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

Abstract

The Colorado River in the southwestern U.S. provides an excellent natural laboratory for studying the origins of a continent-scale river system, because deposits that formed prior to and during river initiation are well exposed in the lower river valley and nearby basinal sink. This paper presents a synthesis of regional stratigraphy, sedimentology, and micropaleontology from the southern Bouse Formation and similar-age deposits in the western Salton Trough, which we use to interpret processes that controlled the birth and early evolution of the Colorado River. The southern Bouse Formation is divided into three laterally persistent members: basal carbonate, siliciclastic, and upper bioclastic members. Basal carbonate accumulated in a tidedominated marine embayment during a rise of relative sea level between ~6.3 and 5.4 Ma, prior to arrival of the Colorado River. The transition to green claystone records initial rapid influx of river water and its distal clay wash load into the subtidal marine embayment at ~5.4-5.3 Ma. This was followed by rapid southward progradation of the Colorado River delta, establishment of the earliest through-flowing river, and deposition of river-derived turbidites in the western Salton Trough (Wind Caves paleocanyon) between ~5.3 and 5.1 Ma. Early delta progradation was followed by regional shut-down of river sand output between ~5.1 and 4.8 Ma that resulted in deposition of marine clay in the Salton Trough, retreat of the delta, and re-flooding of the lower river valley by shallow marine water that deposited the Bouse upper bioclastic member. Resumption of sediment discharge at ~4.8 Ma drove massive progradation of fluvial-deltaic deposits back down the river valley into the northern Gulf and Salton Trough.These results provide evidence for a discontinuous, start-stop-start history of sand output during initiation of the Colorado River that is not predicted by existing models for this system. The underlying controls on punctuated sediment discharge are assessed by comparing the depositional chronology to the record of global sea-level change. The lower Colorado River Valley and Salton Trough experienced marine transgression during a gradual fall in global sea level between ~6.3 and 5.5 Ma, implicating tectonic subsidence as the main driver of latest Miocene relative sea-level rise. A major fall of global sea level at 5.3Ma outpaced subsidence and drove regional delta progradation, earliest flushing of Colorado River sand into the northern Gulf of California, and erosion of Bouse basal carbonate and siliciclastic members. The lower Colorado River valley was re-flooded by shallow marine waters during smaller changes in global sea level ~ 5.1-4.8 Ma, after the river first ran through it, which requires a mechanism to stop delivery of sand to the lower river valley. We propose that tectonically controlled subsidence along the lower Colorado River, upstream of the southern Bouse study area, temporarily trapped sediment and stopped delivery of sand to the lower river valley and nort...

A legacy of geo‐climatic complexity and genetic divergence along the lower Colorado River: Insights from the geological record and 33 desert‐adapted animals

“…Comparison of two models showing initiation age and early evolution of the Colorado River: (a–d) Early‐initiation model (e.g., Dorsey et al, ); (e–h) Late‐initiation model (e.g., Pearthree & House, ). (a) Marine incursion into northern Gulf of California, Salton Trough, and lower Colorado River corridor c. 6.3 Ma (Bouse Formation basal carbonate member).…”

“…By ~6.5–6.3 Ma marine waters inundated a long, narrow fault‐controlled basin north to Palm Springs, California and possibly as far north as Parker, Arizona (Figure a vs. Figure e; Dorsey, Housen, Janecke, Fanning, & Spears, ; Dorsey et al, ; McDougall, Poore, & Matti, ; Umhoefer et al, ; see Figure for locations). Increased monsoonal precipitation and/or increased groundwater flow off the Plateau created a series of large late Miocene lakes in the Lake Mead area (Crossey et al, ; Faulds et al, 2016).…”

“…There are two prevailing models for the birth and early evolution of the Colorado River: (a) “early initiation” at ~5.3 Ma, following roughly 1 Myr of marine deposition in the lower river valley (Figure a–d; Buising, ; Dorsey et al, , , ; McDougall, ; McDougall & Martínez, ; O'Connell, Dorsey, & Humphreys, ); or (b) “late initiation” at ~4.8 Ma, following a very short period (<50 Kyr) of lake deposition in the lower river valley (Figure e–h; Bright, Cohen, Dettman, & Pearthree, ; Bright et al, ; Crow et al, ; Gootee, Pearthree, House, Youberg, O'Connell, et al, ; Gootee, Pearthree, House, Youberg, Spencer, et al, ; House et al, ; Pearthree & House, ; Spencer & Patchett, ; Spencer et al, ). Both models propose emergence of the integrated river network after 6.0 Ma and before 4.5 Ma, but with notable differences within that time frame.…”

“…The Colorado River itself has been the focus of geological and stratigraphic investigations for over 100 years (Dutton, ; Powell, ) and today serves as a testing ground for ideas about the origin and evolution of continental‐scale rivers (e.g., Dorsey, O'Connell, McDougall, & Homan, ; House, Pearthree, & Perkins, ; Howard, House, Dorsey, & Pearthree, ; Lucchitta, ; Meek & Douglass, ; Pearthree & House, ). As a result, the age and evolution of the Colorado River are relatively well‐understood, with some differences between current models (see Background).…”

Mixed carbonate-siliciclastic tidal sedimentation in the Miocene to Pliocene Bouse Formation, palaeo-Gulf of California

“…The focus of this study is the upper Miocene to lower Pliocene Bouse Formation in its southern exposures along the lower Colorado River region, which accumulated at the margin of a tidal strait near the north end of the ancestral Gulf of California (Fig. 1;O'Connell et al, 2017;Dorsey et al, 2018;Garnder & Dorsey, in press). The basal carbonate member of the Bouse Formation is characterised by compositional mixing (sensu Chiarella et al, 2017), reflecting contemporaneous accumulation of carbonate and siliciclastic sediment at laminae and bed scales (Chiarella et al, 2017).…”