DETRITAL ZIRCON EVIDENCE FOR AND AGAINST FLOWAGE OF AN ANCESTRAL MIOCENE COLORADO RIVER INTO THE VIRGIN DEPRESSION

Lucchitta (1984, 1989), Pelletier (2010), Dickinson (2013), and Karlstrom et al. (2013) have proposed that an ancestral Miocene Colorado River crossed the Kaibab uplift, and prior to 5-6 Ma flowed northwest across the Shivwits Plateau to exit the Colorado Plateau into the Virgin depression northwest of the Virgin Mountains. We use ~50 detrital zircon (DZ) samples from the surrounding region to evaluate their hypothesis. No mixing of Virgin River DZ with extraneous Colorado River DZ is required to explain the DZ population in the Miocene–Pliocene Muddy Creek Formation of the Virgin depression with respect to sands from the modern Virgin River or Pliocene–Pleistocene sands deposited within the Virgin depression. But neither is entry of an ancestral Colorado River into a pre-Pliocene Virgin paleodrainage wholly precluded by the DZ evidence, which is ambiguous. On the one hand, no robust statistical distinctions can be drawn for subpopulations of DZ grains older than 75 Ma, forming 90%-99% of total DZ populations, between sands of the modern Colorado and Virgin Rivers, nor between sands of the Miocene–Pliocene Muddy Creek Formation of the Virgin depression and the Miocene Crooked Ridge paleochannel east of the Grand Canyon. On the other hand, robust statistical distinctions for subordinate subpopulations (≤10%) of grains younger than 75 Ma in all sands of the Virgin depression as compared to modern or Pliocene Colorado River sands, or to sands of modern and Miocene Colorado River tributaries, provide a different perspective. For example, Miocene Crooked Ridge and Pliocene Colorado River sands contain ~5% Eo-Oligocene (40-24 Ma) DZ grains whereas Miocene–Pliocene Muddy Creek sands contain <1%, but Muddy Creek sands contain ~10% Miocene (≤23 Ma) DZ grains whereas Miocene Crooked Ridge and Pliocene Colorado River sands contain <1%. Any model for mixing Virgin and Colorado sediment within the Virgin depression that could satisfy the observed proportions of <75 Ma DZ grains would seemingly require Virgin sediment to overwhelm Colorado sediment volumetrically. As that relationship is unexpected and difficult to envision, the DZ evidence for and against the hypothesis of an ancestral Miocene Colorado River that crossed the Shivwits Plateau and entered the Virgin depression is equivocal with existing data.