‘SAGLEK SUPER-RIVER' AND THE EARLY MIOCENE (22-17 MA) BEGINNING OF THE GRAND CANYON

Late Oligocene to Early Miocene rifting established the upper Colorado River basin. Uplift of the shoulders of the rifts warped the Paleogene land surface into a broad, southwest-plunging structural basin. The upper Colorado River followed the main axis of the structural basin while tributary rivers followed second-order structural troughs on its true left bank. Oligocene monadnocks that stand above broad, Middle Miocene paleovalleys show that vigorous Early Miocene erosion stripped 1-2 km from the upper Colorado River basin. The immense sediment flux from 22 to 17 Ma required a through-going river several times larger than the modern upper Colorado, as well as an appropriate sediment depocenter. A set of Middle Miocene paleovalleys that cross the Navajo Reservation merge with a broad bedrock terrace that transects the Grand Canyon about midway in elevation between the rim and the river; that terrace may represent the Middle Miocene floor of the Grand Canyon.

I propose that the Colorado River exited the Early Miocene Grand Canyon to the north and flowed through rift valleys in the Great Basin and Northern Rockies, where it joined the already well-established ‘Saglek River’ basin in northern Montana. Miocene paleovalleys along the reconstructed river basin contain fluvial deposits with provenances that indicate northward flow from the Colorado Plateau to the Great Basin, from the Great Basin to central Montana, from central Montana to Saskatchewan, and from Saskatchewan to the Saglek basin in the Labrador Sea. Balkwill et al. (1990) proposed that a ‘super-river’ of the scale of the Mississippi fed the Saglek basin, the largest Cenozoic depocenter along the eastern seaboard of North America. The scale of the Saglek basin can accommodate the sedimentary volume denuded from the Colorado Plateau during the Early Miocene. Late Cenozoic glaciation, volcanism, and tectonics segmented the ‘super-river’ into the modern drainage basins.