SEQUENCE GEOLOGY IN PLEISTOCENE OUTBURST FLOOD DEPOSITS IN SOUTHEASTERN WASHINGTON

The latest Pleistocene Missoula Floods have been intensively studied since the pioneering work of Bretz in the 1930's, and much is known about the distribution, stratigraphy, sedimentology, source, number and timing of floods, and flood dynamics. Since the late 1970's, several papers have presented data suggesting that earlier, similar episodes of outburst flooding left a widespread, but more obscure record. The deposits of earlier outburst flood episodes are recognized based on a number of criteria, including flood-cut unconformities, diamicts containing clasts of exotic (non-basalt) lithologies, cyclically deposited packets of graded sand-to-clay sediment, caliche soils developed on flood gravels, and clastic dikes truncated by erosion. In some cases, ages of units are constrained by tephra.

Seven critical sites in southeastern Washington and northeastern Oregon demonstrate that outburst flood episodes, including the Missoula Floods, can be distinguished on the basis of internal sedimentology and bounding flood-cut unconformities. Five unconformities delineate four Flood Sequences, at least one of which is reversely magnetized. In addition to the characteristic sediments used to recognize flood deposits, the Sequences include soils, loess, and colluvium deposited after a flood episode. Flood Sequences may ultimately be tied to formally established pedostratigraphic units. Additional field work may reveal the presence of tephras that will help clarify age relationships. Study of outburst flood events in eastern Washington should be facilitated by recognition of the sequence-stratigraphy demonstrated by these sediments.