NUMERIC AGES OF FINAL CROSS-SCABLAND PLEISTOCENE FLOODING, COLUMBIA PLATEAU

Research into timing of final cross-Scabland floods related to recession of the Cordilleran ice sheet is reported. Numeric ages associated with final flooding span a 600 year period of Upper-most Pleistocene flood events. Numeric ages associated with final flooding include a date of 12,800+/-60BP on bone from a ruminant killed by one of the flood events, and a date of 12,130+/-50BP on the extinct Jefferson Ground Sloth (Megalonyx jeffersoni) which was discovered immediately overlying rhythmically bedded flood sediments. At least six successive flood occurrences post-date deposition of the St. Helens Set S tephra (~13,000BP). Sediments deposited as a result of these late floods include pedogenic and geologic evidence which indicate a depositional hiatus between individual flood events.