Cordilleran Section - 115th Annual Meeting - 2019

Paper No. 40-13
Presentation Time: 9:00 AM-3:30 PM


STEELY, Alexander N., Washington Department of Natural Resources, Washington Geological Survey, 1111 Washington St SE, Olympia, WA 98504-7007 and SCHUSTER, J. Eric, Washington Department of Natural Resources, Washington Geological Survey, 1111 Washington St. SE, Olympia, WA 98504-7007

The Port Townsend 1:100,000-scale quadrangle in northwestern Washington contains parts of the Olympic Peninsula, the southern San Juan Islands, and portions of the Puget Lowland and nearby foothills. The quadrangle contains several large cities, including Everett, Anacortes, Port Townsend, Marysville, and Oak Harbor, and is crossed by several active fault zones and long-runout lahar deposits from Glacier Peak have been mapped as far west as Whidbey Island (over 100 mi from their source). The quadrangle also contains one of the largest tectonic boundaries in the state—the southern Whidbey Island fault zone—which divides the mostly Paleogene-age Olympic Mountains from the Mesozoic bedrock of the Puget Lowland.

The purpose of this effort is to combine and update the previous (late 1980s) separate bedrock and surficial compilations with new mapping and regional studies. Of the 30 constituent 7.5-minute quadrangles that contain land, 17 have been mapped since 1989; many adjacent quadrangles have also been recently mapped. Geophysical, paleoseismic, and regional studies completed since the original compilations are also used to update and refine the active fault zones in the area. An initial digital geodatabase of all available mapping was created that included ~120 geologic units. Simplification and merging of units in order to achieve 1:100,000-scale legibility resulted in a total of ~60 units. Faults, folds, and contacts between units have also been simplified. Additionally, ~1,900 attitude measurements, 303 geochronology samples, 60 geochemistry samples, and data from 4 paleoseismic trenches were compiled.

Evidence for at least six major geologic events is preserved in the Port Townsend quadrangle: (1) Quaternary deformation and faulting on the Darrington–Devils Mountain fault zone, Utsalady Point and Strawberry Point faults, southern Whidbey Island fault zone, and unnamed faults and folds in the Strait of Juan de Fuca; (2) Quaternary advance and recession of continental glaciers; (3) a late Eocene pulse of widespread volcanism; (4) Oligocene to Eocene deposition in tectonically controlled basins; (5) Eocene formation and accretion of the basaltic Coast Range terrane (also known as Siletzia) and; (6) Cretaceous assembly and metamorphism of a thick structural stack of Mesozoic terranes.