PROVENANCE, PALEOGEOGRAPHY, AND RESOURCE POTENTIAL OF LATE EOCENE TO EARLY OLIGOCENE FLUVIAL CONGLOMERATES, NE OREGON AND NE CALIFORNIA

On the edge of a large area of Neogene volcanic cover in the NW Basin and Range, scattered outcrops of polymict fluvial conglomerate and sandstone, ≤2 km2, ≤100m thick, nonconformably overlie Cretaceous plutonic rocks. Reported ages are late Eocene-early Oligocene in OR and late Eocene-early-Miocene in CA. Deposits consist of three, unconformity-bounded units (Tc1-3) with similar lithostratigraphy and with conglomerate clast populations (n=4479) that are a mix of local plutonic and metamorphic rocks:PM, first-cycle volcanic rocks:V1, and multicycle, sedimentary and volcanic rocks sourced, in part, from the northern Rocky Mtns:QSV2. Tc1 (≤10m) has pebble conglomerates with variable clast percentages: PM:14-95, V1:2-97, QSV2:3-62, and petrified wood:1-60%, with intact paleologs to 30cm. Individual beds contain ≥7% Fe and lesser Mn oxides and hydroxides as cement, fracture-fill, and ≤4mm spherules. Tc2 and Tc3 erosionally overlie Tc1 and basement and consist of ≈20m thick, fining-upward, channel-fill sequences of braided-meandering, gravel-bed rivers. Tc2 is confined to paleochannels whereas a pebble-cobble lag with Tc3 composition regionally overlies basement and is preserved under Oligo-Miocene volcanic rocks. Tc2 cobble-boulder conglomerates have PM:16-41, V1:3-11, QSV2:59-73. Tc3 pebble-cobble conglomerates have PM:21-59, V1:25-47, QSV2:34-53. Resistant QSV2 clast types such as polymict rhyolitic breccia, and orthoquartzite are common at all locations. Deposits in OR and CA differ in having diverging N-S paleocurrents. In OR, Tc3 is overlain by ≈30 Ma volcanic rocks. In CA, Tc3 is overlain by ≈250m of volcanic conglomerate with interbedded Oligo?-Miocene volcanic flows in 2-5km wide, WSW-flowing, channel-fill complexes. Tc2 and Tc3 are interpreted to record changes in paleoslope and sediment dispersal, coeval intermediate-silicic volcanism (V1), and erosion of ≥1km of late Cretaceous-mid-Eocene nonmarine, forearc deposits (QSV2) that accompanied changes in the Eocene arc, the subducted slab, and the Yellowstone plume between 45-35Ma. Coarse-clastic deposits with potential for groundwater and geothermal resources, gold placers, and base and precious metal mineralization may underlie Neogene rocks in the study area especially where later structures cut Paleogene paleovalleys.