Mining geology of the Pond Creek seam, Pikeville Formation, Middle Pennsylvanian, in part of the Eastern Kentucky Coal Field, USA

International Journal of Coal Geology 41 Ž1999. 25–50

Mining geology of the Pond Creek seam, Pikeville Formation, Middle Pennsylvanian, in part of the Eastern Kentucky Coal Field, USA Stephen F. Greb a

a,)

, John T. Popp

b

Kentucky Geological SurÕey, UniÕersity of Kentucky, 228 Mining and Minerals Building, Lexington, KY, 40506-0107 USA b Mapco Coal, 771 Corporate DriÕe, Ste. 1000, Lexington, KY 40503, USA

Abstract The Pond Creek seam is one of the leading producers of coal in the Eastern Kentucky Coal Field. The geologic factors that affect mining were investigated in several underground mines and categorized in terms of coal thickness, coal quality, and roof control. The limits of mining and thick coal are defined by splitting along the margin of the coal body. Within the coal body, local thickness variation occurs because of Ž1. leader coal benches filling narrow, elongated depressions, Ž2. rider coal benches coming near to or merging with the main bench, Ž3. overthrust coal benches being included along paleochannel margins, Ž4. cutouts occurring beneath paleochannels, and Ž5. very hard and unusual rock partings occurring along narrow, elongated trends. In the study area, the coal is mostly mined as a compliance product: sulfur contents are less than 1% and ash yields are less than 10%. Local increases in sulfur occur beneath sandstones, and are inferred to represent post-depositional migration of fluids through porous sands into the coal. Run-of-mine quality is also affected by several mine-roof conditions and trends of densely concentrated rock partings, which lead to increased in- and out-of-seam dilution and overall ash content of the mined coal. Roof control is largely a function of a heterolithic facies mosaic of coastal–estuarine origin, regional fracture trends, and unloading stress related to varying mine depth beneath the surface. Lateral variability of roof facies is the rule in most mines. The largest falls occur beneath modern valleys and parallel fractures, along paleochannel margins, within tidally affected ‘stackrock,’ and beneath rider coals. Shale spalling, kettlebottoms, and falls within other more isolated facies also occur. Many of the lithofacies, and falls related to bedding weaknesses within or between lithofacies, occur along northeast–southwest trends, which can be projected in advance of mining. Fracture-related falls occur independently of lithofacies trends along northwest–southeast trends, especially beneath modern valleys where overburden thickness decreases sharply. Differentiating

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Corresponding author. E-mail: [email protected]

0166-5162r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 6 - 5 1 6 2 Ž 9 9 . 0 0 0 1 0 - 5

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roof falls related to these trends can aid in predicting roof quality in advance of mining. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Kentucky; roof falls; partings; paleochannel; split; fractures

1. Introduction The Pond Creek–Lower Elkhorn seam is one of the leading producers of coal in the Eastern Kentucky Coal Field, with 12 to 14 MT annual production in the last decade according to Kentucky Department of Mines and Minerals data. The seam has been an important exploration target because it typically has very low sulfur contents Ž0.4–1.7%, mean 0.8%. and ash yields Ž5.1–16.7%, mean 9.6%. according to Kentucky Geological Survey data. Geologic research in several large Pond Creek mines suggested variability in roof quality and coal thickness related to Ž1. deposition in a coastal–estuarine depositional environment and Ž2. the position of the mines beneath drainages. Because of mine access, geologic problems encountered during mining could be documented and are described and interpreted herein. Many of the geologic obstacles are common to Appalachian coal mines and serve as examples of typical mining problems, whereas others are unique to the Pond Creek seam and are related to local conditions during deposition of the Pond Creek peat mire.

2. Location and stratigraphy The study area is located in the Eastern Kentucky Coal Field, which is part of the central Appalachian Basin, one of the leading coal-producing basins in North America. The Eastern Kentucky Coal Field produces 130 MT of coal annually. The Pond Creek and correlative seams account for nearly 10% of that production according to Kentucky Department of Mines and Minerals data. Stratigraphically, the coal bed occurs in the Pikeville Formation of the Breathitt Group Žpreviously Breathitt Formation, see Chesnut, 1992., and is Duckmantian ŽUpper Morrowan. in age ŽFig. 1A.. In the study area, the seam occurs 50 m Ž150 ft. above the base of the Betsie Shale Member, a regionally correlative marine zone ŽRice et al., 1987., and is overlain by similar, but less extensive, marine-fossil-bearing shales of the Crummies Shale ŽChesnut, 1991.. The Pond Creek seam has numerous names, partly because of its irregular distribution. Regionally, it consists of a series of thick coal pods Žmostly in the southeastern part of the state. separated by vast areas of thinner coal ŽFig. 1B.. In the northeasternmost pod, where this study was concentrated, the seam is called the Pond Creek. In much of the area where the coal is called Pond Creek, it occurs below drainage Žbelow the level of the lowest stream bottom.. Because of the hilly topography of eastern Kentucky, the study mines vary in depth from 65 to 365 m Ž200 to 1100 ft. below the surface. Three large, subadjacent room-and-pillar mines were studied. Mining is done by continuous mining machinery, and roof support utilizes point-anchor, fully grouted resin, and combination roof bolts of varying lengths, depending on roof geology. Supplemental

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Fig. 1. Stratigraphy and location of the Pond Creek seam. ŽA. Stratigraphic position of the Pond Creek seam. ŽB. Thickness of the Pond Creek seam and equivalents Žafter Thacker et al., 1998..

support is used in problem areas. Because the mines are still operating, confidentiality of mine locations and specific outlines was required. At least one of the features noted in the mines appears to be unique to the Pond Creek seam and poses an interesting problem for interpretation, whereas other features are more common and should be applicable to mines in other stratigraphic horizons and basins.

3. Previous investigations of the Pond Creek seam Previous studies of the petrography ŽHower and Pollock, 1988., geochemistry ŽHower and Bland, 1989., and palynology ŽHelfrich and Hower, 1990., all summarized by Hower et al., 1991a, of the Pond Creek seam in Pike and Martin Counties, KY, have defined vertical and spatial trends in coal quality and thickness that appear to be related to the development of a local, northeast-plunging structure called the Belfry Anticline. Uplift along the structure was contemporaneous with the deposition of the Pond Creek peat ŽHower et al., 1991a.. Contemporaneous uplift of another local structure in the southwestern part of the coal field, called the Flat Lick Anticline, may have also influenced the thickness of the Blue Gem coal, a Pond Creek correlative ŽRice, 1974.. Studies of petrographic and geochemical variations in the Blue Gem coal ŽFig. 1B. indicate that areas of high-sulfur coal are generally beneath permeable sandstone paleochannels in the immediate mine roof ŽRimmer et al., 1985; Hower et al., 1991b; Hower et al., 1994.. A study of longwall productivity in the Lower Elkhorn coal Ža Pond Creek correlative; see Fig. 1B. noted decreased productivity Ž1. beneath weak shales that were too thin to provide good gob support for the main sandstone roof, Ž2. beneath thin rider coals, and Ž3. across narrow synclinal troughs ŽNelson et al., 1991.. In another mine, the coal above one of these trough-like features Žreferred to as a ‘dip’ or ‘hole’. contained more durains and partings than the area outside of the dip did ŽVogler, 1994..

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The three mines studied in this investigation occur north of the Belfry Anticline, and hence do not show thickness or quality effects related to this structure. In the study area, coal thickness and roof rock variation appear to be related to tidal–estuarine depositional controls, fractures, and possible tectonic controls, which were not discussed in previous investigations of the Pond Creek coal bed. Weak roof shales, rider coals, and narrow troughs or dips do occur in the study mines, and their effects on room-and-pillar mining, rather than on longwall mining, are discussed herein. Similar geologic features are known to occur in mines of correlative coals and other eastern Kentucky coals, so that this investigation serves as an example of the effects of variable geology on mining in the central Appalachians, as well as in other coal basins. 4. Mining geology In the study area, the Pond Creek seam is a mineable coal pod that splits to the south and west, and is bounded to the north and east by a northeast–southwest-oriented trend of thin coal ŽFig. 2A–B.. A cross-section of the coal across the study area shows the splitting margin of the coal and variable roof geology ŽFig. 2C.. Individual mine outlines

Fig. 2. Study area geology. ŽA. Data locations for study area. Mine outlines not shown for reasons of confidentiality. ŽB. Thickness of the Pond Creek seam. Split line shown by saw teeth. ŽC. Cross-section showing splitting margin of coal Žs., multiple riders ŽR., and variable roof geology. Numbers in oval insets correspond to figures in text projected along trend to this section.

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are not shown at the request of the cooperating mines, but the seam is mined or is being mined in most of the area where it is more than 75 cm Ž30 in.. thick. Specific elements of the mining geology are investigated along the line of section and compared with features in other Pond Creek coal mines to document geologic features that affect seam mineability. 4.1. Coal thickness Õariation In the three mines studied, the seam was generally between 90 and 180 cm thick. A split in the seam ŽFig. 2C. limits mining and extends along a similar trend across a broad region ŽThacker et al., 1998.. Thickness variation within the mines can be locally dramatic and is related to elongated dips in coal elevation, elongated trends of rock partings, and paleochannels ŽFig. 2C.. Thickness variations related to splitting, dips, and parting trends are discussed below. Thickness variations related to paleochannels are discussed in the mine-roof variability section. 4.1.1. Coal splitting Along the margin of thick Pond Creek coal, laterally thickening rock partings split the seam and effectively define the limits of mining. Two types of splitting are noted along the western boundary of mined Pond Creek coal. The first occurs where the lower third of the seam abruptly drops in elevation beneath the middle part of the seam to become a leader coal bench. The leader may occur laterally as much as 2 m Ž6.6 ft. below the main coal bench. The intervening clastics generally consist of dark-gray to black, carbonaceous shales. The second type of split occurs in the upper third of the coal ŽFigs. 2 and 3., often along the same trend as the lower split. In upper-seam splits, the top of the mined bed rises in elevation above a rooted claystone. Thickening of the split away from the unsplit coal is generally rapid ŽFig. 3.. Petrographic studies of the coal in one Pond Creek mine indicate that the split is at least locally correlative with a persistent durain layer in the middle of the seam ŽVogler, 1994..

Fig. 3. Generalized mine outline from western margin of the study area showing splitting margin of the coal and northeast–southwest-oriented cutouts and rock partings, relative to areas of thicker- and thinner-than-normal coal. Thickness of split shown in centimeters.

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4.1.2. Interpretation Splits along the margin of the seam record syndepositional clastic influx during peat accumulation. The fact that the lower bench of the seam generally drops in elevation beneath the split, coupled with the occurrence of several elongated dips in coal elevation Ždiscussed next., suggests a possible paleotopographic control on splitting. The dark color of the shale that tends to form the lower split is common in marine or marginal marine shales Žsuch as the overlying Crummies Member. in the central Appalachian Basin. No marine fauna have been found in the lower split, however, and there are few data beyond the upper split line to infer its origin. Thick sandstones have been reported lateral to the lower split in some mining areas ŽVogler, 1994., and suggest a lateral, coarse-grained clastic source, probably some type of channel. Interestingly, even though these splits suggest syndepositional influx of sediments into the Pond Creek mire, the coal has a consistently low ash yield. Either clastics were restricted from entering the main mire for any significant distance, or ash was flushed from the seam through post-depositional processes. 4.1.3. Coal dips In all three mines, the Pond Creek seam exhibits dips, in which the coal elevation drops and then rises along narrow elongated depressions in the floor ŽFig. 4A.. The dips have been called synclinal troughs in another Lower Elkhorn coal mine ŽNelson et al., 1991., and have been called swags ŽWeisenfluh and Ferm, 1991b., swilleys ŽElliott, 1965., and swalleys ŽClarke, 1963. in other basins. The dips in the Pond Creek mines do

Fig. 4. Coal dips. ŽA. Location of major cutout, trends of seam dips, and trends of densely concentrated partings in study area. ŽB. Detail showing trend of seam dip. ŽC. Cross-section of dip showing subtle increase in coal thickness and parting frequency along axis. ŽD. Cross-section of dip showing increased coal thickness and concentration of sandstone lithofacies above the dip ŽSh, Hs, Ss, and Sx are lithofacies discussed in text..

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not tend to rise in elevation prior to dropping into the axial depression, as has been described for swilleys and swalleys in British coal mines ŽClarke, 1963; Elliott, 1965.. Most dips are discontinuous, but at least one can be mapped as it meanders for more than 2 km in the study area. Extensive segments of these dips are oriented along northeast–southwest trends. Most dips are less than 50 m across ŽFig. 4B., and vary in depth from centimeters to as much as 5 m ŽFig. 4C–D.. In cross-section, dips may be asymmetric ŽFig. 4C. or symmetric ŽFig. 4D.. In most dips, the coal thickens toward the axis of the dip. In some dips, the coal increase was only slight, so it could easily be missed if not measured relative to the position within the dip. In other dips, the increase was more than a meter ŽFig. 4D.. Increased coal thickness in dips is generally accompanied by increased parting thickness; one dip exhibits a thick claystone Ž15 cm. toward the axis of the dip. Macroscopic measurements of durain and fusain layers in the coal across a dip in one mine indicate that increased coal thickness is mostly a function of additional coal in the bottom of the seam, and the upper part of the seam is relatively consistent onto the margins of the dip ŽVogler, 1994.; similar conditions are noted in these features in other basins ŽElliott, 1965.. The dips affect mining in several ways. While the coal is being mined, it is difficult to operate equipment safely on the slopes of some dips. Often, extra roof and floor rock must be extracted to improve operating and transportation conditions. The dips are also topographic lows, in which water can collect, causing environmental nuisances. In other mines, roof weaknesses were noted above and along the flanks of dips, caused by increased tensional stresses in the roof ŽNelson et al., 1991; Weisenfluh and Ferm, 1991b.. 4.1.4. Interpretation The dips noted in the Pond Creek mines represent abandoned paleochannels on the pre-Pond Creek mire paleotopography. Continuous trends represent larger drainages, and show a preferred orientation, and the seemingly random orientations of smaller dips probably represent tributaries and paleotopographic depressions. The U-shaped dip in the northern study area ŽFig. 4A. may represent an abandoned stream meander. Coal at the base of dip axes is inferred to have been the initial peat accumulations of the Pond Creek mires, a situation similar to coal-draped scours noted in other parts of the basin ŽGreb and Chesnut, 1992; Eble and Greb, 1997.. As the depressions filled with peat, they were also susceptible to clastic influx, and the resultant seam thickness within the dips often contains more partings than the area outside the dips. The upward decrease in partings within dips reflects infilling of the paleotopography, and a loss of confinement for intermire clastics as well as the peat. In some cases, the dips also affected roof lithofacies. Coarser grained strata often occur above the axis of the dip, whereas finer grained lithofacies occur laterally ŽFig. 4D.. Where coal riders drape lenticular sandstones above dips they may drop in elevation laterally onto the top of the main coal bench away from the dip. Hence, coal thickness approaching a dip may increase because of the addition of a coal rider, decrease where the rider splits from the coal, and then increase again into the dip. In some cases, scouring from overlying facies along the dip axis may also result in a loss of seam height toward the dip axis.

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4.1.5. Densely concentrated rock partings A unique feature of two of the mines studied was narrow belts of densely concentrated rock partings in the coal, which cause narrow trends of increased seam height. The partings occur in two elongated belts 200 to 1400 m wide, one paralleling the main cutout trend, and the second oriented subparallel to Žand possibly cutting across. the trend of the extensive eastern dip ŽFigs. 3 and 4A.. The western parting trend parallels but does not thicken toward or connect to the cutout sandstones, and in many areas the partings are separated from the cutout by 500 to 1000 m of parting-free coal. Likewise, the eastern partings roughly parallel the position of a continuous dip, but may occur in the center or either flank of the dip ŽFig. 4A.. The partings consist of very hard shale, siltstone, and sandstone laminae vertically interbedded with the coal ŽFig. 5A–D.. More than 50 partings may occur within the coal, each parting varying between 2 mm and 10 cm in thickness. In most exposures, very hard shale partings occur in the lower part of the seam Žsome to the base., and sandstone partings occur in the upper part of the seam ŽFig. 5A–D.. Generally, the thickness of partings increases upward, and laterally the extent of the partings increases upward. Upper partings may develop scour bases and truncate underlying laminae ŽFig. 5B–C.. Soft-sediment deformation is common in thicker beds, especially in shaley laminae ŽFig. 5A, C.. Some partings exhibit deformed ripple laminations ŽFig. 5B–C..

Fig. 5. Densely concentrated rock partings in the seam. ŽA. Rock partings showing soft sediment deformation Žarrows.. Seam height is approximately 1.8 m Ž6 ft.. ŽB. More concentrated rock partings, some exhibiting ripples. Seam height is approximately 1.4 m Ž4.6 ft.. ŽC. Margin of western parting concentrations showing millimeter-scale interfingering with coal and increasing width of sandy Žlighter colored. partings upward in the seam. Seam height is 2 m Ž6.9 ft. at this location, but only the upper two-thirds of the seam is shown. ŽD. Sketch from photomosaic of partings exposed in a rib, near the center of the western parting trend, where the upper sandy partings were bowed down beneath a sandstone roof roll.

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Toward the center of the parting trends, the upper laminae may be depressed and truncated by elongated trends of sandstone in the roof, a condition called ‘rolls’ by miners ŽFig. 5D.. Along the margin of parting concentrations, the upward increase in extent may cause the limit of partings to look like a scour margin ŽFig. 5C., but the coal laminae between the partings continue into the surrounding parting-free coal bed. Also, close examination of individual partings shows that they tend to be coarser-grained toward the center of parting concentrations and then fine laterally into carbonaceous shale and bone coal. Generally, the transition from coal with dense partings to coal with no partings occurs within 10 m Ž30 ft.. The parting concentrations are a significant obstacle to mining because they are associated with an increase in the ash yield of the surrounding coal, increased dust emissions during mining, and increased bit usage because of their hardness. Also, because they occur in elongated belts, they create a substantial barrier of unproductive mining that must be crossed to get to better coal. 4.1.6. Interpretation The increased seam thickness along the trend of partings is caused by the uncompactability of the partings relative to lateral peat in the parting-free seam. How the partings became so concentrated in the first place is problematic. Similar parting concentrations have not been reported elsewhere in the coal field. Concentrations of multiple clastic layers could have been introduced into the peat through detrital flooding, upwelling from springs, or injection from lateral sources. Because the coal laminae between partings are continuous and not penetrated by clastics, injection from underlying or overlying sources seems implausible. Hence, the clastic partings were probably deposited detritally. Conversely, each coal laminae continues into the main coal, so they do not represent detrital deposition but accumulation of plant litter and peat between periods of detrital clastic influx. In some areas, the margins of the partings look channel-form ŽFig. 5C., but each clastic layer continues into the surrounding coal, so the partings are not filling erosional channels. Yet because the partings occur in elongated belts, and at least toward the end of peat accumulation were deposited by currents Žas indicated by ripples., they must have occupied some type of drainage or water path in which currents moved and carried sediments from outside the mire into the mire. If clastics were only deposited within these intermire drainages during unusually high water, and peat accumulation was allowed to continue following each flood, a laterally restricted, dense concentration of partings could be formed. As will be discussed in following sections, roof rocks in the study mines contain evidence of tidal laminations, but the interlaminated coal and parting laminations probably do not represent tidal rhythmites. The accumulation of as much as 2 cm of coal between these laminations must surely represent more than a day, or biweekly accumulation of peat, as is common in rhythmites ŽKvale and Archer, 1990; Nio and Yang, 1991; Greb and Archer, 1998.. Possibly, unusually high storm tides or longer tidal-period events periodically led to higher than normal water levels in an estuary or trunk channel marginal to the main mire, which backed up sediment into these small intermire drainages. But it is hard to imagine how any type of intermittent drainage would persist

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in relatively the same location within a water-covered mire throughout the existence of the mire Žpresumably hundreds or thousands of years.. It is also surprising that the parting concentrations occur mostly on the margins of the dips, rather than along the axis of the dips, where a depositional low would seem to have been more likely. Perhaps tension along the flanks of the dips led to periodic tears along the hinge lines of the dips, which could be filled with sediment during floods and then reoccupied by peat-forming plants. Whatever the cause of the concentrations, the location acted as a sink for detritus and grew in scale and energy with time. The axis of the trend of small paleochannels in the roof above the parting concentrations parallels the dip axis ŽFig. 5D., indicating that this trend subsequently became a small channel. Likewise, the trend that parallels the main cutout may represent the position of the pre-channel drainage, which aggraded and shifted laterally to become the major washout channel. In terms of mining, the elongated nature of the partings, regardless of origin, indicates that they can be projected in advance of mining, the same way a paleochannel can. 4.2. Coal quality The Pond Creek coal bed in the study area is a low-sulfur, high-volatile A bituminous coal. All coals are washed, and the average coal product has 6.0% moisture, 7.0% ash, 0.72% sulfur, and a calorific value of 30 MJrkg Ž12,900 Bturlb.. Variations in coal quality are projected from borehole and mine samples. The most significant coal-quality issue is high sulfur, which appears to be linked to the presence of sandstone in the immediate roof. Locally, sulfur contents may increase to more than 1.5% beneath sandstones. Because the Pond Creek coal is generally a compliance product, run-of-mine quality is often the primary quality concern. Under several roof conditions, the amount of roof Žreject. that must be mined with the coal varies, thus decreasing the run-of-mine quality. Especially troublesome are Ž1. transitions between sandstone-dominated and shaledominated roof, Ž2. areas having rider coal within 1 m Ž3 ft. of the top of the seam Ž3. areas having densely concentrated rock partings, and Ž4. areas where seam thickness is less than 1.05 m Ž42 in... 4.2.1. Interpretation Increased sulfur contents beneath sandstones indicate post-depositional movement of sulfide- or sulfate-bearing waters through the permeable roof, as has been documented in the Blue Gem coal, a Pond Creek correlative ŽRimmer et al., 1985.. Sulfides may have been transported from the Crummies Shale Member, a marine unit overlying the sandstones, or from other units following burial. Because the increase in sulfur can be associated with roof type, coal mined beneath a sandstone roof Žhigher sulfur. is blended with coals mined beneath a shale roof Žlower sulfur. to produce a uniform product. Increased ash in the coal is easily lowered by washing. The cost of handling the reject materials and the lowered productivity is high, however, so that careful geological monitoring of roof lithofacies is required. Roof trends are discussed in Section 4.3.

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4.3. Mine-roof Õariability Roof rocks of the Pond Creek study mines consist of laminated, silty, gray shales, laminated siltstones, and fine- to coarse-grained sandstones. Laminated gray shales with abundant disseminated plant debris are the most common roof rock, but lithologic variability is common ŽFig. 2C.. Carbonaceous draw shales Žrash. in the immediate roof may coarsen upward into siltstones and fine-grained sandstones, may be overlain and truncated by intervals of interbedded sandstone and shales Žcalled ‘stackrock’ by miners., or may be overlain and laterally truncated by fine- to medium-grained sandstones. Roof falls are locally common along the transition zone between shale- and sandstone-dominated lithofacies. 4.3.1. Cross-bedded sandstone (Sx) lithofacies and cutouts Cross-bedded sandstones are medium- to coarse-grained, dominated by trough crossbedding, and have north to northeast dip orientations. Cross-bed foresets exhibit common shale drapes ŽFig. 6A., some of which appear to be bundled ŽFig. 6B.. Shale and coal clasts are common at the base of the cross-bedded sandstones, especially in areas where they truncate the underlying coal ŽFig. 6C., termed ‘cutouts.’ One cutout trend can be traced for more than 2 km along a discontinuous northeast–southwest orientation ŽFig. 4A., and is 0 to 60 m Ž200 ft. wide. Cross-sections through the cutout ŽFig. 7A–B. and along the margin of the cutout where it was not mined to access reserves on the other side ŽFig. 7C–E. show that the margin of the cutout is variable along the trend. In most areas, the seam dips in elevation Žas much as 5 m. and increases in thickness before being cut out ŽFigs. 6C and 7A–C.. In one entry, the seam increased in thickness to 3 m Ž9 ft. before splitting off toward the cutout ŽFigs. 6E and 7E.. Examination of the thick coal and the coal above the lateral split showed that Ž1. it was generally duller and more deformed than the main seam ŽFigs. 6D–E and 7A,C–E.; Ž2. the laminae in the coal above the split could not be traced laterally into the main seam where it regained normal thickness ŽFig. 6D.; Ž3. in some entries, it contained numerous slickenside surfaces, and was separated from the main coal by a slickenside plane; Ž4. it often contained circular to lensoid blocks of sandstone with thin coal rinds, which did not occur in the main coal bench ŽFigs. 6E–F and 7C–D.. Not all of the cutouts in the study mines were filled with cross-bedded sandstones. In one narrow, discontinuous cutout parallel to the contact of sandstone- and shaledominated roof ŽFig. 3., the cutout was dominated by laminated shale and siltstone, and then overlain by cross-bedded sandstone ŽFig. 8A–C.. In one entry, the fill consisted of dipping interlaminated shale and siltstone ŽFig. 8B., which coarsened upward into sandstone. Two entries away, the scour fill consisted of shale, but it was overlain and partially truncated by cross-bedded sandstone ŽFig. 8C.. The cross-bedded sandstone in the scour fill laterally graded into more sheet-form sandstones. This cutout was very discontinuous, although a trend of parallel fractures was continuous on either side of the cutout ŽFig. 8A.. 4.3.2. Interpretation Elongated, cross-bedded sandstone-filled cutouts are generally interpreted as paleochannels in coal mines ŽHorne et al., 1978; McCabe and Pascoe, 1979; Moebs and

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Fig. 6. Cross-bedded sandstone ŽSx. lithofacies. ŽA. Shale-draped cross-bed foresets in composite cross-bed in immediate roof. ŽB. Shale-draped foresets showing rhythmic bundling Žarrow. above sandstone roll in the roof. ŽC. Cross-bedded sandstone truncating coal. ŽD. Apparent split in coal along margin of large cutout. ŽE. Jim Hower points to another apparent split where the combined seam height is more than 3 m thick. White dashes indicate coal Žc.. ŽF. Sandstone-filled paleologs in overthrust coal bench.

Ellenberger, 1982; Fielding, 1986; Guion, 1987; Greb, 1991, 1992.. Timing of paleochannel deposition relative to peat accumulation is usually inferred based on the presence or absence of lateral splits and partings into the coal. Fig. 7A–B show the western margin of the sandstone interdigitating with the coal for a short distance. Because no parting continues into the coal, the interfingering could have been caused by injection along a ragged, eroded edge of the coal, rather than by syndepositional influx. Likewise, the apparent split in the uppermost part of the seam along the main cutout trend ŽFig. 7C–E. could be used to infer syndepositional channel formation, although the coal adjacent to the channel has extremely low ash yields, which would not be expected marginal to an active paleochannel. Examination of the split in the upper seam shows that the thickness of the rider bench is not retained laterally into the main coal,

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Fig. 7. Measured sections across major paleochannel margin. Seam thickness variation results from thinning beneath the axis of the sandstone ŽA–E.; thickening along the margin of the sandstone where a rider merges with the coal ŽA, E.; thickening above slickensided glide planes caused by compactional thrusting ŽA–B., and thickening caused by sand injection, overthrusting, or rider merging ŽC–D.. Overthickened seam heights indicated by white arrows.

laminae are not continuous into the lateral coal, and the rider coal has a different texture than the main coal, all indicating that the added coal thickness probably results from the rider bench merging with the Pond Creek coal, rather than the Pond Creek coal splitting. This may seem like a subtle difference, but it means that the split may not be a syndepositional feature, but a post-depositional feature relative to the main coal. Slickensided surfaces separating the upper coal from the main coal bench in some entries suggest that in some areas the upper coal bench was thrust over or slid down on

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Fig. 8. Narrow, fine-grained paleochannel. ŽA. Measured section across narrow paleochannel, showing parallel fracture trend and relation to sandstone–shale trend in roof. ŽB. Cutout shown in ŽA.. Hammer for scale. ŽC. Cutout in another entry with gray shale base Žsh., overlain by sandstone, suggesting increasing energy with aggradation. Hammer for scale.

top of the main bench, because of compaction from the lateral sandstone. Similar overthickened coals have been documented along the margin of the Fire Clay coal bed in another part of the coal field ŽGreb and Weisenfluh, 1996.. The circular to lensoid blocks of sandstone in the overthrust blocks are inferred to be fossilized logs, which were injected with sandstone from the channel during deposition. The fact that logs in the main bench were not similarly injected further suggests that the channel and upper bench accumulated after the main bench had been deposited. Shale-draped foresets and crude bundling of foresets in some cross-bed sets in the sandstone may represent tidal influences in the paleochannels ŽVisser, 1980.. The northern paleocurrent modes are opposite the normal western and southwestern modes for fluvial deposits in the lower Breathitt Group Žsee Greb and Chesnut, 1996., which would be consistent with flood tidal influences. Whether tidal or fluvial in origin, the main paleochannel follows a linear trend, which has been successfully projected in advance of mining. 4.3.3. Sandstone stackrock (Ss) lithofacies This facies, which is known as ‘stackrock’ by the miners, is common in several of the mines and generally occurs as irregular or wedge-shaped deposits bounded on the thinning margin by gray shales, and thickening toward cross-bedded sandstones, as shown in Fig. 4D. Stackrock sandstones generally are sheet form, fine grained and ripple laminated, although locally they contain discontinuous trough cross-beds ŽFig. 9A. separated by thin shale laminations. Ripple cross-lamination orientations from two fall areas in one of the study mines were north-northeast.

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Fig. 9. Sandstone stackrock ŽSs. lithofacies. ŽA. Sheet sandstone roof fall. ŽB. Two parallel trends of sheet sandstones and roof falls. Sheet sandstones occur as channel fills and more typical clastic wedges. Arrows show locations of falls along this trend. ŽC. Cross-section across largest fall area in ŽB., where irregular sheet sandstone was cut by a compactional fault beneath an overlying sandstone.

In most areas where stackrock roof falls are noted, falls occur along the thinning margin of the sheet sandstones where they are overlain by gray shales or coals, or are cut out by lateral sandstones ŽFig. 9B–C.. Roof falls are generally flat-topped and break straight up the ribs. The largest falls not related to fractures in the study mines occurred in the stackrock lithofacies. In one mine, two falls, each more than 30 m Ž100 ft. long and as much as 5.4 m Ž18 ft. in height, occurred in a thick sequence of stackrock, although there were no falls in lateral entries. 4.3.4. Interpretation Thin-bedded, sheet-form sandstones similar to those above the Pond Creek seam have been attributed to levee and splay deposits in coal mines ŽHorne et al., 1978; Moebs and Ellenberger, 1982; Hylbert, 1984; Guion, 1984.. In two of the Pond Creek mines, sandy stackrock facies are elongated in plain view ŽFig. 9B. and appear more like levee deposits than splays, which typically have broader, lobate distributions. If they are levees, the channels they bounded were small Žpossibly crevasse-like., and generally less than 2 m in depth, because the elongated sand pods are generally less than 2 m Ž6 ft. thick. In the southernmost study mine, where the largest stackrock falls occurred, stackrock bedding occurs across a much broader area, without a recognizable channel facies, so it probably was deposited as a splay, or series of stacked splay deposits. Lateral facies have sedimentary features that may suggest tidal influences, so levee deposits may represent tidal channel levees, or tidally reworked crevasse margins. Likewise, splays may be more similar to tidal sandflat deposits than they are to typical splays, but the data are insufficient in the falls studied to differentiate these possible environments. Stackrock roofs fall because the thin beds lack the strength to create a spanning beam across mine entries when bolted, especially across four-way intersections. In general, the closer the spacing of shaley laminae, the weaker the bedding ŽSames and Moebs, 1991..

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In tidal facies, where shale drapes would be common across most sand laminae, the weaknesses are perhaps greater than in fluvially-dominated stackrock facies, because adhesion between layers of strongly contrasting grain size is weak. Where individual beds within stackrock are more than 1 m Ž3 ft. thick, they generally are strong enough to support the roof when bolted ŽMoebs and Ellenberger, 1982; Hylbert, 1984; Greb, 1991, 1992.. Bolting must be completed quickly or the stackrock will sag and individual beds will separate and fail. In some cases, an overlying competent sandstone is present, and the stackrock interval is suspended by roof bolts anchored into the sandstone. Otherwise, resin bolts are used to beam the individual beds into a more competent unit. 4.3.5. Heterolithic stackrock (Hs) lithofacies The term stackrock is also applied to interbedded sandstone and shale, which is dominated by shale rather than sandstone; it is referred to here as heterolithic stackrock. This facies occurs lateral to the thin-bedded sandstone stackrock and above gray shale lithofacies. In most areas where this roof type occurs, spalling and thin sandstone sheet falls are common. This roof type is called ‘catalogue top’ by the miners because it tends to break into thin beds and laminae along shale laminations ŽFig. 10A.. Larger falls have occurred where this facies occurs between the mined coal and a rider in the roof ŽFig. 10B., and where this facies is interbedded with the gray shale lithofacies beneath

Fig. 10. Heterolithic stackrock ŽHs. lithofacies. ŽA. Small falls in miners’ catalogue top. ŽB. Rhythmically laminated fine-grained sandstone and shale exposed in heterolithic stackrock fall beneath coal rider. ŽC. Schematic cross-section of roof fall in one of the study mines occurring in the gray shale ŽSh. and heterolithic stackrock ŽHs. lithofacies. ŽD. Detail of interlaminations from ŽA. showing vertically repeated, crudely rhythmic sandstone and shale laminations in roof.

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sandstone stackrock in the roof ŽFig. 10C.. In these falls, combinations of factors, including compactional slips between the heterolithic stackrock and surrounding facies, resulted in failure. Some of the interlaminated sandstones and siltstones in this facies are rhythmically bedded ŽFig. 10D.. Rhythmites generally consist of bundles of three to five sandstone laminae, vertically thickening and thinning into shale laminae. In Fig. 10B, six bundles occur between ripple-bedded sandstones in the middle of the photograph. In Fig. 10D, at least five bundles occur. Bolting methods for the rhythmically-bedded strata are similar to those discussed for stackrock roofs. If a competent bed occurs above the rhythmites, the catalogue top has been suspended by roof bolts. Often, header boards are used where laminae are spalling. 4.3.6. Interpretation The heterolithic stackrock facies contains rhythmic beds similar to those deposited in tidal environments ŽKvale and Archer, 1990; Nio and Yang, 1991.. The arrangement of three to five laminae bundles in distinct bedsets may represent incomplete annual deposits, with each laminae bundle representing a month of deposition ŽGreb and Archer, 1998.. Regardless of the duration, these rhythmites occur in coal-draped clastic wedges marginal to thin-bedded sheet sandstones ŽFig. 4D. and may represent levees of minor tidal channels, or tidally-reworked splays and crevasse channels. Weaknesses between thin sand laminae and shale are similar to those previously discussed for sandstone stackrock deposits, although the catalogue top may be more susceptible to air slaking and horizontal stress. 4.3.7. Coal rider lithofacies Coal layers or beds that occur above the main seam are termed riders. One to three, thin Ž- 15 cm. coal riders are common within 5 m Ž15 ft. of the top of the Pond Creek seam, and in some areas may merge with the main coal bench. In the study mines, riders often cap laterally thinning stackrock intervals and then drop in elevation to the top of the coal, where they either merge with the coal, or thin above a rooted fireclay ŽFig. 4, Fig. 11A–C.. Fireclays beneath rider coals are generally thin Žless than 30 cm., appear leached, contain abundant carbonaceous root traces, and often exhibit small slickenside planes. 4.3.8. Interpretation Coal riders are common in Appalachian coal mines and represent the accumulation of additional peat mires after burial of the main peat swamp ŽMcCulloch et al., 1975; Moebs, 1977; Horne et al., 1978; Moebs and Ellenberger, 1982; Hylbert, 1984; Weisenfluh and Ferm, 1991a,b; Greb, 1992.. Several riders cap the narrow paleochannels and lateral stackrock facies in the study mines ŽFig. 2C., and in correlative Lower Elkhorn mines ŽNelson et al., 1991.. The riders in the study mines represent peat accumulation on the margins of levees bordering the channels. Where the riders drop toward the coal between channels, they may become part of the mined seam, or pinch out above a rooted fireclay with in situ lycopod stumps; this is indicative of clastic swamp deposits and illustrates paleotopographic control on rider peat accumulation.

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Fig. 11. Coal rider lithofacies. ŽA. Coal rider Žr. exposed in roof fall nearly 2 m above the mined seam, which laterally drops in elevation and pinches out above ŽB. fireclay in immediate roof of the main coal. ŽC. Cross-section in one of the study mines showing thin coal rider dropping in elevation above laterally pinching clastic wedge. Falls occurred sporadically along this trend.

Areas of riders often lead to increased roof-support costs, reduced productivity because of out-of-seam dilution, and increased potential for roof falls. Weaknesses in coal-rider roofs occur because of poor bonding between the thin coals and shales, ancient rooting structures beneath the coals disrupting bedding, slickensides in fireclays beneath coals, and the shale–coal contact concentrating moisture, which promotes shale swelling and continued falls ŽHylbert, 1984; Sames and Moebs, 1991; Weisenfluh and Ferm, 1991a,b; Greb, 1991, 1992.. In general, whether rider roofs in the study mines hold depends on the thickness and strength of the strata between the rider and main seam. Where the rider is less than a meter Ž3 ft. into the roof, it has been removed as draw rock, which increases out-of-seam dilution. Where the rider is more than 1.2 m Ž4 ft. above the main seam, and the interburden is gray shale, 4-ft resin bolts provide adequate support. Gray shales beneath riders are particularly susceptible to seasonal changes in air humidity Žprobably because of increased clay content from rooting., and spalling has been locally common in intake passages. Where combinations of weaknesses lead to falls or spalling, supplemental support Žsteel straps and header boards. are used to support the roof. 4.3.9. Gray shale (Sh) lithofacies The most common roof rock in the study mines is gray, silty shale containing sideritic laminations ŽFig. 2C.. In the immediate roof, this shale commonly contains carbonaceous streaks and abundant plant fossils ŽFig. 12A.. Because the carbonaceous streaks weaken the immediate roof, the lower part is often taken as draw rock during

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Fig. 12. Gray shale ŽSh. lithofacies. ŽA. Abundant plant fossil debris. ŽB. In situ tree stump with Stigmaria roots, called ‘kettlebottoms’ by miners. ŽC. Slumped shale blocks along distal margin of sandstone wedge. Black dashes highlight rotated bedding. White dashes mark the top of the coal. Hammer for scale.

mining. Out-of-seam dilution caused by the draw rock is an important economic factor in Pond Creek mines. Spalling is common in intake passages, and may continue after bolting. At one location where a rider had come near the top of the coal, the overlying gray shale contained a lag of crinoidal debris, and probably is the Crummies Shale Member. But in most of the study mines, the shale contained no marine fauna and the base of the marine member was probably higher. Gray shales may contain in situ lycopod stumps Žkettlebottoms. lateral to areas where riders drop in elevation from above lateral sandy facies ŽFigs. 3 and 12B.. These may result in isolated roof falls. Along shale–sandstone contacts in the roof, the gray shales may be deformed with common slickensides, and in some cases small paleoslumps ŽFig. 12C., which also cause small, local falls. 4.3.10. Interpretation Gray shale roofs are common in Appalachian coal mines. They were deposited from suspension in flood-plain, bay, marsh, and lacustrine environments ŽHorne et al., 1978.. Marine conditions have been inferred for the Crummies Shale ŽChesnut, 1991., but abundant plant fauna and a lack of marine fauna and bioturbation in the shale facies of the study mines indicates that the roof shale was dominantly deposited in nonmarine Žalthough possibly brackish. environments. These environments probably became increasingly marine as base level rose, culminating with transgression of the Crummies seaway. The lag of crinoidal debris seen at the base of a roof shale in one of the mines may represent a ravinement surface or transgressive lag at the base of the shale, where the base of the marine shale cuts into underlying nonmarine shales. Minor problems with spalling in the shales are common to all Appalachian coal mines because moisture concentrates along irregular carbonaceous laminations Žplant fossils. and thin claystones in the shales, and breaks along laminae are accentuated by seasonal moisture changes and shale swelling when the shale is exposed to moisture. Roof bolts themselves may act as points of weakness; many falls break at the bolts ŽAughenbaugh and Bruzewski, 1976; Chugh and Missavage, 1980; Greb, 1991.. Shales also have poor tensile strength, so they are more susceptible to horizontal stresses than sandy lithologies are.

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4.3.11. Fracture trends and oÕerburden thickness The longest falls in the study mines occurred in three sets of main entry passages and were extensive for more than 225 m ŽFig. 13A.. The rock types in the falls consisted mostly of gray shale Žwith locally thin carbonaceous streaks. and heterolithic stackrock lithofacies. These falls were not facies-dependent, and did not occur along northeast– southwest orientations as in other parts of the study mines, but rather along northwest– southeast orientations. A topographic overlay map shows that these large falls occur where the thickness of overburden decreases beneath an overlying drainage ŽFig. 13A.. Falls generally occurred in the middle entries of each panel. In each entry, falls began as fractures that broke up the rib line, commonly called ‘cutters,’ which continued after bolting. Supplemental support methods, including posts, cribs, angle bolts, and truss bolts, were used to keep the mains and a belt line open. Investigations of apparently random falls in shaley and heterolithic stackrock roofs in one mine showed that they were related to fractures. The falls occurred in both east–west- and north–south-oriented passages. Orientations of the fall margins where they broke across entries showed a preferred northwest–southeast direction ŽFig. 13B.. A line through the center of several sets of apparently random falls, parallels the fracture

Fig. 13. Fracture-controlled roof falls. ŽA. Mine map showing large falls below northwest–southeast-oriented modern drainage. Contour lines are in 30 m Ž100 ft. intervals and rise in elevation on both sides of the stream. ŽB. Parallel trends of smaller falls, possibly caused by horizontal stress field. Rose diagram shows orientations of fractures bounding falls, and spalling associated with cutters and roof sag along the trends. ŽC–E. Illustrations of mine roofs from locations in ŽB. showing roof sag and beginning of centerline tension fracture ŽC., falls bounded by slickensided Žs. fracture ŽD., and center-line fracture fall ŽE.. Rib cutters are designated by ‘c.’

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trends Žshaded trends in Fig. 13B.. Individual falls broke along northwest–southeast-oriented fractures ŽFig. 13D. or along the center line ŽFig. 13E.. Subsequent in-mine examination of areas northwest and southeast of known fall areas showed that roof sag along the center line ŽFig. 13C. and small rib cutters occurred along trend ŽFig. 13B.. The roof sag indicates that slippage is occurring along bedding planes in the roof. Continued sag could result in tension fractures along the center line of the entry and more collapse ŽMoebs, 1977.. 4.3.12. Interpretation Many fractures in the Eastern Kentucky Coal Field are caused by the process of unloading or differential gravitational loading beneath modern valleys ŽOverbey et al., 1973; Moebs, 1977; Hylbert, 1984; Kipp and Dinger, 1991; Sames and Moebs, 1989.. High topographic relief results in high lateral compressive stress between the valley and valley walls; in the Appalachians this stress may affect underlying strata to depths of 180 m Ž600 ft. ŽMoebs, 1977; Sames and Moebs, 1991.. In the study mine, large falls occur beneath a major tributary drainage with 90 m Ž300 ft. of relief ŽFig. 13A.. Many modern drainages in the Eastern Kentucky Coal Field are entrenched along preexisting fractures, and this tributary may be an example. The other fracture-bound falls also occurred along northwest–southeast orientations, although along a slightly different trend, and not beneath modern valleys ŽFig. 13B.. In this case, an overlay map of topography shows little correlation, but careful documentation of small falls and roof sag or excessive spalling ŽFig. 13B. helps to define northwest– southeast-oriented trends, which may be caused by regional horizontal stresses oriented perpendicular to fall orientation. Because several parallel trends are noted, the trend can be projected in advance of mining to aid in future mine planning and support plans.

5. Discussion 5.1. Thick coal trends Unlike many seams in the central Appalachian Basin, the Pond Creek is syndepositionally split and contains localized partings ŽFig. 2C., but has very low sulfur contents and ash yields on a washed basis. In other areas where the seam or correlatives are mined, ash yield, parting frequency, and coal thickness have been attributed to syndepositional growth of the Belfry Anticline ŽHower et al., 1991a.. The study mines occur north of the influence of the anticline. In the study area, thicker-than-average seam heights occur because of Ž1. increased coal thickness within dips ŽFig. 14A., Ž2. dense concentrations of rock partings ŽFig. 14B., Ž3. merging rider coals ŽFig. 14C., and Ž4. emplacement of coal blocks on top of the main seam along paleochannel margins ŽFig. 14D.. Each of these factors causes laterally restricted trends of thickness variation, generally along northeast–southwest orientations. In dips, the increased coal thickness occurs toward the base of the seam, suggesting that it was controlled by the paleotopography ŽFig. 14A.. In some dips, the initial accumulations were separated from the main seam, so that the local coal accumulation is

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Fig. 14. Ways in which the Pond Creek coal bed thickens.

called a leader coal ŽFig. 15.. In other dips, peat accumulation was continuous and the extra accumulation is considered part of the main coal. Both the leaders and dips define an uneven surface beneath the seam, inferred to represent paleotopography ŽFig. 15.. The increased coal thickness, splits, and partings in dips illustrate strong paleotopographic control on initial mire accumulations. The dips are also associated with dense concentrations of rock partings. These partings cause increased total seam thickness ŽFig. 14B.. As discussed previously, the unusual rock partings occur parallel to, but not usually within the dips ŽFig. 15.. Hence, they must represent either fractures or breaks in the peat slopes along the margins of the dips, or some type of intermire drainage controlled by the location of the preexisting dip. The dips themselves may have been partly controlled by northeast–southwest-oriented fractures or faults. Although slightly meandering, the dips become parallel along northeast–southwest orientations, parallel to the paleochannel cutouts, in the western part of the study area ŽFig. 4A.. The Pond Creek peat accumulated in an actively subsiding foreland basin, which subsided towards the southeast. Fault-controlled sedimentation has been inferred for other central Appalachian coals that showed northeast–southwest thickness trends ŽStaub et al., 1991; Weisenfluh and Ferm, 1991a; Greb et al., 1999., and may apply here. Interestingly, the total coal isopach shows a northeast–southwest-oriented trend of coal thinning along the southwestern part of the study area ŽFig. 2B.. This parallels the

Fig. 15. Pond Creek coal bed and roof model showing paleotopographic control on coal thickness and roof lithofacies.

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trend of the main cutout and split along the western margin of the study area. These trends, as well as the parallel dip locations and unusual parting concentrations, may represent the subtle expression of fault- or fracture-controlled paleotopography which developed as the basin subsided along a series of parallel block faults toward the southeast. Some of the paleotopographic dips that controlled initial peat accumulation continued to control clastic sedimentation following peat burial ŽFig. 15.. Along several dips, minor channels and lateral levees or splays were concentrated. The levees and splays became topographic highs, which in turn controlled the position of coal riders. Where riders merge with the main coal, total seam thickness increases from above ŽFig. 14C., with little lateral continuation into the main coal. Because the riders often cap clastic wedges centered along the axis of dips, they merge along dip-parallel orientations with the coal, although the trend of merging is significantly more variability than the previously discussed trends because of the irregular margins of the clastic wedges between the riders and the main coal. The main cutout channel trend also follows a northeast–southwest trend. Determining if the dip in coal elevation beneath the cutout sandstone is caused by a preexisting paleotopographic depression or resulted from post-depositional compaction beneath the sand is difficult. Both factors may have been important. But the fact that a parting trend parallels the flank of the cutout, just as the other parting trend flanks an extensive dip, suggests a similar origin ŽFig. 15.. Likewise, the northeast–southwest orientation may indicate that channel position was controlled by a preexisting fracture or fault. Total coal thickness increases along the margin of the channel because of riders dropping or slumping down the margin of the channel onto the lateral coal, and because of compactional overthrusting of coal onto lateral coal ŽFig. 14D.. 5.2. Roof rock trends Just as coal thickness trends correlate with paleotopography and resultant dips, so too do roof rock trends. The lithofacies examined occur in a lateral succession from coarsest to finest, the coarsest facies generally centered above a dip ŽFig. 15.. Roof falls locally occur along the margin between sandstone-dominated and shale-dominated facies, and generally parallel the northeast–southwest orientation previously discussed, except for deviations caused by local irregularities in the sandstone–shale margin. Recognition of these trends has helped predict roof conditions in advance of mining so that potential weaknesses in roof strata can be looked for during mining, and support plans can be adjusted as needed. The most extensive falls in the study mines were not facies-dependent, however, but resulted from a northwest–southeast-oriented regional fracture trend, and depth beneath valleys entrenched along that trend. These falls occur regardless of the lithofacies shown in Fig. 15, but are perhaps worse in shale-dominated lithofacies, which are more susceptible to tensional stresses. Recognition of the fracture trend aided in correlation of apparently random falls in the second mine, and the trend can now be projected in advance of mining. Larger falls can be anticipated at the intersection of the two trends: below valleys, under thinning overburden, along northwest–southeast trends, and along sandstone–shale contacts in the roof.

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Acknowledgements The authors wish to thank the mines that cooperated with this study. Thanks to M.L. Smath of the Kentucky Geological Survey for grammatical review. Thanks to R.F. Rathbone and J.C. Hower for critical review. Thanks also to J.C. Hower for accompanying the authors on one of their mine visits.

References Aughenbaugh, N.B., Bruzewski, R.F., 1976. Humidity effects on coal mine roof stability. US Bureau of Mines Open-File Report 5–78, 164 pp. Chesnut, D.R., Jr., 1991. Paleontological survey of the Pennsylvanian rocks of the Eastern Kentucky Coal Field: Part 1. Invertebrates. Kentucky Geological Survey, Series 11, Information Circular 36, 71 pp. Chesnut, D.R., Jr., 1992. Stratigraphic and structural framework of the carboniferous rocks of the central Appalachian Basin in Kentucky. Kentucky Geological Survey, Series 11, Bulletin 3, 42 pp. Chugh, Y.P., Missavage, R.A., 1980. Effects of moisture on strata control in coal mines. In: Chugh, Y.P., Van Besien, A. ŽEds.., Proceedings, First Conference on Ground Control Problems in the Illinois Coal Basin. Carbondale, Southern Illinois University, pp. 70–88. Clarke, A.M., 1963. A contribution to the understanding of washouts, swalleys, splits, and other seam variations and the amelioration of their effects on mining in South Durham. Mining Engineer 33, 667–706. Eble, C.F., Greb, S.F., 1997. Channel-fill coals on the western margin of the Eastern Kentucky Coal Field. International Journal of Coal Geology 33, 183–207. Elliott, R.E., 1965. Swilleys in the Coal Measures of Nottinghamshire interpreted as palaeo-river courses. The Mercian Geologist 1, 133–142. Fielding, C.R., 1986. Fluvial channel and overbank deposits from the Westphalian of the Durham Coalfield, NE England. Sedimentology 33, 119–140. Greb, S.F., 1991. Roof falls and hazard prediction in eastern Kentucky coal mines. In: Peters, D.C. ŽEd.., Geology in Coal Utilization. American Association of Petroleum Geologists, Energy Minerals Division. Techbooks Publishing, Fairfax, VA, pp. 245–262. Greb, S.F., 1992. Heterogeneity in seam and roof related to mineability prediction: Hazard no. 8 coal, a case study. In: Platt, J., Price, J., Miller, M., Suboleski, S. ŽEds.., One Point Two-New Geologic Perspectives on Central Appalachian Low-Sulfur Coal Supply. American Association of Petroleum Geologists Coal Decisions Forum Publication. Techbooks, Fairfax, VA, pp. 102–124. Greb, S.F., Archer, A.W., 1998. Annual sedimentation cycles in rhythmites of carboniferous tidal channels, in tidalites—processes and products. Society of Economic Paleontologists and Mineralogists Special Publication 61, 75–83. Greb, S.F., Chesnut, D.R. Jr., 1992. Transgressive channel filling in the Breathitt Formation, upper Carboniferous Eastern Kentucky Coal Field, USA. Sedimentary Geology 75, 209–221. Greb, S.F., Chesnut, D.R. Jr., 1996. Lower and lower Middle Pennsylvanian fluvial to estuarine deposition, central Appalachian basin—effects of eustacy, tectonics, and climate. Geological Society of America Bulletin 108, 303–317. Greb, S.F., Weisenfluh, G.A. Jr., 1996. Paleoslumps in coal-bearing strata of the Breathitt Group, Pennsylvanian, Eastern Kentucky Coal Field, USA. International Journal of Coal Geology 31, 115–134. Greb, S.F., Eble, C.F., Hower, J.C., 1999. Depositional history of the Fire Clay coal bed, late Duckmantian, eastern Kentucky, USA. International Journal of Coal Geology 40, 255–280. Guion, P.D., 1984. Crevasse splay deposits and roof-rock quality in the Three Quarters Seam ŽCarboniferous. in the East Midlands Coalfield, UK. International Association of Sedimentologists Special Publications 7, 291–308. Guion, P.D., 1987. Paleochannels in mine workings in the High Hazles Coal ŽWestphalian B., Nottinghamshire Coalfield, England. Journal of the Geological Society ŽLondon. 144, 471–488.

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