PRE-MAZAMA PEDOGENESIS RECORDED IN POST-OUTBURST FLOOD GEOLOGIC DEPOSITS OF THE SCABLAND, COLUMBIA PLATEAU, WASHINGTON

The shallow subsurface of the Scabland region of the Columbia Plateau in central Washington State, displays abrupt change in mode of deposition and sedimentation rates recorded in Pleistocene and early Holocene pedogenic sequences. Local depositional environment change occurred rapidly as a result of catastrophic flood sedimentation, the source of eolian and alluvial parent materials. Cross-Scabland flooding ceased between 12.8 and 12.1 KBP, while catastrophic floods that were restricted to the Columbia River valley post-date the fall of Glacier Peak tephra (11.2 KBP). Pedogenesis into these flood deposits and thin L1 loess at numerous Scabland locations is recorded in two soil-forming periods. The lower soil, termed the Bishop Paleosol, is characterized by a well-developed A horizon and relatively thin Cambic (Bw) or Argillic (Bt) horizons, depending on the depositional environment. The age of this soil is constrained by its relative position between Mt. St. Helens Set S (12.8±60KBP) and Glacier Peak tephras. Separating the two soils at several locations is a post-Glacier Peak tephra period of eolian deposition which may reflect disbursement of Columbia River Valley outburst flood sediments during the Younger Dryas cooling period. The second period of pedogenesis post-dates deposition of Glacier Peak tephra and pre-dates Mazama (7.7KBP) tephra deposition. This soil, termed the Badger Mountain paleosol, is characterized by multiple, stacked buried A (Ab) horizons, Cambic horizons and well developed Argillic horizons which may qualify as Natric soils. These strong B horizons overprint the deepest buried A horizons and the Bishop paleosol in some exposures. The cap of the Badger Mountain soil is characterized by a zone of extremely well-developed cicada burrowing up to 1m thick, indicating a shift to arid conditions after 11.2KBP.