This section describes, in ascending order, the rock units that enclose the Springfield Coal and Galatia paleochannel (Figure 4). To facilitate discussion of the new depositional model, two new members are proposed.
Houchin Creek Coal
Excello Shale Member
Hanover Limestone Member
Delafield Member (New)
Galatia Member (New)
Underclay of Springfield Coal
Like most coal beds in the Illinois Basin, the Springfield normally is underlain by nonfissile mudstone that miners call underclay. Features such as roots, slickensides, microscopic structure, and secondary carbonate nodules demonstrate that the underclay is a paleosol: the product of overprinting by soil development in previously deposited sediment. A paleosol is not a lithostratigraphic unit. From the stratigraphic point of view, the Springfield underclay is considered part of the Delafield Member, except close to the Galatia channel, where the underclay is part of the Galatia Member.
Where the underclay is part of the Delafield Member, it is generally 4.9 to 9.8 ft (1.5 to 3 m) thick and in places reaches 13.1 ft (4 m). Black, organic-rich claystone commonly occurs at the top. The remainder is olive-gray to greenish-gray claystone that grades downward to siltstone or silty mudstone (Figure 9). All except the uppermost 1.0 to 2.0 ft (0.3 to 0.6 m) is generally calcareous and contains irregular nodules of limestone or dolomite. A solid layer of carbonate rock as thick as 3.3 ft (1 m) occurs in places. This is microgranular, argillaceous to silty, and lacks fossils other than root traces. The lower part of the paleosol is commonly riddled with irregular vertical fractures lined with siderite, calcite, and dolomite. These soils have characteristics of Calcisols and Vertisols.
Where underclay belongs to the Galatia Member, it is rarely thicker than 3.3 ft (1 m) thick and less strongly developed than in the Delafield Member. Root traces, slickensides, and granules or nodules of siderite are present, but carbonate nodules are rare and the rock is not calcareous. In places close to the Galatia channel, no paleosol is present, and the Springfield Coal grades downward to laminated, carbonaceous shale. Such soils have characteristics of Protosols.
The Springfield underclay exhibits a complex climatic history. Detailed examination (Rosenau et al. 2013) indicates that the soil formed mostly under a seasonally dry climatic regime, leading to vertic features. Late in its history, the soil underwent pronounced gleying, suggesting a change toward a much wetter climate. In places, the top of the paleosol grades into the base of the coal through a transitional zone of organic-rich shale, suggesting the development of clastic swamps prior to onset of peat formation.
Because the area away from the channel was topographically higher and better drained, soils developed during the entire interval of time while the Galatia precursor channel was being eroded and backfilled. Calcite nodules and caliche layers require subhumid to semiarid climate, strongly seasonal with distinct wet and dry seasons (Cecil and Dulong 2003). In contrast, soil formation within the Galatia meander belt was frequently interrupted by lateral channel shifts. This low-lying, poorly drained, and frequently disturbed area did not permit long-term pedogenesis or caliche development. The youngest landscape, along the riverbanks, saw little or no soil development.
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.
Dykersburg (Shale) Member
Hopkins (1968) named the Dykersburg Shale (Figure 4).
as the unit of gray clastic strata that lies between the Springfield Coal and Turner Mine Shale. As discussed here, the Dykersburg comprises a variety of rock types, ranging from true fissile shale to weakly fissile, laminated, and massive mudstone, siltstone, and sandstone. On this basis, the name “Dykersburg Member” seems more appropriate than “Dykersburg Shale.” However, the latter term has been used by nearly all previous authors.
Chronologically, the Dykersburg is younger than the Springfield Coal, and younger than the Galatia Member and the channels it fills. Cores and exposures in underground mines show the Dykersburg in erosive contact with the Galatia Member. However, the Galatia and Dykersburg Members contain similar rock types and are difficult to distinguish in well logs other than cores. In addition, wedges of Dykersburg locally split the Springfield Coal and separate the coal from its underclay. This is a seeming paradox—a younger unit within and beneath an older one. The unusual processes that caused this anomaly are addressed later in this report.
Thickness and distribution
The Dykersburg is closely associated with the Galatia channel. As mapped by Hopkins (1968) and Eggert (1994), the Dykersburg occupies an irregular southwest-trending belt along the Galatia channel (Figure 14). This belt widens from about 9.3 to 12.4 mi (15 to 20 km) in Indiana to nearly 31.1 mi (50 km) in southern Illinois. Distribution of the shale relative to the channel is not symmetrical. Most of the thick shale is southeast of the channel in Indiana, whereas thick shale in southern Illinois is largely northwest of the channel. In places, the Dykersburg is missing right up to the channel margin. Maximum thickness of the Dykersburg increases from about 14.9 mi (24 m) in Indiana to as much as 23.6 mi (38 m) in Saline County, Illinois.
Beyond areas of thick, continuous Dykersburg, the member occurs as isolated pods and lenses that are typically 3.3 to 9.8 ft (1 to 3 m) thick and several tens of feet (meters) to roughly 328.1 ft (100 m) across. These are known mostly from exposures in underground mines. Including these lenses, the width of the Dykersburg tract is 49.7 to 55.9 mi (80 to 90 km) along the southern outcrop. Extent of Dykersburg lenses farther north is poorly known because less underground mining has taken place there.
Medium to medium-dark gray silty mudstone and fine- to coarse-grained siltstone are the most prevalent rock types. Mudstone generally shows bedding or lamination but is weakly to moderately fissile. Much of the siltstone is nearly massive. Sandstone becomes common where the Dykersburg is thickest. The sandstone is light gray and very fine to fine-grained, occurring as laminae, lenses, interbeds, and larger lens-shaped bodies. Volumetrically minor, dark gray fissile clay-shale (no silt) is mostly confined to the basal part of the Dykersburg, in contact with the Springfield Coal.
Regular bands, nodules, and small concretions of siderite are common in finer grained facies of the Dykersburg. Small pyrite nodules occur in dark, carbonaceous shale, especially near the base of the member.
Sedimentary structures are mostly small scale. Planar, wavy, ripple, flaser, and cross-lamination are most common. Small load casts, ball-and-pillow structures, and deformed or contorted lamination (soft-sediment deformation) also are commonly seen. No cross-bedding in sets thicker than a few centimeters has been encountered.
Much of the lamination in the Dykersburg is strongly rhythmic. Specifically, bundles of thin clay-rich laminae alternate with bundles of thicker silt- or sand-rich laminae (Figures 15 and 16). This style of lamination is diagnostic of tidal settings with neap and spring tidal cycles (Archer and Kvale 1993; Archer et al. 1994, 1995). We have observed examples of tidal rhythmites through most of the geographic extent of the Dykersburg and at various levels in vertical profiles of the unit. The only places where rhythmites are not developed are at the thin edges of the Dykersburg, where the member is reduced to isolated lenses.
Large-scale (feet) inclined bedding has been encountered in underground mines close to the Galatia channel. Bedding of the Dykersburg intersects the top of the coal at angles as steep as 30°, although 15° to 20° is more usual (Figure 16). Such a structure signifies the lateral accretion of wedges of sediment.
Although sandstone occurs mostly in the upper part of the Dykersburg, it locally is found directly on the Springfield Coal. Sandstone bodies are lens shaped in cross section. Their lower contacts are erosive, truncating underlying shale and forming “rolls” in the coal (see below). Some sandstone bodies are convex upward, grading laterally to siltstone and mudstone.
Fossil land plants are widely distributed. They are profuse and beautifully preserved in the basal layers near the Galatia channel, where the Dykersburg is thickest. Fossils are preserved as compressions; that is, they are carbonized or coalified and stand out against the gray shale matrix (Figure 17). In many areas near the channel, fossil tree stumps occur in growth position (Figure 18). Invariably, these are rooted in the top of the Springfield Coal. Rare Stigmaria roots, with rootlets attached, have been found in the plant-rich basal part of the shale, suggesting that a few trees were able to grow during initial stages of Dykersburg deposition.
The only invertebrate fossils are the inarticulate brachiopod Lingula and pectenoid pelecypods, such as Dunbarella. Indicative of brackish water, these forms occur chiefly on the outer margins of the Dykersburg. Trace fossils are uncommon. The few examples we have seen are mostly from cores, in the upper part of the member. They are mostly vertical and inclined burrows, some, such as Teichichnus, with gutter stacking. Horizontal burrows are rare; no trails or feeding traces have been observed. Root traces are nonexistent, except at the top of the Dykersburg directly beneath the Briar Hill Coal.
Miners apply the term “rolls” to bodies of shale, siltstone, and sandstone at or near the top of a coal seam. Rolls filled with Dykersburg Shale are prevalent close to the Galatia channel, where the Dykersburg is thick and relatively coarse grained (siltstone, interlaminated shale and sandstone). They are filled with rock identical to that overlying the coal at the same site. In map view, rolls are elongate and straight to sinuous, locally branching. Rolls commonly occur in parallel sets or swarms. In cross section, they are roughly lens shaped, with “riders” or stringers of coal overlapping one or both margins (Figure 19). Widths range from 3.28 to 32.8 ft (1 to 10 m), lengths from tens to hundreds of feet (meters). Similar rolls disrupt the Herrin Coal elsewhere in west-central and southern Illinois (Krausse et al. 1979; Bauer and DeMaris 1982; Nelson 1983; DiMichele et al. 2007).
Rolls evidently formed during early stages of drowning and burial of the peat deposit. At least some rolls appear to have formed as tidal channels within the mud flats that developed along the flanks of drowning estuaries (e.g., DiMichele et al. 2007; Elrick et al. 2013). Currents scoured the peat, which, although tough and fibrous, was pliable and partially buoyant. Layers of peat were ripped out and clastic sediment filled the resulting voids. After burial, the roll filling compacted less than the surrounding peat, so the roll assumed a lens shape.
Miners use the term “split” for a tabular or wedge-shaped layer of rock within a coal seam. Several types of splits occur in the Illinois Basin and have different modes of origin. Discussed above are the thin layers of dark shale that are interlayered with the Springfield Coal at the margins of the Galatia channel. These simply represent sediment that washed into the peat swamp.
Markedly different are large splits in which the Dykersburg Member intrudes the Springfield Coal. These splits are elongate lenses and tapering wedges of clastic rock that divide the coal along bedding planes. Thickness ranges from a feather-edge to more than 32.8 ft (10 m), with a lateral extent of up to several hundred feet (meters) in some cases. The lithology of splits closely resembles that of the Dykersburg Member overlying the coal at the same locality. Most prevalent is medium gray, silty mudstone and siltstone, but some splits consist of very fine sandstone. The rock may be massive, or it may display indistinct, horizontal to mildly contorted layering. Well-defined tidal lamination is rarely, if ever seen.
A number of features of these splits are noteworthy and significant in interpreting their origin. Although ragged stringers and fragments of coal are abundant, identifiable plant remains are rare. No upright stems or trunks have been observed or reported. Roots and paleosol features are absent (Figures 20 and 21). “Riders” of coal detach into the clastic layer from both above and below. In some cases, coal stringers cross the clastic split diagonally from the upper to the lower layer of coal (Figure 21). Splits of the type described here commonly occur hundreds of feet (meters) to more than 0.6 mi (1 km) away from the Galatia channel and are surrounded by “normal” coal, so the splits lack a direct connection to the channel.
In the past, splits such as these were assumed to record clastic incursions into the swamp during the time of peat formation. Under the prevailing Mississippi Delta analogue, such splits were crevasse splays derived from breaches in natural levees along the main channel. But if clastic sediment was carried into a wetland with standing vegetation, we would expect upright stumps and other plant remains to be preserved in the lower part of the split, as they are in the basal Dykersburg at the top of the Springfield seam. Moreover, when plant growth resumed above the split, roots should have penetrated the latter. The absence of stumps, plant remains, roots, and other soil features casts doubt on splits being crevasse splays.
As an alternative, we suggest that at least some of the siltstone splits near the Galatia channel resulted from rafting of peat layers during deposition of the Dykersburg. As rising waters drowned the peat, trapped air and self-generated gas made the upper layers buoyant. Agitated by tidal currents, upper layers floated free of their substrate, separating along weak bedding planes. Some of the floating layers drifted away, allowing sediment-laden water to enter beneath other peat rafts and deposit clastic layers. Obviously, no vegetation or roots grew in this setting. Stringers of peat could float upward, dangle downward, or occasionally cross from one floating peat mat to another. This activity could take place anywhere in the Dykersburg estuary, not being limited to margins of the channel that existed while the peat was forming.
Major disturbances in coal
Related to the planar “splits” are more dramatic disturbances of the Springfield Coal. In an example from the Millennium Mine in Saline County, Illinois (Figure 22), disturbed coal occupies an arcuate belt that is approximately 820.2 ft (250 m) wide and lies more than 0.6 mi (1 km) west of the Galatia channel (as mapped by the absence of coal). There is no indication that the disturbance connects with the main channel. Within the disturbed area, the Springfield Coal rises abruptly, undulates strongly, and is torn asunder. The siltstone that underlies the coal within the disruption appears identical to siltstone overlying the coal. No roots or paleosol features occur below the coal in the area where it rises. Normal rooted underclay (claystone) occurs below the coal outside the disturbance. The coal splays apart both above and below, and large lenses of siltstone occur within the seam. At the crest of the disturbance, the roof is water-bearing sandstone.
A smaller disturbance occurs near the slope bottom in the same mine. Completely surrounded by normal coal, the disrupted area is ovoid in map view, about 98.4 262.5 ft (30 80 m). In the central part of the disturbance, the coal is torn apart and arches upward on one or both sides. As shown by the sketch (Figure 23), in one place the torn edges overlap one another by about 32.8 ft (10 m), as if repeated by a thrust fault, although no fault is present. The upper end of the torn seam thins markedly, suggesting that the peat (still pliable) was stretched, accounting for the overlap.
Similar in many respects is disrupted coal encountered in the northeast part of the Sahara No. 20 mine, also in Saline County. A map and field sketch portray part of the major disturbance (Figure 24). The entire Springfield Coal seam is torn apart, leaving ragged terminations. The southern end of the torn coal arches up, partly overlapping the northern end. Separating the torn ends is “gray-brown, very hard massive silty mudstone or siltstone, similar to roof material.” (field notes) Although the feature was not clearly exposed, the southern flap of coal appeared to truncate layering of the siltstone beneath. Notice on the map that the major disturbance is entirely surrounded by normal coal. Disturbances of lesser intensity lay north of the main one, surrounded on three sides by unaffected coal. These features lie several hundred feet (meters) west of the Galatia channel margin.
Also in Saline County, a large area of thin, absent, and disturbed coal interrupted the workings of the Dering Coal Company’s No. 2 Mine on the south side of the Galatia channel (Figure 25) The coal company drove entries across the disturbed area, and ISGS geologists Rolf Roley and Gilbert H. Cady sketched what they saw. By chance, an igneous dike intrudes the coal at the eastern edge of the disturbance. On both margins, the coal splits against a blunt, rounded body of gray shale with “varve-like laminae” of light gray siltstone. The sketch indicates that coal and shale interfinger and that shale bedding is deformed in a manner suggesting forcible intrusion.
Another major disturbance was encountered in the northern workings of the Wabash underground mine in Wabash County, Illinois. Meier and Harper (1981) described and illustrated the feature, supplemented by observations by ISGS geologists. Where crossed by mine entries, the disrupted area was about 656.2 ft (200 m) wide, flanked by normal coal on both sides and lying at least 1,968.5 ft (600 m) south of the Galatia channel, as mapped by the absence of coal. Within the disturbance, the coal seam rises sharply and the lower part is torn out in a highly irregular fashion (Figure 26). The rock that replaces and invades the coal is gray siltstone to fine-grained sandstone that is largely massive but that shows contorted lamination and contains many ragged stringers and chunks of coal. No roots or paleosol features were observed below the disrupted coal.
In the past, features such as these were generally interpreted as “splits” that formed when sediment was washed into the peat swamp, accompanied by currents that washed away some of the peat and perhaps an element of “slumping” to account for the observed deformation of layering. Specifically, Meier and Harper (1981, p. 14) invoked “wedges of fine-grained sediment, possibly a crevasse splay or overbank muds derived from the Galatia channel,” subsequently “thrown into a series of ridges and troughs, possibly by gravitational slumping on a slope formed by differential compaction.” Such models view “splitting” as contemporaneous with peat accumulation. But contemporaneous “splitting” cannot account for lifting the peat above its underclay, tearing it partly or entirely apart, and inserting a wedge of Dykersburg-like sediment between the coal and underclay. Explaining such geometry requires a new concept.
We propose that like the small planar “splits,” these dramatic disruptions formed when rising water rafted large masses of peat during the early stages of Dykersburg deposition. Rafting could involve just the upper layers or the entire thickness of the peat deposit. Agitated by tidal currents, floating peat mats frequently tore apart, admitting sediment-laden water beneath the peat. In this manner, thick wedges of mudstone and siltstone could be deposited within the seam and between the peat and the underclay. When currents stretched part of a torn peat mat, the ends of the mats could overlap. As the water level continued to rise, eventually the floating peat mats were completely encased in Dykersburg sediment. Differential compaction then deformed the sediment, perhaps squeezing some of it laterally. As an example, the major disturbance in the Wabash Mine is illustrated to show hypothetical ripping and rafting of peat (Figure 27).
Smaller disturbances that are completely surrounded by “normal” coal (e.g., Millennium slope and Sahara No. 20) probably began as blisters of floating peat that eventually burst, allowing sediment to infiltrate beneath. Some of the larger disruptions, such as the one in the Dering No. 2 mine, seem to involve tidal channels that connect directly to the main Galatia channel.
Absence of natural levees
We have already shown that no natural levees flanked the Galatia channel while the Springfield peat was accumulating. Likewise, no levees existed during deposition of the Dykersburg Member, and this member and this unit appear to have been deposited in estuarine sedimentary environments rather than as crevasse-splay deposits, as proposed by previous authors.
Natural levees of the modern Mississippi Delta are seldom higher than about 6.6 ft (2 m) and are routinely overtopped by the floods that sustain them. Levees are composed of intermixed clay, silt, and sand exhibiting a variety of sedimentary structures, especially climbing ripples. These sediments are intensively rooted and burrowed, and they contain plentiful diagenetic iron carbonate (siderite) and iron oxide nodules (Coleman 1981). Inferred natural levees in Pennsylvanian rocks of eastern Kentucky likewise are mostly less than 4.9 ft (1.5 m) high and 492.1 ft (150 m) wide, although a few are larger. These comprise a poorly sorted, unevenly layered mix of claystone, siltstone, fine sandstone, and thin lenticular coal. Rooting is prevalent. Bedding typically dips gently away from the parent channels (Baganz et al. 1975; Horne et al. 1978).
Abundant continuous cores and underground mine exposures in the Dykersburg Member, including the continuous profile in the Galatia Mine, show no bioturbated strata that could be compared to modern natural levees. Crevasse splays cannot develop without natural levees. As Coleman (1981) explained, a crevasse splay is basically a small delta having its own distributary channels, delta front, and prodelta deposits. Crevasse splays are fan-shaped in map view and wedge-shaped in cross section. The coarsest sediments drop out close to the levee break; as the splay progrades into the bay or marsh, it develops an upward-coarsening profile. Although the Dykersburg Shale locally displays upward-coarsening sequences, in most cases the grain size is either uniform throughout the member or varies in a random fashion.
Origin of the Dykersburg Member
Many features of the Dykersburg Member point to rapid deposition in a tidal regime:
- Burial of tree stumps in the growth position
- Superb preservation of delicate plant fossils
- Sparse brackish-water invertebrate fauna
- Scarce burrowing and the absence of rooting
- Scouring of peat tops (rolls)
- Large-scale splits involving rafted peat
- Prevalent tidal rhythmites
- Common slump and load features
- Large-scale clinoform bedding
The Dykersburg occurs in narrow bands on either side of the Galatia channel. Although the pattern is intricate, the Dykersburg belt overall widens toward the southwest, resembling a funnel in map view (Hopkins 1968). The pattern is consistent with an estuary, as Archer and Kvale (1993) proposed. This estuary developed when a rapid rise of sea level drowned the coastline. The Francis Creek Shale overlying the Colchester Coal and the Energy Shale over the Herrin Coal are close analogues to the Dykersburg, and they share nearly all the features listed above. Estuarine models have been proposed for both the Francis Creek (Kuecher et al. 1990; Baird 1997) and the Energy Shale (DiMichele et al. 2007).
The preservation of upright tree stumps and the packages of neap–spring tidal rhythmites indicate that Dykersburg sedimentation locally was rapid. Although we have not systematically measured and counted neap-spring cycles, they are commonly 0.4 to 2 in. (1 to 5 cm) thick, equating to deposition rates of about 3.3 ft (1 m) in 2 to 10 years. By analyzing rhythmites, Kuecher et al. (1990) determined that parts of the Francis Creek Shale were laid down at a rate of about 3.3 ft (1 m) per year. Of course, such findings do not show that the entire unit was laid down at such a pace. Overall sedimentation rates were limited by the rising base level, tectonic subsidence, and compaction of peat and clastic sediment, which together typically account for millimeters per year. However, in tidal environments, if accommodation space is available, that space can be filled rapidly.
What caused the transition from the sediment-starved “black water” channel during peat formation to a much greater load of coarser sediment is unknown. Although no evidence is available, some Dykersburg sediment may have been transported up the estuary from offshore sources by flood tides. Alternatively, a large increase in fluvial runoff is suggested. A climate shift from ever-wet to a seasonally dry, monsoonal regime accompanied the global deglaciation that brought about sea-level rise. With long annual dry spells, less vegetation cloaked upland source areas. Therefore, the sediment load and transport in the Galatia river system dramatically increased in harmony with sea-level rise in the estuary. This process is consistent with the relationship observed between climate and sediment transport in modern tropical river systems (Cecil and Dulong 2003; Cecil et al. 2003a) and with the potential for a rapid, pulse-like rise of sea-level in association with deglaciation (Archer et al. 2015, 2016).
Turner Mine Shale Member
The Turner Mine Shale is the first marine bed to follow the Springfield Coal or the Dykersburg Shale, or both. It is black, fissile, and phosphatic and lies directly above the Springfield Coal throughout the Illinois Basin wherever the Dykersburg Member is absent. The Turner Mine also overlaps the Dykersburg (Figure 4).
but typically pinches out where the Dykersburg reaches a thickness of 32.8 ft (10 m) or more. The Turner Mine averages about 3.3 ft (1 m) thick, ranging from less than 1 to 7.9 ft (0.3 to 2.4 m) thick. The Turner Mine is readily identified by very high readings on gamma-ray logs (Figure 5) and low readings on density and neutron logs.
The contact between the Turner Mine and Dykersburg Shale varies from rapidly gradational to erosive. Unfortunately, few mines exposing this contact have been active within the last 40 years. In the Eagle No. 2 underground mine in Gallatin County, Illinois, Dykersburg Shale occurred in lenses less than 23.6 in. (60 cm) thick, grading into overlying Turner Mine Shale. Evidently, this mine lay at the eastern depositional limit of the Dykersburg. At the Willow Lake Mine in Saline County, closer to the Galatia channel, lenses of Dykersburg were more numerous and ranged up to 3.3 ft (1 m) thick. Here, the contact was clearly erosive, with the Turner Mine lying against truncated bedding of the Dykersburg.
The gray, nonmarine Energy Shale Member that overlies the Herrin Coal near the contemporaneous Walshville channel is a close analogue to the Dykersburg Shale. The contact of the Energy Shale to the overlying black, phosphatic Anna Shale has been observed and mapped in large areas of several underground mines. This contact is sharply erosive, cutting off Energy Shale bedding at angles as steep as 20 degrees. The Energy Shale can be truncated from more than 16.4 ft (5 m) thick to zero in a lateral distance of 98.4 ft (30 m). Such erosion explains the highly irregular, lobate, and podlike distribution of gray shale as mapped in the mines (Bauer and DeMaris 1982; Nelson et al. 1987). Similar erosion evidently affected the Dykersburg prior to deposition of the Turner Mine Shale. The inferred origin of the Turner Mine Shale is essentially the same as that for the Excello Shale.
St. David Limestone Member
The St. David is marine limestone that overlies the Turner Mine Shale (Figure 4).
. Savage (1927) named the unit; Wanless (1956) designated (but did not describe) a type section in Fulton County, western Illinois. The name “Alum Cave Limestone” has been used for the same unit in Indiana. Wanless (1939) was the first to use Alum Cave in its present sense. The name St. David therefore has priority and will be used in this report.
The St. David is medium to dark gray, argillaceous, lime mudstone to wackestone containing an abundant normal-marine fauna of brachiopods, bivalves, gastropods, cephalopods, ostracods, crinoids, fusulinids, and bryozoans (Savage 1921; Wanless 1957; Shaver et al. 1986).
The St. David is practically coextensive with the Springfield Coal. In the Fairfield Basin and parts of western Kentucky, the limestone is less than 11.8 in. (30 cm) thick and consists of dark gray, argillaceous, fossiliferous lime mudstone to wackestone. On the Eastern Shelf in Indiana the limestone is as thick as 9.8 ft (3 m) but more commonly 1 to 3.9 ft (0.3 to 1.2 m), in two layers separated by thin shale (Wier 1961). In northwestern Illinois, the unit is normally a few inches (centimeters) to about 23 in. (60 cm) thick but locally exceeds 6.6 ft (2 m; Wanless 1957). Thicker St. David, lighter colored and less argillaceous than that found in the basin, also occurs on areas of the Western Shelf where the Springfield Coal is thick enough to mine. Along with the Turner Mine Shale, the St. David pinches out where the Dykersburg Shale is thick (~32.8 ft [10 m] or more).
The St. David closely resembles the older Hanover limestone and probably was deposited under similar conditions. Like the Hanover, the St. David appears to have developed better in the shallow waters of the margins of the Illinois Basin than in the deeper waters of the basin interior.
Savage (1921) named the Canton Shale for the city of Canton in Fulton County, western Illinois. Little has been published about this unit. As depicted by Willman et al. (1975), the Canton Shale occupies the interval between the St. David Limestone and the Briar Hill Coal (Figure 4).
The following information is based on a cursory inspection of selected core records and observations in mines.
Generally, the Canton is an upward-coarsening succession of shale, siltstone, and fine-grained sandstone. Shale in the lower part contains numerous bands and nodules of siderite. Brachiopods and other marine fossils are common near the base. Sandstone in the upper Canton tends to be shaly and thinly layered. A few well records indicate a sharp contact between upper sandstone and lower shale, but deep channel incision is unknown. At the top of the Canton is the weakly developed underclay of the Briar Hill Coal. The thickness of the member is normally 9.8 to 49.2 ft (3 to 15 m), but the Canton reaches 82 ft (25 m) thick in Webster County, Kentucky.
Two problems arise in defining the Canton Shale. One problem, which does not concern this study, arises where the Briar Hill Coal is absent and the Canton cannot be distinguished from the unnamed clastic rocks overlying the Briar Hill position. The other difficulty is separating the Canton from thick Dykersburg Shale where the Turner Mine and St. David Members are missing. Core records indicate that the Canton thins to less than 9.8 ft (3 m) where the Dykersburg is thicker than about 49.2 ft (15 m).
In depositional terms, the Canton Shale is more or less a repetition of the Delafield Member. The unit reflects the shoreline prograding into a shoaling basin during highstand to early regression.
Briar Hill Coal
The youngest unit considered in this report, the Briar Hill Coal (Figure 4)
is thin but widely persistent in southeastern Illinois, southwestern Indiana, and western Kentucky. Glenn (1912) named the coal in Union County, Kentucky; Butts (1925) extended the Briar Hill into southeastern Illinois. The same coal in Indiana has been called the Bucktown Coal Member (Shaver et al. 1970). Because Briar Hill has priority, usage of Bucktown should be discontinued.
The Briar Hill is confined to the southeastern part of the Illinois Basin in approximately the same region as the southeastern area of thick Springfield Coal. It extends above the Galatia channel except where the Dykersburg Member approaches its greatest thickness. Generally, the Briar Hill is a single bench of bright-banded coal between 9.8 and 19.7 in. (25 and 50 cm) thick. The maximum known thickness is about 51.2 in. (130 cm) in Sullivan County, Indiana. Where the Briar Hill is thinner than 9.8 in. (25 cm), the coal commonly becomes shaly. The Briar Hill rests on a weakly developed paleosol, commonly little more than a thin root-penetrated interval of laminated silty mudstone or siltstone. Above the coal is a shaly succession that typically coarsens upward. A thin layer of impure marine limestone or fossiliferous shale may occur at the base. No black phosphatic shale, comparable to the Excello or Turner Mine, accompanies the Briar Hill Coal.
- The Houchin Creek Coal formed as an in situ peat deposit on a vast, level, stable coastal plain.
- The Excello Shale records rapid marine transgression to the point where bottom water became anoxic because of the absence of circulation and the abundance of plant-derived organic matter.
- The Hanover Limestone reflects restoration of normal marine circulation in deep water offshore, probably under a seasonally dry climate.
- The Delafield Member records progradation of the shoreline into brackish water under a falling sea level, essentially filling the basin with clastic sediment.
- 5. The Galatia Member fills an incised valley that was cut and filled during regression to early lowstand. The river meandered actively and carried a heavy load of sand, rapidly backfilling its meander belt.
- Springfield peat formation commenced during lowstand (maximum glaciation) under an ever-wet climate that produced a perennially high water table. Vegetation stabilized meanders of the Galatia channel, which transitioned to a black-water stream that carried only fine-grained sediment. There were no natural levees, but belts of laminated shaly coal flank the Galatia channel.
- The Dykersburg Member records the onset of transgression, which converted the Galatia channel to an estuary and drowned the peat swamp. Vigorous tidal currents dislodged floating mats of peat, creating rolls, splits, and localized major disruption of the seam. As the climate became seasonally dry, the fluvial runoff and sediment load increased. Gray Dykersburg clastics rapidly entombed the peat.
- 8 Deposition of the marine Turner Mine black shale, St. David Limestone, and younger units completed the story.
The Galatia channel provides insights into events that are not recorded in most Carboniferous cyclothems. This example indicates, in conformance with some other studies (e.g., Cecil et al. 1985, 2003b, 2014; Eros et al. 2012; Horton et al. 2012; DiMichele 2014), that glacially driven sea-level fluctuations were linked to climate changes in the tropics. It also refines our understanding of when certain events (such as the development of peat) took place within the eustatic and climatic cycle. These themes will be developed further in a later section of this report. For a more complete understanding of the Galatia channel, it is necessary to investigate other paleochannels related to the Springfield Coal.
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