UPLIFT AND EROSION HYPOTHESIS FOR THE MISSING SECTION OF THE PROTEROZOIC AND PALEOZOIC MIOGEOCLINAL PRISM IN THE SALMON RIVER SUTURE REGION, WESTERN IDAHO

A significant unconformity in the Blue Mountains of northeastern Oregon separates Permian and Triassic metasedimentary rocks intruded by Late Jurassic to Early Cretaceous plutons below from early Tertiary sedimentary rocks above. The eroded surface of the crystalline rocks below the nonconformity displays paleo-potholes, channels, and other dramatic evidence of fluvial erosion. The basal sediments above the unconformity contain an impressive cobble-boulder conglomerate up to 4 m thick as well as sedimentary structures and textures indicating that the area was host to a vigorous high-gradient river system proximal to a rapidly eroding highland. The clast population includes rock types, especially quartzite, that are absent from the Blue Mountains. We infer that (1) clasts of red and white quartzite were derived from the Proterozoic Belt Supergroup, and (2) clasts of mottled black-gray-white chert were derived from authigenic chert in Paleozoic carbonates. After restoring paleomagnetically determined rotation, paleocurrents and channels show that the source of these cobbles lay to the east. Today, east of this paleoriver system, the sedimentary rocks that make up the Proterozoic and Paleozoic miogeocline are missing in the well-known but controversial gap in the Cordilleran miogeocline. We propose that in early Tertiary time, the miogeoclinal prism was present in the gap and was uplifted and rapidly eroded by the river system. Our hypothesis differs from those that explain the gap as having resulted from Precambrian transform faults offsetting the rifted continental margin, Mesozoic strike-slip faulting, or subduction of the miogeocline.