GEOLOGIC MAP OF THE LITTLEROCK QUADRANGLE, THURSTON COUNTY, WASHINGTON

We present a new 1:24,000 scale geologic map of the Littlerock 7.5’ quadrangle, which straddles the eastern Black Hills and southern Puget Lowland southwest of Olympia and was previously mapped at smaller scales. More detailed geologic mapping is needed for informed land management; forestry, agriculture, recreation, ecologically sensitive areas, and development increasingly compete for natural resources. We support the new map with field work, lidar, subsurface exploration data, aeromagnetic data, geophysical explorations, geochemistry, and new ⁴⁰Ar/³⁹Ar, detrital zircon U/Pb, luminescence and radiocarbon dates.

New bedrock geochemistry confirms that the Black Hills are cored by Crescent Formation basalt. North of Mima Creek the geochemical signature is similar to Willapa Hills Crescent Fm.; south of the creek it is more similar to Olympic Mtns. Crescent Fm. Geochemistry and a new 46.0 Ma ⁴⁰Ar/³⁹Ar date suggest that basalt east of the Black River is chemically distinct and younger than Crescent Formation but older than nearby intrusive rocks, though all are likely from the same mantle source. A new 47.4 Ma U/Pb detrital zircon date from McIntosh Fm. tuffaceous marine sedimentary rocks that overlie Crescent Fm. basalt in the southern Black Hills constrains the transition from Siletzia volcanism and accretion to marine sedimentation.

We mapped glacial deposits from the Vashon stade and at least one prior glaciation that was slightly more extensive; these glaciations created ice dammed lakes in the Waddell Creek valley. New OSL and IRSL dates on fluvial sediments suggest that Possession (MIS4) meltwater drained south via the Black River Valley, although Possession ice never came close to the map area.

Geochemistry and clay mineral identification by XRD of thick, deeply weathered, clay rich soils in the Black Hills suggest that most of the Black Hills are unglaciated and have been subaerially weathered since before the Quaternary. Onlapping of Eocene to Miocene marine sediments indicates that the hills have been a topographic high since the Eocene and may never have been buried. Isolated shear exposures and geophysical data suggest a northwest striking, southwest down fault beneath Mima Prairie. This fault may extend structures previously mapped farther southeast, but it has not offset the Vashon outwash terraces.