The Jefferson Mountain area, located at the eastern end of
the Centennial Mountains in Fremont County, Idaho and Beaverhead
County, Montana, is approximately 42 square miles in size. The
area contains exposed metamorphic, sedimentary, and volcanic
rocks ranging in age from Precambrian to Tertiary. Approximately
2,500 feet of Paleozoic and Mesozoic sedimentary rocks lie between
a basal complex of Precambrian metamorphic rocks and overlying
Tertiary volcanics and Quaternary alluvium.
The Precambrian rocks in the area are regionally metamorphosed
sedimentary rocks of the Cherry Creek Group that
include hornblende schists, granite gneisses, quartzites, and
Eleven Paleozoic formations occur in the area, representing
all of the Paleozoic periods except Ordovician and Silurian. The
dominant rock types representing the Paleozoic are dolomites and
limestones, but sandstone and shales make up a significant part of
the section. The Triassic Dinwoody Formation, a silty dolomite, is
the only representative of Mesozoic deposition in the thesis area.
The volcanics in the area are part of the late Tertiary Yellowstone
Tuff and Snake River volcanics of eastern Idaho and northwestern
Pliocene and Early Pleistocene downwarping of the Snake
River Plain resulted in block-faulting along its northeastern margin,
forming the Centennial Range, a large south-tilting cuesta.
The sedimentary rocks were deposited by a series of transgressions
and regressions of marine water during the evolution of
the Cordilleran Geosyncline. The sedimentary rocks represent the
shelf facies along the eastern margin of the miogeosyncline.
Oil and gas possibilities in the area are negligible. Structural
and stratigraphic relationships do not indicate any type of control
for the entrapment of petroleum. The economic potentiality for
phosphate mining in the area is low because of the thinness of the
phosphate-bearing member of the Phosphoria Formation.