LOESS, TEPHRA, AND ACTIVE FAULTING IN THE SOUTHERN PALOUSE: POSSIBLE EVIDENCE OF QUATERNARY NEOTECTONISM IN NE OREGON

Lidar-based mapping of Quaternary loess that blankets lava flows of the early-middle Miocene Columbia River Basalt Group W of the Blue Mountains and E of the Columbia River in NE Oregon reveals a complicated depositional and preservation history influenced by post-middle Miocene folding and faulting. Loess in NE Oregon blankets a tectonically active ~1600 km² region that includes the converging WNW-striking Wallula, NNW-striking Milton-Freewater, and NNE-striking Hite fault zones. The loess in this region is the southern part of the >50,000 km² Palouse where meters to 10s of m of windblown silt cover parts of NE Oregon, E Washington, and N Idaho. Palouse loess was derived primarily from sand-, silt- and clay-rich glacial outburst megaflood slackwater deposits that accumulated in the Umatilla and Pasco Basins of NE Oregon and SE Washington, respectively. Southern Palouse loess is more sand-rich than the northern Palouse and is marked by a distinctive NE-trending, linear-ridge topography with 10s m of relief. Late Pleistocene and Holocene eolian deposits in the Palouse have served as repositories for accumulations of air-fall tephra, the bulk of which originated from explosive volcanic eruptions in the Cascade Range. Two distinctive air-fall tephras are intercalated with loess in the Adams-Waterman area. The older tephra is an ~4 cm thick discontinuous planar deposit correlated with a 171-130 ka tephra (5-42A) recovered from a core at Walker Lake, Nevada. The younger and more voluminous tephra originated from the ~7700 cal yr B.P. Plinian eruption of Mount Mazama. Mazama tephra is cms to decimeters thick, laterally discontinuous, and conspicuously exposed in unvegetated sandy blowouts developed on steep eastern (downwind) flanks of a NNW-trending loess ridge bordering Sand Hollow. The combination of tephra chronostratigraphic-control and mapped bedrock fault traces, suggest that fault-offsets produced a landscape along Sand Hollow that favored heightened accumulation of Mazama tephra. It is possible, though as yet unverified, that other steep, now-vegetated ridge slopes to the east including along Gerking Flat may obscure similar Mazama tephra-dominated blowout deposits genetically linked to systematic Late Pleistocene and Holocene landscape offset by NNW-striking right-lateral oblique-slip faulting.