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March Talk: Utah’s Ancient Mega-Landslides by Bob Biek
March 18, 2021 @ 5:30 pm - 7:30 pm
Utah’s Ancient Mega-Landslides
by Mr. Robert F. Biek, Utah Geological Survey (Co-authors are David B. Hacker, Kent State University, and Peter D. Rowley, Geologic Mapping Inc.)
ABSTRACT: During the late Oligocene to early Miocene, the Marysvale volcanic field of southwestern Utah experienced three consecutive, catastrophic, mega-scale collapse events: the Sevier, Markagunt, and Black Mountains gravity slides, which we refer to collectively as the Marysvale gravity slide complex (MGSC). Each collapse is newly discovered and ranks among Earth’s largest terrestrial landslides; collectively they span more than 8000 km2 with runout distances that exceed 35 km over the < 3° dipping former land surface. The allochthonous stratigraphic sequence, kinematic indicators, and basal structures including clastic dikes, pervasive fragmentation and pseudotachylyte collectively suggest emplacement of each gravity slide occurred at high velocity during a single event.
These gravity slides exhibit the full range of structural features commonly seen in modern landslides, but on a gigantic scale. Each consists of four distinct structural segments: (1) a high-angle breakaway segment, (2) a bedding-plane segment, (3) a ramp segment, and (4) a former land-surface segment where the upper plate moved over the landscape. The gravity slide complex remained undiscovered for so long precisely because of its gigantic size and an initially confusing mix of extensional, translational, and compressional structures overprinted by post-MGSC basin-range tectonism. Each slide is a large contiguous sheet of andesitic lava flows, mudflow breccias, source intrusions, and intertonguing ash-flow tuffs that record southward, gravitationally induced catastrophic failure of the southern flank of the Marysvale volcanic field. Failure was preceded by slow gravitational spreading on the Paunsaugunt thrust fault system. Triggering mechanisms are poorly understood, but the principal zone of failure was in mechanically weak, clay-rich volcaniclastic strata at the base of the volcanic section.
In September 2017 we convened a 6-day GSA-sponsored Thompson Field Forum on the characteristics and implications of the MGSC. New collaborative research now underway includes geologic mapping, further age control, characterization and implications of pseudotachylytes and injectites, and modelling of mobility mechanisms that allowed extraordinary long runouts.
TIME: Please login to meeting a little before 5:30pm.
- 5:30pm – 6:00pm attendees will be randomly assigned to break out rooms for three, ten-minute sessions
- 6:00pm – 6:15pm back to meeting for society business announcements and introductory remarks
- 6:15pm – 7:15pm Talk by Mr. Robert Biek and Q&A
LOCATION: Your own home! This is a Zoom meeting hosted by FLC.
Link to Join Zoom Meeting: https://fortlewis.zoom.us/j/94286084881
Join by phone: +1-346-248-7799
COST: FREE! But please renew your membership! https://fourcornersgeologicalsociet.org/membership/