Geologic map of the Fish Creek Reservoir 7.5' Quadrangle, Blaine County, Idaho by Betty Skipp and Theodore R. Brandt 2012 Scientific Investigations Map 3191 ABSTRACT The Fish Creek Reservoir quadrangle in south-central Idaho lies on the north-central margin of the Snake River Plain at the southern end of the Pioneer Mountains. Rocks exposed in the quadrangle range in age from Paleozoic through Cenozoic. Mesozoic sedimentary rocks are absent, possibly removed by erosion associated with thrusting and subsequent erosion of the Late Cretaceous-Early Tertiary Sevier Orogeny, Mesozoic igneous rocks may be present at depth. Eocene Challis Volcanics of intermediate composition cover much of the area, and Tertiary rhyolitic ignimbrites and Cenozoic basalts are present in the southern half. The region is underlain by Archean and Proterozoic rocks exposed to the northwest in the Pioneer Mountains Core Complex (Dover, 1983; Link and others, 2007; Link and others, 2010). The oldest sedimentary rocks exposed in the quadrangle are two faulted lower Paleozoic sequences, one a western-derived clastic facies, and the other a shelf carbonate facies overlain unconformably by different late Paleozoic sequences. The basinal Permian-Pennsylvanian Wood River Formation overlies the western clastic facies; and the Mississippian Copper Basin Formation, western-derived flysch, overlies the carbonate shelf facies. The steep Pioneer Thrust of the Sevier Orogeny places the Wood River Formation, and the underlying faulted early Paleozoic argillites and quartzites, against the Copper Basin Formation and the underlying faulted carbonate shelf facies. The Early Paleozoic thrusted sequences are exposed in two windows in the southern and central parts of the quadrangle. The two windows formed above probable Tertiary granodiorite intrusives, one encountered in a shallow drill hole, the other inferred. The thrusting in the Early Paleozoic rocks is interpreted to have taken place before the Sevier Orogeny during the Late Devonian to Early Mississippian Antler Orogeny. Interpretive cross sections illustrate these relationships. Thick Mississippian flysch deposits of the western-derived Copper Basin Formation are undisputed evidence of a disturbance at the western margin of Paleozoic Laurentia during latest Devonian and earliest Mississippian time (Antler Orogeny) in this part of Idaho. Antler thrusts, however, have not been identified with certainty in Idaho, but these two structural windows provide additional evidence for this early compressional period. Another intriguing thrust segment recently identified in the Pioneer Mountains Core Complex north of this quadrangle places Archean rocks over Proterozoic rocks (Link and others, 2010). Faults of the core complex itself cut the overlying Sevier Pioneer thrust which has Devonian argillites at its base overlain unconformably by Carboniferous rocks. The question remains open as to the age of the Archean over Proterozoic thrust segment in the core complex, but the possibility remains that it is a basal thrust of the Antler Orogeny. Following the compression of the Sevier Orogeny, the Fish Creek area has been dominated by Cenozoic extension and volcanism. The downwarp of the Snake River Plain, the northeastward passage of the Yellowstone hot spot, and basin-range extension all have contributed to a complex of young normal faults that have created the valley of Fish Creek and capture of the West Fork. All the Miocene ignimbrites have their sources in the calderas of the Snake River Plain (Moye and others, 1988; Morgan and McIntosh, 2005; Kuntz and others, 2007). The region is still expanding within the seismic parabola of the Intermountain seismic belt (Scott and others, 1985; Pierce and Morgan, 1992; Janecke and others, 2007). Minor base metal concentrations have been identified in gossans associated with the Wood River Formation (Otto and Zieg, 2003); minor precious mineral concentrations have been reported in carbonate rocks of the lower Paleozoic Robert Mountains Formation, and Cu, Zn, and As anomalies have been reported in unidentified carbonate rocks. Hot springs found along the margins of the Plain indicate potential sources of geothermal energy. DISCLAIMERS This database, identified as SIM3191, has been approved for release and publication by the Director of the USGS. Although this database has been subjected to rigorous review and is substantially complete, the USGS reserves the right to revise the data pursuant to further analysis and review. Furthermore, it is released on condition that neither the USGS nor the United States Government may be held liable for any damages resulting from its authorized or unauthorized use. Although these data have been processed successfully on a computer system at the U.S. Geological Survey, no warranty, expressed or implied is made regarding the display or utility of the data on any other system, or for general or scientific purposes, nor shall the act of distribution constitute such warranty. The U.S.Geological Survey shall not be held liable for improper or incorrect use of the data described and/or contained herein. Any use of trade, product or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Geological Survey. Although this information product, for the most part, is in the public domain, it also contains copyrighted material as noted in the text. Permission to reproduce copyrighted items for other than personal use must be secured from the copyright owner. DATA FILES The database can be downloaded via the Web from http://pubs.usgs.gov/sim/3191/. ArcInfo export files, ArcView shapefiles, and an ArcGIS geodatabase may be extracted from a zip-compressed file (SIM3191_GIS.zip). In addition, an Adobe Portable Document Format (PDF) file (SIM3191_map.pdf) can be downloaded, from which paper copies may be printed. The database contact is: Theodore R. Brandt 303-236-1901 tbrandt@usgs.gov U.S. Geological Survey Box 25046, Mail Stop 980 Denver, CO 80225 The data were compiled from field mapping. Paul K. Link of Idaho State University and Mitchell W. Reynolds of the U. S. Geological Survey reviewed the geologic map. This geospatial database consists of ArcInfo export files (.e00), ArcView shapefiles, and ArcGIS geodatabase files contained in a zip archive See the list below. Please refer to the file SIM3191.met for detailed metadata documentation for this geospatial database. List of files: 00ReadMe.txt SIM3191_pamphlet.pdf SIM3191_map.pdf SIM3191.met fishbasr.tif fishbasr.tfw SIM3191_GIS.zip: e00 directory: fishgeo.e00 fishpnt.e00 fishxc.e00 fishfold.e00 fishhlsd.e00 wpgcmykg.shd.e00 geol_sfo.lin.e00 geoscamp2.mrk.e00 import.aml shapefiles directory: fishgeo.shp (contains polygon attributes for fishgeo database) fishgeo.shx fishgeo.dbf fishgeo.prj fishgeol.shp (contains line attributes for fishgeol database) fishgeol.shx fishgeol.dbf fishgeol.prj fishpnt.shp (contains point attributes for fishpnt database) fishpnt.shx fishpnt.dbf fishpnt.prj fishxc.shp (contains line attributes for fishxc database) fishxc.shx fishxc.dbf fishxc.prj fishfold.shp (contains line attributes for fishfold database) fishfold.shx fishfold.dbf fishfold.prj SIM3191.gdb directory (contains GIS data as geodatabase feature classes) HOW TO OBTAIN THE DIGITAL FILES The digital files constituting the geologic map database of this report can be obtained via the Internet from the U.S. Geological Survey publications website. Go to the web page at http://pubs.usgs.gov/sim/3191/ and follow the directions to download the files. The main product is a Portable Document Format (.pdf) map, which requires Adobe Acrobat for viewing. Acrobat software runs on a variety of systems, and is available for download free of charge from Adobe at http://www.adobe.com.