PRELIMINARY COSMOGENIC 3HE CHRONOLOGY OF PLEISTOCENE/HOLOCENE INTRACANYON LAVA FLOWS AND OUTBURST-FLOOD DEPOSITS IN THE OWYHEE RIVER BASIN, OREGON

Numerous mass movements and basaltic lava flows have had a pronounced effect on valley and channel morphology of the Owyhee River, a river system in southeastern Oregon. These large volume incursions into the channel have the capacity to inhibit incision by altering channel slope, width, and bed character and burying valley-bottom bedrock under exogenous material. On the other hand, they can also promote incision by generating cataclysmic floods through natural dam failures. River incision, in turn, dictates the pattern and tempo of landscape evolution, including the opportunity and accommodation space for extra-channel incursions. We are studying these feedbacks and the effects of individual mass movements and lava flows on short- and long-term changes to valley morphology, including those processes that inhibit and promote valley incision. To accomplish this, we must establish the chronology of intracanyon lava flows and landslides in the Owyhee River basin, and the fluvial response to these incursions into the river. The use of cosmogenic 3He dating allows us to calculate canyon incision rates, differentiate among channel features, and relate them to specific extrafluvial causes, and delineate the temporal and spatial scales over which these processes operate. Dated surfaces include olivine-rich lava flows/dams and basalt boulders on fluvial terraces, and flood deposits downstream of breached natural dams. Initial 3He results indicate that ages of individual surfaces, such as river terraces and flood bars, can be distinguished within a given location, and that we should be able to correlate fluvial features with causative events. For example, a preliminary cosmogenic age of ~10 ka on an outburst-flood deposit at the informally named Artillery Landslide is consistent with a minimum age of the landslide ca. 7700 cal-yr BP based on Mazama ash from a closed depression on the landslide. The developing chronology indicates a complex history of multiple events of different origins spanning at least the past ~50 ka.