73-1 Stratigraphy and Mapping of the Devils Horn Rhyolites, Goat Rocks Volcanic Complex, WA

Session: Using Volcanic Deposits to Help Us Understand Volcanic and Magmatic Processes (Posters)

Poster Booth No.: 271

Presenting Author:

Isabella Harnett

Authors:

Harnett, Isabella¹, Gaston, Miles², Crane, Matthew³, Johnson, Dushawn⁴, Brunstad, Keith A.⁵

(1) Earth and Atmospheric Sciences, State University of Oneonta, Oneonta, NY, USA, (2) Earth and Atmospheric Sciences, State University of 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 Univeristy of Oneonta, Oneonta, NY, USA,

Abstract:

The Devils Horn Rhyolites (Tpr) are a thick sequence of high-silica rhyolitic deposits dated to the upper Pliocene, with a zircon fission-track age of ~3.2 Ma (Clayton, 1983). These units, up to 650 meters thick, unconformably overlie the Rimrock Lake Inlier beneath Bear Creek Mountain and the Devils Horn in the Goat Rocks volcanic complex (South-Central WA Cascades). The goal of this study is to create stratigraphic columns detailing morphological features within Tpr exposures and develop a preliminary geologic map of Tpr and adjacent volcaniclastic and bedrock units.

With an estimated volume exceeding 20 km³, Tpr likely records a major eruptive event (Clayton, 1983). Evidence of explosive volcanism includes accretionary lapilli-bearing welded tuffs and regional tuff layers rich in lithic fragments and lapilli, likely sourced from the Rimrock Lake Inlier. Samples show variable amounts of quartz, feldspar, oxides, and pumice in a fine-grained ash matrix. Stratigraphic columns were developed using ImgJ, DJI Air 3s drone imagery, and Pixelmator Pro. A ~100 m section north of Conrad Meadows along the south Fork of the Tieton River reveals distinct stratigraphy: the upper ~20 m features sub-rounded to sub-angular cobbles interbedded with fine-grained, light-tan tuff. A ~15–20 m brecciated zone overlies additional fine-grained, massive tuff that appears more resistant than other layers. Beneath this ~ 20-30 m of light tan, fine-grained, thinly bedded tuff extends downward, with possible evidence of faulting and cross bedding. At the base, a light brown to rusty-tan layer of cobbles, pebbles, and fine-grained material may represent lakebed deposits.

Recent fieldwork uncovered undocumented Tpr outcrops mapped with Adobe Photoshop, ArcGIS Pro, DJI Air 3s drone imagery, and Pixelmator Pro. LiDAR data from WA DNR shows large slump blocks may have displaced parts of the unit, inflating apparent thickness; these features were also mapped. While some propose Tpr as caldera-fill from explosive eruptions at Devils Horn (Swanson and Clayton, 1983), another hypothesis suggests valley-fill from Devils Horn or an unknown Goat Rocks source, supported by possible fluvial reworking and laterally continuous layers traceable over long distances.

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

doi: 10.1130/abs/2025AM-10382

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