HYDROGEOLOGY OF THE MARKAGUNT PLATEAU IN THE VICINITY OF MAMMOTH SPRING, DIXIE NATIONAL FOREST, SOUTHWESTERN UTAH

The Markagunt Plateau lies at an altitude of about 9,500 feet in southwestern Utah, within Dixie National Forest. The plateau is capped in large part by Quaternary-age basalts that overlie Eocene-age limestones and clastic rocks of the Claron Formation, which forms the western escarpment of the plateau at Cedar Breaks National Monument. Over a large part of the Markagunt Plateau, dissolution of limestone units within the Claron Formation and subsequent collapse of the overlying basalt have produced a terrain characterized by sinkholes as much as 1,000 feet across and 100 feet deep. Numerous large springs discharge from the Claron Formation on the plateau, including Mammoth Spring, one of the largest springs in Utah, with a discharge that can exceed 300 cubic feet per second. Although discharge from Mammoth Spring is from the Claron, recharge to the spring largely takes place by both focused and diffuse infiltration through the overlying basalt into the limestone. Results of dye tracing to Mammoth Spring indicate that recharge originates largely southwest of the spring outside of the Mammoth Creek watershed, as well as from losing reaches along Mammoth Creek upstream of the spring. Groundwater travel time to Mammoth Spring during the 2009 snowmelt runoff period was no more than 1 week from as far as 9 miles southwest of and 1,900 feet higher than the spring. Temperature and specific-conductance measurements of water from the spring during 2006 to 2009 ranged from 3.3 to 5.4°C and from about 110 to 200 microsiemens/cm (at 25°C), and show an inverse relation to discharge during snowmelt runoff and rainfall events, also indicating short groundwater residence times.

Previous investigations from 1954 to 1958 in the Navajo Lake watershed in the southern part of the Markagunt Plateau have also shown rapid groundwater travel times within the Claron Formation. Results of dye-tracer studies indicate that outflow from Navajo Lake loses into sinkholes and discharges to both Cascade and Duck Creek Springs, which subsequently flow into the Virgin and Sevier River drainage basins, respectively. Apportionment of water to Cascade and Duck Creek Springs was calculated to be about 40 and 60 percent. Groundwater travel times to the springs, 1.2 miles south of and 3.5 miles east of the sinkholes, were about 8.5 and 53 hours, respectively.