NORTHERN ROCKY MOUNTAINS TRANSECT, IDAHO-MONTANA: CONTRIBUTIONS TO A 2D AND 3D NATIONWIDE GEOLOGIC SYNTHESIS

The 2020-2030 strategic plan for the USGS National Cooperative Geologic Mapping Program specifies creation of a seamless, variable scale, 2D and 3D geologic framework of the U.S. As part of that plan, the U.S. GeoFramework Initiative (USGI) uses modern geologic concepts and new data to integrate and resolve geologic discontinuities across geologic provinces and map boundaries. The resulting GIS data will enable fit-for-purpose construction of geologic maps and models for areas of interest across the nation.

The USGI is working on two large swaths in the western United States. The Intermountain West Transect stretches east-west through southern Colorado, Utah, and Nevada into California. The Northern Rocky Mountains Transect (NRMT), highlighted here, runs north-south through eastern Idaho and western Montana.

The NRMT is updating 1:250,000-scale geologic quadrangles across 3 geologically distinct zones (North, Middle, and South). The procedure is to 1) build a GIS foundation by updating legacy data for regional-scale maps to the Geologic Map Schema format; 2) acquire new age, correlation, and geologic map data to update existing 1:250,000-scale quadrangles; 3) produce first-generation geologic maps where they are currently lacking; and 4) produce cross sections for 3-D interpretation.

In the North zone, fieldwork and shared mining industry data are being used to update mapping of the basal Prichard Formation of the Mesoproterozoic Belt Supergroup, which hosts the world-class Pb-Zn-Ag Sullivan deposit in Canada. Geochemical and stratigraphic data are being used to characterize plutonic rocks and to resolve correlation problems in sedimentary units. In the Middle zone, geochronologic and mapping studies are clarifying 1) ages and relations among significant stratigraphic units and terranes, and 2) timing and deformation related to accretion along the Salmon River suture. The goal is to fill map gaps and to constrain the timing and geometry of island arc terrane accretion to Laurentia. In the South zone, fieldwork and subsequent geochronologic analysis are focused on improving the temporal resolution of several previously mapped bodies, including smaller intrusions and metamorphic rocks in the Idaho Batholith and volcanic rocks in the western Snake River Plain and the Mount Bennet Hills of Idaho.