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The spatial and temporal distribution of Pliocene to Holocene Colorado River deposits (southwestern USA and northwestern Mexico) form a primary data set that records the evolution of a continental-scale river system and helps to delineate and quantify the magnitude of regional deformation. We focus in particular on the age and distribution of ancestral Colorado River deposits from field observations, geologic mapping, and subsurface studies in the area downstream from Grand Canyon (Arizona, USA). A new 4.73 ± 0.17 Ma age is reported for a basalt that flowed down Grand Wash to near its confluence with the Colorado River at the eastern end of what is now Lake Mead (Arizona and Nevada). That basalt flow, which caps tributary gravels, another previously dated 4.49 ± 0.46 Ma basalt flow that caps Colorado River gravel nearby, and previously dated speleothems (2.17 ± 0.34 and 3.87 ± 0.1 Ma) in western Grand Canyon allow for the calculation of long-term incision rates. Those rates are ∼90 m/Ma in western Grand Canyon and ∼18–64 m/Ma in the eastern Lake Mead area. In western Lake Mead and downstream, the base of 4.5–3.5 Ma ancestral Colorado River deposits, called the Bullhead Alluvium, is generally preserved below river level, suggesting little if any bedrock incision since deposition. Paleoprofiles reconstructed using ancestral river deposits indicate that the lower Colorado River established a smooth profile that has been graded to near sea level since ca. 4.5 Ma. Steady incision rates in western Grand Canyon over the past 0.6–4 Ma also suggest that the lower Colorado River has remained in a quasi–steady state for millions of years with respect to bedrock incision. Differential incision between the lower Colorado River corridor and western Grand Canyon is best explained by differential uplift across the Lake Mead region, as the overall 4.5 Ma profile of the Colorado River remains graded to Pliocene sea level, suggesting little regional subsidence or uplift. Cumulative estimates of ca. 4 Ma offsets across faults in the Lake Mead region are similar in magnitude to the differential incision across the area during the same approximate time frame. This suggests that in the past ∼4 Ma, vertical deformation in the Lake Mead area has been localized along faults, which may be a surficial response to more deep-seated processes. Together these data sets suggest ∼140–370 m of uplift in the past 2–4 Ma across the Lake Mead region.

Essential features of the previously named and described Miocene Crooked Ridge River in northeastern Arizona (USA) are reexamined using new geologic and geochronologic data. Previously it was proposed that Cenozoic alluvium at Crooked Ridge and southern White Mesa was pre–early Miocene, the product of a large, vigorous late Paleogene river draining the 35–23 Ma San Juan Mountains volcanic field of southwestern Colorado. The paleoriver probably breeched the Kaibab uplift and was considered important in the early evolution of the Colorado River and Grand Canyon. In this paper, we reexamine the character and age of these Cenozoic deposits. The alluvial record originally used to propose the hypothetical paleoriver is best exposed on White Mesa, providing the informal name White Mesa alluvium. The alluvium is 20–50 m thick and is in the bedrock-bound White Mesa paleovalley system, which comprises 5 tributary paleochannels. Gravel composition, detrital zircon data, and paleochannel orientation indicate that sediment originated mainly from local Cretaceous bedrock north, northeast, and south of White Mesa. Sedimentologic and fossil evidence imply alluviation in a low-energy suspended sediment fluvial system with abundant fine-grained overbank deposits, indicating a local channel system rather than a vigorous braided river with distant headwaters. The alluvium contains exotic gravel clasts of Proterozoic basement and rare Oligocene volcanic clasts as well as Oligocene–Miocene detrital sanidine related to multiple caldera eruptions of the San Juan Mountains and elsewhere. These exotic clasts and sanidine likely came from ancient rivers draining the San Juan Mountains. However, in this paper we show that the White Mesa alluvium is early Pleistocene (ca. 2 Ma) rather than pre–early Miocene. Combined 40 Ar/ 39 Ar dating of an interbedded tuff and detrital sanidine ages show that the basal White Mesa alluvium was deposited at 1.993 ± 0.002 Ma, consistent with a detrital sanidine maximum depositional age of 2.02 ± 0.02 Ma. Geomorphic relations show that the White Mesa alluvium is older than inset gravels that are interbedded with 1.2–0.8 Ma Bishop–Glass Mountain tuff. The new ca. 2 Ma age for the White Mesa alluvium refutes the hypothesis of a large regional Miocene(?) Crooked Ridge paleoriver that predated carving of the Grand Canyon. Instead, White Mesa paleodrainage was the northernmost extension of the ancestral Little Colorado River drainage basin. This finding is important for understanding Colorado River evolution because it provides a datum for quantifying rapid post–2 Ma regional denudation of the Grand Canyon region.

New detrital zircon U-Pb age distributions from 49 late Cenozoic sandstones and Holocene sands (49 samples, n = 3922) record the arrival of extra-regional early Pliocene Colorado River sediment at Grand Wash (western USA) and downstream locations ca. 5.3 Ma and the subsequent evolution of the river’s provenance signature. We define reference age distributions for the early Pliocene Colorado River (n = 559) and Holocene Colorado River (n = 601). The early Pliocene river is distinguished from the Holocene river by (1) a higher proportion of Yavapai-Mazatzal zircon derived from Rocky Mountain basement uplifts relative to Grenville zircon from Mesozoic supra crustal rocks, and (2) distinctive (∼6%) late Eocene–Oligocene (40–23 Ma) zircon reworked from Cenozoic basins and volcanic fields in the southern Rocky Mountains and/or the eastern Green River catchment. Geologic relationships and interpretation of 135 published detrital zircon age distributions throughout the Colorado River catchment provide the interpretative basis for modeling evolution of the provenance signature. Mixture modeling based upon a modified formulation of the Kolmogorov-Smirnov statistic indicate a subtle yet robust change in Colorado River provenance signature over the past 5 m.y. During this interval the contribution from Cenozoic strata decreased from ∼75% to 50% while pre-Cretaceous strata increased from ∼25% to 50%. We interpret this change to reflect progressive erosional incision into plateau cover strata. Our finding is consistent with geologic and thermochronologic studies that indicate that maximum post–10 Ma erosion of the Colorado River catchment was concentrated across the eastern Utah–western Colorado region.