STATUS AND FINDINGS FROM 1:24,000 SCALE GEOLOGIC MAPPING IN THE PUGET LOWLAND, WA

A major effort is underway to produce new or updated 1:24,000-scale geologic quadrangle maps of the Puget Lowland. Goals of the mapping effort include 1) a revised Quaternary stratigraphy supported by modern dating techniques (including magnetostratigraphy, tephrochronology, fission track, and thermoluminescence analysis), 2) mapping with attention to hazards analyses and groundwater availability, 3) full use of digital topography for mapping young deposits and as a base for map presentation, and 4) digital compilation and publication of all map data in digital and analog (paper) form. We are using a regionally consistent stratigraphy and are edge-matching all quadrangles for seamless mosaics. Because of increased awareness of geologic hazards and a desire to pursue intelligent mitigation, municipalities and agencies have initiated and partly funded some of the new mapping, which has thus centered on major urban areas and trend of the Seattle fault. As an example of this work, we present the 9-quadrangle block centered on Tacoma WA. Particular features of these maps include: 1) abandonment of much previous pre-last-glacial stratigraphic nomenclature, subdividing older deposits primarily on the basis of radiocarbon age, depositional facies, and/or magnetostratigraphy rather than assigning them to named glacial and interglacial periods; 2) subdivision of the extensive late-glacial recessional outwash sequence south of Tacoma; 3) digitization and stratigraphic revision of the previously published map of Vashon Island (Booth, 1991); 4) recognition and documentation of tectonic structures: inclined beds, folds, and faults; 5) obtaining many new absolute and relative age dates; 6) recognition of multiple tephra and extensive lahar deposits in the pre-last-glacial units. The mapping is part of a collaborative effort involving the University of Washington, US Geological Survey, other universities, City of Seattle, Washington Dept. of Natural Resources, local agencies, and private businesses.