QUATERNARY HISTORY OF MISSOULA FLOODS IN OREGON BASED ON SOILS

The Missoula Floods have a record found in the soils of the Willamette Valley of Oregon. Soils are mainly Inceptisols and some are Alfisols in the flood deposits of the Willamette Silts. A chronosequence has been established with Bw horizons developing in 5000 years and Bt horizons in 12,000 years. Five older paleosols have been found near The Dalles, Oregon that have stage 4 carbonate horizons in the lower two. A tephra identified as 600,000 years old is found in the third paleosol. This sequence of paleosols ranges from the late to the early Pleistocene. Loesses of the Portland Hills also show paleosols that range back to the middle of the Pleistocene. Rhythmites found in Portland and near Dayton, Oregon also show changes in the chemistry of the fine-grained phases of the deposits. From the earlier floods to the latest floods there is an increase of iron and scandium with corresponding decreases in chromium. These values can be used to identify where rhythmites might be in the overall sequence of the 12,700 - 15,300 BP floods of the Missoula Floods.