Since the late 1960s, geologists working in the Illinois Basin have recognized that deposits of thick coal having a relatively low sulfur content (less than 2%) are associated with thick, nonmarine gray mudstone and siltstone overlying the coal. Moreover, these thick, gray sediments are associated with paleochannels that existed during peat formation. Channels contemporaneous with the Herrin and Springfield Coals, the two most important economic seams in the basin, have been identified and mapped in detail. Similar relationships involving the Murphysboro, Colchester, Baker, and Danville Coals also have been documented (Treworgy and Jacobson 1979).
Previous authors have explained these relationships by using a model based on the modern Mississippi delta. They envisioned a channel that periodically broke through its natural levees and discharged sediment-laden water into flanking peat swamps. Resulting “crevasse splays” created clastic “splits” within the peat along the channel margins. However, no natural levees have been found along the paleochannels, significant evidence exists for tidal sedimentation, and further research has shown that the Mississippi delta is probably not a good analogue for Pennsylvanian coal deposits. Hence, the model is in need of revision.
This study describes and explains the Galatia channel, one of the best-known examples of a paleochannel contemporaneous with peat accumulation. Such paleochannels yield key insights into the ways eustasy and climate influence sedimentation, and they add complexity to generalized models of cyclic sedimentation. A new model is presented here, which takes in the complete history of development of the Galatia channel and the landscapes of which it was a part. This model is then applied to other paleochannels in the Illinois Basin, and likely can be applied in other basins.
The Illinois Basin, also called the Eastern Interior Basin, covers much of Illinois along with southwestern Indiana and part of western Kentucky in the east-central United States (Figure 1). The Illinois Basin is an interior cratonic basin that developed progressively throughout Paleozoic time (Leighton et al. 1991). During the Pennsylvanian Period, widespread tectonic deformation took place in the Illinois Basin in response to the ancestral Rocky Mountain Orogeny (McBride and Nelson 1998) and perhaps flexural interactions with the Allegheny Orogeny (Quinlan and Beaumont 1984). In Illinois, the La Salle Anticlinorium, Du Quoin Monocline, Salem and Louden Anticlines, and numerous smaller structures all were active during the Pennsylvanian. In fact, the Springfield Coal thins where it crosses the Louden Anticline and southern part of the La Salle Anticlinorium (Plate 1), evidence that these structures were rising during peat accumulation.
On a global scale, assembly of the supercontinent of Pangaea was well underway by the Middle Pennsylvanian. Southwest of the Illinois Basin, plate collision had closed off the Arkoma Basin, and the Ouachita Mountains were rising (Houseknecht 1983). Tectonic activity was widespread throughout the Midcontinent, including the Illinois Basin (McBride and Nelson 1998). Plate reconstructions show the Illinois Basin close to, or slightly south of, the equator (Scotese 2010; Blakey 2011).
The Springfield Coal MemberClassification differs among the three state geological surveys. Indiana and Illinois regard the Springfield as a formal member, whereas Kentucky classifies all coals informally as beds</ref> of the Carbondale Formation is of late Desmoinesian age (Figure 2), which is equivalent to Asturian (Westphalian D) of Western Europe and to late Moscovian on the global time scale (Davydov et al. 2012). It is informally known as No. 5 Coal in Illinois, Coal V in Indiana, and No. 9 Coal in western Kentucky. The Springfield is correlative with the Summit Coal of the Western Interior Basin and with the Middle Kittanning coal bed of the northern Appalachian Basin on the basis of physical stratigraphy (Wanless 1939), palynology of coal (Peppers 1996), and conodonts (Heckel 2009) and ammonoids (Work et al. 2009) in associated marine rocks.
The Springfield accounts for about 29% of remaining identified Illinois Basin resources and has been the most extensively mined coal seam in the Illinois Basin (Hatch and Affolter 2002). The coal is high-volatile bituminous in rank and generally is bright-banded, having well-developed cleat and lacking significant clastic partings. Thickness varies from about 3.9 to 4.9 ft (1.2 to 1.5 m) in most areas where the coal has been mined. Thicker coal, locally exceeding 9.8 ft (3 m), is confined to the flanks of the Galatia channel.
Nelson, W.J., S.D. Elrick, W.A. DiMichele, and P.R. Ames, 2020, Evolution of a peat-contemporaneous channel: The Galatia channel, Middle Pennsylvanian, of the Illinois Basin: Illinois State Geological Survey, Circular 605, 85 p., 6 pls.
- Treworgy, C.G., and R.J. Jacobson, 1979, Paleoenvironments and distribution of low-sulfur coal in Illinois, in A.T. Cross, ed., Economic geology: Coal, oil and gas: Ninth International Congress on Carboniferous