Lithostratigraphy: McCormick Group
Chronostratigraphy: Paleozoic Erathem >>Pennsylvanian Subsystem >>Atokan Series
Allostratigraphy: Absaroka Sequence
Willman, H. B., Elwood Atherton, T. C. Buschbach, Charles Collinson, John C. Frye, M. E. Hopkins, Jerry A. Lineback, and Jack A. Simon, 1975, Handbook of Illinois Stratigraphy: Illinois State Geological Survey Bulletin 95, 261 p.
M. E. Hopkins and J. A. Simon
The Atokan Series of the Pennsylvanian System (Taff and Adams, 1900, p. 273).
Named for Atoka, Atoka County, Oklahoma.
No type section was designated, but the area near Atoka contains a sequence of a few thousand feet of sandstones and shales generally devoid of invertebrate fossils and truncated at the top by a major fault.
In Illinois the Atokan Series consists of the strata composing the Abbott Formation of the McCormick Group (Willman et al., 1967) (fig. P-2). The base and top have not been defined by fossils, although fossiliferous zones are present within the series.
Extent and thickness
Where marine rocks are abundant, the Atokan is generally characterized as the two sub-zones of the earliest fusiform fusulinids, Profusulinella and Fusulinella, and the top is defined as the strata below the first appearance of Fusulina and Wedekindellina, although some confusion has arisen because Fusulina has been reported in the Atokan of the type area.
Well log characteristics
Plant fossil zones for the Atokan Series are Zones 7 (Megatopteris spp.) and 8 (Neuropteris tenufolia) of Read and Mamay (1964). Spore assemblages are the best means for correlating the Atokan strata in the Illinois Basin with Atokan strata of other areas. They are of greater diversity and their genera are more abundant and more evenly distributed than is true in the Morrowan Series. Laevigatosporites, Calamites, Florinites , and Punctatisporites increase in the upper part at the expense of Lycospora, which, with Densosporites, especially D. annulatus, and Cristatisporites indignibundus, is common in the lower part of the Atokan. Certain species of Radiizonates and Torispora are useful in defining the upper part.
Age and correlation
Environments of deposition
READ, C. B., and S. H. MAMAY, 1964, Upper Paleozoic floral zones and floral provinces of the United States: USGS Professional Paper 454-K, p. K1-K35.
TAFF, J. A., and G. I. ADAMS, 1900, Geology of the eastern Choctaw coal field, Indian Territory: USGS 21st Annual Report, part 2, p. 257-311.
WILLMAN, H. B., et al., 1967, Geologic map of Illinois: Illinois State Geological Survey.
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