NEW GEOLOGIC MAPPING RESULTS FROM STATEMAP PROJECTS IN IDAHO

The Idaho Geological Survey’s STATEMAP program has substantially increased the area covered by new geologic mapping and the number of maps entered into our statewide digital database. A newly reorganized State Mapping Advisory Committee representing government, private, and public sectors has been effective in establishing mapping priorities and assisting the survey in a long-term plan. Current mapping project areas include the Sun Valley urban impact corridor, the Clearwater River corridor, and the Potlatch and Headquarters 30' x 60' quadrangles.

In the Sun Valley project area 1:24,000-scale color digital maps provide the first surficial geologic information available in this rapidly growing urban recreation area. New contributions include a more complete understanding of the geomorphic history of the Wood River Valley and episodes of slope activity in the tributary valleys, and relationship of Snake River Plain volcanism to the gradational history of the Wood River drainage.

Geologic maps in the Clearwater River corridor identify landslides and their genetic relationship to sedimentary units intercalated with Columbia River basalt flows. New surficial geologic digital maps imported into ArcView databases provide the basis for geotechnical interpretations and related planning criteria by the local counties.

New mapping in the Potlatch and Headquarters 30' x 60' quadrangles has identified metasedimentary rocks that are dissimilar to the 1400-1470 Ma Belt Supergroup, with which they have been previously correlated. This Syringa metamorphic sequence is more like amphibolite facies metasedimentary rocks in the northern Monashee Mountains of British Columbia, some of which are at least 1850 Ma in age, and to the 1700 Ma Willyama Supergroup of Australia. A-type granite sills (now augen gneiss) intrude the sequence and a preliminary U-Pb zircon age of 1372 ± 10 Ma has been obtained. New mapping has also identified a major right-lateral mylonitic shear zone that may have been an Eocene extensional transfer zone between the Bitterroot mylonite and the Priest River metamorphic complex.