BASAL LAYERS, INJECTITES, DEFORMED BLOCKS, AND IMPLICATIONS OF THE NEWLY DISCOVERED SEVIER GRAVITY SLIDE, MARYSVALE VOLCANIC FIELD, SOUTHWESTERN UTAH

In 2016, we discovered the early Miocene Sevier gravity slide (SGS) in southwestern Utah and now understand it to rank among Earth’s largest terrestrial landslides. The SGS spans about 2000 km² in the south-central Marysvale volcanic field and exhibits stunning deformation along its basal slip surface. Single-generation basal layers—diamictites that are millimeters to tens of meters thick—and associated injectites provide compelling evidence of overpressured fluids on its basal and subsidiary failure planes. Striated, grooved and fractured slip surfaces indicate emplacement from north to south. Cataclastic and ultracataclastic deformed blocks within the basal layer commonly exhibit length-to-width ratios in excess of 10:1, and many such blocks have delicate “tails” suggestive of shearing or cataclastic growth during transport. We interpret the SGS as a gigantic, contiguous sheet of andesitic lava flows, volcaniclastic rocks, and intertonguing ash-flow tuffs that records southward, gravitationally induced catastrophic failure of the south-central flank of the Oligocene to Miocene Marysvale volcanic field between 25 and 23 Ma. The principal zone of failure was in mechanically weak, clay-rich volcaniclastic strata at the base of the volcanic section. Failure was preceded by gravitational spreading on the Paunsaugunt thrust fault system, which is rooted in Middle Jurassic evaporite-bearing strata at a depth of about 2 km. Triggering mechanisms are poorly understood, but a spatial association of the apparent breakaway area with the 22.9 Ma Monroe Peak caldera is intriguing.

The SGS is cousin to the gigantic, 22 to 21 Ma Markagunt gravity slide (MGS, >5000 km², >95 km long, >35 km runout, estimated volume 3000 km³; dimensions revised from Geology, v. 42, no. 11, p. 943–946). In fact, the SGS was discovered using lessons learned from our mapping of the MGS. That features as large as the MGS can remain undetected despite decades of geologic mapping and research in the area suggests to us that other volcanic fields around the world may hold evidence of as-yet-undiscovered, exceptionally large gravity slides.