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...

Continental‐Scale Landscape Evolution: A History of North American Topography

“…Sedimentary flux was calculated by integrating incision rate in a downstream direction using the landscape evolution model shown in Figure . The tectonic history of western North America complicates interpretation of sedimentary flux observations from, for example, the Los Angeles basin or Baja California (see, e.g., Dorsey et al., ). In contrast, the Gulf of Mexico appears to have acted as a stable depositional sink since Cretaceous times (Galloway et al., ).…”

“…40 Ar/ 39 Ar dating and magnetostratigraphy of tephra deposits within the oldest marine deposits of the Gulf of California suggests that rifting of the Gulf of California initiated by ∼7 Ma. Present-day elevations (>300 m) of the shallow marine Upper Bouse formation implies that uplift of the lower reaches of the Colorado River occurred after ∼4.8 Ma (Dorsey et al, 2018). Quaternary marine terraces along southwestern and eastern coasts of Mexico are elevated by several tens of meters above sea level, an observation that is interpreted as a consequence of uplift at rates of up to 3 m/ka (Ramirez-Herrera et al, 2011;Self, 1977).…”

“…interpretation of sedimentary flux observations from, for example, the Los Angeles basin or Baja California (see, e.g., Dorsey et al, 2018). In contrast, the Gulf of Mexico appears to have acted as a stable depositional sink since Cretaceous times (Galloway et al, 2000).…”

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).…”