STRATIGRAPHY OF THE NEOGENE SIXMILE CREEK FORMATION FROM THE GALLATIN BASIN – IMPLICATIONS FOR MIDDLE MIOCENE TO PLIOCENE LANDSCAPE EVOLUTION IN THE YELLOWSTONE REGION

The Yellowstone region’s remarkable landscape is the result of active geological processes that are attributed to the Yellowstone hotspot encroaching on the base of the North American continental lithosphere. Despite the hotspot’s impact on the landscape, the spatial and temporal patterns of deformation in regions near the hotspot are unclear. The Mid-Miocene to Pliocene Sixmile Creek Formation found in southwest Montana basins is argued to contain a stratigraphic record of paleodrainage and landscape changes caused by the arrival of the hotspot. Here we examine the stratigraphy of the Sixmile Creek Formation in the Gallatin River drainage to test the hypothesis that this sedimentary package records the arrival of the hotspot.

We map four units in the Sixmile Creek Formation in the Madison Bluffs on the western edge of the Gallatin Basin. Unit 1 consists of vertically-stacked, floodplain and channel-fill deposits with lateral accretion surfaces, representing a meandering fluvial system. Conglomerate clasts in Unit 1 are dominantly orthoquartzites and other sedimentary rocks similar to clasts in conglomerates of the underlying Renova Formation. We interpret this to represent reworked Renova formation, perhaps derived from Renova Formation uplifted by the adjacent Elk Creek Fault. Unit 2 consists of several fining-upward sequences and is interpreted to represent a change to a less sinuous fluvial system. Clasts in Unit 2 are dominantly volcanic and metamorphic clasts sourced from the adjacent uplifted Gallatin and Madison Ranges. We interpret this to represent the initiation of motion on the Gallatin Range and Elk Creek faults. Unit 3 documents a decrease in grain size and increase in tuffaceous sand and reworked tephra. The increase in volcanic material is interpreted to be airfall and fluvially-reworked tephra sourced from the hotspot as it approached the Gallatin Basin. Unit 4 is a widespread sheet of cobble conglomerate with dominantly orthoquartzite clasts. The source of Unit 4 is unclear but could represent a change in paleodrainage due to the hotspot. Samples being processed for ⁴⁰Ar/³⁹Ar dating will constrain the Unit 1 to Unit 2 transition and future work will focus on refining the depositional age, provenance, and stratigraphy of the Sixmile Creek Formation in the Gallatin Basin.