SME Utah Section Meeting
Utah Geological Survey
Title:The Gigantic Early Miocene Markagunt and Sevier Gravity Slides, Southwest Utah
Abstract:The newly discovered early Miocene Markagunt (MGS) and Sevier (SGS) gravity slides in southwestern Utah exhibit the full range of structural features commonly seen in modern landslides, but on a gigantic scale; they are among Earth’s largest terrestrial landslides. The MGS consists of four distinct structural segments: (1) a high-angle breakaway segment, (2) a bedding plane segment, (3) a ramp segment, where the slide cuts up-section, and (4) a former land surface segment where the upper plate moved at least 35 km over the Miocene landscape. The MGS remained undiscovered for so long precisely because of its gigantic size (>5000 sq km, >95 km long, estimated volume 3000 cu km) and an initially confusing mix of extensional,translational, and compressional structures overprinted by post-MGS basin-range tectonism. Preliminary mapping of the SGS shows it to be smaller (likely at least 1500 sq km in size) and slightly older. Both slides are large contiguous sheets of andesitic lava flows, volcaniclastic rocks, source intrusions, and intertonguing ash-flow tuffs that record southward, gravitationally induced catastrophic failure of the southern flank of the Oligocene to Miocene Marysvale volcanic field. Failure was preceded by slow gravitational spreading on the Paunsaugunt thrust fault system, which is rooted in Middle Jurassic evaporate-bearing strata at a depth of about 2 km. The MGS was emplaced between 22 to 21 Ma; SGS emplacement is presently constrained between 25 to 23 Ma. The presence of basal and lateral cataclastic layers, clastic dikes,
pseudotachylyte, deformed clasts, and a variety of kinematic indicators suggest that each slide represents a single catastrophic emplacement event from the north to the south. The principal one of failure was in mechanically weak, clay-rich volcaniclastic strata at the base of the volcanic section. Triggering mechanisms are poorly understood.
Date:February 15, 2018
Time:Social Hour—6:00 to 7:00
Dinner—7:00 to 7:30
Presentation—7:30 to 8:30
3665 South West Temple
Salt Lake City, UT 84115
Please park in the north parking lot.
Price:Members and Guests $20.00
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