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Flow Rheology and Encased Xenolith Geochemistry of the Qta2 Tieton Andesite at Bear Creek Mountain

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

Presenting Author:

Mr. Miles Gaston

Authors:

Gaston, Miles¹, Harnett, Isabella², 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) Media Studies, State University of Oneonta, Oneonta, NY, USA, (5) Earth and Atmospheric Sciences, SUNY Oneonta, Oneonta, NY, USA,

Abstract:

The Tieton Andesite lava flows of Bear Creek Mountain, sourced from the Goat Rocks volcanic complex in the south-central Washington Cascades, are among the longest lava flows worldwide, measuring ~74 km (Qta1) and ~52 km (Qta2). These flows erupted in a subduction zone setting where the Juan de Fuca Plate subducts beneath the North American Plate. 40Ar/39Ar geochronology dates Qta1 at 1.64 ± 0.04 Ma and Qta2 at 1.39 ± 0.10 Ma (Wall 2018). Both flows are high-alkali, calc-alkaline pyroxene andesites to trachyandesites with SiO₂ contents ranging from ~59–62 wt%. Estimated volumes are 2.5–6.6 km³, with effusion rates of 11–19 m³/s. Recent fieldwork revealed a distinctive outcrop of Qta2 characterized by a 3.5m thick zone enriched in xenoliths, many of which are encased in nodular structures. The outcrop, located at Section 3 Lake Cirque of Pinegrass Ridge, is ~24m thick and extends laterally >50m. It exhibits crude columnar jointing beginning at 4m above the base, transitioning to platy jointing near the surface. A ~0.5m thick zone of densely clustered, nodule-encased xenoliths was identified ~1.6m above the outcrop base, where samples were collected for geochemical analysis. Nodules are spheroidal, medium-dark grey on fresh surfaces, and range in size (LWH) from ~8x7x5.4 to ~2x2.5x1.5 cm. They consist primarily of plagioclase and pyroxenes, with crystals ≤5 mm in length and ≤2.5mm width. Above the nodule zone, glassy and highly crystalline layers were found within a 3m zone. Above this, tight isoclinal folds, gently inclined to recumbent, were identified within crude columnar jointing. A stratigraphic column was constructed using ImgJ and Pixelmator Pro to document nodule distribution. SEM-EDX analysis (Hitachi 3030 SEM and Bruker EDX) at SUNY Oneonta identified xenoliths as micro-granodiorites, quartz-rich sandstones, and fine-grained, light-tan rhyolites. The current interpretation is that the sandstones originate from the Rimrock Lake inlier, the rhyolites from the Devils Horns Rhyolite (Tpr), and the micro-granodiorites from an unidentified Goat Rocks source. Surrounding nodules are likely quenched Qta2 host lava. Glassy and crystalline layers, along with tight isoclinal folds, may indicate shearing within the flow. These findings provide new insights into the emplacement processes of Qta2.