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31-16 Preliminary geologic map of Bear Creek Mountain, Pinegrass Ridge between the North and South Forks Tieton River, South-Central Washington

Session: Best Student Geologic Map Competition (Posters)

Poster Booth No.: 219

Presenting Author:

Matthew Crane

Authors:

Crane, Matthew¹, Harnett, Isabella², Gaston, Miles³, Johnson, Dushawn⁴, Brunstad, Keith⁵

(1) Earth and Atmospheric Sciences, State University of Oneonta, Oneonta, NY, USA, (2) Earth and Atmospheric Sciences, State University at Oneonta, Oneonta, NY, USA, (3) Earth and Atmospheric Sciences, State University of Oneonta, Oneonta, NY, USA, (4) Earth and Atmospheric Sciences, State University of Oneonta, Oneonta, NY, USA, (5) Earth and Atmospheric Sciences, State University of Oneonta, Oneonta, NY, USA,

Abstract:

The preliminary geologic map presented here builds on previous work by Clayton (1980, 1983), Swanson and Clayton (1983), Gusey et al. (2018), and Wall et al. (2018), focusing on Pinegrass Ridge (PR) between the North and South Forks of the Tieton River, from Bear Creek Mountain to Rimrock Lake. Mapped rock units, from oldest to youngest, include: pre-Tertiary Rimrock Lake Inlier (RLI), volcaniclastics of the proposed Devils Horn Caldera (DHC), Bear Creek Mountain andesites, glacial deposits, and landslide debris. The oldest rocks are the pre-Tertiary RLI units comprising pre-Cascade accreted ophiolitic, arc-complex rocks. The RLI includes arkose, mudstone, chert, conglomerate, meta-volcaniclastics, pillow basalts (greenstone), and minor green tuffs. Overlying these are high-silica rhyolitic volcaniclastics of the ~3.2 Ma Devils Horn Rhyolite (DHR), part of the proposed Devils Horn Caldera (DHC). DHR includes tuff with accretionary lapilli, pumice lapilli tuff, flow-banded rhyolite, pyroclastic breccia, and obsidian. Previous mapping reported its thickness as 900 m, though landslide activity may have caused overestimation. Volcanism at the Goat Rocks complex began ~2.6 Ma, with four eruptive stages: Tieton Peak (~3.0–2.6 Ma), Bear Creek Mountain (>1.6–1.3 Ma), Late Creek (~1.1–0.6 Ma), and Old Snowy Mountain (~0.4–0.1 Ma). The Bear Creek Mountain stage dominates the PR area, including the Tieton Andesite (Qta1 & Qta2), Olivine Basalt (Qob1), Basaltic Andesite (Qob2), and Andesite of Bear Creek Mountain (Qabm). These units cap PR and form its cliff faces. The youngest deposits include glacial tills, alluvium, talus, and landslide debris. New mapping in southern PR reveals increased Qta2 exposures and less glacial till than previously reported. In central PR, a ~3–5 m thick till sequence dominates, while PR margins show extensive post-glacial landslides. This updated mapping aims to refine the distribution of Qta2 to better constrain lava flow volumes and directions. Mapping glacial features such as striations, till ridges, boulder trains, and moraines will help reconstruct glacial history. Additionally, documenting landslide timing and development improves stratigraphic interpretations. For instance, the apparent 900 m thickness of the DHR may be inflated due to landslides. Similar concerns apply to older andesites in paleo-valleys of Bear Creek.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

Preliminary geologic map of Bear Creek Mountain, Pinegrass Ridge between the North and South Forks Tieton River, South-Central Washington

Description

Session Format: Poster

Presentation Date: 10/19/2025

Presentation Room: Hall 1

Poster Booth No.: 219

Author Availability: 9:00–11:00 a.m.

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