GEOLOGIC MAP OF THE BOZEMAN 30’ X 60’ QUADRANGLE, SOUTHWESTERN MONTANA

A geologic map of the Bozeman 30’ x 60’ quadrangle was recently released on open file by the Montana Bureau of Mines and Geology through the STATEMAP component of the U.S. Geological Survey National Cooperative Geologic Mapping Program. The map incorporates new detailed field mapping by the authors with mapping compiled and modified from older sources including numerous thesis and dissertation maps by students of the Indiana University Geologic Field Station.

The Southwest Montana Transverse Zone (SWMTZ), a prominent tectonic feature of the map, separates the southern margin of the Helena Salient of the fold-thrust belt to the north from Rocky Mountain foreland structures to the south. The SWMTZ represents reactivation of the Precambrian Willow Creek fault along which deposits derived from the uplifted crystalline metamorphic basement rock to the south were shed into the Mesoproterozoic Belt basin. The most prominent structures of the Precambrian basement rocks south of the SWMTZ are NW-striking, NE-dipping faults that have experienced several episodes of movement beginning in the Proterozoic.

New geologic mapping and geophysical work by the authors focused on the controls that the Willow Creek fault has had on the SWMTZ, the fold/fault relationships in the fold-thrust belt, the relationship of the NW-striking basement faults to intrusion of the Tobacco Root batholith, and seismicity along the NW-striking faults. Mapping the extensive valley deposits provides geologic information for land-use planning, particularly in the Gallatin Valley which has recently experienced the highest population growth rate in Montana. Mapping valley deposits has also revealed Quaternary faults with the same strike as the Precambrian basement faults, and has led to fossil discoveries such as an Eocene skull of the first legitimate southwestern Montana Metamynodon (amynodontid rhinoceros) fossil, now at the Museum of the Rockies. The map provides continuity across previously unmapped areas of the metamorphic basement in the southern part of the quadrangle, timely for research underway that is leading to reinterpretation of the age and genesis of these rocks.