Multiple-bench architecture and interpretations of original mire phases—Examples from the Middle Pennsylvanian of the Central Appalachian Basin, USA

International Journal of Coal Geology 49 (2002) 147 – 175 www.elsevier.com/locate/ijcoalgeo

Multiple-bench architecture and interpretations of original mire phases—Examples from the Middle Pennsylvanian of the Central Appalachian Basin, USA S.F. Greb a,*, C.F. Eble a, J.C. Hower b, W.M. Andrews a a

Kentucky Geological Survey, 228 MMRB, University of Kentucky, Lexington, KY 40506-0107, USA b Center for Applied Energy Research, University of Kentucky, Lexington, KY 40511, USA Accepted 1 October 2001

Abstract Coal seams often exhibit lateral and vertical variability in composition. When sampled as a whole seam this variability is masked. But if a seam is subdivided into correlateable components, this variability can be tested and better understood. Herein, an architectural approach is used to divide seams into intra-seam components. Clastic splits and mineral partings, as well as persistent fusain and durain layers, can be used as intra-seam bounding units to subdivide a seam into subdivisions called benches. Regional examination of Lower and Middle Pennsylvanian-age coal seams shows that many contain laterally persistent bounding surfaces that can be used to define multiple benches of coal within each seam. Inter-bench analyses from some of the most extensively mined seams in the central Appalachian Basin show that individual benches often have different spatial and quality trends. Hence, some component of whole-seam variability is a function of changes in the relative contribution of these different benches to the seam as a whole. Many coal benches also exhibit intra-bench variation in coal parameters. Intra-bench variation can be analyzed in terms of parameters such as sulfur content and ash yield in order to address changes in coal quality for regional resource evaluation. Intra-bench variation can also be analyzed in terms of a combination of palynologic, petrographic, and geochemical parameters, termed compositional groups, in order to better understand the development of the original mire systems. Compositional groups are defined by ranges of multiple criteria, which are inferred to owe their origin to the mire type in which they formed. Vertical changes in compositional groups within coal benches can be used to infer paleo-edaphic conditions during peat accumulation. If seam thickness is a product of bench configuration, and trends in compositional groups occur in benches, then trends in quality can be marginally predicted based upon seam thickness and inferred bench architecture. Additionally, changes in inter-bench and intra-bench thickness and compositional profiles can be used to infer original depositional controls, such as paleotopography, syndepositional structural movement, and syndepositional clastic influx for more accurate reserve estimates. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Coal; Bench; Architectural-element analysis; Depositional models; Peat models; Coal quality

1. Introduction

*

Corresponding author. Fax: +1-859-257-1147. E-mail address: [email protected] (S.F. Greb).

Architectural analysis of layered clastics involves the ordering of bounding surfaces to delineate a hierarchy of units between those surfaces. Perhaps the most

0166-5162/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 6 - 5 1 6 2 ( 0 1 ) 0 0 0 7 5 - 1

148

S.F. Greb et al. / International Journal of Coal Geology 49 (2002) 147–175

commonly used scale of unit are lithofacies, which exhibit similar composition, and are inferred to represent deposition by the same physical process or suite of processes. Coal beds or seams are commonly defined as a single lithofacies, which in clastic sedimentology may be sufficient. But coal geologists recognize that a single coal bed can represent accumulation of a variety of different types of mires, each representing different suites of processes, which greatly affect the coal quality (e.g., sulfur content, ash yield) and mineability of the coal they form. If these different mire conditions can be identified within the subsequent coal in a structured or ordered manner, quality and thickness trends can be more readily predicted. In the central Appalachian Basin, coals often occur in stratigraphic clusters called coal zones (Fig. 1). Coal zones comprise multiple coal beds in a stratigraphic interval, separated from overlying coal beds or zones by a stratigraphic interval of non-coal-bearing strata. The zones generally have one or more widely continuous coal beds, associated with additional coal beds of lesser continuity (Fig. 1). For the purpose of this report, the term coal bed simply refers to a bed-scale stratigraphic unit of coal, whereas coal seam is used to define the mined unit of coal (shaded boxes in Fig. 1). In many cases, the two terms can be used interchangeably because the mined unit is equivalent to the stratigraphic unit. Regional mapping of coals in the Eastern Kentucky Coal Field indicates that most coal zones have a persistent main bed (CM in Fig. 1) that is extensive for areas of hundreds of square kilometers. These beds may locally be underlain by additional coal beds, generally termed ‘‘leaders’’ (CL in Fig. 1), and overlain by coal beds generally termed ‘‘riders’’ (CR in Fig. 1). Individual leader and rider beds occur across smaller areas than the main beds. In other cases, rider

(CR) or leader (CL) coal beds may merge with the main (CM) coal bed, or two main beds may come close together, such that the coal seam is a composite of completely different coal beds. When these beds combine into a single seam, or where a single bed or seam contains persistent partings, the subcomponents are generally not considered separate beds, but rather are called coal benches (e.g., Staub, 1991). Coal benches are increments of coal beds and seams bound by floor, roof, clastic partings, or persistent and subtle durain (dull coal) layers (dashed lines in Fig. 1). Where coal beds lack partings or persistent identifiable durain layers, the coal bed is equivalent to a coal bench. In other cases, where coal beds contain persistent partings, the coal bed can be subdivided into coal benches (CMa, CMb, in Fig. 1) using the parting as a boundary. Incremental sampling within coal benches has shown that individual benches within a coal seam often exhibit less lateral and vertical variations in coal parameters than that between benches for the whole seam (e.g., Eble et al., 1994). These inter- and intra-bench variations appear to be related to the stacking or architecture of the benches within the coal bed or seam, and may result from origins in different types of paleomires. In order to interpret the original mire types, various criteria are summarized in the following section.

2. Coal depositional models In the last two decades, the recognition that paleomire morphology may significantly control the quality of the coal bed formed has led to increasingly sophisticated attempts at interpreting coals not just as swamps or mires, but specific types of mires. Thin

Fig. 1. Conceptual diagram of coal zones as occur in the Central Appalachian basin, and expanded view of regionally persistent coal beds within the zone, which are commonly comprised of multiple, component benches. In this report, the term coal seam is used to indicate the mined coal (shaded boxes), which may be equivalent to a bed, or may be comprised of multiple beds. CM = main persistent bench or bed, CL = leader bench or bed, CR = rider bench or bed.

S.F. Greb et al. / International Journal of Coal Geology 49 (2002) 147–175

peats, rooted seat earths, and kettlebottoms may form in a wide variety of swamp and marsh settings (e.g., Teichmu¨ller, 1989), but peats thick enough to form mineable Carboniferous coal seams are thought to have accumulated in two basic types of forest mires. Planar, also called topogenous mires or low-lying moors (Teichmu¨ller and Teichmu¨ller, 1982), generally occur at or just below the ground-water table and derive most of their moisture from ground water (Fig. 2A). Because they are ground-water fed they are also termed rheotrophic mires (Moore, 1989). Modern examples of rheotrophic mires include temperateclimate mires of North America (Cameron et al., 1989), the Snuggedy, Okefenokee, and Everglades (Fig. 2A) peats of the southeastern United States (Spackman et al., 1964, 1969, 1974; Cohen, 1974, 1984; Staub and Cohen, 1979), peats of the Orinoco delta in Venezuela (Scheihing and Pfefferkorn, 1984; Wagner and Pfeferkorn, 1997), and peats of the Mississippi delta in North America (Kolb and Van Lopik, 1966; Frazier and Osanik, 1969; Kosters et al., 1987), although the latter deltaic examples are likely to produce carbonaceous shales rather than mineable

149

coal seams (McCabe, 1984). Numerous examples of modern rheotrophic mires are discussed in Gore (1983). Raised mires, also termed domes, high moors (Teichmu¨ller and Teichmu¨ller, 1982), or ombrotrophic mires (Moore, 1989), build up above the topography like sponges and derive their moisture from rainfall in ever-wet climates (Fig. 2B). Modern examples of ombrotrophic mires include temperate climate peats of the Pacific (e.g., Styan and Bustin, 1983), and Atlantic (Chague and Fyfe, 1996) coasts, and tropical climate peats of Indonesia (Anderson, 1964, 1983; Polak, 1975; Clymo, 1987; Esterle et al., 1992). Examples of modern ombrotrophic mires are discussed in Gore (1983). Research on modern Indonesian peat domes indicates that doming is associated with a lateral succession of plant types, a general trend of decreasing tree size, a reduction in nutrients, and increased oxidation from the margin of the mire towards the mire center (Fig. 2B; Anderson, 1964, 1983). The recognition that trends in these parameters might be found in coals led Smith (1957, 1962) to conclude that some of the coal beds in the British coal measures had ombrogenous

Fig. 2. Analogues of forest mires used in interpretations of coal. (A) Rheotrophic coastal model based on mires of the southeastern United States showing lateral trends from mangrove-forest to marshes with islands or hammocks of peat forests (modified from Spackman et al., 1964, 1974). (B) Indonesian ombrotrophic mire (modified from Anderson, 1964).

150

S.F. Greb et al. / International Journal of Coal Geology 49 (2002) 147–175

origins. The domed analogy remained unheeded for many years until the mid-1980s when numerous authors began to infer that various aspects of highquality (low-ash, low-sulfur) coals were the result of ombrogenous peat accumulations (McCabe, 1984, 1987; Cecil et al., 1985; Esterle and Ferm, 1986; Clymo, 1987; Cameron et al., 1989; Esterle et al., 1989). Because the type of water input into ancient mires cannot be directly measured in the subsequent coal bed, a wide variety of analogous criteria has been used to infer rheotrophic and ombrotrophic origins in coals. These criteria and some of the factors that need to be considered when using the criteria are summarized herein. 2.1. Palynological criteria Because miospores remain relatively unaltered after coalification relative to other plant parts, palynology is a useful tool for inferring the types of plants and plant successions that accumulated in the ancient mires represented by coal. For example, lycopod tree spores, such as Lycospora, dominate many Carboniferous coal beds. Lycopod trees appear to have needed everwet substrates for their reproductive strategies (DiMichele and Phillips, 1994), hence, their dominance in a coal can be used to infer dominantly rheotrophic paleomire conditions. Numerous coals have been interpreted as rheotrophic mires, based in part on the abundance of lycopod tree spores (Calder, 1993; Calder et al., 1996; Andrews et al., 1996; DiMichele et al., 1996). Some modern peats show successions of different mire types. The lateral and vertical succession of planar-rheotrophic (mire margin in Fig. 2B) to domed-ombrotrophic (open bog plain in Fig. 2B) mires seen in modern tropical mires was first used as an analogue for Middle Pennsylvanian coals of Great Britain by Smith (1957, 1962). Palynological analyses of Yorkshire coal beds established a common cyclic vertical succession from Lycospora-producing lycopod trees, to Densospore-producing lycopod shrubs, which were inferred to be analogous to the stunted trees in domed mires, back to lycopod trees and standing water conditions, inferred to represent drowning of the mire (Fig. 3). Not all coals show similar successions, however, and there can be significant

Fig. 3. Vertical trend in palynological variation interpreted to represent cyclic mire phases from planar to domed and back to planar mires (modified from Smith, 1957, 1962).

ambiguity concerning paleoecological interpretations of densospores. Densosporites was produced by the small tree Omphalophloios (Bodeodendron/Sporangiostrobus, Wagner, 1989), which appears to have occupied areas of Carboniferous mires that were stressed. The stress may have been caused by (1) depressed water tables and nutrient deficiency as would be expected in domed-ombrogenous mires (Smith, 1962; Eble and Grady, 1990; Pierce et al., 1991), (2) clastic influx and salinity that occurs during transgression (Habib and Groth, 1967), as would occur in a planar-rheotrophic mires; or (3) Omphalophloios abundance may have been partly controlled by climate (Butterworth, 1966). Hence, increasing percentages of densospores by themselves are not a good indicator of ombrotrophic paleomires. 2.2. Ash-yield criteria By far the most widely used criteria to infer original mire morphology are ash yields and clastic parting occurrence. Since rheotrophic mires generally accumulate within depressions (below HW in Fig. 2A) and are fed by ground water, they have a high potential to accumulate mineral matter (both detrital and plantderived), which results in clastic partings and high ash yields in coals. In contrast, ombrotrophic mires build up above the level of clastic influx (above HW in Fig. 2B), so are generally parting free, and are leached of inherent ash derived from wind or the plants themselves by rain water. Ash yields of less than 5% are characteristic of modern Indonesian ombrotrophic peats (Polak, 1975; Cameron et al., 1989; Esterle et al., 1992; Neuzil et al.,

S.F. Greb et al. / International Journal of Coal Geology 49 (2002) 147–175

1993). This distinction has led numerous researchers to use a lack of partings and low ash yield in coal beds as indicators of ombrotrophic origins (Cecil et al., 1985; Clymo, 1987; McCabe, 1987; Staub, 1991; Staub and Esterle, 1992). However, ash yield is not necessarily a diagnostic criteria for distinguishing paleomire types. Although all modern ombrotrophic mires have low ash yields, not all rheotrophic mires have high ash yields. McCabe (1984) noted that low ash yields in coal beds could be caused by (1) peat doming, (2) pH differences between mire and riverine waters, (3) a lack of syndepositional clastic influx during peat accumulation, or (4) by peats of such broad extent that they were too far removed from clastic sources. The inference of pH influences is based on modern analyses in the topogenous mires of the southeastern United States, where pH differences in water chemistry cause clay to flocculate adjacent to the channel margin, apparently limiting extensive inundation into the mire (Staub and Cohen, 1979). This analogy has been used in several studies that have interpreted coal beds as ancient planar, low-ash peats (Litke, 1987; Cairncross and Cadle, 1988; Staub and Richards, 1993). Another complication is that studies of modern ombrotrophic mires (Anderson, 1964; Gore, 1983; Moore, 1989; Chague and Fyfe, 1996) indicate that ombrotrophic mires begin as rheotrophic mires, and may pass laterally into rheotrophic mires (mire margin in Fig. 2B). Although some mires begin and end as rheotrophic types, it seems probable that no coal bed was completely deposited under ombrotrophic conditions across its entire extent. Additionally, changes in ash content that might be used to infer paleomire types must be tempered with the knowledge that mineral matter can be introduced into a peat/coal or leached from the peat/coal by diagenetic fluids during burial, independent of mire origin (Williams and Keith, 1963; Gluskoter and Simon, 1968; Horne et al., 1978; Spears, 1987). 2.3. Petrographic criteria Macerals of coal are analogous to grains in clastics. Vitrinite is a maceral derived from wood and woodlike tissues, whereas inertinite is a maceral derived from oxidized, previtrinite material. The upward decrease in woody material in modern ombrogenous

151

peats has been used as an analogue for the upward decrease in vitrinite macerals within some coals (Esterle and Ferm, 1986; Esterle et al., 1989; Calder, 1993). As vitrinite preservation is enhanced in aqueous conditions (Teichmu¨ller, 1989), an upward-decreasing vitrinite pattern within a seam may suggest temporal moisture stress as well as a decrease in woody material, both consistent with ombrotrophic conditions. However, these petrographic associations are not ubiquitous. For example, increased inertinite contents have been noted where modern forest mires are drowned and succeeded by reed mires (Teichmu¨ller, 1989). Upward dulling has also been noted in coals with ash contents and other criteria typical of rheotrophic conditions (Staub and Richards, 1993). Increased degradation could also be caused by bacterial activity in waters of elevated pH, as would be characteristic of rheotrophic mires (Calder et al., 1996). Hence, increased inertinites at the expense of vitrinite in dulling-upward profiles simply reflects increased oxidation of the original peat, which can occur because of drying and degradation, or because of the influx of well-oxygenated extra-mire waters and flooding. Additionally, drying is not a unique feature of domes (Calder, 1993). Droughts that dry surficial peat in planar mires and crown fires that burn the upper parts of trees in planar mires can cause inertinite-rich layers and upward dulling even in rheotrophic mires (Staub and Cohen, 1979; Cohen et al., 1987; Scott, 1989: DiMichele and Phillips, 1994; Scott and Jones, 1994). 2.4. Compositional group criteria Many of the contradictions that arise from the use of the previously discussed criteria for inferring the original depositional environment of an individual coal bed occur because of (1) the use of only one of the criteria as the basis for interpretations, and (2) inferences on a whole-bed basis, without regard to possible intra-bed or intra-seam variation. Because there are exceptions to each individual criteria, an assimilation of all criteria is advocated in order to decrease the bias of any one criteria to the final interpretation. Smith (1962) used palynology, petrography, and partings to infer paleomire ‘‘phases’’ within British coals. Other studies have used combi-

152

S.F. Greb et al. / International Journal of Coal Geology 49 (2002) 147–175

Table 1 Compositional groups for Middle Pennsylvanian coals of the central Appalachian basin (after Eble and Grady, 1990; Eble et al., 1994) Mid-Pennsylvanian compositional groups

Palynology

Petrography

Dominant spores and plant types

Vitrinite %

Inertinite %

Mixed Palynoflora – High Ash Lycopospora – Vitrinite Dominant Mixed Palynoflora – Vitrinite Dominant Mixed Palynoflora – Low Vitrinite – Low ash

Mixed

Low – Mod (<80%) High (>80%) High (>80%) Low – Mod (< 80%)

Variable (mod – high) Variable (low – mod) Variable (low – mod) High (>15%)

>70% lycopod trees Mixed < 70% lycopod trees Mixed >10% lycopod shrubs, < 70% lycopod trees

nations of criteria to interpret paleomire types (Eble and Grady, 1990; Grady et al., 1993; Andrews et al., 1996; Calder et al., 1996; Hower et al., 1996). Eble and Grady (1990) utilized ash yield, sulfur content, palynological associations, and petrographic variation in compositional groups to derive sets of characteristics that should be analogous to rheotrophic, ombrotrophic, and transitional mires (Table 1). For Middle Pennsylvanian coal beds, four compositional groups: (1) Mixed Palynoflora –High Ash, (2) Lycospora– Vitrinite Dominant, (3) Mixed Palynoflora – Vitrinite Dominant, and (4) Mixed Palynoflora – Low Vitrinite – Low Ash, have been developed as analogies to (1) clastic-influenced and mire-margin planar-rheotrophic mires, (2) planarrheotrophic mires, (3) transitional-mesotrophic mires,

Ash yield

Sulfur content

Analogous position in Fig. 4

High (>10%) Variable (mod – High) Variable (low – mod) Low (<10%)

Variable

Planar mire margin Planar mire Transitional mire Domed mire

Variable Variable Variable (low – mod)

n

5

and (4) domed-ombrotrophic mires, respectively (Table 1, Fig. 4; Eble and Grady, 1990; Eble et al., 1994). In order to determine if a coal bed contains changing mire phases, incremental samples are needed at a smaller scale than the bed or bench. Fifteen to 30 cm (6 to 12 in.) increment samples have been used in several investigations of central Appalachian Basin coals, because there are usually naturally occurring breaks in coal lithotypes at that scale, and because that scale is enough to determine intra-bench variation in coal benches that generally are less than 2 m (6 ft) thick (Eble et al., 1989, 1994; Eble and Grady, 1990, 1993; Staub, 1991; Grady et al., 1992, 1993; Hower et al., 1991, 1992; Staub and Esterle, 1992). Smaller sampling increments produce increased analytical cost

Fig. 4. Compositional groups for Middle Pennsylvanian coals of the Central Appalachian basin can be used as analogues for rheotrophic to ombrotrophic mires.

S.F. Greb et al. / International Journal of Coal Geology 49 (2002) 147–175

and time with diminishing returns for most applications. For applications outside of the Middle Pennsylvanian of Euramerica, the palynological-botanical part of the compositional group (Table 1) would have to be adjusted to plants that would occupy the framework position of modern plants in different types of mires. For non-Carboniferous applications Densosporites-producing lycopods would be replaced by another type of plant in the framework position of the modern Shoria tree in ombrogenous mires. Likewise, Lycospora-producing arborescent lycopods would be replaced by another type of wet-substrate tree for non-Carboniferous rheotrophic mires (Fig. 4). 2.5. Multiple mire complications Aside from the various methods to infer the type of mire in which a coal bed formed, and the realization that cycles of mire phases may occur within coal beds, is the recognition that some coal beds may actually represent different mires, which recurred at the same location, but were separated in time. Tertiary coal beds can be as much as 90 m thick. Modern ombrotrophic mires are generally less than 20 m thick. In fact, ombrotrophic mires may be limited in their potential height by numerous conditions including microbial respiration within the underlying peat (Moore, 1995). Hence, the great thickness of some Tertiary coals suggests that they cannot represent the accumulation of a single peat mire, but rather the accumulation of multiple, stacked, mires (Shearer et al., 1994; Holdgate et al., 1995; Moore, 1995). Thick Tertiary coal beds often contain widespread inorganic partings that can be used to divide the coals into smaller components, or benches. Additionally, oxidized organic partings and organic non-oxidized, but degraded partings also occur within Tertiary coals, which indicate cessation or extreme slowing of peat accumulation. The three types of partings can be used as boundaries to divide a single coal bed into component parts, or benches, each representing the accumulation of a different mire (Shearer et al., 1994). Many of the Lower and Middle Pennsylvanianage, low-ash, low-sulfur coals of the central Appalachian Basin are less than 2 m thick, with a maximum thickness of less than 6 m. These thicknesses are within the range of modern ombrotrophic mires (not

153

compensating for compaction), so that they might represent the accumulation of a single mire. Yet these coal beds also commonly contain extensive inorganic and organic partings similar to those noted in Tertiary coals. In many cases, such partings are analogous to the incursions of Smith (1962). The implication is generally made that a part of the paleomire was buried or oxidized in a short-term event, but then reestablished itself following the incursion. In other cases, however, inorganic partings may be regionally extensive and represent spatial and temporal disconnection between the underlying and overlying coal. In such cases, the underlying and overlying coal benches could represent completely different, albeit stacked mires separated by a longer time than is generally inferred for incursions. The stacked mires could be deposited under slightly different environmental conditions (relative sea level, sediment flux, etc.), which could yield coal benches with independent trends in thickness and quality (ash yield, sulfur content). This is particularly important to understand when assessing thickness and quality trends in multiple-bench coal seams. If some seams represent composite mires, then any trends within the seam need to be evaluated in terms of the component parts of the seam, in other words, as a function of their coal-bench architecture.

3. Purpose The purpose of this investigation is to (1) illustrate how some of the most heavily mined coal seams in the central Appalachian Basin can be divided into naturally occurring benches using an architectural scheme, (2) demonstrate that there is lateral and vertical variation in compositional groups at the bench-scale in these seams, (3) use common patterns of compositional-group trends within the context of an architectural scheme to interpret how some of the coal seams accumulated, and then (4) illustrate through a conceptual model, lateral configurations in coal-bench architecture and how they could be used to infer original depositional controls on peat accumulation. Because an architectural scheme allows variation in coal parameters to be assigned to specific parts of a coal bed or seam, it can lead to more accurate projection of quality and thickness data, based on lithofacies-like components of the coal.

154

S.F. Greb et al. / International Journal of Coal Geology 49 (2002) 147–175

4. Coal-bench-architecture methodology Geological mapping of more than a hundred 7.5min quadrangles, and several coal resource studies in eastern Kentucky, have shown that numerous Middle Pennsylvanian coal beds in the central Appalachian Basin are composed of multiple coal benches separated by mineral partings of varying extents. Just as interpretations of lithofacies in clastic rocks require analysis of the individual beds that make up the facies, so too do coals require analysis of the individual benches that make up the coal bed or seam. If the quality and thickness of individual coal benches were uniform vertically and laterally within a seam there would be no need for an architectural approach. Numerous investigations, however, have shown that coal seams often exhibit what would be classified as interand intra-bench variation in various coal parameters (Renton and Hamilton, 1988; Staub, 1991, 1994; Hower et al., 1991, 1992; Grady et al., 1992; Esterle et al., 1992; Eble et al., 1994; Greb et al., 1999). In a market economy where constraints on coal specifications are stringent, an understanding of inter- and intra-bench seam variability can be critical to successful utilization of a coal resource. To accurately assess inter- and intra-bench variation, a scaled approach to sampling and interpretation is needed. In the examples that follow, coal-bench architecture is defined to develop a framework in which lateral and vertical variation within a coal seam can be analyzed and tested. The architectural approach used herein works at both the local and regional scale as will be shown in several examples. In fact, most interpretations for local applications benefit from an understanding of regional bench-scale variation. For regional analysis, measured sections of coal beds from a broad area are examined to determine the number of beds that can occur in a coal zone, and the number of benches that can occur within an individual bed or seam of the zone. From such sections, bench variability can usually be determined. Where multiple benches are common, the observation has been made that at least one bench is persistent. Additionally, many of the persistent coal beds (CM in Fig. 1) contain a widespread (>25 km2) parting or high-ash durain layer. Where such partings occur, they can be used to divide the bed into benches (e.g., CMa and CMb in Fig. 1).

They can also be thought of as a primary bounding surface in an architectural scheme, and can make a useful datum for testing and interpreting variation between (inter-) and within (intra-) the coal benches that make up the coal bed or seam. These benches may themselves contain local partings that allow the benches to be divided into smaller components (e.g., CMb1 and CMb2 in Fig. 1), if needed for special applications. 4.1. Inter- and intra-bench variation and sampling bias An example from the New Livingston coal is used to illustrate local inter- and intra-bench compositional variation and the importance of sampling bias on possible interpretations of original coal depositional environments (Fig. 5A –D). The New Livingston coal is exposed on the southwestern margin of the Eastern Kentucky Coal Field. The coal bed is usually composed of a single bench of coal (lacking significant partings), but as shown in Fig. 5A, it locally doubles in thickness above scours in the underlying strata. In these circumstances, the coal bed contains a thick, mineral parting that can be used to separate a lower and upper coal bench. The lower coal bench is confined to the scour, while the upper bench is continuous beyond the scour. The coal is sampled above the scour and analyses are interpreted at three scales: full-channel, bench, and intra-bench. When the coal bed was sampled as a full-channel sample above the scour, analyses indicate arboreal lycopod spores in access of 70% and a moderate ash yield, which is a Lycospora– Vitrinite Dominant compositional grouping (Fig. 5B, Table 1). This grouping indicates original planar-rheotrophic mires dominated by lycopod forests (Table 1, Fig. 4). This makes sense, as the coal bed thickens into an obvious depression, and contains a thick parting, both criteria used to interpret rheotrophic conditions typical of planar mires (Table 1). However, this type of whole-bed or wholeseam analysis masks the possibility of smaller scale variation. At this location, the coal can be divided into an upper (NLM) and lower or leader (NLL) coal bench relative to the shale layer in the middle of the bed. When the coal is sampled at the bench scale, the lower bench still contains more than 70% lycopods and so a Lycospora– Vitrinite Dominant compositional group-

S.F. Greb et al. / International Journal of Coal Geology 49 (2002) 147–175

155

Fig. 5. Example of sample scale and compositional group interpretations from the New Livingston (NL) coal in the Mount Vernon quadrangle, along Interstate 75. (A) Upper bench (NLM) is continuous, while lower bench (NLL) is confined to a local scour. (B) Summary of analyses from a full-channel sample through the coal and resultant compositional group. (C) Analyses from two-bench samples to show inter-bench variation and resultant compositional groups. (D) Intra-bench samples used to show intra-bench variation in parameters and compositional groups.

ing, but with a higher ash yield. The upper bench has less than 70% lycopods and is assigned to one of the mixed palynomorphs groupings. Because it has vitrinite contents of greater than 80% and a low ash yield, it is assigned to the Mixed Palynoflora – Vitrinite Dominant compositional grouping (Fig. 5C, Table 1). The high vitrinite content in both benches still suggests standing water cover and rheotrophic mire origins (Fig. 4, Table 1). The lower bench mire was more dominated by lycopod trees, however, and had greater clastic influence than the upper mire. The lower ash yield of the upper bench would probably be interpreted as resulting from less-confined condi-

tions than the bench within the scour. This type of bench-scale variation is common in Lower to Middle Pennsylvanian coal beds of the basin. To test for intra-bench variation, increment sampling within individual benches of the coal was undertaken. Increment sampling within the New Livingston example shows that the lower coal bench consists of the Lycospora – Vitrinite Dominant compositional grouping throughout, but with decreasing clastic influence upward (as determined from ash yield). The upper bench (NLM), however, shows intra-bench variation (Fig. 5D). The lower increments of the upper bench are assigned to the Mixed Palynoflora – Vitrinite

156

S.F. Greb et al. / International Journal of Coal Geology 49 (2002) 147–175

Dominant, and Mixed Palynoflora – Low Vitrinite – Low Ash (ash less than 10%) compositional groupings (Fig. 5D, Table 1). These groupings suggest an early mire phase in which densospore-producing lycopods and ferns prospered, without clastic influence ( < 10% ash yields), possibly developing into a short-term transitional or mesotrophic to ombrotrophic mire (Fig. 4, Table 1). In contrast, the upper part of the upper bench shows a return to the Lycospora – Vitrinite Dominant compositional grouping (Fig. 5D), with increasing lycopod dominance upward. This grouping suggests a return to planar-rheotrophic mire conditions (Fig. 4, Table 1). Because this vertical transition occurs toward the top of the bench, in the top of the bed, it probably represents a response to flooding or drowning of the peat. Hence, the upper bench shows a vertical transition in mire phases, close to the type of cycle envisioned by Smith (1962), but occurring only within the upper part of the main, persistent bench at this location. To make lateral interpretations of coal depositional environments, or to project interpretations for mining applications, the coal bed would need to be sampled laterally at the bench, and intra-bench scale. In practical terms, the number of locations sampled, and number of increments sampled at each location will be dependent on the goal of the study, and the architecture of the coal bed or seam. Not all data locations can be sampled in increments. Likewise, petrographic, and palynologic data will not be available at all locations, nor needed for many practical applications. This is too costly and time intensive. But if bench architecture is defined, existing data can be analyzed in the context of the seam’s architectural framework, and sampling can be directed toward different elements of that architecture. Then if thickness or quality variation is found within a specific part of the architectural framework, it can be projected more accurately as a specific component of the mined seam. In the New Livingston example, coal thickness increases above a scour, but the additional thickness is due to the addition of a coal bench (NLL) of lesser quality than the overlying, persistent bench (NLM). If these benches could be correlated laterally, or along the trend of the scour, we could test or project more meaningful quality trends on the basis of the seam architecture. In the following case studies, vertical variation in compositional groups at the bench- and

intra-bench scale is examined laterally across broader areas to show applications of coal-bench architectural analyses.

5. Architecture of the Fire Clay coal seam In the New Livingston coal bed example, vertical variation in coal-bench architecture was caused by local variation in the paleotopography in which the peat accumulated. But bench-scale variation is not limited to such local features, and can be documented regionally in the most heavily mined coals of the Eastern Kentucky Coal Field. The Fire Clay coal produces 19 to 21 Mt annually, the largest producing coal in the Eastern Kentucky Coal Field. The seam is high volatile A bituminous, generally low in ash yield (mean 10%), and generally low in sulfur content (mean 1%). The seam has been the subject of numerous reports (Eble et al., 1989, 1994; Eble and Grady, 1993; Grady et al., 1993; Andrews et al., 1994; Hower et al., 1994a; Greb et al., 1999; Thacker et al., 2000) and herein the coal is summarized in terms of its overall architecture. Where mined, the Fire Clay coal seam occurs both as a single coal bench and as multiple benches. Across much of its extent, at least two coal benches are separated by a distinctive flint clay and carbonaceous shale parting, informally called the jackrock parting. The jackrock parting is, in part, a tonstein (Bohor and Triplehorn, 1981; Lyons et al., 1992; Greb et al., 1999; Hower et al., 1998). Sanidine from the flint clay parting has been age-dated across the basin and indicates an isochronous surface 311 F 1 million years old (Lyons et al., 1992). The parting is often described as rising and falling in the coal, but as an extensive time line, the surrounding coal benches are more properly interpreted as rising and falling relative to the datum. A sample location from Greb et al. (1999) showed increment sample variation within different benches of the Fire Clay coal. Herein that sample location is analyzed in the same manner as the New Livingston example (Fig. 5A –D), to illustrate a coal with similar bench-scale variation, but with benches not laterally confined to a local scour fill. If the location is sampled on a whole-coal basis, the seam has a low to moderate ash yield (6.9% dry weight), low sulfur content (0.69% dry weight), mixed palynomorphs, and high

S.F. Greb et al. / International Journal of Coal Geology 49 (2002) 147–175

vitrinite contents (68%), which is characteristic of the Mixed Palynoflora –Vitrinite Dominant compositional group (Fig. 6A). This would indicate a rheotrophic mire with standing water cover (Fig. 4, Table 1). If analyzed at the bench scale, the upper bench has lower ash yield (3.4%) and more than 10% small lycopods (11%), which would be in the Mixed Palynomorph – Low Ash compositional grouping (Fig. 6B). The lower bench has ash yields above 10% (13.9%) and increased tree fern spores (16%), typical of the Mixed Palynoflora – High Ash compositional group (Fig. 6B). If analyzed at the intra-bench scale, the upper bench shows a succession of compositional groups that divide the bench into thirds: a lower Mixed Palynoflora – Vitrinite Dominant section, a middle Lycospora – Vitrinite Dominant section, capped by a Mixed Palynomorph – Low Vitrinite – Low Ash section at the top of the bench (Fig. 6C). This would

157

indicate a succession from a rheotrophic to ombrotrophic mire upwards within this bench (Table 1, Fig. 4). In contrast, the lower bench contains an overall higher ash yield and does not contain evidence for ombrotrophic mire development. If the location in Fig. 6 is placed in a cross-section, lateral variation in bench architecture can be examined (Fig. 7). In areas where the Fire Clay coal occurs as a single bench of coal without correlatable partings, the flint clay occurs in the floor of the bed or is absent. Where the coal is separated into two benches, the flint clay generally occurs in the middle or lower part of the bed. The bench above the flint clay (FCMb) is the main, persistent bench. Compositional group profiles from Eble et al. (1994) along the trend of the crosssection (numbered arrows in Fig. 7) indicate that the bench below the parting commonly contains the Mixed Palynoflora – High Ash and Lycospora– Vitri-

Fig. 6. Example of sample scale and compositional group interpretations from the Fire Clay coal. (A) Full-channel sample through the coal and resultant compositional group. (B) Two-bench samples using parting to divide benches and resultant compositional group. (C) Intra-bench samples used to show intra-bench variation in parameters and compositional groups. Data for intra-bench variation from Greb et al. (1999). HW = Hyden West, HE = Hyden East, HS = Hazard South, VI = Vicco quadrangles.

158

S.F. Greb et al. / International Journal of Coal Geology 49 (2002) 147–175

Fig. 7. Cross-section of the Fire Clay coal from location 5 (sample shown in Fig. 6) westward, in order to show lateral variation in bench architecture. The rectangles in the location map represent 7.5-min quadrangles. Compositional group data from Eble et al. (1994) and Greb et al. (1999).

nite Dominant compositional groups. It does not contain the Mixed Palynoflora – Low Vitrinite –Low Ash compositional group. The main bench (FCMb) generally contains the Mixed Palynoflora – High Ash and Lycospora – Vitrinite Dominant compositional groups at the base, as in the underlying bench (FCMa), but may contain the Mixed Palynoflora –Low Vitrinite –Low Ash compositional group in the upper part

of the bench (Fig. 7). The uppermost increment is most often composed of the Lycospora – Vitrinite Dominant compositional group, in some cases, separated from the underlying increments by a thin mineral parting. Another typical architectural configuration that can be seen in Fig. 7 is the local occurrence of rider benches (FCR) that come close to merging with the main bed.

Fig. 8. Cross-section showing merging of the Fire Clay (FC) and Fire Clay Rider (FCR) coal beds and each of their component benches (e.g., FCMa, FCMb) to demonstrate lateral variation in bench architecture.

S.F. Greb et al. / International Journal of Coal Geology 49 (2002) 147–175

Not only can local rider benches merge with the main coal bench of the Fire Clay coal, but sometimes, distinctly different overlying coal beds merge with the Fire Clay coal. In Fig. 8, the Fire Clay coal (FCM) is a single bench to the west and a double bench to the east. A second coal bed, called the Fire Clay Rider (FCRM), drops in elevation and merges with the main coal bench (FCMR) toward the center of the section. At first glance, it would be easy to infer that the Fire Clay coal here is split by syndepositional clastics on either side of the section (espe-

159

cially if the flint clay was not used as a datum). However, the bench configuration or architecture may indicate that the seam here is the result of distinctly different coal beds, and therefore, originally different peat mires that merge together. This type of bench architecture is not uncommon in the central Appalachian Basin. Several studies have shown that the benches above and below the flint clay parting have different quality and spatial trends in different parts of the coal field (Weisenfluh and Ferm, 1991a,b; Eble et al., 1994;

Fig. 9. Average trends in coal quality and compositional groups for the Fire Clay coal where sampled in eastern Kentucky and West Virginia quantified as (A) whole seam, (B) two benches split by flint clay (f ) parting, (C) intra-bench variation with upper bench divided into thirds and lower bench in half, and (D) with rider or upper part of the upper bench split off in samples where there was a parting (p) in the upper part of the seam. N = number of locations, n = number of intervals. Data compiled from Eble and Grady (1993), Grady et al. (1993), Andrews et al. (1994), Eble et al. (1994), Hower et al. (1994a,b), Greb et al. (1999).

160

S.F. Greb et al. / International Journal of Coal Geology 49 (2002) 147–175

Hower et al., 1994a; Greb et al., 1999). A regional compilation of data from these and other studies can be used to compare average trends between and within benches of the Fire Clay coal at a regional scale. If the coal is treated as a single bench, the coal has a mean sulfur content of 1.38 + 2.27 and ash yield of 10.47 + 6.25. It is co-dominated by the Lycospora –

Vitrinite Dominant (43.2%) and Mixed Palynoflora – High Ash (29.6%) compositional groups (Fig. 9A). At the bench scale, the upper, main bench has lower average sulfur contents (0.89 + 0.36 vs. 1.74 + 2.74) and ash yields (8.40 + 4.68 vs. 18.19 + 10.24) than the lower bench (Fig. 9B). The main bench is still dominated by the Lycospora –Vitrinite Dominant (47.2%),

Fig. 10. Average trends in coal quality and compositional groups for the main (upper) bench of the Fire Clay coal as a function of thickness. (A) Trends where bench is >2.4 m. (B) Trends where bench is < 2.4 m. (C) Trends where bench is >2.4 m, and rider or uppermost intervals are separated by a parting. (D) Trends where bench is < 2.4 m, and rider or uppermost intervals are separated by a parting. N = number of locations, n = number of intervals. Data compiled from Eble and Grady (1993), Grady et al. (1993), Andrews et al. (1994), Eble et al. (1994), Hower et al. (1994a,b), Greb et al. (1999).

S.F. Greb et al. / International Journal of Coal Geology 49 (2002) 147–175

with lesser amounts of the Mixed Palynoflora –High Ash (20.3%), and increased amounts of the Mixed Palynoflora – Low Vitrinite –Low Ash (17.3%) compositional groups. The lower bench is dominated by the Mixed Palynoflora – High Ash group (69%), and does not contain the Mixed Palynoflora – Low Vitrinite – Low Ash compositional group (Fig. 9B). This was shown in smaller scale in Fig. 7, but in this analysis appears to be regionally consistent from a sample of 20 locations in which increment samples were taken in eastern Kentucky and West Virginia. Intra-bench variation is tested in two ways. In the first analyses, the upper main bench is divided into thirds and the lower bench in half (dry weight basis). In this scenario, the lowest ash yields are in the middle of the upper bench (6.81 + 5.64), and the highest ash yields are in the lower bench (both halves greater than 17%) (Fig. 9C). Sulfur contents are lowest in the middle increment of the upper bench (0.77 + 0.30) and highest in the lower bench (both halves greater than 1.5%). Also, the greatest variance in ash yield and sulfur content is in the two halves of the lower bench. In terms of total regional occurrences of previously interpreted compositional groups, the upper bench shows a trend of upward decreasing Mixed Palynoflora – High Ash, and upward increasing Mixed Palynoflora – Low Vitrinite – Low Ash compositional groups (Fig. 9C). The upper third of the upper bench contains the greatest percentage of the Mixed Palynoflora – Low Vitrinite –Low Ash compositional group (29.6% vs. 14.5%, 7.7%). In contrast, both halves of the lower bench are similarly dominated by the Mixed Palynoflora – High Ash compositional group (72% and 66% in Fig. 9D). In the second analysis, recognition of persistent partings in the upper part of the upper bench are used to split an upper component or possibly rider bench from the main bench increments. Regionally, these rider benches (or uppermost parts of the upper, main bench) have significantly higher ash yields (13.34 + 7.61) than the underlying main bench (6.21 + 4.96, 6.93 + 5.68, 11.04 + 7.0 in Fig. 9D). Likewise, sulfur contents are significantly higher and variable (2.24 + 7.64) than the rest of the bench (0.85 + 7.64, 0.78 + 0.34, 0.77 + 0.33 in Fig. 9D). In terms of compositional groups, benches above a parting in the upper bench are dominated by the Lycospora –Vitrinite Dominant compositional group (81%), and the percentage

161

of the Mixed Palynoflora – Low Vitrinite – Low Ash compositional group in the upper third of the upper bench increases to 38%. Linking these types of variation to the bench architecture provides a powerful tool for accurate reserve characterization and quality projections. In the case of the Fire Clay coal, a common perception in one mining area was that as the seam thinned its quality increased. This was documented on a wholeseam basis. But, in terms of the coal’s bench architecture, the quality increased where the coal thinned because the lower bench (statistically higher in ash than the overlying bench) pinched out. Beyond the point of lower bench termination, quality did not continue to increase. In fact, lateral thinning of the upper, main bench is accompanied by an increase in ash yield. A test of thick (>2.4 m) vs. thin ( < 2.4 m) upper bench coal shows that both sample sets exhibit intrabench variation, but only the upper third of the thick coal samples contain appreciable amounts (38% compared to 15%) of the Mixed Palynoflora – Low Vitrinite –Low Ash compositional group that is related to the lowest sulfur contents and ash yields (Fig. 10A). This indicates that where the seam contains a thick, upper bench component it is more likely to have lower ash yields. Likewise, if a rider or upper part of the upper main bench is split from the main bench based on a parting in the upper third of the bench, the uppermost split or rider bench does not contain the Mixed Palynoflora – Low Vitrinite – Low Ash compositional group where thin or thick (Fig. 10C,D). Where thick, the upper third of the main bench contains the Mixed Palynoflora – Low Vitrinite –Low Ash compositional group in 50% of the locations sampled. Where thin, the Mixed Palynoflora –Low Vitrinite –Low Ash compositional group only occurs in 18% of the locations (Fig. 10D). Another noticeable difference in thick and thin coals is that where thin, ash yields are overall higher, and particularly higher in the lowest increment of the bench (Fig. 10A –D).

6. Architecture of the Lower Elkhorn coal seam Not all partings are as obvious a major bounding surface as the tonstein in the Fire Clay coal. Most

162

S.F. Greb et al. / International Journal of Coal Geology 49 (2002) 147–175

partings are less extensive, and can only be correlated based on similarity of thickness and lithology. Another type of bounding surface that can be documented in coal beds of the basin are extensive durains. Durains are dull, inertinite-rich coal lithotypes. They are generally formed from oxidation of the peat surface (Teichmu¨ller, 1989). Coals often contain multiple durain layers. Some seams have persistent durain layers that can be used as bounding surfaces in a coal-bench architecture analysis. One such seam is the Lower Elkhorn coal bed. The Lower Elkhorn coal bed is another heavily mined seam, producing 12 to 14 Mt annually, second in eastern Kentucky. The coal has been the subject of numerous reports (Hower and Pollock, 1988; Hower and Bland, 1989; Helfrich and Hower, 1991; Hower et al., 1991; Nelson et al., 1991; Vogler, 1994; Thacker et al., 1998) and the following is a summary of those studies. The Lower Elkhorn is high-volatile B to medium-volatile bituminous, generally low in ash content (mean 9.6%), and generally low in sulfur content (mean 0.8%). Regional resource analysis of

the coal shows that it is widespread across the southeastern part of the coal field and consists of multiple beds or benches in a zone (Thacker et al., 1998). Some areas mine a coal seam that is a combination of rider benches and a main bench, others, a leader and main bench (Fig. 11A). By using an architectural scheme, the seam can be mapped regionally in the context of different combinations of benches that comprise the seam (Fig. 11B). As was seen in the Fire Clay coal, these benches have different characteristics (Fig. 11C). The best quality in terms of sulfur content and ash yield is the main, persistent bench (LEM). The highest sulfur contents and ash yields occur in rider (LER) benches. Again, an understanding of bench architecture is critical to understanding and interpreting variation in the seam. At the scale of a mine, the seam generally appears as a single bench, with only very local additions of thin rider benches (Fig. 12A). In the northern part of the coal field the seam may contain a thick durain layer that laterally grades into shale, called the ‘‘middleman’’ by miners (mm in Fig. 12B). Partings below

Fig. 11. Lower Elkhorn coal-bench architecture. (A) Schematic diagram of regional bench architecture based on correlations of more than 1200 borehole records in eastern Kentucky. Gray shaded boxes show which component benches comprise the seam as mined at different locations. (B) Map of bench architecture for Pike County, KY, showing the lateral bench composition of the seam in different areas (after Thacker et al., 1998). (C) Average trends in Lower Elkhorn coal quality when quantified as a whole seam (total), as combinations of benches as shown in the map view (LEMbR1,2, LEMab, LEMb), and as individual benches (LEMa, LEMb, etc.).

S.F. Greb et al. / International Journal of Coal Geology 49 (2002) 147–175

163

Fig. 12. Cross-sections illustrating bench architecture of the Lower Elkhorn coal: (A) Section from northern outcrop areas showing persistent, but laterally splitting main bench (LEM), local-hole filling benches (LEL), and rider benches (LER). (B) Detail of ‘‘dip’’ with lower bench thickening into paleotopographic depression (after Greb and Popp, 1999). mm = Middleman durain. (C) Splitting and merging of component benches across a line of section in Pike County, KY, showing lateral variation in benches that comprise the Lower Elkhorn coal seam.

the middleman are variable but not uncommon. Partings above the middleman are rare (Vogler, 1994). Hence, the middleman durain divides the seam/bed into two benches (LEMa, LEMb) with different parting frequencies. In several areas, riders have been noted that merge with the upper coal bench (Fig. 12A,C). In others, the seam thickens into ‘‘dips’’ or ‘‘swales’’ (Fig. 12B), and thickening occurs through the addition of coal and partings in the lower bench or possibly through the addition of a leader bench confined to the depression (Vogler, 1994; Greb and Popp, 1999). Laterally, the coal may split into two distinct benches or beds (LEMa and LEMb in Fig. 12C). Another aspect of coal-bench architecture analysis that can be demonstrated in the Lower Elkhorn coal is the detection of subtle structural influences. Hower et al.’s (1991) report on the Pond Creek coal bed (a Lower Elkhorn synonym) divided the coal into lithotype groupings, which was essentially an intra-bench analysis. Using lithotype groupings, the study noted vertical and lateral petrographic differences in the coal relative to the Belfry Anticline (BA in Fig. 13). From

a coal-bench architecture perspective, there were differences in the number of benches, bench thickness, and intra-bench petrographic variation from the crest to the flank of the structure. Using data not illustrated in that report, intra-bench changes in compositional groupings off the structure can also be noted (Fig. 13). The best-quality, Mixed Palynoflora – Low Vitrinite – Low Ash compositional group is limited to areas farthest off the anticline. At first glance, it looks like this grouping begins at the base of the coal, which would be significantly different than the Fire Clay coal bed example. But, laterally, additional benches occur below the Densosporites-bearing, Mixed Palynoflora – Low Vitrinite – Low Ash compositional grouping (Hower et al., 1991), as well as numerous local occurrences of depression-filling benches as in Fig. 12B (Nelson et al., 1991; Vogler, 1994; Greb and Popp, 1999). Inferred syndepositional structural controls based on what would be considered inter- and intra-bench changes in coal seams have also been noted for the Taylor, Hazard, Hazard No. 7, Hazard No. 8 (Hower et al., 1992), Fire Clay (Weisenfluh and

164

S.F. Greb et al. / International Journal of Coal Geology 49 (2002) 147–175

Fig. 13. Cross-section of the Lower Elkhorn coal near the Belfry anticline showing changes in bench thickness and intra-bench compositional groups.

Ferm, 1991a,b; Greb et al., 1999), Skyline and Coalburg (Greb and Weisenfluh, 1996), and No. 5 Block (Hower et al., 1994b) coals.

7. Discussion Many of the most extensively mined coals in the central Appalachian Basin can be divided into distinct benches of different scales and extents. Each bench represents accumulation of at least a different phase of mire, and in some cases, may represent completely different mires, which recurred in a similar location, but were temporally separated. These individual benches often exhibit different trends in characteristics. A better understanding of these bench-scale characteristics is needed to more accurately assess coal reserves and interpret original peat accumulation. The idea that many of the low-ash, low-sulfur coals of the region formed in ombrotrophic mires is only partly correct. Most of the coals that contain compositional groups indicative of domal origin only contain those increments where the coal is thick (Fig. 10A – D; Staub, 1991; Greb et al., 1999). Many of these partly domed benches exhibit a vertical planar to domed, and back to planar cycle similar to the idea of cyclic mire phases proposed by Smith (1957, 1962), as previously discussed. Smith’s conceptual diagram for these coals, however, is a vertically symmetrical cycle between planar, domed, and planar mires. In central Appalachian basin coals, the sequence is almost never symmetrical within a seam, and often is vertically asym-

metric within individual benches of a seam (Figs. 5– 7). Additionally, partings, which are the incursion phases of Smith (1957, 1962), sometimes have widespread extent, as in the Fire Clay coal examples, and do not always appear to be related to short-term syndepositional clastic influx. Some may represent longer-term clastic sedimentation and development of new paleotopographic platform for the accumulation of an entirely new mire. Hence, vertical changes in Lower to Middle Pennsylvanian mire phases do occur in this basin, but in a manner slightly different than that proposed by Smith (1957, 1962). More importantly, bench-scale architectural analyses indicate that the domal increments are both laterally and vertically limited in extent. In order to discuss how bench architecture can be applied to understanding peat accumulation and coal resource analyses in the basin, a general model of the vertical and lateral changes in paleomire phases noted is presented (Fig. 14A –H). 7.1. Lower to Middle Pennsylvanian mire phases 7.1.1. Pioneering mire phase — leader and lower main benches The topographic surface on which peat accumulates is a primary control on subsequent peat thickness and peat types in modern mires (Spackman et al., 1964, 1974; Staub and Cohen, 1979; Teichmu¨ller and Teichmu¨ller, 1982; Styan and Bustin, 1983; McCabe, 1984). Not surprisingly, many Lower to Middle Pennsylvanian coals exhibit variable thickness related to paleotopography. In most coals, the lowest increment

S.F. Greb et al. / International Journal of Coal Geology 49 (2002) 147–175

165

Fig. 14. Possible depositional sequence interpreted for thick, mined, Middle Pennsylvanian coals in the central Appalachian Basin based on common patterns of inter- and intra-bench compositional group trends (A – H are described in text).

samples are composed of the Mixed Palynoflora – High Ash or Lycospora – Vitrinite Dominant compositional groupings (Figs. 5D, 6C, 7). Ash yields, parting frequency, and contributions of various spore taxa are often laterally variable. In some cases, cannel coals and canneloid shales are reported at the base of seams (e.g., Greb et al., 1999). This variability can be interpreted in terms of lateral confinement by paleotopography (Fig. 14A – C). In some cases, confinement occurs above local scours as in the New Livingston (Fig. 5A – D) and Lower Elkhorn coals (Fig. 11B) and in others, across broader areas as in the lower bench of the Fire Clay coal (Figs. 7 and 8C). The colonizing lycopod Paralycopodites is often abundant, as are pteridosperms and calamites, in leader benches and the lowest increments of Middle Pennsyl-

vanian coal seams (Calder, 1993; Calder et al., 1996; DiMichele and Phillips, 1994; Eble et al., 1994). Spores of these plants were also abundant in previous studies of local, scour-draping coals that obviously fill paleotopographic depressions (Eble and Greb, 1997). In the context of mire phases, the carbonaceous shales and coaly shales (rashes) that underlie or occur at the base of some coals may represent clastic swamps that preceded or led to the development of the pioneering phase of the main mire as base level rose and ponded the pre-peat surface (Fig. 14A). 7.1.2. Local incursions — partings in leader and lower main benches In many cases, the lower increments of a singlebench coal or the lower benches of multiple bench

166

S.F. Greb et al. / International Journal of Coal Geology 49 (2002) 147–175

coals will have the highest ash yields of the seam. Partings are also common in these benches (Fig. 14B). Partings may be laterally confined in obvious depressions as in the New Livingston (Fig. 5A) and Lower Elkhorn (Fig. 12A,B) examples, or bound by coal as may happen in the lower bench of the Fire Clay coal (location 3 in Fig. 7). Laterally restricted and locally abundant partings have been noted in the lower portions of many coal seams in the basin (Esterle et al., 1992; Hower et al., 1991, 1992; Grady et al., 1992; Eble et al., 1994). These represent the incursion phases of Smith (1957, 1962), and their limited extent indicates lateral confinement or varying trophic levels across the peat. Lateral to the incursions, peat accumulation is inferred to have been uninterrupted. 7.1.3. Regional incursion — horizontal surface datum In some cases, conditions for peat accumulation did not proceed beyond the pioneering phase and resultant coals were confined to local depressions (Greb and Chesnut, 1992). In more widespread coals, however, the sheer extent of the coal suggests that peats infilled the low-relief topographic surfaces on which they formed and continued to accumulate or keep pace with base-level rise (Fig. 14C). Partings that accumulated prior to infilling were confined as discussed previously. But once a flat surface was achieved, flood waters were unconfined. In this context, widespread partings can be seen not solely as a function of the veracity of the clastic influx, but also of the low relief of the topographic surface on which they were deposited. In the case of the Fire Clay coal bed, a regional parting (Figs. 7, 8, 9B – D) consists of a carbonaceous shale and tonstein, which has been interpreted as drowning of the lower bench swamp due to rising ground-water table, followed by regional volcanic ash fall (Greb et al., 1999). It is extensive across parts of three states. In the other coal beds, a tonstein is lacking, but regional to semi-regional durains and clastic partings divide overlying parting-free increments of the bench from underlying, locally partingrich parts of the bench. The durain in the Lower Elkhorn coal bed, which may be extensive for several counties, grades laterally into shale and a clastic split suggesting that it also records an extensive influx of extra-mire oxygenated sediment and water (Vogler, 1994). Other coals such as the Stockton (Eble and

Grady, 1993) and Hazard No. 8 (Greb, 1992; Hower et al., 1992) have regionally extensive partings towards the middle of the seam. The base of these partings marks a point in each seam where there was essentially a widespread horizontal surface datum upon which an extensive parting could be deposited, although peat accumulation continued. 7.1.4. Regional rheotrophic forest mire phase — main bench In the Fire Clay coal bed, the coal bench above the tonstein is extensive. Basal increments of the bench where thick, and the entire bench where thin are dominated by the Lycospora – Vitrinite Dominant compositional group with lesser amounts of the Mixed Palynoflora – Low Vitrinite – Low Ash compositional group (Fig. 10A –D). In the Lower Elkhorn coal bed, the main bench is extensive, and that part of the bench above the ‘‘middle-man’’ durain seems least variable in its characteristics (Vogler, 1994; Greb and Popp, 1999). Hence, benches developed above the horizontal surface datum generally continued or began as widespread, rheotrophic forest mires, rather than locally restricted pioneering mires (Fig. 14E). Sampling of persistent, main benches of different coal beds in the basin suggests that most were dominated by widespread planar-rheotrophic to possibly transitional-mesotrophic, lycopod-dominant forest mires (Eble et al., 1989, 1994; Eble and Grady, 1990, 1993; Grady et al., 1992, 1993; Staub and Richards, 1993; Greb et al., 1999). Extensive, main benches of coal beds may also contain partings. These represent clastic incursions, fires, or surfaces of oxidation, and are especially prevalent near thinning or splitting margins of main benches. Numerous studies have also documented widespread, thick, main benches lacking partings that were interpreted as rheotrophic forest paleomires. Eble et al. (1994) noted stacked ‘‘half cycles’’ of Lycospora – Vitrinite Dominant and Mixed Palynoflora– High Vitrinite compositional groupings which appeared to indicate that the rheotrophic paleomire was developing toward mesotrophic conditions but that a flood or other edaphic change returned the mire to rheotrophic forest-mire conditions. These half cycles often occur without obvious partings. The lack of partings in some widespread, thick, main benches of coal, especially where palynological analysis indi-

S.F. Greb et al. / International Journal of Coal Geology 49 (2002) 147–175

cates an edaphic change, supports the idea that in some cases, the sheer extent of the forest paleomire may have aided in buffering the inner mire (thicker coal) from clastic incursions. 7.1.5. Ombrotrophic forest mire phase — main bench The main benches of several of the most heavily mined Middle Pennsylvanian coal seams in the basin exhibit lateral and vertical intra-bench variation from increments with Lycospora – Vitrinite Dominant to Mixed Palynoflora –Low Vitrinite –Low Ash compositional groupings. The latter are indicative of ombrotrophic mire development (Table 1, Fig. 4). Where the Mixed Palynoflora –Low Vitrinite –Low Ash has been noted, it generally occurs in only part of the extensive main coal bench (Figs. 6, 7, 9C,D, 10A –D, 13). In multiple-bench coals, it is most likely to occur in the bench above the regional parting or horizontal surface datum. This is consistent with an inference that domedombrotrophic mires should produce ‘‘clean’’ coals of greater spatial extent than planar-rheotrophic mires (Cecil et al., 1985; Esterle and Ferm, 1986; Esterle et al., 1992; Grady et al., 1992; Staub and Esterle, 1992; Eble et al., 1994). The domed increments indicate accumulation of a shrub- or stunted-forest paleomire that built up above the topography, and restricted clastic influx to areas along the margins of the domes (Fig. 14F). Laterally, where the beds or benches thin, the coals tend to be dominated by coal increments indicative of planar-rheotrophic mires (Fig. 14F). 7.1.6. Drowning phase — upper main bench The tops of coal seams represent the end of peat accumulation, marking the point in time when the rate of accumulation did not keep pace with rising base level (Fig. 14G). This can occur because of compactional subsidence, tectonic subsidence, fires and droughts that degrade the peat surface, incursions of salt water into fresh-water mires, and unusual incursions such as tonsteins or extremely large flooding events. In singlebenched coals containing increments of inferred domal origin, the domal increments may extend to the top of the bench, or may be capped by increments indicative of planar or mixed origin (Eble et al., 1989, 1994; Eble and Grady, 1990, 1993; Grady et al., 1992, 1993; Greb et al., 1999). In the Fire Clay coal, an extensive durain occurs at the top of the domal increments in at least two counties (Eble et al., 1994; Hower et al., 1994a).

167

Whether this inertinite-rich layer was formed by (1) drying of the ombrogenous peat surface, (2) fires, or (3) resulted from oxidation of the peat surface when the dome collapsed or deflated, and was flooded by lateral extra-mire waters is debatable. Regardless, that part of the seam that was formed by an ombrogenous peat was superceded by planar peats in rheotrophic mires; hence, the peat was once again water covered, prior to burial (Fig. 14G). Comparison of existing compositional group columns from coals of the basin (Eble et al., 1989, 1994; Eble and Grady, 1990, 1993; Grady et al., 1992, 1993) indicates a wide variety of transitions between the upper parts of main coal benches and overlying clastics. In some cases, partings increase in frequency upward or the coal grades vertically into a carbonaceous shale, suggesting gradual inundation by rising base level. The parting used to separate an extra bench from the upper part of the main bench in the Fire Clay coal bed analyses (Figs. 9D and 10C,D) represents flooding of the underlying mire, and in some cases may mark the beginning of the drowning phase of the upper bench. The Lycospora – Vitrinite Dominant compositional group is most common in these intervals, indicating a return to planar-rheotrophic conditions before final inundation (channel margin in Fig. 14G). In some areas, parting-free, low- to moderate-ash increments in the uppermost sample intervals of the main coal bench are sharply overlain by siliciclastics, without an obvious return to a rheotrophic mire stage. This may be a function of sample size, or may indicate rapid inundation. For parts of the Fire Clay coal bed, it has been suggested that dome collapse or deflation might have led to rapid inundation of lateral clastics that had been confined by the dome resulting in sharp contacts between the top of the main bench and the Mixed Palynoflora – Low Vitrinite – Low Ash compositional group and overlying sandstone roof rock (Greb et al., 1999). The wide variety of uppermost compositional group/roof rock contacts, however, indicates that there were probably many different mechanisms or styles of dome collapse and deflation relative to base-level rise and subsequent flooding of the mires. 7.1.7. Pioneering mire phase — rider benches A complication to the idea of cyclic mire phases in the central Appalachian Basin is that in areas where the

168

S.F. Greb et al. / International Journal of Coal Geology 49 (2002) 147–175

most heavily mined seams are thickest, they may include a rider coal bench or bed ((Figs. 7, 8, 13); Hower et al., 1991; Weisenfluh and Ferm, 1991a,b; Thacker et al., 1998; Greb et al., 1999). These rider coals may be separated from the main bench by a persistent, often wedge-shaped parting or split, or they may merge with the main coal bed. Riders generally do not stay at the seam level for great distances and laterally will rise above clastic wedges to as much as 15 m above the main coal bench or bed. Limited analysis of what would be considered rider benches in the basin indicates that they are generally higher in sulfur content and ash yield than underlying coal benches (Esterle et al., 1992; Grady et al., 1992; Hower et al., 1992; Eble et al., 1994). Those rider coals that have been examined palynologically, generally belong to the Mixed Palynoflora – High Ash or Lycospora – Vitrinite Dominant compositional groupings (Eble et al., 1994), similar to leader and lower benches, indicating a return to a pioneering mire phase (Table 1, Fig. 14H). Some of the uppermost intervals of the main coal benches that were separated for analysis in Fig. 10C,D above a persistent parting may represent rider coal benches merging with the main coal bench. These were also dominated by the Lycospora –Vitrinite Dominant compositional grouping. Just as leader coal benches are inferred to infill the paleotopography, rider benches and beds draped the paleotopography that developed following burial of the underlying mire (e.g., the main coal bench mire).

In many cases, splays and other sedimentary bedforms that buried the underlying mires had considerable relief. Peats which accumulated on the distal toes of these bedforms sometimes developed directly above the pre-existing mire. In some cases, cannel coals developed on top of the underlying coal bed. In other cases, peats themselves did not accumulate, but rather clastic mires accumulated in the new paleotopographic depressions. These left in situ tree stumps called kettlebottoms, and sometimes thin-rooted claystones inferred to be short-term paleosols. 7.2. Conceptual bench-architecture models In the central Appalachian Basin, the vertical and lateral changes in mire phases discussed in the previous section occur at inter-bench and intra-bench scales. Analysis of inter- and intra-bench compositional group variation shows similar trends, in which there are vertical changes in quality parameters and compositional groups through the main bench, but not necessarily in rider or leader benches (Figs. 5– 13). Fig. 15(A – H) is a conceptual model of bench architectures documented in some Middle Pennsylvanian coals of the central Appalachian Basin. Single-bench, double-bench, and bench-complex architectures are recognized. Each can occur laterally within the same coal. This is important to consider because a single coal bed may consist of different component benches across a region. Single-bench coals may be equivalent

Fig. 15. Conceptual model of coal bench architecture relative to a horizontal datum, and the types of inter- and intra-bench variability expected (A – H are described in text).

S.F. Greb et al. / International Journal of Coal Geology 49 (2002) 147–175

to the lowest bench at one location and the upper bench at another, as shown conceptually in Fig. 15(A,B). Likewise, double-bench coals may represent different combinations of benches as shown in Fig. 15(D,E). The mined seam, therefore, may consist of components of multiple beds and benches. Common bench configurations are shown in Fig. 15(A –H). 7.2.1. Single-bench architecture Single-bench coals, lacking partings, are common among the thinner coal beds of the central Appalachian Basin. In fact, in terms of occurrence they probably represent the dominant architectural scheme in the basin. Coal availability studies from the basin almost always show a higher volume of thin coals (less than 14 in.) than any other thickness category and these are almost always single benches (e.g., Brant et al., 1983). Intra-bench compositional analyses indicate that these single-bench coal beds are generally dominated by moderate- to high-ash, pioneering-rheotrophic, or forest-mire phases, either as local depression and scour filling (Fig. 15B), or more widespread mires above relatively low-relief paleotopography (Fig. 15A). These are illustrated as occurring above and below the relative horizontal datum (the level at which paleotopography was filled and extensive partings or peat could accumulate). Where thick (greater than 50 cm), some singlebench coals may have developed to the ombrotrophic forest mire phase (Fig. 15C). In single-bench coals where the domal compositional group occurs, it almost always is in the upper third of the bench, and never is the sole compositional group of the bench. Hence, the vertical variation is similar to the type of cyclic mire phase proposed by Smith (1957, 1962) for British coals, but more asymmetric than previously illustrated. 7.2.2. Double-bench architecture Double-benched coals are very common in the thicker, mined seams in the basin. In some cases, both benches will be dominated by planar-rheotrophic mire phases (Fig. 15D). When the upper bench is the most regionally continuous of the two benches, it is generally lower in ash content and sulfur yield than the lower bench. In some cases, the upper bench developed into a domal-ombrogenous phase, again generally in only a fraction of the bench (Fig. 15E).

169

Conceptually, this domal grouping can be thought of as marking the point in the bench at which conditions were right to allow peat to accumulate above the water table (i.e., above a point of regional clastic influx). Complications in architectural interpretation can occur where rider benches merge with main benches. In these cases, the domed increments occur in the local lower bench. This happens in the Fire Clay coal bed, but in the Fire Clay coal the readily identifiable flint clay allows definitive evidence that the local lower bench is actually the main regional bench. This can be more difficult to ascertain in coals with less distinctive partings. In the Lower Elkhorn coal bed, the main bench contains the domal compositional group in the lower part of the main bench. Lower benches are not as widespread as in the Fire Clay coal, but occur as depression-fills locally. Hence, the lowest local bench of the Lower Elkhorn coal bed is not necessarily the regionally lowest bench. Again, a regional perspective on bench configuration can aid in interpreting local bench-architecture variability for interpretations of original paleomire development. 7.2.3. Bench-complex architecture Bench complexes are less widespread than either the single- or double-bench architecture. In bench complexes, at least two widespread bounding units can be correlated, and sometimes more. Additional benches may come from below the main bench or from above (Fig. 15G,H). The most common occurrence is probably the addition of rider benches, but leader benches have also been locally documented. Generally, bench complexes are not laterally extensive, with riders and leaders splitting back off, pinching out, or being truncated by lateral clastic strata. In fact, a lateral change from single- to double-bench to bench-complex-architecture is essentially the criteria used to interpret syndepositional paleotopographic, clastic influx, syndepositional faulting or limited accommodation influences on paleomire accumulation (Fig. 16A – E). The most commonly cited reason for a lateral increase in coal benches is syndepositional clastic influx (Horne et al., 1978; Donaldson, 1979; McCabe, 1984; Greb, 1991, 1992). An increase in parting frequency and thickness can indicate lateral paleochannels and other clastic sources. In terms of coalbench architecture, the coal bed generally shows

170

S.F. Greb et al. / International Journal of Coal Geology 49 (2002) 147–175

Fig. 16. Conceptual diagrams of the different ways in which bench complexes could form (A – E described in text).

similar bench configurations toward and then away from an intervening paleochannel (generally a sandstone that truncates the seam) (Fig. 16A). Another key point is that ash yield often increases within each of the benches toward the paleochannel and may increase in coal increments below the partings. Increment sampling at the bench and intra-bench scale is required to indicate in which part of the coal bed the ash yield is increasing. Not all bench complexes represent syndepositional sedimentation from bounding channels. Some bench complexes may result from paleotopographic infilling (e.g., Eble and Greb, 1997). In this case, benches increase in number below the horizontal surface datum, essentially having filled in the holes in the paleotopography (Fig. 16B). Lower benches formed under these circumstances will generally exhibit high vertical and lateral variability in thickness and quality, with a greater tendency toward more frequent or thicker partings and higher ash yields. Paleotopographic influences may also occur from above, as happens when a rider draping a splay comes close to merging or merges with the main coal bench. In this case, the extensive rheotrophic forest-mire terminated, a new topographic surface was created during burial of the forest mire, and the rider-pioneering mire draped the new topography (Fig. 16B). This is subtly different than a split caused by syndepositional clastic influence in which the mire continues lateral to the split and then reoccupies its previous position above the sediments deposited during the incursion.

Differentiating leader or rider merging from a syndepositional split can sometimes be done by examining thickness trends lateral to the point of merger or split. If the coal thickness above and below the split equals the thickness of the single-bench coal laterally, it probably represents a split from syndepositional influx, especially if it retains the thickness for some distance laterally. If the thickness of the single bench is not equivalent to the benches bound by the split, the change in bench configuration might be due to merging. Merging of leaders with the main coal bed or bench should generally result in short-term introduction of a lower coal bench. This happens where a pioneering mire draped a paleotopographic surface, and the upper part of that surface was the platform upon which the main coal bench accumulated. Merging of rider beds or benches with the main coal bed or bench generally results in short-term introduction of an upper or rider bench to the main bench from above. Rider merging can occur when post-depositional rider mires draped bedforms that thinned distally above the preexisting mire (main bed or bench). Coal benches formed as a result of merging can be thought of as different mires or mire phases, depending on whether peat accumulation was completely terminated between the two benches, or only briefly interrupted. Where the additional benches represent completely different mires they can have very different characteristics from the main bench and each will retain those characteristics (or trends of characteristics) past the point at which the bench complex develops. In

S.F. Greb et al. / International Journal of Coal Geology 49 (2002) 147–175

contrast, a conventional split, formed from the incursion of clastics into a peat mire in which peat accumulation was only briefly interrupted or in which peat accumulation was continuous lateral to the split, should result in lateral trends toward the clastic source in benches above and sometimes below the split. In other seams, bench complexes may develop because of syndepositional faulting or structural movement (Fig. 16C,D). Bench complexes developed by faulting may be hard to differentiate from bench complexes developed by lateral clastic influx. In fact, syndepositional faulting can trap the position of drainages that flood lateral mires (e.g., Weisenfluh and Ferm, 1991a,b). To some extent, differentiation of faulting from clastic influx is a question of scale and trend. Abrupt lateral changes in coal-bench architecture are perhaps more consistent with faulting than flooding. Exceptions would be where lateral migration of a channel followed clastic influx into the mire, or faulting fixed subsequent channel position. Both could lead to abrupt contacts between multiple bench coals and paleochannel deposits rather than faults. Sudden lateral changes in bench thickness or bench number mapped along relatively linear or rectangular trends have been used to infer syndepositional faulting controls on several coals in the basin (Ferm and Staub, 1984; Hower et al., 1991, 1992; Weisenfluh and Ferm, 1991a,b; Staub, 1994; Greb et al., 1999). Bench complexes formed through clastic influx from rivers and other clastic sources would be expected to result in more gradual rates of bench splitting along curvilinear to irregular trends as has been documented in many coal beds. In order to determine the lateral rate of change in bench number or thickness, however, closely spaced data is needed, either in mine, or in outcrop exposure. Where data is widely spaced it may not be possible to determine the lateral rate at which multiple benches occur, or what type of trend they occur along. Limited accommodation space can also result in coal-bench complexes. Faulting, structure, and paleotopography can result in areas of limited accommodation space. Faulting can juxtapose different beds into similar stratigraphic positions. Syndepositional faulting or structure movement might also result in a low area in which multiple peats accumulated relative to a higher area where a single peat subsequently accumulated (Fig. 16E). The difference in accommoda-

171

tion space could result in different coal beds, separated in time, but merging to form a single seam comprised of a bench complex. Significantly, each of the beds would have its own trend and bench, each having a different depositional history and likely to differ with respect to thickness and quality trends. This can complicate interpretations of coal deposition and reserve estimates. For example, the Stockton and Coalburg coal beds have been shown to locally come together and form a thick low-ash, multi-benched seam. Laterally, they are distinctly separate coal beds separated by 8 to 15 m of siliciclastics. Interpretations from increment sampling of the combined seam suggest that the coal at this location may record the stacking and flooding of as many as five individual ombrotrophic mires (Grady et al., 1992; Eble and Grady, 1993). The Fire Clay and Fire Clay Rider (not simply a rider bench) coal beds also have been shown to merge locally (Weisenfluh and Ferm, 1991a,b), although they generally are separated by 8 to 16 m of siliciclastics. It would be a mistake to interpret the combined bench complex in these two examples as the accumulation of a single coal, even though in both examples the coals are mined as single seams, and appear as single, albeit overly thick, coal beds. Rather, the individual beds in the examples accumulated as temporally separate mires, each with its own trend, deposited under different depositional controls, which came together because of limited accommodation space, and persistence of, or reestablishment of, peat-forming conditions.

8. Summary Many of the most extensively mined coal seams in the central Appalachian Basin consist of multiple coal benches. Vertical sampling of these seams indicates inter- and intra-bench variability in thickness, quality, and other parameters, which are masked when the coal is analyzed on a whole-coal basis. Analyses of intra-seam variation in compositional groups indicate common repetitive trends that can be used to infer a cycle of mire phases during deposition from planar to domed, and back to planar, but the cycle is not regionally consistent. Coal-bench architectural analyses can be used to show the lateral extent of the

172

S.F. Greb et al. / International Journal of Coal Geology 49 (2002) 147–175

component phases within the benches of the seam. Analyses of some of the major mined seams in the basin indicate that the domal phase was temporally restricted to generally small parts of the seam as a whole, and additionally was laterally restricted. In multi-bench coals, the domal phase is usually restricted not only to a small part of the seam, but to a single bench of the seam. This is significant, as it illustrates the importance of recognizing patterns in bench-scale components of the seam. Recognition of common coal-bench architecture patterns can provide models for the accumulation of widespread, low-sulfur, low-ash coals, which incorporate and explain lateral variation in thickness, quality, petrography, and palynology as a function of the juxtaposition of successive benches of coal. Each bench represents accumulation of a different mire phase, and in some cases, a completely different mire reestablished at the same location but temporally separated from the underlying mire. Because each of the coal benches may have its own quality characteristics, it is important to understand the juxtaposition of benches at the scale of a mine, relative to more regional relations of the benches. In the Fire Clay and Lower Elkhorn coal beds, the areas of thickest coal are rarely the areas of best coal quality, because of the addition of poorer quality coal benches to the main, persistent bench. The important aspect of the architectural method is that it creates a framework in which quantitative testing of different subsets of the database can be made in the context of bench configuration. Coal seams can be mapped as a function of their architecture. Where there is variation between benches this method will allow for more accurate quality projections and reserve estimates. If trends in quality, compositional groups, or other parameters can be defined within a particular bench of a coal seam, then correlations of that particular bench can aid in more accurate projections of the salient parameters. This is significant because the thickness data by which most correlations are made is always more abundant than quality, petrographic, palynologic, and geochemical data. If quality is a function of bench configuration, and bench configuration is reflected in coal thickness, then thickness data can be used to map areas of different bench configurations. Marginal approximations of expected quality trends can then be based on available data for that

particular bench configuration. By correlating these data in a larger architectural framework, the best use can be made of available data.

Acknowledgements The authors graciously thank the many individuals from the mining community that gave us access to their property, provided data, and shared the wisdom of their experience. We thank James C. Cobb and Gerald A. Weisenfluh of the Kentucky Geological Survey; Jim for his enthusiasm for our architectural methodology, and Gerry for his critical review of the draft manuscript, and discussions of bench configurations in the basin. We thank John Calder and James Staub for their critical reviews of the manuscript. Lastly, we acknowledge the shared wisdom and advice of John C. Ferm, with whom we all discussed various aspects of our coal research, and whose many contributions to applied coal geology we can only hope to emulate. His insight and experience will be missed.

References Anderson, J.A.R., 1964. The structure and development of peat swamps in Sarawak and Brunei. Journal of Tropical Geography 18, 7 – 16. Anderson, J.A.R., 1983. The tropical pet swamps of western Malesia. In: Gore, A.J.P. (Ed.), Mires: Swamps, Bog, Fen, and Moor. Ecosystems of the World, vol. 4B, pp. 181 – 199. Andrews Jr., W.M., Hower, J.C., Hiett, J.K., 1994. Lithologic and geochemical investigations of the Fire Clay coal bed, southeastern Kentucky, in the vicinity of sandstone washouts. International Journal of Coal Geology 26, 95 – 115. Andrews Jr., W.M., Hower, J.C., Ferm, J.C., Evans, S.D., Sirek, N.S., Warrell, M., Eble, C.F., 1996. A depositional model for the Taylor coal bed, Martin and Johnson Counties, eastern Kentucky. International Journal of Coal Geology 31, 1512 – 1567. Bohor, B.F., Triplehorn, D.M., 1981. Volcanic origin of the flint clay parting in the Hazard No. 4 (Fire Clay) coal bed of the Breathitt Formation in eastern Kentucky. In: Cobb, J.C., Chesnut Jr., D.R., Hester, N.C., Hower, J.C. (Eds.), Coal and Coalbearing Rocks of eastern Kentucky, Annual Geological Society of America Coal Division field trip. Kentucky Geological Survey, Ser. 11, Field Trip Guidebook, pp. 49 – 54. Brant, R.A., Chesnut, D.R., Frankie, W.T., Portig, E.R., 1983. Coal resources of the Hazard District, Kentucky. University of Kentucky Institute for Mining and Minerals Research, Energy Resource Series, 49 pp.

S.F. Greb et al. / International Journal of Coal Geology 49 (2002) 147–175 Butterworth, M.A., 1966. The distribution of densospores. The Paleobotanist 15, 16 – 28. Cairncross, B., Cadle, A.B., 1988. Palaeoenvironmental control on coal formation, distribution and quality in the Permian Vryheid Formation, east Witbank Coalfield, South Africa. International Journal of Coal Geology 9, 343 – 370. Calder, J.H., 1993. The evolution of a ground-water influenced (Westphalian B) peat-forming ecosystem in a piedmont setting – the No. 3 seam, Springhill Coalfields, Cumberland Basin, Nova Scotia. In: Cobb, J.C., Cecil, C.B. (Eds.), Modern and Ancient Coal-forming Environments. Geological Society of America Special Paper, vol. 286, pp. 153 – 180. Calder, J.H., Gilbing, M.R., Eble, C.F., Scott, A.C., MacNeil, D.J., 1996. The Westphalian D fossil lepidodendrid forest at Table Head, Sydney Basin, Nova Scotia: Sedimentology, paleoecology, and floral response to changing edaphic conditions. International Journal of Coal Geology 31, 277 – 313. Cameron, C.C., Esterle, J.S., Palmer, C.A., 1989. The geology, botany, and chemistry of selected peat-forming environments from temperate and tropical latitudes. International Journal of Coal Geology 12, 105 – 156. Cecil, C.B., Stanton, R.W., Neuzil, S.G., Dulong, F.T., Ruppert, C.F., Pierce, B.S., 1985. Paleoclimate controls on Late Paleozoic sedimentation and peat formation in the central Appalachian basin (U.S.A.). International Journal of Coal Geology 5, 195 – 230. Chague, G.C., Fyfe, W.S., 1996. Geochemical and petrographical characteristics of a domed bog, Nova Scotia: A modern analogue for temperate coal deposits. Organic Geochemistry 24, 141 – 158. Clymo, R.S., 1987. Rainwater-fed peat as a precursor of coal. In: Scott, A.C. (Ed.), Coal and Coal-bearing Strata — Recent Advances, vol. 32. Geological Society Special Publication, London, pp. 17 – 23. Cohen, A.D., 1974. Petrography and paleoecology of Holocene peats from the Okefenokee swamp – marsh complex of southern Georgia. Journal of Sedimentary Petrography 44, 716 – 726. Cohen, A.D., 1984. The Okefenokee swamp: a low sulfur end member of a shoreline-related depositional model for coastal plain coals. In: Rahmani, R.A., Flores, R.M. (Eds.), Sedimentology of Coal and Coal-bearing Sequences. Special Publication of the International Association of Sedimentologists, vol. 7, pp. 231 – 240. Cohen, A.D., Spackman, W., Raymond, R.J., 1987. Interpreting the characteristics of coal seams from chemical, physical, and petrographic studies of peat deposits. Scott, A.C. (Ed.), Coal and Coal-bearing Strata — Recent Advances, vol. 32. Geological Society Special Publication, London, pp. 107 – 125. DiMichele, W.A., Phillips, T.L., 1994. Paleobotanical and paleoecological constraints on models of peat formation in the Late Carboniferous of Euramerica. Palaeogeography, Palaeoclimatology, Palaeoecology 106, 39 – 90. DiMichele, W.A., Eble, C.F., Chaney, D.S., 1996. A drowned forest above the Mahoning coal (Conemaugh Group), Upper Pennsylvanian) in eastern Ohio, USA. International Journal of Coal Geology 31, 249 – 276. Donaldson, A.C., 1979. Origin of coal seam discontinuities. In:

173

Donaldson, A.C., Presley, M.W., Renton, J.J. (Eds.), Carboniferous Coal Short Course and Guidebook. West Virginia Geological and Economic Survey, Bulletin B, vol. 37-1, pp. 102 – 132. Eble, C.F., Grady, W.C., 1990. Paleoecological interpretation of a Middle Pennsylvanian coal bed in the Central Appalachian Basin, U.S.A. International Journal of Coal Geology 16, 255 – 286. Eble, C.F., Grady, W.C., 1993. Palynologic and petrographic characteristics of two middle Pennsylvanian coal beds and a probable modern analogue. In: Cobb, J.C., Cecil, C.B. (Eds.), Modern and Ancient Coal-forming Environments. Geological Society of America Special Paper, vol. 286, pp. 119 – 138. Eble, C.F., Greb, S.F., 1997. Channel-fill coals along the western margin of the Eastern Kentucky Coal Field. International Journal of Coal Geology 33, 183 – 207. Eble, C.F., Grady, W.C., Gillespie, W.H., 1989. Palynology, petrography, and paleoecology of the Hernshaw – Fire Clay coal bed in the central Appalachian Basin. In: Cecil, C.B., Eble, C. (Eds.), Carboniferous Geology of the eastern United States. 28th International Geological Congress Field Trip Guidebook, vol. T352. American Geophysical Union, Washington, DC, pp. 133 – 142. Eble, C.F., Hower, J.C., Andrews Jr., W.M., 1994. Paleoecology of the Fire Clay coal bed in a portion of the Eastern Kentucky Coal Field. Palaeogeography, Palaeoclimatology, and Palaeoecology 106, 287 – 305. Esterle, J.S., Ferm, J.C., 1986. Relationship between petrographic and chemical properties and coal seam geometry, Hance Seam, Breathitt Formation, southeastern Kentucky. International Journal of Coal Geology 6, 199 – 214. Esterle, J.S., Ferm, J.C., Tie, Y.-L., 1989. A test for the analogy of tropical domed peat deposits to ‘‘dulling up’’ sequences in coal beds: preliminary results. Organic Geochemistry 14, 333 – 342. Esterle, J.S., Gavett, K.L., Ferm, J.C., 1992. Ancient and modern environments and associated controls on sulfur and ash in coal. In: Platt, J., Price, J.P., Miller, M., Suboleski, S. (Eds.), 1.2 — New Perspectives on Central Appalachian Low-sulfur Coal Supplies. TechBooks, Coal Decisions Forum Publication, Fairfax, VA, pp. 55 – 76. Ferm, J.C., Staub, J.R., 1984. Depositional controls on mineable coal beds. In: Rahmani, R.A., Flores, R.M. (Eds.), Sedimentology of Coal and Coal-bearing Sequences. Special Publication of the International Association of Sedimentologists, vol. 7, pp. 275 – 289. Frazier, D.E., Osanik, A., 1969. Recent peat deposits – Louisiana coastal plain. In: Dapples, E.C., Hopkins, M.E. (Eds.), Environments of Coal Deposition. Geological Society of America Special Paper, vol. 114, pp. 63 – 86. Gluskoter, H.J., Simon, J.A., 1968. Sulfur in Illinois coals. Illinois State Geological Survey, Circular 432, 28 pp. Gore, A.J.P. (Ed.), 1983. Mires: Swamp, Bog, Fen and Moor, Ecosystems of the World. Elsevier, New York, 440 pp. Grady, W.C., Eble, C.F., Ashton, K.C., 1992. Coal supplies for the 1990’s: a re-evaluation of Kanawha Formation splint coals in central and southern West Virginia. In: Platt, J., Price, J.P., Miller, M., Suboleski, S. (Eds.), 1.2 — New Perspectives on Central Appalachian Low-sulfur Coal Supplies. TechBooks, Coal Decisions Forum Publication, Fairfax, VA, pp. 77 – 102.

174

S.F. Greb et al. / International Journal of Coal Geology 49 (2002) 147–175

Grady, W.C., Eble, C.F., Neuzil, S.G., 1993. Brown coal maceral distributions in a modern domed tropical Indonesian peat and a comparison with maceral distributions in middle Pennsylvanianage Appalachian bituminous coal beds. In: Cobb, J.C., Cecil, C.B. (Eds.), Modern and Ancient Coal-forming Environments. Geological Society of America Special Paper, vol. 286, pp. 63 – 82. Greb, S.F., 1991. Roof falls and hazard prediction in Eastern Kentucky coal mines. In: Peters, D.C. (Ed.), Geology in Coal Utilization. TechBooks Publishing, American Association of Petroleum Geologists, Energy Minerals Division, 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.P., Miller, M., Suboleski, S. (Eds.), 1.2 — New Perspectives on Central Appalachian Low-sulfur Coal Supplies. TechBooks, Coal Decisions Forum Publication, Fairfax, VA, pp. 102 – 124. Greb, S.F., Chesnut Jr., D.R. 1992. Transgressive channel filling in the Breathitt Formation (upper Carboniferous) Eastern Kentucky Coal Field, U.S.A. Sedimentary Geology 75, 209 – 221. Greb, S.F., Popp, J.T., 1999. Mining geology of the Pond Creek seam, Pikeville Formation, Middle Pennsylvanian, in part of the Eastern Kentucky Coal Field. International Journal of Coal Geology 41, 25 – 50. Greb, S.F., Weisenfluh, G.A., 1996. Paleoslumps in coal-bearing strata of the Breathitt Group (Pennsylvanian) in the Eastern Kentucky Coal Field, U.S.A. International Journal of Coal Geology 31, 115 – 134. Greb, S.F., Eble, C.F., Hower, J., 1999. Depositional history of the Fire Clay coal bed (Late Duckmantian), eastern Kentucky, U.S.A. International Journal of Coal Geology 40, 255 – 280. Habib, D., Groth, P.K.H., 1967. Paleoecology of migrating Carboniferous peat environments. Palaeogeography, Palaeoclimatology, and Palaeoecology 3, 185 – 195. Helfrich, C.T., Hower, J.C., 1991. Palynological and petrographic variation in the Pond Creek coal bed, Pike County, Kentucky. Organic Geochemistry 17, 153 – 159. Holdgate, G.R., Kershow, A.P., Sluiter, I.R.K., 1995. Sequence stratigraphic analysis and the origins of Tertiary brown coal lithotypes, Latrobe Valley, Gippsland Basin, Australia. International Journal of Coal Geology 28, 249 – 275. Horne, J.C., Ferm, J.C., Carrucio, F.T., Baganz, B.P., 1978. Depositional models in coal exploration and mine planning in the Appalachian region. American Association of Petroleum Geologists Bulletin 62, 2379 – 2411. Hower, J.C., Bland, A.E., 1989. Geochemistry of the Pond Creek coal bed, Eastern Kentucky Coal Field. International Journal of Coal Geology 11, 205 – 226. Hower, J.C., Pollock, J.D., 1988. Petrology of the Pond Creek coal bed in eastern Kentucky. Organic Geochemistry 12, 297 – 302. Hower, J.C., Pollock, J.D., Griswold, T.B., 1991. Structural controls on the petrology and geochemistry of the Pond Creek coal seam, Pike and Martin counties, eastern Kentucky. In: Peters, D.C. (Ed.), Geology in Coal Resource Utilization. TechBooks, Fairfax, VA, pp. 413 – 426. Hower, J.C., Trinkle, E.J., Pollock, J.D., Helfrich, C.T., 1992. In-

fluence of penecontemporaneous tectonism on thickness and quality of Breathitt Formation coals, eastern Kentucky. In: Platt, J., Price, J.P., Miller, M., Suboleski, S. (Eds.), 1.2 — New Perspectives on Central Appalachian Low-Sulfur Coal Supplies. TechBooks, Coal Decisions Forum Publication, Fairfax, VA, pp. 143 – 171. Hower, J.C., Andrews Jr., W.R., Wild, G.D., Eble, C.F., Dulong, F.T., Salter, T.L., 1994a. Quality of the Fire Clay coal bed, southeastern Kentucky. Journal of Coal Quality 13, 13 – 26. Hower, J.C., Eble, C.F., Rathbone, R.F., 1994b. Petrology and palynology of the No. 5 Block coal, northeastern Kentucky. International Journal of Coal Geology 25, 171 – 193. Hower, J.C., Eble, C.F., Pierce, B.S., 1996. Petrography, geochemistry, and palynology of the Stockton coal bed (Middle Pennsylvanian), Martin County, Kentucky. International Journal of Coal Geology 31, 195 – 215. Hower, J.C., Ruppert, L.F., Eble, C.F., 1998. Lanthanide, yttrium, and zirconium anomalies in the Fire Clay coal bed, eastern Kentucky. International Journal of Coal Geology 39, 141 – 153. Kolb, C.R., Van Lopik, J.R., 1966. Depositional environments of the Mississippi River deltaic plain: Southeastern Louisiana. In: Shirley, M.L. (Ed.), Deltas in their Geologic Framework. Houston Geological Society, Houston, TX, pp. 17 – 61. Kosters, E.C., Chmura, G.L., Bailey, A., 1987. Sedimentary and botanical factors influencing peat accumulation in the Mississippi Delta. Journal Geological Society of London 144, 423 – 434. Litke, R., 1987. Petrology and genesis of Upper Carboniferous seams form the Ruhr Region, West Germany. International Journal of Coal Geology 7, 147 – 184. Lyons, P.C., Outerbridge, W.F., Triplehorn, D.M., Evans Jr., H.T., Congdon, R.D., Capiro, M., Hess, J.C., Nash, W.P., 1992. An Appalachian isochron — A kaolinized Carboniferous air-fall volcanic-ash deposit (tonstein). Geological Society of America Bulletin 104, 1515 – 1527. McCabe, P.J., 1984. Depositional environment of coal and coalbearing strata. In: Rahmani, R.A., Flores, R.M. (Eds.), Sedimentology of Coal and Coal-bearing Sequences. Special Publication of the International Association of Sedimentologists, vol. 7, pp. 13 – 42. McCabe, P.J., 1987. Facies studies of coal and coal-bearing strata. In: Scott, A.C. (Ed.), Coal and Coal-Bearing Strata — Recent Advances, vol. 32. Geological Society Special Publication, London, pp. 51 – 66. Moore, P.D., 1989. The ecology of peat-forming processes — A review. International Journal of Coal Geology 12, 89 – 103. Moore, P.D., 1995. Biological processes controlling the development of modern peat-forming ecosystems. International Journal of Coal Geology 28, 99 – 110. Nelson, J.S., Mullennex, R.H., Miller, M.S., 1991. Geological modeling techniques for evaluation of productivity-related longwall mining roof conditions — A case study. In: Peters, D.C. (Ed.), Geology in Coal Resource Utilization. TechBooks, American Association of Petroleum Geologists, Energy Minerals Division, Fairfax, VA, pp. 1263 – 1286. Neuzil, S.G., Supardi, Cecil, C.B., Kane, J.S., Soedjono, K., 1993. Domed peat in Indonesia and its implication for the origin of

S.F. Greb et al. / International Journal of Coal Geology 49 (2002) 147–175 mineral matter in coal. In: Cobb, J.C., Cecil, C.B. (Eds.), Modern and Ancient Coal-forming Environments. Geological Society of America Special Paper, vol. 286, pp. 23 – 44. Pierce, B.S., Stanton, R.W., Eble, C.F., 1991. Facies development in the Lower Freeport coal bed, west-central Pennsylvania, USA. International Journal of Coal Geology 18, 17 – 43. Polak, W., 1975. Character and occurrence of peat deposits in the Malaysian tropics. In: Bartstra, G., Casparie, W. (Eds.), Modern Quaternary Research in Southeast Asia. Balkema, Rotterdam, pp. 77 – 81. Renton, J.J., Hamilton, W.H., 1988. Petrographic zonation within the Waynesboro coal. International Journal of Coal Geology 10, 261 – 274. Scheihing, M.H., Pfefferkorn, H.W., 1984. The taphonomy of land plants in the Orinoco Delta: a model for the incorporation of plant debris in clastic sediments of Later Carboniferous age of Euramerica. Review of Palaeobotany and Palynology 41, 205 – 240. Scott, A.C., 1989. Observations on the nature and origin of fusain. International Journal of Coal Geology 12, 443 – 475. Scott, A.C., Jones, T.P., 1994. The nature and influence of fire in Carboniferous ecosystems. Palaeogeography, Palaeoclimatology, Palaeoecology 106, 91 – 112. Shearer, J.C., Staub, J.R., Moore, T.A., 1994. The conundrum of coal bed thickness: a theory for stacked mire sequences. Journal of Geology 102, 611 – 617. Smith, A.H.V., 1957. The sequence of microspore assemblages associated with the occurrence of crassidurite in coal seams of Yorkshire. Geological Magazine 94, 345 – 363. Smith, A.H.V., 1962. The paleoecology of Carboniferous peats based on miospores and petrography of bituminous coals. Proceedings of the Yorkshire Geological Society 33, 423 – 463. Spackman, W., Scholle, D.W., Taft, W.H., 1964. Field guide to environments of coal formation in southern Florida. Geological Society of America Field Trip Guidebook 10, Pre-Convention Field Trip, Nov. 16 – 18, 67 pp. Spackman, W., Riegel, W.L., Dolson, C.P., 1969. Geological and biological interactions in the swamp – marsh complex of southern Florida. In: Dapples, E.C., Hopkins, M.E. (Eds.), Environments of Coal Deposition. Geological Society of America Special Paper, vol. 114, pp. 1 – 35. Spackman, W., Cohen, A.D., Given, P.H., Casagrande, D.J., 1974. Comparative study of the Okefenokee Swamp and the Everglades-mangrove-marsh complex of southern Florida. Geological Society of America Field Trip Guidebook 6, Pre-convention Field Trip, Nov. 15 – 17, 1974, 265 pp. Spears, D.A., 1987. Mineral matter in coals, with special reference to the Penine coal fields. In: Scott, A.C. (Ed.), Coal and Coalbearing Strata — Recent Advances, vol. 32. Geological Society of London, Special Publication, pp. 171 – 185. Staub, J.R., 1991. Comparisons of central Appalachian Carboniferous coal beds by benches and a raised Holocene peat deposit. International Journal of Coal Geology 18, 45 – 69. Staub, J.R., 1994. Mine level, analysis of planar and raised peat deposition in a wave- and tide-influenced deltaic shoreline setting — Beckley bed (Westphalian A), southern West Virginia. Pa-

175

laeogeography, Palaeoclimatology, Palaeoecology 106, 203 – 221. Staub, J.R., Cohen, A.D., 1979. The Snuggedy Swamp of South Carolina, a back-barrier coal forming environment. Journal of Sedimentary Petrology 49, 133 – 144. Staub, J.R., Esterle, J.S., 1992. Evidence for a tidally influenced upper Carboniferous ombrogenous mire system — Upper bench, Beckley bed (Westphalian A), southern West Virginia. Journal of Sedimentary Petrology 62, 411 – 428. Staub, J.R., Richards, B.K., 1993. Development of low-ash, planar peat swamps in an alluvial-plain setting — The No. 5 Block beds (Westphalian D) of southern West Virginia. Journal of Sedimentary Petrology 63, 714 – 726. Styan, W.B., Bustin, R.M., 1983. Sedimentology of Fraser River Delta peat deposits: a modern analogue of some deltaic coals. International Journal of Coal Geology 3, 101 – 143. Teichmu¨ller, M., 1989. The genesis of coal from the viewpoint of coal petrography. International Journal of Coal Geology 12, 1 – 87. Teichmu¨ller, M., Teichmu¨ller, R., 1982. The geological basis of coal formation. In: Stach, E., Mackowsky, M.-T., Teichmu¨ller, M., Taylor, G.H., Chandra, D., Teichmu¨ller, R. (Eds.), Stach’s Textbook of Coal Petrology, 3rd edn. Gebru¨der Borntraeger, Berlin, pp. 5 – 86. Thacker, E.E., Weisenfluh, G.A., Andrews Jr., W.A., 1998. Total coal thickness of the Lower Elkhorn coal in eastern Kentucky. Kentucky Geological Survey, Series 11, Map and Chart Series 20, 1 sheet. Thacker, E.E., Weisenfluh, G.A., Greb, S.F., Esterle, J.A., 2000. Total coal thickness of the Fire Clay and Fire Clay Rider coals in eastern Kentucky. Kentucky Geological Survey, Series 12, Map and Chart Series 5, 1 sheet. Vogler, P.D., 1994. Depositional model of the Pond Creek seam, eastern Kentucky, based on megascopic and microscopic analysis. Master’s Thesis, Department of Geological Sciences, University of Kentucky, Lexington, 178 pp. Wagner, R.H., 1989. A late Stephanian forest mire with Sporangiostrobus fossilized by volcanic ash fall in the Puertallano Basin, central Spain. International Journal of Coal Geology 12, 523 – 552. Wagner, T., Pfeferkorn, H.W., 1997. Tropical peat occurrences in the Orinoco Delta: preliminary assessment and comparison to Carboniferous coal deposits. Proceedings of the XIII International Congress on the Carboniferous and Permian, Prace Pan’stwowego Instytutu Geologicznego, CL V11, 161 – 168. Weisenfluh, G.A., Ferm, J.C., 1991a. Roof control in the Fireclay Coal Group, southeastern Kentucky. Journal of Coal Quality 10, 67 – 74. Weisenfluh, G.A., Ferm, J.C., 1991b. Application of depositional models to mining problems. In: Peters, D.C. (Ed.), Geology in Coal Utilization. TechBooks Publishing, American Association of Petroleum Geologists, Energy Minerals Division, Fairfax, VA, pp. 189 – 201. Williams, E.G., Keith, M.L., 1963. Relationship between sulfur in coals and the occurrence of marine roof beds. Economic Geology 58, 720 – 729.