Galatia Channel:Similar Channels Affecting Other Coal Seams

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Similar Channels Affecting Other Coal Seams

Several well-documented paleochannels in the Illinois Basin existed contemporaneously with peat deposits older and younger than the Springfield (Figure 39). The Galatia channel provides an apt model for comparison.

  • Figure 39 Map of the Illinois Basin showing channels and gray shale wedges affecting the Murphysboro, Colchester, Herrin, Baker, and Danville Coals.

Colchester Coal and Francis Creek Shale

Colchester Coal and Francis Creek Shale

Herrin Coal, Energy Shale, and Walshville Channel

Herrin Coal, Energy Shale, and Walshville Channel

Baker Coal and Winslow-Henderson Channel

Baker Coal and Winslow-Henderson Channel

Danville Coal

The Danville (Figures 2 and 40) is the next major coal bed above the Baker. It is the youngest widespread, economically important seam in the Illinois Basin. The Danville is thin and rather patchy in western Illinois, reaching 3.3 ft (1 m) thick in small areas. Coal thick enough to mine (generally 3.9 to 5.9 ft [1.2 to 1.8 m] thick) is largely confined to a belt running north-northwest along the east side of the basin from Gibson County, Indiana, into northern Illinois. Coal-thickness patterns (Korose et al. 2002) indicate that much of the thickest coal has been eroded east of the present outcrop. Most Danville Coal has a high sulfur content, but low-sulfur (locally <0.5%) coal occurs in Knox and Sullivan Counties, Indiana, and in the bordering part of eastern Illinois (Harper and Eggert 1995; Harper 1998; Korose et al. 2002).

  • Figure 2 Correlation chart showing position of key units within the Pennsylvanian Subsystem. Global and provincial stage boundaries and ages in millions of years (Ma) are from Gradstein et al. (2012)
  • Figure 40 Stratigraphic column showing the units mentioned in the section on channels affecting coal seams other than the Springfield.

Overlying the Danville is a complex succession of gray clastic rocks that thickens from less than 3.3 ft (1 m) in parts of western Illinois to as much as 229.7 ft (70 m) in the central Fairfield Basin of southeastern Illinois. Where it is thin, the shale is dark colored and, in places, is black, fissile shale similar to the Excello and Turner Mine Shales. Eastward, the interval changes to gray mudstone, siltstone, and sandstone arranged in multiple upward-coarsening cycles. The coarsest facies occur in Knox County, Indiana, coinciding with the only known area of low-sulfur Danville Coal. Abundant fossil plants, tree stumps in growth position, and tidal rhythmites are much in evidence in underground mines here. Rolls are also common, as are wedge-shaped siltstone splits and a large-scale coal-seam disruption that probably involved peat rafting (Figure 46). However, no channels contemporaneous with the Danville have been encountered.

  • Figure 46 Disruption of the Danville Coal, with the seam “split” by a thick wedge of mudstone. Note the ragged splaying of coal into mudstone, with a thin coal stringer crossing diagonally from the lower to upper bench. The site is the box cut at a portal of the Prosperity Mine in Gibson County, Indiana.

Evidently, Knox County lay near the mouth of a large estuary that discharged sediment onto the Danville peat from the east. Rapid burial under freshwater to slightly brackish conditions resulted in low-sulfur coal. Tidal currents agitated the peat near the estuary mouth, whereas deeper in the basin, the peat was quietly buried under fine mud.

Murphysboro Coal and Oraville Channel

The Murphysboro Coal, in the upper Tradewater Formation, presents some similarities to the Colchester, Springfield, Herrin, and Danville Coals in that thick, low-sulfur coal is associated with a gray shale “wedge” and a contemporaneous channel. However, the Murphysboro is highly lenticular and has been less thoroughly studied than the other examples, so the relationship of coal to gray shale and the channel is not completely understood.

Jacobson (1983) documented that thick, low-sulfur Murphysboro Coal in southwestern Illinois flanks a feature that he named the Oraville channel (Figures 39 and 47). In fact, coal of mineable thickness (up to 8.2 ft [2.5 m]) is confined to a small area near the channel. Sulfur content is low to moderate (1 to 2.5%) near the channel where thick, nonmarine gray mudstone (unnamed) overlies the coal. Elsewhere, the coal is topped by marine black shale and limestone, and its sulfur content is greater than 3% (Jacobson 1983). Away from the Oraville channel, the Murphysboro has a highly patchy distribution (Treworgy and Bargh 1984). Only small, isolated areas of thick coal are known. Little significant mining has taken place away from the channel.

  • Figure 39 Map of the Illinois Basin showing channels and gray shale wedges affecting the Murphysboro, Colchester, Herrin, Baker, and Danville Coals.
  • Figure 47 Map showing the thickness of the Murphysboro Coal near the Oraville channel in Jackson and Perry Counties, southwestern Illinois. From Jacobson (1983).
  • Figure 48 Interpretive cross section of the Oraville channel.

The Murphysboro undergoes dramatic splitting near the Oraville channel margins, where underclay is absent or weakly developed (Figure 48). Upright lycopsid tree stumps are common above the lower coal bench. The gray mudstone exhibits tidal rhythmites and bears a prolific, well-preserved flora dominated by Macroneuropteris scheuchzeri, a plant believed to have been tolerant of coastal, perhaps brackish-water, conditions (Falcon-Lang 2009). The Oraville channel follows the downthrown side of a monocline that was active during deposition of the Murphysboro Coal (Nelson et al. 2011).

Ostensibly, the Oraville is another fluvial channel that became an estuary, like the Galatia and Walshville channels. However, nothing is known of the presumed fluvial “precursor” channel, and the nature of “splitting” coal is poorly understood. Further study of the Murphysboro is required to integrate this unit into a general model.

Primary Source

W. John Nelson, Scott D. Elrick, William A. DiMichele, and Philip R. Ames xxxx, Evolution of a Peat-Contemporaneous Channel: The Galatia Channel, Middle Pennsylvanian, of the Illinois Basin FINISH CITATION