GEOLOGIC MAP OF THE LAKE TAHOE BASIN, CALIFORNIA AND NEVADA – MAP NO. 4 IN THE 1:100,000 SCALE REGIONAL GEOLOGIC MAP SERIES

One objective of the California Geological Survey (CGS) is to prepare and maintain a database of geologic mapping throughout the state. This is done primarily through the Regional Geologic Mapping Project in cooperation with the U.S. Geological Survey (USGS) through STATEMAP and SCAMP. The compilation of the geology of the Lake Tahoe Basin was supported in part by the USGS, U.S. Forest Service and prepared in cooperation with the Nevada Bureau of Mines and Geology.

In 1997 a Lake Tahoe multi-agency data clearinghouse was established by the USGS to coordinate activities within the Lake Tahoe Basin and create a natural resource GIS database. Noting inconsistencies with the existing digital geologic coverage, CGS proposed a project to prepare a new digital geologic map and a seamless database that could be used by planning and decision-making agencies in the basin. Existing published and unpublished geologic maps were digitized, compiled, and merged into a single digital database. Although presented at 1:100,000 scale, the spatial resolution of the data is good to 1:62,500 scale and in many areas 1:24,000 scale.

The geologic history of the area is complex. Paleozoic and Mesozoic rocks occur as isolated remnants intruded by the granitic rocks of the Sierra Nevada batholith. During the Tertiary volcanism was widespread, characterized by andesitic volcanic flows and debris derived from nearby volcanic centers with local volcanic activity continuing into the Pleistocene. Faulting and glaciation have also played a significant part in the development and shaping of the basin. One striking feature of this geologic map is the incorporation of USGS imagery of the lake bottom as well as the shaded-relief image on land. Young-looking fault features are visible in the lake bottom and the large embayment on the western shore is attributed to a landslide. Investigations continue to evaluate these hazards and their potential.

The Geologic Map of the Lake Tahoe Basin is the fourth map in the CGS's 1:100,000 scale Regional Geologic Map Series. Map images are accessible through the CGS Preliminary Geologic Map website: http://www. conservation.ca.gov/cgs/rghm/rgm/preliminary_geologic_maps.htm.