57-6 STRATIGRAPHY AND GEOCHRONOLOGY OF THE NEOGENE SIXMILE CREEK FORMATION FROM THE GALLATIN VALLEY – IMPLICATIONS FOR MIDDLE MIOCENE TO PLIOCENE LANDSCAPE EVOLUTION, SOUTHWEST MONTANA

Session: Advances and Applications in Geochronology for Interpreting Stratigraphic and Basin Records, Part II

Presenting Author:

Authors:

Abstract:

The mid-Miocene to Pliocene Sixmile Creek Formation and the underlying unconformity that separates it from the Eocene to early-Miocene Renova formation are found in basins throughout southwest Montana. The Sixmile Creek Formation and unconformity are argued to preserve a stratigraphic record of changing tectonic and landscape conditions associated with the arrival of the Yellowstone hotspot and the initiation of Basin and Range-style extension. We break the Sixmile Creek Formation up into Units 1-4 from oldest to youngest in the Madison Bluffs on the western edge of the Gallatin Valley based on distinct changes in the stratigraphy. Sanidine ⁴⁰Ar/³⁹Ar and detrital zircon geochronology constrain the age of the unconformity and initiation of deposition to between 21.5 and 17 Ma, and the end of deposition to after 7 Ma in the Madison Bluffs area. Lithofacies analysis in Units 1-3 suggests a fluvial depositional system. Unit 4 is a widespread pediment gravel with a pediment surface inset on top. Detrital zircon spectra are similar throughout the entire Sixmile Creek Formation, except with the addition of younger zircon grains up-section. Clasts counts, paleocurrents, and detrital zircon age spectra are consistent with sourcing of sediment from local uplifted fault blocks and from rocks upstream in the Gallatin and Madison River catchments. We suggest that a drainage pattern similar to the modern-day Gallatin and Madison River systems was established by the time the Sixmile Creek Formation was deposited. This is consistent with paleocurrent and clast count data from the underlying Renova Formation showing that the Paleogene drainage system of the region was similar to that of the modern-day drainage pattern. The appearance of clasts that are sourced from uplifted fault blocks in the Sixmile Creek Formation and the presence of the basal unconformity is interpreted to signify the initiation of movement on nearby Basin and Range-style faults. This constrains the onset of Basin and Range-style extension in the region to between 21.5-17 Ma, which is similar with the onset of Basin and Range extensional province south of the Snake River Plain.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9281

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