Abstract
Determining the paleogeographic evolution of the Lake Mead region of southern Nevada and northwest Arizona is crucial to understanding the geologic history of the Southwest, including the evolution of the Colorado Plateau and Grand Canyon. The approximately 25-17 Ma Rainbow Gardens Formation in the Lake Mead region as well as the informally named, roughly coeval Jean Conglomerate and the approximately 24-19 Ma Buck and Doe Conglomerate southeast of Lake Mead hold the only stratigraphic evidence for the Cenozoic pre-extensional geology and paleogeography of this area. We use new sedimentologic and stratigraphic data to better define the paleogeography of the southern part of the Rainbow Gardens basin and surrounding region, test the hypothesis that extension began in the southern Lake Mead region approximately 19 Ma, and examine how Rainbow Gardens strata inform the formation of the Grand Canyon debate. Sandstone provenance and detrital zircon data confirm that sediment was sourced primarily from Paleozoic strata exposed in surrounding Sevier and Laramide uplifts and active volcanic fields to the north. In addition, a distinctive signal of coarse sediment derived from Proterozoic crystalline basement first appeared in the southwestern corner of the basin approximately 25 Ma at the beginning of Rainbow Gardens deposition and then prograded north and east approximately 19 Ma across the southern half of the basin. Streams carrying crystalline basement sediment either travelled around the western end of an escarpment of Permian strata on the Kingman uplift or breached the escarpment. Deposition of the Rainbow Garden Formation in general, and the 19 Ma progradational pulse in particular, may reflect tectonic uplifts just prior to onset of rapid extension at 17 Ma as supported by both thermochronology and sedimentary data. These data negate the California and Arizona River hypotheses for an "old" Grand Canyon and also negate models that the Rainbow Gardens Formation was the depocenter for a 24-18 Ma Little Colorado paleoriver flowing west through East Kaibab paleocanyons. Instead, provenance and paleocurrent data suggest local to regional sources for deposition of the Rainbow Gardens Formation atop a stripped, low-relief western Colorado Plateau surface and preclude any significant input from a regional through-going paleoriver entering the basin from the east or northeast.