THE AMERICAN ASSOCIATION OF PETROLEUM GEOLOGISTS BULLETIN

DECEMBER 1978 VOLUME 62, NUMBER 12

Depositional Models In Coal Exploration and Mine Planning In Appalachilan Region'

J. C. HORNE, J. C. PERM, F. T. CARUCCIO, and B. P. BAGANZ2

Abstract Geologic studies in the Appalachian region have shown that many parameters of coal beds (thick­ness, continuity, roof and floor rock, sulfur and trace-element content, and ash) can be attributed to the de­positional environment in which the peat beds formed and to the tectonic setting at the time of deposition. With an understanding of the depositional setting of the coal seam and contemporaneous tectonic influences, the characteristics and variability of many of these pa­rameters can be predicted.

Coals formed in "back-barrier" environments tend to be thin, laterally discontinuous, high in sulfur, and to exhibit severe roof problems. Therefore, they are not generally Important as minable coals. Coal beds de­posited in the "lower delta-plain" environment are rela­tively widespread with fewer roof problems but gener­ally are thin and show a highly irregular pattern of sulfur and trace-element distribution. Conversely, "up­per delta plain-fluvial" coals are low in sulfur, are thick locally, but are commonly discontinuous laterally. De­spite these problems, some "lower delta-plain" and "upper delta plaln-fluvial" coals are successfully mined. However, most important seams in the Appala­chian area are in the transitional zone between these two environmental fades. In this transition zone, thick coals attain a relatively high degree of lateral continuity and are usually low In sulfur.

Contemporaneous tectonic influences are super­posed on changes in seam character attributed to vari­ations in environments of deposition. Rapid subsi­dence during sedimentation generally results in abrupt variations in coal seams but favors lower sulfur and trace-element content, whereas slower subsidence fa­vors greater lateral continuity but higher content of chemically precipitated material.

INTRODUCTION

In the past, the role of geology in coal explora­tion, mine planning, and mine development has been relatively insignificant. Primarily responsi­ble for this situation has been the simplicity of geologic concepts necessary to conduct these op­erations. Briefly, these concepts can be stated: (1) coals occur in beds or layers that are underlain and overlain by shales, sandstones, and lime­stones in varying proportions; (2) each coal bed can be given a name, and certain quality charac-

© Copyright 1978. The American Association of Petroleum Geologists. All rights reserved.

AAPG grants permission for a sin^^le photocopy of this article for research purposes. Other photocopying not allowed by the 1978 Copyright Law is prohibited. For more than one photocopy of this article, users should send request, article identification number (see below), and $3.00 per copy to Copyright Clearance Center. Inc.. One Park Ave., New York, NY 10006.

•Manuscript received, December 13, 1977; accepted, June 15, 1978.

^Department of Geology, University of South Carolina, Columbia. South Carolina 29208.

This paper is the result of many years of study of coal beds in the Appalachian region. In this effort, many more people have contributed than can be properly acknowledged. However, we would like particularly to thank J. M. Coleman and S. M. Gagliano of the Louisiana State University Coastal Studies Institute; A. D. Cohen. J. R. Staub, and D. A. Corvinus of the University of South Carolina; and R. S. Saxena of Superior Oil Co. for information concerning modern peat-forming environments. In addition, information about coal quality, thickness variability, and roof quality has been obtained from the theses and dissertations by G. Geidel, D. J. Howell, D. Mathew, R. A. Melton, G. W. Pedlow, IIL and J. M. Sewell of the University of South Carolina.

Geologic studies of the coal-bearing strata in the Appalachian basin have been financed through funds from the Environmental Protection Agency, National Science Foundation, and U.S. Bureau of Mines. In addition, we gratefully acknowledge both data and input (engineering and geologic) from the personnel of: the Westmoreland Coal Co., Consolidation Coal Co., Eastern Associated Coal Co.. Bethlehem Steel Corp., United States Steel Corp., Massey Coal Co.. New River Coal Co.. Kerr-McGee Corp., Zapata Corp.. Slab Fork Coal Co.. Allied Chemicals Co., Olga Coal Co., Gaddy Engineering Co., Beaver Land Co., Berwind Land Co.. and Pocahontas Land Co.

With this tremendous amount of industrial cooperation, it is understandable that the confidentiality of data must be maintained. Therefore, in some examples used in this paper, the names and locations of the coals have been deleted or disguised.

Finally, we are grateful to George deVries Klein of the University of Illinois and Edward Belt of Amherst College for critically reviewing the manu.script. Their suggestions have helped to improve the paper.

Article Identification Number 0149-1423/78/B012-()0Ol$03.l)O/()

2379

2380 J. C. Home et al

teristics commonly are associated with this name; and (3) coal beds (and adjoining rocks) common­ly are folded into broad anticlines and synclines and, in places, are displaced by faults.

Basically, the thickest, most persistent, and best quality coal seams were found to follow these concepts reasonably well. However, thickening, thinning, pinchouts, and changes in coal quality did occur, but these occurrences appeared to be random. In addition, when unexpected problems were encountered, ingenious and often expensive engineering techniques provided solutions to most of them.

Today, in many areas, the easily mined, high-quality coals are nearing exhaustion, and the in­creased demand for clean, nonpolluting, safe en­ergy brings a need for new approaches to explora­tion and mining that will make development of formerly unminable seams a profitable venture. Hence, the coal explorationist now must consider such matters as roof and floor control, methane problems, and sulfur and trace-element distribu­tions as well as problems of continuity and thick­ness of coal seams. Because most practical appli­cations occur in relatively small areas of approximately 15,000 acres (6,100 ha.) or less, all of the preceding factors require a high level of precision.

Investigations in the Appalachian region by the Carolina Coal Group of the University of South Carolina have shown that one of the most critical determinants of seam character at this level of investigation is the depositional environment of the coal and enclosing strata. These studies indi­cate that the topographic surface on which the coal swamp developed was a major factor in con­trolling its thickness and extent, whereas the envi­ronments of deposition of the sediments that cov­ered the peat strongly influenced both roof conditions in mines and many aspects of coal quaUty.

Contemporaneous tectonic influences are su­perposed on changes in seam character attributed to variations in environments of deposition. Rapid subsidence during sedimentation results generally in abrupt variations in coal-seam geom­etry and petrography but may favor lower sulfur and trace-element contents, whereas slower subsi­dence rates favor greater lateral continuity but higher contents of sulfur and other chemically precipitated material.

Thus, the principal objectives of this paper are to show the manner in which the depositional en­vironment influences the thickness, extent, qual­ity, and potential minability of coal seams, and also, how the tectonic setting modifies these vari­ations.

CRITERIA FOR RECOGNITION OF DEPOSITIONAL ENVIRONMENTS

The principal criteria for the delineation of de­positional environments are readily illustrated in the coal-bearing parts of the Carboniferous of eastern Kentucky and southern West Virginia (Table 1). The identification of these various pa-leoenvironments in the Carboniferous strati-graphic section is based on the recognition of var­ious counterparts m modern fluvial, deltaic, and barrier systems. Figure 1 shows all the compo­nents of these depositional systems but is not meant to imply thai they are actually contempo­raneous. This figure is based mainly on studies of modern environments of deposition, but includes data from mine maps where coal has been worked out, as well as from maps developed from borehole and outcrop information. The lower part of the figure shows a cross section through these environments with particular emphasis giv­en to the thickness and extent of peat (coal) units. This cross section was derived mostly from strip-mine highwalls, large highway cuts, and closely spaced borehole cross sections, as well as from borehole cross sections from modern coastal ar­eas.

On the left of Figure 1 is the barrier environ­ment. In the Appalachian Carboniferous, barrier environments (Fig. 2) are not important in terms of minable coals and are not discussed in detail in this paper. However, this environment is impor­tant because barrier sands seal off the oxidizing effects of seawater and promote peat formation landward.

The principal criteria for recognizing barrier environments are the lateral and vertical relations of sedimentary structures and textural sequences as well as the mineralogy of the sandstones. In a seaward direction, the sandstones become finer grained and intercalate with red and green calcar­eous shales and carbonate rocks with marine fau­nas whereas, landward, they grade into dark-gray lagoonal shales with brackish-water faunas. Be­cause of wave and udal reworking, sandstones of the barrier system are more quartzose and better sorted than those of the surrounding environ­ments even though both types had the same source area.

Landward, the barrier environments grade into the lagoonal back-barrier environments (Fig. 3). The characteristics of this setting have been de­scribed by Home et al (1974). The principal com­ponents of this environment are sequences of or­ganic-rich dark-gray shales and siltstones which are directly overlain by thin laterally discontinu­ous coals or burrowed sideritic zones. These la-

Deposltional Models In Coal Exploration 2381

Table 1. Criteria for Recognition of Depositional Environments*

I .

I I .

I I I .

IV.

V.

Recognition F luv ia l and Character is t ics Upper Delta

Plain

Coarsening Upward

A. Shale and S l l t s tone sequences

1. Greater than 50 feet

2. 5 t o 25 feet

B. Sandstone sequences

1. Greater than 50 feet

2. 5 to 25 feet

Channel Deposits

A. Fine-grained abandoned f i l l

1 . Clay and s i l t

2. Organic debris

B. Act ive sandstone f i l l

1 . Fine-grained

2. Medium- and coarse­grained

3. Pebble lags

4. Coal spars

Contacts

A. Abrupt (scour)

B. Gradational

Bedding

A. Cross-beds

1 . Ripples

2. Ripple d r i f t

3. Trough cross-beds

4. Graded beds

5. Point-bar accret ion

6. I r regu la r bedding

Levee Deposits

2-3

4

2-3

3-4

4

3

3

3

3

1

2

1

1

1

1

2-3

1

2

2-1

1

3

1

1

Trans i t iona l Lower Delta

Plain

2

3-4

2-1

3-2

4

3-2

2-3

2-3

2-3

2

2

2-3

1

1

1

2

1

2-1

2

1-2

3

2

2

Lower Delta Plain

1

2-1

2-1

2-1

2-1

2-1

1-2

1-2

1-2

2-3

2-3

3

2

2

2

2-1

1

1

2-3

2-1

2-1

3-4

3-2

Back-Barrier

2-1

2-1

2-1

2

3

2

2

2

2-3

2-3

2-3

3

2-3

2-3

2

2

1-2

i

3-2

2

3-2

3-4

3-2

Bar r ie r

3-2

3-2

3-2

2-1

2-1

2

3-2

3-2

3

2

2

2-3

3-2

3-2

2-1

2

1-2

1

3-2

2-1

3-2

3-4

3-2

VII.

A. Irregularly interbedded 1 sandstones and shales, rooted

Mineralogy of Sandstones

A. Lithic graywacke 1

B. Orthoquartzites 4

Fossils

A. Marine 4

B. Brackish 3

C. Fresh 2-3

D. Burrow 3

1-2

1

4

3-2

2

3-2

2

3-2

1-2

4-3

2-1

2

3-4

1

3

1-2

1-2

2-3

4

1

3

1

1-2

2-3

4

1

*Explanation: 1. Abundant 2. Common 3. Rare 4. Not Present

2382 J. C. Home et al

AREA INFLUENCED BY .MARINE TO BRACKISH WATER.

AREA INFLUENCED BY FRESH WATER-

BAR-1 RIER I

BACK- I BARRIER I

LOWER DELTA PLAIN

ITRANSITIONALi I LOWER I DELTA

UPPER DELTA PLAIN-

FLUVIAL,

ORTHOQUARTZITE SANDSTONE

GRAYWACKE SANDSTONE

COAL

0 10 KILOMETERS MILES

FIG. 1—Depositional model for peat-forming (coal) environments in coastal regions. Upper part of figure is plan view showing sites of peat formation in modern environments; lower part is cross section (AA) showing, in relative terms, thickness and extent of coal beds and their relations to sandstones and shales in different environments (modified from Perm, 1976).

goonal to bay-fill sequences (Fig. 4) become coar­ser upward, are extensively burrowed, and com­monly contain marine to brackish faunas. Sea­ward, they intertongue with orthoquartzitic sandstones of barrier origin; in a landward direc­tion, they intercalate with subgraywacke sand­stone of fluvial-deltaic origin. The lagoonal de­posits are 25 to 80 ft (7.5 to 24 m) thick and 3 to 15 mi (5 to 25 km) wide.

The orthoquartzitic sandstones which inter­tongue with the dark-gray lagoonal bay fill are of three general types. The first type consists of ex­tensive sheets of plane-bedded orthoquartzites with rippled and burrowed upper surfaces. These beds dip gently (2 to 12°) in a landward direction (Fig. 3A). Similar features are present in modern barrier washovers into open-water lagoons (Schwartz, 1975). The second type consists of wedge-shaped bodies that extend nearly horizon­

tally in a landward direction for up to 3 mi (5 km; Fig. 3). Near the main body of orthoquartzite, they are up to 20 ft (6 m) thick but thin abruptly and continue as nearly horizontal thin sheets 2 to 3 ft (1 m) thick. In the thicker parts of the deposit, bedding consists predominantly of planar to fes­toon cross-beds with amplitudes of 18 to 24 in. (45 to 60 cm) and landward dip directions. Simi­lar features have been observed in flood-tidal del­tas in modern lagoons (Hubbard and Barwis, 1976).

The third type of orthoquartzite intertonguing with the dark lagoonal shales is tidal-channel de­posits that may scour up to 40 ft (12 m) into un­derlying strata (Fig. 3B). These deposits common­ly are associated with the inclined sheet sands or the wedge-shaped bodies; in addition they occur as isolated units. Associated levee deposits are ab­sent or inconspicuous. Near the main sandstone

Depositional Models in Coal Exploration 2383

I ~ Z ^ SILTSTONE

: , , ^ BlPPttO OK FLASEH-1 BEOOEO SILTSTONE

P T v i J ^ l SANDSTONE F> * ! * i WI>H GRAVEL

EXPLANATION [:-:-:-:-:j SHALE U^"^ *

SANDSTONE COAL PENECONTEMPOSANEOUS

DEFORMATION STRUCTURES

SANDSTONE, RIPPLED ^ 7 7 ROOTED ZONE J^ MARINE FOSSILS

FIG. 2—Barrier model. Depositional composite of exposures near Monteagle, Tennessee, showing shoreface, bar­rier, and back-barrier environments (Ferm et al, 1972).

bodies, the orthoquartzites contain herringbone, festoon cross-bedding; grain size decreases up­ward in the unit. However, not all channels are filled with sandstone; many are filled with dark-gray shales, siltstones, coal, or slump blocks.

Carboniferous lower delta-plain deposits of eastern Kentucky have been described by Baganz et al (1975). These deposits are dominated by thick coarsening-upward sequences of shale and siltstone (Fig. 5A) which range in thickness from 50 to 180 ft (15 to 55 m) and in lateral extent from 5 to 70 mi (8 to 110 km). Recent counterparts of these sequences are forming in interdistributary bays and prodeltas of modern lower delta plains (Coleman et al, 1969).

In the lower part of these bay-fill sequences, dark-gray to black clay shales are the dominant lithologies; some irregularly distributed lime­stones and siderites are present also. In the upper part of these sequences, sandstones with ripples and other current-related structures are common, reflecting the increasing energy of the shallower water as the bay fills with sediment. Where the bays filled sufficiently to form a surface upon which plants could take root, coals formed. How­ever, where the bays did not fill completely, or­

ganisms reworked the subaqueous subsurface, and burrowed, sideritic cemented sandstones were formed.

This general coarsening-upward pattern of in­terdistributary bays is broken in many places by tongues of coarse-grained detritus introduced by crevasse splays (Fig. 5B). Chemically precipitated iron carbonate is common in persistent bands or as large concretions (up to 3 ft or 1 m in diame­ter) along bedding surfaces. Undoubtedly, these secondary siderite concretions formed and Hthi-fied early as evidenced by the compaction of en­closing shales and siltstones around them.

Commonly, the bay-fill sequences contain ma­rine and/or brackish water fossils and burrow structures. These fossils usually are most abun­dant in the basal clay shales but also may be pres­ent throughout the sequence.

Overlying and laterally equivalent to the bay-fill sequences are lithic graywacke sandstone bod­ies 1 to 3 mi (1.5 to 5 km) wide and 50 to 90 ft (15 to 25 m) thick. Recent counterparts of these de­posits are forming at the mouths of distributaries in modern lower delta plains (Saxena and Ferm, 1976). These distributary-mouth bar sandstones (Fig. 6) are widest at the base and have gradation-

2384 J. C. Home et al

ORTHOQUARTZITE SANDSTOME

•DARK-GRAY SHALE AND SILTSTONE

RED AND GREEN SHALE

lOOi T T T ROOTING

->./- BURROW STRUCTURE

Mjr- CROSS - BEDS

^— RIPPLES FEET

SCALES

ISO

WAVE TRAINS MARSH

EBB-TIDAL DELTA

BARRIER ISLAND

FLOOD-TIDAL DELTA SANDSTONE

OFFSHORE] SHALE

SHORE FACE SANDSTONE

ORTHOQUARTZITE

GRAY SHALE

g g j j RED AND GREEN SHALE

g ^ LIMESTONE

— COAL TTTT ROOTING -^^ BURROW STRUCTURE

TIDAL CHANNEL SANDSTONE

LAGOONAL SHALE

FESTOON CROSS-BEDS

PLANAR CROSS-BEDS

RIPPLES

METERS

KILOMETERS

FIG. 3—A, Back-barrier deposits including storm washovers, tidal channels, and flood-tidal delta exposed in clay pit along Interstate 64 west of Olive Hill, Kentucky. Side panels based on greater than 95% exposure (Home and Ferm, 1976). B, Barrier and back-barrier environments including tidal channels and flood-tidal deltas exposed in Carter Caves State Park region near Olive Hill, Kentucky (Home and Ferm, 1976).

Depositional Models in Coal Exploration 2385

COAL SEAT ROCK. CLAYEY

SILTSTONE WITH QUARTZOSE SANDTONE FLASERS

CLAY SHALE WITH SIDERITE BANDS,BURROWED, FOSSILIFEROUS

COAL SEATROCK.CLAYEY

SANDSTONE,QUARTZOSE PLANAR ACCRETION BEDS

SHALE AND SILTSTONE.COARSENING UPWARD, BURROWED

CLAY SHALE, SIDERITE BANDS, LIMESTONE, BURROWED, FOSSILIFEROUS COAL SEAT ROCK,CLAYEY SANDSTONE, QUARTZOSE, FINING UPWARD, RIPPLED AND CROSS-BEDDED

SILTSTONE WITH SANDSTONE FLASERS BURROWED SIDERITIC SANDSTONE

SANDSTONE QUARTZOSE, CROSS-BEDDED

SHALE AND SILTSTONE,COARSENING UPWARD, BURROWED

CLAY SHALE, SIDERITE BANDS, BURROWED, FOSSILIFERO

WAMP BANDONED TIDAL CHANNEL

^ ^ n ^ ^ l D A L FLAT

— LAGOON

STORM WASHOVERS

^LAGOON

SWAMP

TIDAL CHANNEL

TIDAL FLAT

FLOOD-TIDAL \- DELTA

LAGOON

FIG. 4—Generalized vertical sequence through back-barrier deposits in Carboniferous of eastern Ken­tucky and southern West Virginia.

al lower and lateral contacts. Grain size increases upward in the sequence and toward the center of the bar. Laterally persistent fining-upward graded beds are common on the flanks of the bars as are oscillation and current-rippled surfaces, whereas multidirectional festoon cross-beds are prevalent in the central part of the bar. In the central area, there is little lateral continuity of beds owing to multiple scouring by flood currents. Slumps and flow rolls are associated with the flanks and front of the mouth bar where the sediment interface steepened beyond the angle of repose. Fossils and burrow structures are generally absent within the bar deposits but, where subaerial levees are con­structed protecting the interdistributary areas from the rapid influx of detrital sediments, organ­isms returned and burrowed the flanks of the bar.

Distributary channels in the lower delta plain are characterized by two types of sedimentary fill: active and abandoned. Because channels in the lower delta plain are straight with little ten­dency to migrate laterally, active channel-fill de­posits containing point-bar accretion beds are not common. Where present, these deposits consist of sandy sequences up to 60 ft (18 m) thick and

1,000 ft (300 m) wide and grade upward from coarse to fine with trough cross-beds in the lower part and ripple drift in the upper. The basal con­tact, which is scoured along an undulating or wavy surface, in many places truncates the under­lying distributary-mouth bar and bay deposits. Commonly, pebble-lag conglomerates are present at the base of the channel deposits as are coal "spars" which represent compressed pieces of wood or bark.

Because of the rapid abandonment of distribu­taries, fine-grained clay plugs are the predomi­nant type of channel fill in the Carboniferous lower delta-plain deposits of eastern Kentucky. These abandoned fills (Fig. 7) are comprised of clay shales, siltstones, and organic debris which settled from suspension in the ponded water of the abandoned distributary. In some places, thick organic accumulations (now coal) filled these holes. The clay shales commonly are root pene­trated or burrowed. The only coarse-grained sedi­ments present in the abandoned channels are thin-rippled and small-scale cross-bedded sands and silts which probably were deposited during floods or at sites near the distributary cutoff.

2386 J. C. Home et al

Coal Seat rock, clayey Sandstone, fn. to rned.-grained, multi­

directional planar and festoon cross-beds Sandstone, fine-grained, rippled Sandstone, fine-grained, graded beds Sandstone, flow rolIs Sandstone, fine-grained, flaser-bedded and siltstone

Silty Shale and Siltstone with calcareous concretions thin-bedded, burrowed, occasional fossil

Clay Shale with siderite bands, burrowed, fossiliferous

Distributary-Mouth Bar

L Distal Bar

Interdistributary Bay Or Prodelta

SAND 1 SILT CLAY

Coal Rooted Sandstone Sandstone, f i ne -g ra ined , cl imbing r ipp les Sandstone, f i ne to medium-grained Sandstone, med.-grained, festoon cross-beds Congl. Lag, siderite pebble, coal spar Sandstone, Siltstone, graded beds

Sandstone, flow rolls Sandstone, S i l t s t one , f laser-bedded S i l t s tone and S i l t y Shale thin-bedded,

burrowed Burrowed s i d e r i t i c Sandstone Sandstone, f ine-gra ined Sandstone, f i ne -g ra ined , r ipp led

S i l t y Shale and S i l t s tone wi th calcareous concret ions, th in-bedded, burrowed

Clay Shale wi th s i d e r i t e bands burrowed, f o s s i l i ferous

Channel

D is t r i bu ta ry -Mouth Bar

Distal Bar

n te rd i s t r i bu ta r y

Crevasse Splay

I n t e rd i s t r i bu ta r y Bay Or Prodelta

FIG. 5—Generalized vertical sequences through lower delta-plain deposits in eastern Kentucky. A, Typical coarsening-upward sequence. B, Same sequence interrupted by splay deposit (Baganz et al, 1975).

In the Carboniferous lower delta-plain deposits of eastern Kentucky, levees are thin and poorly developed, the largest being about 5 ft (1.5 m) thick and 500 ft (150 m) wide. Levees consist of poorly sorted, irregularly bedded, partially rooted siltstones and sandstones. These beds display a pronounced dip (about 10°) away from the asso­ciated channel (Fig. 7). Coal beds, other than

those associated with abandoned fills, are thin but relatively widespread parallel with distribu­tary trends.

The final major component of the lower delta plain is the crevasse splay (Fig. 8). These deposits contain all the characteristics of coarsening-up­ward minideltas. They become gradationally finer grained away from the breached levee to where

Depositional Models in Coal Exploration

DISTRIBUTARY CHANNEL LEVEE

2387

CREVASSE SPLAY

MUDS

DISTRIBUTARY-MOUTH BAR SANDS

SANDSTONE

^SHALE AND SI ITS TONE

f " T T ROOT STRUCTURES

T^ BURROW STRUCTURES

BEDDING

RIPPLES

CROSS-BEDS

FLOW ROLLS

200

100 300

FIG. 6—Distributary-mouth bar sandstone exposed in interval below lower Elkhorn coals along U.S. Highway 23 north of Pikeville, Kentucky. Side panels of block diagram based on greater than 90% exposure (Baganz et al, 1975).

FIG. 7—Abandoned channel fill with thin levees near Ivel, Kentucky. Levee dips away from channel.

2388 J. C. Home et al

LEVEE

DISTRIBUTARY CHANNEL

CREVASSE

123 SANDSTONE

• SPtAY SItTSTONE AND SHALE l^yj BAYFILL SHALE

• i COAL

SCALES

5 • IN FEET

) 250 5C FIG. 8—Crevasse-splay deposits exposed in interval above upper Elkhorn Nos. 1 and 2 coals along U.S. Highway 23 near Betsy Layne, Kentucky. Side panels of block diagram based on greater than 80% exposure (Baganz et al, 1975).

they grade laterally into interdistributary bay-fill sequences. Commonly, an abandoned channel fill occurs in a splay which formed as a result of the closing of the crevasse in the levee. Carboniferous splays vary in size with thicknesses up to 40 ft (12 m) and horizontal extents ranging from 100 ft (30 m) to 5 mi (8 km).

In contrast to the thick fine-grained bay-fill se­quences of the lower delta-plain deposits, the eastern Kentucky Carboniferous upper delta plain-fluvial deposits are dominated by linear, lenticular bodies of sandstone which, in cross sec­tion (Fig. 9), are 50 to 80 ft (15 to 25 m) thick and 1 to 7 mi (1.5 to 11 km) wide. These sandstone bodies contain scoured bases, sharply truncating the surface upon which they lie, but laterally, in the upper part, they intertongue with gray shales, siltstones, and coal beds (Fig. 10). The sandstone mineralogy varies from lithic graywackes to ar-koses; grain sizes are predominantly medium to coarse. Above the scoured base, grain size dimin­ishes upward within these sandstones; abundant pebble lags and coal "spars" are present in the

lower part. Bedding in these sandstone bodies is massive, with thick festoon cross-beds in the low­er part; upward, these massive beds merge into point-bar accretion beds (average dip of 17°) con­taining smaller scale festoon cross-beds. These beds are overlain by partially rooted sandstones and siltstones with climbing ripples. All of these characteristics, in addition to the lateral relations, suggest a high-energy channel flanked by swamps, small ponds, and lakes (Fig. 11). The up­ward-widening cross-sectional shape of the sand­stone bodies and the point-bar accretion beds in­dicate that meandering was important in the development of these deposits. These sandstone bodies show an en echelon arrangement suggest­ing episodes of lateral jumping of channels into adjoining backswamps.

Backswamp deposits consist of sequences which, from base up, are comprised of seat earth, coal, shale with abundant plant fossils and rare freshwater pelecypods, siltstone, sandstone, seat earth, and coal. The sandstone thickens laterally and merges with the major sandstone bodies. The

NORTH

1000 2000

-rIO M

5 0 0 M i5» LEGEND

E m SANDSTONE

Lin SANDSTONE AND SILTSTONE

£13 SHALE AND SILTSTONE

^ SIDERITE SANDSTONE

^a BLACK SHALE

F ^ PLANT SHALE

M~2 BONE SHALE

^m COAL

SOUTH

D (D

5" 3 m

I O o S. m X •o o"

5' 3

FIG. 9—Upper delta plain-fluvial deposits exposed along U.S. Highway 23 south of Louisa, Kentucky. Cross section is based on more than 60% exposure along highway (Home and Baganz, 1974). 00

2390 J. C. Horne et al

COAL WITH CLAY SPLIT SEAT ROCK, CLAYEY

SANDSTONE AND SILTSTONE, CLIAABING RIPPLES,ROOTED

SANDSTONE,MEDIUM TO COARSE GRAINED, FESTOON CROSS-BEDDED

COAL WITH SEAT-ROCK SPLITS SEAT ROCK, SILTY

SANDSTONE AND SILTSTONE, CLIMBING RIPPLES, ROOTED

SANDSTONE MEDIUM TO COARSE GRAINED FESTOON CROSS-BEDDED

CONGLOMERATE LAGSIDERITE PEBBLES SLUMPS SILTSTONE, THIN-BEDDED

COAL WITH CLAY SPLITS

BACKSWAMP LEVEE

CHANNEL

FLOOD PLAIN BACKSWAMP

LEVEE

CHANNEL

^^rr LAKE FLOOD PLAIN BACKSWAMP

FIG. 10—Generalized vertical sequence through upper delta plain-fluvial deposits of eastern Kentucky and southern West Virginia.

SWAMP POINT BAR

LEVEE

SCALES

1.; i SANDSTONE

\5^ SILTSTONE AND SHALE

o.c. PEBBLE LAG — COAL ^^'' ROOTING -^^- TROUGH CROSS-BEDS : / 5 BEDDING PLANES

301

METERS

lOOn

FEET 50

L=_ L. 300

METERS

5 0 0

FEET

1000

FIG. 11—Reconstruction of upper delta plain-fluvial environments as exposed in interval around Haz­ard No. 6 coal along Daniel Boone Parkway and Kentucky Route 15 northeast of Hazard, Kentucky. Side panels of diagram based on greater than 65% exposure.

Depositional iModels in Coal Exploration 2391

thin (5 to 15 ft; 1.5 to 4.5 m), fine-grained, up­ward-coarsening sequences are typical deposits of open-water bodies, probably shallow ponds or lakes. The lateral extent of these deposits is only 1 to 5 mi (1.5 to 8 km).

Levee deposits consist of poorly sorted, irregu­larly bedded sandstones and siltstones that are extensively root penetrated. They are thickest (up to 25 ft; 8 m) near active channels, and decrease both in grain size and thickness away from the channels. The levee deposits also display a promi­nent dip (up to 10°) away from the channel. Coals in the upper delta plain-fluvial deposits are locally thick (up to 32 ft; 10 m) but are laterally discontinuous (sometimes pinching out within 500 ft; 150 m).

Between the lower and upper delta-plain de­posits of the eastern Kentucky Carboniferous is a transitional zone that exhibits characteristics of both the lower and upper delta-plain sequences (Figs. 12, 13). The fine-grained bay-fill sequences are thinner (5 to 25 ft; 1.5 to 7.5 m) than those of the lower delta plain. However, unlike the thin bay-fill sequences of the upper delta plain, they contain marine to brackish faunas and are exten­sively burrowed (Fig. 14).

Channel deposits (Fig. 15) exhibit features of lateral migration such as point-bar accretion beds similar to the channels of the upper delta plain, but these transitional delta-plain channels are fin­er grained than those of the upper delta plain. These channel deposits are single-storied se­quences having one direction of lateral migration, whereas upper delta-plain channel sandstones are multistoried units with many directions of lateral migration. The levees associated with these chan­nels are thicker (5 to 15 ft; 1.5 to 4.5 m) and more extensively root penetrated than those of the low­er delta plain. Thin (5 to 15 ft; 1.5 to 4.5 m) splay sandstones are common in these deposits but are less numerous than in the lower delta plain, yet they are more abundant than those of the upper delta plain.

Because many of the interdistributary bays filled with sediment in the transitional zone be­tween the lower and upper delta plains, a wide­spread platform developed upon which peat (coal) swamps formed. The resultant coals are thicker and more widespread than the coals of the lower delta plain. Most of the economically im­portant coal seams of the Appalachian region are in this transitional zone between lower and upper delta-plain environments.

INTERACTION OF DEPOSITIONAL ENVIRONMENT AND TECTONIC SETTING

Superposed on changes in lithologic character which can be attributed to variations within and

between depositional environments are those ef­fects that arise from broad-scale contemporane­ous tectonic influences. This point is illustrated by a generalized regional cross section of the Car­boniferous from Bluefield in southern West Vir­ginia to Pittsburgh, Pennsylvania (Fig. 16). South of the Paint Creek fault zone, the section thickens greatly in response to an increased rate of subsi­dence (Ferm, 1976).

This large differential rate of subsidence from south to north produced very pronounced effects on depositional environments and, consequently, on the characteristics of distribution and quality of the enclosed coal seams. In the southern area of more rapid subsidence, the depositional facies are stacked on each other and exhibit slow rates of progradation, whereas in the more stable (less rapid subsidence) platform area on the north the depositional facies prograde very abruptly over this shelf. The transition from upper delta-plain to barrier environments occurs in a distance of approximately 10 to 15 mi (16 to 24 km) in the south, whereas on the more stable platform on the north the same environmental transition oc­curs very gradually over a distance greater than 60 mi (96 km). The net effect of this change is that, generally, the minable coals of southern West Virginia display a much more restricted lateral distribution than those of western Pennsyl­vania.

An equally important consequence of differen­tial regional subsidence is the sulfur content of coals. Coals of southern West Virginia, like those of western Pennsylvania, show an increase in to­tal sulfur (and reactive pyrite) as they pass from upper delta-plain to back-barrier environments. However, in the south, the effect is muted by the rapid rate of sedimentation; the coals from south-em West Virginia are well known for their low sulfur content. In contrast, the coals of the Pitts­burgh area, which were deposited on a stable platform where the rates of sedimentation were lower and chemical activity was higher than in southern West Virginia, generally have a higher sulfur content. The same effect may be expected with the minor trace elements.

In addition to the regional influences of con­temporaneous tectonism on depositional facies, detailed local sedimentary responses to move­ments of basement features can be identified. Al­though most of the basement faults in eastern Kentucky do not offset the deposits of the coal measures, there is ample evidence of sedimentary responses to these contemporaneously active structures. Figure 17 is a regional cross section (constructed from over 400 highway roadcuts) of the coal measures exposed along U.S. Highway 23 between Pikeville, Kentucky, on the south and

2392 J. C. Home et al

I D i i i i i o

Depositional Models in Coal Exploration 2393

COAL.SEAT ROCK CLAYEY

SANDSTONE.FINE-GRAINED, RIPPLED

SHALE AND SILTSTONE, COARSENING UPWARD SIDERITE BANDS, BURROWED COAL SEAT ROCK SILTY

SANDSTONE AND SILTSTONE,CLIMBING RIPPLES, ROOTED SANDSTONE FINE TO MEDIUM-GRAINED FESTOON CROSS-BEDOED CONGLOMERATE LAG,SIDERITE PEBBLES COAL.SEAT ROCK

SHALE AND SILTSTONE, COARSENING UPWARD BURROWED COAL.SEAT ROCK CLAYEY

SANDSTONE F INEGRAINED RIPPLED

SHALE AND SILTSTONE COARSENING UPWARD SIDERITE BANDS. BURROWED

COAL WITH SEAT ROCK SPLITS

SANDSTONE AND SILTSTONE, CLIMBING RIPPLES, ROOTED

SANDSTONE, FINE TO MEDIUM-GRAINED FESTOON CROSS-BEDDED

CLAY SHALE, BURROWED

COAL

CREVASSE SPLAY INTERDISTRIBUTARY BAY

LEVEE

CHANNEL

:^?^?7Z^tREVASSE SPLAY INTERDISTRIBUTARY BAY

fREVASSE SPLAY INTERDISTRIBUTARY BAY

LEVEE

CHANNEL

FIG. 13—Generalized vertical sequence through transitional lower delta-plain deposits of eastern Ken­tucky and southern West Virginia.

Louisa, Kentucky, on the north. Although this di­agram has been generalized, many relations be­tween the basement structures and lithologic vari­ations can be observed. Terrigenous clastic wedges thin or pinch out, and coal beds may thin or merge over these flexures. In addition, the in­tensity of root penetration also may increase over the structural highs indicating longer exposure and deeper soil development.

Finally, the most obvious feature is the "stack­ing" or localization of channeling along the flanks of these flexures, which is emphasized on Figure 18, a block diagram of the area near the Blaine-Woodward fault. This fault is also shown at the north end of the regional cross section (Fig. 17) in the area 3 mi (5 km) south of Louisa, Ken­tucky. Regional paleocurrent analyses indicate the channels shown on Figure 18 were carrying sediment from southeast to northwest. However, just south of the Blaine-Woodward fault, the pa­leocurrent directions indicate the channels were deflected to the west. Thus, the channels were lo­calized on the downthrown (southern) side of the fault. This area should be avoided from the stand­point of coal exploration because the coals have been removed by the channeling.

APPLICATION OF DEPOSITIONAL MODELS

The deUneation of depositional environments can be applied to produce predictive models that

are of economic significance in coal exploration and mine planning.

Variations in Thiciiness and Extent

The three-dimensional shape (thickness and lateral extent) of coal bodies is affected directly by the depositional setting in which they accumu­lated. The depositional environments that pre­cede, coexist with, or are postdepositional modify the shape of the coal bodies, as do the internal processes active within the coal swamps. The sed­imentary environments that immediately precede the coal swamp shape the topography on which the swamp develops. This topography affects most directly variations in coal thickness, al­though to a lesser extent it also affects the lateral continuity of the seam. Those environments that coexist laterally with the peat (coal) swamp, as well as internal processes within the swamp such as the plant growth, plant decay, fires, and water flow, directly affect the lateral continuity of the coal-forming deposits and, to a lesser extent, thickness variations of the seams. Following burial of peat (coal) beds, the processes of postde­positional environments, such as channeling, may impinge downward and modify the upper surface of the deposits. These processes cause local thin­ning and the interruption of the lateral continuity of seams by channel "washouts" (the removal of coal by channel scouring).

2394 J. C. Home et al

CREVASSE LEVEE SPLAY SWAMP

POINT BAR

100

30a FEET 50

METERS

\~~] SANDSTONE

I ] SANDSTONE AND SILTSTONE

^ ^ SHALE

— COAL r7Tf ROOTING ~-^ BURROW STRUCTURE ^ MARINE FOSSIL

^5?? BEDDING PLANES V > ^ TROUGH CROSS-BEDS

FIG. 14—Reconstruction of transitional lower delta-plain environments as exposed along U.S. Highway 23 near Sitka, Kentucky. Side panels based on greater than 70% exposure (Home and Perm, 1976).

I FEET 5ol SCALES

L 0 1000 2000 FEET

On a regional scale, depositional models can be used to predict the trends of coal bodies. These models are useful in an initial phase of coal explo­ration. Moreover, locally, they permit a detailed understanding of variations in coal thickness and lateral continuity that can aid in mine planning and development.

Regionally, back-barrier coal bodies formed landward of contemporaneous or preexisting bar­rier systems. The coal swamps developed on plat­forms that evolved as the result of infilling of the lagoons behind the barriers. Coexisting and post-depositional tidal channels may modify the back-barrier coals into pod-shaped bodies. However, the trend of these pod-shaped seams parallels the trend of the associated barrier systems, and bar­riers, most commonly, are elongate parallel with depositional strike.

By contrast, in river-dominated lower delta-plain deposits of the Appalachian Carboniferous, the coal bodies are elongate parallel with deposi­tional dip. This trend exists because the only sites where peat swamps can develop are on the nar­row, poorly developed levees along the distribu­tary channels. These river-dominated lower delta-plain channels generally are straight and rapidly prograde seaward in the direction of depositional dip. For this reason, the coals that develop in this environment are continuous laterally in the direc­tion of depositional dip but discontinuous parallel with depositional strike, being interrupted by in-terdistributary bay-fill deposits. These seams commonly are relatively thin and contain numer­ous splits caused by crevasse splays that breached the poorly developed levees along the distributary channels.

Depositional Models in Coal Exploration 2395

ms^Mi^^^ )CXZ3EX3dCSZ3dCCZZaSDDCZ3X3SE ^^'TrTTTTTTT-T-T-^rT-T-T—T-T—7—r-T—?—r-?— f t- ^ ,» -r f -t—7—-< • < <•'•!. f v '̂ " "^^ >".<"<

• • lOOn 2 0 n

SANDSTONE

SANDSTONE AND SIITSTONE SHALE AND SILTSTONE

BURROWED SIDERITE SANDSTONE

o.o* PEBBLE LAG

— — COAL

jrr ROOTING

< 2 l ^ SLUMP STRUCTURE 600

1000 2000

FIG. 15—Fine-grained point-bar, channel, and backswamp deposits exposed along Interstate 64, 4 mi west of Rush, Kentucky (Home and Ferm, 1976).

Upper delta plain-fluvial coals also tend to parallel depositional dip. However, they are not so laterally continuous in that direction as the coals of the lower delta plain. These seams occur in pod-shaped bodies that accumulated on flood plains adjacent to coexisting meandering chan­nels. As a result, coals formed in this setting dis­play abrupt variations in thickness over short lateral distances with numerous splits occurring in the coals near the levees of the contemporane­ously active channels. In addition, postdeposi-tional channeling may interrupt further the lateral continuity of these seams by causing "washouts."

Within the transition zone between the lower and upper delta plains, many of the large interdis-tributary bays have filled with sediment providing a broad platform upon which widespread coal swamps can develop. In this depositional setting, the resultant coal bodies are extensive laterally with an inclination to be slightly elongate parallel with depositional strike. Similar to the upper delta plain-fluvial coals, these transitional lower delta-plain seams develop splits adjacent to levees of

coeval channels, and in some places, they contain "washouts" where later periods of channeling have scoured through the coals. Although the other depositional settings contain many econom­ic coals, most of the important widespread coals of the Appalachian region have accumulated in this transition zone between the lower and upper delta plains.

Thus, in an initial exploration phase, an under­standing of the controls depositional environ­ments exert on the shape of coal bodies is impor­tant in designing a drilling program that can trace the trends of coal bodies. However, at the stage of mine planning and development, a detailed knowledge of the influence of depositional envi­ronments on variations in coal thickness is most critical.

The Beckley coal of southern West Virginia il­lustrates these characteristics. Figure 19 is a pa-leogeographic reconstruction of the depositional setting of the Beckley coal. This reconstruction is based on data from 1,000 core holes in a 400 sq mi (1,000 sq km) area. Regionally, this coal accu-

POCAHONTAS BASIN DUNKARP BASIN

c**ro wci KM ot cicx

SCALES 0 50 KILOMETERS

0>

AUfCHCNT

POTTS VIILI

MISSlSSIfPlAN

BLUEFIELD

o 3 (D <5.

F I G . 16—Cross section of Pottsville and Allegheny Format ions from vicinity of Bluefield, West Virginia, to Pit tsburgh, Pennsylvania, showing general a r rangement of coal beds a n d depositional envi ronments in which they formed (after Ferm and Cavaroc , 1969).

REGIONAL CROSS SECTION ALONG U.S. 23

PIKEVILLE TO LOUISA, KENTUCKY

BtAINE WAIBRIDGE

SYSTEM • f fTyv»^< t < t '>i.yT y >

LEGEND

'_ Q ORTHOOUARTZITE

, SANnSIONE , — — ^ . . ^ - y — - f ' • - • : " J ^ * ^

- ^\\ r-^'-r-'-i''^'^'^,—-^ V ^ t 1 bHAlE ANO SIlISTONf ,» \ ?^ '—•—J I ^' » ; ••• < • »' A • r f , > ^ . ) < ^ . ^ ^ f^^£^ BURROWED SIOEKIIE

PAINT CREEK IRVINE

SYSTEM

~=H IIMESTCINE

^ 1 COAl

[ " ^ ' ^ ROOTS

f O

a

I m X

•o

o" Q>

0 KIIOMETERS 10

SCALES

FIG. 17—Regional cross section of lateral relations of coal beds and enclosing lithologies exposed along U.S. Highway 23 between Louisa, Kentucky, on north and Pikeville, Kentucky, on south. Cross section is constructed from over 400 highway roadcuts (Home et al, 1976a).

u

2398 J. C. Home et al

LEGEND

CZ] SANDSTONE

TTf

SHALE AND SILTSTONE

COAL

ROOTS

< ^ DIRECTION OF PALEOCURRENTS

360 0 500 1000

FIG. 18—Localization and deflection of channeling along Blaine-Walbridge fault south of Louisa, Kentucky. Side panels based on greater than 60% exposure (Home et al, 1976a).

mulated landward (south) of a barrier system trending from east-northeast to west-southwest (Galloway, 1972; Robinson, 1975). The trend of this back-barrier coal body parallels closely that of the associated barrier system, although it is ab­sent in places owing to concomitant and later tidal channels that produced "want areas" (areas of little or no coal). Thus, the Beckley coal be­haves as other back-barrier coals with respect to coal-body shape and regional trend, and present and future exploration programs should be de­signed to take advantage of these characteristics.

At the lease-tract level (15,000 acres; 6,000 ha., or less), an understanding of coal-thickness varia­tions is important economically. To depict the in­fluence depositional environments exert in con­trolling coal-thickness variations, the details of a mined-out area of the Beckley coal were used (Fig. 20).

Regionally, this mined-out area of the Beckley coal is situated in a back-barrier depositional set­

ting (Fig. 19). The coarsening-upward sequence that was deposited over the preexisting coal (front panels. Fig. 20) formed as a result of the infilling of the lagoonal area landward of the associated barrier system. As the lagoonal area filled with sediment, tidal flats (flasered siltstone and sand­stone) and salt marshes became established be­tween intervening tidal channels (upper surface, Fig. 20A). This setting provided the topography upon which the subsequent coal swamp formed.

Initially a freshwater marsh and/or swamp de­veloped on the high areas over the previous salt-marsh surface and eventually spread over most of the area. As plant growth continued, the smaller channels and the upper parts of the larger chan­nels became clogged with organic material, and only the major tidal channels continued to re­main open (Fig. 20B). Consequently, the thick­ness variations in the resultant coal deposit reflect very closely the influence of the preexisting depo­sitional topography, with the thicker coal occur-

Depositional Models in Coal Exploration 2399

r ^ ORTHOQUARTZITIC L 2 J SANDSTONE

FIASERED SILTSTONE

tfVKLl COAL < 2'

COAL>2'

SCALES 0 5

1 1

KILOMETERS

0 3.5 '1 1

AREA DETAILED BLOCK DIAGRAM

MILES

FIG. 19—Regional depositional setting of Beckley coal and surrounding lithologies. Re­construction is based on data from 1,000 core holes in 400 sq mi (1,000 sq km) area. Area enclosed by heavy lines is detailed on Figure 26.

COAL SWAMP

X' COAL THICKNESS

5-lOft

SANDSTONE

F ~ n FLASER-BEDDED SILTSTONE b ^ AND SANDSTONE

p S i l SHALE AND SILTSTONE

2000 0 600

FIG. 20—Block diagrams showing detailed relations of depositional topography and coal thickness. Front panels of block diagrams are reconstructed from core-hole (average spacing, one record per 500 ft or 150 m) and mine data, whereas plan views are reconstructed from mine maps. Depositional topography shown on surface of diagram A is residual topography after regional dip has been removed by trend-surface program. On block diagram B, coal thicknesses were contoured from thicknesses recorded on mine maps; within mine, elevations of base of coal and thickness were recorded by engineers approximately every 75 ft (25 m). Regional setting of these detailed diagrams of Beckley is shown on Figure 19.

ring in the former lows and the thinner coals over the highs. The coal is absent or badly split in places where a few contemporaneous tidal chan­nels remained active.

As observed in this example, at the lease-tract level, coal-thickness variations are closely related to the preexisting depositional topography. This topography is the result of the depositional envi­ronments that existed prior to coal formation. In addition, the shape of the coal body is modified by coexisting and postdepositional environments such as channels. If these factors are considered during mine planning, the main tunnels of the mine could be designed to maximize economi­cally the recovery of the thicker bodies of coal while avoiding the "want areas."

COAL QUALITY: SULFUR PROBLEMS

Iron disulfides (FeSi) are present in coals either as marcasite or pyrite. They occur as euhedral grains, coarse-grained masses (greater than 25ft) which replace original plant material, coarse­

grained platy masses (cleats) occupying joints in the strata, and framboidal pyrite (Fig. 21; Caruc-cio et al, 1977). The last is in clusters of spherical agglomerates comprised of 0.25/1 grains of iron disulfide and is disseminated finely throughout the coal and associated strata. Of these four basic types, only the framboidal form decomposes rapidly enough to produce severe acid mine drainage in the absence of carbonate material (Caruccio, 1970) and is so disseminated through the coal that it cannot be removed in the 1.50-density sink fraction in washability tests.

Research by Love (1957). Love and Amstutz (1966), Cohen (1968), Rickard (1970), Berner (1971, Chap. 10), and Javor and Mountjoy (1976) suggests that the framboidal form of pyritic sulfur is produced by sulfur-reducing microbial organ­isms which are found in marine to brackish wa­ters but not fresh water. Mansfield and Spack-man (1965), working with selected bituminous coals from western Pennsylvania, have shown petrographically that coals formed under the in-

Depositlonai Models in Coal Exploration 2401

SECONDARY REPLACEMENT

CLEAT COATS

EUHEDRA FRAMBOIDAL

PRIMARY

FIG. 21—Forms of pyrite that occur in coals.

fluence of marine water contained more sulfur than those formed in fresh water. Similar sulfur variations were reported by Cohen (1968) and Cohen et al (1971) in the modern peats of the Everglades.

Among Carboniferous coal-bearing rocks in western Pennsylvania, Williams and Keith (1963) demonstrated statistically that coals having roof rocks of marine or brackish-water origin contain more sulfur than those with roof rocks of fresh­water origin. On the basis of research in the Car­boniferous of eastern Kentucky and southern West Virginia, Ferm et al (1976) and Caruccio et al (1977) have established that sulfur present in the framboidal form of iron disulfide is most strongly associated with roof rocks deposited in marine to brackish-water environments. Sim larly, in the Everglades (Cohen et al, 1971) and

along the South Carolina coast (Corvinus and Cohen, 1977), it has been documented that peats with high sulfur contents in the form of framboi­dal pyrite are formed where the marshes are being transgressed by marine to brackish-water envi­ronments. The only exception occurs where a suf­ficient thickness of sediment is introduced early enough to shield the peat from the marine to brackish waters.

Thus, the environments of deposition of the sediments that overlie the coal are more impor­tant to the distribution of the type and amount of sulfur in the coal than the environments of depo­sition of the sediment on which the coal devel­oped. Consequently, coals that accumulated in areas under marine influence such as back-barrier and lower delta-plain environments are likely to be overlain by marine to brackish sediments and

2402 J. C. Home et al

contain high amounts of disseminated pyritic sul­fur in the reactive framboidal form.

Coals that amassed in the transitional lower delta-plain environment were farther from marine influences and, generally, contain less framboidal pyritic sulfur. However, some of these coals are overlain by sediments that were deposited in shal­low-marine to brackish-water bays. That these bays were open to marine influences is shown by the marine to brackish faunas preserved in the strata. Where this marine to brackish roof rock is present, the pyritic sulfur in the underlying coal increases greatly, most of it being present as framboidal pyrite. For this reason, the distribu­tion of disseminated pyritic sulfur is highly varia­ble in the transitional lower delta plain, although, overall, deposits in this environment are lower in pyritic sulfur than those in the more marginal ma­rine environments.

Upper delta-plain to fluvial environments sel­dom are transgressed by marine to brackish wa­ters, and almost all coals formed in these deposi-tional settings are low in pyritic sulfur. In addition, most of the iron disulfide present is of secondary origin in the forms of massive plant replacements and cleat fillings.

At the lease-tract level, an understanding of the controls that the depositional setting exerts on the

distribution of the amount of sulfur and the type of pyrite can permit the exploration for low-sulfur coals in areas where the sulfur content is usually high. This strategy can be illustrated by an exam­ple from the Carboniferous of the eastern United States. In this example, based on 450 core holes in a 200 sq mi (500 sq km) area, the coal accumulat­ed in a lower delta plain environment. Where ov­erlain by marine to brackish roof rock, coals formed in this depositional setting commonly dis­play a propensity toward high (greater than 2%) sulfur contents with most of the sulfur (greater than 75%) in the form of framboidal pyrite (Ca-ruccio et al, 1977). However, when splay deposits are introduced early and are of sufficient thick­ness, they shield the coal from the sulfur-reducing bacteria, and the sulfur content remains low (less than 1%; Home et al, 1976b).

An east-west cross section (Fig. 22) through the exploration area shows a fossiliferous hmestone and black shale that he directly on coal X in the eastern part of the cross section. However, the limestone and black shale rise stratigraphically above the coal to the west, being separated by an intervening wedge of terrigenous clastic sediment. The distribution and thickness of this detrital wedge, as well as the area where the limestone and black shale directly overlie the coal, are

EAST-WEST GEOLOGIC CROSS SECTION

pg5^ LIMESTONE

B l COAL

BLACK SHALE ^TT ROOTING

SANDSTONE

•SHALE AND SILTSTONE

I00|

FEET

50| L 3 0 | L_ 000 0 3

FEET KILOMETERS SCALES

FIG. 22—Cross section showing distribution of lithologies overlying coal X. Location of cross section shown on Figure 23.

Depositional Models in Coal Exploration 2403

shown in Figure 23. That the detrital sediment was introduced early and shielded the coal from the marine to brackish waters is demonstrated by the fact that the deposits of these waters (the limestone and black shale) overlie the terrigenous clastic rocks. This configuration indicates that the detrital influx occurred before or during marine inundation.

Figure 24 is a reconstruction of the deposi­tional setting immediately after the formation of coal X. It is based on data related to lithologic and sediment-thickness variations. These data suggest that the levees of a distributary channel in the southwestern part of the area were breached several times forming large splay deposits in the north and east over the coal and into the inter­vening interdistributary bay. In areas removed from this detrital influx, fossiUferous limestone and black shale were deposited from the marine to brackish waters of the bay.

Figure 25 illustrates the distribution of the sul­fur in coal X that cannot be removed in the 1.50-density sink fraction of washability tests. As ex­pected, the coal in the eastern part of the area, where it is overlain by roof rock of marine to brackish origin, is high in sulfur (greater than 2%) with most of the pyritic sulfur in the form of disseminated framboids. On the west and south where the coal is overlain by the wedge of terrige­

nous clastic sediment, the sulfur content decreas­es to less than 1%.

This example demonstrates the importance of splay deposits in the formation of pockets of low-sulfur coal of sufficient areal extent to be eco­nomic in the lower delta-plain setting, normally a high-sulfur coal realm. Because splay deposits form adjacent to the distributary channels in this depositional setting, drilling programs should be devised to define these features. In this manner, the areas of the lower delta plain with the greatest potential for low-sulfur coal can be delineated.

The relations shown in this example illustrate the closely parallel distributions of coals with dis­seminated pyritic sulfur and roof rock of marine to brackish origin. Moreover, when terrigenous clastic sediment is introduced early and is of suf­ficient thickness, the sulfur content in the under­lying coal remains low. With a knowledge of these characteristics and an understanding of the depositional setting, exploration programs can be designed to outline areas of low-sulfur coal in what is most commonly a high-sulfur coal prov­ince.

ROOF CONDITIONS

In the mines of southern West Virginia and eastern Kentucky, roof quality is dependent on the interrelations of rock types, syndepositional

SCALES

1 T miles

0 5 kilometers

pnqq LIMESTONE AND ^ 3 BLACK SHALE

^ ^ Otf - IOft

I I lOft-20Jt

[ ] greater than 20 ft

—^— line of cross section

FIG. 23—Thickness of terrigenous clastic wedge of sediment between coal X and overlying marine limestone and black shale. Location of cross section in Figure 22 shown by heavy line.

2404 J. C. Home et al

SCALES

0 5 kilomalars

DEPOSITIONAL

ENVIRONMENTS

OF ROOF ROCK

( N

FIG. 24—Reconstruction of depositional setting immediately after formation of coal X. Diagram is based on data related to lithologic and sediment-thickness variations.

SCALE

0 1 2 3 milM

0 S kilomatars

SULFUR PERCENT

1 1 iMslhanl

E ' 2 S 2 3 m a 4 [ •* ' ] gr«al«r than 4

N

FIG. 25—Distribution of sulfur in coal X that cannot be removed in 1.50-density sink fraction of washability tests.

Depositional Models in Coal Exploration 2405

structures, early postdepositional compactional traits, and later tectonic features (Ferm and Mel­ton, 1975). Because most of the deposits of the coal measures in this region are terrigenous clas­tic rocks, rock types are contingent upon grain size and degree of cementation. Most commonly, the syndepositional features are burrow and root structures, bedding, and slickensided surfaces in clayey rooted zones. Where less compactible rocks such as sandstone are surrounded by more compactible types such as shales and siltstones, differential compactional features occur. Super­posed on these characteristics are later tectonic structures such as jointing and fracturing.

The best quality roof conditions in this region of the Appalachians occur in hard graywacke sandstones that are more than 10 ft (3 m) thick and extend horizontally more than 2,000 ft (600 m). These sandstones were deposited in active, laterally migrating channels. This type of channel is predominantly in upper delta plain-fluvial and transitional lower delta-plain depositional set­tings. Lag deposits, composed of shale and coal pebbles, commonly formed near the base of the channels. These lags can weaken the sandstone and cause roof problems.

Unjointed, well-cemented, orthoquartzitic sandstones, with similar thickness and areal ex­tent such as the graywacke sandstones, also may provide excellent roof conditions. Unfortunately, they usually are jointed and fractured, and in this state, the resulting blocks come loose causing se­vere roof falls. These quartzose sandstones nor­mally are most abundant in back-barrier deposi­tional settings in close proximity to the associated barrier system.

In flat-bedded sandstones and interbedded sandstones and shales, the roof quality is depen­dent on bed thickness. If the beds are less than 2 ft (0.6 m) thick, parting separations can occur along bedding planes, making bolting necessary. Where the beds are 2 to 10 ft (0.6 to 3 m) thick, the roof conditions are excellent because bridging strengths are sufficient to prevent falls. However, where bed thicknesses exceed 10 ft (3 m), slicken­sided surfaces may develop owing to differential compaction, and failure may occur along these surfaces. Flat-bedded sandstones and interbed­ded sandstones and shales are most common in the flanks of distributary-mouth bars and in splay deposits. Predominantly, these features are devel­oped best in lower delta-plain sequences, but they also may be present in the transitional lower del­ta-plain setting.

Coarsening-upward rock sequences that grade from shale upward through shales with thin sand­stone streaks (flasers) to interbedded sandstone and shale, capped by sandstones, provide few

roof-support problems. However, separations at sandstone-shale bedding planes can produce roof falls. Hence, roof bolting is an essential precau­tion. Coarsening-upward rock sequences are characteristic of bay-fill deposits. Thick wide­spread bay-fill units dominate the lower delta-plain depositional setting, but they are also abun­dant in lagoonal bay fills of the back-barrier set­ting. To a lesser extent, they are present in the thin bay fills of the transitional lower delta plain.

In some places, the coals are overlain by a brit­tle, nonbedded, carbonaceous black claystone that is jointed (called "cube rock" by miners). Blocks of this "cube rock" may come loose sud­denly from the roof causing dangerous falls. Thus, this lithology always should be bolted and, in places, it may have to be removed to prevent dangerous roof conditions. These carbonaceous black shales are the result of the low-energy re­working of the upper surface of peats during the drowning phase of coal swamps. They are pres­ent, to a limited degree, in all the coal-forming environments. However, carbonaceous shales are developed most extensively in the transitional lower delta-plain setting, and they may also be abundant in lower delta-plain deposits.

Another roof problem occurs where fine­grained rocks such as shales, siltstones, and shales with sandstone streaks are extensively burrowed. The burrow structures can reduce significantly the strength of these fine-grained rocks and cause roof falls. Bolting is a necessity but often is insuf­ficient to prevent falls and, in some places, the underlying coal must be abandoned. Extensively burrowed fine-grained rocks are formed where sedimentation rates are low and/or infaunal ac­tivity is intensive. The environments that are open to marine or brackish waters, such as the back-barrier, lower delta plain, and transitional lower delta plain are most likely to fulfill these criteria.

Some of the poorest roof conditions occur where the coal is overlain by seat earths (silty clays that are extensively root penetrated). These root-penetrated, fine-grained rocks are crosscut by slickensided planes which commonly intersect at angles ranging between 90 and 120° and may display pronounced local vectoral attributes (Ferm and Melton, 1975). However, any regional orientation of the slicked surfaces is lacking. Be­cause of the slickensided surfaces and the exten­sive rooting, such fine-grained seat earths possess little strength. So, when they are present above coals, no amount of bolting will prevent roof falls. Either this material must be removed, or the coal beneath has to be abandoned.

Although the origin of these slickensided sur­faces is not known, similar features are reported in the root-penetrated swamp soils of the Missis-

2406 J. C. Home et al

sippi delta (Coleman et al, 1969). Rooting is abundant in areas that are more continually ex­posed. Thus, the upper delta plain-fluvial envi­ronment and transitional lower delta-plain envi-roimient have the largest potential for seat earths to develop over coals.

Frequently, upright stumps of trees remained when a coal swamp was buried by fine-grained terrigenous clastic deposits. Ultimately, the cores of these stumps filled with sediment and, with time, the bark surrounding the sediment altered to a thin film of coal. When the underlying coal is removed, the stumps (called "kettles" by miners) remain in the roof of the mine. Because the thin film of coal has little strength and, like most trees, the diameter of the stumps increases downward, these "kettles" may fall suddenly of their own weight. As they usually weigh several hundred pounds, they can easily kill or severely injure a worker. For this reason, they should be bolted or removed immediately when they are encountered. Although these buried stumps are present in all the coal-forming environments, they are most abundant in the upper delta plain-fluvial and transitional lower delta-plain settings owing to the broad flood-plain platforms for plant growth and the rapid rates of sedimentation during floods.

In areas where less compactible coarse-grained rocks (principally sandstones) are present as dis­crete bodies in more compactible fine-grained sediments, slickensided surfaces form at the con­tact between the lithologies. Zones of weakness are developed along these surfaces, and separa­tions may cause severe roof falls. This situation occurs only in environments with high shale-to-sandstone ratios, such as the lower delta-plain and back-barrier depositional settings.

Another place where severe roof problems may develop is where channel-bank slump blocks form the roof over the coal. The slickensided planes present with these disturbed blocks are analogous to slicked surfaces associated with modern channel-bank slumps. Because of the numerous slickensided surfaces and the size of the blocks, severe roof problems can be antici­pated wherever these slumps are encountered. Roof bolting and bracing are of little use, and the area of the slump blocks should be avoided. Channel-bank slump blocks (Fig. 26) develop normally on the cutbank side of laterally migrat­ing, meandering, stream channels. This type of channel is most common in the upper delta plain-fluvial and transitional lower delta-plain environ­ments. In addition, cutbanks and slump blocks may be present in the meandering tidal channels of the back-barrier setting.

Finally, some of the most severe roof problems arise where rider coals have formed within 20 ft (6 m) above the main seam, and the intervening rock type is dominantly fine-grained material such as shale or siltstone. Because the rider coals and underlying root-penetrated clays have little strength, they provide zones of weakness along which separations can occur. When these separa­tions develop, severe roof falls evolve and encom­pass all the material up to the rider seam. Such areas should be circumvented wherever possible.

The rider seams developed in areas where the levees of sediment-laden channels were crevassed and detritus splayed over the adjoining coal swamps. After the floodwaters subsided, the swamps reestablished themselves, and peat, from which the rider coals formed, accumulated. This situation is common in any of the delta-plain en­vironments.

As shown previously, most of the features of roof conditions can be related to depositional or early-stage compactional processes. It appears probable that later tectonic events may have ac­centuated these early traits, but the basic charac­teristics seem to have been established during or shortly after the sediments were deposited. Thus, by depicting the depositional setting, much can be predicted about the lateral distribution of roof types, and potential roof problems can be antici­pated (Table 2).

To demonstrate how depositional environ­ments can affect roof conditions in underground mines, a case history of a roof problem is illus­trated for a mine in the Cedar Grove coal of southern West Virginia. On the basis of regional exploration data, the depositional setting in which the Cedar Grove formed was the transitional low­er delta plain (Fig. 27). In this area, peat (coal) accumulation was interrupted at many localities by terrigenous clastic sediment that splayed over the coal swamp. The sediment for these splays originated from the waters of the distributary channel located in the northern part of the area. After the periods of splaying, the swamp reestab­lished itself, and a thin rider coal developed over the splay deposits (Fig. 28A).

Between the splays, peat accumulation contin­ued uninterrupted, and economically thick bodies of coal were amassed. In the area of exploration, there were two bodies of thick coal. Separating these two bodies of thick coal is a zone where the coal has been split into two thinner seams by a splay deposit (Fig. 28A). On the basis of detailed exploratory drilling, a company developed a mine in the western pocket of thick coal. In addition, the company's property encompassed a sizable part of the eastern body of thick coal and, ulti-

Depositional Models in Coal Exploration 2407

FIG. 26—Exposure of slump deposits along base of cutbank side of channel near Ashland, Kentucky.

Table 2. Abundance of Potent ia l Roof Problems Related to Deposit ional Environments

Roof Problems Back-Barrier Lower Delta Transitional Lower Fluvial and Upper Plain Delta Plain Delta Plain

Clay-r1ch Rooted Zones

Slump Blocks

Rider Coals within 20 f t . (6 meters) of Mined Coal (Intervening Material Fine-grained)

Kettles (Upright Stumps) in Fine-grained Rock

Sandstone Channels Enclosed in Shales

Jointed Orthoquartzitic Sandstones

Cube Rock (Non-bedded Black Claystones)

Conglomerate and Plant Debris Lags

Fine-grained Burrowed Zones

3-2

1-2

3-2

2

1

2-1

2

3-2

1-2

2

3-2

2-1

2

1-2

3

2-1

2-3

1-2

2-1

2-1

1-2

2-1

2

3-4

1

2-1

2-1

1

1-2

1

1-2

3-2

1

3-2

1. Abundant 2. Common 3. Rare 4. Not Present

2408 J. C. Home et al

FIG. 27—Transitional lower delta-plain setting in which Cedar Grove coal of southern West Virginia accumu­lated. Area enclosed in heavy lines is location of mine property shown in Figure 29. Cross-section location in Figure 28 is shown by heavy line XX'.

mately, they planned to extend their mine into that area.

With the continued removal of coal from the thick western pocket, the mine eventually began to impinge onto the edge of the splay (Fig. 29). This splay divided the coal into two benches with the interval of sediment between the benches in­creasing toward the center of the splay (Fig. 28A). For this reason, as mining proceeded into the splay, the percentage of rejects increased until it became uneconomical to continue mining both benches of coal.

Initially, the company tried to circumvent the splay by going around its southern terminus but, unfortunately, ran out of property before reach­ing the end of the splay. Then, they decided to extend the mine to the eastern body of thick coal by driving a tunnel through the splay. As neither bench of the coal was of economic thickness by itself, the company chose to have the tunnel fol­low the thicker of the two benches (the lower seam) so they could continue to mine some coal. In this manner, the economic losses incurred in cutting this tunnel were to be minimized. Figure

28B is a cross section showing the planned route of the tunnel.

As might be expected, severe roof falls were encountered almost immediately as tunneling proceeded under the splay. The falls encom­passed all the material up to the rider coal. Un­daunted, the engineers pulled out and tried again and, then, a third time. Each time, severe roof falls occurred.

Finally, after considerable expense and loss of equipment, the engineers for the company were convinced that this was not the way to reach the eastern body of coal. At this point, a study of roof conditions related to depositional setting was commissioned. From this study, a revised plan for reaching the eastern pocket of coal was proposed. This plan took into account the geologic patterns imposed by the depositional setting. Thus, rather than go under the splay and contend with severe roof problems, it was proposed that the tunnel go directly over the top of the splay where roof con­ditions were favorable.

The company engineers and miners were cau­tioned strongly to go straight over the top of the

Deposltlonal Models in Coal Exploration 2409

SANDSTONE AND SILTSTONE

SHALE AND SILTSTONE

3 0 |

15| 2ol SCALE I loU 'N «ET

^ / J COAL 5000

IN METERS

T T - ? ROOTING

mm — HEIGHT OF ROOF BOLTS 1000

PROPOSED ROUTE OF TUNNEL

FIG. 28—A, Cross section of splay deposit that crosses mine property and splits Cedar Grove coal. Location of cross section is shown on Figures 27 and 29. B, Location of proposed tunnel under splay deposit; dashed line is height to which roof bolts are driven. C, Location of proposed tunnel over splay deposit.

2410 J. C. Home et al

FIG. 29—Detailed diagram of mine property showing location of mined-out Cedar Grove coal and position of cross-section XX'.

splay and not follow the rider coal down into the central gut of the splay (Fig. 28C). It is difficult to convince mine owners and operators that it will be profitable economically in the long term to mine rock even for a short time. However, if they had followed that rider coal into the gut of the splay, they would have encountered considerable difficulty in getting the machinery back out. Con­tinuous mining machines may do well going downhill, but they do not function well going up­hill, especially when it is the steep side of a chan­nel, and the floor is clay.

Fortunately, the warnings were heeded, and this story has a happy ending. The tunnel over the top of the splay has been completed, and coal is being removed from the eastern body of thick coal.

SUMMARY

Increased demands for energy in the face of diminishing supplies of readily available liquid hydrocarbons have turned the attention of the en­ergy industries to coal. Geologic studies in the Appalachian region have shown that many of the characteristics of coal beds—thickness, continui­

ty, roof and floor rock, ash, sulfur, and trace-ele­ment contents—can be attributed to the environ­ments in which the peat beds accumulated and to the tectonic setting at the time of deposition. These studies indicate that the topographic sur­face on which the coal swamp developed is a ma­jor factor in controlling its thickness and lateral extent, whereas the environments of deposition of the sediments deposited on top of the peat mark­edly influenced many aspects of coal quality and roof conditions within mines. Rapid subsidence during sedimentation results in abrupt lateral variations in coal seams but favors lower sulfur and, probably, trace-element contents, whereas slower subsidence rates favor greater lateral con­tinuity of seams but higher content of chemically precipitated material.

Thus, a knowledge of depositional environ­ments and contemporaneous tectonic influences should aid in the exploration and development of economic coal bodies.

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Depositional Models in Coal Exploration 2411

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