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.