NEW CONSTRAINTS ON THE EXTENT, AGE, AND EMPLACEMENT HISTORY OF THE MARKAGUNT MEGABRECCIA, UTAH'S LARGEST GRAVITY SLIDE

New geologic mapping of the Panguitch 30´x60´ quadrangle―which straddles the Basin and Range and Colorado Plateau Provinces in southwest Utah and includes the distal, southwestern Marysvale volcanic field―constrains the extent, age, and emplacement history of the early Miocene Markagunt Megabreccia (MM). The MM is a catastrophically emplaced gravity slide of Miocene and Oligocene ash-flow tuffs (originally erupted from calderas on the Utah-Nevada border) and intermediate volcanic and volcaniclastic rocks from the southwestern Marysvale volcanic field. The basal detachment is in Eocene-Oligocene Brian Head Formation, a clay-rich, non-resistant volcaniclastic unit that forms the base of the volcanic section in southwest Utah. The MM is exposed over an area of at least 800 km² mostly in the Markagunt Plateau; remnants of probable distal debris avalanche deposits along its south margin and hypothesized parts buried under Parowan Valley suggest its original extent exceeded 1300 km².

Newly discovered MM exposures in Summit Canyon and reinterpreted exposures in nearby Parowan Canyon show that the MM overlies 22.03-Ma Harmony Hills Tuff; at Haycock Mountain, the MM overlies stream gravels that contain rounded clasts of Harmony Hills Tuff, showing that the MM must be younger than 22 Ma. Previously, the 23.8-Ma Leach Canyon Formation was the youngest unit known to be overlain by the MM and the MM was interpreted by some to be older than the 22.8-Ma Haycock Mountain Tuff, which we map as part of the MM.

What had previously been interpreted as autochthonous Isom Formation at the MM reference section near Panguitch Lake is now known to be part of the gravity slide. These exposures of cataclasite, associated basal breccia and clastic dikes, and brittle microstructures provide strong evidence of catastrophic emplacement by gravity sliding, not by slow gravitational spreading or creep nor by seismically cycled thrust faulting. This includes a single, northward-tilted panel (45 km²) of resistant Isom Formation at Haycock Mountain whose lower few meters is cataclasite that grades upward into otherwise undisturbed Isom. Furthermore, these exposures demonstrate southward transport of the MM, not northward transport as originally inferred.