GEOLOGIC MAP OF THE VIOLET PRAIRIE 7.5-MINUTE QUADRANGLE, THURSTON AND LEWIS COUNTIES, WASHINGTON

The Violet Prairie quadrangle straddles Thurston and Lewis counties along western Washington’s economically vital I-5 corridor. This 1:24,000-scale geologic map aims to identify hazards and improve geologic understanding for private and public resource management and planning. A synthesis of new and existing geologic information provides fresh insight into the glacial deposits, bedrock units, and structural geology of the study area. Glacial deposits of Vashon age and at least one pre-Vashon ice incursion into the quadrangle are likely northern-sourced but may include some Cascades-sourced, pre-Vashon deposits. Outwash provides ample gravel resources and highly productive but easily polluted aquifers. Field- and lidar-informed mapping suggests that some landslides may be seismically triggered. Lidar also improved mapping of mima mounds and confirmed that they are found only on outwash surfaces. New radiocarbon ages indicate that despite ample outwash, most surficial sediments in the Skookumchuck River valley are late Holocene alluvium. Seven new U-Pb analyses from the upper McIntosh, lower Skookumchuck, and lower and middle Lincoln Creek Formations provide improved benchmarks for ages, sediment sources, and biostratigraphic correlations throughout southwest Washington of: (1) the Eocene McIntosh and Skookumchuck Formations, (2) the Eocene–Oligocene Lincoln Creek Formation, (3) the Miocene Wilkes Formation, and (4) the Pleistocene Logan Hill Formation. New geochemical data suggest that all volcanic rocks in the quadrangle are Northcraft Formation. The data also suggest geochemical distinctions between sedimentary rock units, but more work is needed to attribute these to either unit distinctions or depositional environment distinctions. Mapped faults are reinterpreted as northeast-dipping dextral-reverse.