Abstract

Salt Lake City is the capital of Utah and is a major financial, trade, and transportation center for the western United States. Founded in 1847, Salt Lake City presently (1990) has a population of about 158,000. However, the metropolitan area of the city, which includes most of the Salt Lake Valley, contains both incorporated and unincorporated suburbs that increase the population to nearly 705,000.

Geologic exposures in the Salt Lake City region record a long history of sedimentation and tectonic activity extending back to the Precambrian Era. Today, the city lies above a deep, sediment-filled basin flanked by two uplifted range blocks, the Wasatch Range and the Oquirrh Mountains. The Wasatch Range is the easternmost expression of major Basin and Range extension in north-central Utah, and is bounded on the west by the Wasatch fault zone (WFZ), a major zone of active normal faulting. During the late Pleistocene Epoch, the Salt Lake City region was dominated by a succession of inter-basin lakes. Lake Bonneville was the last and probably the largest of these lakes. By 11,000 years BP, Lake Bonneville had receded to approximately the size of present-day Great Salt Lake. Lake Bonneville sediments bury most older deposits in the valley below an elevation of about 5,200 ft (1,585 m). Lake sediments include near-shore beach, delta, spit, and bar deposits and silt and clay deposited in deeper water. Post-Bonneville deposits include Holocene alluvium along the Jordan River and its tributaries and alluvial fans along mountain fronts.

Repeated normal-slip faulting has occurred at the ground surface in northern Utah during late Pleistocene and Holocene time. Most of this activity has been on the WFZ which traverses the Salt Lake City metropolitan area. West-facing scarps of a few to tens of feet high are common, as are graben, horsts, and other fault-related features. Paleoseismic data show that the average recurrence interval for surface-fault displacement on the Salt Lake City segment of the WFZ is 4,000 ±1,000 years. However, the city can expect to experience strong ground shaking associated with a large earthquake somewhere on the WFZ every 340 to 415 years. The West Valley fault zone (WVFZ) is an east-dipping, normal fault that trends to the north-northwest through the central part of the Salt Lake Valley. The WVFZ has had at least six surface-faulting events in the past 13,000 years. Despite the close proximity to active faults, Salt Lake City has not yet been subjected to a large, destructive earthquake. Felt events have occurred, but only a few have caused appreciable damage.

Geologic units in the Salt Lake City metropolitan area generally provide adequate foundation conditions. The principal foundation problems are compressible, low bearing-strength soils; collapse-prone soils; and liquefaction. Some shale units and the soils derived from them may be expansive. Use of underground space is restricted to the nearby Wasatch Range and Oquirrh Mountains and includes storage of documents and valuables, water storage, and mining-related uses.

Numerous geologic hazards exist in the Salt Lake City metropolitan area. Movement on faults may cause ground rupture, ground shaking, tectonic displacement, ground failure including liquefaction, and seiches on the Great Salt Lake. Steep slopes create the potential for landslides, rock falls, debris flows, and snow avalanches. Streams and the Great Salt Lake experience flooding, and high ground-water conditions are common.

Water for the Salt Lake City metropolitan area comes principally from streams in the Jordan and Colorado River drainages. Ground water from wells and springs is also used. Major surface storage reservoirs are considered inadequate and additional storage is being constructed. Basin-fill aquifers provide the largest existing source of stored water.

Most of the Salt Lake City metropolitan area is served by public sewers. Wastewater is treated at municipal treatment plants and solid waste is placed in county and municipal sanitary landfills. Hazardous waste sites include disposal sites for cement kiln dust; mine, smelter, and oil refinery wastes; and various chemical wastes. Three hazardous waste disposal sites are on the Environmental Protection Agency's current Superfund National Priority List.

Mineral resources have played an important role in the development of Salt Lake City. A variety of salines and metals are recovered from Great Salt Lake brines. The Bingham mining district in the Oquirrh Mountains is one of the world's largest copper producers. The Big and Little Cottonwood mining districts and the Hot Springs mining district, all in the Wasatch Range, are no longer active, but have produced a variety of precious and base metals. Industrial rocks and minerals include cement, construction aggregate, crushed stone, industrial sand, and clay. The oil and gas potential of the Salt Lake Valley has not been thoroughly explored, but probably is low. Geothermal water is used to heat greenhouses and part of the Utah State Prison.

The Great Salt Lake presents Salt Lake City with unique geologic hazards and engineering geology problems. Fluctuations in lake level occur daily, seasonally, and on a long-term basis. The rise of the lake during the period 1983–1985, due to above-normal precipitation, caused over $240 million in damages and initiated construction of the West Desert Pumping Project. In addition to flooding, development near the lake must also consider the effects of earthquakes on the lake and sensitive lake-bottom sediments, low bearing-strength soils, and rafting-ice impact on in-lake structures.

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