MIS 4 AND OLDER PLEISTOCENE DEPOSITS IN THE PUGET LOWLAND, WASHINGTON STATE

A stratigraphic framework was developed for the last million years of geologic history in the Puget Lowland, based on global climate patterns, abundant new absolute and relative dates, and decades of detailed field mapping. New geologic units have been identified and others have been clarified and their understanding expanded. Useful techniques include luminescence radiocarbon, fission-track, and Ar/Ar dating; paleomagnetic correlations, palynology, macrofossil identification, provenance, and field-measured sections. Data suggest that the Vashon stade (Marine Isotope Stage 2 ice sheet) ended quite abruptly, nearly simultaneously in the northern and southern Lowland. The prior MIS 4 ice sheet advance, named the “Possession glaciation,” extending to within about 25 km of the Vashon limit, with glacial conditions from about 61 to 76 ka. An ELA of about 1240 m brings the reconstructed ice sheet into balance at maximum extent; assuming a summer MAT of about 7 degrees C cooler than at present. MIS 4 fine-grained glacial deposits are abundant in the Puget Lowland. The Whidbey Formation (MIS 5) and Double Bluff Drift (MIS 6), previously recognized only north of Seattle, have now been newly identified in the southern Puget Lowland, over 60 km from their type section on Whidbey Island. Newly identified stratigraphic units include deposits from the MIS 7 interglacial period, informally named the “Hamm Creek formation” for 200-ka pumice deposits in Seattle; deposits from the MIS 8 glacial period, collectively named the “Defiance drift”; and the “Gig Harbor gravel,” which may be part of MIS 8 or even older. In addition, reversely magnetized deposits, assumed to be older than 780 ka based on limiting luminescence ages, have been identified in multiple locations along the Tacoma and Gig Harbor coastline. Many unconformities exist in the Pleistocene record of the Puget Lowland, making stratigraphic correlation difficult even with absolute dating. Each time-stratigraphic unit is an unconformity-bounded sequence with widely distributed but discontinuous deposits. Glaciotectonic and tectonic discontinuities further confound correlations, although large-scale folding is evident and consistent with tectonic compression in western WA.