A NEW TERRACE CHRONOLOGY AND LANDSCAPE DEVELOPMENT MODEL FROM THE BURNT RIVER, EASTERN OREGON, USA.

Few constraints on the timing and rates of fluvial terrace formation exist from unglaciated river catchments in the inland Pacific Northwest (east of the cascades). Little is known with regards to when in the Late Quaternary terraces form and under what climatic regime strath surfaces are cut and river incision occurs across this region. Available studies contain poorly resolved geochronologic data lacking the precision necessary to understand the link between Quaternary climate and landscape development. New 1:24,000-scale geologic mapping conducted along the Burnt River in eastern Oregon revealed numerous late Quaternary strath terraces. We constructed a ca. 500 ka record of fluvial incision along the Burnt River that is chronologically anchored by both OSL and tephrochronology. We developed a conceptual model of fluvial terrace formation for unglaciated catchments using these ages. Our model suggests that strath terraces formed in response to glacial-interglacial climate fluctuations. Terrace straths were beveled during interglacial intervals. Increased rates of vertical incision and strath capture in the landscape appears to occur during the transition into a glacial climate regime. This model is germane to other non-glaciated catchments in eastern Oregon and Washington, providing a regionally applicable tool for future research. We use our terrace and geochronology data to constrains the rate of incision of the Burnt River, which is downcutting at 0.15 to 0.3 m kyr^-1. This incision appears to reflect to a first-order local base level adjustment tied to movement along the newly mapped Durkee fault and second-order regional base level control imposed by the downcutting of the Snake River at the upper end of Hells Canyon.