LATE QUATERNARY EOLIAN SAND DUNE, LOESS, AND DUST GENERATED FROM TEXTURALLY DIVERSE LATEST GLACIAL OUTBURST FLOOD DEPOSITS IN THE QUINCY BASIN, WASHINGTON

The late Quaternary sedimentary history of the Quincy Basin, NW Columbia Plateau, Washington, yields insights into the nature and timing of latest stage glacial outburst flooding and the role that flood deposition had on subsequent eolian activity, including sand dune/sheet, loess, and dust generation. The latest outburst floods transported sediment into the Quincy Basin via the Columbia River and Channeled Scabland distributaries including the Grand Coulee. Floodwaters entering the Quincy Basin produced complexly interstratified and widespread gravel/sand bars and sheets and sand-silt/clay slackwater rhythmites. Stratigraphic relations at a site in the north-central part of the basin reveal at least eight outburst flood pulses following accumulation of Mt St. Helens set S tephra (ca. 12.9 ¹⁴C yr BP). The first seven pulses are recorded in a < 2 m thick succession of fining-up sand-silt/clay rhythmites, whereas the eighth is a > 10 m thick accumulation of gravel and sand strata and cross-strata whose paleocurrents indicate SW and WSW transport. While such variably distributed outburst flood sediment has been a potential source for eolian deflation and atmospheric dust since the end of flooding, only locations not armored by gravel have done so. A stabilized 4 m high parabolic dune capped by a 0.8 m thick soil with a calcic horizon in the north-central Quincy Basin contains a 30 cm thick bed of cicada-burrowed, pumiceous, crystal-rich tephra characteristic of the Glacier Peak 11.2 ka ¹⁴C BP eruptions. The relatively concentrated nature of the tephra bed (2 m below the surface) and well-developed modern soil suggest this dune has been stable since the early Holocene. The cicada burrows are evidence of shrub vegetation (likely Artemisia) growing under arid conditions. Thinning and fining of loess east and northeast of the Quincy Basin indicates the basin has been a loess/dust source since the last glacial maximum (LGM). Concentrations of coarse-grained outburst flood sediment at the surface, however, reduced silt/clay availability and kept L1 loess (post-LGM) deposits in this region < 1.5 m thick. By contrast, coeval L1 loess deposition on the southern Columbia Plateau exceeded 4 m.