ROCKFALL AND LANDSLIDE HAZARDS OF THE CANYONS OF THE UPPER VIRGIN RIVER BASIN NEAR ROCKVILLE AND SPRINGDALE, UTAH

The spectacular beauty of the steep canyons of the upper Virgin River basin is famous because of Zion National Park. But the same geologic and topographic conditions that make the area beautiful also make it highly susceptible to two widely different types of hazards: rockfalls and landslides. This was dramatically emphasized on October 18, 2001, when a one-story home in Rockville was struck by a 200-300 ton boulder. The homeowner escaped within inches and seconds from injury! A cliff of the Shinarump Conglomerate Member of the Upper Triassic Chinle Formation gave way and the resulting slab fell vertically, hit the slope at the cliff base, and shattered into several pieces, one of which bounced over a stone wall into the home. Its source was Rockville Bench, towering about 65 m above the valley floor. Four sets of joints, at about 45 degrees to each other, cut the rocks, but the primary control was the through-going N. 40 W. set, which paralleled the Bench face. Similar hazards between Rockville and the entrance of Zion Canyon 8 km to the NE include: (1) large angular rocks, many of which have moved historically, littering slopes of the incompetent Moenkopi Formation below the Shinarump; (2) Springdale is bounded on its NW and SE sides by landslides, including one of 14 million cubic meters that destroyed 3 homes on September 2, 1992, and another that moved in May 1988; and (3) a slide of 84,000 cubic meters in Zion Canyon that blocked the canyon, stranded tourists, and dammed the Virgin River on April 12, 1995, only the latest of many historic (1923, 1941) and Quaternary landslides and landslide lakes there. The causes of these continually moving mantles of debris are a gypsiferous and bentonitic lower Mesozoic sedimentary section, rapid downcutting in a semi-arid environment, and flat-lying alternating massive sandstones and incompetent shales. As development continues, increasing numbers of people are placed in danger.