M. E. Hopkins and J. A. Simon
The Morrowan Series of the Pennsylvanian System (Adams, 1904, p. 28) is named for Morrow, Washington County, Arkansas.
The type exposure on Hale Mountain lies a short distance from Morrow, Arkansas.
The Caseyville Formation of the McCormick Group (fig. P-2) constitutes the entire Morrowan Series in Illinois (Willman et al., 1967). The top of the series may not correlate precisely with the top in the Midcontinent section.
The series consists of marine and nonmarine rocks.
Marine fossils are rare in the Caseyville, but the Sellers Limestone Member in southeastern Illinois contains a Morrowan fauna. The type region of the Morrowan in many places contains Millerella, a nonfusiform fusulinid that is also found in the Chesterian Series and in younger Pennsylvanian sediments and is the most advanced fusulinid found in Morrowan rocks. However, no fusulinids have been found in the Illinois Morrowan. The upper limit of the Morrowan in fusulinid-bearing rocks is placed at the base of the first strata containing Profusulinella.
The compression plant fossils of the Morrowan Series are in Zone 6 (Neuropteris tennesseeana and Mariopteris pygmaea) of Read and Mamay (1964). The roof shale of the Baldwin coal, which occurs in the Morrowan type section, contains a plant-impression flora similar to that above the Gentry Coal Member in southeastern Illinois. The relative ages of strata within the Morrowan Series in Illinois can best be determined from the spores, which are abundant in Caseyville sediments, especially the coals. The Morrowan Series in Illinois is dominated by the genus Lycospora, which in some coals makes up as much as 80 percent of the spore population. Spores of herbaceous lycopods -- Densosporites, Cristatisporites , and Radiizonates -- are also abundant. D. sinosus, R. striatus, and R. irregularis are useful guide fossils for the lower part of the Morrowan, whereas C. indignibundus is typically found in the upper part. Schulzospora ranges up to the top of the Morrowan. Triquitrites, Laevigatosporites, and Endosporites are found in the lower part of the Morrowan but are very rare.
ADAMS, G. I., 1904, Zinc and lead deposits of northern Arkansas: USGS Professional Paper 24, 118 p.
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.
WILLMAN, H. B., et al., 1967, Geologic map of Illinois: Illinois State Geological Survey.
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