St. David Limestone Member
Nelson, W.J., P.H. Heckel and J.M. Obrad, 2022, Pennsylvanian Subsystem in Illinois: Illinois State Geological Survey, Bulletin (in press).
Savage (1927) showed the St. David Limestone on a stratigraphic column for Fulton County, Illinois. Wanless (1931) discussed the “St. David cyclical formation” and again illustrated the unit on a graphic column. Wanless (1939) summarized the regional features and correlation of the St. David Limestone.
St. David is a town (population of about 600) located 5 mi (8 km) south of Canton in Fulton County, Illinois. Welsh coal miners who settled in 1865 named the community for the patron saint of Wales (Callary 2009).
The name “Periwinkle shale” (or “Penniwinkle shale”) is widely used in western Kentucky, where this unit is typically reduced to thin shale containing gastropods and other marine fossils. The Indiana Geological Survey has long used the name Alum Cave Limestone Member for this unit, but the name Alum Cave is hereby abandoned because St. David has priority both by first usage and by the first formal publication with a stratotype.
The section is in a ravine sloping southeast to Big Creek in the SE¼ SE¼ of sec. 17, NE¼ NE¼ of sec. 20, and NW¼ NW¼ of sec. 21, T 6 N, R 4 E, Fulton County, western Illinois. The section spans strata from the Canton Shale at the top to the Pleasantview Sandstone at the base.
Wanless (1957, p. 197, geologic section 21).
Soil Conservation Service borehole B-41 was drilled in sec. 27, T 9 S, R 4 E, Williamson County, Illinois. The St. David Limestone is 1.7 ft (52 cm) thick and occurs in the depth interval from 51.0 to 52.7 ft (Figure 4-60). This core doubles as the type section of the Turner Mine Shale Member, which underlies the St. David.
Core from B-41 is archived at the ISGS Geological Samples Library under call number C-12753. Logs from this and other cores in the same series are on file at the ISGS Geological Records Unit and can be viewed on the ISGS website.
Except where cut out in paleochannels, the St. David overlies the Turner Mine and underlies the Canton Shale Member. It is rarely in contact with other units.
Extent and thickness
The St. David is practically coextensive with the Springfield Coal. In the Fairfield Basin and most of western Kentucky, the limestone is less than 1 ft (30 cm) thick and consists of dark gray, argillaceous, fossiliferous lime mudstone to wackestone. On the Eastern Shelf in Indiana, the limestone is as thick as 11.8 ft (3.6 m), but more commonly 1 to 4 ft (0.3 to 1.2 m), in two layers separated by thin shale (Wier 1961; Shaver et al. 1986). In northwestern Illinois, the unit is normally a few inches to 2 ft (~10 to 60 cm) thick but locally reaches 7 ft (2.1 m; Wanless 1957). Thicker St. David, lighter colored and less argillaceous than that found in the basin, also occurs on areas of the Western Shelf where the Springfield Coal is thick enough to mine. Locally, the St. David nearly merges with the younger Percy Limestone. The overall pattern, in common with several Middle Pennsylvanian limestone units, is that the St. David tends to be thicker around the margins of the Illinois Basin than in the interior.
The St. David is composed largely of medium to dark gray, argillaceous, fossiliferous lime mudstone to skeletal wackestone. The structure varies from massive to indistinctly bedded or nodular. The clay content tends to increase toward the top and base. Chiefly in Indiana, the member may comprise two or more beds of limestone alternating with calcareous shale. Where the unit is thinnest, as in Kentucky, limestone grades to calcareous shale.
Both contacts are conformable. Most commonly, they are sharp, but either or both may be gradational. The lower contact tends to be knobby and the upper one more nearly planar.
Well log characteristics
Where the unit is thick enough to register, the log response is typical for limestone.
The St. David contains an extensive marine fauna dominated by brachiopods and mollusks. Savage (1921) and Wanless (1958) published lengthy lists; the latter counted 98 species. The brachiopods Marginifera splendens, Mesolobus mesolobus, Chonetina verneuliana(?), and Dictyoclostus americanus are characteristic of the St. David in northwestern Illinois, along with the nautiloid Liroceras sp. on the upper surface (Wanless 1957). From Indiana outcrops, Wier (1961) listed the same brachiopods and also Composita subtilita, Derbyia crassa, Dictyoclostus portlockensis, Marginifera muricatina, Neospirifer cameratus, and Squamularia perplexa together with fusulinids, gastropods, pelecypods, crinoids, and ostracods.
Age and correlation
Although richly fossiliferous, the St. David contains few forms suitable for zonation. Ammonoids and conodonts from the Turner Mine Shale and spores from the Springfield Coal provide tighter age constraints. The Houx Limestone (Cline 1941) in the Midcontinent is correlative with the St. David Limestone (Weller et al. 1942).
Environments of deposition
As noted above, the St. David carries a normal marine fauna. Shallow water on the margins of the basin was more conducive to limestone deposition than was the deeper Fairfield Basin.
This unit may have been quarried informally for building stone. Where thick, it is a competent member in the roofs of underground coal mines.
- Callary, E., 2009, Place names of Illinois: Urbana, University of Illinois Press, 425 p.
- Cline, L.M., 1941, Traverse of upper Des Moines and lower Missouri Series from Jackson County, Missouri to Appanoose County, Iowa: American Association of Petroleum Geologists Bulletin, v. 25, p. 23–72.
- Nelson, W.J., 1983a, The Turner Mine Shale Member: A newly named stratigraphic unit of the Carbondale Formation, in R.J. Jacobson, ed., Geologic Notes: Illinois State Geological Survey, Circular 529, p. 11–12.
- 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.
- Shaver, R.H., C. H. Ault, A.M. Burger, D.D. Carr, J.B. Droste, D.L. Eggert, H.H. Gray, D. Harper, N.R. Hasenmueller, W.A. Hasenmueller, A.S. Horowitz, H.C. Hutchison, B. Keith, S.J. Keller, J.B. Patton, C.B. Rexroad, and C.E. Wier, 1986, Compendium of Paleozoic rock-unit stratigraphy in Indiana—A revision: Indiana Geological Survey, Bulletin 59, 203 p.
- Wanless, H.R., 1931, Pennsylvanian cycles in western Illinois: Illinois State Geological Survey, Bulletin 60, p. 179–193.
- Wanless, H.R., 1939, Pennsylvanian correlations in the Eastern Interior and Appalachian coal fields: Geological Society of America, Special Paper 17, 130 p.
- Wanless, H.R., 1957, Geology and mineral resources of the Beardstown, Glasford, Havana, and Vermont Quadrangles: Illinois State Geological Survey, Bulletin 82, 233 p.
- Wanless, H.R., 1958, Pennsylvanian faunas of the Beardstown, Glasford, Havana, and Vermont Quadrangles: Illinois State Geological Survey, Report of Investigations 205, 59 p.
- Weller, J.M., H.R. Wanless, L.M. Cline, and D.G. Stookey, 1942, Interbasin Pennsylvanian correlations, Illinois and Iowa: American Association of Petroleum Geologists Bulletin, v. 26, p. 1585–1593.
- Wier, C.E. 1961, Stratigraphy of the Carbondale and McLeansboro Groups in southwestern Indiana: Indiana Geological Survey, unpublished bulletin, 147 p. and unnumbered appendix, http://hdl.handle.net/2022/3612 (accessed October 5, 2020).
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