CURRENT MAPPING IN THE TWIN FALLS REGION OF THE IDAHO GEOLOGICAL SURVEY’S STATEMAP PROGRAM

The Idaho Geological Survey has been a partner in the USGS STATEMAP program since its inception in 1993. The funded projects vary in scale, scope, and geologic province, but the following themes underpin the Survey’s geologic mapping program: (1) advancement of geologic understanding, (2) cutting-edge digital cartography and databases, (3) new mapping of 1:24,000-scale bedrock and surficial geologic maps, and (3) new mapping of 1:100,000-scale geologic maps to meet the long-term goal of digital geologic coverage of the entire state. A geologic map of the Twin Falls 30' x 60' quadrangle is one of several high-priority projects, and will be completed in 2005. More than half of the thirty-two 7.5' quadrangles will be published in preparation for compiling the entire 30' x 60' quadrangle. All of the geologic information is drawn and digitized at 1:24,000 scale and the various sources coded for the Twin Falls database. Examples of completed geologic maps of 7.5' quadrangles show the scope and character of map units, and the quality of the all-digital CAD layout and print-on-demand paper maps.

The project area encompasses the juncture of the eastern and western Snake River Plain, a prime agricultural area with a growing population, and outlets of the Snake River Plain aquifer into the canyon of the Snake River. Most of the area has not been mapped in detail since the 1970s. The new geologic mapping is helping build a better understanding the structural and geomorphic evolution of this geologically important region.

In addition to intensive field work and an evaluation and compilation of previous mapping, we are applying the results of chemical and paleomagnetic analyses and radiometric dating to several mapping goals: separating Miocene felsic units and understanding their relationship to Snake River Plain calderas; refining Pliocene basaltic units and associated basin-fill sediments; detailed mapping of Pleistocene basalt flows and tracing to source volcanoes; mapping of loess, dune sand, and soils; and mapping details of the features and deposits of the Bonneville Flood.