GENETIC RELATIONS BETWEEN CATASTROPHIC GLACIAL OUTBURST FLOOD SEDIMENT SOURCES, QUATERNARY EOLIAN DEPOSITS, AND ATMOSPHERIC DUST AT THE HANFORD REACH NATIONAL MONUMENT, WASHINGTON

Catastrophic glacial outburst flood deposits exposed along the Columbia River within Hanford Reach National Monument have been a major sediment source for dune sand, sand sheets, loess and atmospheric dust in east-central Washington throughout the Quaternary. Approximately eight glacial outburst flood events are recognized in a 15 to 20 m thick succession of rhythmites exposed at Hanford Reach. Each rhythmite consists of ripple cross-laminated fine-sand that grades upward into horizontally-laminated to massive silt. The ripple cross-laminated facies comprise approximately 60 % of the volume of the rhythmite and consists of 66 % sand-, 28 % silt-, and 6 % clay-sized grains. The horizontally-laminated to massive facies, which comprises the other 40 % of the rhythmite volume, is composed of 27 % sand, 60 % silt, and 13 % clay. These rhythmites are preserved in two 1 to 2 km wide, paleovalleys incised in the Pliocene-Pleistocene Ringold Formation and in a 2 to 3 m thick deposit that caps the Ringold Formation. Ringold Formation sedimentary deposits are also a likely source of silt to clay sized eolian sediment, but are less significant eolian contributors because they often are obscured beneath flood deposits.

The grain size distribution of samples collected from soil auger borings in parabolic dune sand, sand sheets, and loess up to 18 km downwind from outburst flood sediment sources reveal a > 50% depletion of 4- 62.5 micron sized particles. Dune and sand sheet deposits most proximal (0-10km) to outburst flood source sediment are composed on average of 92 % sand, 6 % silt and 2 % clay; this grain size distribution is enriched in sand-sized content compared to flood sources. More distal (10-18 km from source) loess deposits are composed on average of 38 % sand, 50 % silt, and 12 % clay, a grain size distribution that reveals deposition of finer-grained particles. An unknown proportion of particles < 10 microns in diameter that were contained in outburst flood source sediment contributed to heightened atmospheric dust emissions during episodes of regional aridity and eolian activity that characterized the Columbia Basin during the early-middle Holocene. Should prolonged, severe drought return to this region the potential exists for much expanded eolian activity and emissions of atmospheric dust.