The California River and its role in carving Grand Canyon

Abstract

Recently published thermochronological and paleoelevation studies in the Grand Canyon region, combined with sedimentary provenance data in both the coastal and interior portions of the North American Cordillera, place important new constraints on the paleohydrological evolution of the southwestern United States. Review and synthesis of these data lead to an interpretation where incision of a large canyon from a plain of low elevation and relief to a canyon of roughly the length and depth of modern Grand Canyon occurred primarily in Campanian time (80-70 Ma). Incision was accomplished by a main-stem, NE-fl owing antecedent river with headwaters on the NE slope of the North American Cordillera in California, referred to herein after its source region as the California River. At this time, the river had cut to within a few hundred meters of its modern erosion level in western Grand Canyon, and to the level of Lower Mesozoic strata in eastern Grand Canyon. Subsequent collapse of the head waters region into a continental border land and coeval uplift of the Cordilleran foreland during the Laramide orogeny reversed the river's course by Paleogene time. After reversal, its terminus lay near its former source regions in what is now the Western Transverse Ranges and Salinian terrane. Its headwaters lay in the ancient Mojave/Mogollon Highlands region of Arizona and eastern California, apparently reaching as far northeast as the eastern Grand Canyon region. This system is herein referred to after its source region as the Arizona River. From Paleogene through late Miocene time, the interior of the Colorado Plateau was a closed basin separated from the Arizona River drainage by an asymmetrical divide in the Lees Ferry-Glen Canyon area, with a steep SW fl ank and gently sloping NE fl ank that drained into large interior lakes, fed primarily by Cor dilleran/Rocky Mountain sources to the north and west, and by recycled California River detritus shed from Laramide uplifts on the plateau. By Oligocene time, the lakes had largely dried up and were replaced by ergs. By mid-Miocene time, a pulse of unroofi ng had lowered the erosion level of eastern Grand Canyon to within a few hundred meters of its present level, and the Arizona River drainage below modern Grand Canyon was deranged by extensional tectonism, cutting off the supply of interior detritus to the coast. Increasing moisture in the Rocky Mountains in late Miocene time reinvigorated fl uvio lacustrine aggradation NE of the asymmetrical divide, which was fi nally overtopped between 6 and 5 Ma, lowering base level in the interior of the plateau by 1500 m. This event reintegrated the former Arizona drainage system through a cascade of spillover events through Basin and Range valleys, for the fi rst time connecting sediment sources in Colorado with the coast. This event, combined with the intensifi cation of summer rainfall as the Gulf of California opened, increased the sediment yield through Grand Canyon by perhaps two orders of magnitude from its Miocene nadir, giving birth t...

Response to Comments on “Apatite 4 He/ 3 He and (U-Th)/He Evidence for an Ancient Grand Canyon”

“…These data contradict the prevailing view of post-6 Ma carving of the entire canyon and instead support an ancient canyon model (2). Below, we first offer an alternative explanation for the geological observations of Lucchitta (3).…”

“…Combined with a more arid Miocene climate, such a system would not transport substantial fluvial detritus to the trough. Moreover, the evidence of any limited aggradation would have been removed during the 530-m excavation of the Grand Wash Trough after integration of the modern, far larger Colorado River (2,9). Pierce Canyon fan deposition in the Grand Wash Trough was accomplished by a high-gradient stream issuing from a steep channel and yet was restricted to within a few kilometers of its canyon.…”

The Paleogene California River: Evidence of Mojave-Uinta paleodrainage from U-Pb ages of detrital zircons

“…1 for locations of control samples) contain a much larger proportion of Grenville age (1315-1000 Ma) grains, minor but ubiquitous populations of Neoproterozoic (650-550 Ma) and Paleozoic (500-400 Ma) grains, and much fewer grains younger than 275 Ma than our Colton samples ( Fig. 2C; Dickinson and Gehrels, 2008a, 2008b, 2009. While recycling of Grenville, Neoproterozoic, and Paleozoic age grains from Mesozoic strata of the Colorado Plateau could account for the few grains of those ages in our Colton samples, no such recycling could account for the abundance of arc-derived grains in the Colton, or for the preponderance of pre-Grenville (1800-1600 Ma) grains in the Proterozoic subpopulation of Colton zircons (Fig.…”

“…Others have suggested Laramide paleorivers fl owing northeast through Arizona (e.g., Potochnick, 2001), which may have carved an ancestral Grand Canyon (Wernicke, 2009) and then reversed direction as early as to the Cretaceous−Tertiary boundary (Wernicke, 2009) or as late as Miocene time (Potochnik, 2001). Unlike the Cretaceous California River described by Wernicke (2009), however, we fi nd evidence of a river transporting arc-derived sediment from the Mojave region to northeast Utah during the Paleogene, with little addition of sediments from units being eroded in the foreland. Nonetheless, extension and lowering of areas adjacent to the Colorado Plateau alone could not have reversed the direction of a California River debouching in northern Utah (cf.…”

“…DR1-DR6). Of 248 detrital zircon grains that provided acceptable U-Pb ages and had uncertainties that either overlapped concordia or were close enough to meet standard acceptance criteria (Dickinson and Gehrels, 2008a), 131 grains were younger than 275 Ma and 117 grains were older than 275 Ma, the age of the oldest magmatic arc assemblages along the Cordilleran margin (Dickinson and Gehrels, 2008a, 2008b, 2009). …”