Only since 5-6 Ma has the Colorado River flowed through the western Grand Canyon into the Grand Wash Trough at the eastern end of Lake Mead. Before then, the river may have flowed through a paleocanyon transiting the Kaibab uplift at a stratigraphic level above the present eastern Grand Canyon to cross the Shivwits Plateau north of the western Grand Canyon and enter the Virgin River drainage, which exits the Colorado Plateau into the Virgin River depression north of Lake Mead. U-Pb age spectra of detrital zircons from Miocene-Pliocene basin fill of the Muddy Creek Formation in the Virgin River depression preclude any paleo-Colorado River sand in Muddy Creek exposures but fail to show that a Miocene paleo-Colorado River never flowed into the Virgin River depression, because all exposed Muddy Creek horizons sampled for detrital zircons postdate ca. 6 Ma. Detrital-zircon populations from available surface collections cannot test for the presence of Colorado River sediment within unexposed lower Muddy Creek or sub-Muddy Creek strata in the subsurface of the Virgin River depression. Alternate scenarios include rejection of paleo-Colorado flow into the Virgin River drainage at any time, or exit of the pre-6 Ma paleo-Colorado River from the upper Virgin paleodrainage toward the northwest into the eastern Great Basin without flowing southwest into the Virgin River depression through the Virgin River gorge, which may be younger than ca. 6 Ma.
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