GEOLOGIC MAP OF THE CENTRALIA 7.5-MINUTE QUADRANGLE, LEWIS COUNTY, WASHINGTON
New mapping and geophysical data build on previous work by characterizing geologic structures in greater detail, refining fault locations, and updating structural interpretations. Northwest- and west-striking oblique faults in the quadrangle may be capable of producing damaging earthquakes and may be responding to a local transpressive stress regime. Newly acquired gravity, ground magnetic, and passive seismic geophysical data constrain the locations of several concealed fault strands, and strongly suggest a structural high—the Lincoln Creek uplift—underlies the Chehalis River valley from Chehalis to Fords Prairie, WA.
Radiometric dating (U-Pb) of detrital zircons also refines the stratigraphic framework for southwest Washington. Analyses reveal new maximum depositional ages for the base and middle of the Lincoln Creek Formation (36.70±0.32 Ma and 30.58±0.35 Ma, respectively), the previously unrecognized Wilkes Formation (8.57±0.92 Ma and 7.54±1.04 Ma), and the Logan Hill Formation (1.68±0.97 Ma youngest age).
Numerous previously unrecognized landslides are identified from lidar and field mapping. The lower contact of the early Pleistocene Logan Hill Formation is commonly concealed by landslides, which suggests that the base of this formation may form a slide plane for landslides and may warrant future geotechnical work.