Redefining the age of the lower Colorado River, southwestern United States

Abstract

Sanidine dating and magnetostratigraphy constrain the timing of integration of the lower Colorado River (southwestern United States and northern Mexico) with the evolving Gulf of California. The Colorado River arrived at Cottonwood Valley (Nevada and Arizona) after 5.24 Ma (during or after the Thvera subchron). The river reached the proto–Gulf of California once between 4.80 and 4.63 Ma (during the C3n.2r subchron), not at 5.3 Ma and 5.0 Ma as previously proposed. Duplication of section across newly identified strands of the Earthquake Valley fault zone (California) probably explains the discrepancy. The data also imply the start of focused plate motion and basin development in the Salton Trough (California) at 6–6.5 Ma and relative tectonic stability of the southernmost part of the lower Colorado River corridor after its integration. After integration, the Colorado River quickly incised through sediment-filled basins and divides between them as it also likely excavated Grand Canyon (Arizona). The liberated sediment from throughout the system led to deposition of hundreds of meters of Bullhead Alluvium downstream of Grand Canyon after 4.6 Ma as the river adjusted to its lower base level.

Early Pliocene Marine Transgression into the Lower Colorado River Valley, Southwestern USA, by Re-Flooding of a Former Tidal Strait

“…The observation that this unit overlies cross-bedded channel sands and floodplain fines of the earliest Colorado River requires a dramatic change of depositional environment in the southern Blythe basin, and shows that that shallow marine inundation into the lower valley took place after the river first ran through it. This is a critical event in the punctuated early development of the Colorado River that has received some attention (Dorsey et al, 2018) and is currently debated (Crow et al, 2021;Dorsey et al, 2021b). Below we explore the significance of the upper bioclastic member with discussion of the basal unconformity, paleoenvironmental interpretation, depositional processes, dynamics of back-stepping and retreat of the Colorado River delta, and regional tectonic controls on this unique episode of marine transgression into the lower reaches of the Colorado River valley.…”

“…This chronology is consistent with delivery of Colorado River sand to the Salton Trough starting 5.3 Ma as determined from biostratigraphy, paleomagnetism, and U-Pb dating of interbedded tuffs (Dorsey et al, 2007(Dorsey et al, , 2011Miranda-Martínez, 2019). U-Pb dating of detrital zircon (Cloos, 2014) and 40 Ar/ 39 Ar dating of detrital sanidine (Crow et al, 2021) suggest a somewhat younger age for birth of the river and first arrival of Colorado River sand in the Salton Trough. This suggestion remains speculative in light of incomplete datasets and conflicting interpretations (e.g., Dorsey et al, 2021b).…”

“…Existing evidence indicates that the lower reaches of the valley were subsequently re-flooded by a marine transgression that led to deposition of the upper bioclastic member of the southern Bouse Formation at ~ 5 Ma, followed by resumption of sediment discharge and final establishment of the modern Colorado River system (Dorsey et al, 2018). This sequence of events is important for understanding how tectonically controlled marine connections influenced the initiation and early development of the lower Colorado River, but the processes and chronology of evolving environments are currently debated (Gootee et al, 2019;Crow et al, 2021;Dorsey et al, 2021b). This study was undertaken to test the marine reflooding hypothesis through integration of stratigraphic, sedimentologic, and paleontologic data from the upper bioclastic member of the Bouse Formation in the lower Colorado River valley (Figs.…”

Rise of the Colorado Plateau: A Synthesis of Paleoelevation Constraints From the Region and a Path Forward Using Temperature-Based Elevation Proxies

“…The modern CR was integrated from its headwaters in the Rocky Mountains through GC coincident with development of the Salton Trough by ∼6 Ma, recorded by CR deposits in the Muddy Creek Formation (e.g., Longwell, 1946;Lucchitta, 1972;Karlstrom et al, 2014). The CR reached its current base-level in the Gulf of California at 4.80-4.63 Ma (Crow et al, 2021).…”

A constraint on post–6 Ma timing of western Grand Canyon (Arizona, USA) incision removed: Local derivation indicated by ca. 5.4 Ma fluvial deposits below Shivwits Plateau basalts north of Grand Canyon

“…5.24 Ma (Crow et al, 2018(Crow et al, , 2019 and 4.49 Ma (Faulds et al, 2001(Faulds et al, , 2016. Integration of the lower Colorado River through Cottonwood Valley (roughly 185 km downstream of where the Colorado River exits Grand Canyon) after 5.24 Ma and into the proto-Gulf of California between 4.80 and 4.63 Ma (Crow et al, 2021) would suggest the Colorado River was integrated through Grand Canyon on the early end of this range. Incision rates calculated from nearby Grand Wash gravels that are capped by a dated basalt flow suggest influence of local Colorado River incision by 4.73 ± 0.074 Ma, though no Colorado River gravels are preserved below the basalt (Crow et al, 2019).…”