POSSIBLE MIDDLE MIOCENE STRATH TERRACES IN THE GRAND CANYON LEAD TO THE VIRGIN MOUNTAINS: DID THE COLORADO RIVER FLOW INTO THE GREAT BASIN FROM 17 TO 6 MA?

Preliminary observations suggest that a broad fluvial erosional surface, a strath terrace, may transect the familiar landscape of Grand Canyon National Park. The terrace smoothly declines to the west as it crosses bedrock structures of the Kaibab Plateau. Between Desert View Tower and Bass Canyon, the terrace forms the well-known bench at the top of the Redwall Limestone, but it cuts across bedding on the west flank of the Kaibab Plateau to occupy the Esplanade surface at the top of the Supai Group in western Grand Canyon. East of Desert View Tower, the strath crosses into the Bright Angel Shale at the apex of the East Kaibab Monocline, then may cross the north edge of Nankoweap Mesa at the level of the Coconino Sandstone. In western Grand Canyon, 17-Ma Grand Pipe volcanic breccia may date the strath to Middle Miocene. Normal faults offset the strath, but the strath cross-cuts Laramide folds. The strath appears to bend north at Grand Wash to cross the Virgin Mountains near Mesquite, Nevada. The Middle Miocene Colorado River may have flowed along the strath into the Great Basin, where it may have fed a complex system of lakes at a base level of about 4000 feet, depositing such units as the Muddy Creek Formation. Then, at about 5-6 Ma, faulting associated with opening of the Gulf of California may have captured the Colorado River at Grand Wash and dramatically lowered the base level, so that the deeper half of the Grand Canyon incised the Miocene strath.