Presentation Type

Poster

Abstract

The Miocene Sixmile Creek Formation in Montana contains distinctive river cobbles of coarse-grained sandstone and conglomerate that include small grains of black chert, but the source of these cobbles has not been confirmed. The Paleozoic Diamond Peak Formation of Nevada may be one possible source of these cobbles. It is widespread in central Nevada's mountains but is unlike any bedrock formations found in Montana. If cobbles in the Sixmile Creek Formation were derived from central Nevada, the Miocene river that carried them would have had to have crossed the present positions of the Snake River Plain and the Idaho-Montana Continental Divide. The purpose of our study is to test the hypothesis by examining particularly distinctive cobbles along the proposed Miocene river path. We compared cobbles collected from the Sixmile Creek Formation at locations from Nevada to Great Falls, Montana, and compared these to each other and to samples collected from proposed source rocks at Pequot Summit, Nevada. We measured the percentage of black chert grains in each rock sample, as well as the shapes and size ranges of the chert grains. Our results support the hypothesis of a great Miocene river that flowed through Montana from headwaters as far south as central Nevada (Sears, 2014). This would have been possible in the Miocene because tectonics had not yet formed the Snake River Plain and modern Continental Divide. The river implied by this model would have been a tributary to the pre-ice age Bell River basin of Canada, which rivaled the modern Amazon River basin in scale. This research could lead to new understanding of the paleotopography and paleofluvial systems of the western interior of North America.

Category

Physical Sciences

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Apr 11th, 3:00 PM Apr 11th, 4:00 PM

Do Montana's Sixmile Creek Cobbles Have Nevada Origins? Evidence for Headwaters of the Miocene Bell River Basin

The Miocene Sixmile Creek Formation in Montana contains distinctive river cobbles of coarse-grained sandstone and conglomerate that include small grains of black chert, but the source of these cobbles has not been confirmed. The Paleozoic Diamond Peak Formation of Nevada may be one possible source of these cobbles. It is widespread in central Nevada's mountains but is unlike any bedrock formations found in Montana. If cobbles in the Sixmile Creek Formation were derived from central Nevada, the Miocene river that carried them would have had to have crossed the present positions of the Snake River Plain and the Idaho-Montana Continental Divide. The purpose of our study is to test the hypothesis by examining particularly distinctive cobbles along the proposed Miocene river path. We compared cobbles collected from the Sixmile Creek Formation at locations from Nevada to Great Falls, Montana, and compared these to each other and to samples collected from proposed source rocks at Pequot Summit, Nevada. We measured the percentage of black chert grains in each rock sample, as well as the shapes and size ranges of the chert grains. Our results support the hypothesis of a great Miocene river that flowed through Montana from headwaters as far south as central Nevada (Sears, 2014). This would have been possible in the Miocene because tectonics had not yet formed the Snake River Plain and modern Continental Divide. The river implied by this model would have been a tributary to the pre-ice age Bell River basin of Canada, which rivaled the modern Amazon River basin in scale. This research could lead to new understanding of the paleotopography and paleofluvial systems of the western interior of North America.