Surficial Geology of the Newly Annexed Area of Kirkland, Washington King County, USA

Abstract

The 2014 Oso landslide brought nationwide attention to the geologic hazards of Washington State, impelling 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. Geologic hazards within the City include but are not limited to landsliding and earthquake-induced ground failure. City engineers, planners, and emergency managers will use these products for assessing and mitigating hazards, prioritizing facility upgrades, and other city projects. Also, these products will allow experts to address risks and make recommendations on construction projects developed within hazard areas according to the City’s zoning code. An updated geologic map will also allow nterested persons, including students and Kirkland residents the opportunity to learn about local geology. The work is necessary to upgrade the City’s Growth Management Act (GMA) products. This information is useful to determine areas that can support high-density growth in a rapidly growing city. My map was compared to Minard’s 1983 surficial geology map of the area. Additionally, my map extends the area of the Troost and Wisher 2010 map. Kirkland’s geologic history plays a major role in shaping the city’s landscape. Multiple glacial advances and retreats left a tangle of silt, sand and gravel deposits of various densities. The annexation area consists of two broad upland plateaus divided by glacially derived channels. The surficial geology of these plateaus is a blanket of Vashon till overlying a thick layer of Vashon advance outwash. Exposed in some areas is a deposit of Vashon Lawton Clay. The clay is very stiff to hard and marks the transition from non-glacial to earliest glacial time. Pre-Vashon deposits are exposed in some areas. The channels are lined with Vashon recessional outwash and recessional lacustrine deposits. The recessional lake deposits discovered were recessional Lake Bretz (120-15 feet), Lake Totem (160-180 feet) and Lake Juanita (50-90 feet). Additionally, paleotopography is found on the eastern slopes of the upland plateaus. Landslides are abundant on the steep non- consolidated slopes.

To create the 2017 map, I prepared base maps with the 2016 LIDAR data, conducted new field mapping, managed the input of geotechnical data into the GeoMap NW database, and completed a geomorphic analysis. Datapoints used to draw geologic unit polygons consisted of more than 250 field stations and more than 2000 subsurface investigations. Field data were acquired by interpreting excavations at construction sites and by exploring area gullies to find outcrops. Borehole logs, acquired from several different entities, were the main source of geologic data for the map.

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 interpolating geology to complete the map.