Galatia Channel:Herrin Coal, Energy Shale, and Walshville Channel
Herrin Coal, Energy Shale, and Walshville Channel
The gray, nonmarine Energy Shale and Walshville channel (Figures 42 and 43) are close analogues of the Dykersburg Shale and Galatia channel. Belts of thick Herrin Coal (6.6 to locally more than 13.1 ft [2 to 4 m]) flank the channel, and shaly or “split” coal flanks channel margins. Away from the channel, the Herrin has a black shale–limestone roof and uniformly high sulfur content. Near the channel, wedges of the gray Energy Shale intervene between coal and black shale. The sulfur content of the coal is moderate to low (0.5 to 2.5%) where the Energy Shale is thicker than about 19.7 ft (6 m; Johnson 1972; Allgaier and Hopkins 1975; Treworgy and Bargh 1984; Nelson et al. 1987).
The Energy Shale consists of gray mudstone, siltstone, and sandstone as thick as 121.4 ft (37 m). It is thickest and coarsest near the channel, becoming thin and lenticular at its outer limits. The sparse fauna comprises brackish-water forms such as Lingula, Dunbarella, Myalina, the brachiopod Leaia tricarinata, and rare cephalopods and eurypterids. Fossil land plants are abundant and well preserved, including stands of upright stumps (mainly lycopsids) rooted at the top of the coal (DiMichele and DeMaris 1987; DiMichele et al. 2007). Locally, the Energy Shale contains siderite concretions, similar to those of Mazon Creek, containing fossils of plants and soft-bodied invertebrates (Gastaldo 1977; Baird et al. 1985). Shared with the Dykersburg are features including tidal rhythmites (Archer and Kvale 1993; DiMichele et al. 2007), a variety of rolls (Edwards et al. 1979; Krausse et al. 1979; Bauer and DeMaris 1982; Nelson 1983), and a major coal disruption that suggests large-scale rafting of peat (Nelson 1983, p. 21–23). Away from the Walshville channel, the Energy Shale is eroded into pods beneath the black, fissile, marine Anna Shale. The Anna–Energy contact is clearly erosional, having locally more than 16.4 ft (5 m) of relief with bedding of the Energy Shale truncated at a 20-degree angle. Finally, the Anna and overlying marine Brereton Limestone onlap and pinch out against the tidally deposited, gray siltstone and sandstones that fill the core of the Walshville channel (Bauer and DeMaris 1982; DeMaris et al. 1983; DeMaris 2000). The Turner Mine Shale and St. David Limestone pinch out in similar fashion where the Dykersburg Member is thick.
Although no precursor Walshville channel has been mapped, core drilling in several areas confirms a deeply incised, sandstone-dominated valley fill below the belt of thick Herrin Coal. As with the Galatia, the precursor Walshville channel is considerably wider than the contemporaneous portion (as defined by the absence of Herrin Coal). Moreover, the Walshville channel, like the Galatia, exhibits looping meanders that did not migrate laterally.
Authors through the 1980s attributed the Energy Shale, like the Dykersburg, to crevasse splays and related environments in a river-dominated deltaic setting similar to the modern Mississippi (Johnson 1972; Allgaier and Hopkins 1975; Nelson 1983; Palmer et al. 1985; Treworgy and Jacobson 1985; Burk et al. 1987). Archer and Kvale (1993) pointed out flaws in this model, including the absence of natural levees, the absence of unequivocal evidence for crevasse splays, and the presence of a strong tidal signature. Their model calls for a mud-dominated, tropical estuary or tidal delta having a small to moderate tidal range.
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., C.P. Korose, C.A. Chenoweth, and D.L. North, 2000, Availability of the Springfield Coal for mining in Illinois, map summary of Illinois Minerals 118: Illinois State Geological Survey, 1 sheet, 1:500,000.