Early Pliocene marine transgression into the lower Colorado River valley, southwestern USA, by re-flooding of a former tidal strait
Posted on 07.12.2021 - 12:16
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 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.
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Dorsey, Rebecca J.; Braga, Juan Carlos; Gardner, Kevin; McDougall, Kristin; O'Connell, Brennan (2021): Early Pliocene marine transgression into the lower Colorado River valley, southwestern USA, by re-flooding of a former tidal strait. Geological Society of London. Collection. https://doi.org/10.6084/m9.figshare.c.5740426.v1
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Rebecca J. Dorsey
Juan Carlos Braga
upper bioclastic membersymmetrical gravelly ripplesserpulid worm tubesresults provide evidenceregional tectonic subsidenceminor red mudstonelevel rise producedfossils include upwardbranching segmented corallinebounded tectonic lowlandsorted gravelly grainstonelaminated bioclastic grainstonesediment output duemarine waters inundatedmarine waters floodedlower colorado riverlike red algaeknown modern relativesmarine calcareous algaepebbly grainstonesediment trappingmarine foraminifersmarine connectionsmarine baywide rangeupstream basinstsunamis generatedtransgression resultedstudy exploressouthwestern usasedimentary processesripple crossrelative seamatrix conglomeratelike stratificationfirst integratedestuary seawardechinoid spinesdelta backsteppingcritical testc </