The Need for Geologic Maps in Nevada--Protecting the Environment and Efficiently Using and Conserving Resources

Spring 1990

Geologic maps are essential for Nevada. They provide basic information needed for protecting our environment and efficiently using and conserving our resources. In Nevada, geologic maps are needed to

In addition, geologic maps are the principal sources of geologic information for nearly all basic and applied earth-science research and decision making. Major environmental, scientific, and political issues, such as global climate change, effects of acid rain, unequal distribution of critical resources, and extinctions of species, require the fundamental data from geologic maps.

A National Geologic Mapping Initiative is being prepared jointly by the Association of American State Geologists and the U. S. Geological Survey. The initiative calls for joint federal and state funding of a mapping program that will cover the entire country. In Nevada, our most detailed statewide mapping at this time is at a scale of 1:250,000 (1 inch equals approximately 4 miles). Although this scale is useful for certain applications, it is insufficient for most problems facing us today.

We are proposing a program of systematic mapping at scale of 1:24,000 (1 inch equals 2,000 feet), the most detailed scale at which USGS topographic maps are available for the entire state. Surficial (Quaternary) units as well as bedrock will be mapped. New technologies using digital cartography and remote sensing for lithologic and structural mapping will be integrated with conventional field mapping. Both detailed and reconnaissance mapping will be done at a scale of 1:24,000. These maps will be made available generally as black-and-white hard copies and as digital products. Compilation into full-color geologic maps for publication will be at a scale of 1:100,000.

The benefits of geologic mapping greatly outweigh the costs. Nevada is covered by 62 1:100,000- scale, 30- by 60-minute quadrangles and 1,980 1:24,000-scale, 7.5-minute quadrangles. We presently have acceptable geologic mapping on less than 10% of the 7.5-minute quadrangles. All 1: 100,000- scale quadrangles need to be mapped. We estimate that it will cost, on the average, approximately $500,000 to map each 30- by 60-minute quadrangle. The total program in Nevada will cost approximately $31 million. A few Nevada-specific examples illustrate the benefits.

The U.S. Department of Housing and Urban Development has underwritten construction of low-income housing in the city of North Las Vegas. Damage totaling approximately $12 million has occurred in one subdivision because of land subsidence. Although the land subsidence is caused by groundwater withdrawal, the most striking movement is focused along Quaternary faults. With better geologic mapping and better zoning or building regulations, this type of damage could be avoided. We estimate that better geologic mapping to identify the Quaternary faults that may be problems in the expanding metropolitan area of Las Vegas Valley could save several hundreds of millions of dollars in property damage.

Much of the mineral wealth of Nevada is undiscovered, buried beneath sediments in valleys or beneath young volcanic rocks. Geologic mapping is necessary to determine thicknesses of these overlying rocks and structural controls that may be visible through the overlying rocks or that may be extended into the valleys from outcrop areas in the mountains. On the basis of past production, identified resources, and distribution of young sedimentary and volcanic rocks, we estimate that the value of undiscovered mineral resources in Nevada is between $120 billion and $1.2 trillion. Geologic mapping is needed to help find these resources and to extract them in an environmentally safe manner.

Disposal of hazardous chemical, biological, and nuclear waste requires knowledge of geologic containment. Geologic mapping is an integral part of understanding the processes that could allow the wastes to migrate beyond the disposal sites. Several billion dollars will be spent before decisions are made as to where these wastes should be stored. Detailed geologic maps should be used in screening sites and in deciding how to restore contaminated sites.

Nevada experiences, on the average, a magnitude 7 or greater earthquake once every 27 years. Judging from the October 17, 1989 Loma Prieta earthquake near San Francisco (magnitude 7.1), damage to the metropolitan areas of Reno, Carson City, or Las Vegas could well be in excess of $10 billion per event, and loss of life could be substantial. Detailed geologic mapping, especially of sediments and soils younger than 10,000 years, helps to define the areas of high seismic risk.

In summary, geologic mapping is vital to Nevada, it is cost effective, and a program is being planned to meet Nevada's needs.

---Jonathan G. Price, Director/State Geologist