm/yr is inferred. This revised younger age for the Boleo Formation implies marine incursion in the central Gulf of California at ca. 6.3 Ma, ~1 m.y. younger than previously thought. We envision that regional marine flooding occurred during a very short (<100 k.y.) event that inundated a narrow tectonic trough over a distance of at least ~1000 km along the plate boundary from the central Gulf of California to the Salton Trough and reaching into the present-day Lower Colorado River Valley. This study also demonstrates the utility of large-volume and large-n detrital zircon studies in establishing the ages of sedimentary successions deposited over very short time spans (<1 m.y.) and/or during relative lulls in magmatism and geomagnetic reversals.

Abstract Marine straits and seaways are known to host a wide range of sedimentary processes and products, but the role of marine connections in the development of large river systems remains little studied. This study explores a hypothesis that shallow-marine waters flooded the lower Colorado River valley at c. 5 Ma along a fault-controlled former tidal strait, soon after the river was first integrated into the northern Gulf of California. The upper bioclastic member of the southern Bouse Formation provides a critical test of this hypothesis. The upper bioclastic member contains wave ripple-laminated bioclastic grainstone with minor red mudstone, pebbly grainstone with hummocky cross-stratification (HCS)-like stratification and symmetrical gravelly ripples, and calcareous-matrix conglomerate. Fossils include upward-branching segmented coralline-like red algae with no known modern relatives but confirmed as marine calcareous algae, echinoid spines, barnacles, shallow-marine foraminifers, clams, and serpulid worm tubes. These results provide evidence for deposition in a shallow-marine bay or estuary seaward of the transgressive backstepping Colorado River delta. Tsunamis generated by seismic and meteorological sources likely produced the HCS-like and wave-ripple cross-bedding in poorly-sorted gravelly grainstone. Marine waters inundated a former tidal strait within a fault-bounded tectonic lowland that connected the lower Colorado River to the Gulf of California. Delta backstepping and transgression resulted from a decrease in sediment output due to sediment trapping in upstream basins and relative sea-level rise produced by regional tectonic subsidence.