Mesozoic Thrust Belt

by Paul Link & Laura DeGrey, Idaho State University

Mesozoic Idaho-Wyoming Fold and Thrust Belt

Fold and Thrust Belt Structures in Idaho

References

PDF Slideshows: Thrust Belt, South of the SRP and North of the SRP
Flythroughs: Blackfoot River, Portneuf, Bear River, Big Lost River, Salmon, Middle Fork Salmon, Raft River Valley, Henry's Fork, Clearwater, Salmon Falls Creek, Snake 1, Snake 2, Big Wood River, Teton Valley

Vocabulary Words

foreland basin

thrust fault

orogeny

orogenic belt

imbricated

passive margin

 

Mesozoic Idaho-Wyoming Fold and Thrust Belt

The fold and thrust belt of the Cr


etaceous Sevier orogenic belt extends from eastern Idaho into western Wyoming, north-central Utah, and southwest Montana. The thrust belt is covered or cut by the Snake River Plain and previously extended continuously into central Idaho. As part of the Cordilleran orogenic belt, it extends from Alaska to Mexico.

The Cordilleran orogenic belt formed during subduction of the Farallon plate beneath North America, with associated intrusion of batholiths like the Idaho batholith, overthickening of crust east of the magmatic belt, and development of a thrust and fold belt within the Paleozoic passive margin strata to the east.

In Idaho the thrust faults trend northwest-southeast, similar to the orientation of faults within the Basin and Range Province. In fact, much of the Basin and Range represents extension of the area formerly shortened by thrusting. In several places the Basin and Range normal faults reactivate previous thrust faults.

Map of the Thrust Belt in Idaho, north of the Snake River Plain. Figure from Link and Janecke (1999).

Map showing the main part of the Idaho-Wyoming Thrust Belt, south of the Snake River Plain.

Folding and thrust faulting occurred during the Cretaceous to Paleocene Sevier Orogeny (~130-60 Ma). The Sevier Orogeny was a time of active compression, in response to the Pacific Plate subducting beneath the North American Plate. The thrust faults have moved their respective thrust sheets with up to 50 miles of displacement and the large folds have a amplitude of 5,000 to 10,000 feet.

The thrust belt in SE Idaho contains, from west to east, the Paris thrust, the Meade thrust, and the Absaroka Thrust (see cross section below). The other major thrust faults, located in Wyoming, are the Crawford Thrust, the continuation of the Absaroka Thrust, the Darby or Hogsback Thrust, and the Prospect Thrust (see map above).

The thrust faults of the "Thrust Belt" are considered shallow, or "thin-skinned", meaning they are not very deep, and that the fault geometry flattens at depth and follows layers of weakness along bedding planes in the Paleozoic and Proterozoic strata.

Timing of thrusting in southeast Idaho thrust belt, from oldest to youngest.

Paris Thrust Middle-Early Cretaceous 120-113 Ma
Meade Thrust Late-Early Cretaceous 113-98 Ma
Crawford Thrust Late Cretaceous 87-84 Ma
Absaroka Thrust late Cretaceous 84-74 Ma
Hogsback (Darby) Thrust Paleocene 63-57 Ma
Prospect Thrust Eocene 57-52 Ma
 

These thrust faults are the bottom boundary of each thrust sheet of the same names, which contain all of the passive margin sedimentary rocks above the fault. Each thrust sheet is 10,000 to 20,000 feet thick (Dixon, 1982). Thrusting migrated in an eastward progression, subsequently stacking the thrust sheets on top of one another. On the continent-side of each of these imbricated thrust sheets, there becomes a structural foreland basin, which receives sediment shed from the thrust sheet (DeCelles, 2004).

In central Idaho the thrusts are less well dated, but generally follow the same pattern, of becoming younger to the east, always putting older rocks on younger. From west to east the faults are the Pioneer, Copper Basin , Hawley Creek, Cabin and Tendoy thrusts (Link et al., 1988; Rodgers et al., 1995). Rodgers and Janecke (1992) summarize the geometry of thrust ramps in the northern Idaho thrust belt. Because the Copper Basin Group is confined to the foot wall of the Copper Basin thrust, and the Milligen Formation and Sun Valley Group are confined to the hanging wall, the geometry is more complex.


Fold and Thrust Belt Structures in Idaho

 

Teton, Bonneville, Caribou, and Bear Lake Counties have been merged to show the fold and thrust belt in southeastern Idaho.

 

 

Mesozoic Continental Strata

Rocks of Mesozoic age in the southeast Idaho thrust belt include thick limestones of Triassic and Jurassic age and even thicker sandstones and shales of Cretaceous age. They were deposited before and during the thrust deformation, in foreland basins which subsided because they were loaded down by the weight of advancing thrust sheets (Armstrong and Oriel, 1986).

Parts of the Triassic and Jurassic section were deposited in continental fluvial environments under a warm climate, so they are a deep red color. The source area for the deltaic Cretaceous strata was uplifted parts of the Sevier thrust belt to the west. As a result these sandstones are rich in sedimentary lithic fragments, in particular chert. They often have a speckled “salt and pepper” appearance, from the black chert lithic fragments.

In SE Idaho these Mesozoic rocks are exposed on Highway 89 east of Montpelier, and on the Tincup highway east of Wayan. The most spectacular single place to see Mesozoic rocks is Raymond Canyon, Wyoming, just south and east of Geneva, Idaho.

For individual county maps and explanations, visit the Digital Atlas of Idaho website.


References

Armstrong, F. C., and Oriel, S. S., 1965, Tectonic Development of Idaho-Wyoming Thrust Belt: American Association of Petroleum Geologists Bulletin, v. 49, p. 1847-1866

DeCelles, P. G., 2004, Late Jurassic to Eocene Evolution of the Cordilleran Thrust Belt and Foreland Basin System, Western U.S.A. : American Journal of Science, v. 304, p. 105-168

Dixon , J. S., 1982, Regional Structural Synthesis, Wyoming Salient of Western Overthrust Belt: The American Association of Petroleum Geologists Bulletin, v. 66, no. 10, p. 1560-1580.

Fortsch, D. E., and Link, P. K., 1999, Regional Geology and Fossil Sites from Pocatello to Montpelier, Freedom, and Wayan, Southeastern Idaho and Western Wyoming, in Hughes, S. S., and Thackray, G. D., eds., Guidebook to the Geology of Eastern Idaho: Idaho Museum of Natural History, Pocatello, p. 281-294.

Kellogg, K.S., Rodgers, D.W., Hladky, F.R., Kiessling, M.A., and Riesterer, J.W., 1999, The Putnam Thrust Plate, Idaho - Dismemberment and Tilting by Tertiary Normal Faults in Hughes, S. S., and Thackray, G. D., eds., Guidebook to the Geology of Eastern Idaho: Idaho Museum of Natural History, Pocatello, p. 97-114.

Link, P.K., Skipp, B., Hait, M.H., Jr., Janecke, S., and Burton, B.R., 1988, Structural and stratigraphic transect of south-central Idaho: A field guide to the Lost River, White Knob, Pioneer, Boulder and Smoky Mountains in Link, P.K. and Hackett, W.R., eds., Guidebook to the geology of central and southern Idaho: Idaho Geological Survey Bulletin 27, p. 5-42.

Link, P. K., and Janecke, S. U., 1999, Geology of East-Central Idaho: Geologic Roadlogs for the Big and Little Lost River, Lemhi, and Salmon River Valleys, in Hughes, S. S., and Thackray, G. D., eds., Guidebook to the Geology of Eastern Idaho: Idaho Museum of Natural History, Pocatello, p. 295-334.

Rodgers, D.W. and Janecke, S.U., 1992,. Tertiary paleogeologic maps of the western Idaho-Wyoming-Montana thrust belt, in Link, P.K., Kuntz, M.A. and Platt, L.B., eds., Regional Geology of eastern Idaho and western Wyoming , Geological Society of America Memoir 179, p. 83-94.

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 1o x 2o quadrangle, south-central Idaho: U.S. Geological Survey Bulletin 2064-B, p. B1-B18.

Skipp, B.A., 1987, Basement thrust sheets in the Clearwater orogenic zone, central Idaho and western Montana: Geology, v.15, p. 220-224.

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Continue to Module 6 - Idaho Batholith