New evidence for pre-Wisconsin flooding in the channeled scabland of eastern Washington

Flood-gravel deposits capped by loess sequences that display well-developed argillic, calcic, and petrocalcic paleosols indicate pre-Wisconsin catastrophic flooding in the Cheney-Palouse tract of the channeled scabland, eastern Washington. The most complete stratigraphic exposure reveals two flood-gravel units, one of pre-Wisconsin age and the other representing the last major phase of scabland flooding (late Wisconsin). The two gravel units are separated by three loess units. Each period of loess deposition was followed by a soil-forming interval. The older of the two flood-gravel units contains cobbles of an early pre-Palouse Formation (that is, pre-Bull Lake) loess. It is capped by a loess unit displaying superimposed argillic and petrocalcic soil horizons. Above the petrocalcic horizon are two younger layers of loess (the Palouse Formation), each of which displays a paleosol having a weakly developed argillic B horizon and a calcic C horizon. These units are overlain by gravel from the last major phase of scabland flooding, which is, in turn, overlain by late Wisconsin and Holocene loess. The earliest flood probably carved part of the Cheney-Palouse scabland morphology during a glaciation prior to that responsible for the Palouse Formation.