GEOLOGIC MAP OF THE SHELTON VALLEY 7.5’ QUADRANGLE, MASON COUNTY, WASHINGTON

We present a 1:24,000-scale geologic map of the Shelton Valley 7.5’ quadrangle, which straddles the northern Black Hills and the western Puget Lowland, just west of the town of Shelton. We support the map with new field work, geochemistry, palynology, and U-Pb, luminescence, radiocarbon, biostratigraphic chronology, lidar, and existing subsurface explorations.

Our findings include that bedrock geochemistry and two new 50.51 and 50.14 Ma 40Ar/39Ar dates support the notions that (1) basalt in the northern Black Hills is part of the Crescent Formation and (2) these rocks may be slightly younger than earlier analyses suggested. Subsurface sediment north of the Black Hills is largely sourced from the basalt-rich Olympic Mountains. New radiocarbon, luminescence and pollen analyses suggest that alpine glaciers from the Olympic Mountains advanced into the north half of the map area and deposited voluminous drift there. Sediment farther south in the Black hills is mostly polymict, northern-sourced material deposited by incursions of the Cordilleran ice sheet, or locally reworked from such glacial sediment. Geologic mapping suggests that the southern limit of the Vashon ice advance is slightly farther south than previously inferred.

Intense deformation of Quaternary sediment in the quadrangle is collocated with strong northwest-trending geophysical anomalies previously interpreted as the Olympia structure. Orientations of deformation measured in both Quaternary and bedrock exposures are consistent with strain expected for the kinematics previously proposed for the Olympia structure. However, glaciotectonic forces cannot be ruled out as a possible source of the observed deformation.