The Rock Creek area consists of 53 square miles located in
the eastern foothills of the Pioneer Mountains, Beaverhead County,
Montana. Approximately 8, 500 feet of late Paleozoic and early
Mesozoic contact-metamorphosed sedimentary rocks and late Mesozoic
sedimentary rocks are exposed in the area of study.
Most of the late Paleozoic and Mesozoic rocks were deposited
along the eastern edge of the Cordilleran miogeosyncline. The oldest
exposed rocks in the map area belong to the Amsden Formation
of Late Mississippian to Early Pennsylvanian age. Other late Paleozoic
rock units include the Quadrant Formation of Pennsylvanian
age and the Phosphoria Formation of Permian age. Mesozoic formations
include the Dinwoody Formation of Triassic age and the
Kootenai Formation and Colorado Group of Cretaceous age.
The Tertiary is represented by basin deposits and basaltic
andesite extrusives. Unconsolidated Quaternary glacial and fluvial
deposits complete the stratigraphic succession.
Granodiorite intrusive rocks occupy the southwestern corner
of the area mapped and compose part of the eastern margin of the
Mount Torrey Batholith, which is probably genetically related to the
Boulder Batholith. Along the margin of the intrusive, contact metamorphism
has destroyed the original sedimentary characteristics of
the late Paleozoic and early Mesozoic Formations.
During the Late Cretaceous Laramide orogeny the sedimentary
rocks were tilted and faulted. The structures in the area are related
to the main episode of Laramide deformation, emplacement of the
Mount Torrey Batholith, and later phases of structural adjustment.
The main structures are the Lost Creek tear fault and a northwest-trending
high-angle reverse fault. Minor folds are present in the
eastern half of the mapped area.