127-5 Volcanism Before the Big Slide: A Study of the Volcanic Suite Preceding the Black Mountains Gravity Slide, Utah, USA

Session: Mineralogy, Geochemistry, Petrology, and Volcanology Student Session

Presenting Author:

Alexander Smith

Authors:

Smith, Alexander Stone¹, Rivera, Tiffany², Biek, Robert F.³, Malone, David H.⁴, Hacker, David B.⁵, Braunagel, Michael⁶, Griffith, William Ashley⁷

Abstract:

The Marysvale volcanic field (MVF), Utah, contains three mega-scale gravity slides that occurred in the late Oligocene to early Miocene. Two of the three slides in the Marysvale Gravity Slide Complex (MGSC) have been dated using high-precision ⁴⁰Ar/³⁹Ar dating. Results from earlier work suggests that the Seiver gravity slide (SGS) was emplaced at 25.25 ± 0.05 Ma and the Markagunt gravity slide (MGS) at 23.05 +0.22/-0.20 Ma (Holliday et al., 2022; Rivera et al., 2024). Ages were determined by dating the pre-deformation and post-deformation tuffs below and above the slide plane, along with dating of the syn-deformation pseudotachylyte that formed along subsidiary slip surfaces. The third slide, the Black Mountains gravity slide (BGS), has been constrained between 21 and 22 Ma using the maximum depositional U/Pb dates of detrital zircons from post-slide Quaternary tuffaceous sandstones (Stevens et al., 2023). 

 

In an effort to better constrain the volcanic history leading up to the BGS, this study uses U/Pb geochronology of igneous and detrital zircons from five lithologies in the allochthonous block of the BGS. From lowest to highest in the block, the units are (1) the basal layer of the BGS, (2) a lithic tuff from the Brian Head formation, (3) the Wah Wah tuff member of the Needles Range Group, (4) a lahar within the Mt. Dutton formation, and (5) a local ash-flow tuff. Dates were determined by LA-ICP-MS at the Arizona LaserChron Center. Results indicate a younging of strata up-section from the basal layer. The basal layer here displays the characteristic older-on-younger age relationship, found throughout the MGSC, relative to the underlying 23.6-24.0 Ma Bear Valley formation. Interestingly, the basal layer contains zircons from the 30.4 Ma Wah Wah tuff and 1.7 Ga Yavapai Proterozoic province but lacks zircons from the underlying Bear Valley sandstone. We compare our results with Braunagel et al. (2025), in which they discuss the increase of the mean age of zircons within the basal layer as a consequence of slide deceleration. Additionally, we compare the geochemistry of two sections found in the BGS with nearby volcanic centers, locating the source of the strata using whole rock elemental analysis. 

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

doi: 10.1130/abs/2025AM-6445

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