SURFICIAL GEOLOGY OF THE NEWLY ANNEXED AREA OF KIRKLAND, WASHINGTON, KING COUNTY, USA

The 2014 Oso landslide brought nationwide attention to the geologic hazards of Washington State, impelling the City of Kirkland officials to create a new geologic map of recently annexed areas and update prior 2010 geologic map products to protect the community and upgrade the City’s Growth Management Act (GMA) products. Geologic hazards within the City include, but are not limited to, landsliding and liquefaction. This information is useful to determine areas that can support high-density growth in a rapidly growing city.

To create the 2017 map, we prepared base maps using the 2016 LIDAR data, conducted new field mapping, managed the population of geotechnical data into the GeoMapNW database, and completed a geomorphic analysis. Datapoints used to draw geologic unit polygons consisted of more than 270 field stations and more than 5500 subsurface investigations. Field data was acquired by making observations in excavations at construction sites and by mapping outcrops in gullies, ravines, and roadcuts. Boring logs, acquired from several different entities, were the main source of geologic data for the map.

Kirkland’s geologic history plays an important role in shaping the City’s landscape. Multiple glacial advances and retreats left a tangle of silt, sand and gravel deposits of varied densities and ages. The annexation area consists of two broad upland plateaus divided by glacially derived outburst channels. The surficial geology of these plateaus consists of a blanket of Vashon till overlying Vashon advance outwash of variable thickness and pre-Vashon deposits. Locally the Vashon Lawton Clay is present and together with pre-Vashon deposits, forms a relatively impermeable barrier to groundwater resulting in springs on steep hillsides. The channels are lined with Vashon recessional outwash and recessional lacustrine deposits. Landslides are abundant on the steep non-consolidated slopes. Holocene and Vashon recessional deposits are susceptible to liquefaction.

The main limitation to this project was the lack of exposures in the well-established neighborhoods of the annexation area. Subsurface reports filled in much of the data gaps that existed due to this limitation. However, areas with no surficial geologic information exist throughout the mapping area, which led to extrapolating geology to complete the map.