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The Bear Valley Formation (Fm.) is a distinctive eolian sandstone interbedded with thick volcanic rocks of the Marysvale volcanic field of southwest Utah, the southern part of which failed during eruptive activity along three mega-scale gravity slides. The formation is as thick as 300 m and extends over an area of >2,500 km²in the Black Mountains and Markagunt Plateau. The Bear Valley Fm. is composed of tuffaceous sandstone interbedded with tuff, conglomerate, and polymict volcanic mudflow breccias. The sandstone beds are lithic arenite and lithic wacke that occur as massive beds with large-scale cross bedding. The Bear Valley Fm. occurs in the upper plate of the Markagunt gravity slide and is in both the upper and lower plates of the Black Mountains gravity slide. We used laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to acquire U/Pb dates of detrital zircons (N = 3, n = 346) from the autochthonous Bear Valley Fm. at Kane Spring and Jako Wash in the Black Mountains and the allochthonous Bear Valley at Sandy Wash in the central Markagunt Plateau. All samples are dominated by Oligocene zircons with maximum likelihood ages for deposition ranging from 23.6 to 24.0 Ma. The western-most sample from Jako Wash also preserves a slightly older group of zircons, indicating derivation from either the underlying Wah Wah Springs Fm. or another unit erupted from the Indian Peak caldera complex to the west. Thus, the upper Bear Valley Fm. was deposited within ~400 kyr before the emplacement of the Markagunt gravity slide at 23 Ma, reflecting accelerated uplift of the northern Marysvale complex that ultimately resulted in collapse and slide emplacement. 

Abstract. The Marysvale volcanic field in southwestern Utah hosts three large-volume gravity slides: the Sevier (SGS), the Markagunt (MGS), and the Black Mountains (BGS). The gravity slides are composed of lahar deposits, lava flows, and ash-flow tuffs erupted from former stratovolcanoes and other vents during the Oligocene and Miocene. The ash-flow tuffs are prime targets for dating to constrain the age of the gravity slides because some ash-flow tuffs are deformed within the slides, whereas others are undeformed and cap the slides. Furthermore, the gravity slides produced pseudotachylyte during slide motion, a direct indicator for the timing of each slide. This work provides new 40Ar/39Ar dates for several ash-flow tuffs and pseudotachylyte for the SGS, along with U/Pb zircon dates for one deformed tuff and alluvium near the slide plane. Results show that the slide was emplaced at 25.25 ± 0.05 Ma and was immediately followed by the eruption of the Antimony Tuff at 25.19 ± 0.02 Ma. The model presented here suggests that the intrusion of magma related to the Antimony Tuff acted as a triggering mechanism for the slide and that slide movement itself led to decompression melting and eruption of the Antimony Tuff. This sequence of events occurred on a geologically rapid timescale and may have been virtually instantaneous.