Karst Aquifers: Basin and Range and Bear River Range Carbonate Aquifers Active

In the Basin and Range, bedrock is present in the uplifted blocks of the mountain ranges and beneath the basin fill in the valleys. Some of the bedrock consists of consolidated carbonate rocks that are relatively impermeable unless extensively fractured. Fracturing in carbonate rocks (limestone and dolomite) may enable groundwater to circulate through the fractures where dissolution can enlarge and increase the size and number of pathways for water movement through the rock. Such dissolution eventually can change a relatively impermeable carbonate rock into a permeable water-yielding unit.

Carbonate rocks predominate in a 20,000- to 30,000-foot thick sequence of Paleozoic and Lower Mesozoic rocks in an extensive area of western Utah and southern and eastern Nevada. The location of solution-altered zones of enhanced permeability within these carbonate rocks is poorly known. However, some data indicate that groundwater might flow between basins through permeable carbonate rocks in the mountains of west-central Utah, and water might flow from recharge areas in the mountains to local basins through permeable carbonate rocks bordering the northeastern part of the aquifer system. Small ephemeral streams and water flowing through fractured bedrock generally recharge the aquifers near the mountain fronts and together constitute mountain-front recharge.

Interbasin flow is a significant component of recharge or discharge in areas of solution-altered carbonate rocks, primarily in western Utah, where groundwater flows through deep, enlarged bedrock fractures from basin to basin or under several basins and discharges at distant points. Few data exist to document the location or magnitude of interbasin flow. However, several springs in western Utah likely yield water from carbonate rocks and have a combined discharge of about 45,000 acre-feet per year.

The alpine Bear River Range consists in large part of a thick sequence of carbonate rocks (more than 3,000 feet of limestone and dolomite) that range in age from Cambrian to Mississippian (Dover, 1987). The sequence is composed of eight principal geologic units, all of which are capable of transmitting water along dissolution-enhanced fractures, faults, and bedding planes. The formations make up the upper part of the Logan Peak syncline, a large regional structure that influences the movement of groundwater in much of the region.

In the Bear River Range, karst features include large springs that discharge along major rivers, losing streams in tributary drainages, caves and pits, blind valleys, sinkholes, dolomite pavement, and surficial karst (karren). Glaciation occurred above 8,000 ft during the Pleistocene, resulting in destruction of karst landforms that developed during interglacial periods. Speleothem age-dating, fluvioglacial deposits in caves, and deranged topography indicate that existing karst features, particularly caves, are largely remnants of former karst landscapes. Karst systems in alpine terrains are substantially different from those in relatively flat-lying strata in more temperate regions. Characteristics of alpine karst systems include a large component of vertical solution development and a thick unsaturated (vadose) zone, steep hydraulic gradients, spring discharge that responds primarily to snowmelt runoff, pit development in high-altitude meadows, and cold-temperature dissolution of carbonate rocks.