TRACKING THE UPLIFT OF YELLOWSTONE'S THERMAL DOME IN SW MONTANA

Preliminary observations suggest that there is a significant height difference between Miocene-age river terraces in southwestern Montana and similar river terraces farther north. It is hypothesized that this dissimilarity is due to an increase in fluvial incision rates corresponding to surface uplift attributed to the Yellowstone thermal dome. Commonly, the terraces are overlain by Miocene Sixmile Creek Formation and cut underlying Miocene Renova Formation. This interface, known as the Renova unconformity, represents a nearly one million year gap spanning from 17.5 Ma to 16.8 Ma and provides a consistent reference surface throughout the field area for height measurement. Active normal faulting farther south has resulted in the location of the unconformity high up in mountains as well as at the bottom of gravel-filled grabens. Mapping the elevation of this surface with respect to current river elevations will provide a structure contour map of the million-year-spanning unconformity. Because the surface to be mapped is dated to 17Ma, any structure recorded will have occurred in the past 17 million years. Also, the stratigraphy of the Sixmile Creek Formation further constrains the start of deformation in some areas to 4Ma. Tectonic reconstruction of the field area will be completed, thus measuring fault displacement. Total surface uplift and extension will be calculated as part of the reconstruction. An integral aspect of this investigation relies on gaining subsurface control of the Renova unconformity where it is buried deep beneath the surface. To this end, existing gravity-anomaly data will be analyzed and interpreted. Multiple theories concerning the origin and migration of the Yellowstone hotspot hinge on the shape and magnitude of its thermal dome. A systematic analysis of Miocene river terrace elevation has never been performed for the area surrounding Yellowstone. If successful, this investigation will serve as a quantitative test of existing Yellowstone uplift boundaries.