QUATERNARY GLACIAL GEOLOGY, AS DELINEATED FROM SOIL DEVELOPMENT, OF A WATERSHED ON THE EASTERN FLANKS OF THE CASCADES, CENTRAL OREGON

Four distinct glacial deposits are identified in the Squaw Creek watershed, draining an eastern portion of South Sister in Central Oregon. This study points out a more extensive delineation of glacial units than previoulsy identified in tills alone by using outwash terraces. Differences in outwash deposits are distinguished primarily by soil silica development, and further subdivided based on soil properties of clay % and bulk outcrop color (BOC). A Profile Development Index (PDI) is applied to further establish relative ages. The youngest deposits are identifeid as Suttle Lake glacial deposits, after Scott (1977), associated with marine oxygen isotope stage (MIS) 2. The intermediate deposit, informally referred to as Sisters outwash, associated with MIS 4, postdate an older unit, Jack Creek glacial deposits (Scott, 1977), associated with MIS 6. The oldest deposit, recognized in one locale, is informally referred to as the Melvin Springs unit, potentially MIS 8 or greater in age.

Suttle Lake deposits are characterized with soils having no silica development, a Bw horizon, 10YR 5/7 to 10YR 4/6 colors and up to 10% clay, developed from a 10YR 3/3 parent material with 3% clay. BOC is 10YR 4/3. Total PDI averages 0.13. Sisters outwash deposits have silica Stage I development, scale-like coatings on underside of stones, advancing to silica stage II, pendants developed on undercoatings. Bwq horizons are present with up to 12% clay and 10YR 6/3 to 10YR 4/4 colors. BOC is 2.5Y 4/4. Total average PDI is 0.16. Jack Creek deposits have silica Stage I and II, developing into silica Stage III, matrix cementation. A Btq horizon, with up to 19% clay, overlays Bwq horizons with colors ranging from 10YR 5/4 to 10YR 6/2.5 and 2.5Y 5/2.5, reddening, then lightening compared to unweathered parent material. BOC is lighter at 2.5Y 5/2.5. Total averavge PDI is 0.21. The Melvin Springs outrwash deposit has silica Stages I, II and III developing into Stage IV, an indurated cement. Horizonation could not be determined because of induration. BOC is slightly reddening at 2.5Y 5/4. Total PDI average is 0.36. Regressive pedogenesis, by color lightening, dominates a silica rich environment. The progression of silica development and regressive pedogenesis appears to be a useful tool to evaluate time progression and deposit age.