Unnamed gray shale member - Carbondale Formation

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Lithostratigraphy: Carbondale Formation >>Unnamed gray shale member - Carbondale Formation
Chronostratigraphy: Paleozoic Erathem >>Pennsylvanian Subsystem >>Desmoinesian Series
Allostratigraphy: Absaroka Sequence

Primary source

Nelson, W.J., P.H. Heckel and J.M. Obrad, 2022, Pennsylvanian Subsystem in Illinois: Illinois State Geological Survey, Bulletin (in press).

Contributing author(s)

W.J. Nelson


No name is proposed at this time.

Type section

None is proposed at this time. The ISGS did not acquire the cores that contain this unit. W.J. Nelson and Joseph A. Devera of the ISGS described the cores; the company furnished gamma-ray logs. These are on file at the Geological Records Unit of the ISGS and are available online from the ISGS website.

Reference section


Stratigraphic relationships

Figure 4-45. Graphic logs from three cores drilled by Rust E & I in St. Clair County, Illinois, illustrating the unnamed gray shale that locally occurs between the Houchin Creek Coal and the Excello Shale. © University of Illinois Board of Trustees.

The unnamed gray shale occurs as lenses between the Houchin Creek Coal and Excello Shale Members (Figure 4-45).

Extent and thickness

At present, this member is known only from four boreholes. Three of these were cores drilled by Rust E & I: number B-106 in sec. 20, B-101 in sec. 21, and B-108c in sec. 28, all in T 3 S, R 6 W of St. Clair County, Illinois (Figure 4-45). The thickness of the gray shale ranges from 9.75 to 16.2 ft (2.97 to 4.94 m) in the three Rust E & I cores. The fourth hole is Northern Illinois Gas boring no. 9, 150 mi (240 km) north of the other three holes in sec. 11, T 24 N, R 6 E, McLean County. The core log indicates gray shale 3.0 ft (90 cm) thick.


The shale is medium to dark gray, slightly silty and micaceous (in part), fissile, and well laminated. Siderite bands and nodules are present, along with small pyrite nodules. In one core, the shale becomes black and highly fissile at the base.




The contact to the Houchin Creek Coal is sharp; the contact to the Excello Shale is sharp to gradational.

Well log characteristics

The gamma-ray log response is typical for gray shale. The basal black shale in hole #B-106C does not exhibit a higher than normal gamma-ray reading.


A few plant remains and small, thin-shelled bivalves were observed.

Age and correlation

No data are available, except as provided by the enclosing units. No analogous unit is known in other basins.

Environments of deposition

Lithology and fossils suggest a quiet, fresh- to brackish-water setting. The environment is likely analogous to other “gray shale wedges,” such as the Francis Creek, Dykersburg, and Energy Shales.

Economic importance




ISGS Codes

Stratigraphic Code Geo Unit Designation