PRESENT AND FUTURE OF GEOLOGIC MAPPING IN THE PACIFIC NORTHWEST (Invited Presentation)

Geologic maps are essential to (a) identify and mitigate hazards to human life, property, and essential infrastructure; (b) identify and conserve groundwater resources; and (c) explore for and exploit industrial and metallic minerals, aggregate, and energy resources. All efforts depend upon a framework understanding of Earth history and process that comes from geologic mapping.

The 2012 geologic map of Idaho is at 1:750K scale. Oregon released OGDC-6, at variable scale, in 2015. The 2005 geologic map of Washington, at 1:500K, is built on earlier 1:100K and 1:250K compilations. All may be viewed interactively and underlying GIS data are online. These maps are built upon, and are being improved by, 1:24K quadrangle mapping that presently covers less than 1/5^th of the 3-state area. Such large-scale maps are essential for the uses identified above. New large-scale maps are published at a rate of 10-25 per year, with support and coordination provided by the National Cooperative Geologic Mapping Program.

In the near future motivations for mapping are unlikely to change. Mapping will focus on urban areas and areas with groundwater concerns, with infrastructure subject to seismic hazard, with potential for other Earth resources. Scientific topics likely to be addressed by future mapping include: (1) Geometry, kinematics, and mechanics of diffuse plate boundary deformation. (2) Onshore-offshore geometry and history of the upper plate of the Cascadia subduction zone, including elevation of the Cascade forearc to become the Coast Ranges. (3) Evolution of the Columbia River Basalt (CRBG) mantle plume head and its connection with the Yellowstone system. (4) Geometry, history, and dynamics of the south margin of the Cordilleran ice sheet.

Recent developments support improved geologic mapping. High-resolution topography enables better and faster surficial mapping. High-resolution gravity and magnetic surveys clarify subsurface geometry. U-PB microanalyses improve age resolution for continental strata. Optical and exposure dating techniques fill in the pre-¹⁴C hole. New work on geochemistry and paleomagnetism raises the possibility of close dating and flow-by-flow stratigraphy of the CRBG: time lines that span 3 states will be a world-class resource for studies of regional deformation.