268-6 Layer by Layer: Differentiating Yellowstone’s Eruptions through Remapping, Petrology, and Geochronology

Session: Old and the New, Long and the Short: Perspectives on Integration of Timescales of Magmatic Processes: Special Session Related to MGPV Awards to Madison Myers and Anita Grunder (Posters)

Poster Booth No.: 204

Presenting Author:

Liv Wheeler

Authors:

Wheeler, Liv¹, Henderson, Stacy², Myers, Madison³, Wilson, Colin J.N.⁴, Arnold, Liam R. ⁵, Luft, Lexi⁶, Nolander, Faith E.⁷, Gormley, Lynne⁸

(1) Montana State University, Bozeman, MT, USA, (2) Montana State University, Bozeman, MT, USA, (3) Montana State University, Bozeman, MT, USA, (4) Victoria University, Wellington, New Zealand, (5) Montana State University, Bozeman, MT, USA, (6) Montana State University, Bozeman, MT, USA, (7) Central Washington University, Ellensburg, WA, USA, (8) Montana State University, Bozeman, MT, USA,

Abstract:

Recent remapping of the Lava Creek Tuff (LCT; 0.631 Ma), Yellowstone, WY, along the NW caldera boundary has determined that a vitrophyre previously identified as LCT member A represents a new pyroclastic deposit within the Mount Jackson Rhyolite series (MJR; 1.28-0.70 Ma). ⁴⁰Ar/³⁹Ar dating on this deposit yields 0.7926 ± 0.0042 Ma (2σ), predating the LCT and unmatched by any dated MJR lava. This discovery suggests that MJR activity, previously characterized as entirely effusive, also produced explosive eruptions. Recognizing these smaller-scale explosive products calls for reevaluation of Yellowstone’s eruptive history and style. This research presents results from an eight-week remapping effort, focusing on descriptions of LCT and MJR units at a higher resolution (1:24,000 scale) for an updated geologic map. Multiple stratigraphic sections reveal intra-unit variability, demonstrating that the LCT and MJR are not laterally consistent across this field area. At Purple Mountain, the LCT lower member A ignimbrite can be divided into four distinct units based on phenocryst content and welding textures (10–30% crystals; weak to strong welding; crystal sizes 0.1–0.3 cm). These are (from base to top): (1) a crystal-rich, fiamme-absent unit, (2) a crystal-rich, fiamme-rich unit, (3) a crystal-rich layer containing black recycled rhyolitic lithics, and (4) a crystal-poor, pink pumice-bearing unit. These units build upon descriptions in Christiansen (2001) but offer finer resolution to correlate across the field area. To the west at Gneiss Creek, three distinct ignimbrite layers are recognized (from base to top): (1) a lower densely welded, crystal-rich unit, (2) a crystal-poor, fiamme-rich unit, and (3) an upper variably welded, pumice-rich unit. Together, these sections highlight unrecognized intra-unit complexity and ambiguous flow boundaries within current maps. In the Mount Jackson region, a well-exposed contact between a MJR lava and overlying LCT includes a 20-30 cm pyroclastic layer composed of components matching the lower MJR flow. Based on crystal counts and sizes (plagioclase-rich, 0.5-0.7 cm) we suggest that this may be another explosive MJR. Further ⁴⁰Ar/³⁹Ar geochronology and detailed petrological comparisons are planned to distinguish this unit from documented MJR lavas and constrain its timing in Yellowstone’s history. These findings support a revision of MJR-LCT relationships and demonstrate the importance of detailed field mapping in understanding Yellowstone’s volcanic history.

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

doi: 10.1130/abs/2025AM-9470

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.