March 2019 

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Utah’s Markagunt and Sevier Gravity Slides—New
Discoveries and New Research on the World’s Largest
Terrestrial Landslides

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Utah Geological Survey

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Robert Biek


11 March 19




The early Miocene Markagunt
(MGS) and late Oligocene 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. 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 at least 35 km over the Miocene
landscape. The 22 to 21 Ma MGS remained undiscovered for so long precisely because
of its gigantic size (>5000 km2, >95 km long, estimated volume 3000 km3) 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 (>2000 km2) and slightly older (25 to 23 Ma). Both slides are
large contiguous sheets 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.
The MGS (discovered in 2014) and SGS (discovered in 2016) provide stunning
examples of gravity-slide structures so large that they may be mistaken for tectonic
features. However, basal and lateral cataclastic breccias, clastic dikes, and
pseudotachylytes indicate catastrophic emplacement events. Triggering mechanisms
are poorly understood, but the principal zone of failure was in mechanically weak, clayrich
volcaniclastic strata at the base of the volcanic section.


Bill Lund

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