ALLUVIAL STRATIGRAPHY, DISTAL SOURCES, AND INDURATION IN SUPRABASALT SEDIMENTS IN THE WALLA WALLA BASIN, WASHINGTON AND OREGON: REVISITING AND REVISING A LAYER-CAKE STRATIGRAPHIC MODEL

Terrigenous clastic sediments (up to 800 ft thick) overlie the Columbia River Basalt Group (CRBG) in the Walla Walla Basin of southeastern Washington and northeastern Oregon. Most of this sediment sequence is described as clay, silt, sand, and gravel forming a series of strataform units. From oldest to youngest these units are the old clay of Pleistocene age, old gravel of Pleistocene age, and Recent alluvium. The old gravel and Recent alluvium typically are interpreted as stream deposits that were locally derived off adjacent CRBG highlands while the old clay is typically interpreted as a basin-wide lacustrine deposit. In the past two years the sedimentary geology of the Walla Walla Basin has been revisited for work being done to support aquifer and surface water-groundwater interaction studies for the Walla Walla Watershed Planning Unit and the Walla Walla Basin Watershed Council. This work, which has focused on interpreting the geologic framework of the sediment hosted aquifers in the Basin, is revealing a more complex stratigraphy for these sediments than previously described. Significant observations include: (1) Clay, silt, sand, and gravel strata traditionally assigned to the old clay unit and old gravel unit form a complex intercalated sequence more typical of a fluvial-alluvial basin fill sequence rather than the previously interpreted simple strataform sequence. (2) Intervals of arkosic sand and micaceous mudstone suggest source terranes for these sediments other than simply nearby basalt highlands. (3) The induration found in these strata suggests much of the interval is probably older than Pleistocene age. (4) The youngest sediments in the Basin, consisting of Quaternary alluvial deposits, loess, and cataclysmic flood deposits, form a complex interstratified sequence overlying the much thicker, older indurated strata.