A Depositional Model for the Mid-Westphalian a to Late Westphalian B Coal Measures of South Wales

Journal of the Geological Society, London, Vol. 150, 1993, pp. 1121-1136, 12 figs. Printed in Northern Ireland

A depositional model for the Mid-Westphalian A to late Westphalian B Coal Measures of South Wales

A. J. HARTLEY Department of Geology, University of Wales College of Cardiff, PO Box 914, Cardiff CF1 3YE, UK Present address, Production Geoscience Unit, Department of Geology and Petroleum Geology, Kings College, University of Aberdeen, Aberdeen AB9 2UE, UK

Abstract: Mid-Westphalian A to late Westphalian B sediments of the South Wales Coalfield comprise mainly lacustrine, raised mire and flood deposits developed in an upper coastal plain environment. Occasional high sinuosity, fine-grained channel systems drained the subdued topography of the Wales-Brabant Massif to the north and east of the basin. Low sinuosity channels with lithic, coarse-grained fills drained a rising source to the south. Direct marine influence was restricted to a single marine band. Facies distributions were controlled by (1) local scale (102-103 m 2) autocyclic sedimentary proc- esses, (2) regional scale (10km 2) synsedimentary fault activity and differential subsidence and 3) basin-wide scale (102 km2) response to changes in relative sea-level. Base-level changes are recorded by basin-wide bivalve horizons (up-dip or landward extension of marine bands) developed in lacustrine mudstones above basin-wide coal seams. Peat accumulation was terminated by a water table rise related to base-level rise. Following flooding, re-establishment of the clastic supply resulted in the gradual infilling of the basin-wide lake to produce coarsening-upwards cyclothems or parasequences. A eustatic origin for parasequence development is supported by the development of identical facies associations and parasequences in coeval sediments from the Pennine and Midland Valley Coalfields.

Silesian sediments in South Wales are exposed in a source resulted in an influx of coarse, immature lithic structurally complex E-W trending synclinorium (Fig. 1). detritus and a change to alluvial braidplain sedimentation Over the last century the commercial exploitation of these (Pennant Measures) with no marine influence (Kelling 1974, coal-bearing sediments has resulted in the development of 1988; Jones 1989b). Alluvial sedimentation persisted until an extensive basin-wide data base (e.g. Robertson 1933; the early Stephanian (Kelling 1974; Cleal 1978; Jones 1989a, Moore & Cox 1943; Moore 1945, 1947; Blundell 1952; b; Hartley 1993) although a change from braidplain to Woodland & Evans 1964; Parry 1966; Thomas 1967, 1974; dominantly floodplain sedimentation took place in the late Archer 1968; Squirrel & Downing 1969; Barclay 1989 and Westphalian D (Hartley 1993). Throughout, the Silesian unpublished British Coal data). In recent years this has been basin depocentre was located in the Swansea area (Fig. 1) as used to help elucidate the Silesian sedimentological and marked by rapid decreases in thickness to the east and to a tectonic development of the area (e.g. Kelling 1974, 1988; lesser extent to the north and west (Woodland & Evans Jones 1989a, b, 1991; Hartley & Warr 1990; Hartley 1993). 1964; Squirrel & Downing 1969; Thomas 1974). The gross These studies have shown that Silesian sediments were changes in thickness and depositional environment have deposited in a foreland basin initiated in the early Namurian been related to synsedimentary tectonic activity associated and developed to the north of the Variscan orogen and with the northward migration of the Variscan orogen south of the cratonic Wales-Brabant Massif (Kelling 1988; (Kelling 1988; Gayer & Jones 1989; Jones 1989b; Hartley Gayer & Jones 1989; Hartley 1993; Fig. 1). Basin initiation 1993). followed an early Namurian inversion event which marked Following deposition, the coalfield was strongly de- the switch from a Dinantian extensional to a Silesian formed in the late Stephanian to early Permian Variscan compressional tectonic regime (Hartley & Warr 1990). Orogeny. The structure of the coalfield has been extensively In general, the 3.2 km thick Silesian basin-fill sequence described (e.g. Trotter 1947; Owen 1953, 1974; Owen & shallows and coarsens upwards. Namurian sediments were Weaver 1983; Gayer & Jones 1989; Jones 1989a, b; 1991; deposited in a shallow marine deltaic/wave-influenced Cole et al. 1991; Frodsham etal. 1992). Deformation took environment (for further details see Oguike 1969; George the form of a major linked thrust system in the incompetent 1970; Jones 1974; Kelling 1974; Hartley 1993). From late Lower and Middle Coal Measures with shortening locally up Namurian to early Westphalian C times lower and upper to 55% (Frodsham et al. 1992). Northward verging delta/coastal plain environments predominated with varying structures extend to the northern margin of the coalfield and amounts of marine influence (Kelling 1974; Jones 1989b). reactivated ENE-WSW Caledonoid basement structures During this period sediment was mainly derived from the (Tawe, Neath and Carreg Cennan Disturbances; Fig. lb). In north and the east and to a lesser extent from the south the south of the coalfield a major southward-verging thrust (Bluck & Kelling 1963; Kelling 1974; Jones 1989b; Hartley system developed as a passive roof duplex beneath the 1993). In early Westphalian C times uplift of a southerly Upper Coal/Pennant Measures (Jones 1989b, 1991). Thrusts 1121

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Fig. 1. (a) Map showing currently and previously worked coalfields of Great Britain (dense stipple) together with known Silesian palaeogeographic highs (light stipple), based on Moses (1981) and S0u h l l i Guion & Fielding (1988). (b) Geological map of the South Wales Coalfield showing the main structures. The productive measures L;oaiT~elcl - .-I: --.--... ~.". l'~-& Basin ": -~ ~ ~ interval lies within the Lower and Middle Coal Measures. CCD, Carreg Cennan disturbance; TD, Tawe disturbance; PBF, Pwllau Culm Bach fault; L-DF, Llanwonno-Daren-Ddu fault System; DF, Dinas Basin fault; MGF, Moel Gilau fault; PA, Pontypridd anticline; GS, Gelligaer syncline; CCA, Cardiff-Cowbridge anticline; UA, Usk 0 100 km 1 1 I axis.

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are cut by, or terminate against, ubiquitous NW-SE and coalfield respectively. The productive measures contain only N-S dextral strike-slip and/or extensional cross-faults a single marine band (Amman/Vanderbeckei marine band, (Trotter 1947; Jones 1991; R.A. Gayer, pers. comm. 1992). Fig. 2) but are sandwiched between two marine Despite the large data base available for the South Wales band-bearing sequences found in late Namurian to mid Coalfield, only the general synthesis of Kelling (1974) and Westphalian A and late Westphalian B to early Westphalian the work of Jones (1989a, b) in the eastern part of the C sediments respectively (Fig. 2). coalfield have attempted to examine controls on facies distribution during sedimentation. Here, a detailed sedim- Facies analysis entological study of the stratigraphic interval containing the main productive coal seams of the South Wales Coalfield Detailed facies analysis of the productive measures interval (informally termed the productive measures) ranging from based on visits to British Coal opencast sites supplemented mid-Westphalian A to late Westphalian B (Fig. 2) is by fieldwork, published data and British Coal borehole and presented and the controls on sedimentation assessed and deep mines data has revealed the presence of eight distinct compared with other British Silesian coal-bearing basins. Of facies. particular interest and relevance to other coal-bearing sequences are the relative influences of base-level changes, (1) Mire. This facies comprises coal seams ranging from inherited basement topography and active faulting in 0.02 to 6.5 m thick (generally 0.3 to 1.2 m). Beds may merge controlling facies distributions in a delta/coastal plain upwards into carbonaceous (sometimes canneloid) mudst- environment. ones of facies 3 or downwards into rootleted sediments of facies 2. Seams vary in lateral extent; many of the thicker (>1 m) and some of the thinner seams can be traced across Productive measures the coalfield (Figs 3 & 4) although the quality and thickness The productive measures range from the Five Feet/Gellideg of the seams can vary considerably. Seam splits are common (mid-Westphalian A) to the Two Feet Nine Group (late and some seams are only locally developed (over a few Westphalian B) of coal seams (Fig. 2) and contain the main square kilometres). Coal quality is particularly good with seams exploited in the coalfield. The interval ranges in low ash and sulphur contents (average 6.2 and 0.9% repec- thickness from 330 m in the centre of the coalfield (Swansea tively; Adams 1967). area) to 63 and 230 m on the east and west sides of the The miospore content of the productive measures seams is largely dominated (35-60%) by Densosporites spp. (Smith & Butterworth 1967). Smith (1962, 1968) and Fulton (1987) have shown that Densosporites spp. dominated miospore ,~v F- assemblages within coal seams record the development of climax flora within long residence histosols formed in raised I- Ca 305 .... mires, an interpretation favoured for the productive measures seams. Raised mires develop from low-lying mires (see McCabe 1984 for model of raised mire formation) and Upper Cwmgors Marine Band are only developed in areas where precipitation exceeds M M M M evaporation...... M M The probability that most of the main productive measures seams developed as raised mires can explain their Z low ash and sulphur content. Modern raised mires in SE < Cefn Coed (Aegiranum) M M M--M Marine Band Asia have an average elevation of some 3-7.5 m above flood _J M M TWO FEET NINE level (Anderson 1964) preventing flood events from FOUR FEET 308 - "I- ~.~ ,.~ breaching the mire and depositing clastic detritus. In SIX FEET 13_ RED VEIN addition, preservation of clastic detritus in raised mires is I-- ,8 ~.~ NINE FEET unlikely as the strong acidity of mire groundwaters (pH C BUTE LLI M--M Amman Marine Band 3.3-4.5) is such that even if sediment was deposited it would YARD

SEVEN FEET be subject to dissolution (McCabe 1984). 311 - FIVE FEET GELLIDEG (2) Seatearth. Rootleted mudstones, siltstones and oc- ,8 ,.~ GARW casional fine-grained sandstones characterize this facies. The facies always occurs beneath coal seams, although coals are

Margam Marine Band not always developed above it. Laminae are frequently

315 - C M M Gastrioceras listeri M.B. disturbed and carbonaceous rootlet casts of Stigmaria are M-' M 319 Z M M < common. Siderite nodules commonly replace rootlets. The 137 B facies is interpreted as a siliciclastic palaeosol or gley (Ful-

G. subcrenatum M.B. ton 1987; Besly & Fielding 1989). 7< A (3) Lacustrine. This facies forms the bulk of the productive Fig. 2. Stratigraphy of the South Wales Coalfield showing position measures and ranges in thickness from 0.1 to 10m. It is of the productive measures interval, main coal seams and marine dominated by black organic-rich and grey parallel laminated bands (M.B.) based on Woodland & Evans (1964). Time scale mudstones together with occasional siltstones and fine- based on Lippolt et all (1984). Note the position of bivalve horizons grained sandstones. Where the facies occurs above coals, (horizontal comma ornament) above coal seams. black, fissile, ferruginous (locally pyritic), and sometimes

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Fig. 3. Representative logged sections of the productive measures interval from opencast coal sites (main part of the Westphalian B interval). The base of section A is taken at the Amman/Vanderbeckei Marine Band. Arrows indicate palaeocurrent directions. Note major low-sinuosity channel-fill sequences (facies 7) developed at the top of section B and above the stwrin seam in section D. National grid references for sites are: Cynheidre/Ffos Las [SN 494076]-Gilfach lago [SN 609124]-Garnant [SN 685127]; Nant Helen [SN 825112].

canneloid, organic-rich mudstones up to 50cm thick (ave- supported filter-feeding bivalves. Waters above the bottom rage 15 cm) are developed which pass upwards into dark were sufficiently oxygenated to support Naiadites and fish. grey organic-poor mudstones and grey siltstones with oc- The grey mudstones, siltstones and sandstones reflect a casional plant fragments. The black mudstones commonly higher clastic input. The coarser, fining-upwards units contain delicate plant fragments, some fish debris, 'Esthe- represent waning flow deposits generated by fluvial flood ria', ostracodes, some Xiphosura (Dix & Pringle 1929) and events. Laminated mudstones record suspension fallout bivalves mainly of the genera Naiadites, Anthracosia, between flood events. Similar facies have been described Anthraconaia and to a lesser extent Anthracosphaerium from the Silesian of NE England by Fielding (1984a, his (Woodland & Evans 1964; George 1970). The siltstone beds facies 7 and 8) and Haszeldine (1984, his facies 1 and 2). are up to 0.15 m thick (average 7 cm) and can be traced laterally over hundreds of metres suggesting a sheet-like (4) Subaqueous and subaerial flood. Beds of fine-grained geometry. Thin (up to 2 cm), horizontally laminated and/or sandstone and siltstone 0.1-1.25 m thick with a sheet-like cross-laminated fining-upwards units of fine-grained sand- geometry characterize this facies. The facies is normally stone to siltstone are developed (Fig. 5a,c,e). Siderite- interbedded with lacustrine sediments. Disseminated plant cemented beds (after siltstone) and nodular horizons a few fragments are common and rootlets are occasionally de- centimetres thick are common. Bioturbation is rare. veloped. In detail the facies can be subdivided into fining- These sediments are considered to represent deposition and coarsening-upwards beds. Fining-upwards beds of fine- in a lacustrine environment. The black, organic-rich, locally grained sandstone to siltstone have sharply conformable to canneloid mudstones were deposited in sediment-starved mildly erosive bases and often show an upward transition lakes with poorly oxygenated floors (no bioturbation) which from current ripple to parallel lamination (Fig. 5b,e). Clim-

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Gellideg e~ Fig. 4. Correlated generalized sections of the productive measures interval across the South Wales Coalfield, (a) Gwendraeth Valley [SN 500111] partly based on Archer (1968) and field data, Llanedi/Cwmgwili [SN 609075] and Treforgan [SN 803055] based on unpublished British Coal data, Hirwaun [SN 960055] partly based on Robertson (1933) and field data, Rhymney [SO 103058] and Abergavenny [SO 257101] based on Barclay (1989); (b) Margam [SS 841853], Llanharan [SS 997825] and Rhondda Fach [ST 011959] based on Woodland & Evans (1964), Deep Navigation [ST 101974] and Hafodyrynys [ST 246978] based on Squirrel & Downing (1969). Correlation is based on extensive workings (most of the productive measures seams have been worked across the eastern half of the coalfield and along the north crop), British Geological Survey correlations and British Coal data.

bing ripple lamination may be present above the base and upwards sequences record the gradual abandonment or below the current rippled unit. Coarsening-upwards beds of switching of a distributary feeder channel. siltstone to fine-grained sandstone record an upward transi- tion in sedimentary structures from parallel laminated lac- (5) Major high-sinuosity channels. This facies is charac- ustrine mudstones and siltstones through parallel to current terized by extensive inclined heterolithic stratification (IHS) to climbing ripple laminated siltstones and fine-grained up to 6 m in height and developed in channels >50 m wide sandstones. in transverse section (Fig. 5c). IHS is composed of siltstone, This facies represents the unconfined sheet flow of fine-grained sandstone and occasionally mudstone; internal sediment from fluvial channels at high flood stage. erosion surfaces are common. Coarse-grained sandstones Fining-upwards beds reflect the waning flow strength of the may be present towards the channel base. Occasionally, flood event (cf. McKee et al. 1967). In contrast structureless mudstone channel-fill units are present. Plant coarsening-upwards beds are thought to record increasing fragments are common within channel-fill sequences. The flow strength. The association of this facies with lacustrine facies is only occasionally developed in the productive me- sediments suggests that the flood events were subaqueous, asures and represents major, long-lived, high sinuosity, although occasional rootlets indicate that water depths were mixed-load fluvial channel systems, with inclined hetero- minimal at times. Flood events are thought to represent lithic stratification reflecting point bar development. more proximal equivalents to the lacustrine waning flow Mudstone-filled channels record the development of deposits of facies 3. suspended-load dominated channel systems. Beds of this facies may be isolated or stacked. St~cked sequences composed of both coarsening and fining-upwards (6) Minor high-sinuosity channels. Facies 6 is characterized beds are up to 10 m thick and may coarsen or fine upwards by fine-grained sandstones, siltstones and mudstones dis- overall. Large-scale coarsening-upwards sequences are playing inclined heterolithic stratification. Transverse chan- interpreted as lacustrine delta deposits. Occasionally they nel width is up to 10 m with IHS up to 1.5 m in height. The may be topped by trough cross-stratified, fine-grained facies represents a small-scale, short-lived, high sinuosity sandstones interpreted as mouth bar deposits. Fining- distributary system. Whilst it is possible that this facies may

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(c)

(d) (e) Fig. 5. Field photographs of typical facies associations in the productive measures interval. (a) Thin (mm scale) cross-laminated siltstone and sandstone units interbedded with dark grey horizontally-laminated mudstone, lacustrine facies (Garnant opencast coal site [SN 685127]); (b) fine-grained cross- and horizontally-laminated sandstone, subaqueous and subaerial flood facies (Ffos Las opencast coal site [SN 494076]); (e) major high sinuosity channel filled entirely by siltstone eroding into horizontally laminated mudstones of the lacustrine facies, note the low angle accretion surfaces (dipping to right), Pen Bryn Oer [SO 103058] opencast coal site; (d) faulted, laterally extensive, thinly bedded fine-medium-grained cross-stratified and horizontally stratified sandstones, minor low sinuosity channel facies (Garnant opencast coal site); (e) typical exposure of the productive Coal Measures, horizontally laminated grey siltstones and mudstones (lacustrine facies) interbedded with thin laterally extensive fine grained sandstone beds (subaqueous flood facies, arrowed) Gilfach Iago opencast coal site [SN 609124].

1126

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represent a crevasse channel supplied by a facies 5 fluvial peat accumulation was terminated by flooding of the mire, system (cf. Fielding 1986), due to the nature of exposure it although locally, flood deposits (facies 4) may overlie coal is impossible to demonstrate a causal relationship between seams. Bivalve horizons appear to be best developed in the the two facies. centre of the basin. Major fluvial channel deposits occur at different levels (7) Major low-sinuosity channels. Sets of cross-strata up to between coals (Fig. 3). Channels occasionally erode down 80 cm thick forming >8 m thick erosively based channel-fill into underlying seams resulting in washouts (Fig. 3). sequences (Fig. 3) characterize this facies. Sets may fine Washouts are recorded during mining, but care must be upwards and are developed in medium-granule grade (oc- taken to differentiate between fluvial and tectonic washouts. casionally pebbly), carbonaceous and lithic-rich material Tectonic 'washouts' occur were coal seams are absent with cosets a few metres thick. Internal erosion surfaces may through normal faulting or progressive easy slip thrusting be present and are overlain by a lag of reworked ironstone (Frodsham et al. 1992). In general, washouts occur on a nodules, quartz pebbles and plant material including oc- small scale (5-20 m in width) and are localized (over a few casional logs. The facies is only occasionally present in the kilometres) probably reflecting minor channel activity (Fig. productive measures (sections B and D of Fig. 3) and 6), although it should be noted that washouts may only represents the development of major, long-lived, low sinu- represent part of the channel width. Occasionally, washouts osity, bedload dominated fluvial channel systems. Cross- can be mapped over larger areas (tens of square strata record the migration of sinuous-crested, in-channel kilometres), particularly in the eastern part of the coalfield mega-ripples. (Jones 1989b;), reflecting major channel activity.

(8) Minor low-sinuosity channels. This facies comprises Provenance erosively-based, sheet-like, channel-fill units with high width to height ratios (>15:1). Channel-fill units are up to 1.75 m The limited palaeocurrent data available for the productive thick and composed of two to five sets of coarse-grained, measures interval suggest that the major fluvial systems lithic-rich, trough cross-strata up to 55 cm thick (Fig. 5d). were derived from the east, north and to a lesser extent The facies represents low sinuosity, short-lived, poorly from the south (Bluck & Kelling 1963; Kelling 1974; Jones channelised flood events. The development of this facies at 1989b; Hartley 1993). Palaeocurrent data indicate that the the same horizon as a major low sinuosity channel in the lithic-rich, bedload-dominated low sinuosity channels of west of the coalfield (Fig. 3) suggests that sediment may facies 8 were derived from the south (Fig. 3). The have been supplied from facies 7 channels. However, as for composition of these southerly derived sandstones closely facies 5 and 6, a causal relationship cannot be established between facies 7 and 8.

White 4' Facies relationships c k I !!!i!i!i!iii!i!:!::::':" Facies relationships for the productive measures interval are illustrated in Figs 3 and 4. Sedimentation was dominated by mire, seatearth and lacustrine facies, with some flood deposits and only limited fluvial channel development. Conventionally, workers have regarded the productive measures as comprising numerous coarsening-upwards !iiiii' cycles or cyclothems developed between coal seams and related to the progressive infilling of a lake or !iii!i iiii ii iii ,, interdistributary bay (e.g. Trueman 1947; Woodland & Evans 1964; Archer 1968; Squirrel & Downing 1969; George 1970; Thomas 1974; Barclay 1989). The cyclothems are equivalent to parasequences as defined by Posamentier -n et al. (1988). Whilst coarsening-upwards parasequences (lacustrine delta deposits) comprise the main interseam sequence, examination of Fig. 3 indicates that seams (and associated seatearths) also overlie lacustrine shales or overbank flood deposits. Consequently, it is likely that changes in lake level (water table level) were locally important in promoting mire development. Parasequences range between 5 and 30 m in thickness and can show large local lateral thickness variations. For example a 5 m thick parasequence with a thick coal seam may be traced (through coal seam splits) over a few kms into three or four Fig. 6. Map of seam washouts at Nant Helen opencast coal site [SN parasequences with combined thicknesses up to 30m 825112]. Note that washouts run parallel to the strike of the Pwllau (Woodland & Evans 1964). Coals in these thicker Bach Fault (see Fig. 3 for stratigraphic position). Washouts mapped parasequences are generally thin and of poor quality. Figure from over 500 borehole records (unpublished British Coal data). 4 also shows that bivalve horizons of the lacustrine facies Different stippled zones show different areas of 'washout' of coal commonly overlie major basin-wide coal seams (not the seams (named) caused by erosion related related to fluvial channel thinner seams of thicker parasequences), suggesting that activity.

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resembles those of the overlying Pennant Measures (Fig. 2), thickness variations can be attributed to autocyclic also derived from the south (Kelling 1974; Jones 1989a). It sedimentary effects such as: (1) differential compaction of is probable, therefore, that the early low sinuosity fluvial sediment of different grain sizes, e.g. la.custrine delta systems were sourced from the southerly landmass which sandstones and siltstones and adjacent contemporaneous later, following further uplift, supplied detritus to the basin lacustrine mudstones or coarse-grained fluvial channel during Pennant Measures deposition. The presence of this deposits and adjacent overbank fines; (2) inherent variations landmass is supported by palaeocurrent data from shallow in the thickness of peat accumulations (particularly in raised water and deltaic Westphalian sediments in the Culm Basin mires, e.g. Wilford 1961; Anderson 1964) that would be of SW England (at present 75 km south of South Wales, Fig. enhanced during subsequent compaction; (3) fluvial channel 1), which were derived from the north (Elliott 1976; avulsion or migration. Edmonds et al. 1979). In contrast, sediment supplied from the Wales-Brabant Massif to the north and east of the basin (2) Regional scale (tens of km 2) was compositionally mature, predominantly fine-grained and transported by high sinuosity fluvial systems. This suggests Two isopach maps for the productive measures interval (Fig. that the topography of the Wales-Barbant Massif was 7) illustrate a general thickness increase towards the relatively subdued such that a large-scale fluvial distributary Swansea-Gower area (cf. Thomas 1974), although regional network was not developed. A similar scenario is envisaged variations are apparent, particularly in the central and for the northern margin of the Massif (the southern flank of eastern part of the coalfield. Detailed examination of mine the Pennine Basin). During Westphalian A and B times the records suggests that these thickness variations can be Massif had a subdued topography, small areal extent and accounted for by synsedimentary cross-fault and fold supplied only limited amounts of sediment northwards, activity, and deposition over relatively slowly subsiding consequently major distributary channels were not de- basement highs. veloped on the northern edge of the massif (Fulton & Williams 1988; Guion & Fielding 1988). a ~ ooO~~-~~~ j j:~ ._/. Depositional setting The restriction of marine fauna to a single horizon (Amman/Vanderbeckei Marine Band) and the pre- dominance of coal seams and bivalve faunas indicate that direct marine influence (i.e. deposition of marine rocks) was minimal during productive measures deposition. The dominance of fine-grained sediments, absence of marine N sedimentation, development of thick, extensive coals and low rate of occurrence of major fluvial channels suggests an upper coastal/delta plain environment (see Jones 1989b). The scale of the limited major fluvial channel systems in the studied interval is significantly smaller than those described from sediments of equivalent age in the Pennine Basin which range from hundreds to thousands of metres in width (Haszeldine 1983; Fielding 1984b; Guion 1984; Guion & Fielding 1988). The catchment areas of the terrains supplying sediment to the South Wales Coalfield basin were of insufficient size to develop the large-scale distributary systems characteristic of an upper delta plain setting in the Pennine Basin (Guion & Fielding 1988). Consequently, an upper coastal plain traversed by high sinuosity, short-headed fluvial channels derived from the north and east and low sinuosity channels derived from a developing higher-relief source to the south is envisaged during productive measures Fig. 7. Isopach maps (in metres) for the productive measures times. interval showing regional thickness variations for (a) Five Feet/Gellideg to Amman (Vanderbeckei) marine band interval and (b) Amman marine band to Two Feet Nine interval. Dots refer to position of collieries used as control points. For details of Controls on sedimentation abbreviations used see Fig. lb. Note that the isopachs parallel the Factors operating on three different scales controlled trend of the Usk anticline and the NNW-SSE cross-faults and sedimentation during deposition of the productive measures. thickening associated with the Gelligaer syncline. Regional variations are not well delineated in the west of the Coalfield as (1) Local scale (102-103 m 2) extensive deformation and more limited exploitation in the west do not permit the construction of sufficiently detailed isopach maps. Across individual and adjacent opencast sites the thick- Thicknesses corrected for tectonic dip and repetition due to thrust nesses of coals and interseam strata can vary by an average faulting, data from Squirrel & Downing (1964, 1969), Woodland & of about 20% (up to 50% in places), although tectonic Evans (1964), Parry (1966), Adams (1967), Archer (1968), Barclay effects such as layer-parallel shortening (Frodsham et al. (1989) and unpublished British Coal borehole data. For 1992) can often complicate thickness estimates. Local abbreviations see Fig. lb.

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Synsedimentary fault and foM activity. Figures 8 and 9 ill- W E • m m ustrate a westerly increase in sediment thickness across the AMMAN MARINE BAND Llanwonno-Daren-Ddu and Dinas Faults and the coin- UPPER AMMAN RIDER ~-~" ~ ~'- cidence of the split axis (point of splitting) of the upper and 0~-~ LLANWONNO lower leaves of the Nine Feet seam with the position of the N-DDU fault. This coincidence suggests that faulting was synchro- ~J FAULT-SYSTEM nous with sedimentation, as the slow rates of peat accu- mulation and largely fiat topography of a mire are likely to "~,,~.~ SE V~,,;" ~qq~. FAULT L have been highly sensitive to fault-related subsidence (Bro- ,r~L o~,," ~'~'R" LOWF--9'5x" "~ CONTROL POINTS(COLLIERIES) adhurst & France 1986). Consequently, as noted by Fulton & Williams (1988), where seam splitting results from con- Fig. 9. East-West cross-section showing thickness variations temporaneous faulting, the line of splitting should coincide between coal seams (named) in the upper part of the Westphalian with the fault line. Figure 9 also shows that the growth A across the Llanwonno-Darren-Ddu and Dinas fault systems (see sequences are restricted to certain stratigraphic horizons and Fig. lb for location of section). Section is hung on the Amman that therefore, fault movement was episodic. Also, the Marine Band. Data based on Woodland & Evans (1964), Parry present day downthrow direction of the Llanwonno-Daren- (1966) and Squirrell & Downing (1969). Ddu and Dinas Fault systems is to the east whilst the inferred Silesian downthrow direction was to the west. offset of washouts oriented parallel to the inferred fault However, cross-faults have a long history including several scarp suggests that episodic fault movement resulted in the possible phases of reactivation (Jones 1991), consequently formation of topographic lows in the hanging-wall block the present sense of displacement is unlikely to reflect that close to the fault (roll-over or reverse drag effect, e.g. of Silesian times. Barnett et al. 1987) into which rivers were attracted (cf. Synsedimentary faulting is also indicated by the Hartley & Gillespie 1990). A similar situation was described orientation of fluvial washouts parallel to the Pwllau Bach from the Durham Coalfield by Fielding (1984b), who Fault, a major cross-fault at Nant Helen open cast coal site suggested that the vertical stacking and/or offset of (Fig. 6). Detailed exploration borehole coverage of the site distributary channels was caused by enhanced subsidence allowed the identification of three sedimentary coal seam associated with synsedimentary fault activity. washouts at different stratigraphic levels, subsequently A N-S cross-section across E-W folds (Fig. 10), shows a confirmed during excavation. The washouts formed by general thinning of strata across the Pontypridd Anticline incision of fluvial channel systems and have a NNW-SSE with marked thickening of strata into the Bettws-Tonyrefail orientation parallel to the Pwllau Bach Fault. Palaeocur- Syncline (the western extension of the Llantwit-Caerphilly rents where measurable were also to the north. The vertical Syncline), a feature also seen on the isopach maps (Fig. 7). This observation concurs with the work of Jones (1989a) further to the east, who showed that growth folding 90 O0 10 00 1 associated with the Pontypridd Anticline and Gelligaer and \ " • • Llantwit-Caerphilly Synclines actively controlled seam- splitting and drainage patterns during late Westphalian A to late Westphalian C times. Jones (1989a) suggested that the " growth folds were generated by the compressional J.~ -- ~ o~ • • -c,, \0 \~ FAULT SYSTEM reactivation of E-W basement structures during the early

N S • O-,',t-O • -,t- ~ • -~" OOe • • • "~" AMMAN MARINE BAND 90 UPPER AMMAN RIDER

KM 10 I X ~o~,~,og\\\ %-%. ~// ~~LLANTWIT- • Control Points (Collieries) Contours in tens of metres SYNCLINE ~~ [

~%'et SEVEN lOT10m ~1 km Fig. 8. Isopach map of thickness variations between the upper and lower leaves of the Nine-Feet seam across the Llanwonno-Daren- X Ddu fault system and Dinas Fault. Note the coincidence of the seam BErrws- -,It- CONTROLPOINTS (COLLIERIES) split axis with the location of the Llanwonno-Daren-Ddu fault TONYREFAIL SYNCLINE trace, suggesting synsedimentary fault movement (cf. Fulton & Williams 1988) and the absence of thickness changes across the Fig. 10. North-south cross-section showing thickness variations in Dinas Fault for this stratigraphic interval (compare with Fig. 9). coal seams (named) across predominantly E-W-trending folds (see Thicknesses from Woodland & Evans (1964), corrected for tectonic Fig. 1 for location of section). Section is hung on the Amman dip and repetition of strata due to thrust faulting. Numbers at edge Marine Band. Data based on Woodland & Evans (1964) and Parry of box refer to 10 km squares of national grid sheet SS. (1966).

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stages of the Variscan orogeny. It is possible therefore, that interval for parasequence development of between 200 000 the cross-faults were active as synsedimentary transtensional and 350000 years can be determined. This recurrence structures as their N-S to NNW-SSE orientation is highly interval is within the 100000 to 400 000 recurrence interval oblique to the inferred northward transport direction of estimated for marine band-bearing Upper Carboniferous Variscan deformation. cyclothems in North America (Busch & Rollins 1984; Heckel 1986) and the Pennine basin (Collier et al. 1990; Inherited basement topography. The isopach maps (Fig. 7) Maynard & Leeder 1992). show a marked eastwards thinning of strata. Figures 3 and 9 McCabe (1984) noted that peat accumulation can occur illustrate that the thinning results from stratal attenuation faster than most normal subsidence rates (particularly in a without a break in deposition (Squirrel & Downing 1964, raised mire setting) and drowning of the mire will only occur 1969). Thinning is accounted for by an increase in coal seam if subsidence is unusually rapid or there is a sudden rise in thickness through a gradual decrease in interseam clastic the water table due to other factors. It is probable, material and amalgamation of seam splits. The absence of therefore, that the development of basin-wide coal seams unconformities or intermittent rapid growth sequences on and bivalve horizons was related to base level changes the eastern margin of the coalfield suggests that sediment possibly driven by eustatic fluctuations. Ramsbottom (1984) draped pre-existing topography which had a persistent resis- used a model based on eustatic fluctuations to explain Coal tance to subsidence. The presence of thick coal seams (up to Measures sedimentation in South Wales. He suggested that 6.5 m) reflects the development of a stable environment a rise in water table (caused by rising sea-level) would where the interplay between water table level, peat ac- impede drainage thus preventing the oxidation of organic cumulation and subsidence rate was such that large thick- matter and promoting the build up of thick peats and nesses of peat could accumulate uninterrupted by clastic proposed that basin-wide coal seams in effect represent sedimentation for thousands of years. This eastern basement 'failed' marine bands. It is considered that these basin-wide high is thought to be related to the Lower Palaeozoic inlier base-level changes are driven by eustatic fluctuations of the Usk Axis (Fig. lb). The axis is known to have had a (outlined below) and represent the main large-scale control strong influence on Silesian sedimentation (Moore & Blun- on productive measures sedimentation. In other Silesian dell 1952; George 1956) and is likely to have closely in- coal-bearing basins delta lobe progradation and switching fluenced the development of the eastern margin of the were considered to represent the primary controls on basin. This influence is supported by the productive me- sedimentation (e.g. Guion & Fielding 1988), however, it is asures isopachs of the Gellideg-Amman Marine Band inter- likely that the development of these other large-scale fluvial val (Fig. 7), which lie parallel to the trend of the anticline. systems was also controlled by base-level changes.

(3) Basin-wide scale A depositional model for the productive measures The basin-wide extent of major coal seams and bivalve A possible eustatic origin for basin-wide coal seams is horizons suggests an allocyclic control on long term raised supported by their association with the overlying bivalve mire and lake development. Possible allocyclic controls horizons. These horizons are thought to represent include: changes in climate, changes in sediment supply, correlative up-dip equivalents of marine bands (in contrast variations in subsidence rates related to changes in the to Ramsbottom 1984 who regarded the coals as marine band nature of the lithospheric load and relative sea-level equivalents). This hypothesis is supported by the work of fluctuations. Climatic changes are thought to be unimpor- Calver (1968, 1969), Ramsbottom (1969) and Fulton & tant, as a humid, non-seasonal equatorial climate prevailed Williams (1988) who showed that Silesian marine bands are over much of North America and Europe during the characterized by lateral faunal changes when traced Westphalian (Chaloner & Creber 1973; Phillips et al. 1985; landwards. In particular, the most 'terrestrial' faunal belt is Besly 1987) although seasonal rainfall in the northern characterized by bivalves and fish, which passes basinwards catchment area to the Pennine Basin was identified by into 'Estheria' and ostracode faunal belts that developed in Broadhurst et al. (1980) and Broadhurst (1988). Subsidence brackish water conditions between mires and the sea (Calver rates were laterally variable across the basin due to inherited 1968, 1969). Bivalve horizons therefore are interpreted to basement features and active faults, however, burial history represent brackish/freshwater upper delta/coastal plain curves for different points across the basin for Westphalian equivalents of marine bands developed on the lower A to C times reveal that in any one place subsidence rates delta/coastal plain and record time periods when relative were constant (Kelling 1988; Jones 1989b). Also, during sea-level rise exceeded subsidence. productive measures deposition, sedimentation rates kept Direct marine influence on the upper coastal plain was pace with subsidence as no gross change in depositional limited to a single marine band, although relative sea-level setting occurred. Sediment supply to the basin during fluctuations had a considerable control on water table levels. productive measures deposition showed no significant long Peat accumulation was terminated by a rising water-table term changes (Kelling 1974), although in the short term (Fig. lla), although the increased salinities associated with sediment supply was strongly influenced by high frequency the bivalve fauna, will have resulted in groundwater base-level fluctuations (see below). Fluctuations in climate, conditions which the bulk of Westphalian arborescent sediment supply and subsidence rate can also be discounted lycopods could not tolerate (Scott 1978; Read & Forsyth on the basis that these mechanisms are unlikely to change 1991). The water table rise resulted in drowning of the mire rapidly or regularly enough to produce 10 largely basin-wide to form a basin-wide brackish-freshwater lake in which 5-30m thick parasequences (Fig. 2). Assuming a time clastic input was sufficiently low as to allow the period of between 2 and 3.5 Ma (time period based on establishment of bivalve communities (Fig. llb). Following Lippolt et al. 1984; Fig. 2) an approximate recurrence lake development, a gradual re-establishment of clastic

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sediment supply recorded by a change from black to grey a ~ 2__ mudstones and siltstones resulted in the death of bivalve communities. Continued clastic sediment supply resulted in the gradual infilling of the basin-wide lake, formation of numerous localised lakes and re-establishment of dis- tributary channel systems (Fig. l lc). The re-establishment of the clastic supply could have Increasing ,~..,,.'-" c~ .~ )//~ resulted from a base-level fall. However, the large-scale, sea-level ~ ~'~< ") ... v 0 ' synchronous basin-wide incision by fluvial channel systems expected during a base-level fall (Posamentier & Vail 1988; Van Waggoner et al. 1990), has not been observed (cf. Busch & Rollins 1984; Read & Forsyth 1991). Also, coal seam washouts are relatively rare and channel incision, where observed, is only of the order of a few metres. This suggests that the rate of base-level fall was relatively slow or negligable, such that the balance between subsidence and sediment supply was maintained, or alternatively, that the b ~ magnitude of base-level change may not have been sufficient to induce incision prior to the next flooding event (see Mitchum & Van Waggoner 1991 for a fuller discussion). The subsequent development across the basin of low-lying mires overlying lacustrine delta, flood and fluvial deposits represents an abrupt facies change. Conditions suitable for mire development are likely to be associated with an increase in water table level resulting in ponding of clastic supply thus aiding plant colonisation and initial mire ~ ~ ~ // establishment. Raised mires then developed over the low-lying mire, with mire growth sustained by an increasing water table level and the high precipitation rates of a tropical equatorial climate. Mire growth was subsequently outpaced by a rising water table driven by a sea-level rise. The above idealized cycle of water table rise-infill and subsidence-water table rise, represents a possible e eustatically-generated mechanism of explaining the develop- ment of parasequences within the productive measures interval (see also Busch & Rollins (1984) and Goodwin & Anderson (1985) for a similar analysis). However, as Woodland & Evans (1.964) noted and as can be seen from sea-level ,~,..~v f Fig. 3 'complete' parasequences rarely occur. The absence of uniform parasequence development can be attributed to minor topographic variations on a low-lying coastal plain generated by the local and regional facies controls outlined previously and supplemented by variations in the magnitude of base-level rises. For example, the enhanced, but episodic subsidence associated with synsedimentary tectonism can account for lateral thickness changes, seam splitting and lateral increase in parasequence numbers. As bivalve horizons are thought to be essentially Fig. 11. Schematic diagrams of the influence of relative sea-level isochronous, their distribution will reflect the surface rise on deposition in the South Wales Coalfield basin. (a) Onset of topography of the upper coastal plain. They have a varied relative sea-level rise, peat accumulation (development of raised distribution (see Woodland & Evans 1964, their plate III) mires) promoted by rise in water table level, fluvial systems 'backed which is likely to again reflect local and regional topographic up' decreasing clastic input into the basin. (b) Relative sea-level rise variations. In addition, areas of enhanced subsidence may exceeds subsidence/peat aggradation ('R' inflection point where the rate of relative sea-level rise exceeds the rate of sediment be preferentially colonised by bivalve communities, accumulation see Posamentier et al. (1988) for further details), particularly as the magnitude of Silesian sea-level rises resulting in flooding of the coastal plain, cessation of peat which generated the European Silesian marine bands varied accumulation, virtually no clastic input and establishment of considerably (Calver 1969; Ramsbottom 1979; Maynard & basin-wide brackish lake. (¢) Relative sea-level highstand followed Leeder 1992). Consequently, relatively small sea-level rises by a relative sea-level fall results in re-establishment of the clastic are thought to have resulted in the restriction of bivalve supply and progradation, gradual silting up of the lake and death of horizons to the basin depocentre. Many of the bivalve bivalve communities (locally in areas with very low clastic input horizons do not extend over the eastern flanks of the basin isolated bivalve communities may survive for a short while). (see Squirrel & Downing 1969 and Barclay 1989 for further Aggradation results in subaerial exposure and plant colonisation details; Fig. 4) suggesting that the topographic high related leading to the gradual establishment of low-lying mires. to the Usk Axis was not submerged during most water table

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rises. The lack of mire flooding on the eastern edge of the Butterworth 1967 for lower Westphalian A palynological basin aided the development of thick coal seams (up to data). 6.5 m). However, water table fluctuations did affect peat accumulation in the east of the coalfield. Clymo (1987) has shown that because of bacterial decay, the accumulation of Eustatic fluctuations and comparison with other peats with a thickness sufficient enough to form coals over British Silesian coalfields 3 m thick is not possible without an accompanying water Silesian marine bands and parasequences are thought to have table rise. been generated by eustatic fluctuations driven by the growth Coal seam quality in the South Wales Coalfield is and decay of the Gondwanan ice cap (Wanless & Shepherd controlled by depositional setting. Deposition during the 1936; Crowell 1978; Busch & Rollins 1984; Heckel 1986; productive measures interval took place on an upper coastal Veevers & Powell 1987; Leeder 1988; Collier et al. 1990). plain with very limited direct marine influence. Coals Consequently, marine bands and brackish/freshwater deposited in the lower delta/coastal plain environments of bivalve horizon equivalents should be present in all Silesian late Namurian to mid Westphalian A and late Westphalian upper delta/coastal plain successions. In order to test this B to early Westphalian C times are generally thin, of poor hypothesis the stratigraphy of the Pennine and Midland quality and have not been worked commercially. This Valley Basins has been compared with that of South Wales contrast in seam quality is likely to be related to the amount (Fig. 12). Significantly, these basins have different tectonic of marine influence. Of the two environments, the lower settings. The South Wales Coalfield developed as a coastal plain will suffer greater marine inundation such that compressional foreland basin, the Pennine Coalfield as an the recurrence time between marine flood events will be extensional basin (Leeder 1982 1988) and the Midland smaller and residence time of marine waters longer than on Valley Coalfield as a strike-slip basin following an earlier the upper coastal plain. This will reduce the time available phase of extension (Read 1988), although it should be noted for mire development particularly as saline groundwaters that Stedman (1988) considers the Midland Valley Coalfield will restrict the development of coal-forming flora (Read & to have formed under an extensional tectonic regime. Forsyth 1991) until pore waters have been replaced by Each marine band within the Westphalian has a freshwater through precipitation and fluvial recharge. distinctive faunal assemblage which allows detailed correla- Consequently, it is likely that the resulting lower delta plain tion between the coalfields of Britain (Ramsbottom et al. coal seams will be thin. The limited time available for mire 1978). Consequently strata of equivalent age to the development meant that raised mires were not developed as productive measures of the South Wales Coalfield can be the characteristic miospore assemblage of raised mires precisely identified in other basins. In both the Midland (Densosporites spp.) was largely absent from seams Valley and Pennine Basins mid Westphalian A to early deposited on the lower coastal plain (see Smith & Westphalian C sediments are characterized by thick,

PENNINE BASIN

SOUTH NOTTS.- YORKS. LANCS. CUMBERLAND DURHAM MIDLAND WALES DERBYS. VALLEY

~///H//////H/////,M////H/////////H///

VANDERBECKEI * , ~ ~ ; ~ ~

• * * I ~"~ t*t } : I UPPERCOASTAL / : ] "-.L. ~ i " i _.__...-----q~M---q DELTA PLAIN , , • • --M--~ • _ ,,

A E ~ ~ ~ E ! f [

M ~ .... tll M . /// ......

--M, ~M: ---- ~I I LOWERCOASTAL / -~ I ,,, 1 I I ~ L? "-~ DELTA PLAIN M ---M:

=_ • 0 --M- z M • i Z . 100 __--M ~- * Metres fitamarne 'estheria'ban band lingula band 200 non marine bivalve band 3OO

--M____..~

--M-- Fig. 12. Comparison of stratigraphic sequences from the Westphalian A and B of the South Wales, Midland Valley and Pennine Coalfields. Midland Valley based on Lumsden & Calver (1958) and Read (1988), Pennine Coalfield in general based on Ramsbottom et al. (1978), position of bivalve horizons in Yorkshire and Nottinghamshire after Edwards (1951), Cumberland after Taylor (1961), Durham after Smith & Francis (1967) and in Lancashire after Wright et al. (1927), Tonks et al. (1931), Jones et al. (1938) and Earp et al. (1961). Note the position of bivalve horizons above coal seams.

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extensive (102-103 km 2) coals commonly overlying the infill horizons are taken to represent the onshore or upper deposits of shallow lakes (Guion & Fielding 1988) and delta/coastal plain equivalents of a more basinward or lower overlain by bivalve horizons. These have been interpreted as delta/coastal plain marine band development. upper delta plain deposits (Guion & Fielding 1988; Read (4) Idealized cyclothem development can be related to 1988). Unfortunately, the bivalve faunas are not suitably water table fluctuations driven by eustatic changes. Mire diagnostic to permit more refined correlation of individual establishment was aided by a rise in water-table level with horizons between coalfields. However, the stratigraphic and decreased clastic input promoting plant colonisation. With sedimentological similarities between the Pennine, Midland increasing water-table levels raised mires developed. Mire Valley and South Wales basins suggests that the growth was terminated by flooding as the water-table rise development of extensive coal seams in the Pennine and outpaced mire growth. This resulted in basin-wide, Midland Valley Coalfields was significantly influenced by brackish-freshwater lake development with communities of eustatically-generated water table rises. It should be noted bivalves and extremely low clastic input. Subsequent that, like the South Wales Coalfield autocyclic controls, re-establishment of the clastic supply resulted in the fault-related subsidence and inherited basement topography formation and infilling of lakes and death of bivalve significantly affected the development and distribution of communities. This phase, which is characteristic of the coals and bivalve horizons in the Pennine (Fielding 1984b; coarsening-upwards part of the parasequence, took place Guion & Fielding 1988) and Midland Valley Coalfields while water table levels, clastic supply and subsidence rates (Forsyth & Brand 1986; Read 1988). Fielding (1984b) and were roughly balanced. An increase in water table level Guion & Fielding (1988) considered the major controls on marks the onset of the succeeding cycle. Idealized para- Westphalian upper delta plain sedimentation in the Pennine sequences are rarely developed as water table rises were Basin to be tectonic activity, and progradation and switching superimposed on a pre-existing topography resulting from of deltaic systems. In contrast the work of Elliott (1968 local and regional controls on sedimentation. 1969) and Rippon (1984, 1985) emphasized the role of (5) The Pennine and Midland Valley Silesian Coalfields autocyclic sedimentary processes over synsedimentary have closely similar facies developments to South Wales. tectonism in controlling facies geometries in the Pennine Thick, basin-wide coals top coarsening-upwards cycles and Basin. In both cases however, eustatic fluctuations were in turn are overlain by extensive bivalve horizons. Like considered to be of only minor importance. In contrast, it South Wales, these cycles are thought to reflect eustatically- appears that, as in South Wales, upper delta/coastal plain driven water table rises, in addition, the presence of lateral sedimentation (including delta lobe progradation and facies changes suggests that water-table rises were switching) in the Pennine and Midland Valley Coalfields was superimposed on a low relief (up to a few metres at most) largely controlled by small-scale eustatic fluctuations. pre-existing topography. On a larger scale all three basins show virtually (6) The Pennine, Midland Valley and South Wales synchronous changes from lower to upper delta plain Coalfields all show a synchronous change from lower to sedimentation in the mid Westphalian A with a subsequent upper delta/coastal plain deposition in the mid-Westphalian change back to lower delta plain sedimentation in late A and return to lower delta/coastal plain deposition in the Westphalian B times (Fig. 12). These synchronous changes late Westphalian B. This synchronous change in three suggest a long term eustatic control on lower and upper separate basins reflects a long term eustatic control on delta/coastal plain sedimentation in the Westphalian deposition. A tectonic control on cycle development can be particularly as these eustatic signatures are developed in discounted as the three basins were developed in markedly basins with different tectonic regimes. different tectonic settings (compressional in South Wales, extensional in the Pennines and strike-slip and/or extensional in the Midland Valley). In conclusion, both small- and large-scale eustatic Conclusions fluctuations superimposed on variable topographies either (1) Deposition during the productive measures interval inherited or related to active tectonism were the primary (mid-Westphalian A to late Westphalian B) of the South control on environment and facies development in the Wales Coalfield took place on an upper coastal plain Silesian of Britain. Delta lobe progradation and switching characterized by mire, seatearth and lacustrine facies with previously thought along with major tectonic events to be less frequent flood facies and only limited small-scale (80 m the principal controls on facies distribution in the British wide) fluvial channel development. Coal Measures are a function of glacio-eustatic fluctuations. (2) Facies are organized into vertical cyclothems or parasequences which record a change from coal to brackish/freshwater bivalve-bearing lacustrine mudstones to lacustrine siltstones to flood deposits to seatearths and back This work was supported by a NERC special basin dynamics grant. to coals. Thanks to the Cardiff Coalfield Research Team for field assistance (3) Controls on sedimentation were operative at different and discussion. R.A. Gayer, S. Leigh and S. Flint commented on spatial scales: (a) local-scale controls including autocyclic an earlier draft of this work. British Coal kindly allowed access to sedimentary processes and subsidence patterns, (b) regional currently working opencast sites and unpublished data. Discussions scale including synsedimentary growth faults and folds, and with R. Thewlis (British Coal) helped to clarify the validity of coal deposition over inherited basement topography, and (c) seam correlations in South Wales. Thanks to B. Fulton for drafting. basin-wide scales including glacio-eustatic fluctuations The comments of I. Fulton and J. Rippon (British Coal) on earlier operative on short and long term time scales. Eustatic draft were appreciated although they do not necessarily share the controls are inferred from the basin-wide development of interpretations presented here. G. Kelling and an anonymous major coal seams and overlying bivalve horizons. Bivalve referee are thanked for their comments.

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Received 13 April 1992; revised typescript accepted 20 February 1993.

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