We recently released four new 1:24,000-scale geologic maps covering parts of Thurston, Pierce, Lewis, Kittitas, and Stevens Counties.
The new geologic maps include:
- Geologic map of the Bald Hill 7.5-minute quadrangle, Thurston, Pierce, and Lewis Counties, Washington: Map Series 2023-03, 1 sheet with 37 p. text. [Click to download zip file]
- Geologic map of the Harts Lake 7.5-minute quadrangle, Pierce and Thurston Counties, Washington: Map Series 2023-04, 1 sheet with 16 p. text. [Click to download zip file]
- Geologic map of the Kittitas and East Kittitas 7.5-minute quadrangles, Kittitas County, Washington: Washington Geological Survey Map Series 2023-05, 1 sheet with 32 p. text. [Click to download zip file]
- Geologic map of the Adams Mountain and Hunters 7.5-minute quadrangles, Stevens County, Washington: Map Series 2023-06, 1 sheet with 56 p. text. [Click to download zip file]
You can access and search through all geologic maps from the Washington Geological Survey via an interactive map here: Washington State Geologic Map application
Below we include additional information about each publication, including the abstract, suggested citation, and a direct download link.
Geologic map of the Bald Hill 7.5-minute Quadrangle, Pierce, Thurston, and Lewis Counties, Washington
Abstract
We present a geologic map of the Bald Hill quadrangle located along the Nisqually River in Washington’s southeastern Puget Lowland. The map describes the age, stratigraphy, and distribution of glacial, fluvial, and volcanic deposits along the southern margin of the Tacoma basin. We use 38 geochemical analyses to characterize volcanic, volcaniclastic, and intermediate igneous intrusive rocks of the Eocene Northcraft Formation. The volcanic rocks are mostly andesitic but range from basalt to rhyolite. These rocks commonly include propylitic alteration, especially near intrusions. The volcaniclastic rocks range from welded to unwelded tuff, tuff breccia, and coarse, bouldery lahar deposits. Eleven new ages from the Northcraft Formation range from 42.5 ±0.1 Ma to <35.2 ±1.2 Ma.
Our analysis of existing aeromagnetic and new gravity data suggests that the map area marks the southeastern terminus of the Olympia fault. Joint two-dimensional forward modeling of aeromagnetic and gravity data permits either a faulted or folded southern edge of the Tacoma basin. New detrital zircon ages reveal that sediment (unit PLmc), which closely resembles the Mashel Formation in outcrop, is Pliocene—significantly younger than the fossil-based Miocene age of the Mashel Formation. Definitive determination of whether unit PLmc is equivalent to the Mashel Formation awaits improved geochronologic age control on the Mashel Formation type section. Based on three radiocarbon dates, heavy mineral content in pumice pebbles, and geochemical analyses of three clasts, we associate lahar deposits near the northern map boundary with Mount Rainier’s late Holocene Summerland eruptive period. These deposits document infrequent but life-threatening volcanic hazards. Vashon glacial deposits, Holocene alluvium, and underlying Cascade Range-derived sediment provide productive aquifers. High arsenic content in groundwater in the northeastern part of the map area may be associated with unit PLmc.
Suggested Citation
Polenz, Michael; Hladky, F. R.; Bauer, A. L.; Lau, T. R.; Tepper, J. H.; Nesbitt, E. A.; Legorreta Paulín, Gabriel, 2023, Geologic map of the Bald Hill 7.5-minute quadrangle, Thurston, Pierce, and Lewis Counties, Washington: Washington Geological Survey Map Series 2023-03, 1 sheet, scale 1:24,000, with 37 p. text. [https://www.dnr.wa.gov/publications/ger_ms2023-03_geol_map_bald_hill_24k.zip]
Click here to download the publication as a zip file:
https://www.dnr.wa.gov/publications/ger_ms2023-03_geol_map_bald_hill_24k.zip
Geologic map of the Harts Lake 7.5-minute Quadrangle, Pierce and Thurston Counties, Washington
Abstract
We present a geologic map of the Harts Lake quadrangle in Washington’s southern Puget Lowland. We combine new geologic mapping, well log data, and geophysical modeling to better understand the glacial history, volcanic hazards, and local faulting in the map area. Quaternary glacial drift covers most of the map area, with a veneer of ablation till covering most of the glaciated and fluted surfaces. We interpret deposits along the Nisqually River and Tanwax Creek that were previously mapped as Miocene Mashel Formation (Walters, 1965) as younger Pleistocene lahars and alluvium from the Cascade Range. A review of cuttings and well logs from the E.F.E. Willhoite et al. No. 1 oil exploration well, drilled by the Humble Oil & Refining Company in 1961, helped inform the cross section and geophysical model for the quadrangle, providing new insight into the structure of the subsurface. Some water wells in the southeast portion of the map area produce water with arsenic concentrations above the Washington State Department of Health’s recommendations for drinking water. We believe that the high concentration of arsenic in these wells may be related to the decay of organic material that was preserved by Pleistocene or older Pliocene–Miocene volcanic mudflow deposits.
Suggested Citation
Contreras, T. A.; Goughnour, R. L.; Lau, T. R., 2023, Geologic map of the Harts Lake 7.5-minute quadrangle, Pierce and Thurston Counties, Washington: Washington Geological Survey Map Series 2023-04, 1 sheet, scale 1:24,000, with 16 p. text. [https://www.dnr.wa.gov/publications/ger_ms2023-04_geol_map_harts_lake_24k.zip]
Click here to download the publication as a zip file:
https://www.dnr.wa.gov/publications/ger_ms2023-04_geol_map_harts_lake_24k.zip
Geologic Map of the Kittitas and East Kittitas 7.5-minute Quadrangles, Kittitas County, Washington
Abstract
New geological and geophysical investigations of the Kittitas and East Kittitas quadrangles refine Neogene stratigraphy and characterize geologic structures in southern Kittitas Valley. New whole rock geochemistry (n=329) locally refines the middle Miocene chemostratigraphic framework of the Columbia River Basalt Group (CRBG). These strata are tilted toward Kittitas Valley by faulting and folding at the mountain front. Abundant suprabasalt late Miocene to Pliocene volcaniclastic strata are less steeply tilted.
From twelve U-Pb analyses, we interpret eruptive ages of ~15.95 Ma for a tuff at the top of the Vantage Member and ~8.72 Ma for a pumice within the upper Ellensburg Formation. Younger sedimentary interbeds of the Ellensburg Formation are more common and thicker compared to older interbeds, suggesting a migrating and (or) deepening depositional center. Between ~5.4 and ~3.3 Ma, deposition transitioned from largely Cascade-sourced sediments of the Ellensburg Formation to basalt-derived sediments of overlying Pliocene(?)–Quaternary deposits.
We identify reverse and thrust faults with oblique-slip on the flanks of bedrock highs. We interpret these range front faults as young, in-sequence thrust-type structures in a compressional or transpressional regime. Fault-related folds are associated with these structures, including several plunging, northwest- through west-trending anticlines and synclines. We map numerous northerly striking oblique-slip faults with varying offsets, especially where the folds of the Boylston Mountains bend northward.
Geophysical modeling of gravity and aeromagnetic data suggests laterally abrupt thickness changes in basaltic units. These thickness changes may be related to (1) ramp-flat geometries under uplifts; and (or) (2) periods of syn-eruptive development of local accommodation space, where CRBG rocks experienced concurrent footwall thickening (growth strata) and hanging-wall thinning (uplift-related erosion).
Suggested Citation
Sadowski, A. J.; Wetherell, L. R.; Anderson, M. L.; Powell, J. E., 2023, Geologic map of the Kittitas and East Kittitas 7.5-minute quadrangles, Kittitas County, Washington: Washington Geological Survey Map Series 2023-05, 1 sheet, scale 1:24,000, with 32 p. text. [https://www.dnr.wa.gov/publications/ger_ms2023-05_geol_map_kittitas_e_kittitas_24k.zip]
Click here to download the publication as a zip file:
https://www.dnr.wa.gov/publications/ger_ms2023-05_geol_map_kittitas_e_kittitas_24k.zip
Geologic Map of the Adams Mountain and Hunters 7.5-minute quadrangles, Stevens County, Washington
Abstract
New geologic mapping, geochemistry, and geochronology, and compilation of existing geologic maps, improve our understanding of the >1.3 billion-year geologic history in the Adams Mountain and Hunters 7.5-minute quadrangles.
Metasedimentary and metavolcanic rocks of the Deer Trail Group and Windermere Group are the oldest in the area (~1.3–0.7 Ga). These rocks are typically foliated or cleaved, and contain evidence for the rifting of Rodina. The Addy Quartzite straddles the Precambrian–Paleozoic boundary and underlies a thick sequence of metacarbonate and shale, which are exposed in a now-vertical sequence of thrust sheets. Paleozoic metasedimentary and metavolcanic rocks of the Roberts Mountains allochthon are found in the northern part of the map area.
Two voluminous granitic plutons intrude the Paleozoic and older rocks: a 100–105 Ma zoned pluton near the Germania mine, which hosts tungsten and molybdenum mineralization (in addition to other critical and commodity elements), and a 71–74 Ma pluton near Fruitland, which is locally associated with mineralized skarn.
Eocene andesitic to rhyolitic flows and tuffs were rapidly emplaced 51.5–52.2 Ma on the exhumed 71–74 Ma pluton and are found in fault-bounded half grabens that presumably developed above the Kettle detachment fault. The volcanic rocks are similar to the Sanpoil Volcanics farther west; both are adakites and likely formed by partial melting of continental crust and not from subduction.
Much of the map area was repeatedly covered by continental glaciers during the Pleistocene, the most recent of which left a variety of deposits from lacustrine beds of Glacial Lake Columbia, to ice-marginal kame terraces and deeply incised bedrock gorges that record glacial retreat.
Suggested Citation
Steely, A. N., 2023, Geologic map of the Adams Mountain and Hunters 7.5-minute quadrangles, Stevens County, Washington: Washington Geological Survey Map Series 2023-06, 1 sheet, scale 1:24,000, with 56 p. text. [https://www.dnr.wa.gov/publications/ger_ms2023-06_geol_map_adams_mountain_hunters_24k.zip]
Click here to download the publication as a zip file:
https://www.dnr.wa.gov/publications/ger_ms2023-06_geol_map_adams_mountain_hunters_24k.zip
WASHINGTON STATE GEOLOGY NEWS 