THE PRICHARD AND ALTYN FORMATIONS
The Prichard Formation, 1.375 to 1.4 Ga sandstone and siltstones, is only found on the western side of the park and is believed to be the age equivalent of the Altyn Formation, found in the eastern portion of Glacier National Park. This formation has been reported to contain microfossils and pseudomicrofossils (Horodyski, 1981). The microfossils found in the dark gray mudstones of the Prichard Formation are of interest due to the fact that they demonstrate the effects of burial metamorphism on organic-walled microfossils. The fossils, which consist of black carbonaceous films, are very rare and poorly preserved, with only 12 known thus far. Due to the extent of their altered state, these fossils cannot be identified to the genus level and are therefore not useful for biostratigraphic correlations. The pseudomicrofossils from the formation occur as spheroids and filaments and illustrate an occurrence of non-biogenic carbonaceous microstructures that could be mistaken as authentic fossils (Horodyski 1981, 1993a).
The Altyn Formation, found in the eastern portion of Glacier National Park, is composed predominantly of 1.350 to 1.450 Ga limestones and dolomite. When the park was studied by paleontologists such as Charles Walcott around 1914, this formation was often referred to as the “Newland Limestone,” a formation known from the Big Belt and Little Belt Mountains. The Fentons noted a stratigraphic error made by Walcott in assigning Weedia tuberose to the Altyn Formation. They reassigned this genus to the Siyeh Formation and also identified Beltina cf. danaii in the park (Fenton and Fenton, 1931, 1937; Horodyski, 1985a, 1993b). However, this remains an important discovery in that it is one of the earliest published reports of fossils from the park.
One of the significant contributions to the park’s paleontology in the 1930s was the description of a massive bed of stromatolites located near Apikuni Falls (also known as Appekunny Falls) in the upper Altyn Limestone (Fenton and Fenton, 1931). These columnar stromatolites are located in a light gray to tan limestone that is some 6 m thick at the foot of Apikuni Mountain. They were named Collenia columnaris by Fenton and Fenton, with two other locations containing C. columnaris identified from within the park (Fenton and Fenton, 1931, 1937; Horodyski, 1977). Another stromatolite group occurring above the C. columnaris zone was assigned to Baicalia by White (1970). Ross (1959) notes that the zones are well developed on both Apikuni (“Appekunny”) and Divide Mountains, but poorly developed or absent in other areas, making it a discontinuous zone in the Altyn Formation. Horodyski (1976a) studied these stromatolites in great detail, describing three macrostructural varieties that occur in this horizon. In 1957, Rezak reassigned Collenia columnaris to Collenia frequens without explanation, assigning it as a “zone” due to its presence in two new locations (Ross and Rezak, 1959). Horodyski (1985a) took this a
step further and referred to these stromatolites as “highly elongated, inclined stromatolites,” rather than referring to them by a genus name. However, Horodyski still referred to the group Baicalia interchangabally with “branching stromatolite” (left; Horodyski, 1985a).Baicalia is a branching columnar stromatolite forming in subtidal areas, where C. columnaris is a highly elongated, unbranched, columnar stromatolite living in quiet waters below the tidal zone. These stromatolites are tightly packed next to one another and would have formed reeflike masses similar to those seen in the vicinity of the contemporary Bahamas.
"Top view" of Baicalia group
During their time in the park Fenton and Fenton (1931, 1937) also identified three new species from the Altyn Formation: Newlandia sarcinula, Collenia albertensis and Morania antique, although Rezak (1957) reassigns Collenia albertensis to
Collenia frequens. White (1974a,b, 1979) also reports on an assemblage of microfossils that are comparable to modern blue-green algae and unicellular green algae from black chert found within the Altyn Formation (Horodyski, 1993b). Circular "trace fossils" found in the 1960s from the Altyn Formation have since been found to be pseudofossils that were made by modern bees (right; Sando, 1972).------------------------------------------------------------------------------------------------
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.
Fenton, C.L. and Fenton, M.A., 1937, Belt Series of the north: Stratigraphy, sedimentation, paleontology: Geological Society of America Bulletin, v. 48, p. 1873-1969.
Horodyski, R.J., 1976a, Stromatolites from the Middle Proterozoic Altyn Limestone, Glacier National Park, Montana, in Walter, M.R., ed., Stromatolites: Amsterdam, Elsevier, Developments in Sedimentology 20.
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., 1981, Pseudomicrofossils and altered microfossils from a Middle Proterozoic Shale, Belt Supergroup, Montana: Precambrian Research, v. 16, p. 143-154.
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.
Ross, C.P., 1959, Geology of Glacier National Park and the Flathead region, northwestern Montana: U.S. Geological Survey Professional Paper, 296, 125 p.
Ross, C.P., and Rezak, R., 1959, The rocks and fossils of Glacier National Park: The story of their origin and history: U.S. Geological Survey Professional Paper 294-K, p. 401-439.
Sando, W., 1972, Bee-nest pseudofossils from Montana, Wyoming, and South-West Africa: Journal of Paleontology, v. 46, no. 3, p. 421-425.
White, B., 1970, Algal stromatolites, depositional environments and age of the Altyn Formation of Montana: Geological Society of America, Abstracts with Programs, v. 2, p. 719-720.
White, B., 1974a, Microfossils from the Late Precambrian Altyn Formation of Glacier National Park, Montana: Geological Society of America, Abstracts with Programs, v. 6, p. 85.
White, B., 1974b, Microfossils from the Late Precambrian Altyn Formation of Montana: Nature, v. 247, p. 452-453.
White, B., 1979, Stratigraphy and microfossils of the Precambrian Altyn Formation of Glacier National Park, Montana, in Linn, R.M., ed., Proceedings of the First Conference of Scientific Research in the National Parks, v. 2, p. 727-735.
White, B., 1984, Stromatolites and associated facies in shallowing-upward cycles from the Middle Proterozoic Altyn Formation of Glacier National Park, Montana: Precambrian Research, v. 24, no. 1, p. 1-26.
White, B. and Pedone, V.A., 1975, A new microfossil locality in the Precambrian Altyn Formation of Montana: American Association of Petroleum Geologist- Society for Sedimentary Geologist (AAPG-SEPM), Abstracts with Programs, v. 2, p. 80-81.
No comments:
Post a Comment