| Abstrakt: |
Ash Creek drainage basin, about 30 miles long and 15 miles wide, extends along Interstate Highway I-15 down a steep topographic slope from southern Iron County to just east of St. George, in southern Washington County. The basin is south of Cedar basin, which contains Cedar City, and between the Pine Valley Mountains on the west and the Colorado Plateau on the east. The small towns of Kanarraville and New Harmony are in the north, Pintura is near the center, and Anderson Junction and Toquerville are in the south. The basin axis parallels the great north northeast-striking Hurricane fault on the east, which marks the transition from the Basin and Range to the Colorado Plateau. The Pine Valley Mountains is a large basin range uplifted by many faults, including one newly found, so that Ash Creek basin is a complex graben between two highlands. The southern end of the basin is the Virgin River, into which Ash Creek flows. The present topography has been created mostly in the last 10 million years by basin-range tectonism. Basin-fill sediments and basalt flows were deposited in the basin and are in turn underlain by two thick older (faulted) aquifers, the Miocene Pine Valley laccolith/Pine Valley Latite and the Jurassic Navajo Sandstone. Water resources in Ash Creek basin would seem to be limited, considering what remains of the surface water and what has been previously estimated for the groundwater. Yet we interpret, based largely on geology and fracture-flow analysis, that previous estimates of groundwater volumes were too conservative. Significant groundwater crosses into the basin from Cedar basin, most of it from precipitation on the Hurricane Cliffs that moved by surface flow to the Hurricane fault zone at the base of the Cliffs, then by fracture flow southward within the fault zone. Groundwater also must flow in thick Paleozoic aquifers beneath the Kolob terrace, then west across the Hurricane fault into the Miocene and Jurassic aquifers that are juxtaposed by the fault. Yet the greatest recharge must come from the Pine Valley Mountains, where precipitation enters a highly faulted, excellent (brittle) Miocene igneous aquifer that forms much of the range, then down gradient to the basin. Finally, the graben of the Ash Creek basin is a complex hash of faults that brecciate the underlying thick rock aquifers and create large volumes of permeable rock for groundwater storage. Water-chemistry data indicate that the highest quality groundwater is on the western side of the Ash Creek basin, where recharge is the greatest; and that wells on faults there not only have the highest yields, but also higher quality water because fracture flow is rapid. Tritium values indicate that virtually all streams, springs, and shallow wells in the basin have modern water. In addition to existing groundwater resources, two previous studies recommended drilling and testing "new" (unappropriated) groundwater from Permian to Mississippian aquifers, most of them in the footwall of the Hurricane fault. As suggested by a previous worker, water discharged from two large springs on the eastern side of the basin is derived largely from three perennial tributaries that drain the Pine Valley Mountains, yet deeper fresh groundwater that was noticed in oil wells that tested the Hurricane fault is probably derived from surface water and groundwater that entered the fault in the northern part of the basin. We conclude that moderately deep fresh groundwater moves south by fracture flow in the Hurricane fault, perhaps all the way to the Grand Canyon. Furthermore, groundwater in the southern end of the basin is carried by faults and the Navajo Sandstone southward beneath the Virgin River, formerly recharging the Sand Hollow well field, then continues still farther south along faults and in aquifers that underlie the Navajo Sandstone. [ABSTRACT FROM AUTHOR] |
