Six Cordilleran Paleorivers that Connected Deforming Highlands in Idaho to Depocenters in California, Oregon, Washington, and Wyoming

Detrital zircon U-Pb ages and other data provide evidence for two voluminous, short-lived, anomalous pulses of sediment delivery from eroding highlands in Idaho to depocenters on the Pacific coast and in Wyoming, one in Campanian time (ca. 85-80 Ma) and one in Early-Middle Eocene time (ca. 53-47 Ma). This presentation will focus on the implications of these data for source regions in Idaho. In the Campanian, the Proterozoic Belt Supergroup basin in Idaho was strongly deformed and uplifted during shortening in the Sevier retroarc. Erosion of Belt strata fed three paleoriver systems. One system flowed E and sourced a major nonmarine megafan in the Cordilleran foreland basin. The Kione River flowed SW to northern California, where it sourced a major delta and submarine fan complex within the Great Valley forearc basin. The Swakane River flowed NW into Washington, sourcing the protolith for the high-grade Swakane gneiss. In the Eocene, central Idaho was re-deformed by major extension in the Bitterroot, Anaconda, Clearwater, and Priest River metamorphic core complexes (53-40 Ma) and by major volcanism in the Challis volcanic field (51-43 Ma). Three Eocene paleorivers delivered Idaho detritus to the Great Valley, to the Tyee forearc basin in Oregon, and to the Green River basin in Wyoming. The six paleorivers help in reconstructing the paleogeography of the US northern Rocky Mountains region. Selected implications are: (1) Campanian time may have been a period of particularly intense Sevier deformation and erosion in central Idaho. (2) The Kione paleoriver from Idaho to California is evidence of a lowland (or deep canyon) that may have formed the northern edge of an elevated Nevadaplano. (3) A ca.-48-Ma paleodrainage divide (Challis age) may have passed between the Atlanta and Bitterroot lobes of the Idaho batholith, because the Tyee basin received little Bitterroot-age zircon. (4) Bimodal, rift-related, 1380-Ma plutonic rocks that intruded Belt strata are an important record of the later stages of the breakup of the Columbia supercontinent. 1380 Ma detrital zircons are surprisingly abundant in Campanian strata and suggest that the 1380 Ma plutonic rocks were once more extensive than currently appreciated and may have been largely removed by erosion and/or assimilated into post-Campanian plutons of the Idaho batholith.