New detrital sanidine geochronology from the Bouse Formation in Blythe basin supports recent research that redefined the age of lower Colorado River integration. Sanidine dating along with magnetostratigraphy indicates that the Colorado River arrived at Cottonwood Valley after 5.24 Ma and reached the proto–Gulf of California between 4.80 and 4.63 Ma. Earlier studies had proposed that initial integration to the evolving proto–Gulf of California occurred at 5.3 Ma; this was based largely on magnetostratigraphy tied to ~2 Ma old ashes ~4 km higher in the section than the deposits containing the first Colorado River sand. The discrepancy in ages was ascribed to duplication of section across newly identified faults in the Fish Creek-Vallecito basin. Detrital sanidine dating from near the first arriving Colorado River sand in this basin placed a much tighter constraint on the timing of integration than was previously available. The new detrital sanidine results reported here come from near the base of a thick section of siliciclastic Bouse Formation in the northern Blythe basin, which was the last in a series of basins that the Colorado River filled before spilling into the proto–Gulf of California. New single grain sanidine ages indicate a maximum depositional age of 4.8 ± 0.1 Ma for this section. This age should approximately correspond with the first arrival of Colorado River sediment to that basin. Previous dating of single sanidine grains from the Bouse Formation indicated a similar age; currently 6 grains have been dated at younger than 5.3 Ma with increasing precision. Because the Colorado River had to pass through this basin prior to reaching the proto-Gulf of California, deposition should predate not postdate integration, supporting the new younger age. These new results and previous work are consistent with downward integration of the Colorado River through a series of closed basins in the lower Colorado River corridor, culminating in integration to the proto–Gulf of California between 4.8 and 4.63 Ma. During and after integration, the Colorado River quickly incised through sediment-filled basins and the liberation of sediment from throughout the entire system led to deposition of hundreds of meters of Bullhead Al­luvium starting around 4.6 Ma as the river adjusted to its lower base level.

First posted October 13, 2020 For additional information, contact: Contact Information, Geology, Minerals, Energy, & Geophysics Science CenterU.S. Geological Survey2255 N. Gemini DriveFlagstaff, AZ 86001-1600 This geologic map includes a trove of stratigraphic and geomorphic information that chronicles the inception and evolution of the lower Colorado River. The map area is located near the south end of the Lake Mead National Recreation Area about 80 km (50 mi) downstream from Hoover Dam. It spans parts of northwestern Arizona and southern Nevada near the south end of Cottonwood Valley. The map includes the Spirit Mountain SE 7.5' quadrangle and the southern part of the Spirit Mountain NE 7.5' quadrangle. The map area contains well-exposed Neogene and Quaternary strata and associated geomorphic features that record and are critical in dating the arrival of the Colorado River in the early Pliocene and the subsequent history of the river and its landscape through the Holocene. The valley is bounded on the west by the Newberry Mountains (Nevada) and on the east by the Black Mountains (Arizona) and includes part of Lake Mohave, a reservoir created by the completion of Davis Dam in 1951. This map does not include the geology of the reservoir floor and focuses only on surficial deposits.

Detrital sanidine dating coupled with magnetostratigraphy indicates that the Colorado River was first integrated from the Colorado Plateau to the proto-Gulf of California at least half a million years later than previously argued. In Cottonwood Valley, 40Ar/39Ar dating of a 5.37 Ma ash in pre-Colorado River axial-basin deposits plus magnetostratigraphic analyses indicate that the overlying Bouse Formation, which records arrival of the Colorado River, was deposited after the beginning of the Thvera subchron, which started at 5.24 Ma. Detrital sanidine in the Bullhead Alluvium, the first coarse-grained aggradational package of the Colorado River, indicates a maximum depositional age of 4.6 Ma for that unit in the same area. At Split Mountain Gorge, new detrital sanidine dating coupled with previously published magnetostratigraphy and detrital zircon dating of Imperial Group sediments indicate that the first Colorado River sediment arrived at the proto-Gulf of California between 4.8 and 4.63 Ma (during the C3n.2r subchron), not at 5.3 Ma as has been previously proposed. The new geochronology supports models for rapid downward integration of this continental-scale river system extending its reach from Cottonwood Valley after 5.24 Ma to the opening Gulf of California between 4.8 and 4.6 Ma. This is consistent with the previously dated 5.0-4.9 Ma Lawlor tuff interbedded in the Bouse Formation at the highest levels in the Blythe basin, which records the last filling of that basin prior to integration of the river system to the proto-Gulf of California. Additionally, the data suggest there was little or no hiatus between integration of the Colorado River, incision into the siliciclastic Bouse Formation, and initial deposition of the Bullhead Alluvium, which seems to be a response to rapid profile changes caused by integration.

The Castle Rock quadrangle is in the northeast corner of Chemehuevi Valley, California and Arizona.It includes the Colorado River's entrance to the valley at the mouth of Topock Gorge and the northern outskirts of Lake Havasu City, Arizona, and the Chemehuevi Indian Tribe community of Havasu Lake, California.The map includes large parts of the Chemehuevi Indian Reservation and the Havasu National Wildlife Refuge.Upon its exit through the mouth of Topock Gorge, the Colorado River enters Chemehuevi Valley where its floodplain (now submerged under Lake Havasu) is flanked by alluvial piedmonts of the Chemehuevi and Mohave Mountains to the west and east, respectively.This abrupt transition offers a useful perspective into the structural evolution of the Colorado River extensional corridor and of the Colorado River itself.It contains key structural and stratigraphic elements recording a complex history of Cretaceous plutonism and deformation, significant tectonic extension, volcanism, and sedimentation in the Miocene, and, ultimately, the evolution of the Colorado River from the latest Miocene to the present.Lake Havasu submerged the axis of Chemehuevi Valley following the completion of Parker Dam in 1938, and the Colorado River now feeds a verdant delta marsh that composes part of the map.Important bedrock units include the Cretaceous Chemehuevi Mountains Plutonic Suite, the 18.78 Ma Peach Spring Tuff, and thick overlying sequences of interlayered Miocene megabreccia and fanglomerate.The exposure of these units is closely linked to extension along the Chemehuevi-Whipple Mountains detachment fault system.The complex bedrock geologic framework serves as the structural and topographic foundation for the key strata chronicling the evolution of the lower Colorado River.Important stratigraphic units that bear on its evolution to the present day include the Bouse Formation, the Bullhead Alluvium, and the Chemehuevi Formation.The map area also contains the river's modern delta at the head of Lake Havasu.

New age constraints allow for a refined assessment of the timing of Colorado River integration through a series of Miocene-aged extensional basins downstream from Grand Canyon. Here, we summarize existing constraints from upstream to downstream, present new Ar/Ar and paleomagnetic analyses, and describe their implications for Colorado River integration. In eastern Lake Mead, dated pre- and syn-Colorado River deposits record river arrival between 6 and 4.5 Ma. In western Lake Mead and Black Canyon, dated basalt flows graded to well above river level support significant river incision after 4.9 Ma. In Parker Valley, midway through the river corridor, a paleomagnetic reversal in the Bouse Formation, which records integration, suggests river arrival there before 5 Ma. Further south in Palo Verde Valley, the 4.9 Ma Lawlor tuff is intercalated with Bouse Formation at the highest elevation outcrops in the valley. First arrival of Colorado River sand to the proto-Gulf of California is thought have occurred at 5.3 Ma based on magnetostratigraphy, but detrital zircon dating suggests a younger maximum depositional age. A recent model used these existing constraints and field observations to suggest initial sediment arrival to the proto-Gulf of California followed by upstream sediment trapping and marine reflooding of the basin in the Palo Verde area. New Ar/Ar dating of a reworked ash in pre-Colorado River deposits in Cottonwood Valley, between Black Canyon and Parker Valley, and paleomagnetic analysis of the ash and overlying sediments indicates an eruption age of 5.35 Ma and deposition during the Thvera subchron. Multiple soil horizons between the ash and the base of the Bouse Formation require that the Colorado River arrived in this area well after 5.24 Ma, the beginning of the Thvera. Preliminary magnetostratigraphy on low elevation basal Bouse deposits in Palo Verde Valley indicate that Colorado-River-derived sediment there is mostly to entirely reversed polarity, consistent with deposition after the end of the Thvera, at 5 Ma. At least one reversal is present lower in the section, where both marine and lacustrine deposition have been argued. Together the data suggests more recent integration of the Colorado River to the proto-Gulf of California and that reflooding of the Palo Verde Valley might not be necessary.