Geology of Reno and Truckee Meadows, Nevada, United States of America

Abstract

Reno, situated within the Truckee Meadows, just a few miles east of the California state line, is located between two markedly different physiographic provinces. One province is the Sierra Nevada (to the west), a region characterized with high mountains and heavy winter snow. To the east is the semi-arid Basin and Range province. Consequently, Reno, and the surrounding area, contains a diverse geology and a highly varied and interesting suite of geotechnical challenges, which greatly influence land-use considerations in this rapidly growing urban area. Reno owes its origins to the immigrant trails from the Midwest to California in the 18409s and to the Comstock Lode silver discovery in the nearby Virginia Mountains, in the 18509s. Officially, it9s existence began in 1868, with the arrival of the Central Pacific Railroad and the need for a station to join with the future Virginia and Truckee Railroad to service the Comstock Lode. Truckee Meadows is a fault-bounded basin surrounded by mountain ranges composed mainly of Tertiary volcanic rocks and inliers of pre-Tertiary volcanic and granitic rocks. The Virginia Range, to the east, is composed entirely of hydrothermally altered Oligocene and Miocene volcanic rocks forming the famous Comstock silver mining district. On the west, the Carson Range segment of the Sierra Nevada is composed largely of Tertiary volcanic rocks, overlying older granites, granodiorites, and metamorphic rocks. A thick sequence (in excess of 330 m [1,000 ft]), of glacial-fluvio, alluvial fan, and lacustrine deposits of late Tertiary to Quaternary age underlies the Truckee basin. At the southern end of the Truckee Meadows is located Steamboat Hot Springs, which has been utilized for commercial and recreational purposes since the 18609s. Presently three small (12.5, 7 and 5.5 MW) electric power plants make use of this geothermal resource. Seismologically active, Reno lies along the northern segment of the Sierra Nevada fault zone, a series of north-south trending faults, separating the Sierra Nevada from the Great Basin. Fault movement is predominantly dip-slip, based on surface displacements, although focal plane solutions suggest a component of strike-slip motion. No large magnitude earthquakes (MS>7) have been noted in the Reno area during historic time. However, several moderate-sized earthquakes with surface rupture events were associated with the 1869, 1950, and 1966 events. Long-term recurrence intervals for large-magnitude earthquakes on this segment of the Sierra Nevada fault zone are believed to be of the order of several thousand years. Several rock and soil formations pose potential geotechnical problems or hazards. Foundation problems related to expansive soils are mainly due to the hydrothermal alteration of andesites of the Alta Formation and surface weathering of other volcanic rocks. Floodplain and lacustrine deposits in the Truckee basin are composed of clayey silts, silty sands and large areas of fine sands which are susceptible to liquefaction where the ground-water surface is close to the topographic surface. Two major slope stability hazards in the hills surrounding the basin are recognized: rockfall from the well-jointed and slabby volcanic units and debris flows in the weathered rock, lahars, and surficial deposits which mantle the hill slopes. Major slope instability is restricted to steeper hill slopes and the Truckee River canyon. Snow avalanche hazards exist in the winter on the highways leading from Reno to Lake Tahoe and into California. Flash floods and floodplain hazards are historically a common occurrence. These events are due to severe summer thunderstorms, heavy spring rains, or rapidly melting snow packs. Even though a 1963 channelization of the Truckee River has reduced the hazard, over $16 million dollars of flood-related damage occurred as recently as 1986. Numerous small and large-scale metal mines exist in the hills surrounding Reno, producing cinnabar, lead, silver and gold. The mines are now inactive but their presence produces mine-related hazards of ground subsidence or the dramatic and rapid collapse of their shafts, especially where workings underlie buildings and other structures. In common with many other cities in the west, a dependable and adequate water supply is at a premium. Reno9s water reservoir is the winter snow pack of the Sierra Nevada, which, in a good year, can exceed 1,000 cm (± 480 in.). The Truckee River is fed from snowmelt and Reno derives its water either directly from the river or from wells in the alluvium which borders the river. Overall water quality is good, but localized geothermal discharges can adversely affect ground-water quality. Solid waste, in excess of 220 tonnes (1,200 tons)/day, is placed in two sanitary landfills east of the city. Liquid aqueous waste requires special handling and after waste-water treatment, its sludge is taken to one of the landfills.