WHEN MOUNTAINS CROSS RIVERS: PLIOCENE TRUNCATION OF THE ANCESTRAL UPPER MISSOURI IN THE WAKE OF THE YELLOWSTONE HOTSPOT, MONTANA/IDAHO CONTINENTAL DIVIDE

Structurally disturbed Miocene/Pliocene fluvial gravel beds that are interlayered with Yellowstone hotspot (YHS) volcanics of the Heise volcanic field document the tectonic truncation of a major north-flowing paleoriver at the Continental Divide near Monida Pass on the Montana/Idaho border. The distinctive gravel includes stream-rounded pebbles that may have been derived from the Roberts Mountain allochthon and Antler foredeep of central Nevada and the Neoproterozoic/Early Paleozoic Cordilleran miogeoclinal quartzite wedge of SE Idaho, western Utah, and eastern Nevada. The gravel comprises a facies of the Miocene/Pliocene Sixmile Creek Formation, which filled a 200-m deep, 10-km wide paleovalley across western Montana. The paleovalley follows the trend of the upper Missouri drainage basin from Monida Pass in the Rocky Mountains to Fort Benton on the Great Plains. Fluvial gravel preserved in Saskatchewan shows that the river continued into Canada before it was diverted into the Mississippi basin along the SW margin of the Laurentide continental ice sheet. In SW Montana, the paleoriver followed a Miocene graben system that was superimposed on a Paleogene rift system. In the grabens, the Sixmile Creek Formation typically overlies, with angular unconformity, Eocene volcanics and Eocene-early Miocene ash-rich beds of the Renova Formation and its equivalents. By about 5 Ma, faulting associated with the YHS diverted the paleoriver around the SE end of the rising Tendoy Range so that it crossed its older path near Dell, Montana, in the Red Rock graben valley. Uplift of the Centennial Range then pinched off the paleoriver at Monida Pass by about 4 Ma. South of the Divide, collapse of the Heise mega-calderas tilted the gravel into the Snake River Plain, where it was buried by Pleistocene basalt.