THE PENULTIMATE GLACIATION AND MID- TO LATE-PLEISTOCENE STRATIGRAPHY IN THE CENTRAL PUGET LOWLAND, WASHINGTON

Recent geologic mapping and stratigraphic work provides new insights into mid- and late-Pleistocene glaciations of the central Puget Lowland of Washington. Time-stratigraphic units and unconformity-bounded sequences are best used to define glacial and interglacial strata here. For all but the most recent deposits, chronologic control is needed to positively identify those units because of multiple unconformities. New units are recognized and defined, and previously identified units have been extended geographically, correlated to marine isotope stages using absolute ages, and or re-named. Interglacial deposits correlative with MIS 3 are recognized throughout the Puget Lowland. Strata correlative with the Whidbey Formation, MIS 5e, are recognized in the Seattle, Tacoma, and Olympia areas; well south of the more northerly type section. Possession-age, MIS 4, glacial deposits of till and outwash are recognized in Seattle, Tacoma, and Olympia, extending the glacier much further south than previously thought. Unlike from the Vashon Glaciation, facies of the Possession Drift are geographically orientated. Till, correlative with the Double Bluff glaciation, MIS 6, is newly recognized in Tacoma. Deposits correlating with MIS 7 and 8, are identified; previously none had been recognized. The Hamm Creek interglaciation is informally named and is recognized in the Woodinville, Seattle, and Federal Way areas. This deposit, first noted for thick ash and pumice in south Seattle, dates to about 200,000, MIS 7. Newly recognized, the Defiance glaciation dates to around 250,000, MIS 8. First noted at Pt. Defiance in Tacoma, it is also encountered throughout the area of the Tacoma uplift. Importantly, use of the term “Kitsap Formation” should be discontinued, since it represents multiple climate sequences. At its type section, the Kitsap Formation includes two interglacial deposits separated by a glacial deposit, all of indeterminate age. The name historically has been used for Olympia-age as well as Whidbey-age deposits and to label fine-grained material between glacial deposits. These findings help to better define the glacial history of the Central Puget Lowland and to better compare global climate patterns to signals in the Puget Lowland.