2380 J. C. Home et alteristics commonly are associated with this name; CRITERIA FOR RECOGNITION OFand (3) coal beds (and adjoining rocks) common- DEPOSITIONAL ENVIRONMENTSly are folded into broad anticlines and synclinesand, in places, are displaced by faults. The principal criteria for the delineation of de- Basically, the thickest, most persistent, and best positional environments are readily illustrated inquality coal seams were found to follow these the coal-bearing parts of the Carboniferous ofconcepts reasonably well. However, thickening, eastern Kentucky and southern West Virginiathinning, pinchouts, and changes in coal quality (Table 1). The identification of these various pa-did occur, but these occurrences appeared to be leoenvironments in the Carboniferous strati-random. In addition, when unexpected problems graphic section is based on the recognition of var-were encountered, ingenious and often expensive ious counterparts m modern fluvial, deltaic, andengineering techniques provided solutions to barrier systems. Figure 1 shows all the compo-most of them. nents of these depositional systems but is not Today, in many areas, the easily mined, high- meant to imply thai they are actually contempo-quality coals are nearing exhaustion, and the in- raneous. This figure is based mainly on studies ofcreased demand for clean, nonpolluting, safe en- modern environments of deposition, but includesergy brings a need for new approaches to explora- data from mine maps where coal has beention and mining that will make development of worked out, as well as from maps developed fromformerly unminable seams a profitable venture. borehole and outcrop information. The lowerHence, the coal explorationist now must consider part of the figure shows a cross section throughsuch matters as roof and floor control, methane these environments with particular emphasis giv-problems, and sulfur and trace-element distribu- en to the thickness and extent of peat (coal) units.tions as well as problems of continuity and thick- This cross section was derived mostly from strip-ness of coal seams. Because most practical appli- mine highwalls, large highway cuts, and closelycations occur in relatively small areas of spaced borehole cross sections, as well as fromapproximately 15,000 acres (6,100 ha.) or less, all borehole cross sections from modern coastal ar-of the preceding factors require a high level of eas.precision. On the left of Figure 1 is the barrier environ- Investigations in the Appalachian region by the ment. In the Appalachian Carboniferous, barrierCarolina Coal Group of the University of South environments (Fig. 2) are not important in termsCarolina have shown that one of the most critical of minable coals and are not discussed in detail indeterminants of seam character at this level of this paper. However, this environment is impor-investigation is the depositional environment of tant because barrier sands seal off the oxidizingthe coal and enclosing strata. These studies indi- effects of seawater and promote peat formationcate that the topographic surface on which the landward.coal swamp developed was a major factor in con- The principal criteria for recognizing barriertrolling its thickness and extent, whereas the envi- environments are the lateral and vertical relationsronments of deposition of the sediments that cov- of sedimentary structures and textural sequencesered the peat strongly influenced both roof as well as the mineralogy of the sandstones. In aconditions in mines and many aspects of coal seaward direction, the sandstones become finerquaUty. grained and intercalate with red and green calcar- Contemporaneous tectonic influences are su- eous shales and carbonate rocks with marine fau-perposed on changes in seam character attributed nas whereas, landward, they grade into dark-grayto variations in environments of deposition. lagoonal shales with brackish-water faunas. Be-Rapid subsidence during sedimentation results cause of wave and udal reworking, sandstones ofgenerally in abrupt variations in coal-seam geom- the barrier system are more quartzose and betteretry and petrography but may favor lower sulfur sorted than those of the surrounding environ-and trace-element contents, whereas slower subsi- ments even though both types had the samedence rates favor greater lateral continuity but source area.higher contents of sulfur and other chemically Landward, the barrier environments grade intoprecipitated material. the lagoonal back-barrier environments (Fig. 3). Thus, the principal objectives of this paper are The characteristics of this setting have been de-to show the manner in which the depositional en- scribed by Home et al (1974). The principal com-vironment influences the thickness, extent, qual- ponents of this environment are sequences of or-ity, and potential minability of coal seams, and ganic-rich dark-gray shales and siltstones whichalso, how the tectonic setting modifies these vari- are directly overlain by thin laterally discontinu-ations. ous coals or burrowed sideritic zones. These la-
Deposltional Models In Coal Exploration 2381 Table 1. Criteria for Recognition of Depositional Environments* Recognition F l u v i a l and Transitional Lower Delta Back-Barrier Barrier Characteristics Upper Delta Lower Delta Plain Plain Plain I. Coarsening Upward A. Shale and S l l t s t o n e 2-3 2 1 2-1 3-2 sequences 1. Greater than 50 f e e t 4 3-4 2-1 2-1 3-2 2. 5 t o 25 f e e t 2-3 2-1 2-1 2-1 3-2 B. Sandstone sequences 3-4 3-2 2-1 2 2-1 1. Greater than 50 f e e t 4 4 2-1 3 2-1 2. 5 t o 25 f e e t 3 3-2 2-1 2 2 II. Channel Deposits A. Fine-grained abandoned 3 2-3 1-2 2 3-2 fill 1. Clay and s i l t 3 2-3 1-2 2 3-2 2. Organic debris 3 2-3 1-2 2-3 3 B. Active sandstone fill 1 2 2-3 2-3 2 1. Fine-grained 2 2 2-3 2-3 2 2. Medium- and coarse- 1 2-3 3 3 2-3 grained 3. Pebble lags 1 1 2 2-3 3-2 4. Coal spars 1 1 2 2-3 3-2III. Contacts A. Abrupt (scour) 1 1 2 2 2-1 B. Gradational 2-3 2 2-1 2 2 IV. Bedding A. Cross-beds 1 1 1 1-2 1-2 1. Ripples 2 2-1 1 i 1 2. Ripple d r i f t 2-1 2 2-3 3-2 3-2 3. Trough cross-beds 1 1-2 2-1 2 2-1 4. Graded beds 3 3 2-1 3-2 3-2 5. Point-bar accretion 1 2 3-4 3-4 3-4 6. I r r e g u l a r bedding 1 2 3-2 3-2 3-2 V. Levee Deposits A. Irregularly interbedded 1 1-2 3-2 sandstones and shales, rooted Mineralogy of Sandstones A. Lithic graywacke 1 1 1-2 3 3 B. Orthoquartzites 4 4 4-3 1-2 1VII. Fossils A. Marine 4 3-2 2-1 1-2 1-2 B. Brackish 3 2 2 2-3 2-3 C. Fresh 2-3 3-2 3-4 4 4 D. Burrow 3 2 1 1 1*Explanation: 1. Abundant 2. Common 3. Rare 4. Not Present
2382 J. C. Home et al AREA INFLUENCED BY AREA INFLUENCED .MARINE TO BRACKISH WATER. BY FRESH WATER- UPPER BAR-1 BACK- I LOWER ITRANSITIONALi DELTA PLAIN- RIER I BARRIER I DELTA PLAIN I LOWER I FLUVIAL, DELTA ORTHOQUARTZITE SANDSTONE GRAYWACKE COAL SANDSTONE 0 10 KILOMETERS MILESFIG. 1—Depositional model for peat-forming (coal) environments in coastal regions. Upper part of figure is planview showing sites of peat formation in modern environments; lower part is cross section (AA) showing, in relativeterms, 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 coar- tally in a landward direction for up to 3 mi (5 km;ser upward, are extensively burrowed, and com- Fig. 3). Near the main body of orthoquartzite,monly contain marine to brackish faunas. Sea- they are up to 20 ft (6 m) thick but thin abruptlyward, they intertongue with orthoquartzitic and continue as nearly horizontal thin sheets 2 tosandstones of barrier origin; in a landward direc- 3 ft (1 m) thick. In the thicker parts of the deposit,tion, they intercalate with subgraywacke sand- bedding consists predominantly of planar to fes-stone of fluvial-deltaic origin. The lagoonal de- toon cross-beds with amplitudes of 18 to 24 in.posits are 25 to 80 ft (7.5 to 24 m) thick and 3 to (45 to 60 cm) and landward dip directions. Simi-15 mi (5 to 25 km) wide. lar features have been observed in flood-tidal del- The orthoquartzitic sandstones which inter- tas in modern lagoons (Hubbard and Barwis,tongue with the dark-gray lagoonal bay fill are of 1976).three general types. The first type consists of ex- The third type of orthoquartzite intertonguingtensive sheets of plane-bedded orthoquartzites with the dark lagoonal shales is tidal-channel de-with rippled and burrowed upper surfaces. These posits that may scour up to 40 ft (12 m) into un-beds dip gently (2 to 12°) in a landward direction derlying strata (Fig. 3B). These deposits common-(Fig. 3A). Similar features are present in modern ly are associated with the inclined sheet sands orbarrier washovers into open-water lagoons the wedge-shaped bodies; in addition they occur(Schwartz, 1975). The second type consists of as isolated units. Associated levee deposits are ab-wedge-shaped bodies that extend nearly horizon- sent or inconspicuous. Near the main sandstone
Depositional Models in Coal Exploration 2383 EXPLANATION P T v i J ^ l SANDSTONE I~Z^ SILTSTONE F> * ! * i W I > H GRAVEL [:-:-:-:-:j SHALE U^"^ * :,,^ BlPPttO OK FLASEH- PENECONTEMPOSANEOUS SANDSTONE COAL 1 BEOOEO SILTSTONE DEFORMATION STRUCTURES SANDSTONE, RIPPLED ^ 7 7 ROOTED ZONE J^ MARINE FOSSILSFIG. 2—Barrier model. Depositional composite of exposures near Monteagle, Tennessee, showing shoreface, bar- rier, and back-barrier environments (Ferm et al, 1972).bodies, the orthoquartzites contain herringbone, ganisms reworked the subaqueous subsurface,festoon cross-bedding; grain size decreases up- and burrowed, sideritic cemented sandstonesward in the unit. However, not all channels are were formed.filled with sandstone; many are filled with dark- This general coarsening-upward pattern of in-gray shales, siltstones, coal, or slump blocks. terdistributary bays is broken in many places by Carboniferous lower delta-plain deposits of tongues of coarse-grained detritus introduced byeastern Kentucky have been described by Baganz crevasse splays (Fig. 5B). Chemically precipitatedet al (1975). These deposits are dominated by iron carbonate is common in persistent bands orthick coarsening-upward sequences of shale and as large concretions (up to 3 ft or 1 m in diame-siltstone (Fig. 5A) which range in thickness from ter) along bedding surfaces. Undoubtedly, these50 to 180 ft (15 to 55 m) and in lateral extent from secondary siderite concretions formed and Hthi-5 to 70 mi (8 to 110 km). Recent counterparts of fied early as evidenced by the compaction of en-these sequences are forming in interdistributary closing shales and siltstones around them.bays and prodeltas of modern lower delta plains Commonly, the bay-fill sequences contain ma-(Coleman et al, 1969). rine and/or brackish water fossils and burrow In the lower part of these bay-fill sequences, structures. These fossils usually are most abun-dark-gray to black clay shales are the dominant dant in the basal clay shales but also may be pres-lithologies; some irregularly distributed lime- ent throughout the sequence.stones and siderites are present also. In the upper Overlying and laterally equivalent to the bay-part of these sequences, sandstones with ripples fill sequences are lithic graywacke sandstone bod-and other current-related structures are common, ies 1 to 3 mi (1.5 to 5 km) wide and 50 to 90 ft (15reflecting the increasing energy of the shallower to 25 m) thick. Recent counterparts of these de-water as the bay fills with sediment. Where the posits are forming at the mouths of distributariesbays filled sufficiently to form a surface upon in modern lower delta plains (Saxena and Ferm,which plants could take root, coals formed. How- 1976). These distributary-mouth bar sandstonesever, where the bays did not fill completely, or- (Fig. 6) are widest at the base and have gradation-
2384 J. C. Home et al lOOi SCALES ORTHOQUARTZITE TTT ROOTING SANDSTOME ->./- BURROW STRUCTURE DARK-GRAY SHALE • AND SILTSTONE Mjr- CROSS - BEDS RED A N D GREEN ^— RIPPLES ISO SHALE FEET WAVE TRAINS MARSH BARRIER ISLAND EBB-TIDAL DELTA FLOOD-TIDAL DELTA SANDSTONEOFFSHORE] LAGOONAL TIDAL CHANNEL SHALE SANDSTONE SHALE SHORE FACE SANDSTONE FESTOON CROSS-BEDS ORTHOQUARTZITE PLANAR CROSS-BEDS GRAY SHALE RIPPLESggjj RED A N D GREEN SHALE METERSg ^ LIMESTONE— COALTTTT ROOTING KILOMETERS-^^ BURROW STRUCTUREFIG. 3—A, Back-barrier deposits including storm washovers, tidal channels, and flood-tidal delta exposed in claypit along Interstate 64 west of Olive Hill, Kentucky. Side panels based on greater than 95% exposure (Home andFerm, 1976). B, Barrier and back-barrier environments including tidal channels and flood-tidal deltas exposed inCarter Caves State Park region near Olive Hill, Kentucky (Home and Ferm, 1976).
Depositional Models in Coal Exploration 2385 COAL SEAT ROCK. CLAYEY WAMP BANDONED SILTSTONE WITH QUARTZOSE SANDTONE FLASERS TIDAL CHANNEL ^ ^ n ^ ^ l D A L FLAT CLAY SHALE WITH SIDERITE BANDS,BURROWED, FOSSILIFEROUS — LAGOON COAL SEATROCK.CLAYEY SANDSTONE,QUARTZOSE PLANAR ACCRETION BEDS STORM WASHOVERS SHALE AND SILTSTONE.COARSENING UPWARD, BURROWED ^LAGOON CLAY SHALE, SIDERITE BANDS, LIMESTONE, BURROWED, FOSSILIFEROUS COAL SEAT ROCK,CLAYEY SWAMP SANDSTONE, QUARTZOSE, FINING UPWARD, RIPPLED AND CROSS-BEDDED TIDAL CHANNEL SILTSTONE WITH SANDSTONE FLASERS TIDAL FLAT BURROWED SIDERITIC SANDSTONE SANDSTONE QUARTZOSE, CROSS-BEDDED FLOOD-TIDAL - DELTA SHALE AND SILTSTONE,COARSENING UPWARD, BURROWED LAGOON CLAY SHALE, SIDERITE BANDS, BURROWED, FOSSILIFERO FIG. 4—Generalized vertical sequence through back-barrier deposits in Carboniferous of eastern Ken- tucky and southern West Virginia.al lower and lateral contacts. Grain size increases 1,000 ft (300 m) wide and grade upward fromupward in the sequence and toward the center of coarse to fine with trough cross-beds in the lowerthe bar. Laterally persistent fining-upward graded part and ripple drift in the upper. The basal con-beds are common on the flanks of the bars as are tact, which is scoured along an undulating oroscillation and current-rippled surfaces, whereas wavy surface, in many places truncates the under-multidirectional festoon cross-beds are prevalent lying distributary-mouth bar and bay deposits.in the central part of the bar. In the central area, Commonly, pebble-lag conglomerates are presentthere is little lateral continuity of beds owing to at the base of the channel deposits as are coalmultiple scouring by flood currents. Slumps and "spars" which represent compressed pieces offlow rolls are associated with the flanks and front wood or bark.of the mouth bar where the sediment interface Because of the rapid abandonment of distribu-steepened beyond the angle of repose. Fossils and taries, fine-grained clay plugs are the predomi-burrow structures are generally absent within the nant type of channel fill in the Carboniferousbar deposits but, where subaerial levees are con- lower delta-plain deposits of eastern Kentucky.structed protecting the interdistributary areas These abandoned fills (Fig. 7) are comprised offrom the rapid influx of detrital sediments, organ- clay shales, siltstones, and organic debris whichisms returned and burrowed the flanks of the bar. settled from suspension in the ponded water of Distributary channels in the lower delta plain the abandoned distributary. In some places, thickare characterized by two types of sedimentary organic accumulations (now coal) filled thesefill: active and abandoned. Because channels in holes. The clay shales commonly are root pene-the lower delta plain are straight with little ten- trated or burrowed. The only coarse-grained sedi-dency to migrate laterally, active channel-fill de- ments present in the abandoned channels areposits containing point-bar accretion beds are not thin-rippled and small-scale cross-bedded sandscommon. Where present, these deposits consist of and silts which probably were deposited duringsandy sequences up to 60 ft (18 m) thick and floods or at sites near the distributary cutoff.
2386 J. C. Home et al Coal Seat rock, clayey Sandstone, fn. to rned.-grained, multi- Distributary- directional planar and festoon cross-beds Mouth Bar Sandstone, fine-grained, rippled Sandstone, fine-grained, graded beds Sandstone, flow rolIs Sandstone, fine-grained, flaser-bedded L Distal Bar and siltstone Silty Shale and Siltstone with calcareous Interdistributary concretions thin-bedded, burrowed, Bay occasional fossil Or Prodelta Clay Shale with siderite bands, burrowed, fossiliferous SAND 1 SILT CLAY Coal Rooted Sandstone Channel Sandstone, f i n e - g r a i n e d , climbing r i p p l e s Sandstone, f i n e to medium-grained Sandstone, med.-grained, festoon cross-beds Distributary- Congl. Lag, siderite pebble, coal spar Mouth Bar Sandstone, Siltstone, graded beds Sandstone, flow rolls Distal Bar Sandstone, S i l t s t o n e , flaser-bedded S i l t s t o n e and S i l t y Shale thin-bedded, nterdistributary burrowed Burrowed s i d e r i t i c Sandstone Sandstone, f i n e - g r a i n e d Crevasse Splay Sandstone, f i n e - g r a i n e d , r i p p l e d S i l t y Shale and S i l t s t o n e w i t h calcareous Interdistributary c o n c r e t i o n s , t h i n - b e d d e d , burrowed Bay Or Clay Shale w i t h s i d e r i t e bands burrowed, Prodelta f o s s i l iferous 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 those associated with abandoned fills, are thinof eastern Kentucky, levees are thin and poorly but relatively widespread parallel with distribu-developed, the largest being about 5 ft (1.5 m) tary trends.thick and 500 ft (150 m) wide. Levees consist of The final major component of the lower deltapoorly sorted, irregularly bedded, partially rooted plain is the crevasse splay (Fig. 8). These depositssiltstones and sandstones. These beds display a contain all the characteristics of coarsening-up-pronounced dip (about 10°) away from the asso- ward minideltas. They become gradationally finerciated channel (Fig. 7). Coal beds, other than grained away from the breached levee to where
Depositional Models in Coal Exploration 2387 DISTRIBUTARY CHANNEL LEVEE CREVASSE SPLAY MUDS DISTRIBUTARY-MOUTH BAR SANDS 200 SANDSTONE BEDDING 100 300 ^SHALE AND RIPPLES SI ITS TONE CROSS-BEDS f " T T ROOT STRUCTURES FLOW ROLLS T^ BURROW STRUCTURESFIG. 6—Distributary-mouth bar sandstone exposed in interval below lower Elkhorn coals along U.S. Highway 23north 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 SCALES • SPtAY SItTSTONE AND SHALE l^yj BAYFILL SHALE • i COAL 5• IN FEETFIG. 8—Crevasse-splay deposits exposed in interval above upper Elkhorn Nos. 1 and 2 coals along U.S. Highway ) 250 5C23 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 lower part. Bedding in these sandstone bodies issequences. Commonly, an abandoned channel fill massive, with thick festoon cross-beds in the low-occurs in a splay which formed as a result of the er part; upward, these massive beds merge intoclosing of the crevasse in the levee. Carboniferous point-bar accretion beds (average dip of 17°) con-splays vary in size with thicknesses up to 40 ft (12 taining smaller scale festoon cross-beds. Thesem) and horizontal extents ranging from 100 ft (30 beds are overlain by partially rooted sandstonesm) to 5 mi (8 km). and siltstones with climbing ripples. All of these In contrast to the thick fine-grained bay-fill se- characteristics, in addition to the lateral relations,quences of the lower delta-plain deposits, the suggest a high-energy channel flanked byeastern Kentucky Carboniferous upper delta swamps, small ponds, and lakes (Fig. 11). The up-plain-fluvial deposits are dominated by linear, ward-widening cross-sectional shape of the sand-lenticular bodies of sandstone which, in cross sec- stone bodies and the point-bar accretion beds in-tion (Fig. 9), are 50 to 80 ft (15 to 25 m) thick and dicate that meandering was important in the1 to 7 mi (1.5 to 11 km) wide. These sandstone development of these deposits. These sandstonebodies contain scoured bases, sharply truncating bodies show an en echelon arrangement suggest-the surface upon which they lie, but laterally, in ing episodes of lateral jumping of channels intothe upper part, they intertongue with gray shales, adjoining backswamps.siltstones, and coal beds (Fig. 10). The sandstone Backswamp deposits consist of sequencesmineralogy varies from lithic graywackes to ar- which, from base up, are comprised of seat earth,koses; grain sizes are predominantly medium to coal, shale with abundant plant fossils and rarecoarse. Above the scoured base, grain size dimin- freshwater pelecypods, siltstone, sandstone, seatishes upward within these sandstones; abundant earth, and coal. The sandstone thickens laterallypebble lags and coal "spars" are present in the and merges with the major sandstone bodies. The
NORTH SOUTH 1000 2000 D (D -rIO M 5" 3 m 500M i5» LEGEND I Em SANDSTONE O o Lin SANDSTONE A N D SILTSTONE S. m £13 SHALE A N D SILTSTONE X •o ^ SIDERITE SANDSTONE o" 5 ^a BLACK SHALE 3 F^ PLANT SHALE M~2 BONE SHALE ^m COALFIG. 9—Upper delta plain-fluvial deposits exposed along U.S. Highway 23 south of Louisa, Kentucky. Cross section is based on more than 60% 00 exposure along highway (Home and Baganz, 1974).
2390 J. C. Horne et al COAL W I T H CLAY SPLIT SEAT ROCK, CLAYEY BACKSWAMP SANDSTONE A N D SILTSTONE, LEVEE CLIAABING RIPPLES,ROOTED SANDSTONE,MEDIUM TO COARSE GRAINED, FESTOON CROSS-BEDDED CHANNEL FLOOD PLAIN COAL WITH SEAT-ROCK SPLITS BACKSWAMP SEAT ROCK, SILTY SANDSTONE A N D SILTSTONE, CLIMBING RIPPLES, ROOTED LEVEE SANDSTONE M E D I U M TO COARSE GRAINED FESTOON CROSS-BEDDED CHANNEL CONGLOMERATE LAGSIDERITE PEBBLES SLUMPS ^^rr LAKE SILTSTONE, THIN-BEDDED FLOOD PLAIN COAL WITH CLAY SPLITS 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 301 lOOn 5^ SILTSTONE AND SHALE METERS FEET 50 o.c. PEBBLE LAG — ^^ -^^- :/5 COAL ROOTING TROUGH CROSS-BEDS BEDDING PLANES L=_ L. METERS 300 500 FEET 1000 FIG. 11—Reconstruction of upper delta plain-fluvial environments as exposed in interval around Haz- ard 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 2391thin (5 to 15 ft; 1.5 to 4.5 m), fine-grained, up- between depositional environments are those ef-ward-coarsening sequences are typical deposits of fects that arise from broad-scale contemporane-open-water bodies, probably shallow ponds or ous tectonic influences. This point is illustratedlakes. The lateral extent of these deposits is only 1 by a generalized regional cross section of the Car-to 5 mi (1.5 to 8 km). boniferous from Bluefield in southern West Vir- Levee deposits consist of poorly sorted, irregu- ginia to Pittsburgh, Pennsylvania (Fig. 16). Southlarly bedded sandstones and siltstones that are of the Paint Creek fault zone, the section thickensextensively root penetrated. They are thickest (up greatly in response to an increased rate of subsi-to 25 ft; 8 m) near active channels, and decrease dence (Ferm, 1976).both in grain size and thickness away from the This large differential rate of subsidence fromchannels. The levee deposits also display a promi- south to north produced very pronounced effectsnent dip (up to 10°) away from the channel. on depositional environments and, consequently,Coals in the upper delta plain-fluvial deposits are on the characteristics of distribution and qualitylocally thick (up to 32 ft; 10 m) but are laterally of the enclosed coal seams. In the southern areadiscontinuous (sometimes pinching out within of more rapid subsidence, the depositional facies500 ft; 150 m). are stacked on each other and exhibit slow rates Between the lower and upper delta-plain de- of progradation, whereas in the more stable (lessposits of the eastern Kentucky Carboniferous is a rapid subsidence) platform area on the north thetransitional zone that exhibits characteristics of depositional facies prograde very abruptly overboth the lower and upper delta-plain sequences this shelf. The transition from upper delta-plain(Figs. 12, 13). The fine-grained bay-fill sequences to barrier environments occurs in a distance ofare thinner (5 to 25 ft; 1.5 to 7.5 m) than those of approximately 10 to 15 mi (16 to 24 km) in thethe lower delta plain. However, unlike the thin south, whereas on the more stable platform onbay-fill sequences of the upper delta plain, they the north the same environmental transition oc-contain marine to brackish faunas and are exten- curs very gradually over a distance greater thansively burrowed (Fig. 14). 60 mi (96 km). The net effect of this change is Channel deposits (Fig. 15) exhibit features of that, generally, the minable coals of southernlateral migration such as point-bar accretion beds West Virginia display a much more restrictedsimilar to the channels of the upper delta plain, lateral distribution than those of western Pennsyl-but these transitional delta-plain channels are fin- vania.er grained than those of the upper delta plain. An equally important consequence of differen-These channel deposits are single-storied se- tial regional subsidence is the sulfur content ofquences having one direction of lateral migration, coals. Coals of southern West Virginia, like thosewhereas upper delta-plain channel sandstones are of western Pennsylvania, show an increase in to-multistoried units with many directions of lateral tal sulfur (and reactive pyrite) as they pass frommigration. The levees associated with these chan- upper delta-plain to back-barrier environments.nels are thicker (5 to 15 ft; 1.5 to 4.5 m) and more However, in the south, the effect is muted by theextensively root penetrated than those of the low- rapid rate of sedimentation; the coals from south-er delta plain. Thin (5 to 15 ft; 1.5 to 4.5 m) splay em West Virginia are well known for their lowsandstones are common in these deposits but are sulfur content. In contrast, the coals of the Pitts-less numerous than in the lower delta plain, yet burgh area, which were deposited on a stablethey are more abundant than those of the upper platform where the rates of sedimentation weredelta plain. lower and chemical activity was higher than in Because many of the interdistributary bays southern West Virginia, generally have a higherfilled with sediment in the transitional zone be- sulfur content. The same effect may be expectedtween the lower and upper delta plains, a wide- with the minor trace elements.spread platform developed upon which peat In addition to the regional influences of con-(coal) swamps formed. The resultant coals are temporaneous tectonism on depositional facies,thicker and more widespread than the coals of the detailed local sedimentary responses to move-lower delta plain. Most of the economically im- ments of basement features can be identified. Al-portant coal seams of the Appalachian region are though most of the basement faults in easternin this transitional zone between lower and upper Kentucky do not offset the deposits of the coaldelta-plain environments. measures, there is ample evidence of sedimentary responses to these contemporaneously activeINTERACTION OF DEPOSITIONAL structures. Figure 17 is a regional cross sectionENVIRONMENT AND TECTONIC SETTING (constructed from over 400 highway roadcuts) of Superposed on changes in lithologic character the coal measures exposed along U.S. Highwaywhich can be attributed to variations within and 23 between Pikeville, Kentucky, on the south and
Depositional Models in Coal Exploration 2393 COAL.SEAT ROCK CLAYEY S A N D S T O N E . F I N E - G R A I N E D , RIPPLED CREVASSE SPLAY SHALE AND SILTSTONE, COARSENING UPWARD SIDERITE BANDS, BURROWED INTERDISTRIBUTARY BAY COAL SEAT ROCK SILTY SANDSTONE A N D SILTSTONE,CLIMBING LEVEE RIPPLES, ROOTED SANDSTONE FINE TO M E D I U M - G R A I N E D CHANNEL FESTOON CROSS-BEDOED CONGLOMERATE LAG,SIDERITE PEBBLES COAL.SEAT ROCK SHALE AND SILTSTONE, COARSENING UPWARD :^?^?7Z^tREVASSE SPLAY BURROWED INTERDISTRIBUTARY BAY COAL.SEAT ROCK CLAYEY SANDSTONE F I N E G R A I N E D RIPPLED fREVASSE SPLAY SHALE A N D SILTSTONE COARSENING UPWARD INTERDISTRIBUTARY BAY SIDERITE BANDS. BURROWED COAL WITH SEAT ROCK SPLITS LEVEE SANDSTONE AND SILTSTONE, CLIMBING RIPPLES, ROOTED CHANNEL SANDSTONE, FINE TO MEDIUM-GRAINED FESTOON CROSS-BEDDED CLAY SHALE, BURROWED COAL FIG. 13—Generalized vertical sequence through transitional lower delta-plain deposits of eastern Ken- tucky and southern West Virginia. Louisa, Kentucky, on the north. Although this di- are of economic significance in coal exploration agram has been generalized, many relations be- and mine planning. tween the basement structures and lithologic vari- ations can be observed. Terrigenous clastic Variations in Thiciiness and Extentwedges thin or pinch out, and coal beds may thin The three-dimensional shape (thickness andor merge over these flexures. In addition, the in- lateral extent) of coal bodies is affected directly tensity of root penetration also may increase over by the depositional setting in which they accumu- the structural highs indicating longer exposure lated. The depositional environments that pre-and deeper soil development. cede, coexist with, or are postdepositional modify Finally, the most obvious feature is the "stack- the shape of the coal bodies, as do the internaling" or localization of channeling along the processes active within the coal swamps. The sed-flanks of these flexures, which is emphasized on imentary environments that immediately precedeFigure 18, a block diagram of the area near the the coal swamp shape the topography on whichBlaine-Woodward fault. This fault is also shown the swamp develops. This topography affectsat the north end of the regional cross section (Fig. most directly variations in coal thickness, al- 17) in the area 3 mi (5 km) south of Louisa, Ken- though to a lesser extent it also affects the lateraltucky. Regional paleocurrent analyses indicate continuity of the seam. Those environments thatthe channels shown on Figure 18 were carrying coexist laterally with the peat (coal) swamp, assediment from southeast to northwest. However, well as internal processes within the swamp suchjust south of the Blaine-Woodward fault, the pa- as the plant growth, plant decay, fires, and waterleocurrent directions indicate the channels were flow, directly affect the lateral continuity of thedeflected to the west. Thus, the channels were lo- coal-forming deposits and, to a lesser extent,calized on the downthrown (southern) side of the thickness variations of the seams. Following fault. This area should be avoided from the stand- burial of peat (coal) beds, the processes of postde-point of coal exploration because the coals have positional environments, such as channeling, maybeen removed by the channeling. impinge downward and modify the upper surface of the deposits. These processes cause local thin-APPLICATION OF DEPOSITIONAL MODELS ning and the interruption of the lateral continuity The deUneation of depositional environments of seams by channel "washouts" (the removal ofcan be applied to produce predictive models that coal by channel scouring).
2394 J. C. Home et al CREVASSE SWAMP SPLAY LEVEE POINT BAR~~] SANDSTONE 100I ] SANDSTONE AND SILTSTONE 3 0Ia F E 5ol ET FEET 50 SCALES^ ^ SHALE L— COAL r7Tf ROOTING METERS 0 1000 2000~-^ BURROW STRUCTURE FEET ^ MARINE FOSSIL^5?? BEDDING PLANES V>^ TROUGH CROSS-BEDSFIG. 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). On a regional scale, depositional models can be By contrast, in river-dominated lower delta-used to predict the trends of coal bodies. These plain deposits of the Appalachian Carboniferous,models are useful in an initial phase of coal explo- the coal bodies are elongate parallel with deposi-ration. Moreover, locally, they permit a detailed tional dip. This trend exists because the only sitesunderstanding of variations in coal thickness and where peat swamps can develop are on the nar-lateral continuity that can aid in mine planning row, poorly developed levees along the distribu-and development. tary channels. These river-dominated lower delta- Regionally, back-barrier coal bodies formed plain channels generally are straight and rapidlylandward of contemporaneous or preexisting bar- prograde seaward in the direction of depositionalrier systems. The coal swamps developed on plat- dip. For this reason, the coals that develop in thisforms that evolved as the result of infilling of the environment are continuous laterally in the direc-lagoons behind the barriers. Coexisting and post- tion of depositional dip but discontinuous paralleldepositional tidal channels may modify the back- with depositional strike, being interrupted by in-barrier coals into pod-shaped bodies. However, terdistributary bay-fill deposits. These seamsthe trend of these pod-shaped seams parallels the commonly are relatively thin and contain numer-trend of the associated barrier systems, and bar- ous splits caused by crevasse splays that breachedriers, most commonly, are elongate parallel with the poorly developed levees along the distributarydepositional strike. channels.
Depositional Models in Coal Exploration 2395ms^Mi^^^)CXZ3EX3dCSZ3dCCZZaSDDCZ3X3SE ^^TrTTTTTTT-T-T-^rT-T-T—T-T—7—r-T—?—r-?—f t- ^ , -r f » -t—7—-< • < <••!. f v ^ " "^^ >".<"< • SANDSTONE o.o* PEBBLE LAG lOOn 20n • SANDSTONE AND SIITSTONE SHALE AND SILTSTONE —— jrr COAL ROOTING BURROWED <2l^ SLUMP STRUCTURE 600 SIDERITE SANDSTONE 1000 2000FIG. 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 coeval channels, and in some places, they contain parallel depositional dip. However, they are not "washouts" where later periods of channeling so laterally continuous in that direction as the have scoured through the coals. Although the coals of the lower delta plain. These seams occur other depositional settings contain many econom- in pod-shaped bodies that accumulated on flood ic coals, most of the important widespread coals plains adjacent to coexisting meandering chan- of the Appalachian region have accumulated in nels. As a result, coals formed in this setting dis- this transition zone between the lower and upper play abrupt variations in thickness over short delta plains. lateral distances with numerous splits occurring Thus, in an initial exploration phase, an under- in the coals near the levees of the contemporane- standing of the controls depositional environ- ously active channels. In addition, postdeposi- ments exert on the shape of coal bodies is impor- tional channeling may interrupt further the lateral tant in designing a drilling program that can trace continuity of these seams by causing "washouts." the trends of coal bodies. However, at the stage of Within the transition zone between the lower mine planning and development, a detailed and upper delta plains, many of the large interdis- knowledge of the influence of depositional envi- tributary bays have filled with sediment providing ronments on variations in coal thickness is most a broad platform upon which widespread coal critical. swamps can develop. In this depositional setting, The Beckley coal of southern West Virginia il- the resultant coal bodies are extensive laterally lustrates these characteristics. Figure 19 is a pa- with an inclination to be slightly elongate parallel leogeographic reconstruction of the depositional with depositional strike. Similar to the upper delta setting of the Beckley coal. This reconstruction is plain-fluvial coals, these transitional lower delta- based on data from 1,000 core holes in a 400 sq plain seams develop splits adjacent to levees of mi (1,000 sq km) area. Regionally, this coal accu-
POCAHONTAS BASIN DUNKARP BASIN 0> AUfCHCNT POTTS VIILI MISSlSSIfPlAN o 3 (D <5. BLUEFIELD c * * r o w c i KM ot cicx 0 50 SCALES KILOMETERSF I G . 16—Cross section of Pottsville a n d Allegheny Formations from vicinity of Bluefield, West Virginia, to Pittsburgh, Pennsylvania, showing general arrangement of coal beds a n d depositional environments in which they formed (after Ferm a n d Cavaroc, 1969).
REGIONAL CROSS SECTION ALONG U.S. 23 PIKEVILLE TO LOUISA, KENTUCKY f O BtAINE a WAIBRIDGE SYSTEM • f f T y v » ^ < t < t >i.yT y > LEGEND _ Q ORTHOOUARTZITE I , SANnSIONE , ——^..^-y—-f•-•: " J ^ * ^ m - ^ r-^-r--i^^^,—-^ V ^ X t 1 bHAlE ANO SIlISTONf ,» ?^ •o —•—J I ^ » ; ••• < • » A • r f , > ^ . ) < ^ . ^ ^ f^^£^ BURROWED SIOEKIIE o" PAINT CREEK Q> ~=H IIMESTCINE IRVINE ^ 1 COAl SYSTEM ["^^ ROOTS 0 KIIOMETERS 10 SCALESFIG. 17—Regional cross section of lateral relations of coal beds and enclosing lithologies exposed along U.S. Highway 23 between Louisa, Kentucky, u on north and Pikeville, Kentucky, on south. Cross section is constructed from over 400 highway roadcuts (Home et al, 1976a).
2398 J. C. Home et al LEGEND CZ] SANDSTONE SHALE AND SILTSTONE COAL TTf ROOTS 360 0 500 1000 < ^ DIRECTION OF PALEOCURRENTS 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 ting (Fig. 19). The coarsening-upward sequencetrending from east-northeast to west-southwest that was deposited over the preexisting coal (front(Galloway, 1972; Robinson, 1975). The trend of panels. Fig. 20) formed as a result of the infillingthis back-barrier coal body parallels closely that of the lagoonal area landward of the associatedof the associated barrier system, although it is ab- barrier system. As the lagoonal area filled withsent in places owing to concomitant and later sediment, tidal flats (flasered siltstone and sand-tidal channels that produced "want areas" (areas stone) and salt marshes became established be-of little or no coal). Thus, the Beckley coal be- tween intervening tidal channels (upper surface,haves as other back-barrier coals with respect to Fig. 20A). This setting provided the topographycoal-body shape and regional trend, and present upon which the subsequent coal swamp formed.and future exploration programs should be de- Initially a freshwater marsh and/or swamp de-signed to take advantage of these characteristics. veloped on the high areas over the previous salt- At the lease-tract level (15,000 acres; 6,000 ha., marsh surface and eventually spread over most ofor less), an understanding of coal-thickness varia- the area. As plant growth continued, the smallertions is important economically. To depict the in- channels and the upper parts of the larger chan-fluence depositional environments exert in con- nels became clogged with organic material, andtrolling coal-thickness variations, the details of a only the major tidal channels continued to re-mined-out area of the Beckley coal were used main open (Fig. 20B). Consequently, the thick-(Fig. 20). ness variations in the resultant coal deposit reflect Regionally, this mined-out area of the Beckley very closely the influence of the preexisting depo-coal is situated in a back-barrier depositional set- sitional topography, with the thicker coal occur-
Depositional Models in Coal Exploration 2399 r ^ ORTHOQUARTZITIC SCALES L 2 J SANDSTONE 0 5 1 1 FIASERED KILOMETERS SILTSTONE 0 3.5 tfVKLl COAL < 2 1 1 MILES AREA COAL>2 DETAILED BLOCK DIAGRAMFIG. 19—Regional depositional setting of Beckley coal and surrounding lithologies. Re-construction is based on data from 1,000 core holes in 400 sq mi (1,000 sq km) area. Areaenclosed 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 600FIG. 20—Block diagrams showing detailed relations of depositional topography and coal thickness. Front panels ofblock 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 isresidual topography after regional dip has been removed by trend-surface program. On block diagram B, coalthicknesses were contoured from thicknesses recorded on mine maps; within mine, elevations of base of coal andthickness were recorded by engineers approximately every 75 ft (25 m). Regional setting of these detailed diagramsof Beckley is shown on Figure 19.ring in the former lows and the thinner coals over grained platy masses (cleats) occupying joints inthe highs. The coal is absent or badly split in the strata, and framboidal pyrite (Fig. 21; Caruc-places where a few contemporaneous tidal chan- cio et al, 1977). The last is in clusters of sphericalnels remained active. agglomerates comprised of 0.25/1 grains of iron As observed in this example, at the lease-tract disulfide and is disseminated finely throughoutlevel, coal-thickness variations are closely related the coal and associated strata. Of these four basicto the preexisting depositional topography. This types, only the framboidal form decomposestopography is the result of the depositional envi- rapidly enough to produce severe acid mineronments that existed prior to coal formation. In drainage in the absence of carbonate materialaddition, the shape of the coal body is modified (Caruccio, 1970) and is so disseminated throughby coexisting and postdepositional environments the coal that it cannot be removed in the 1.50-such as channels. If these factors are considered density sink fraction in washability tests.during mine planning, the main tunnels of the Research by Love (1957). Love and Amstutzmine could be designed to maximize economi- (1966), Cohen (1968), Rickard (1970), Bernercally the recovery of the thicker bodies of coal (1971, Chap. 10), and Javor and Mountjoy (1976)while avoiding the "want areas." suggests that the framboidal form of pyritic sulfur is produced by sulfur-reducing microbial organ-COAL QUALITY: SULFUR PROBLEMS isms which are found in marine to brackish wa- Iron disulfides (FeSi) are present in coals either ters but not fresh water. Mansfield and Spack-as marcasite or pyrite. They occur as euhedral man (1965), working with selected bituminousgrains, coarse-grained masses (greater than 25ft) coals from western Pennsylvania, have shownwhich replace original plant material, coarse- 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 along the South Carolina coast (Corvinus andthan those formed in fresh water. Similar sulfur Cohen, 1977), it has been documented that peatsvariations were reported by Cohen (1968) and with high sulfur contents in the form of framboi-Cohen et al (1971) in the modern peats of the dal pyrite are formed where the marshes are beingEverglades. transgressed by marine to brackish-water envi- Among Carboniferous coal-bearing rocks in ronments. The only exception occurs where a suf-western Pennsylvania, Williams and Keith (1963) ficient thickness of sediment is introduced earlydemonstrated statistically that coals having roof enough to shield the peat from the marine torocks of marine or brackish-water origin contain brackish waters.more sulfur than those with roof rocks of fresh- Thus, the environments of deposition of thewater origin. On the basis of research in the Car- sediments that overlie the coal are more impor-boniferous of eastern Kentucky and southern tant to the distribution of the type and amount ofWest Virginia, Ferm et al (1976) and Caruccio et sulfur in the coal than the environments of depo-al (1977) have established that sulfur present in sition of the sediment on which the coal devel-the framboidal form of iron disulfide is most oped. Consequently, coals that accumulated instrongly associated with roof rocks deposited in areas under marine influence such as back-barriermarine to brackish-water environments. Sim and lower delta-plain environments are likely tolarly, in the Everglades (Cohen et al, 1971) and be overlain by marine to brackish sediments and
2402 J. C. Home et alcontain high amounts of disseminated pyritic sul- distribution of the amount of sulfur and the typefur in the reactive framboidal form. of pyrite can permit the exploration for low-sulfur Coals that amassed in the transitional lower coals in areas where the sulfur content is usuallydelta-plain environment were farther from marine high. This strategy can be illustrated by an exam-influences and, generally, contain less framboidal ple from the Carboniferous of the eastern Unitedpyritic sulfur. However, some of these coals are States. In this example, based on 450 core holes inoverlain by sediments that were deposited in shal- a 200 sq mi (500 sq km) area, the coal accumulat-low-marine to brackish-water bays. That these ed in a lower delta plain environment. Where ov-bays were open to marine influences is shown by erlain by marine to brackish roof rock, coalsthe marine to brackish faunas preserved in the formed in this depositional setting commonly dis-strata. Where this marine to brackish roof rock is play a propensity toward high (greater than 2%)present, the pyritic sulfur in the underlying coal sulfur contents with most of the sulfur (greaterincreases greatly, most of it being present as than 75%) in the form of framboidal pyrite (Ca-framboidal pyrite. For this reason, the distribu- ruccio et al, 1977). However, when splay depositstion of disseminated pyritic sulfur is highly varia- are introduced early and are of sufficient thick-ble in the transitional lower delta plain, although, ness, they shield the coal from the sulfur-reducingoverall, deposits in this environment are lower in bacteria, and the sulfur content remains low (lesspyritic sulfur than those in the more marginal ma- than 1%; Home et al, 1976b).rine environments. An east-west cross section (Fig. 22) through the Upper delta-plain to fluvial environments sel- exploration area shows a fossiliferous hmestonedom are transgressed by marine to brackish wa- and black shale that he directly on coal X in theters, and almost all coals formed in these deposi- eastern part of the cross section. However, thetional settings are low in pyritic sulfur. In limestone and black shale rise stratigraphicallyaddition, most of the iron disulfide present is of above the coal to the west, being separated by ansecondary origin in the forms of massive plant intervening wedge of terrigenous clastic sediment.replacements and cleat fillings. The distribution and thickness of this detrital At the lease-tract level, an understanding of the wedge, as well as the area where the limestonecontrols that the depositional setting exerts on the and black shale directly overlie the coal, are EAST-WEST GEOLOGIC CROSS SECTION I00| L L_ SANDSTONE pg5^ LIMESTONE FEET 30| SHALE AND B l COAL 50| • SILTSTONE BLACK SHALE ^TT ROOTING 000 0 3 FEET SCALES KILOMETERS FIG. 22—Cross section showing distribution of lithologies overlying coal X. Location of cross section shown on Figure 23.
Depositional Models in Coal Exploration 2403shown in Figure 23. That the detrital sediment nous clastic sediment, the sulfur content decreas-was introduced early and shielded the coal from es to less than 1%.the marine to brackish waters is demonstrated by This example demonstrates the importance ofthe fact that the deposits of these waters (the splay deposits in the formation of pockets of low-limestone and black shale) overlie the terrigenous sulfur coal of sufficient areal extent to be eco-clastic rocks. This configuration indicates that the nomic in the lower delta-plain setting, normally adetrital influx occurred before or during marine high-sulfur coal realm. Because splay depositsinundation. form adjacent to the distributary channels in this Figure 24 is a reconstruction of the deposi- depositional setting, drilling programs should betional setting immediately after the formation of devised to define these features. In this manner,coal X. It is based on data related to lithologic the areas of the lower delta plain with the greatestand sediment-thickness variations. These data potential for low-sulfur coal can be delineated.suggest that the levees of a distributary channel in The relations shown in this example illustratethe southwestern part of the area were breached the closely parallel distributions of coals with dis-several times forming large splay deposits in the seminated pyritic sulfur and roof rock of marinenorth and east over the coal and into the inter- to brackish origin. Moreover, when terrigenousvening interdistributary bay. In areas removed clastic sediment is introduced early and is of suf-from this detrital influx, fossiUferous limestone ficient thickness, the sulfur content in the under-and black shale were deposited from the marine lying coal remains low. With a knowledge ofto brackish waters of the bay. these characteristics and an understanding of the Figure 25 illustrates the distribution of the sul- depositional setting, exploration programs can be fur in coal X that cannot be removed in the 1.50- designed to outline areas of low-sulfur coal in density sink fraction of washability tests. As ex- what is most commonly a high-sulfur coal prov- pected, the coal in the eastern part of the area, ince. where it is overlain by roof rock of marine to brackish origin, is high in sulfur (greater than 2%) ROOF CONDITIONS with most of the pyritic sulfur in the form of In the mines of southern West Virginia and disseminated framboids. On the west and south eastern Kentucky, roof quality is dependent on where the coal is overlain by the wedge of terrige- 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 sectionFIG. 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 ( NFIG. 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 N 0 S kilomatars SULFUR PERCENT 1 1 iMslhanl E 2 S 2 3 ma 4 [•*] gr«al«r than 4FIG. 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 2405structures, early postdepositional compactional roof-support problems. However, separations attraits, and later tectonic features (Ferm and Mel- sandstone-shale bedding planes can produce roofton, 1975). Because most of the deposits of the falls. Hence, roof bolting is an essential precau-coal measures in this region are terrigenous clas- tion. Coarsening-upward rock sequences aretic rocks, rock types are contingent upon grain characteristic of bay-fill deposits. Thick wide-size and degree of cementation. Most commonly, spread bay-fill units dominate the lower delta-the syndepositional features are burrow and root plain depositional setting, but they are also abun-structures, bedding, and slickensided surfaces in dant in lagoonal bay fills of the back-barrier set-clayey rooted zones. Where less compactible ting. To a lesser extent, they are present in therocks such as sandstone are surrounded by more thin bay fills of the transitional lower delta plain.compactible types such as shales and siltstones, In some places, the coals are overlain by a brit-differential compactional features occur. Super- tle, nonbedded, carbonaceous black claystoneposed on these characteristics are later tectonic that is jointed (called "cube rock" by miners).structures such as jointing and fracturing. Blocks of this "cube rock" may come loose sud- The best quality roof conditions in this region denly from the roof causing dangerous falls.of the Appalachians occur in hard graywacke Thus, this lithology always should be bolted and,sandstones that are more than 10 ft (3 m) thick in places, it may have to be removed to preventand extend horizontally more than 2,000 ft (600 dangerous roof conditions. These carbonaceousm). These sandstones were deposited in active, black shales are the result of the low-energy re-laterally migrating channels. This type of channel working of the upper surface of peats during theis predominantly in upper delta plain-fluvial and drowning phase of coal swamps. They are pres-transitional lower delta-plain depositional set- ent, to a limited degree, in all the coal-formingtings. Lag deposits, composed of shale and coal environments. However, carbonaceous shales arepebbles, commonly formed near the base of the developed most extensively in the transitionalchannels. These lags can weaken the sandstone lower delta-plain setting, and they may also beand cause roof problems. abundant in lower delta-plain deposits. Unjointed, well-cemented, orthoquartzitic Another roof problem occurs where fine-sandstones, with similar thickness and areal ex- grained rocks such as shales, siltstones, and shalestent such as the graywacke sandstones, also may with sandstone streaks are extensively burrowed.provide excellent roof conditions. Unfortunately, The burrow structures can reduce significantlythey usually are jointed and fractured, and in this the strength of these fine-grained rocks and causestate, the resulting blocks come loose causing se- roof falls. Bolting is a necessity but often is insuf-vere roof falls. These quartzose sandstones nor- ficient to prevent falls and, in some places, themally are most abundant in back-barrier deposi- underlying coal must be abandoned. Extensivelytional settings in close proximity to the associated burrowed fine-grained rocks are formed wherebarrier system. sedimentation rates are low and/or infaunal ac- In flat-bedded sandstones and interbedded tivity is intensive. The environments that are opensandstones and shales, the roof quality is depen- to marine or brackish waters, such as the back-dent on bed thickness. If the beds are less than 2 barrier, lower delta plain, and transitional lowerft (0.6 m) thick, parting separations can occur delta plain are most likely to fulfill these criteria.along bedding planes, making bolting necessary. Some of the poorest roof conditions occurWhere the beds are 2 to 10 ft (0.6 to 3 m) thick, where the coal is overlain by seat earths (siltythe roof conditions are excellent because bridging clays that are extensively root penetrated). Thesestrengths are sufficient to prevent falls. However, root-penetrated, fine-grained rocks are crosscutwhere bed thicknesses exceed 10 ft (3 m), slicken- by slickensided planes which commonly intersectsided surfaces may develop owing to differential at angles ranging between 90 and 120° and maycompaction, and failure may occur along these display pronounced local vectoral attributessurfaces. Flat-bedded sandstones and interbed- (Ferm and Melton, 1975). However, any regionalded sandstones and shales are most common in orientation of the slicked surfaces is lacking. Be-the flanks of distributary-mouth bars and in splay cause of the slickensided surfaces and the exten-deposits. Predominantly, these features are devel- sive rooting, such fine-grained seat earths possessoped best in lower delta-plain sequences, but they little strength. So, when they are present abovealso may be present in the transitional lower del- coals, no amount of bolting will prevent roof falls.ta-plain setting. Either this material must be removed, or the coal Coarsening-upward rock sequences that grade beneath has to be abandoned.from shale upward through shales with thin sand- Although the origin of these slickensided sur-stone streaks (flasers) to interbedded sandstone faces is not known, similar features are reportedand shale, capped by sandstones, provide few in the root-penetrated swamp soils of the Missis-