GEOLOGIC MAPPING AT 1:12,000 SCALE ACROSS THE CITY OF SEATTLE

Detailed geologic maps of the City of Seattle are being prepared to provide both the raw data and the geologic interpretations needed to characterize the material properties and topography of geologic materials within the most densely populated part of the Puget Lowland of western Washington. The same urban development that has obscured much of the ground surface in this urban environment has also created a tremendous source of near-surface and subsurface data, permitting abundant verification of observed or inferred surficial deposits. We are compiling a GIS/Oracle database that houses raw data, metadata, and interpreted geologic information for many thousands of data points. From these data we are producing a set of "traditional" surficial geologic maps, but with a level of detail and supporting data that are unprecedented in the region.

Using as our template the four 7.5-minute quadrangles that almost entirely cover Seattle, map production is proceeding at a rate of about one quadrangle per year. The first two quadrangles, Seattle SW and Seattle NW, demonstrate the utilization and display of surface exposures and subsurface data sources; the integration of analytic methods (on these maps, radiometric dating, thermoluminescence dating, and diatom and pollen analyses); and the incorporation of geophysical information, most critically here in the delineation of the Seattle fault zone as it traverses the populated areas of southwest Seattle. From this variety of data sources, we are producing 1:12,000-scale geologic maps, digitized both for GIS display and for incorporation in subsequent 3-D representations. Collaboration with other geoscientists in the USGS, UW, WA DNR, private consultants, and other universities and state geological surveys has been invaluable, as has been the integration of this detailed mapping within the context of a regional geologic framework for the central Puget Sound region.