GEOLOGIC MAP OF CURLEW VALLEY DRAINAGE BASIN, NORTHERN UTAH AND SOUTHERN IDAHO, AND HYDROGEOLOGIC IMPLICATIONS

Ground-water issues in Curlew Valley, including decreased flow at Locomotive Springs at the north end of Great Salt Lake and declining levels and quality in wells, are the impetus for this project. The compiled geologic map is one component of a larger study designed to delineate aquifers and evaluate boundary conditions for a future ground-water flow model of the Curlew Valley aquifer system. The map is the surface-drainage boundary of this 3100-km2 Y-shaped basin in northern Utah and southern Idaho, and includes all or parts of thirty two 1:24,000-scale quadrangles with nine geologic map sources, including unpublished mapping by workers at the USGS and BYU-Idaho. New mapping of some surficial deposits and bedrock was required.

Curlew Valley is in the northeastern Basin and Range Province. Most ranges consist of limestone and sandstone of the Permian-Pennsylvanian Oquirrh Group; Precambrian metamorphic and igneous rocks and lower Paleozoic metamorphic rocks are exposed in the Raft River Range in the eastern part of the map area. Late Tertiary to early Quaternary sedimentary and volcanic deposits are exposed along range margins and form the main basin-fill deposits. Quaternary deposits of Lake Bonneville overlie the Tertiary basin fill. In Utah, basalt is interbedded with the basin fill. Major deformation events include the Late Jurassic-early Tertiary Sevier orogeny; early Tertiary extension on low-angle normal faults in the Raft River and Black Pine Mountains; and late Tertiary to Quaternary high-angle normal faulting and basaltic volcanism.

Quaternary-Tertiary basin fill and Quaternary basalt are the principal aquifers in the Curlew Valley ground-water system. Flow paths and distribution of confined and unconfined zones are likely complex due to facies variations within the basin fill and the interlayering of basin fill and basalt flows. Locomotive Springs issue from the southern margin of the youngest basalt shield, and are supplied by two regional flow systems that originate in the northern two arms of the "Y" and are separated by a partially concealed bedrock high. Delineation of the subsurface extent and thickness of the basalt and basin fill, and definition of the structure and hydrologic role of the bedrock high and associated faults are the major challenges of the project.