RECOGNITION AND CORRELATION OF PLEISTOCENE GLACIAL OUTBURST FLOOD DEPOSITS IN SOUTHEASTERN WASHINGTON

The late Pleistocene Missoula Floods are well-known and well-studied; more attention has recently been given to the erosional and depositional features representing floods that pre-date the late Pleistocene events. Repeated glaciation in the Cordillera raises the possibility that ice dams could have formed many times during the Pleistocene, and consequently that outburst Flood Sequences could have developed on many occasions. Given the physiographic makeup of the Columbia Basin, with a single outlet at Wallula Gap, if outburst floods occurred within the watershed of the Columbia River, they could produce deposits similar in character and distribution to those of the Missoula Floods. However, as a result of post-flood processes, these deposits might be modified significantly.

Studies in the Walla Walla Valley and surrounding region have identified sediment sequences displaying features that are related to outburst flood and post-flood processes, including graded, rhythmically bedded sediments, clasts of exotic (non-basalt) lithology contained in diamicts, and clastic dikes truncated by flood-cut unconformities. At least four Flood Sequences are recognized: the late Pleistocene Missoula Floods (represented by the Touchet Beds of southeastern Washington) and three Sequences that are demonstrably older. One of these (but not the oldest) is at least 780 ka based on paleomagnetic evidence. Flood Sequences can be correlated on the basis of stratigraphic position, clast composition, and similarity of sedimentary features. They can be separated on the basis of the bounding unconformities, of which five are recognized. Recognition of the Flood Sequence-bounding unconformities may provide a useful tool for correlation of the various Flood Sequences recognized in slackwater, main-channel and channel margin flood areas of the Columbia Basin.