UPDATED GEOLOGIC MAP OF THE PORT TOWNSEND 1:100,000-SCALE QUADRANGLE, WASHINGTON
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.