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Idaho Basement Rocks

Compiled by Laura DeGrey and Paul Link, Idaho State University

Geology of Core Complexes

Geology of the Teton Range

Geology of the Albion Range Metamorphic Core Complex

Geology of the Pioneer Range Metamorphic Core Complex

logo PDF Slideshows: Albions, Tetons, and Pioneer Mountains
plane Flythroughs: Clearwater River, Salmon River, Middle Fork Salmon River, Big Lost River, Raft River Valley, Big Wood River

Vocabulary Words to Know

- normal fault

- fold and thrust belt

- detachment fault

- hanging wall

- plutonic rocks

- stock

- metamorphic core complex

 

The Teton, Albion, and Pioneer Ranges contain exposures of Archean and Paleproterozoic metamorphic and plutonic rocks. The Albion and Pioneer Ranges contain metamorphic core complexes, which expose middle-crustal metamorphic rocks in the center, beneath younger sedimentary rocks along a low-angle normal fault. The Archean and Proterozoic metamorphic rocks contain gneiss and schist that originated as part of the Archean continental crust as well as Paleoproterozoic juvenile volcanic arc rocks. They have been subsequently metamorphosed and intruded, notably in Cretaceous, Eocene and Oligocene time. They are exposed today from Neogene regional extension of the Basin and Range Province.

Geology of Core Complexes

The general core complex model includes older, middle-crustal crystalline basement rock within the "core" of the complex. The core complex is surrounded by younger Paleozoic sediments that once overlayed the basement rock and have been moved from the top by extensional faulting. The core complex formed as the continental crust was overthickened by compressional forces of the fold and thrust belt activity or intrusive igneous processes, and has subsequently had a chance to "relax" following the cessation of compression.

 

This relaxation of the crust caused extensional forces which produce low-angle detachment faults and normal faults that uncovered the middle-crustal crystalline basement rocks. Extension of this nature occurred during the development of the Basin and Range Province. (see Module 9)

Drawing showing the anatomy of a typical core complex. Figure from a sketch by Paul K. Link.


Geology of the Teton Range

The Teton Range is located on the east side of the Idaho-Wyoming fold and thrust belt, and is located north of Jackson, Wyoming, just east of the Idaho-Wyoming border. Location map to the right.

The Teton Range is made up of Archean crystalline basement and overlying Paleozoic sedimentary rocks. The Teton Range has been uplifted by younger Basin and Range faulting. Archean and Paleozoic rocks of the range crest have been offset ~5 km upwards from their counterparts in Jackson Hole, which are on the hanging wall of the Teton normal fault. This fault forms the eastern boundary of the Teton Range and the western hanging wall boundary of the Miocene to Holocene Jackson Hole sedimentary basin.

Recent movement along the Teton fault is related to active Basin and Range extension and faulting related to migration of Snake River Plain volcanism. The rate of movement along the Teton fault has been a rapid ~1.7 ft/1000 yr for at least ~10 Ma.

Map of the Idaho-Wyoming Fold and Thrust Belt above. A simplified geologic map of the same region to the right.

An annotated PDF of the Teton Range, with photographs taken by Paul K. Link.

Photo of Grand Teton. Click image for a larger image.


Geology of the Albion Range Metamorphic Core Complex

The Albion Range is located in southern Idaho, in southern Cassia County. The Albion Mountains contain rocks that range in age from Archean to Oligocene (Link and Phoenix, 1996). The Archean Green Creek Complex contains metamorphic basement rock and is overlain by Proterozoic and Paleozoic strata. The rocks have been subsequently intruded by 32-28 Ma granitic Almo pluton, which is exposed in the City of Rocks.

On the edges of the "Silent" City of Rocks Archean Green Creek metamorphic complex is intruded by the Oligocene granitic Almo Pluton. The Archean and Neoproterozoic rocks in the center of the Albion Range core complex were unroofed by high-angle normal and low-angle detachment faults by around 35 Ma.

Geologic map of Cassia Country. Notice that the Albion Mountains run southwest to northeast in the center of the county. Map from the Digital Atlas of Idaho.

 

 

An annotated PDF slide show describing some of the Albion Range geology, with photographs by Paul K. Link.


Table of Geologic history of the Albion Range. Redrawn from a figure by Dave Miller, (USGS, 1983).


Geology of the Pioneer Range Metamorphic Core Complex

The Pioneer metamorphic core complex is located in central Idaho, in southern Custer County. The center of the Pioneer core complex contains Archean and Neoproterozoic gneiss and Eocene granitic rocks. Surrounding the center of the core complex, separated by low-angle detachment and high-angle normal faults, lie sheets of Proterozoic and Paleozoic shallow-water strata, deep-water Mississippian Copper Basin Group, Eocene conglomerate of Smiley Creek and overlying Challis Volcanic Group.

Geologic map of Custer Count. Map from the Digital Atlas of Idaho. http://imnh.isu.edu/digitalatlas/counties/geomaps/geomap.htm

In the Cretaceous, west of the Pioneer Mountains, Archean continental crust was intruded by the Cretaceous Idaho batholith. This was part of the Cordilleran orogeny, which produced regional crustal thickening. After these compressional forces ceased, around ~ 50 Ma, normal faults and low-angle detachment faults developed, including the Wildhorse Detachment fault. These faults slid the Proterozoic and Paleozoic sedimentary rocks in a northwest direction, off the top of the core complex. The Summit Creek stock in both the upper and the lower plate provides a pin point for >10 km of top-to-the northwest translation across the Wildhorse Detachment (Link and Phoenix, 1996).

In late Eocene, eruption of the Challis Volcanic Group and associated felsic plutons covered central Idaho and the Pioneer Mountains. Also at this time, approximately 48 Ma, the Summit Creek stock intruded the core of the Pioneer Mountains. Around 44 Ma, late-stage granitic intrusions and rhyolitic volcanism related to the Trans-Challis fault system occurred. Final uplift as well as the unroofing of the Pioneer Core Complex, including the erosion of the Summit Creek stock, occurred by 37-34 Ma (Link and Phoenix, 1996).

An annotated PDF of the Pioneer Range , with photographs by Paul K. Link.


References

Digital Atlas of Idaho, http://imnh.isu.edu/digitalatlas, managed by Idaho State University Department of Geosciences and hosted by the Idaho Museum of Natural History, Pocatello, ID.

Link, P. K. and Phoenix, E. C., 1996, Rocks, Rail, & Trails, Second Ed.: Idaho Museum of Natural History, Pocatello, ID, 193 p.

Miller, David M., Armstrong, R.L., Compton, R.R., and Todd, V.R., 1983, Geology of the Albion-Raft River-Grouse Creek Mountains area, northwestern Utah and southern Idaho: in Gurgel, K.D., ed, Geologic excursions in the overthrust belt and metamorphic core complexes of the Intermountain region: Utah Geological and Mineral Survey Special Studies 59, p. 1-63.

Orr, W. N. and Orr, E. L., 2002, Geology of the Pacific Northwest, Second Edition: McGraw-Hill Higher Education, New York , NY , 337 p.

O’Neill, R.L., and Pavlis, T.L., 1988, Superposition of Cenozoic extension on Mesozoic compressional structures in the Pioneer Mountains metamorphic core complex, central Idaho: Geological Society of America Bulletin, v. 100, p. 1833-1845.

Rodgers, D.W. Link, P.K., and Huerta, A.D., 1995, Structural framework of mineral deposits hosted by Paleozoic rocks in the northeastern part of the Hailey 1° x 2° quadrangle, south-central Idaho: U.S. Geological Survey Bulletin 2064-B, p. B1-B18.

Wust, S.L., and Link, P.K., 1988, Field guide to the Pioneer Mountains core complex, South-central Idaho, in Link, P.K., and Hackett, W.R., eds., Guidebook to the geology of central and southern Idaho, Idaho Geological Survey Bulletin 27, P. 43-54.

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Continue to Module 2 - Belt Supergroup