Big Creek Shale Member
Nelson, W.J., 2020, Pennsylvanian Subsystem in Illinois: Illinois State Geological Survey Bulletin (in press).
Edited and figures drafted by Jennifer M. Obrad.
Savage (1921) published several sections that include this unit but did not name it. Savage (1927) applied the name “Big Creek Shale” on charts (p. 309, 313) and once, in passing, in text (p. 315). On the charts, the Big Creek Shale lies above the Cuba (now Vermilionville) Sandstone and below the Herrin Coal.
The shale is named for a stream in Fulton County, Illinois.
None. The type area is along Big Creek in T7N, R4E in Fulton County (Kosanke et al. 1960).
Core from the ISGS #1 McCormick boring, which was drilled 2,800 ft (853.4 m) from the north line, 2,300 ft (701.0 m) from the west line of sec. 21, T 8 S, R 8 E, Gallatin County, southern Illinois (ISGS county no. 25246). The Big Creek Member extends from the base of the Herrin Coal at 268.0 ft (81.7 m) to the top of the Briar Hill Coal at 323.5 ft (98.6 m; Figure 4-61). Thus, the member is 55.5 ft (16.9 m) thick.
The typed core description and geophysical log are on file at the ISGS Geological Records Unit and are available on the ISGS website. The McCormick core is archived at the ISGS Geological Samples Library in Champaign under storage number C-15339.
Previous authors consistently placed the Big Creek Member above the Vermilionville Sandstone and below the Herrin Coal. Willman et al. (1975, p. 191) wrote, “A shale is present at this position throughout much of the state, but the name [Big Creek] has been used only in western Illinois.” Without explanation, Willman et al. (1975) placed the Big Creek Shale above the Briar Hill Member. The Big Creek Member is hereby considered to comprise all fine-grained clastic strata that lie above the Briar Hill Coal (where present) and underlie (1) the overlying, erosive-based Walshville Member, (2) the Percy Limestone Member, or (3) the Herrin Coal Member. Where the Briar Hill is missing, the Canton and Big Creek Members generally cannot be differentiated.
Extent and thickness
Strata of Big Creek age occur more or less throughout the basin, but the member loses its identity where the Springfield and/or Briar Hill and Herrin Coals are missing. Specifically, this region includes most of the Western Shelf southeast of the Illinois River. In northern Illinois, the Big Creek varies from zero to about 30 ft (9 m) thick. More than double that thickness is attained in the southern Fairfield Basin, but the usual range is about 20 to 50 ft (6 to 15 m).
Descriptions in the literature are cursory. Savage (1921) and Wanless (1957) mentioned shale that is gray, soft, nonfossiliferous, and locally sandy in the lower part. Willman and Payne (1942) reported laminated, carbonaceous shale in alternating layers of medium to dark gray, greenish gray, and black overlying the Spring Lake (Briar Hill) Coal in southern LaSalle County. In the deeper part of the basin, the Big Creek is a typical upward-coarsening succession topped by laminated siltstone or sandstone. Shale at the base is commonly dark gray to black and fissile, but not “sheety” or phosphatic. Thin basal marine limestone or fossiliferous, calcareous shale may be present. Upward, the strata grade from sideritic, dark gray shale through silty shale, siltstone, and fine-grained sandstone. Near the top, the rock grades back to claystone that is heavily overprinted with paleosol features: the underclay of the Herrin Coal.
In northern and western Illinois, the lower contact with the Vermilionville Sandstone may be either gradational or sharp and erosive. In the deep basin, the contact to the Briar Hill Coal generally is sharp. The upper contact may be erosive to the unnamed sandstone or younger valley-fill deposits and either sharp or gradational to the Percy Limestone and Herrin Coal.
Well log characteristics
In most places, the log profile is typical for an upward-coarsening succession.
As observed in cores, plant fragments are common along with a variety of trace fossils. Unspecified marine to brackish-water invertebrates occur near the base.
Age and correlation
Part of the Labette Shale in Missouri appears to be correlative to the Big Creek (Figure 4-72).
Environments of deposition
Where it overlies channels of the Vermilionville, the Big Creek represents late stages of channel filling and abandonment. In the Fairfield Basin, this unit broadly comprises a prograding deltaic wedge. Subjected to subaerial exposure and pedogenesis, the upper part is transitional to the Herrin Coal.
- Kosanke, R.M., J.A. Simon, H.R. Wanless, and H.B. Willman, 1960, Classification of the Pennsylvanian strata of Illinois: Illinois State Geological Survey, Report of Investigations 214, 84 p. and 1 pl.
- Savage, T.E., 1921, Geology and mineral resources of the Avon and Canton Quadrangles: Illinois State Geological Survey, Bulletin 38, p. 209–271, 1 pl., 1:62,500.
- Savage, T.E., 1927, Significant breaks and overlaps in the Pennsylvanian rocks of Illinois: American Journal of Science, v. 14, p. 307–318.
- Wanless, H.R., 1957, Geology and mineral resources of the Beardstown, Glasford, Havana, and Vermont Quadrangles: Illinois State Geological Survey, Bulletin 82, 233 p.
- Willman, H.B., E. Atherton, T.C. Buschbach, C. Collinson, J.C. Frye, M.E. Hopkins, J.A. Lineback, and J.A. Simon, 1975, Handbook of Illinois stratigraphy: Illinois State Geological Survey, Bulletin 95, 261 p.
- Willman, H.B., and J.N. Payne, 1942, Geology and mineral resources of the Marseilles, Ottawa, and Streator Quadrangles: Illinois State Geological Survey, Bulletin 66, 388 p., 29 pls.
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