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Time represented by coal seams in the coal measures of England
International Journal of Coal Geology, 6 (1986) 43-54 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
TIME REPRESENTED ENGLAND
FREDERICK
43
BY COAL SEAMS IN THE COAL MEASURES OF
M. BROADHURST’
and ANTHONY A. FRANCE2
’ Department of Geology, University, Manchester Ml 3 9PL, England z Geological Services, National Coal Board, Bickershaw Colliery,
Leigh,
Lancashire,
England (Received April 25,1985;
revised and accepted September 23, 1985)
ABSTRACT Broadhurst, F.M. and France, A.A., 1986. Time represented by coal seams in the Coal Measures of England. Int. J. Coal Geol., 6: 43-54. The relationship between interseam sediment thickness and features of coal seams such as seam splits and partings are examined with respect to possible controls of sedimentation during the Upper Carboniferous in the Lancashire Coalfield, England. The coal seams are characterised by lateral changes (mostly seam splits) which take place across more or less straight boundaries, with a northeast-southwest trend, explained as due to contemporary movement on buried faults during the time of peat formation. Some of the seam splits develop and thicken to the northwest, others to the southeast. Interseam sediments generally reveal a patchy development in terms of thickness distribution and show little evidence of synsedimentary tectonism. The apparent restriction of fault control to coal formation is explained by relatively slow accumulation rates of peat and therefore maximum exposure to the possibility of contemporary movement on underlying faults. Interseam sediments, on the other hand, represent relatively short episodes of deposition and so involved minimum exposure to control by contemporary fault movement.
INTRODUCTION
It has long been known that the coal-bearing sediment sequences of the British Coal Measures accumulated under humid conditions in deltaic and fluvial settings where progressive subsidence has provided for peat formation, burial and ultimate preservation as coal (e.g. see Raistrick and Marshall, 1939). The rate of peat formation at the present time varies greatly (Moore and Bellamy, 1974) and depends not only on rates of plant production but also on rates of decomposition of peat during and after burial (Given and Dickinson, 1973). However, an average rate of peat formation today is thought to be about 1 mm yr-’ (see Stach et al., 1982). Compaction of peat to form coal has been considered by previous authors. Ryer and Langer (1980) list results of previously determined peat-coal thickness
0166-5162/86/$03.50
o 1986 Elsevier Science Publishers B.V.
44 ratios and give the median value of 7:1 (these authors give the peat-coal thickness ratio for a Cretaceous coal in Utah as 11:1). The rate of coal fo rmatio n may thus be of the order of 1 m in 7000 years. Rates o f accumulation of ancient (compacted) interseam sediments have been investigated hitherto mainly on the basis of palaeontological evidence. In man y sequences the presence of r o o t e d upright fossil trees indicates rapid deposition of the sediments around them to permit of such preservation. One such tree is known to a proved height of over 8 m and a probable height of at least 12 m (Broadhurst and Magraw, 1959). In some cases seasonal control of sedimentation has been proposed on the basis of palaeozoological evidence, for instance, in Illinois (Zangerl and Richardson, 1963) where the Mecca Quarry shale was f or m e d at an average rate of 1 m per 13 years. An o th er case of possible seasonal sedimentation, in England (Broadhurst et al., 1980), involving the penetration of sandstones by bivalve escape shafts, suggests an average rate of sediment form at i on of 1 m in 5 years. Such estimates of interseam sedimentation rate are two or three orders o f magnitude greater than those for rates of coal formation. Direct comparison of the rate of f or m a t i on of coal seams with that of interseam sediments can be made in cases of seam splits, well-known in all coal-bearing sequences. A celebrated case is that of the Thick Coal of the South Staffordshire Coalfield in England. This coal splits laterally into several seams separated from each ot her by varying thicknesses o f clastic sediments. The Thick Coal varies between 6 and 10 m in thickness whilst the corresponding sequence of coals and sediments totals approximately 50 m o f which coal accounts for a b o u t 6 m at most (Mitchell and Stubblefield, 1945). This confirms t hat the deposition o f interseam sediments was a much more rapid process than was the accumulation of peat. SYNSEDIMENTARY TECTONISM Tectonic control of sedimentation has been established as an i m p o r t a n t factor in the f or m a t i on of some coal-bearing sequences (e.g. in the Appalachian region, H o m e , 1979). Assuming that tectonism occurred at r a n d o m intervals, then sediments accumulated over long periods of time would be more likely to contain evidence of c o n t e m p o r a r y tectonism than sediments f or m e d within short periods of time. This paper is concerned with the e x t e n t to which the consequence of c o n t e m p o r a r y (synsedimentary) tectonism can be recognised (and the relative rates of sedimentation t h e r e b y inferred) in a typical sequence of coal-bearing sediments from the Upper Carboniferous (Westphalian A and B) of the Lancashire Coalfield in England. SOUTH LANCASHIRE COALFIELD, ENGLAND The area chosen for this study lies within the South Lancashire Coalfield
45
of England, within the Pennine Carboniferous Province (see Trueman, 1947; Calver, 1969; R a m s b o t t o m et al., 1978), Fig. 1. The area covers 196 km 2 (about 75 square miles). The sediments have a southerly and easterly dip; the workable coals crop o u t and are closest to the surface in the north and west. Here the seams have been worked to virtual exhaustion b u t in the central part of the area the mines (shown in Fig. 1) are currently active and to the south, at depth, lie potential reserves for the future. The major faults in the area trend northwest--southeast. This area has been selected for study because synsedimentary tectonism has already been established elsewhere in the same coalfield (Broadhurst and Simpson, 1983) and because extensive data are available concerning the stratigraphy. This information is available from seam maps, plans of working and abandoned mines, shaft records, borehole logs, together with geologic maps and explanatory memoirs of the British Geological Survey and other literature, notably Hickling (1927). The sequence of coal-bearing sediments involved in this study is shown in Fig. 2. All the named coal seams have been worked in the area b u t numerous other, thin, seams are n o t shown. The sequence of most economic interest at present and for the future extends from the Plodder coal at the
Fig. 1. L o c a t i o n o f s t u d y area, its m i n e s and w o r k i n g s with respect to the Lancashire Coalfield, England. Major faults in the Coalfield are s h o w n . Ordnance Survey co-ordinates in the area are indicated.
46
London Delf CROMBOUKE* RAMS* ~ HIGHER/LOWER FLORIDA* -~--Vanderbeckei Marine Band ] ~¢~ 100 metres
WIGAN FOUR FEET* Wigan T w o F e e t I TRENCHERBONE/ PEACOCK* King Plodder
[] Important sandstones
Fig. 2. Generalised Westphalian sequence in the Lancashire Coalfield, England. Coals marked by an asterisk have been worked over large areas. The Vanderbeckei Marine Band is the stratigraphic boundary between Westphalian A (below) and Westphalian B (above). base u p to t h e L o n d o n D e l f s e a m . O f this s e q u e n c e a b o u t 7% is coal, m o s t l y in s e a m s 50 c m or m o r e in thickness. O f t h e coals n a m e d in Fig. 2 o n l y t h o s e m a r k e d b y an asterisk have b e e n w o r k e d e x t e n s i v e l y t h r o u g h o u t the area a n d these are o f p a r a m o u n t interest t o this s t u d y . T h e s e a m s are o f t e n k n o w n b y various n a m e s in d i f f e r e n t p a r t s o f t h e coalfield (see T o n k s et al., 1 9 3 1 ; J o n e s et al., 1 9 3 8 ; R i d g w a y , 1983). T h e s e q u e n c e c o n t a i n i n g the m o s t e x t e n s i v e l y w o r k e d coals is s a n d w i c h e d b e t w e e n t h e P e a c o c k coal at t h e base a n d the C r o m b o u k e s e a m at t h e t o p . This succession is generally o f t h e o r d e r o f 3 0 0 m in t h i c k n e s s to t h e n o r t h a n d 2 0 0 m or less to t h e s o u t h o f t h e area r e p r e s e n t e d in Fig. 1. FEATURES OF THE SUCCESSION BETWEEN THE PEACOCK AND CROMBOUKE SEAMS
The sequence f r o m the Peacock seam up to the Wigan F o u r F e e t seam (Fig. 3) Figure 3 shows t h i c k n e s s variations a n d o t h e r f e a t u r e s o f the succession f r o m t h e P e a c o c k u p t o t h e Wigan F o u r F e e t s e a m . T h e T r e n c h e r b o n e a n d P e a c o c k s e a m s are s e p a r a t e d b y m o r e t h a n 15 m o f s e d i m e n t in t h e n o r t h w e s t b u t c o n v e r g e t o t h e s o u t h e a s t a n d finally unite to f o r m o n e s e a m (Fig. 3a). A b o v e t h e T r e n c h e r b o n e - P e a c o c k s e a m s t h e succession u p to t h e base o f t h e Wigan F o u r F e e t s e a m c o n t a i n s a thin b u t p e r s i s t e n t s e a m , t h e Wigan T w o F e e t , w h i c h , a l t h o u g h w o r k e d o n l y locally, has b e e n identified o v e r a wide area. Figures 3b a n d c s h o w t h i c k n e s s variations o f t h e i n t e r s e a m s e d i m e n t s b e l o w a n d a b o v e t h e Wigan T w o Feet.
47
30
Feet°:tar? t~~ ® _
_
_
_
Wigan
Two Feet-Trencherbone Interval t h i c k n e s s
Okra
e
E_. WIGAN Wigan
•1
tel • g
seam
2
FOUR FEET Two Feet
Trencherbone Rock sandstone TRENCHERBONE PEACOCK
Failure Peacock "
1 seam
(~
Diagrammatic representation of succession not to scale
Trencherbone - Peacock Interval thickness
Location of Perkside I'sopachs in metres
Colliery
Data points indicated Areas of maximum thickness ruled
Fig. 3. T h e Peacock-Wigan F o u r F e e t interval, a. T h i c k n e s s of i n t e r s e a m s e q u e n c e bet w e e n t h e P e a c o c k a n d T r e n c h e r b o n e seams, b. T h i c k n e s s o f i n t e r s e a m s e q u e n c e bet w e e n T r e n c h e r b o n e a n d Wigan T w o F e e t seams, c. T h i c k n e s s o f i n t e r s e a m s e q u e n c e b e t w e e n Wigan T w o F e e t a n d Wigan F o u r F e e t seams. Map area as in Fig. 1.
The interseam sequence below the Wigan Two Feet (Fig. 3b) is unusual in that it is generally thinner to the northwest, the reverse of the general trend. The explanation lies, apparently, in the development of a sandstone, the Trencherbone Rock, which is particularly thick in the northeast. This
48
•
•
10
•
.
•
°
3
"
"..
1 seam
Sandstone
"
--
1
;.
o
y
"
-....:..;!...
(~)
4 5
/'0'1
~j
. ~ ~ 0 ¢ ,
Thickness of sediment wedge between the Florida seams
I°
Con je'c tur e-'d ~'ine
•"
\
•~
~
"
~140
Thickness of parting in Lower Florida seam
130
j.
S
s
s
m m
110
sss
%
8O
9O
S
s
S m
S Sm
m S
S S
Ill
®
Thickness of interval between Lower Florida and Wigan Four Feet seams
100
s ~
S S SSS S
@
\
)J~i
s//
m ~mm
m
m
S
S:
rr~ S m
S
Sandstone or conglomerate
m : mudstone
m
77" Roof roll HIGHER
FLORIDA
LOWER
FLORIDA
Snon;
Vandebeckei
Marine
WIGAN
FIVE
FEET
WIGAN
FOUR FEET
I
m
Washout
S
Band
Roof, Wigan Four Feet seam
*
L o c a t i o n of p a r k s i d e
Data
points
Colliery
indicated
0 km 2 i
Diagrammatic (not
representation
of s u c c e s s i o n
Isopachs
i
i
in m e t r e s
to scale) A r e a of m a x i m u m
thickness
shaded
Fig. 4. The Wigan F o u r F e e t - H i g h e r F l o r i d a interval, a. Nature o f the r o o f o f the Wigan F o u r F e e t seam. b. Thickness o f interseam s e q u e n c e b e t w e e n the Wigan F o u r F e e t and t h e L o w e r Florida seams, c. T h i c k n e s s o f p a r t i n g in the L o w e r Florida seam. d. Thickness o f s e d i m e n t b e t w e e n t h e L o w e r and Higher F l o r i d a seams.
49 sandstone generally lies only a short distance above the Trencherbone seam but, in places, descends to form r o o f rolls and washouts in the coal. The interseam sequence above the Wigan Two Feet (Fig. 3C) is dominated b y mudstones and thickness variations are patchy. Of particular interest is the union of the two seams in one borehole in the southwest. The sequence from the Wigan Four Feet seam up to the Higher Florida seam (Fig. 4) Above the Wigan Four Feet there is an extensive sandstone which rests directly on the coal over large areas and frequently descends into the coal to form r o o f rolls (infilled erosional channels in the top of the seam) and, in many instances, washouts (infilled erosional channels cut entirely through the coal) (Fig. 4a). The interseam sequences separating the Wigan Four Feet and Lower Florida seams (Fig. 4b) contain the Vanderbeckei Marine Band which is taken as the boundary between Westphalian A and Westphalian B (see R a m s b o t t o m et al., 1978). The overall thickness variation in these interseam sediments is patchy. The Higher and Lower Florida seams are separated by over 10 m of sediment in the north (Fig. 4d) b u t converge steadily southwards. A line of union is predicted in this direction b u t has not y e t been proved. Although sandstone is known in the sediment wedge between the Florida coals it is only developed locally (Fig. 4d) and is n o t responsible for overall thickness changes. The Lower Florida seam itself splits (Fig. 4c) to the east where the two c o m p o n e n t seams are known as the White (above) and the Black (below). The sequence from the Higher Florida seam up to the Crombouke seam (Fig. 5) Figure 5 shows seam characteristics and interseam thicknesses for the succession from the Higher Florida up to the Crombouke seam. The two most important coals in this sequence (the Rams, Fig. 5b and Crombouke, Fig. 5d) both show marked changes in character either along or across a northeast--southwest strip of ground. Along this strip a parting within the Rams seam expands to over a metre and the total thickness of the seam, at 2 m and more, is a maximum for the area (Fig. 5b). The character of the Crombouke seam {Fig. 5d) also changes along this strip of ground; in the north a split develops towards the southeast; in the south there is a rapid increase of overall ash content towards the southeast. The r o o f and floor of the Rams and Crombouke seams also show unusual features within the northeast--southwest strip of ground. The r o o f of the Rams (Fig. 5b) shows the development of sandstone in the roof with local washouts which trend north--south. This alignment is close to that of a depression (swilley) in the floor of the Crombouke seam. Rock rolls in the Cromb o u k e follow the northeast--southwest trend.
50
of a~i'~%
,..~.~e~
,o"~ "': D e p r e s s i o n in f l o o r of seam ...(swilley)
Crombouke coal
J~Rock
r o l l s in r o o f of seam
I
I Crc
\
".¢~
~'~
"
I " ~er.ng 0 " .
"'-""
"
"-'~" •
•.
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/ ~ , _ ~s~t>/ \ v
,(..s"
.
S"
• I I
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~ , ' ~ ~\
I
"; ~° ,,
-" .
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~.\
\
"-....-/.
}
.
I \Areas where coal "', e x c e e d s 2m t h i c k n e s s
Rams
Area of s a n d s t o n e S roof and w a s h o u t s
coal
® o
CROMBOUK
t Rams-H;gher
E
Florida interval (thickness)
)RAMS P e e l Hall Rock ( S a n d s t o n e ) P e m b e r t o n Rock ( S a n d s t o n e ) HIGHER FLORIDA
* Location of Parkaide Colliery Data Points indicated
Diagrammatic representation (not to s c a l e )
9kp?
of s u c c e s s i o n I 8 o p a c h s in m e t r e s
A r e a s of m a x i m u m t h i c k n e s s ruled
Fig. 5. T h e H i g h e r F l o r i d a - C r o m b o u k e interval, a. T h i c k n e s s o f i n t e r s e a m s e q u e n c e b e t w e e n t h e H i g h e r F l o r i d a a n d R a m s seams, b. T h i c k n e s s o f p a r t i n g a n d t h i c k n e s s o f coal in t h e R a m s seam. c. T h i c k n e s s o f i n t e r s e a m s e q u e n c e b e t w e e n t h e R a m s a n d C r o m b o u k e s e a m s , d. Lateral c h a n g e s in t h e C r o m b o u k e seam.
51
Thickness variations in the interseam sequences associated with the Higher Florida and Crombouke coals are shown in Figs. 5a and 5c. The interval between the Higher Florida and Rams coals (Fig. 5a) shows a general thickening to the north which is explained by the development there of two sheet sandstones (the Peel Hall Rock above, the Pemberton Rock below). To the northwest the Pemberton Rock descends to form the r o o f of the Higher Florida seam and in places forms washouts and r o o f rolls in the coal. The succession between the Rams and Crombouke seams (Fig. 5 c ) i s dominated by siltstones and mudstones. Thickness variations are patchy and range generally between 30 and 60 m. Sandstones occur but are local in e x t e n t and do n o t account for overall thickness variation. The thickness variations in the interseam sediments associated with the Higher Florida to Crombouke coals show no evidence of change along the northeast--southwest strip of ground characterised by splits, etc., in the associated coal seams. LATERAL SEAMS
VARIATION
WITHIN
INTERSEAM
SEDIMENTS
AND
WITHIN COAL
A notable feature of the interseam sediments involved in this study is the patchy distribution of thickness variations (see Figs. 3b, c; 4b; 5a, c) suggesting that deposition resulted from the deltaic/fluvial processes of channel-switching with the passage of time. However lateral changes in thickness and ash content of coal seams and the development of seam splits tend to occur across narrow belts or lines with a general northeast-southwest trend (Fig. 6). With the exception of the Florida case, the lines of split,etc., are superimposed through the sediment stack. The subparallel nature and superimposed location of these lines of split,etc., suggest that t h e y result from intermittent movement on one or more buried faults with a northeast--southwest trend. Formation of the seam splits, etc. by differential compaction of interseam sediments is n o t indicated. An important feature of the seam splits in the area is that the direction towards which t h e y split is n o t constant. In two cases (TrencherbonePeacock, Floridas) the split is to the northwest; in two cases (White and Black, Crombouke} the split is to the southeast. The Rams seam presents a split and rejoin situation. If movement on buried faults is the cause of these seam splits then the sense of movement on the fault or faults apparently changed with time. There is no suggestion here of the operation o f a growth fault. There is no evidence of faults developed in association with the seam splits. In contrast is the case of the Union seam split in the Burnley area of the Lancashire coalfield where the line of split is closely associated with the Deerplay Fault. Broadhurst and Simpson (1983) attribute the Union split to early movement on this fault which was later reactivated
52
/~ • .
;~,.o~ *"
"'G
~aras
o,,O,
f
....'i: , / White/Black
,¢e,~"
~./.~'~
j..
Okm 2 i
i
i
_L/ --'"
l.Location
of P a r k s i d e
Colliery
Fig. 6. Lines of split (Florida seams, White and Black, Trencherbone and Peacock) and trend of split-and-join (Rams) and split-and-deterioration (Crombouke).
Arrows indicate
direction of thickening sediment wedges within seam splits or direction of coal deterioration. Map area as in Fig. 1. to e x t e n d the fault plane upwards into stratigraphic levels above the Union sealn.
The major faults f o u n d in the area of coalfield under study trend northwest--southeast (Fig. 1) and so are bot h later in origin and unrelated to the p r o p o s ed buried faults. DISCUSSION If c o n t e m p o r a r y m o v e m e n t on buried faults influenced subsidence during times o f growth and accumulation of peats then the question arises as to wh y such influence is n o t so evident in the interseam successions where thickness variations are generally p a t c h y and n o t related to straight boundaries. The fluvio-deltaic e nvi r onm e nt o f deposition of the interseam successions would be sensitive to minor changes in surface levels (caused b y m o v e m e n t on buried faults) in just the same way as were the peats. Areas o f subsidence would become areas of sedimentation with a more or less straight margin associated with the fault. Such fault m o v e m e n t has been invoked to explain the vertical stacking of channel-fills f o u n d on one side o f the Blaine-Walbridge fault in K e n t u c k y ( H o m e et al., 1978). The northeast--southwest trend to the isopachs on the Trencherbone-Wigan Two Feet interval (Fig. 3b) may be the consequence of c o n t e m p o r a r y fault m o v e m e n t and subsidence of the ground to the northwest. But with this possible exception, the interseam sediments show no evidence o f contemporary movements. The irregular, patchy, distribution o f sediment thicknesses suggests, rather, that the bulk o f deposition resulted from the deltaic/ fluvial processes of channel-switching.
53
The evidence from thickness variations in interseam sediments and from lines of split in coal seams suggests that any movement on buried faults occurred mainly during times of peat (coal) formation. The question arises as to why this should be. Previous estimates of the rates of coal formation (of the order of 1 m in 7000 years) and of certain interseam sediments (1 m in 5 years; 1 m in 13 years) suggest an explanation. The probability that basement fault movements were largely restricted to times of peat formation at the surface increases with the proportion of depositional time occupied by peat growth. Most of the depositional time represented by this coal-bearing succession was apparently occupied by peat formation despite the fact that coal now represents only about 7% of the total succession. ACKNOWLEDGEMENTS
We are indebted to Dick Elliott (formerly Chief Geologist, National Coal Board) for helpful discussions and to Tony Adams and Morven Simpson (both of the Department of Geology, University of Manchester) and to Stuart Haszeldine of B&oil, Glasgow, for their constructive comments concerning the manuscript. One of us (FMB) is grateful to Bob Hoare (National Coal Board) for his help; he also acknowledges an award from the Thomas J. Dee Fund from the Field Museum, Chicago, where part of this work was carried out. We acknowledge the help of Keith Whitworth (National Coal Board who has kindly provided a computer check on our isopach work). Thanks are also due to Eric Ridgway (National Coal Board) for help concerning plans of abandoned workings. We are grateful for the assistance of Phil Stubley (drafting) and Patricia Crook (typing).
REFERENCES Broadhurst, F.M. and Magraw, D., 1959. On a fossil tree found on an opencast coal site near Wigan, Lancashire. Liverpool Manchester Geol. J., 2: 155-158. Broadhurst, F.M. and Simpson, I.M., 1983. Syntectonic sedimentation, rigs and fault reactivation in the Coal Measures of Britain. J. Geol., 91: 330-337. Broadhurst, F.M., Simpson, I.M. and Hardy, P.G., 1980. Seasonal sedimentation in the Upper Carboniferous of England. J. Geol., 88: 639-651. Calver, M.A., 1969. Westphalian of Britain. C. R., 6me Congres International Stratigraphie Geologic Carbonifere, Sheffield, 1967, 1: 233-254. Given, P.H. and Dickinson, C.H., 1973. Biochemistry and microbiology of peats. In: E.A. Paul and A. Douglas McLaren (Editors), Soil Biochemistry, III. Dekker, New York, N.Y., pp. 123-212. Hickling, G., 1927. Sections of Strata of the Coal Measures of Lancashire. Lancashire and Cheshire Coal Association, Newcastle-on-Tyne, 270 pp. tectonism. In: J.C. Horne, J.C., 1979. Sedimentary responses to contemporaneous Ferm and J.C. Horne (Editors), Carboniferous Depositional Environments in the Appalachian Region. Carolina Coal Group, University of South Carolina, pp. 259265.
54 Horne, J.C., Ferm, J.C., Caruccio, F.T. and Baganz, B.P., 1978. Depositional models in coal exploration and mine planning in Appalachian region. Bull. Am. Assoc. Pet. Geol., 62: 2379--2411. Jones, R.C.B., Tonks, L.H. and Wright, W.B., 1938. Wigan district. Mere. Geol. Surv. U.K., 244 pp. Mitchell, G.H. and Stubblefield, C.J., 1945. The geology of the northern part of the South Staffordshire Coalfield (Cannock Chase region). Geol. Surv. Great Britain Wartime Pamphlet, No. 43. Moore, P.D. and Bellamy, D.J., 1974. Peatlands. Elek Science, London, 221 pp. Raistrick, A. aad Marshall, C.E., 1939. The Nature and Origin of Coal and Coal Seams. English Universities Press, London, 282 pp. Ramsbottom, W.H.C., Calver, M.A., Eagar, R.M.C., Hodson, F., Holliday, D.W., Stubblefield, C.J. and Wilson, R.B., 1978. A correlation of Silesian rocks in the British Isles. Spec. Rep. Geol. Soc. London, No. 10, 82 pp. Ridgway, E.W., 1983. Correlation and nomenclature of coal seams in the Lancashire and Cheshire Coalfields. National Coal Board, 59 pp. Ryer, T.A. and Langer, A.W., 1980. Thickness change involved in the peat-to-coal transformation for a bituminous coal of Cretaceous age in central Utah. J. Sediment. Petrol., 50: 987--992. Stach, E., Mackowsky, M.-Th., Teichmtiller, M., Taylor, G.H., Chandra, D. and Teichmiiller, R., 1982. Stach's Textbook of Coal Petrology, 3rd Edition. Gebruder Borntraeger, Berlin, 535 pp. Tonks, L.H., Jones, R.C.B., Lloyd, W. and Sherlock, R.L., 1931. The geology of Manchester and the southeast Lancashire Coalfield. Mere. Geol. Surv. U.K., 240 pp. Trueman, A.E., 1947. Stratigraphical problems in the coalfields of Great Britain. Q. J. Geol. Soc. London, 103: lxv-civ. Zangerl, R. and Richardson, E.S., Jr., 1963. The paleoecological history of two Pennsylvanian black shales. Fieldiana: Geology Memoirs, 4, Chicago Nat. Hist. Museum, 352 pp.
TIME REPRESENTED ENGLAND
FREDERICK
43
BY COAL SEAMS IN THE COAL MEASURES OF
M. BROADHURST’
and ANTHONY A. FRANCE2
’ Department of Geology, University, Manchester Ml 3 9PL, England z Geological Services, National Coal Board, Bickershaw Colliery,
Leigh,
Lancashire,
England (Received April 25,1985;
revised and accepted September 23, 1985)
ABSTRACT Broadhurst, F.M. and France, A.A., 1986. Time represented by coal seams in the Coal Measures of England. Int. J. Coal Geol., 6: 43-54. The relationship between interseam sediment thickness and features of coal seams such as seam splits and partings are examined with respect to possible controls of sedimentation during the Upper Carboniferous in the Lancashire Coalfield, England. The coal seams are characterised by lateral changes (mostly seam splits) which take place across more or less straight boundaries, with a northeast-southwest trend, explained as due to contemporary movement on buried faults during the time of peat formation. Some of the seam splits develop and thicken to the northwest, others to the southeast. Interseam sediments generally reveal a patchy development in terms of thickness distribution and show little evidence of synsedimentary tectonism. The apparent restriction of fault control to coal formation is explained by relatively slow accumulation rates of peat and therefore maximum exposure to the possibility of contemporary movement on underlying faults. Interseam sediments, on the other hand, represent relatively short episodes of deposition and so involved minimum exposure to control by contemporary fault movement.
INTRODUCTION
It has long been known that the coal-bearing sediment sequences of the British Coal Measures accumulated under humid conditions in deltaic and fluvial settings where progressive subsidence has provided for peat formation, burial and ultimate preservation as coal (e.g. see Raistrick and Marshall, 1939). The rate of peat formation at the present time varies greatly (Moore and Bellamy, 1974) and depends not only on rates of plant production but also on rates of decomposition of peat during and after burial (Given and Dickinson, 1973). However, an average rate of peat formation today is thought to be about 1 mm yr-’ (see Stach et al., 1982). Compaction of peat to form coal has been considered by previous authors. Ryer and Langer (1980) list results of previously determined peat-coal thickness
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o 1986 Elsevier Science Publishers B.V.
44 ratios and give the median value of 7:1 (these authors give the peat-coal thickness ratio for a Cretaceous coal in Utah as 11:1). The rate of coal fo rmatio n may thus be of the order of 1 m in 7000 years. Rates o f accumulation of ancient (compacted) interseam sediments have been investigated hitherto mainly on the basis of palaeontological evidence. In man y sequences the presence of r o o t e d upright fossil trees indicates rapid deposition of the sediments around them to permit of such preservation. One such tree is known to a proved height of over 8 m and a probable height of at least 12 m (Broadhurst and Magraw, 1959). In some cases seasonal control of sedimentation has been proposed on the basis of palaeozoological evidence, for instance, in Illinois (Zangerl and Richardson, 1963) where the Mecca Quarry shale was f or m e d at an average rate of 1 m per 13 years. An o th er case of possible seasonal sedimentation, in England (Broadhurst et al., 1980), involving the penetration of sandstones by bivalve escape shafts, suggests an average rate of sediment form at i on of 1 m in 5 years. Such estimates of interseam sedimentation rate are two or three orders o f magnitude greater than those for rates of coal formation. Direct comparison of the rate of f or m a t i on of coal seams with that of interseam sediments can be made in cases of seam splits, well-known in all coal-bearing sequences. A celebrated case is that of the Thick Coal of the South Staffordshire Coalfield in England. This coal splits laterally into several seams separated from each ot her by varying thicknesses o f clastic sediments. The Thick Coal varies between 6 and 10 m in thickness whilst the corresponding sequence of coals and sediments totals approximately 50 m o f which coal accounts for a b o u t 6 m at most (Mitchell and Stubblefield, 1945). This confirms t hat the deposition o f interseam sediments was a much more rapid process than was the accumulation of peat. SYNSEDIMENTARY TECTONISM Tectonic control of sedimentation has been established as an i m p o r t a n t factor in the f or m a t i on of some coal-bearing sequences (e.g. in the Appalachian region, H o m e , 1979). Assuming that tectonism occurred at r a n d o m intervals, then sediments accumulated over long periods of time would be more likely to contain evidence of c o n t e m p o r a r y tectonism than sediments f or m e d within short periods of time. This paper is concerned with the e x t e n t to which the consequence of c o n t e m p o r a r y (synsedimentary) tectonism can be recognised (and the relative rates of sedimentation t h e r e b y inferred) in a typical sequence of coal-bearing sediments from the Upper Carboniferous (Westphalian A and B) of the Lancashire Coalfield in England. SOUTH LANCASHIRE COALFIELD, ENGLAND The area chosen for this study lies within the South Lancashire Coalfield
45
of England, within the Pennine Carboniferous Province (see Trueman, 1947; Calver, 1969; R a m s b o t t o m et al., 1978), Fig. 1. The area covers 196 km 2 (about 75 square miles). The sediments have a southerly and easterly dip; the workable coals crop o u t and are closest to the surface in the north and west. Here the seams have been worked to virtual exhaustion b u t in the central part of the area the mines (shown in Fig. 1) are currently active and to the south, at depth, lie potential reserves for the future. The major faults in the area trend northwest--southeast. This area has been selected for study because synsedimentary tectonism has already been established elsewhere in the same coalfield (Broadhurst and Simpson, 1983) and because extensive data are available concerning the stratigraphy. This information is available from seam maps, plans of working and abandoned mines, shaft records, borehole logs, together with geologic maps and explanatory memoirs of the British Geological Survey and other literature, notably Hickling (1927). The sequence of coal-bearing sediments involved in this study is shown in Fig. 2. All the named coal seams have been worked in the area b u t numerous other, thin, seams are n o t shown. The sequence of most economic interest at present and for the future extends from the Plodder coal at the
Fig. 1. L o c a t i o n o f s t u d y area, its m i n e s and w o r k i n g s with respect to the Lancashire Coalfield, England. Major faults in the Coalfield are s h o w n . Ordnance Survey co-ordinates in the area are indicated.
46
London Delf CROMBOUKE* RAMS* ~ HIGHER/LOWER FLORIDA* -~--Vanderbeckei Marine Band ] ~¢~ 100 metres
WIGAN FOUR FEET* Wigan T w o F e e t I TRENCHERBONE/ PEACOCK* King Plodder
[] Important sandstones
Fig. 2. Generalised Westphalian sequence in the Lancashire Coalfield, England. Coals marked by an asterisk have been worked over large areas. The Vanderbeckei Marine Band is the stratigraphic boundary between Westphalian A (below) and Westphalian B (above). base u p to t h e L o n d o n D e l f s e a m . O f this s e q u e n c e a b o u t 7% is coal, m o s t l y in s e a m s 50 c m or m o r e in thickness. O f t h e coals n a m e d in Fig. 2 o n l y t h o s e m a r k e d b y an asterisk have b e e n w o r k e d e x t e n s i v e l y t h r o u g h o u t the area a n d these are o f p a r a m o u n t interest t o this s t u d y . T h e s e a m s are o f t e n k n o w n b y various n a m e s in d i f f e r e n t p a r t s o f t h e coalfield (see T o n k s et al., 1 9 3 1 ; J o n e s et al., 1 9 3 8 ; R i d g w a y , 1983). T h e s e q u e n c e c o n t a i n i n g the m o s t e x t e n s i v e l y w o r k e d coals is s a n d w i c h e d b e t w e e n t h e P e a c o c k coal at t h e base a n d the C r o m b o u k e s e a m at t h e t o p . This succession is generally o f t h e o r d e r o f 3 0 0 m in t h i c k n e s s to t h e n o r t h a n d 2 0 0 m or less to t h e s o u t h o f t h e area r e p r e s e n t e d in Fig. 1. FEATURES OF THE SUCCESSION BETWEEN THE PEACOCK AND CROMBOUKE SEAMS
The sequence f r o m the Peacock seam up to the Wigan F o u r F e e t seam (Fig. 3) Figure 3 shows t h i c k n e s s variations a n d o t h e r f e a t u r e s o f the succession f r o m t h e P e a c o c k u p t o t h e Wigan F o u r F e e t s e a m . T h e T r e n c h e r b o n e a n d P e a c o c k s e a m s are s e p a r a t e d b y m o r e t h a n 15 m o f s e d i m e n t in t h e n o r t h w e s t b u t c o n v e r g e t o t h e s o u t h e a s t a n d finally unite to f o r m o n e s e a m (Fig. 3a). A b o v e t h e T r e n c h e r b o n e - P e a c o c k s e a m s t h e succession u p to t h e base o f t h e Wigan F o u r F e e t s e a m c o n t a i n s a thin b u t p e r s i s t e n t s e a m , t h e Wigan T w o F e e t , w h i c h , a l t h o u g h w o r k e d o n l y locally, has b e e n identified o v e r a wide area. Figures 3b a n d c s h o w t h i c k n e s s variations o f t h e i n t e r s e a m s e d i m e n t s b e l o w a n d a b o v e t h e Wigan T w o Feet.
47
30
Feet°:tar? t~~ ® _
_
_
_
Wigan
Two Feet-Trencherbone Interval t h i c k n e s s
Okra
e
E_. WIGAN Wigan
•1
tel • g
seam
2
FOUR FEET Two Feet
Trencherbone Rock sandstone TRENCHERBONE PEACOCK
Failure Peacock "
1 seam
(~
Diagrammatic representation of succession not to scale
Trencherbone - Peacock Interval thickness
Location of Perkside I'sopachs in metres
Colliery
Data points indicated Areas of maximum thickness ruled
Fig. 3. T h e Peacock-Wigan F o u r F e e t interval, a. T h i c k n e s s of i n t e r s e a m s e q u e n c e bet w e e n t h e P e a c o c k a n d T r e n c h e r b o n e seams, b. T h i c k n e s s o f i n t e r s e a m s e q u e n c e bet w e e n T r e n c h e r b o n e a n d Wigan T w o F e e t seams, c. T h i c k n e s s o f i n t e r s e a m s e q u e n c e b e t w e e n Wigan T w o F e e t a n d Wigan F o u r F e e t seams. Map area as in Fig. 1.
The interseam sequence below the Wigan Two Feet (Fig. 3b) is unusual in that it is generally thinner to the northwest, the reverse of the general trend. The explanation lies, apparently, in the development of a sandstone, the Trencherbone Rock, which is particularly thick in the northeast. This
48
•
•
10
•
.
•
°
3
"
"..
1 seam
Sandstone
"
--
1
;.
o
y
"
-....:..;!...
(~)
4 5
/'0'1
~j
. ~ ~ 0 ¢ ,
Thickness of sediment wedge between the Florida seams
I°
Con je'c tur e-'d ~'ine
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~140
Thickness of parting in Lower Florida seam
130
j.
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s
s
m m
110
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9O
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s
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S Sm
m S
S S
Ill
®
Thickness of interval between Lower Florida and Wigan Four Feet seams
100
s ~
S S SSS S
@
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m
m
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rr~ S m
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Sandstone or conglomerate
m : mudstone
m
77" Roof roll HIGHER
FLORIDA
LOWER
FLORIDA
Snon;
Vandebeckei
Marine
WIGAN
FIVE
FEET
WIGAN
FOUR FEET
I
m
Washout
S
Band
Roof, Wigan Four Feet seam
*
L o c a t i o n of p a r k s i d e
Data
points
Colliery
indicated
0 km 2 i
Diagrammatic (not
representation
of s u c c e s s i o n
Isopachs
i
i
in m e t r e s
to scale) A r e a of m a x i m u m
thickness
shaded
Fig. 4. The Wigan F o u r F e e t - H i g h e r F l o r i d a interval, a. Nature o f the r o o f o f the Wigan F o u r F e e t seam. b. Thickness o f interseam s e q u e n c e b e t w e e n the Wigan F o u r F e e t and t h e L o w e r Florida seams, c. T h i c k n e s s o f p a r t i n g in the L o w e r Florida seam. d. Thickness o f s e d i m e n t b e t w e e n t h e L o w e r and Higher F l o r i d a seams.
49 sandstone generally lies only a short distance above the Trencherbone seam but, in places, descends to form r o o f rolls and washouts in the coal. The interseam sequence above the Wigan Two Feet (Fig. 3C) is dominated b y mudstones and thickness variations are patchy. Of particular interest is the union of the two seams in one borehole in the southwest. The sequence from the Wigan Four Feet seam up to the Higher Florida seam (Fig. 4) Above the Wigan Four Feet there is an extensive sandstone which rests directly on the coal over large areas and frequently descends into the coal to form r o o f rolls (infilled erosional channels in the top of the seam) and, in many instances, washouts (infilled erosional channels cut entirely through the coal) (Fig. 4a). The interseam sequences separating the Wigan Four Feet and Lower Florida seams (Fig. 4b) contain the Vanderbeckei Marine Band which is taken as the boundary between Westphalian A and Westphalian B (see R a m s b o t t o m et al., 1978). The overall thickness variation in these interseam sediments is patchy. The Higher and Lower Florida seams are separated by over 10 m of sediment in the north (Fig. 4d) b u t converge steadily southwards. A line of union is predicted in this direction b u t has not y e t been proved. Although sandstone is known in the sediment wedge between the Florida coals it is only developed locally (Fig. 4d) and is n o t responsible for overall thickness changes. The Lower Florida seam itself splits (Fig. 4c) to the east where the two c o m p o n e n t seams are known as the White (above) and the Black (below). The sequence from the Higher Florida seam up to the Crombouke seam (Fig. 5) Figure 5 shows seam characteristics and interseam thicknesses for the succession from the Higher Florida up to the Crombouke seam. The two most important coals in this sequence (the Rams, Fig. 5b and Crombouke, Fig. 5d) both show marked changes in character either along or across a northeast--southwest strip of ground. Along this strip a parting within the Rams seam expands to over a metre and the total thickness of the seam, at 2 m and more, is a maximum for the area (Fig. 5b). The character of the Crombouke seam {Fig. 5d) also changes along this strip of ground; in the north a split develops towards the southeast; in the south there is a rapid increase of overall ash content towards the southeast. The r o o f and floor of the Rams and Crombouke seams also show unusual features within the northeast--southwest strip of ground. The r o o f of the Rams (Fig. 5b) shows the development of sandstone in the roof with local washouts which trend north--south. This alignment is close to that of a depression (swilley) in the floor of the Crombouke seam. Rock rolls in the Cromb o u k e follow the northeast--southwest trend.
50
of a~i'~%
,..~.~e~
,o"~ "': D e p r e s s i o n in f l o o r of seam ...(swilley)
Crombouke coal
J~Rock
r o l l s in r o o f of seam
I
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.
I \Areas where coal "', e x c e e d s 2m t h i c k n e s s
Rams
Area of s a n d s t o n e S roof and w a s h o u t s
coal
® o
CROMBOUK
t Rams-H;gher
E
Florida interval (thickness)
)RAMS P e e l Hall Rock ( S a n d s t o n e ) P e m b e r t o n Rock ( S a n d s t o n e ) HIGHER FLORIDA
* Location of Parkaide Colliery Data Points indicated
Diagrammatic representation (not to s c a l e )
9kp?
of s u c c e s s i o n I 8 o p a c h s in m e t r e s
A r e a s of m a x i m u m t h i c k n e s s ruled
Fig. 5. T h e H i g h e r F l o r i d a - C r o m b o u k e interval, a. T h i c k n e s s o f i n t e r s e a m s e q u e n c e b e t w e e n t h e H i g h e r F l o r i d a a n d R a m s seams, b. T h i c k n e s s o f p a r t i n g a n d t h i c k n e s s o f coal in t h e R a m s seam. c. T h i c k n e s s o f i n t e r s e a m s e q u e n c e b e t w e e n t h e R a m s a n d C r o m b o u k e s e a m s , d. Lateral c h a n g e s in t h e C r o m b o u k e seam.
51
Thickness variations in the interseam sequences associated with the Higher Florida and Crombouke coals are shown in Figs. 5a and 5c. The interval between the Higher Florida and Rams coals (Fig. 5a) shows a general thickening to the north which is explained by the development there of two sheet sandstones (the Peel Hall Rock above, the Pemberton Rock below). To the northwest the Pemberton Rock descends to form the r o o f of the Higher Florida seam and in places forms washouts and r o o f rolls in the coal. The succession between the Rams and Crombouke seams (Fig. 5 c ) i s dominated by siltstones and mudstones. Thickness variations are patchy and range generally between 30 and 60 m. Sandstones occur but are local in e x t e n t and do n o t account for overall thickness variation. The thickness variations in the interseam sediments associated with the Higher Florida to Crombouke coals show no evidence of change along the northeast--southwest strip of ground characterised by splits, etc., in the associated coal seams. LATERAL SEAMS
VARIATION
WITHIN
INTERSEAM
SEDIMENTS
AND
WITHIN COAL
A notable feature of the interseam sediments involved in this study is the patchy distribution of thickness variations (see Figs. 3b, c; 4b; 5a, c) suggesting that deposition resulted from the deltaic/fluvial processes of channel-switching with the passage of time. However lateral changes in thickness and ash content of coal seams and the development of seam splits tend to occur across narrow belts or lines with a general northeast-southwest trend (Fig. 6). With the exception of the Florida case, the lines of split,etc., are superimposed through the sediment stack. The subparallel nature and superimposed location of these lines of split,etc., suggest that t h e y result from intermittent movement on one or more buried faults with a northeast--southwest trend. Formation of the seam splits, etc. by differential compaction of interseam sediments is n o t indicated. An important feature of the seam splits in the area is that the direction towards which t h e y split is n o t constant. In two cases (TrencherbonePeacock, Floridas) the split is to the northwest; in two cases (White and Black, Crombouke} the split is to the southeast. The Rams seam presents a split and rejoin situation. If movement on buried faults is the cause of these seam splits then the sense of movement on the fault or faults apparently changed with time. There is no suggestion here of the operation o f a growth fault. There is no evidence of faults developed in association with the seam splits. In contrast is the case of the Union seam split in the Burnley area of the Lancashire coalfield where the line of split is closely associated with the Deerplay Fault. Broadhurst and Simpson (1983) attribute the Union split to early movement on this fault which was later reactivated
52
/~ • .
;~,.o~ *"
"'G
~aras
o,,O,
f
....'i: , / White/Black
,¢e,~"
~./.~'~
j..
Okm 2 i
i
i
_L/ --'"
l.Location
of P a r k s i d e
Colliery
Fig. 6. Lines of split (Florida seams, White and Black, Trencherbone and Peacock) and trend of split-and-join (Rams) and split-and-deterioration (Crombouke).
Arrows indicate
direction of thickening sediment wedges within seam splits or direction of coal deterioration. Map area as in Fig. 1. to e x t e n d the fault plane upwards into stratigraphic levels above the Union sealn.
The major faults f o u n d in the area of coalfield under study trend northwest--southeast (Fig. 1) and so are bot h later in origin and unrelated to the p r o p o s ed buried faults. DISCUSSION If c o n t e m p o r a r y m o v e m e n t on buried faults influenced subsidence during times o f growth and accumulation of peats then the question arises as to wh y such influence is n o t so evident in the interseam successions where thickness variations are generally p a t c h y and n o t related to straight boundaries. The fluvio-deltaic e nvi r onm e nt o f deposition of the interseam successions would be sensitive to minor changes in surface levels (caused b y m o v e m e n t on buried faults) in just the same way as were the peats. Areas o f subsidence would become areas of sedimentation with a more or less straight margin associated with the fault. Such fault m o v e m e n t has been invoked to explain the vertical stacking of channel-fills f o u n d on one side o f the Blaine-Walbridge fault in K e n t u c k y ( H o m e et al., 1978). The northeast--southwest trend to the isopachs on the Trencherbone-Wigan Two Feet interval (Fig. 3b) may be the consequence of c o n t e m p o r a r y fault m o v e m e n t and subsidence of the ground to the northwest. But with this possible exception, the interseam sediments show no evidence o f contemporary movements. The irregular, patchy, distribution o f sediment thicknesses suggests, rather, that the bulk o f deposition resulted from the deltaic/ fluvial processes of channel-switching.
53
The evidence from thickness variations in interseam sediments and from lines of split in coal seams suggests that any movement on buried faults occurred mainly during times of peat (coal) formation. The question arises as to why this should be. Previous estimates of the rates of coal formation (of the order of 1 m in 7000 years) and of certain interseam sediments (1 m in 5 years; 1 m in 13 years) suggest an explanation. The probability that basement fault movements were largely restricted to times of peat formation at the surface increases with the proportion of depositional time occupied by peat growth. Most of the depositional time represented by this coal-bearing succession was apparently occupied by peat formation despite the fact that coal now represents only about 7% of the total succession. ACKNOWLEDGEMENTS
We are indebted to Dick Elliott (formerly Chief Geologist, National Coal Board) for helpful discussions and to Tony Adams and Morven Simpson (both of the Department of Geology, University of Manchester) and to Stuart Haszeldine of B&oil, Glasgow, for their constructive comments concerning the manuscript. One of us (FMB) is grateful to Bob Hoare (National Coal Board) for his help; he also acknowledges an award from the Thomas J. Dee Fund from the Field Museum, Chicago, where part of this work was carried out. We acknowledge the help of Keith Whitworth (National Coal Board who has kindly provided a computer check on our isopach work). Thanks are also due to Eric Ridgway (National Coal Board) for help concerning plans of abandoned workings. We are grateful for the assistance of Phil Stubley (drafting) and Patricia Crook (typing).
REFERENCES Broadhurst, F.M. and Magraw, D., 1959. On a fossil tree found on an opencast coal site near Wigan, Lancashire. Liverpool Manchester Geol. J., 2: 155-158. Broadhurst, F.M. and Simpson, I.M., 1983. Syntectonic sedimentation, rigs and fault reactivation in the Coal Measures of Britain. J. Geol., 91: 330-337. Broadhurst, F.M., Simpson, I.M. and Hardy, P.G., 1980. Seasonal sedimentation in the Upper Carboniferous of England. J. Geol., 88: 639-651. Calver, M.A., 1969. Westphalian of Britain. C. R., 6me Congres International Stratigraphie Geologic Carbonifere, Sheffield, 1967, 1: 233-254. Given, P.H. and Dickinson, C.H., 1973. Biochemistry and microbiology of peats. In: E.A. Paul and A. Douglas McLaren (Editors), Soil Biochemistry, III. Dekker, New York, N.Y., pp. 123-212. Hickling, G., 1927. Sections of Strata of the Coal Measures of Lancashire. Lancashire and Cheshire Coal Association, Newcastle-on-Tyne, 270 pp. tectonism. In: J.C. Horne, J.C., 1979. Sedimentary responses to contemporaneous Ferm and J.C. Horne (Editors), Carboniferous Depositional Environments in the Appalachian Region. Carolina Coal Group, University of South Carolina, pp. 259265.
54 Horne, J.C., Ferm, J.C., Caruccio, F.T. and Baganz, B.P., 1978. Depositional models in coal exploration and mine planning in Appalachian region. Bull. Am. Assoc. Pet. Geol., 62: 2379--2411. Jones, R.C.B., Tonks, L.H. and Wright, W.B., 1938. Wigan district. Mere. Geol. Surv. U.K., 244 pp. Mitchell, G.H. and Stubblefield, C.J., 1945. The geology of the northern part of the South Staffordshire Coalfield (Cannock Chase region). Geol. Surv. Great Britain Wartime Pamphlet, No. 43. Moore, P.D. and Bellamy, D.J., 1974. Peatlands. Elek Science, London, 221 pp. Raistrick, A. aad Marshall, C.E., 1939. The Nature and Origin of Coal and Coal Seams. English Universities Press, London, 282 pp. Ramsbottom, W.H.C., Calver, M.A., Eagar, R.M.C., Hodson, F., Holliday, D.W., Stubblefield, C.J. and Wilson, R.B., 1978. A correlation of Silesian rocks in the British Isles. Spec. Rep. Geol. Soc. London, No. 10, 82 pp. Ridgway, E.W., 1983. Correlation and nomenclature of coal seams in the Lancashire and Cheshire Coalfields. National Coal Board, 59 pp. Ryer, T.A. and Langer, A.W., 1980. Thickness change involved in the peat-to-coal transformation for a bituminous coal of Cretaceous age in central Utah. J. Sediment. Petrol., 50: 987--992. Stach, E., Mackowsky, M.-Th., Teichmtiller, M., Taylor, G.H., Chandra, D. and Teichmiiller, R., 1982. Stach's Textbook of Coal Petrology, 3rd Edition. Gebruder Borntraeger, Berlin, 535 pp. Tonks, L.H., Jones, R.C.B., Lloyd, W. and Sherlock, R.L., 1931. The geology of Manchester and the southeast Lancashire Coalfield. Mere. Geol. Surv. U.K., 240 pp. Trueman, A.E., 1947. Stratigraphical problems in the coalfields of Great Britain. Q. J. Geol. Soc. London, 103: lxv-civ. Zangerl, R. and Richardson, E.S., Jr., 1963. The paleoecological history of two Pennsylvanian black shales. Fieldiana: Geology Memoirs, 4, Chicago Nat. Hist. Museum, 352 pp.