GEOLOGIC MAPS ACROSS THE SEATTLE FAULT ZONE: ACTIVE TECTONICS, A NEW DATABASE SCHEMA, AND A LIDAR BASEMAP

New 1:24,000-scale geologic maps of the Suquamish and Bremerton East 7.5-minute quadrangles portray active tectonism in the Seattle fault zone. The area is a rolling upland at elevations of 100 to 500 feet dissected by the deep troughs of Puget Sound and collateral waterways. Intermittent exposures along shoreline bluffs and gullies display thick Quaternary glacial and non-glacial deposits that overlie Miocene fluvial and Eocene-Oligocene marine strata in an uplift above the Seattle fault. A N-dipping homocline, floored by N-dipping roof thrusts antithetic to the blind, S-dipping Seattle fault, contains progressively rotated strata: Miocene beds are near vertical whereas ~18 ,000 ka (calibrated) Esperance Sand dips a few degrees to the N.

The maps were synthesized from outcrop observations, a database of boring and well logs, and an interpretation of high-resolution lidar topography. Many shallow holes were augered to delineate contacts between Vashon till and older units in the glaciated uplands, where dense forest and suburban development cloak the landscape. Locations of outcrop observations, boring and well logs, and auger holes are noted on the maps. Wash boundaries portray the relatively large uncertainty of upland contacts between latest Pleistocene Vashon till and older units.

GIS databases for the maps implement a schema that embodies the best practices of our workgroup and is informed by recommendations of the North American Data Model working group and the National Geologic Map Database project. This schema, implemented in ArcGIS v 9.2, captures all the content of a traditional paper map (map graphic, Correlation of Map Units diagram, Description of Map Units, Explanation of map symbols, cross-sections, analytical tables) with the exceptions of extended map text, accessory figures, and some map-collar information.

Geology is portrayed on bases that include both shaded relief and cartographic contours calculated from lidar topography. PLSS lines are shown but roads and streams are not, as available digital depictions of roads and streams are insufficiently accurate to align with the lidar topography.