We’re pleased to announce the release of two new geologic maps!
Geologic map of the Adna 7.5-minute quadrangle, Lewis County, Washington
New 1:24,000-scale geologic mapping of the Adna quadrangle in southwest Washington focuses on improving geologic understanding of natural hazards that may pose a risk to people and infrastructure, including a proposed flood-retention structure near Pe Ell. Potential hazards include: (1) landslides due to ground instability and (2) seismicity associated with the Doty–Salzer Creek fault system.
New mapping and geophysical data refine previous work by characterizing geologic structures in greater detail, refining fault locations, and updating structural interpretations. West- and northwest-striking oblique reverse faults in the quadrangle may be capable of producing damaging earthquakes. Newly acquired gravity and ground-magnetic geophysical data constrain the locations of fault strands and strongly support 4,000 to 6,000 ft of structural relief on the Lincoln Creek uplift in the north-central map area.
Radiometric dating (U-Pb) of detrital zircons refines the stratigraphic framework for southwest Washington. Analyses reveal new maximum depositional ages for the Lincoln Creek Formation (<30.4 ±5.2 Ma), the Astoria(?) Formation (<14.8 ±3.5 Ma and <15.3 ±1.7 Ma), the Wilkes Formation (<6.2 ±1.0 Ma, <13.4 ±1.0 Ma, and <13.6 ±1.9 Ma), and the Logan Hill Formation (<1.6 ±0.5 Ma). U-Pb age spectra of detrital zircons identify marine rocks as the Eocene McIntosh Formation (<40.5 ±3.6 Ma and <38.5 ±4.2 Ma).
Suggested Citation: Sadowski, A. J.; Becerra, R. I.; Toth, C. H.; Polenz, Michael; Anderson, M. L.; Lau, T. R.; Nesbitt, E. A.; Tepper, J. H.; DuFrane, S. A., 2019, Geologic map of the Adna 7.5-minute quadrangle, Lewis County, Washington: Washington Geological Survey Map Series 2019-01, 1 sheet, scale 1:24,000. [http://www.dnr.wa.gov/publications/ger_ms2019-01_geol_map_adna_24k.zip]
Geologic map of the Rochester 7.5-minute quadrangle, Thurston and Lewis Counties, Washington
The Rochester quadrangle straddles Thurston and Lewis Counties west of Washington’s economically vital I-5 freeway. This map supports private and public resource management and planning with insights into geologic deposits, structures, and hazards. Field- and lidar-informed mapping improve identification of landslides and Mima mounds, confirming that the latter occur only on outwash. Vashon Drift and deposits from at least one earlier Cordilleran ice incursion reached their southern limit in the quadrangle. Outwash provides ample gravel resources and a productive aquifer. New detrital zircon U-Pb ages provide maximum depositional ages for: (1) the Lincoln Creek Formation type section near Galvin (<28.6 ±2.1 Ma) and Helsing Junction (<20.3 ±1.7 Ma), confirming that the formation ranges into the Miocene; (2) the Astoria(?) Formation south of Helsing Junction (<22.1 ±1.5 Ma), suggesting that its age may overlap with that of the Lincoln Creek Formation; (3) the Logan Hill Formation on Michigan Hill (<1.9 ±0.8 Ma), where a lack of Miocene zircons suggests that sedimentary provenance differs from Logan Hill Formation farther south; (4) strongly weathered drift from northern-sourced, pre-Vashon ice incursion(s) (<2.4 ±1.6 Ma; <3.0 ±0.7 Ma; <0.8 ±1.0 Ma; <1.4 ±0.2 Ma); and (5) a relict terrace on Grand Mound (unit Qoa, <0.3 ±0.5 Ma), which implies about 100 ft of net valley-floor lowering during the last approximately 0.7 Ma. New geochemical data suggest that volcaniclastic rocks in the Astoria(?) Formation are derived from the volcanic rocks of Grays River. Faults in the map area are interpreted in the context of regional north–south compression.
Suggested Citation: Polenz, Michael; Toth, C. H.; Samson, Catherine; Sadowski, A. J.; Becerra, R. I.; Lau, T. R.; Anderson, M. L.; Nesbitt, E. A.; Tepper, J. H.; DuFrane, S. A.; Legorreta Paulin, Gabriel, 2019, Geologic map of the Rochester 7.5-minute quadrangle, Thurston and Lewis Counties, Washington: Washington Geological Survey Map Series 2019-02, 1 sheet, scale 1:24,000. [http://www.dnr.wa.gov/publications/ger_ms2019-02_geol_map_rochester_24k.zip]