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Thickness and distribution
Many authors have mapped this major coal deposit. Comprehensive reports by Treworgy and Bargh (1984) and Treworgy et al. (2000) include statewide maps at 1:500,000 scale. Hatch and Affolter (2002) published (in digital form) a Springfield thickness map that covers the entire Illinois Basin.
Two large regions of thick Springfield Coal have been mapped (Figure 10). The larger of the two covers nearly all of the Fairfield Basin in Illinois, along with practically all of Indiana and western Kentucky within the coal outcrop. The coal is 39.4 in. (100 cm) or thicker across about 80% of this region, and it is consistently 47.2 to 59.1 in. (120 to 150 cm) across large areas. The thickest coal, locally exceeding 118.1 in. (300 cm), is found close to the margins of the Galatia channel. Coal thicker than 70.9 in. (180 cm) flanks the channel almost continuously in both Indiana and Illinois.
The second area of thick coal encompasses the part of north-central Illinois roughly bounded by Springfield, Decatur, Bloomington, and Peoria. Where it has been mined, the coal is generally 47.2 to 70.9 in. (120 to 180 cm) thick. The coal is thinner than 11.8 in. (30 cm) on most of the Western Shelf except a small area near the outcrop in Perry and Randolph Counties (Figure 10).
Tectonic influence on coal thickness is evident. Not only the coal, but also the interval between the Springfield and Herrin Coals thins abruptly crossing the Du Quoin Monocline from east to west. The Springfield also thins markedly across the Louden Anticline and La Salle Anticlinorium, but not the Salem Anticline.
Focusing on the Galatia channel, coal thicker than 65.8 in. (167 cm) closely corresponds to the limits of the precursor channel (Figure 10, Plate 1). The floodplain atop the old meander belt clearly was conducive to forming and preserving peat. It is likely that peat formation commenced earlier on this flood plain than on adjacent higher ground.
Two narrow, sinuous belts of thick coal that intersect the main belt from the northwest in Hamilton County (Plate 1) probably represent small tributaries that joined the precursor channel. Potter’s 1962 and 1963 maps do not show these features because Potter mapped sandstone thickness, not channels.
Hopkins (1968) mapped several areas of “thin and split” coal flanking the Galatia channel. Details of these areas are rather elusive because mines do not enter them and few cores have been drilled. One area where core drilling confirms abnormally thin coal is east of the channel in T8S, R6E, Saline County. Here, coal progressively thins from the edges inward, and is absent or reduced to isolated stringers in the center. As the coal thins, the upper part becomes shaly, grading into gray shale above. Another such area, in western White County (Figure 12), has the arcuate shape of an oxbow lake.
These observations suggest that thin coal near the channel represents low-lying areas where standing water inhibited plant growth and peat production. Many such areas probably were meanders abandoned shortly before the onset of peat accumulation.
Shaly coal bordering channel
Bordering the entire length of the Galatia channel on both sides are belts of shaly coal several hundred feet (meters) wide. These exhibit a gradual lateral transition from coal without clastic layers to interlaminated coal and shale (Figure 11). Clastic layers steadily increase in number and thickness toward the channel, although details differ from one place to another.
At American Coal’s Galatia Mine, shale laminae first appear at the top of the seam. Approaching the channel, more and more shale layers appear, reducing the height of salable coal. Near the channel border is less than 0.5 m (1.6 ft) of shale-free coal at the base of the seam, resting on underclay. Shale laminae are dark gray to black and loaded with carbonized plant remains, grading to dull or “bone” coal alternating with vitrain (bright coal) in laminae a few millimeters thick. Lamination is highly tabular, and the transition from bright coal to dull coal to shale is very gradual.
Cady (1919) wrote of the old Galatia Colliery, The lower 6 inches to nearly 3 feet [15 to 90 cm] of the coal contains layers of carbonaceous shale or “bone” that render that part of the bed unmarketable. The middle of the bed is generally fairly clean for a thickness of 3 to 5 feet [90 to 150 cm]. The upper part of the bed is again interbedded with shale, the partings increasing in number and thickness to the top of the bed, which in this mine is about 6 feet [180 cm] thick. The actual position of the top of the bed is rather difficult to ascertain because stringers of coal apparently leading out from the coal bed can be traced to as much as 5 or 6 feet [150 to 180 cm] above the coal, and in places possibly as much as 10 feet [300 cm]. This shale contains a large amount of organic material, and impressions of leaves and stems are exceedingly numerous in the roof of the entries” (p. 51). Similar conditions were encountered elsewhere in Saline County along both margins of the channel. Mines where shaly coal was encountered during the early to middle 20th century include the Galatia Colliery, Peabody No. 47, Sahara No. 16, and Sahara No. 14 west of the channel and Peabody No. 43 and Sahara No. 9 east of the channel. Several cored test holes also record shaly coal fringing the channel. In most cases, the shale partings are most numerous at the top of the seam, but in some places, they occur in the lower part also. Where records are available, shaly coal rested on typical underclay [ISGS field notes, open files].
The transition from coal to shale at channel’s edge was formerly well exposed in the Prosperity Mine of Five Star Mining Company in Pike County, Indiana. In one area within 984 ft (300 m) of the channel, the seam was more than 3 m thick and shale laminae were confined to the uppermost 7.9 in. (20 cm). In another area of the mine, shale laminae appeared first near the base of the seam and made a vertical transition between weakly fissile, rooted shale below through shaly coal to normal coal above. The shaly zone thickened gradually toward the channel.
Belts of shaly coal record constant infiltration of water bearing fine suspended sediment into the peat swamp bordering the open-water channel. It is interesting that in the 47 years since Hopkins proposed the crevasse-splay model, no natural levees have been found. The margins of the channel have been densely core-drilled because of the thick low-sulfur coal adjacent to the channel and for engineering design in underground mines. During our extensive studies in active mines and those by other geologists, no levee facies have been encountered in cores anywhere along the length of the channel. Being subaerial features, natural levees would display intensive rooting, burrowing, and probably evidence of weathering. Clastic layers within and above the Springfield Coal do not contain such features. Therefore, we surmise that vegetation flourished in standing water up to the margins of the flowing Galatia river. A buffer zone of clastic (non-peat-accumulating) wetlands lay between the channel and the peat. The plants and their interlocking roots filtered out fine clastic sediment derived from the channel. In addition, changes in acidity from peat mire through clastic wetlands to the active channel may have caused clays to flocculate along the channel margin rather than in the peat itself, following a model that has been proposed for other low-ash, low-sulfur coal deposits (Staub and Cohen 1979).
Relationship of coal to channel
The path of the Galatia channel where the Springfield Coal is missing is consistently about 0.6 mi (1 km) wide and describes a series of broad, simple, open meanders without significant cross-cutting of meanders or abandoned meanders (Plate 1). The lack of complexity indicates that the river did not migrate laterally while peat was forming or subsequent to peat formation. Evidently, interlocking roots of growing plants and tough, matted peat stabilized its banks. The river course, which freely migrated before peat began to form, was locked into place throughout the time of Springfield peat accumulation. In a similar fashion, modern domed peat deposits have partially encroached on an infilled valley that was incised during Late Pleistocene lowstand on the Rajang River delta of Sarawak, East Malaysia. One distributary, the Lassa, has been abandoned and overtopped with peat. Maps of coastal Sarawak show river bends, and in some cases individual meanders “locked” into place by peat deposits (Staub and Esterle 1993, 1994; Staub and Gastaldo 2003).
Previous authors generally depicted the Galatia channel as filled with sandstone. Drill holes that penetrate the channel encounter a variety of clastic rocks, ranging from shale and claystone to siltstone, sandstone, and conglomerate. Because the Springfield is absent, ascertaining which part of the channel-fill was deposited while peat formed is not easy. By far, the best record is from the Galatia underground mine in Saline County, Illinois. To facilitate underground haulage and ventilation, the company drove a set of mine entries completely across the channel. A detailed profile (Figure 12) is based on exposures in these headings, combined with closely spaced cores that were drilled to plan the channel crossing. In the heart of the channel, rhythmically laminated siltstone to very fine sandstone of the Dykersburg Member rests on dark, carbonaceous shale and claystone of the Galatia Member with an erosional contact. Near the north end of the crossing, the carbonaceous shale and claystone grade laterally into shaly Springfield Coal. The erosive contact between Dykersburg siltstone and Galatia dark shale was observed both in the mine and in drill cores (Figure 13).
These observations are consistent with the fact that along the entire length of the Galatia, the coal intergrades with shale or claystone rather than siltstone and sandstone. Together, these findings indicate that during the time of peat formation, the Galatia river carried mostly (and possibly only) clay, rafted bits of peat and vegetation, and finely dispersed organic matter. This light sediment load contrasts with the heavy bed load of sand in the “precursor” channel prior to peat formation. Thus, the Galatia during the time of peat accumulation may have been a “black-water stream” similar to many found in modern ever-wet, densely vegetated tropical wetlands (Cecil et al. 2003). Modern black-water rivers are restricted to regions having an ever-wet or perhumid climate, meaning that rainfall exceeds evapotranspiration the year around. Such a climate promotes lush vegetation growth, which stabilizes the landscape and inhibits soil erosion. In contrast, soil erosion and the fluvial sediment load peak under a monsoonal regime having prolonged alternating wet and dry seasons. A long annual dry season leaves much of the ground unprotected from severe gullying during the rainy season (Cecil 1990; Cecil and Dulong 1993; Cecil et al. 1993; Staub and Esterle 1993, 1994; Staub and Gastaldo 1993). Therefore, climate change from a seasonally dry or monsoonal regime to ever-wet conditions likely heralded Springfield peat formation.
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