Early Pliocene marine transgression into the lower Colorado River valley, southwestern USA, by re-flooding of a former tidal strait

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 ∼ 5 Ma along a fault-controlled former tidal straight, soon after the river was first integrated to 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 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 meteorologic 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.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5740426

Straits and seaways: controls, processes and implications in modern and ancient systems

“…The papers included in the current volume provide an excellent overview of state-of-the-art knowledge of modern (Andreucci et (3) tectonic and climatic controls on the evolution of straits and seaways (Cavazza and Longhitano 2022;Dorsey et al 2022); and (4) their feedback with climate changes, paleogeographical reconstructions and preservation of associated deposits (Bahr et al 2022;Kalifi et al 2022;Palcu and Krijgsman 2022). Dalrymple (2022) provides a comprehensive review of the morphology, physical oceanography and facies of modern straits and seaways.…”

“…This work also documents the presence of tidal sandstone ridges included in the transgressive interval of the Cook Formation and their time-equivalence with the marginal-marine facies of the northernmost Tilje Formation in the Haltenbanken region. Dorsey et al (2022) describe the stages of flooding and re-flooding of a tectonically controlled narrow incision from the lower Colorado river valley in southwestern USA. The focus is on Miocene-Pliocene deposits of the Bouse Formation, within which facies and marine fauna descriptions indicate the flooding/transgression on top of a non-marine/fluvial system.…”