Paleogeomorphology and evolution of the early Colorado River inferred from relationships in Mohave and Cottonwood valleys, Arizona, California, and Nevada

Abstract

Geologic investigations of late Mioceneearly Pliocene deposits in Mohave and Cottonwood valleys provide important insights into the early evolution of the lower ColoradoRiver system. In the latest Miocene these valleys were separate depocenters; the fl oor of Cottonwood Valley was ~200 m higher than the fl oor of Mohave Valley. When Colorado River water arrived from the north after 5.6 Ma, a shallow lake in Cottonwood Valley spilled into Mohave Valley, and the river then fi lled both valleys to ~560 m above sea level (asl) and overtopped the bedrock divide at the southern end of Mohave Valley. Sediment-starved water spilling to the south gradually eroded the outlet as silici clastic Bouse deposits filled the lake upstream. When sediment accumulation reached the elevation of the lowering outlet, continued erosion of the outlet resulted in recycling of stored lacustrine sediment into downstream basins; depth of erosion of the outlet and upstream basins was limited by the water levels in downstream basins. The water level in the southern Bouse basin was ~300 m asl (modern elevation) at 4.8 Ma. It must have drained and been eroded to a level <150 m asl soon after that to allow for deep erosion of bedrock divides and basins upstream, leading to removal of large volumes of Bouse sediment prior to massive early Pliocene Colo rado River aggradation. Abrupt lowering of regional base level due to spilling of a southern Bouse lake to the Gulf of California could have driven observed upstream river incision without uplift. Rapid uplift of the entire region immediately after 4.8 Ma would have been required to drive upstream incision if the southern Bouse was an estuary. PREVIOUS WORKThe Bouse Formation was fi rst thoroughly described and defi ned by seminal geohydrology studies conducted in the 1960s (Metzger, 1968;. The studies correlated Bouse outcrops within and between basins based on similar physical characteristics, including basal carbonate For permission to copy, contact editing@geosociety.org

Punctuated Sediment Discharge During Early Pliocene Birth of the Colorado River: Evidence from Regional Stratigraphy, Sedimentology, and Paleontology

“…This conclusion is similar to that of previous workers (Buising, 1988(Buising, , 1990Pearthree and House, 2014), though the paleoenvironment of the receiving body of water (lake vs. marine) is debated (e.g. Bright et al, 2016;O'Connell et al, 2017).…”

“…A rise in relative sea level caused deepening-up from intertidal to subtidal conditions over a large area from Parker, Ariz., south to Cibola and Milpitas Wash, possibly inundating areas a far west as Amboy in a tideswept shallow marine embayment or saline lakes (Miller et al, 2014). Areas north of Parker may have experienced carbonate deposition in one or more lakes during this period Pearthree and House, 2014), though the timing of deposition in the northern lakes remains uncertain. The width and location of the marine connection from the Cibola-Milpitas area south to the Salton Trough is not well known due to lack of stratigraphic control in that area, but a passage through the Buzzards Peak area is plausible based on evidence for broad post-Bouse uplift and erosion along the Chocolate Mountains anticlinorium (Beard et al, 2016).…”

“…River catchments may grow by upwardmigrating headward erosion and stream capture in both tectonically active and stable settings, as seen in examples from southern Spain (Stokes et al, 2002), southeast Asia (Clark et al, 2004), Africa (Goudie, 2005), western Himalayas (Clift and Blusztajn, 2005), and the southeastern U.S. (Prince et al, 2011). River integration and growth can also occur by downward-propagating linkage of basins that fill and overflow with water and sediment, as documented for the Rio Grande in New Mexico (Chapin and Cather, 1994;Connell et al, 2005;Repasch et al, 2017) and Colorado River Pearthree and House, 2014), and suggested for the Snake River in Idaho and Oregon (Van Tassell et al, 2001;Wood and Clemens, 2002).…”

The Colorado River and its deposits downstream from Grand Canyon in Arizona, California, and Nevada

“…These ongoing research topics are greatly informed by detailed outcropscale studies, analytical studies, and an understanding of the regional patterns in the extent of Colorado River deposits. Field and subsurface observations (for example, Metzger, 1968;Buising, 1990;House and others, 2005;House and others, 2008;Pearthree and House, 2014;Gootee and others, 2016), and geochronology (for example, House and others, 2008;Dorsey and others, 2011;Sarna-Wojcicki and others, 2011) have led to an increasingly refined understanding of the sequence and timing of river incision and deposition in the area. The main stratigraphic packages deposited by the Colorado River have been grouped by House (2016) as the LOCO strata, which includes (from oldest to youngest): the Bouse Formation, the Bullhead Alluvium, the Palo Verde alluvium, the Chemehuevi Formation, the Riverside alluvium (not yet included in our LOCO group compilation), and the Blythe alluvium.…”

“…The Bouse Formation consists of carbonates and interbedded siliciclastic mud and sand, including abundant Colorado Plateau derived grains that were deposited as the Colorado River filled and spilled southward through a series of formerly internally drained basins to the protoGulf of California (Metzger, 1968;Buising, 1990;House and others, 2008;Pearthree and House, 2014;Spencer and others, 2013;Kimbrough and others, 2015). Previous work indicates a maximum age for the Bouse Formation in Cottonwood and Mohave valleys of 5.6 Ma (House and others, 2008).…”