Chronology, Lithology and Paleoenvironmental Interpretation of the Penultimate Ice-Sheet Advance into the Puget Lwoland, Washington
Troost, Kathy Goetz
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University of Washington Abstract Chronology, Lithology and Paleoenvironmental Interpretations of the Penultimate Ice-Sheet Advance into the Puget Lowland, WA Kathy Goetz Troost Chair of Supervisory Committee: Dr. Derek B. Booth Earth and Spaces Sciences Stratigraphic nomenclature and chronology has been a problem for geologists seeking to understand the Quaternary history of the Puget Lowland of Washington State for over one hundred years. Accurate identification of deposits, and accurate age assignments for those deposits, can help us to better understand the paleoclimate of the region, the physical characteristics of deposits encountered during infrastructure projects, the timing of tectonic deformation and recent seismicity, and our geological history. Each of these has profound consequences on our daily lives. In this dissertation I have established a stratigraphic framework for the last million years of geologic history in the Puget Lowland, based on global climate patterns, abundant new absolute dates, and detailed mapping that synthesizes my work and the work of others over the last century. Two decades of detailed field mapping, absolute and relative dating, and analyses have identified new geologic units and clarified and expanded our understanding of many others that had been previously identified but imprecisely described or understood. The techniques I have made use of include luminescence dating, paleomagnetic correlations, radiocarbon dating, fission-track and Ar/Ar dating, palynology, macrofossil identification, provenance, and field-measured sections. My data suggest that the Vashon stade (representing the regional advance of the Marine Isotope Stage 2 ice sheet) ended quite abruptly, nearly simultaneously in the north and south Lowland. The prior MIS 4 ice sheet advance, named the “Possession glaciation,” was of similar extent to that of the Vashon stade, extending to within about 25 km of the Vashon limit. 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. With diligence and knowledge, however, we can continue to improve our geologic understanding. Society’s future economic progress and human well-being in the Puget Lowland region depends in large measure on our ability to predict and plan for global climate changes, geologic hazards, and the complexity of the ground beneath us. Understanding the relatively recent geologic history of the region, of which this dissertation is a contribution, is a necessary step in that process.