Unnamed clastic member - Carbondale Formation

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Lithostratigraphy: Carbondale Formation >>Unnamed clastic 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


Pending further investigation, no formal name is proposed. This unit is unnamed in the Midcontinent.

Type section


Reference section


Stratigraphic relationships

This unit comprises shale, mudstone, siltstone, and sandstone between the Wheeler (older) and Bevier Coal Beds, which together comprise the Survant Coal Member. Where either coal bed is absent, the clastic unit cannot be differentiated from those above or below, or both.

Extent and thickness

The interval has not been mapped in Illinois. A survey of core records and unpublished cross sections shows a thickness range from 1 to 2 in. (3 to 5 cm) to a maximum of 40 ft (12 m). The unit is widely distributed in the range of 15 to 30 ft (4.5 to 9 m) thick. Local thickness changes can be abrupt. “Merged” and “split” Survant are encountered in widely separated areas of the basin. Available data give the impression of an intricate isopach pattern with no obvious relationship to tectonic features.

Several Indiana Geological Survey publications allude to the “rock parting” or “split” that impeded underground mining in the Survant Coal. Where the Survant was a single seam, it carried a “rock layer” as thin as ¼ inch (6 mm) near the middle or slightly below the middle of the seam. This abruptly thickened to a “split” many feet (meters) thick. North of Terre Haute, the boundary of the “split” area ran north–south and the “split” thickened westward to at least 28 ft (8.5 m). To the south in Sullivan and Greene Counties, a north–south tract of “split” coal averaging about 2 mi (3 km) wide separated two areas of united Survant Coal. South of Vincennes, the Survant is consistently “split” into two seams, again with no apparent relation to structure (Harper 1985, 1988a, 1988b; Friedman 1989; Eggert 1994; Harper and Eggert 1995).

Wright (1965, plate 20) presented an isopach map of this unit in Missouri, southern Iowa, and extreme northeastern Kansas. She reported a maximum thickness of 79 ft (24 m). Wright further remarked that the clastic unit extends farther south in Kansas but cannot be defined because the Bevier Coal is absent there. Where mapped, the interval thickens toward the south.


Where the unit is thin, it consists of dark-colored, carbonaceous shale or claystone that can be massive to fissile. Plant remains are abundant, including roots, stems, and foliage. As the unit thickens to more than about 5 ft (1.5 m), it takes on an upward-coarsening profile, but sandstone seems to be confined to areas where the unit is thicker than 15 to 20 ft (4.5 to 6 m). At the base is thin (1 to 2 ft, 30 to 60 cm) shale that is dark gray to black and fissile but not slaty. This shale contains a brackish-water fauna (see below) and rarely, fully marine fossils. Upward, the interval grades to gray silty shale, siltstone, and shaly sandstone that exhibits burrows and tidal rhythmites. The underclay of the Bevier Coal is at the top of the succession. Better developed than the Wheeler underclay, this is typically 1 to 3 ft (30 to 90 cm) thick and comprises blocky to massive slickensided mudstone to siltstone containing abundant plant roots and, in the lower portion, siderite nodules.

Incised valleys, filled largely with sandstone, locally cut down from near the base of the Bevier Coal. These valleys truncate the Wheeler coal position and, in places, cut nearly as deeply as the Colchester Coal. Examples were observed on subsurface cross sections in Edwards and Wabash Counties, Illinois. Across the border in Gibson County, Indiana, Eggert (1994, figure 6) mapped a “fluvial sandstone-filled channel associated with the Survant Coal” that is about 1 to 6 mi (2 to 10 km) wide and, in places, deeper than 75 ft (23 m). Eggert (1994) considered the channel contemporaneous with the upper (Bevier) bench of the Survant, but his electric-log cross section (plate 1) indicates that more likely, the channel is older than the Bevier and younger than the Wheeler.




Bounded by the Wheeler and Bevier Coal Beds or their horizons, both contacts are sharp and readily identified on well logs. However, where one or both coal beds are missing, this interval cannot be differentiated from the Purington Shale below and the Lagonda Member above.

Well log characteristics

Bounded by the Wheeler and Bevier Coal Beds or their horizons, both contacts are sharp and readily identified on well logs. However, where one or both coal beds are missing, this interval cannot be differentiated from the Purington Shale below and the Lagonda Member above.


As observed in drill cores, plant foliage, stems, and roots are plentiful where the interval is thin. The basal dark shale in thicker successions yields Lingula, Orbiculoidea, pectenoid and mytiloid bivalves, ostracods, and conchostracans(?). Marine bioclasts, such as echinoderm fragments and articulate brachiopods, rarely occur in a thin basal calcareous zone.

Age and correlation

Known fossils in the clastic unit are long ranging. The age and correlation of the enclosing Survant Coal Member are well constrained by palynology.

Environments of deposition

Given its regional continuity (Kansas to Indiana), the unnamed clastic unit apparently reflects a minor eustatic transgressive–regressive cycle. The Wheeler peat apparently developed on a rolling, uneven surface. Upon transgression, low areas were flooded, higher areas were barely submerged, and much of the Wheeler peat was eroded and removed. The water depth was never great enough (perhaps 32.8 to 49.2 ft, 10 to 15 m) to establish the stratified water column needed for “hot” phosphatic black shale, and Illinois was close enough to land areas that the water generally remained brackish. By late regression, depressions were largely filled with sediment and soil development commenced, providing a nearly level substrate for development of the Bevier Coal Bed.

Economic importance




  • Eggert, D.L., 1994, Coal resources of Gibson County, Indiana: Indiana Geological Survey, Special Report 50, 36 p. and 1 pl.
  • Friedman, S.A., 1989, Geology and coal deposits of the Clinton area, west-central Indiana: Indiana Geological Survey, Special Report 42, 49 p, 3 tables, and 5 pls.
  • Harper, D., 1985, Coal mining in Vigo County, Indiana: Indiana Geological Survey, Special Report 34, 67 p.
  • Harper, D., 1988a, Underground mines in the Survant Coal Member (Pennsylvanian) of Indiana; Indiana Geological Survey, Special Report 41, 19 p.
  • Harper, D., 1988b, Coal mining in Sullivan County, Indiana: Indiana Geological Survey, Special Report 43, 48 p.
  • Harper, D., and D.L. Eggert, 1995, Coal mining in Knox County, Indiana: Indiana Geological Survey, Special Report 54, 23 p.
  • Wright, C.R., 1965, Environmental mapping of the beds of the Liverpool cyclothem in the Illinois Basin and equivalent strata of the northern mid-continent region: Urbana, University of Illinois, Ph.D. thesis, 100 p.

ISGS Codes

Stratigraphic Code Geo Unit Designation