AMAZON-SCALE, MID-CENOZOIC “BELL RIVER” DRAINAGE BASIN OF NORTH AMERICA: IMPLICATIONS FOR TECTONICS OF THE WESTERN INTERIOR

Pleistocene continental glaciation destroyed the “Bell River” drainage basin of Canada (named from R. Bell, who first proposed it in 1895). From Eocene through Pliocene time the continental-scale paleo-river, with headwaters in the Cordilleran highlands, discharged into a large delta in the northern Labrador Sea. The paleo-drainage system was analogous in scale and importance to the Amazon basin of South America, which could provide a new template to constrain the timing and amplitude of post-mid-Cenozoic tectonic displacements that have shaped the modern landscape of the western interior of North America. The southern tributary of the basin may have drained the SW Colorado Plateau, carved part of Grand Canyon, followed a rift system on the east side of the Nevadaplano, and left identifiable remnant fluvial deposits in Nevada, Utah, Idaho, Montana, and Saskatchewan. In SW Montana, these deposits comprise the Eocene-early Miocene Renova Formation and the unconformably-overlying middle Miocene-late Pliocene Sixmile Creek Formation. Ash-rich Renova mudstone may represent distal fluvial deposits derived in part from the mega-calderas of the southern Nevada and Utah ignimbrite flare-up, transported northward in a broad Paleogene rift system. Stream-rounded Sixmile Creek gravel may have been derived in part from the Basin-Range of Nevada and Utah. The unconformity between the dominantly fine-grained Renova Formation and the conglomeratic Sixmile Creek Formation coincided with the initiation of Basin-Range faulting and, in Montana, records a tectonically-driven drop in base-level within the paleo-drainage basin. We propose that the paleo-drainage basin was dissected by Basin-Range faulting in the Great Basin (~ 20-15 Ma), cross-cut by the Yellowstone hotspot track in Idaho (~ 7 Ma), diverted from Grand Canyon to the Gulf of California (~5-6 Ma), and deflected by continental glaciation in Montana (< 1 Ma).