COSMOGENIC NUCLIDE CONSTRAINTS ON THE GEOMORPHIC EVOLUTION OF THE COLUMBIA PLATEAU, WASHINGTON USA

The Columbia River drains the Columbia Plateau, a landscape formed due to emplacement of flood basalts during the Miocene. Much of our knowledge of the geomorphic evolution of the Columbia Plateau is based on study of the Missoula floods, which occurred at the end of the Pleistocene during Marine Isotope Stage (MIS) 2. However, geological evidence indicates there were pre-MIS 2 outburst floods on the plateau, and it was suggested by J Harlan Bretz on the basis of differentially weathered scabland surfaces, that scabland of different ages is a general feature of the landscape. However, we have a poor understanding of the relative ages of the different scablands and, further, there are almost no constraints on the longer-term, pre-Pleistocene geomorphic evolution of the plateau. We analyzed ³He from basalt bedrock and ¹⁰Be in granitic erratics to constrain the timing and rates of bedrock erosion on the plateau. High elevation scabland topography at Babcock Bench and Blackrock Coulee—two locations where Bretz indicated there were scablands of different ages—have ¹⁰Be exposure ages that date to MIS 2. Hence we suggest that the differential weathering of scabland surfaces arises due to basalt flows with different mechanical strength or floods spaced by—at most—a few thousand years, rather than due to floods widely separated in time. After correcting for production by muons, ³He concentrations in nearly all the 60 basalt samples collected from multiple flood channels indicate scabland topography on the plateau formed during MIS 2. However, three samples have much higher ³He concentrations. One sample is from an area east of Grand Coulee that was inundated, though not eroded by floods and two samples are from scabland topography on upper Babcock Bench, which we infer are from an exhumed Miocene unconformity on the basis of our ¹⁰Be date and local basalt flow stratigraphy. The ³He concentrations in these three samples indicate the Columbia Plateau has been eroding at an exceptionally low rate of only a few meters per million years. Hence our analyses indicate that landscape evolution on the Columbia Plateau proceeded very slowly from the Miocene until the late Pleistocene, and that much of the topographic relief on the plateau was generated recently by the Missoula floods.