Ancient, newly discovered gigantic landslides resulting from catastrophic failure of the Marysvale volcanic field, Utah

October 16th (Wednesday), 12:15 p.m., Senior Center Jackson Hole – Open to Public. Presentation: “Ancient, newly discovered gigantic landslides resulting from catastrophic failure of the Marysvale volcanic field, Utah”, Presented by Bob Biek, Utah Geological Survey.

Lunch is available from the Senior Center, though you may bring your own.  $4 for those >60, $8 for those <60

Three newly discovered gigantic landslides in southwestern Utah exhibit the full range of structural features commonly seen in modern landslides, but on an enormous scale. Each slide is nearly 60 miles long with runouts (transport distances) over the former land surface of at least 20 miles; together they span 3000 square miles and rank among Earth’s largest terrestrial landslides. These slides resulted from catastrophic failure of the southern flank of the Marysvale volcanic field, near the end of its peak magmatic activity about 18 to 25 million years ago. The principal zone of failure was in mechanically weak, clay-rich sedimentary strata at the base of the volcanic section. The landslide masses become younger westward, and each is spatially associated with one of the largest volcanic features in the field, namely Yellowstone-like calderas and Utah’s largest exposed batholith (granitic intrusion, the roots of ancient volcanoes). Failure was preceded by slow gravitational spreading on the Paunsaugunt thrust fault system.

The discovery of these gigantic landslides offers a beautiful and classic example of the use of multiple-working hypotheses, continually refined after each season’s field studies and applied by field geologists working to understand one of Utah’s most enigmatic and complex terranes. This talk will highlight the decades-long sequence of discovery, show stunning examples of deformed volcanic rocks and gigantic landslide structures, and summarize ongoing research on some of Earth’s most remarkable geologic features—gigantic, long-runout landslides.  Video