THE LAST-STANDING HYPOTHESIS ABOUT THE PATH AND INTEGRATION OF THE LATE MIOCENE COLORADO RIVER OFF THE COLORADO PLATEAU

The path and fate of the pre-Grand Canyon upper Colorado River in the Miocene is one of the main conundrums to be solved in the history of the drainage. Reviewing potential paths of the paleoriver, whether done by workers in the past, or now, leaves two remaining viable hypotheses. This includes ruling out the Bidahochi Fm as a terminal deposit, considering the existing research indicating extremely slow accumulation rates, the small volume of the deposit, and sedimentology indicating deposition largely in small, locally sourced fluvial systems. The remaining two hypotheses are: 1) drainage into the Basin and Range to the northwest of Grand Canyon proposed by Ivo Lucchitta, and 2) infiltration and dissipation of the paleo-upper river in the region of todays western Grand Canyon proposed by Charles Hunt.

Research has been completed testing the first hypothesis of a paleoriver terminating in the internal basins of the Muddy Creek Fm in the northern Lake Mead region. Sedimentology and sand and heavy-mineral petrology were studied along a series of 11 sites in an E-W transect from the hypothetical apex to distal region of the paleodelta. Results indicate a pattern of sedimentation, paleocurrents, and provenance that matches present-day drainage patterns from local-mountain sources of Paleozoic carbonate and Tertiary volcanic rocks along with a notable contribution by a paleo-Virgin River. Thus the Virgin has had its present course off the plateau since the Miocene and there is no distinct evidence for a paleo-Colorado. This leaves one remaining hypothesis—Hunt's termination in the southwest plateau. This may seem unlikely at first, but it fits several lines of evidence, including karst in Paleozoic rocks, Neogene paleotopography, the necessity of earlier superposition over the Kaibab upwarp, and massive Miocene spring and evaporite deposition in the Grand Wash Trough and Red Lake Basin just off the southwest edge of the plateau. It furthermore provides earlier groundwater drainage off the plateau, which provides by far the most physically viable condition for the subsequent integration-capture of the upper river into the lower Colorado at the end of the Miocene.