Shale Facies and Mine Roof Stability: a Case Study from the Illinois Basin

Shale facies and mine roof stability: A case study from the Illinois basin

JOHN A. BREYER Department of Geology, Texas Christian University, Fort Worth, Texas 76129

ABSTRACT Coal in eastern Macoupin County and western below are based primarily on detailed descrip- Montgomery County, Illinois (Fig. 1). It demon- tion of core from 11 boreholes, ~75 m of core in The Hornsby area in south-central Illinois strates the importance of detailed facies analysis all. The cored intervals ranged in length from 4 contains one of the largest deposits of low- for efficient mine planning by showing the rela- to 10 m and included an average of 7 m of roof sulfur coal in the Illinois basin coal field. Min- tionship between shale facies and roof falls in the strata. The core consisted mainly of strata of the ing of the resource has not proceeded apace Hornsby area. Energy Shale Member of the Carbondale For- with the demand for low-sulfur coal, mainly For this study, I was given access to geophysi- mation (Desmoinesian) but included strata of because of fear of unstable roof conditions cal logs and core from numerous boreholes, and the Lawson Shale Member, which directly over- beneath the Energy Shale (Desmoinesian). I made two visits to the workings to examine the lies the Energy Shale where the intervening ma- Three different facies of the Energy Shale strata in the mine roof. The facies descriptions rine units are missing (Fig. 2). overlie the Herrín Coal in the Hornsby area. All three facies are fine-grained, argillaceous rock, but their response to mining is different. The laminated shale facies has a pervasive lamination and in many places is cut by small faults and joints which make the rock prone to slabbing and potting prior to bolting. The gray shale facies that is interbedded with the laminated shale facies lacks pervasive lamination, and fractures are not well developed in it. The gray shale facies generally provides good roof. The carbonaceous shale facies had not yet been encountered in mine workings in the Hornsby area when this study was completed. Lamination in the carbonaceous shale facies is better developed than that in the gray shale facies, but it is not as well developed as that in the laminated shale facies. When the carbonaceous shale facies is the roof rock, the extensive roof control measures that were implemented for mining beneath the laminated shale facies will probably not be required.

INTRODUCTION

Mining of the estimated 1.17 billion tons of coal containing <2.5% sulfur in the Hornsby area in south-central Illinois has not proceeded [i|| wedges of Energy Shale along Walshville channel apace with demand for low-sulfur coal, mainly because of fear of unstable roof conditions be- Q small, isolated lenses and pods of Energy Shale neath the Energy Shale (Nelson, 1987). Infor- mation in the public domain on mining Figure 1. Distribution of the Energy Shale along the trace of the Walshville channel in conditions in the Hornsby area is limited. This central and southern Illinois. Deposits of Energy Shale interpreted as lacustrine delta complex paper presents the results of a detailed facies in Coles County and adjoining counties not shown. Mac., Macoupin County; Mon., Mont- study of roof strata above the Herrín (No. 6) gomery County; Jef., Jefferson County (after Damberger and others, 1980).

Geological Society of America Bulletin, v. 104, p. 1024-1030, 5 figs., 2 tables, August 1992.

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commonly 2-4 mm thick and are made up of many very thin, planar, horizontal laminae. Some sets of laminae are graded, showing an upward decrease in grain size and a concomitant increase in organic-matter content. The dark- gray layers are most commonly < 1 mm thick. Flattened, fallen tree trunks and upright, hollow tree stumps filled with laminated shale can be seen in this facies in the roof of the mine. Branchiopod valves are common in core and also on small pieces of rock of this facies that have fallen from the roof.

Figure 2. Schematic east-west cross section showing the relationships between the strati- Carbonaceous Shale Facies (F2) graphic units mentioned in the text. The maximum thickness of the Energy Shale is 30 m. The Walshville channel is 2 to 8 km across (after Nelson, 1983). This facies consists of light-gray mudstone in dark-gray claystone. The claystone is more common than the mudstone, giving the facies a distinct dark aspect. The light-gray layers are most commonly 1-2 mm thick and are made up PREVIOUS WORK roof rock. The dark-gray shale facies is variable of very fine laminae. The dark-gray layers typi- as roof material. Joints are more regularly de- cally range in thickness from 5-10 mm. Most of the previous work on the geology of veloped and more closely spaced in it than in the Bioturbation is conspicuous. The burrows are the Energy Shale has been in mines in the Qual- medium-gray shale facies. The rock is prone to similar to those seen in the laminated shale facies ity Circle area of southern Illinois (Fig. 1). Pre- slabbing in some areas but is stable in others. but are more common. Bioturbation disrupts the vious workers have recognized two major facies The medium-gray shale facies usualy presents no lamination in the layers of light-gray mudstone within the Energy Shale: a dark-gray shale facies problems to mining; although over an extended and also disrupts layering, mixing mudstone and and a medium-gray shale facies (Krause and period, slaking can cause large roof falls. claystone (Table 1). others, 1979, Damberger and others, 1980). Previous work has established the broad dep- Branchiopods and plant debris are common When present, the dark-gray shale facies lies be- ositional setting of the Energy Shale, but the on core breaks. The plant debris includes recog- neath the medium-gray shale facies, which specific environments in which the shale ac- nizable plant fragments, not just abraded or forms by far the greater part of the Energy Shale. cumulated in the Hornsby area have not been comminuted organic matter. The dark-gray shale facies is usually <0.6 m determined (Nelson, 1987). Two aspects of the thick but can be as much as 1.5 m thick. The regional geology constrain the interpretation of Gray Shale Facies (F3) medium-gray shale facies is from 5 to 15 m thick the Energy Shale in the Hornsby area. First, a or more. The medium-gray shale facies grades vast, low-lying, fresh-water, coastal swamp This facies has an indistinct layering resulting into coarser sediments laterally and vertically. In dominated by arborescent lycopods (DiMichele from slight changes in color, but it lacks the some places near the margins of the Walshville and Phillips, 1985) covered much of the present distinct dark layers of the laminated shale and channel a third facies, planar-bedded siltstone area of the Illinois basin coal field in the late carbonaceous shale facies. It is a medium dark- and sandstone, abruptly overlies or replaces the Middle Pennsylvanian (Desmoinesian). Second, gray to medium light-gray, massive to finely medium-gray shale facies. a large river, comparable to the modern Missis- laminated shale. Plant debris occurs as abraded The dark-gray shale facies is thought to have sippi, flowed from north to south through the and comminuted organic matter ("coffee accumulated in fresh to brackish water (Krause peat swamp in central and southern Illinois grounds") disseminated throughout the sedi- and others, 1979) in shallow ponds within the (Nelson, 1983, 1987; Treworgy and Jacobson, ment. No branchiopod valves were found in this Herrin peat swamp (Edwards and others, 1979; 1985). Today this ancient river is represented by facies. Burk and others, 1987). The bulk of the Energy the sandstone of the Walshville channel that in- Shale, identified variously as "medium-gray terrupts and splits the Herrin Coal (Fig. 2). The Gray Shale with Sand Streaks (F4) shale facies" (Krause and others, 1979; Dam- course of the river passed a few kilometers to the berger and others, 1980), "light-gray shale unit" east of the Hornsby area in what is now western This facies consists mainly of medium-gray to (Edwards and others, 1979), or just "Energy Montgomery County (Fig. 1). medium dark-gray mudstone to siltstone. It also Shale" (Johnson, 1972; Allgaier and Hopkins, contains thin streaks of light-colored sand up to 1975), has been taken to represent splay deposits FACIES DESCRIPTIONS 5 mm thick. Foreset laminae can be recognized associated with the Walshville channel. Burk in some of the thicker streaks of sand. No bio- and others (1987) recognized a proximal splay Laminated Shale Facies (Fl) turbation is evident in this facies. Plant debris is facies with three subfacies and a distal splay fa- present as "coffee grounds" scattered through cies in exposures of the Energy Shale in surface This facies consists of alternating, discrete lay- the sediment and as coarse, abraded fragments mines in Jefferson County in southern Illinois. ers of light-gray mudstone and dark-gray clay- in concentrations on some bedding planes. Krause and others (1979) and Damberger stone (Table 1). The layering is distinct and Small pyrite nodules and a variety of soft- and others (1980) described the behavior of the seldom disrupted by bioturbation or soft sedi- sediment deformation features are common in dark-gray shale and medium-gray shale facies as ment deformation. The light-gray layers are this facies (Table 1).

Geological Society of America Bulletin, August 1992

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Fades code Fl F2 F3 F4 F5 F6

Fades name Laminated Carbonaceous Gray Gray shale with Greenish-gray Shale with shale fades shale fades shale fades sand streaks shale fades slickensides

Grain size, Dark-gray claystone Light-gray mudstone Medium dark-gray to Medium gray to medium Dark green to light Greenish gray to dark color in light-gray mudstone in dark-gray claystone medium light-gray shale dark-gray mudstone to green, mottled claystone gray shale siltstone with thin streaks to mudstone. Alternating discrete Dark daystone more of light-colored sand layers of light mudstone common than light and dark claystone give mudstone, giving the the fades a distinct fades a distinct dark banded aspect. aspect

Layering and Light-gray layers commonly Light-colored layers Lacks the distinct dark Sand streaks are up to Massive to faintly None evident in core lamination 2-4 mm thick, but usually range in thickness layers of Fl and F2 5 mm thick, but most are laminated may occur in groupings from 1-5 mm and clmm thick. Foreset in which thicknesses of alternate with dark Indistinct layering resulting laminae are recognizable in either 5-8 mm or > 10 layers ranging in thickness from slight changes some of the thicker mm are more common from 5-10 mm. in color streaks.

Dark-gray layers most Light-gray layers are Layers massive to Lamination in shale often clmm thick typically 1-2 mm thick finely laminated ranges from horizontal and are made up of to slightly inclined Light-gray layers made very fine laminae. (

Fossils Flattened, fallen tree Want debris includes Abraded and comminuted Plant debris found as None evident in core Roots (?) trunks and upright recognizable plant plant debris ("coffee "coffee grounds" scattered hollow tree stumps fragments, not just grounds") disseminated throughout the shale abraded or comminuted throughout and as coarse, abraded Branchiopod valves organic matter. fragments in concentrations common, completely Branchiopod valves not on some bedding cover some bedding Branchiopod valves and found in this fades planes surfaces plant debris common on core breaks

The largest branchiopod valves are in F2.

Bioturbation Layering and lamination Conspicuous bioturbation None evident in core None evident in core Extensive burrow mottling Massive to churned seldom disrupted by bio- burrows similar to those in fabric resulting from turbation Fl but more common disruption by roots and other pedogenic Small burrows, < 1 mm Burrows usually disrupt processes in diameter, in upper only the lamination at the part of light-colored top and bottom of the layers thicker layers (3-5 mm) of light-gray mudstone, but all of the lamination in thinner layers (1-2 mm) is disrupted when burrows are present.

Bioturbation also disrupts layering, mixing claystone and mudstone, and can obliterate layering (see Layering and lamination above).

Soft sediment Layering seldom Layering seldom None evident in core Differential compaction None evident in core None evident in core deformation disrupted by soft disrupted by soft of shale laminae sediment deformation sediment deformation around sandstone lenses

Contorted sand streaks

Concentrations of microfaults

Lower part of graded Some mudstone layers Small pyrite nodules Isolated framboids of Pedogenic slickensides sets shows orange have an orange discolor- common in this fades pyrite throughout shale discoloration from ation, which against the Peds (?) disseminated siderite. dark background of the Pyrite framboids fill a claystone layers, seems few small burrows. more vivid than in Fl. Calcite concretions

1026 Geological Society of America Bulletin, August 1992

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Greenish-Gray Shale Facies (F5) Shale with slickensides (F6) ,, ^ Gradua| transition

1 ^ Abrupt transition This shale is a dark-green to light-green, massive to faintly laminated, mottled claystone to mudstone. Isolated framboids of pyrite are present throughout the shale, and concentrations of framboids fill small burrows. The calcite concretions in the shale are probably associated with the overlying paleosol and are not a primary feature of this facies.

Shale with Slickensides (F6)

This facies consists of greenish-gray to dark- gray shale with slickensides. The fabric is massive to churned. Tentative identification of peds and roots is possible.

FACIES RELATIONS

Six facies are present in the shale wedge Figure 3. Facies relationship diagram displaying the data in the transition count matrix in above the Herrin Coal in the Hornsby area schematic form. The diagram is arranged to suggest the typical sequence of facies. It also (Table 1). The coal is usually overlain by either shows variations in the facies sequence. A few incidental transitions are omitted for clarity. the carbonaceous shale facies or the laminated Arrows show the number of times one facies passes upward into another. shale facies (Table 2 and Fig. 3). These facies alternate with abrupt transitions and then give TABLE 2. TRANSITION COUNT MATRIX« way to the gray shale facies. The gray shale facies either passes gradually upward into gray Facies code Name Fl F2 F3 F4 F5 F6 shale with sand streaks or is overlain by laminated shale or carbonaceous shale. The greenish- Fl Laminated shale 34(1) KD 1 0 0 F2 Carbonaceous shale 30(1) 8 2 1 0 gray shale facies and shale with slickensides cap F3 Gray shale 2(1) 2 (4) 2(1) 0 the sequence. The makeup of the shale wedge F4 Gray shale with sand streaks 0 1 1(3) 0 0 F5 Greenish-gray shale 0 0 0 0 (1) changes across the mine property (Fig. 4). In F6 Shale with slickensides 0 0 0 0 0 F7 Herrin (No. 6) Coal 4 5 2 0 0 0 general, the gray shale facies, which makes up

most of the thickness of the Energy Shale on the «Rows in the matrix show the underlying facies, and the columns show the overlying facies for the transitions between facies seen in 75 m of core from 11 drill holes. west side of the property, gives way to the car- Numbers in parentheses indicate gradual transitions; other numbers indicate abrupt transitions. bonaceous shale facies to the east, but many individual units within the shale wedge cannot be correlated even between closely spaced bore- height were filled by sediment settling from resent either the fine tail of the size distribution holes (Fig. 4). suspension. in the flood sediment or the return to normal The dark layers in these facies represent the lake sedimentation. The latter is more likely the FACIES INTERPRETATION normal or background sedimentation in the case (see below). lake. The sediment is finer grained than that in Bioturbation is common in the light-colored Laminated Shale and Carbonaceous Shale the light-colored layers and contains more or- layers only in the carbonaceous shale facies Facies (F1 and F2) ganic matter. The absence of lamination suggests where thin, light-colored layers adjoin thick, that the rate of sedimentation was slow, not ex- dark layers. Here the length of time between The laminated shale facies and the carbona- ceeding the rate of infaunal reworking. The floods was great enough and the amount of sed- ceous shale facies are lake deposits. Valves of the plant debris in these layers is larger and less iment introduced during floods small enough late Middle Pennsylvanian branchiopod, Leaia damaged than that in the light-colored layers. It that infaunal reworking could disrupt, and in tr¡carina ta, are common in both facies. This might represent material blown into the lake some cases dissipate, the sediment in the flood small crustacean is thought to have lived in from the adjoining swamp. layer. In contrast, the layering in the laminated standing bodies of fresh water (J. G. Maisey, The light-colored layers in the carbonaceous shale facies suggests more frequent and larger 1985, written commun.). The fine grain size of shale and laminated shale facies are flood depos- floods, and consequently the flood layers were the sediment also suggests quiet water deposi- its. The grading evident within some layers indi- not extensively bioturbated between floods. tion. The horizontal lamination within the light- cates a pulse of sediment-laden water entering The laminated shale facies and the carbona- er colored layers in both facies is the result of the lake and then segregation within the water ceous shale facies alternate with one another. sediment settling from suspension onto an even column as the grains settled to the bottom. The Each facies contains both light-colored and dark surface. The lake must have been at least 1 m thin, distinct, dark layers between the light- layers. The laminated shale facies represents deep, because hollow, upright stumps of this colored layers in the laminated shale facies rep- times or places within the lake when or where

Geological Society of America Bulletin, August 1992

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§=| Laminated shale (F1) QJ Herrin Coal (F7) Relative position of drill holes, Hornsby area, Macoupin County, IL 8 m Carbonaceous shale (F2) [Xj Missing core

0 1 2 N Gray shale (F3) 1— 'l'I t km ' . Gray shale with sand streaks (F4) • % • 7 1 2 3 ? | Greenish-gray shale (F5) • •s Shale with slickensides (F6)

Figure 4. Graphic logs of core from eight boreholes in the Hornsby area showing the makeup of the shale wedge above the Herrin Coal. Facies F1 through F4 are in the Energy Shale Member of the Carbondale Formation. Facies F5 and F6 are in the Lawson Shale Member.

1028 Geological Society of America Bulletin, August 1992

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deposition from flood water spilling out of a interpretation of the environment of deposition subsidence began to outpace sedimentation nearby river overwhelmed the background sed- of this facies. generally and marine strata were deposited over imentation in the lake. The carbonaceous shale The shale with slickensides is an underclay. the Energy Shale and the Herrin Coal through- facies represents times or places within the lake Slickensides form in some modern soil profiles out much of central and southern Illinois. when or where background sedimentation was in response to repeated wetting and drying of the not overwhelmed by deposition from flood soil material (Ahmad, 1983; Miller, 1983). This MINING GEOLOGY waters. shale and the calcite concretions in the greenish- gray shale below it resemble the underclay be- Energy Shale forms the roof for the Herrin Gray Shale Facies and Gray Shale with neath the Herrin Coal. Coal in the mine in the Hornsby area and across Sand Streaks (F3 and F4) the mining property. Three facies directly overlie Comparison with Quality Circle Area the coal: the laminated shale facies, the gray The gray shale facies and the gray shale with shale facies, and the carbonaceous shale facies. sand streaks are crevasse splay deposits. These The Hornsby area is 100 to 150 km up pa- Coal was being mined from beneath strata of the mudstones and siltstones are the coarsest sedi- leoslope from the Quality Circle area in south- laminated shale and gray shale facies when this ment in the core I examined. F3 and F4 overlie ern Illinois where the Energy Shale has been study began. The carbonaceous shale facies had the other facies with abrupt contacts, but the studied in the most detail (Fig. 1). The Energy not yet been encountered in the roof in the mine transitions between F3 and F4 are gradual (Fig. Shale in the Hornsby area differs from that to when this study was completed. It is present 3). Small-scale, coarsening-upward sequences the south in three ways. First, two lacustrine above the coal to the east, in unmined areas, (3-6 m) formed by the gradual transition from facies are present in the Hornsby area. The lam- where it also makes up more of the total thick- gray shale to gray shale with sand streaks record inated shale facies has the pervasive lamination ness of the Energy Shale (Fig. 4). the advance of splays into the lake. Small-scale, typical of the lacustrine "dark-gray shale facies" All three facies are fine-grained, argillaceous fining-upward sequences (1-2 m) formed by the of the Quality Circle area, but the carbonaceous rock, but their response to mining is different. gradual transition from gray shale with sand shale facies does not. Second, the lake facies are The laminated shale facies has a pervasive lami- streaks to gray shale record waning currents or much thicker in the Hornsby area. I measured 7 nation and in many places is cut by small faults the abandonment of a particular lobe of the m of the laminated and carbonaceous shale fa- and joints which make the rock prone to slab- splay, cies in one core, and thicknesses of 2 m (twice), bing and potting prior to bolting; where these These facies must represent distal splay depos- 3 m, and 6 m in other cores. The "dark-gray fractures are absent, the facies provides good its. They contain little sand, and the foreset shale facies" is usually <0.6 m thick in the Qual- roof (J. T. Padgett, 1986, written commun.). laminae in the thicker (3-5 mm) streaks of sand ity Circle area (Krause and others, 1979; Dam- The gray shale facies lacks pervasive lamination are the only indication of traction currents. The berger and others, 1980). Third, the lake facies and is less often cut by joints and faults. It gener- thinner streaks of sand could result from deposi- are interbedded with the splay facies (Fig. 4). In ally provides good roof. The laminated shale tion from suspension. The indistinct lamination one core, lake and splay facies alternate five facies parts along bedding planes, exposing flat in the gray shale facies suggests that the grains times in 8 m of core. Lake facies overlie splay surfaces in the roof when falls occur. Falls in the were not able to sort themselves before coming facies in 5 of the 11 cores I examined in detail. gray shale facies do not expose flat surfaces in to rest on the bed. This indicates a high concen- The lacustrine "dark-gray shale facies" always the roof. Failure surfaces in the gray shale facies tration of suspended sediment and rapid deposi- lies below the splay deposits of the "medium- are characterized by plumose fractures typical of tion. The abundant soft-sediment deformation gray shale facies" in the Quality Circle area rock lacking a well-developed internal fabric. features in the gray shale with sand streaks like- (Krause and others, 1979; Damberger and and Lamination in the carbonaceous shale facies is wise suggest rapid deposition and accumulation others, 1980). better developed than in the gray shale facies, of water-saturated sediments. but not as well developed as in the laminated shale facies. Extensive roof control measures, The character and distribution of plant debris DEPOSITIONAL HISTORY in these splay facies also point to conditions such as those implemented when mining beneath the laminated shale facies, will probably different from those that prevailed during the The Herrin peat swamp covered much of the not be required when the carbonaceous shale deposition of the laminated shale and carbona- present area of the Illinois basin coal field in facies is the roof rock. ceous shale facies. The plant debris is pulverized the late Middle Pennsylvanian (Desmoinesian). and disseminated throughout the sediment, that The course of a large river, marked today by the Detailed facies analysis is necessary to under- in the lake deposits is concentrated in particular sandstone of the Walshville channel, passed stand the behavior of the Energy Shale as roof laminae or layers and includes recognizable through the peat swamp a few kilometers to the rock in the Hornsby area—and this understand- plant fragments in addition to "coffee grounds." east of the Hornsby area, in what is now western ing is necessary for efficient mine planning. Min- Montgomery County (Fig. 1). Subsidence was ing conditions will vary from difficult beneath Greenish-Gray Shale Facies and Shale with higher in areas adjacent to the river than elsewhere the laminated shale facies to generally good be- Slickensides (F5 and F6) beneath the peat swamp (Nelson, 1987). The neath the gray shale facies. Extensive (and ex- shale facies in the Hornsby area accumulated in pensive) roof control measures will be required The greenish-gray shale was deposited in a a lake that formed as the peat swamp alongside when mining beneath the laminated shale facies. quiet-water environment. The extensive burrow the river drowned (Fig. 5). Upright tree trunks If the laminated shale facies formed the roof mottling suggests that either the rate of sedimen- filled with laminated shale indicate rapid drown- over most of a mine property in the Hornsby tation in this environment was very slow or the ing of the swamp, probably resulting from in- area, the cost of roof control would probably sediment supported a flourishing infauna. The creased subsidence along the course of the prohibit mining. The distribution of facies can- features in core do not allow a more specific Walshville channel (Nelson, 1987). Eventually not be predicted ahead of mining in the Hornsby

Geological Society of America Bulletin, August 1992

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area, but a "rough feel" for the area covered by each facies can be obtained from development drilling data, and cost estimates and mine plans can be prepared accordingly. H. H. Read's oft-quoted remark about granite holds for shale as well. All fine-grained, argillaceous rock cannot be safely dismissed as "just shale"—in the Illinois basin coal field or elsewhere.

ACKNOWLEDGMENTS

Most of the material in this article was origi- nally prepared as a consulting report in 1985. W. Matt Bergeron, Chief Geologist, EXXON Coal and Minerals Company, secured approval for the release of the information in the consulting report. I thank Matt for his willing assistance and EXXON Coal and Minerals Company for granting me permission to publish. Heinz Dam- berger, Peter McCabe, John Ferm, and Art Syl- vester provided suggestions which greatly im- proved the quality of the manuscript.

REFERENCES CITED

Ahmad, N, 1983, Vertisols, in Wilding, L. P., Smeck, N. E, and Hall, G. F„ eds., Pedogenesis and soil taxonomy II, the soil orderc: Amsterdam, The Y Netherlands, Elsevier, p. 91-123. Allgaier, G. J., and Hopkins, M. E., 1975, Reserves of the Herrin (No. 6) Coal in the Fairfield Basin in southeastern Illinois: Illinois State Geological Survey Circular 489, 31 p. Burk, M. K., Deshowitz, M. P., and Utgaard, J. E., 1987, Facies and depositional environments of the Energy Shale Member (Pennsylvanian) and their relationships to low-sulfur coal deposits in southern Illinois: Jour- nal of Sedimentary Petrology, v. 57, p. 1060-1067. Damberger, H. H„ Nelson, W. J., and Krause, H.-F., 1980, Effect of geology on roof stability in room-and-pillar mines in the Herrin (No. 6) Coal of Illinois, in Chugh, Y. P.. and Van Besien, A., eds., First Conference on Ground Control Problems in the Illinois Coal Basin, Proceedings: Carbondale, Illinois, Southern Illinois University, p. 14-32. Lake facies Lake level DiMichele, W. A., and Phillips, T. L., 1985, Arborescent lycopod reproduction and paleoecology in a coal swamp environ ment of late Middle Pennsyl- vanian age (Herrin Coal, Illinois, U.S.A.): Review of Paleobotany and Palynology, v. 44, p. 1-26. Edwards, M. J., Langenheim, R. L„ Nelson, W. J, and Ledvina, C., 1979, Splay facies "i Upright trees, logs Lithologic patterns in the Energy Shale Member and the origin of "rolls" in the Herrin (No. 6) Coal Member, Pennsylvanian, in the Orient No. 6 mine, Jefferson County, Illinois: Journal of Sedimentary Petrology, v. 49, p. 1005-1014. ^ v ^ Johnson, D. O., 1972, Stratigraphic analysis of the interval between the Herrin Peat 3—^ Peat surface (No. 6) Coal and the Piasa Limestone in southwestern Illinois [Ph.D. thesis): Urbana, Illinois, University of Illinois, 105 p. Krause, H.-F., Damberger, H. H., Nelson, W. J, Hunt, S. R„ Ledvina, C. T„ Treworgy, C. G„ and White, W. A., 1979, Roof strata of the Herrin (No. 6) Coal and associated rock in Illinois: Their geology and Figure 5. Depositions)! history of the Energy Shale in the Hornsby area, eastern Macoupin stability—Summary report: Illinois State Geological Survey Mineral Notes 72,54 p. County and western Montgomery County, Illinois. A. Peat accumulates in a vast, low-lying Miller, B. J., 1983, Ultisols, in Wilding, L. P., Smeck, N. E„ and Hall, G. F., eds.. Pedogenesis and soil taxonomy II, the soil orders: Amsterdam, The fresh-water swamp which covers most of central and southern Illinois, including the study area Netherlands, Elsevier, p. 283-323. in Macoupin County. B. The peat swamp drowns and a lake forms over a large area to the east Nelson, W. J., 1983, Geologic disturbances in Illinois coal seams: Illinois State Geological Survey Circular 530,47 p. of the study area. Lake sediments accumulate over the peat and in hollow, upright trees in the 1987, The Hornsby district of low-sulfur Herrin Coal in central Illinois (Christian, Macoupin, Montgomery and Sangamon Counties): Illinois drowned swamp. Branchiopods flourish in the waters of the lake. Peat continues to accumulate State Geological Survey Circular 540, 40 p. in the study area. C. The lake continues to encroach on the peat swamp. Lake sediments Treworgy, C. G., and Jacobson, 1985, Paleoenvironments and distribution of low-sulfur coal in Illinois, in Cross, A. T., ed.. Economic geology: Coal, collect in topographic lows within the swamp, especially when sediment-laden flood waters oil and gas: Compte Rendu: International Congress on Carboniferous raise lake level. Peat is still forming on topographic highs within the swamp. Carbonaceous Stratigraphy and Geology, 9th, v. 4, p. 349-359. shale and laminated shale are still accumulating in the main body of the lake. D. Lake level continues to rise. Crevasse splays supplied with sediment from the Walshville channel pro- grade into the lake and across the peat swamp. The splays cover the lake sediments and bury MANUSCRIPT RECEDED BY THE SOCIETY SEPTEMBER 26, 1990 REVISED MANUSCRIPT RECEIVED DECEMBER 30, 1991 the remaining topographic highs in the swamp. MANUSCRIPT ACCEPTED JANUARY 8, 1992

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7 Geological Society of America Bulletin, August 1992

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