GEOLOGICAL ROAD GUIDE to QUAIL CREEK STATE PARK, WASHINGTON COUNTY, UTAH Robert F

Geologic Road, Trail, and Lake Guides to Utah's Parks and Monuments 2000 Utah Geological Association Publication 29 P.B. Anderson and D.A. Sprinkel, editors GEOLOGICAL ROAD GUIDE TO QUAIL CREEK STATE PARK, WASHINGTON COUNTY, UTAH Robert F. Biek Utah Geological Survey Salt Lake City, Utah INTRODUCTION Quail Creek State Park lies astride one of the most remarkable geologic features in southwestern Utah. The park is cradled in the eroded core of the Virgin anticline, a generally symmetrical fold formed during the middle Cretaceous to early Tertiary Sevier orogeny. The flanks of the fold provide spectacular exposures of parts of the late Early Triassic Moenkopi Formation and overlying Late Triassic Chinle Formation, and Quaternary boulder deposits at the northwest end of the reservoir provide a glimpse into the rock types of the distant Pine Valley Mountains. Quail Creek State Park is also the site of a major engineering catastrophe that occurred on January 1, 1989, when the Quail Creek south dike suddenly failed. Quail Creek State Park is a mecca for fishing, boating, and swimming, but despite its truly classic geologic setting, there are currently no developed trails in the park. Still, the views from the park road alone provide a remarkable glimpse into the heart of the Virgin anticline and the geology of southwestern Utah (figure 1) (Biek, 1999). A geologic map and stratigraphic column for the park are shown in figures 2 and 3. MILEAGE DESCRIPTION INTERVAL/CUMULATIVE 0 0 BEGIN road log at the junction of Utah 9 and 318 near the axis of the Virgin anticline. PROCEED NORTH ON UTAH 318 to Quail Creek State Park. The road to the park heads north over the Shnabkaib Member of the Moenkopi Formation. Note microbiotic soils developed on weathered gypsiferous Shnabkaib strata. 1.0 1.0 TURN RIGHT into parking area at the Quail Creek south dike. STOP 1. GENERAL GEOLOGY OF QUAIL CREEK STATE PARK AND CATASTROPHIC FAILURE OF THE QUAIL CREEK SOUTH DIKE. 1 R. F. Biek Geological Road Guide To Quail Creek State Park, Washington County, Utah Figure 1. Topographic map showing Quail Creek State Park and the surrounding area, with route for this road log. 2 P.B. Anderson and D.A. Sprinkel, editors 2000 Utah Geological Association Publication 29 ated ated photograph shows the view looking northeast toward Quail Creek Reservoir. Creek Quail toward northeast looking the view shows photograph Figure 2. Geologic map and cross section of the Quail Creek State Park area in Washington County, Utah. The accompanying annot accompanying The Utah. County, Washington in area State Park section of the Quail Creek cross and map 2. Geologic Figure 3 R. F. Biek Geological Road Guide To Quail Creek State Park, Washington County, Utah Figure 3. Rocks present in Quail Creek State Park and surrounding area. Only those intervals highlighted in color are present in the park; enclosing rock units are found in nearby areas. 4 P.B. Anderson and D.A. Sprinkel, editors 2000 Utah Geological Association Publication 29 This parking area straddles the axis of the Virgin anticline, a 30-mile (48-km) long, northeast-trending, generally symmetrical fold formed during the Sevier orogeny. In fact, Quail Creek State Park owes its existence to the Virgin anticline, which, with two small dams, forms a natural catchment for offline storage of Virgin River water (figures 4 and 5). The anticline is made all the more visible by the resistant Shinarump Conglomerate Member of the Chinle Formation, which forms a prominent carapace along the central portion of the fold (figure 6). The Virgin anticline is truly a classic geologic feature, and because it is so clearly expressed, and only sparsely covered by vegetation and Quaternary deposits, it is visited by geology field camp teachers and students from around the country. Figure 4. Aerial view to the north of the Virgin anticline and Quail Creek Reservoir. Utah Highway 9 cuts across the Virgin anticline at the bottom of the photograph. The snow-covered Pine Valley Mountains are to the west and the Kolob Canyons section of Zion National Park is to the east. Photo by Janice Higgins, Utah Geological Survey. The Virgin anticline has three similar structural domes along its length. From south to north these are Bloomington dome, Washington Dome, and Harrisburg Dome, each of which is cored by the Harrisburg Member of the Kaibab Formation (late Early Permian) (figure 7). A number of west-dipping thrust faults repeat Triassic and Jurassic strata on the northwest flank of the anticline, near Leeds (Proctor, 1953; Proctor and Brimhall, 1986; Biek, 1997, 1998). The northeast-plunging nose of the Virgin anticline is complicated by numerous normal faults and subsidiary folds, which likely formed during the Sevier orogeny (Biek, 1998). One such down-to-the-east normal fault that displaces Shinarump strata is visible at the crest of the anticline at the north end of the reservoir (figure 6). An example of thrust fault deformation is described below at mile 1.5. At 12:30 a.m. on January 1, 1989, the Quail Creek south dike failed catastrophically, unleashing a torrent of water and causing millions of dollars of damage. Fortunately, the downstream area was evacuated in time to avoid fatalities. Approximately 25,000 acre-feet (30,492,000 m3) of water - more than half the reservoir's capacity - flowed through a breach in the dike over a 12-hour period (figure 8). 5 R. F. Biek Geological Geological Biek R. F. 6 Road GuideQuail To Creek State Park, WashingtonCounty, Utah Figure 5. View northeast to Quail Creek State Park, with Utah Highway 9 in the foreground. Note the Abacon-striped@ appearance of the Shnabkaib Member; the yellowish-brown sandstone, locally known as the APurgatory sandstone,@ near the base of the upper red member; and the cliff-forming Shinarump Conglomerate. Note also the small normal fault and deeply dissected landslide at the north end of the reservoir. P.B. Anderson and D.A. Sprinkel, editors 2000 Utah Geological Association Publication 29 Shinarump Conglomerate landslide upper red member Shnabkaib Member Figure 6. View northeast from the top of the Shinarump cliff, immediately west of the Quail Creek south dam parking area. Note how the Shinarump Conglomerate forms a resistant cap over the more easily eroded Moenkopi Formation. Note also the old, deeply eroded landslide just north of Quail Creek Reservoir. The base of the landslide is in Shnabkaib strata, which are comparatively weak and unstable. Figure 7. View to the southwest along the axis of the Virgin anticline. Harrisburg Dome and Washington Dome form the low hills in the middle distance; the east limb of the anticline is also clearly visible. 7 R. F. Biek Geological Road Guide To Quail Creek State Park, Washington County, Utah Figure 8. Quail Creek south dike, shortly after its catastrophic failure on January 1, 1989. Note how flood waters scoured the bedrock clean of loose, overlying sediments just below the dike. Photo courtesy of Ben Everitt, Utah Division of Water Resources. The original Quail Creek south dike was a 78-foot (24-m) high, 2,000-foot (610- m) long earthen dam constructed in 1984. The dam was poorly designed and seepage under the dam occurred immediately after filling the reservoir. Seepage and sinkhole development increased in succeeding years despite efforts to seal the leaks (O'Neill and Gourley, 1991; Gourley, 1992). Seepage water probably passed under the dike, or at the dike-foundation contact, and began to erode the dike materials and in-situ soils. Ultimately, seepage and erosion of the dike and foundation materials continued and accelerated until caving occurred on a developing opening in the dike. Frantic last-minute efforts to stem the seepage were unsuccessful and the dike finally breached. Evidence of the flood caused by the catastrophic failure of the Quail Creek south dike is still visible downstream from the new dam, where the Shanbkaib Member was scoured clean of overlying loose bedrock and sediment. The dike's failure was principally due to poor foundation design and construction coupled with limited oversight by engineering geologists (O'Neill and Gourley, 1991; Gourley, 1992). Factors that contributed to the failure of the dike included the assumption of a low-permeability foundation, inadequate preparation of the foundation, and placement of unprotected, erodible embankment material on the dam's foundation. The dike was built mostly on the Shnabkaib Member of the Moenkopi Formation, except for the southeast abutment which lies on the "Purgatory Sandstone" of the upper red member. Both of these units contain abundant joints, and the Shnabkaib Member contains abundant gypsum, which readily dissolves in water. Joints allowed water to infiltrate rapidly, creating dissolution channels up to 70 feet (21 m) deep below the dike. The new dike, called the Quail Creek south dam, was completed in 1990 as a roller-compacted concrete gravity dam (Payton, 1992). The dimensions of the dam are basically the same as the old dike, except that it now includes a new impermeable cutoff trench up to 75 feet (23 m) deep, which is designed to prevent water from seeping under the dam (figure 9). 8 P.B. Anderson and D.A. Sprinkel, editors 2000 Utah Geological Association Publication 29 Figure 9. Cutoff trench being excavated at the bottom of the new Quail Creek south dam in January 1990. Photo by Bill Lund, Utah Geological Survey. After stop, CONTINUE NORTH ALONG UTAH 318. As you head north from the parking lot, note the foot trails in delicate microbiotic soils north of the overlook. 0.5 1.5 Small turn out along east side of road. West of the road, two small, west-dipping thrust faults associated with formation of the Virgin anticline displace the Shnabkaib Member, and, farther north, the upper red member and the Shinarump Conglomerate (figure 10).

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