EVALUATING EVIDENCE: THE CALIFORNIA/ARIZONA PALEORIVER MODEL FOR AN OLD GRAND CANYON

One model for an old Grand Canyon (Wernicke, 2011, GSAB) postulates a late Cretaceous CA paleoriver and a mid-Tertiary AZ paleoriver that flowed NE, then SW, respectively, through a paleocanyon with almost the same depth and location as modern-day Grand Canyon. The proposed CA/AZ paleoriver model has led to claims that "Grand Canyon is as old as the dinosaurs". This model may raise misconceptions about evolution of this iconic landscape. All agree that significant Sevier-Laramide relief existed and that Early Tertiary paleocanyons flowed NE off the Mogollon highlands in central AZ, but controversies persist about the timing of Colorado Plateau uplift, the age and 3-D geometry of Tertiary paleodrainages, and the age(s) of Grand Canyon carving.

The 80-70 Ma CA paleoriver hypothesis requires reconciliation of the following: 1) Paleoelevation studies are in conflict, but alternative models suggest multi-stage surface uplift rather than a single-stage Laramide surface uplift; 2) A rich dataset of combined Apatite Fission Track and Apatite Helium data indicate that 2-3 km of sedimentary rocks covered much of the Grand Canyon region at 60 Ma. 3) Older cooling ages on upthrown blocks of Laramide monoclines and northerly cliff retreat may explain the observed pattern of Laramide AFT cooling ages.

Reversal of the CA paleoriver and inception of a Paleogene AZ paleoriver is proposed with a W-flowing AZ river/canyon system in existence until mid-Miocene time. However: 1) proposed correlation of quartzite clasts in the early Tertiary Sespe Formation of California with sources in an already >half-carved eastern Grand Canyon is non-unique given quartzite sources in the Mojave; 2) the Muddy Creek constraint still remains solidly in the way of models for an AZ paleoriver at Grand Wash Cliffs. 3) AHe and AFT data indicate several km of sedimentary rocks covered much of the Marble Canyon area until after about 25 Ma, although significant paleorelief and paleovalleys were present.

The alternate model is for a < 6 Ma Grand Canyon that was mainly carved by the Colorado River, perhaps linking older paleovalley segments. This is supported by data for semi-steady incision rates over the last 4 Ma, evidence for mantle-driven Neogene uplift of the western Colorado Plateau, and geomorphic evidence for a young Grand Canyon landscape.