Geology of Wind River Canyon, Wyoming (cont.)
5 Archean Rocks
The combination of the Laramide uplift, the Boysen fault and the incision of the Wind River result in the exposure of Archean rocks in the southern Wind River canyon area. The Archean rocks are mainly amphibolite-facies metamorphic rocks (hornblende amphibolite and biotite-quartz schists) with granite intrusions.
All these rocks have been described in detail by Gwynne (1938), Condie (1967) and Granath (1975). The two latter authors agree that hornblende-cummingtonite amphibolites and biotite schists are metasedimentary, deriving from graywackes and shales, while the massive plagioclase-hornblende amphibolites must be ortho-amphibolites (with an igneous origin as basalt or gabbro). Granath (1975) goes further, suggesting that all these Archean rocks are part of a greenstone belt, as commonly found in the Superior province.
Mueller et al. (1985) recognized three different compositions in the metavolcanic amphibolites (basaltic, basaltic-andesitic and dacitic). They obtained a U-Pb age of 2900 Ma for the dacitic rocks; since they are interlayered with the other rocks, this age should apply to the whole sequence. The same workers dated the metamorphic event at 2750 Ma. The granite was dated at 2730 Ma by Stuckless et al. (1986).
Figure 5-1. Northern end of the Great Unconformity in the Wind River canyon, Wyoming.
View to the east at 43° 27' 54" N. Northern end of the exposure of the Great Unconformity in the Wind River canyon. The name of the Great Unconformity comes from the widespread worldwide flooding of eroded Archean and Proterozoic rocks in Cambrian time. Here the Cambrian Flathead sandstone unconformably overlies an Archean granite. The upper part of the Archean granite is weathered. _f : Cambrian Flathead sandstone. Ag : Archean granite. Dotted line : unconformity.
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Figure 5-2. The Great Unconformity in the Wind River canyon at 43° 27' 30" N.
View to the west. Deformed dark grey amphibolite schist is intruded by massive pink quartz-monzonite intrusions and pegmatite dikes. The intrusions probably belong to a large pluton surveyed in the central Owl Creek Mountains. _f : Cambrian Flathead sandstone. Aqm : Archean quartz-monzonite. Aa : Archean amphibolite. Dotted line : unconformity.
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Figure 5-3. Fine-grained Archean plagioclase-hornblende amphibolite in the Wind River canyon.
View at 43° 27' 30" N on the eastern side of the Wind River canyon.
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Figure 5-4. Blocky Archean amphibolite in the Wind River canyon.
View at 43° 27' 30" N on the eastern side of the canyon Deformed and boudinaged quartz-monzonite dikes cut through the amphibolite.
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Figure 5-5. Pegmatite dike in amphibolite in the Wind River canyon.
View at 43° 27' 30" N on the eastern side of the canyon. Pegmatite dike between a gneissose-textured plagioclase-hornblende amphibolite and a black massive amphibolite.
Figure 5-6. The Boysen fault at the southern end of Wind River canyon.
View to the northwest from 43 26' 35" N. The Boysen fault is a normal fault with a ~600 m displacement (see more on p. 1). The photo shows the footwall, made of Archean rocks. Paleozoic rocks have been removed by erosion. The Wind River canyon is here at its narrowest point between two nearly vertical cliffs. At lower right is the entrance of the tunnel of U.S. Hwy. 20.
Figure 5-7. The hanging wall of Boysen fault.
View to the southwest from 43 26' 35" N. Archean rocks don't emerge above ground here. Cambrian and Mississippian formations resisted erosion, unlike on the footwall (preceding figure). Bighorn dolomite is also present, at the base of the Madison limestone. Mm : Mississippian Madison limestone. _g : Cambrian Gallatin limestone. _gv : Cambrian Gros Ventre formation. _f : Cambrian Flathead sandstone.
References:
Condie, K. C., 1967, Petrologic reconnaissance of the Precambrian rocks in Wind River Canyon, central Owl Creek Mountains, Wyoming, Wyoming University Contributions to Geology, v. 6, no. 2, p. 123-129.
Fanshawe, J. R., 1939, Structural geology of Wind River Canyon area, Wyoming American Association of Petroleum Geologists Bulletin, v. 23, p. 1439-1492.
Keefer, W.R., and van Lieu, J.A., 1966, Paleozoic Formations in the Wind River Basin, Wyoming, U.S. Geological Survey Professional Paper 495-B, 66 p.
Granath, J.W., 1975, Wind River Canyon: an example of a greenstone belt in the Archean of Wyoming, U.S.A., Precambrian Research, v. 2, p. 71-91.
Gwynne, C. S., 1938, The granites in the Wind River Canyon, Wyoming, Bull. Geol. Soc. America, v. 49, p. 1417-1424.
Love, J.D., Christiansen, A.C., Brown, T.M., and Earle, J.L., 1979, Preliminary geologic map of the Thermopolis 1 degree x 2 degree quadrangle, central Wyoming, U.S. Geological Survey, Open-File Report OF-79-962, Scale: 1:250,000.
Maughan, E.K., 1972, Geologic map of the Wedding of the Waters quadrangle, Hot Springs County, Wyoming, U.S. Geological Survey, Geologic Quadrangle Map GQ-1042, Scale: 1:24,000.
Maughan, E.K., 1987, Wind River Canyon, Wyoming, in Beus, S.S., ed., Centennial Field Guide Volume 2, Rocky Mountain Section, p. 191-196.
Mueller, P. A., Peterman, Z. E., and Granath, J. W., 1985, A bimodal Archean volcanic series, Owl Creek Mountains, Wyoming Journal of Geology, v. 93, p. 701-712.
Stuckless, J.S., Miesch, A.T., Wenner, D.B., Hedge, C. E., Simmons, K.R., Nkomo, I.T., and Butt, K.A., 1986, Chemical and isotopic studies of granitic Archean rocks, Owl Creek Mountains, Wyoming, USGS Professional Paper 1388-A-C, 48p.
Thaden, R.E., 1980, Geologic map of the Birdseye Pass quadrangle, showing chromolithofacies and coal beds in the Wind River Formation, Fremont and Hot Springs Counties, Wyoming, U.S. Geological Survey, Geologic Quadrangle Map GQ-1537, Scale: 1:24,000.