32-6 Paleomagnetism of the Junction Butte Basalt, Yellowstone National Park and the Hepburn Mesa Basalt, Paradise Valley, Montana

Session: Latest Research Advances in Structural Geology and Tectonics (Posters)

Presenting Author:

Authors:

Abstract:

The Junction Butte Basalt (JBB) has long been recognized as one of the oldest Quaternary volcanic units of the Yellowstone Plateau volcanic field in Yellowstone National Park (YNP). Stratigraphically, it overlies volcanic rocks of the Absaroka Volcanic Supergroup and underlies the 2.08-Ma Huckleberry Ridge Tuff, as well as other younger volcanic rocks. The JBB consists of thick (up to 60 m) canyon-filling flows that exhibit moderately to well-developed columnar jointing. Petrologically, it is aphanitic, with sparse to moderately abundant plagioclase phenocrysts and olivine in the matrix. The Hepburn Mesa Basalt (HMB) consists of a terrace-capping, columnar-jointed flow in Paradise Valley near Emigrant, Montana, approximately 40 km north of YNP. It overlies andesitic breccias or ancestral Yellowstone River gravels. The HMB is aphanitic and characterized by sparse plagioclase, pyroxene and olivine phenocrysts. As part of a paleomagnetic study of Quaternary volcanic rocks associated with Yellowstone Plateau volcanism, samples were collected from the JBB near Tower Junction, YNP, Wyoming (referred to as the Overhanging Cliff Flow) and the HMB in the southern Paradise Valley, Montana.  Paleomagnetic results from the JBB yield an in situ site-mean direction of declination (D) = 197.8°, inclination (I) = -62.9°, k = 178.4, a₉₅ = 4.5° (n = 7/7 samples) and the HMB yields a site-mean of D = 201.4°, I = -67.0°, k = 131.3, a₉₅ = 4.9° (n = 14/14 samples). Thermomagnetic data from HMB samples are consistent with high-Ti titanomagnetite with Curie temperatures ranging from 280° to 350°C, characteristic of rapidly quenched basalt flows. The paleomagnetic results from the two flows, although spaced approximately 54 km from each other, yield identical reverse polarity remanent magnetizations, whereas paleomagnetic data from younger Quaternary basalts in northern YNP and the Gardner, Montana, area yield normal polarity magnetizations. A preliminary whole rock ⁴⁰Ar/³⁹Ar date of 2.2 Ma ± 0.2 Ma was reported for the HMB, whereas K-Ar dates from flows of the JBB are 2.15 Ma and 2.01 Ma. The paleomagnetic data and geochronologic data are consistent with eruption of these basalts during the Matuyama reverse polarity chron. The similarity of isotopic dates and the identical remanent magnetizations indicate that both flows erupted at about the same time and may be correlative indicating that pre-Huckleberry basaltic volcanism was more widespread than previously thought.

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

doi: 10.1130/abs/2025AM-8918

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