PLIOCENE TRUNCATION OF A MAJOR NORTH-FLOWING RIVER SYSTEM BY THE YELLOWSTONE HOTSPOT TRACK NEAR MONIDA PASS, IDAHO/MONTANA

Recent fieldwork along the Montana/Idaho Continental Divide near Monida Pass indicates that a large river with tributaries in Nevada and Utah flowed north across the present locations of the Eastern Snake River Plain and Continental Divide prior to being cross-cut by the Yellowstone hotspot track, less than 4.2 million years ago. A stream-rounded gravel deposit, > 800 m thick, underlies a wide section of the Divide. The gravel has north-directed paleo-flow indicators and contains cobbles with apparent provenance in the Great Basin of Nevada and Utah. The gravel deposit is faulted and dips south from the Divide toward the Eastern Snake River Plain, where it is onlapped by horizontal Pleistocene basalt flows. The gravel is interlayered with tuff and basalt of the Heise volcanic field, ranging in age from 6.8 to 4.2 Ma. It contains cobbles of apparent Heise tuff and possible Eocene Challis rhyolite. Large SE-facing slump-folds bisect the tilted gravel deposit. Quartzite cobbles in the gravel are commonly deformed by pressure-solution pits, small normal and reverse faults, and slickensides. These kinematic indicators record down-slope slip normal to the slump-folds. There appears to be a dramatic change in cobble provenance above the 4.45 Ma Kilgore tuff, with cobbles having a possible Nevada provenance dropping out of the cobble assemblage, while cobbles with possible Utah provenance continue upward through the section. This may record the truncation of the Nevada tributary of the paleo-river by the Kilgore caldera as the hotspot track intersected the paleo-valley. The thick gravel and tuff deposit plunges into a region characterized by sinking streams, suggesting that it may comprise a significant component of the Eastern Snake River Plain aquifer.