Thursday, September 4, 2008

Glacier National Park Geology & Paleontology: Part 5 - Snowslip & Shepard

*Please remember that all collecting of rocks, minerals, plants, fossils or cultural objects (i.e. arrowheads) from National Park Service land is illegal and punishable by a fine*


THE SNOWSLIP AND SHEPARD FORMATION

In publications from the 1930s to 1976, the Snowslip and Shepard Formations are often grouped together and referred to as the Missoula Group. In 1977, the current formation names were proposed. The Snowslip Formation is exposed locally at high elevations within the park and forms the base of the Missoula Group. This one billion year old formation contains calcitic or dolomitic red and green argillites, siltstones and sandstones and represents a subtidal to intertidal setting with occasional subaerial exposure. Pseudocolumnar and mound-shaped stromatolites, or stromatoloids, are known from five locations, with filamentous and pillar-shaped microfossils detected from a stromatolite (or stromatoloid) in the lower part of the Snowslip Formation (Horodyski, 1977, 1983a, 1985a, 1993a,b). Rezak (1957) describes the stromatolites Collenia undosa, Collenia symmetrica and Cryptozoon occidentale from locations along Highway 2 on the southern border of the park (above, below).

The Shepard Formation is highly eroded, existing only in higher elevations within the park. It is predominantly composed of dolomite, siltstones, argillite and quartzite and overlies the 1.5 to 1.845 Ga Purcell Lava (Aleinikoff et al., 1996). Fenton and Fenton (1931) report several
species of stromatolites from the Shepard Formation: Collenia parva, Collenia clappii and Collenia undosa. They also describe “problematic structures” from the base of the Shepard, later known as “molar-tooth structures,” also noted by Horodyski (Fenton and Fenton, 1931; Horodyski, 1985a). Mound-shaped stromatolites were also located in this formation by Horodyski (1982a) on Reynolds Mountain.

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Aleinikoff, J.N., Evans, K.V., Fanninc, C.M., Obradovich, J.D., Ruppel, E.T., Zieg, J.A. and Steinmetz, J.C., 1996, SHRIMP U-Pb ages of felsic igneous rocks, Belt Supergroup, western Montana: Geological Society of America, Abstracts with Programs, v. 28, p. 376.

Fenton, C.L. and Fenton, M.A., 1931, Algae and algal beds in the Belt Series of Glacier National Park: Journal of Geology, v. 39, p. 670-686.

Horodyski, R.J., 1977, Environmental influences on columnar stromatolite branching patterns: Examples from the Middle Proterozoic Belt Supergroup, Glacier National Park, Montana: Journal of Paleontology, v. 51, p. 661-671.

Horodyski, R.J., 1982a, Problematic bedding-plane markings from the Middle Proterozoic Appekunny Argillite, Belt Supergroup, Northwestern Montana: Journal of Paleontology, v. 56, p. 882-889.

Horodyski, R.J., 1983a, Sedimentary geology and stromatolites of the Middle Proterozoic Belt Supergroup, Glacier National Park, Montana: Precambrian Research, v. 20, p. 391-425.

Horodyski, R.J., 1985a, Stromatolites and Paleontology of the Middle Proterozoic Belt Supergroup, Glacier National Park, Montana, in Whipple, J.W., O.B. Raup, Kelty, T., Davis, G., and Horodyski, R., eds., A field guidebook to the geology of Glacier National Park, Montana and vicinity: Society for Sedimentary Geologist (SEPM) midyear meeting field guide, 19 p.

Horodyski, R.J., 1993a, Paleontology of Proterozoic shales and mudstones: Examples from the Belt Supergroup, Chuar Group, and Pahrump Group, western U.S.A: Precambrian Research v. 61, no. 3-4, p. 241-278.

Horodyski, R.J., 1993b, Precambrian paleontology of the western conterminous United States and northwestern New Mexico, in Reed, J.C., ed., Precambrian of the Conterminous United States: Boulder, Geological Society of America, Geology of North America, v. C-2, p. 558-565
(microfiche appendix of 77 pages).

Rezak, R., 1957, Stromatolites of the Belt Series in Glacier National Park and vicinity, Montana: U.S. Geological Survey Professional Paper 294-D, p. 127-154.

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