GEOLOGIC MAP OF THE KOOSKIA 30' X 60' QUADRANGLE: A PROJECT 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 (4) new mapping and compilation of 1:100,000-scale quadrangles to meet the long-term goal of digital geologic coverage of the entire state. The Kooskia quadrangle is the most recent example of our 1:100,000-scale mapping and compilation efforts.

The new map of the Kooskia quadrangle includes information from previous maps by the Idaho Geological Survey, the U.S. Geological Survey, graduate students, and other previously unpublished sources. The compilation was supplemented with extensive field investigations by five geologists in the summer of 2002. As with the other 30’ x 60’ quadrangles, the geologic information is initially compiled and digitized at 1:24,000-scale and coded as to its various sources when digitized.

A new series of rocks, tentatively termed the Golden metamorphic sequence, was identified in the southern part of the Kooskia quadrangle in 2002. Although previously assigned to the Syringa sequence, it differs from the Syringa in having thicker and more feldspathic quartzite intervals, minimal amounts of schist, and more biotite-feldspar-quartz gneiss. Field checks of previous mapping and additional mapping of Columbia River basalt units were used to modify and refine units and contacts. As a result, some Wanapum and Saddle Mountains units were found in areas where they were previously unrecognized. Separating the underlying Grande Ronde magnetostratigraphic units proved especially difficult because of conflicting and inconsistent magnetometer readings throughout much of the section. The Kooskia map is the first compilation of Quaternary geology mapped previously in the Selway-Bitteroot Wilderness under the Wilderness Program and includes new glacial geology in the Crags portion of this area.