DETAILED STRIP MAP OF ROADCUT GEOLOGY OF IDAHO STATE HIGHWAY 14: A WINDOW INTO MIGMATITE AND LEUCOSOME VARIETIES OF THE ELK CITY REGION, NORTH-CENTRAL IDAHO

Geologic mapping in northern Idaho is frequently challenging due to dense tree coverage and deep soil development, which obfuscates critical geology. When available, roadcuts along transportation corridors can offer rare exposures in these hard to map areas. A 3.5 mile roadcut along Idaho State Highway 14, which follows the South Fork of the Clearwater River in north central Idaho, fortuitously cuts across the structural grain of a migmatite complex that includes high-grade metamorphic rocks coupled with igneous rocks. This exposure serves as an exemplar of the utility of roadcuts for mapping in heavily forested regions, which can be exploited and used as ‘rosetta stones’ for deciphering covered geology.

Rocks exposed in this roadcut include Mesoproterozoic sillimanite-grade metasedimentary and metaigneous rocks, Mesozoic(?) migmatite, Late Cretaceous granites of the Atlanta Lobe of the Idaho batholith, and lesser abundances of Cenozoic sediments and intermediate shallow intrusive rocks. While the western part of the roadcut is dominated by massive Late Cretaceous granite, the eastern portion is defined by steeply dipping metamorphic rocks with abundant, but highly variable, leucocratic bodies and horizons.

The architecture of this migmatite complex is defined by a tonalitic migmatite core that is exposed across ~3.5 miles of roadcut high walls. this migmatite characterized by very coarse-grained to pegmatitic, highly contorted and folded, leucosomes with envelopes of biotite dominant melanosomes; meter-scale xenoliths of metasedimentary and metaigneous are also common. The high-grade gneiss, feldspathic quartzites, and calc-silicate granofels that bound the migmatite core exhibits abundant lit-par-lit injection of granitic material, which forms cm-scale leucocratic boudins to meter-scale massive sheets. The contacts between the metamorphic rocks and the micaceous granite of the Idaho batholith are typically defined as large-scale igneous breccias. Without the access granted by this roadcut, the tectonic history of this area would be near impossible to interpret.