A reasonable interpretation of the geologic history of the Colorado River in the Basin and Range province can be put together by considering the work done by several geologists in the Imperial Valley, California, the Parker–Blythe–Cibola area, California and Arizona, and, the Lake Mead and Hualapai Plateau areas, Arizona.

In the Imperial Valley, the Imperial Formation, whose age has been interpreted variously as late Miocene or early Pleistocene but most commonly Pliocene, records a transgression of the marine waters of the Gulf of California and deposition of sediments very probably of Colorado River origin. These sediments contain reworked Late Cretaceous foraminifers derived from the Mancos Shale of the Colorado Plateau. Later sediments record a gradual change from marine to continental conditions.

In the Parker–Blythe–Cibola area, extending along the Colorado River, the Bouse Formation (Metzger, 1968), of Pliocene age, records a marine transgression in the form of an embayment of the Gulf of California. A possible northward decrease in salinity, evidenced by faunas in the Bouse Formation, and a volume of sediments too large to be accounted for by local sources, suggest that a large river emptied into the Bouse embayment from the north. Although other streams and washes may have contributed sediments to the embayment, it is probable that the main source was the ancestral Colorado River. The Bouse Formation does not contain Mancos-type foraminifers but, in some surface exposures, does include Late Cretaceous coccoliths, probably of relatively local derivation. Deposits younger than those of the Bouse Formation consist of Colorado River alluvium generally unconformable on the Bouse, but locally conformable.

In the Lake Mead area, deposits as young as 18 to 20 m.y. indicate drainage northward, across the present site of Lake Mead, and northeastward onto the Colorado Plateau. None of these drainages can be interpreted as being an ancestral Colorado River. The next youngest deposit is the Muddy Creek Formation, which was laid down in interior basins formed when basin-range faulting disrupted the older topography and drainage some time after 18 to 20 m.y. ago. There was no Colorado River at this time. Deposition of the Muddy Creek Formation ended some time after 10.6 m.y. ago. Younger deposits reflect the Colorado River which, by 3.3 m.y. ago, was a well-incised stream flowing within 350 ft of its present grade.

Deposits and erosion surfaces on the Hualapai Plateau reflect drainage northeastward onto the region of the present plateau from the Basin and Range province until about 18 m.y. ago, when the two provinces became separated topographically by movements along the Grand Wash fault. No Colorado River existed before the faulting. After faulting, drainage became interior. In none of the areas is there evidence to suggest that the Colorado River has departed appreciably from its present course.

This information leads to the following hypothesis: until about 10.6 m.y. ago, or shortly thereafter, there was no Colorado River. After that date, waters of the Gulf of California invaded the Bouse embayment, and the ancestral Colorado River became established in the Lake Mead area. The river emptied into the embayment, which it progressively filled with its sediments from north to south. At this time, the head of the river had not yet reached areas where the Mancos Shale cropped out (probably east of the Kaibab upwarp). As the sediment fill continued to build toward and into the Imperial Valley area, the headwaters of the river breached the uplift and reached the Mancos Shale. The sediments then continued to build southward, filling the head of the Gulf of California, a process still going on. By 3.3 m.y. ago, the river was a well-established stream in the Lake Mead area, where it had cut to within a few hundred feet of its present grade.

By this interpretation neither the Bouse Formation nor the part of the Imperial Formation containing Colorado River material is older than 10.6 m.y. If true, the Bouse Formation is in part older than and in part equivalent in time to the Imperial Formation.

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