NEW GEOLOGIC MAP OF THE BURNS MOUNTAIN 7.5' QUADRANGLE, SOUTHWEST MONTANA

The Burns Mountain 7.5' quadrangle was mapped by the Montana Bureau of Mines and Geology (MBMG) to enhance map coverage of the Tertiary volcanogenic units and to support the seamless 1:100,000 scale map of the State, a long-term MBMG priority supported by the U.S. Geological Survey (USGS) STATEMAP program. Field studies were conducted in 2019, using a smartphone GPS and conventional field sheets to collect field data. The data were then digitized to the USGS Geologic Map Schema (GeMS) standard for map publication at 1:24,000 scale. Bulk-rock geochemical analyses and U-Pb zircon geochronology were used to characterize and correlate the Tertiary map units.

The Burns Mountain quadrangle is in Beaverhead County, approximately 15 km (9.3 mi) west of Dillon, Montana, spanning parts of the Beaverhead Valley, Armstead Hills, and Blacktail Mountains. The oldest bedrock units exposed are minor occurrences of Paleozoic Quadrant Formation and Late Cretaceous sedimentary-volcanogenic deposits of the Beaverhead Group and Bannack Group (75.3 Ma). Eocene volcanic rocks of the informal Dillon Group unconformably rest on the Cretaceous and older units, comprising dacite porphyry (50.0–49.2 Ma), mafic lava flows, and intercalated rhyolitic lava and tuff deposits (48.4–46.6 Ma). The Eocene units occupy the footwall of the Muddy-Grasshopper fault and were likely emplaced during the onset of local extensional faulting. Clastic sedimentary deposits of the Renova Formation (~25.4–21.0 Ma) onlap the tilted footwall of the Muddy-Grasshopper fault, unconformably resting on the older Cretaceous–Eocene volcanogenic units. Poorly lithified deposits of the Rattlesnake Creek gravel unconformably rest on and are faulted against the Renova Formation and older map units. Eastward dipping pediment surfaces, often covered with a veneer of gravel (< 2 m), are formed on the Tertiary sedimentary units. Two parallel, northwest-striking normal faults cross the quadrangle, displacing the Late Cretaceous through Miocene map units. Glacial outwash sourced from the southern Pioneer Mountains underlies the north part of the quadrangle. Extensive landslide deposits are mainly formed in Tertiary volcanic deposits.