JANUARY 5, 2009, STATE ROUTE 14 MILEPOST 8 ROCK FALL, CEDAR CANYON, IRON COUNTY, UTAH

On January 5, 2009, a large rock mass detached from a cliff in Cedar Canyon 8 miles east of Cedar City, Utah, and disintegrated into tens of large boulders and hundreds of smaller ones before burying approximately 750 feet of Utah State Route 14 (SR-14). Rock-fall volume was estimated at 60,000 yd3. The triggering mechanism was likely recurring freeze-thaw cycles and ice wedging in bedrock joints.

Cedar Canyon incises the Markagunt Plateau at the transition between the Basin and Range and Colorado Plateau physiographic provinces. Deformed sedimentary rocks are exposed in the lower several miles of the canyon; however, at the rock-fall site, the canyon is in the Colorado Plateau proper, and rock units are largely undeformed, dipping a few degrees eastward. Site stratigraphy includes the cliff-forming Tibbet Canyon Member of the Straight Cliffs Formation, which sourced the rock fall, underlain by the slope-forming Tropic Shale and Dakota Formation, all Cretaceous in age. The 600-foot-thick Tibbet Canyon is limey sandstone, the 30-foot-thick Tropic is sandy mudstone and muddy sandstone, and the >600-foot-thick Dakota is mostly mudstone.

Just west of the rock fall, SR-14 begins climbing the south wall of Cedar Canyon. The Tropic Shale and Dakota Formation are susceptible to landsliding, and SR-14 crosses a large rotational landslide at the rock-fall site. The rock mass that detached from the overlying cliff fell onto the landslide below. There was no evidence of renewed landslide movement.

A magnitude 1.7 earthquake occurred at 6:04 p.m. in Cedar Canyon – 11 minutes before the Iron County Sheriff's dispatch received notification of the rock fall. The University of Utah Seismograph Stations stated that the rock fall may have caused the earthquake. The seismic waveforms are atypical for a tectonic earthquake, and large solution uncertainty allows for a surface origin. Other constraints on event timing are a Utah Department of Transportation (UDOT) snowplow that passed through the area at 5:30 p.m. and the 911 call at 6:15 p.m.

UDOT began clearing and repair of SR-14 immediately, and the highway was reopened six days later. UDOT also installed concrete Jersey barriers to reduce the potential for boulders remaining on the slope above the road to reach the roadway in the future.