Pennsylvanian Subsystem

Chronostratigraphy: Paleozoic Erathem >>Pennsylvanian Subsystem

Primary source

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.

Contributing author(s)

M. E. Hopkins and J. A. Simon

Name

History/background

The name "Pennsylvanian Series" was introduced by H. S. Williams in a report on Washington County, Arkansas (Simonds, 1891, p. xiii), to designate strata generally called "Coal Measures". Later, the coal fields of Pennsylvania were named as the type area (Williams, 1891, p. 83). The name "Pennsylvanian" has been used in Illinois since the first report of the present Geological Survey (Weller, 1906a), but the terms "Coal Measures" and/or "Upper Carboniferous" were still used in many early reports of the Survey. The Pennsylvanian was classified as a series in Illinois until about 50 years ago and since then has been regarded as a system (fig . P-2).

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  Fig. P-2 -- Classification of the Pennsylvanian System. In the graphic column, blank space indicates gray shale. Named members are listed to the right of the graphic column.

Stratigraphic relationships

A major unconformity underlies the Pennsylvanian System (fig. P-4) and separates the Absaroka Sequence, which includes all the Pennsylvanian strata, from the Kaskaskia Sequence below. Pennsylvanian rocks overlie the youngest strata of the Mississippian System in extreme southern Illinois and progressively overlap older strata northward. In the extreme northern part of the coal basin, Pennsylvanian rocks directly overlie rocks of Ordovician and Silurian age.

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  Fig. P-4 -- Geologic map of the sub-Pennsylvanian (sub-Absaroka) surface (after Willman et al., 1967).

The upper surface of the Pennsylvanian rocks is an eroded post-Pennsylvanian, pre-Pleistocene surface modified in part by further erosion during the Pleistocene (fig. 10). In a relatively small area in Adams, Pike, and Brown Counties in western Illinois, the Pennsylvanian is overlain by Gulfian (upper Cretaceous) rocks (Frye et al., 1964).

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  Fig. 10 -- Generalized areal geology of the bedrock surface (Willman and Frye, 1970).

Because of the abrupt and distinct vertical variations in lithology and the widespread lateral continuity of many of the Pennsylvanian units, their classification has undergone many changes (fig. P-5). The present Pennsylvanian classification was adopted (Kosanke et al., 1960) to make it conform to the Illinois State Geological Survey policy on stratigraphic nomenclature (Willman et al., 1958) (fig. 16).

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  Fig. 16 -- Development of the stratigraphic classifications used in Illinois. The references include reports that introduced changes in the classification and others that show typical usage.
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  Fig. P-5 -- Development of Classification of the Pennsylvanian System (after Kosanke et al., 1960).

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  Fig. P-7 -- Arrangement of lithologic units in a cyclothem (after Willman and Payne, 1942).

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  Table 3 -- Alphabetic list of named cyclothems in Illinois, showing formation, distribution, and principal references (after Kosanke et al., 1960).

Extent and thickness

Strata of the Pennsylvanian System constitute the bedrock in about two-thirds of the area of Illinois (36,806 out of 56,400 square miles) and underlie all or parts of 86 of the 102 counties of the state (fig. P-1).

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  Fig. P-1 -- Thickness of the Pennsylvanian System.

Throughout most of the area they cover in Illinois, Pennsylvanian strata are concealed by unconsolidated Pleistocene deposits, but in many areas they have been exposed by stream erosion (fig. P-3).

• Fig. P-3 -- Exposures of Pennsylvanian rocks.
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  Fig. P-3A -- Battery Rock Sandstone Member of the Caseyville Formation capping an escarpment 8 miles north of Cave in Rock, Hardin County.
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  Fig. P-3B -- Strata overlying the Herrin (No. 6) Coal Member of the Carbondale Formation (at the man's feet), including the Bankston Fork Limestone Member (BF), Conant Limestone Member (C), Jamestown Coal Member (J, 1 inch thick), gray calcareous shale (gs), Brereton Limestone Member (B); in a strip mine southwest of Belleville, St. Clair County.
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  Fig. P-3C -- Mecca Quarry Shale Member (M) of the Carbondale Formation in which a large limestone concretion is present (at the hammer), overlying the Colchester (No. 2) Coal Member (C) just south of Viola, Mercer County.
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  Fig. P-3D -- The Sub-Absaroka unconformity showing Pennsylvanian strata (Caseyville Formation) truncating Mississippian strata (Kinkaid Limestone) in a roadcut of U.S. Highway 51, north of Anna, Union County.
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  Fig. P-3E -- Asymmetrical current ripple marks and weak current laminations (parallel to hammer) in thin-bedded sandstone in the Caseyville Formation, near Pomona, Jackson County.
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  Fig. P-3F -- Quartz-pebble conglomerate in the Caseyville Formation near Shiloh Hill, Randolph County.

Lithology

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Fossils

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  Fig. P-6 -- Typical Pennsylvanian fossils.

Environments of deposition

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  Fig. 12 -- Principal geologic structures of Illinois.
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  Fig. P-8 -- Structure of the top of the Herrin (No. 6) Coal Member (based on map by K. E. Clegg).

References

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ISGS Codes

ISGS Code and Symbol

Stratigraphic Code	Geo Unit Designation
1520