STRATIGRAPHY AND STRUCTURAL GEOLOGY OF THE NORTH FORK OF THE SALMON RIVER AND VICINITY, IDAHO

New geologic mapping of 450 square kilometers along the North Fork of the Salmon River reveals a homoclinal sequence of >10,500 meters of feldspathic quartzites. We correlate the lower part of the section to the Mesoproterozoic Gunsight Formation of the Lemhi Group, the middle to the Swauger and Lawson Creek formations, and the upper part to the newly recognized and stratigraphically overlying quartzite of Jahnke Lake (Burmester et al., 2013). This newly recognized stratigraphic section requires two important revisions to previous structural interpretations of the area. First, near Gibbonsville, the homoclinal stratigraphic sequence is interrupted by a large north-south striking graben partly filled with Challis volcanics. The graben initiates near Lost Trail Pass to the north, and is bounded to the east by the mineralized Anderson Creek fault. Second, south of Gibbonsville, a major east-west striking normal fault, which we name the Lick Creek fault, places Swauger Formation quartzite over rocks we tentatively correlate to the Yellow Lake unit or underlying Big Creek Formation of the Lemhi Group. If the correlation is correct, the fault must cut out >7000 meters of Proterozoic section, and so is likely a major regional structure. To the east, the fault is buried by Tertiary and Quaternary sediments of the Big Hole basin, but may merge with the Anaconda detachment fault at depth. If this interpretation is correct, like the Bitterroot detachment fault, the Anaconda detachment fault also swings west along its southern edge.