NORTH-FLOWING PALEO-RIVER ON THE MONIDA PASS SEGMENT OF THE MONTANA-IDAHO CONTINENTAL DIVIDE: DOWN-STREAM REACH OF THE EARLY COLORADO RIVER SYSTEM DESTROYED BY BASIN-RANGE FAULTING?

A thick fluvial conglomerate interlayered with Miocene and Pliocene tuff forms the Monida Pass segment of the Montana-Idaho Continental Divide in the SE Beaverhead Mountains, on the north flank of the Snake River Plain. This study constrains age and source of this previously undated deposit in order to test the hypothesis of linkage between the Late-Oligocene – early Miocene Grand Canyon and the Labrador Sea of Canada (Sears, 2013). We revise its stratigraphic assignment from the Cretaceous/Paleocene Beaverhead Group to the Neogene Sixmile Creek Formation. The deposit records the syntectonic deposition of a major influx of immature sediment in an active Basin-Range half-graben within a north-flowing drainage system of continental-scale. Stream-rounded gravel was deposited from the Middle Miocene to the Pliocene, attaining a thickness of 800 m. Coarse sediment is largely derived from proximal mass wasting events and recycled from the basal Beaverhead Group. Detrital zircon analysis of a feldspathic quartzite cobble identifies the Brigham Group of the Pocatello area, south of the Snake River Plain, as a likely source. Detrital zircons of anomalous ages from matrix sand suggest long-distance northern transport from the central Basin and Range. This major drainage, analogous and likely continuous with that of the Sixmile Creek Formation in SW Montana, persisted until uplift associated with passage of the Yellowstone volcanic system halted deposition between 4.5 and 4.1 Ma. Approximately 1 km of uplift along the Middle Creek Butte fault led to Pleistocene exposure of the deposit. The revised stratigraphic assignment of the deposit constrains Neogene tectonics in the northern Basin and Range and provides a test for the hypothesis that places the headwaters of the drainage in the early Grand Canyon region before progressive truncation by Basin and Range faulting.