Paper No. 7
Presentation Time: 2:45 PM


SEARS, James, Dept. of Geosciences, University of Montana, 32 Campus Drive #1296, Missoula, MT 59812-1296,

A recent hypothesis that the Late Oligocene-Early Miocene Colorado River may have flowed north out of western Grand Canyon to follow a rift system through Nevada, Idaho, and Montana to join the pre-Pleistocene Bell River of Canada requires the river to have crossed the present positions of the Snake River Plain (SRP) and Idaho/Montana Continental Divide. Miocene beds in the ancestral upper Missouri River basin of Montana contain stream-rounded gravel that appears to have been derived from the Antler orogenic belt of Nevada and from miogeoclinal quartzites of SE Idaho and Utah. The gravel occurs in fluvial deposits of the Sixmile Creek Formation that are up to 200-m thick where preserved in grabens and that are constrained in age to Middle and Late Miocene. The river gravel fills a broad channel that was cut during Late Oligocene/Early Miocene time. Paleocurrent measurements and clast-size distributions indicate NE flow. Pebbles consistently match bedrock lithologies that occur to the SW of sample sites. Recent mapping has traced remnants of the fluvial gravel for 750 km, from the Great Plains at Wood Mountain, Saskatchewan, SW to the SRP. Near the Continental Divide, the gravel encloses ca 6-Ma tuff and basalt, and is capped by the 4.45 Ma Kilgore Tuff. The gravel and volcanics are faulted and folded over broad anticlines between the plunging ends of the Centennial and Beaverhead Ranges, and descend into the SRP beneath a veneer of Plio-Pleistocene basalts. Windows through SRP basalts reveal the distinctive gravel 100 km south of the Divide. There appears to have been a paleo-confluence near Mud Lake, Idaho, where a paleoriver bearing Antler clasts from the SW joined one bearing miogeoclinal clasts from the SE. The paleoriver was still flowing into Montana after 6 Ma, but not after 4.45 Ma. When the paleo-river was blocked by the rising Centennial Range, the stream changed its course to the SW and evidently drained into Lake Idaho. If the Grand Canyon was being carved by 23-17 Ma, as implied by published apatite dating, the Colorado River may have flowed north into the coeval paleovalley of Nevada, Idaho, Montana, and Canada, until cut off by basin-range faulting at about 17 Ma. The northern paleovalley was dissected by basin-range faulting, cross-cut by the Yellowstone hotspot track, and deflected by Pleistocene continental ice sheets.