The Westphalian succession of the Glyn-neath area, South Wales: an internationally important geological resource

The Westphalian succession of the Glyn-neath area, South Wales: an internationally important geological resource Benjamin G. Evans', Christopher J. Clealt, Barry A. Thomas" & W. A. P. Wimbledon3 EVANS, B. G., CLEAL, C. J., THOMAS, B. A. & WIMBLEDON, W. A. P. 2003. The Westphalian succession of the Glyn-neath area, South Wales: an internationally important geological resource. Proceedings of the Geologists' Association, 114, 291-305. The Glyn-neath area has the only natural exposures in the whole of Europe showing an essentially continuous section through the lower and middle Westphalian Stages (Pennsylvanian, Upper Carboniferous). There is a change from the littoral and lower delta-plain deposits of early Westphalian age, through the development of fluvio-Iacustrine, coal-bearing deposits that make up most of the Productive Coal Formation, to the coarse alluvial clastic sediment of the South Wales Pennant Formation. The geology of the Westphalian Series is important because of the presence of economic coal deposits and its bearing on the Variscan evolution of Europe. This succession represents the closest analogue for the 'icehouse' conditions of today, when there were extensive polar ice and extensive tropical forestation. The Glyn-neath exposures provide a unique resource for the study of a critical part of the geological evolution of Europe and for helping to understand environmental changes occurring today. 1 Department of Biodiversity and Systematic Biology, National Museums and Galleries of Wales. Cathays Park, Cardiff CFlO 3NP, UK 2Institute of Rural Studies. University of Wales Aberystwyth, Llanbadarn Fawr, Aberystwytli SY233AL, UK 3Countryside Council for Wales, Unit 4 Castleton Court, Fortran Road, St Mellons, Cardiff crs OLT, UK

1. INTRODUCTION

Pennsylvanian (Late Carboniferous) coal-bearing strata occur between Britain and Poland in a broad belt across western and central Europe, which formed part of the Variscan Foreland (Gayer et al., 1993). Because of the economic importance of these coals, the geology of these deposits has been extensively described, mainly from borehole data and mine workings. With the decline in exploitation of these coals, the study of such anthropogenic exposures is becoming more difficult and presents a real difficulty for developing ideas on the Pennsylvanian evolution of Europe. Only in Britain can alternative natural outcrops be found. A survey of the British outcrops of Pennsylvanian strata found the best exposed section through the lower and middle Westphalian Series to be along the Cwm Gwrelych-Nant Llyn Fach streams in South Wales (C1eal & Thomas, 1996). The site has been designated as a Site of Special Scientific Interest and is now a candidate for the lUGS World Geosites inventory (Cleal & Thomas, 1998). Despite this, the stratigraphy of the site is not fully known. Generalized logs of parts of this sequence were published in Robertson (1932) and parts of the site were mentioned briefly by Barclay et al. (1988). Leitch et al. (1958) published a detailed Proceedings of the Geologists' Association, 114, 291-305.

stratigraphical description of the lowermost part of the succession, while the higher parts of the succession are described in unpublished theses by Matthews (1955) and Thomas (1967). The Countryside Council for Wales consequently funded a two-year investigation into its geology and the geoconservation potential of the site (Evans et al., 2000, 2001) and this paper gives the stratigraphical results of the work. 2. LOCALITY DETAILS AND GEOGRAPHICAL SETTING

The exposures lie near the small town of Glyn-neath, 40 km northwest of Cardiff (Fig. 1). They occur below the Rhigos escarpment, close to the northern edge of the South Wales Coalfield, where streams on the north-facing side of the Neath Valley flow along deep gorges and ravines that are the result of dramatic glacial and fluvial erosion. The most complete section is exposed in the Cwm Gwrelych and Cwm Ceffyl valleys, along which the Nant Gwrelych and Nant Llyn Fach streams run [Ordnance Survey Grid Reference SN 896059]. This was supplemented, especially for the higher parts of the sequence, by sections in Cwm Gwrach [SN 871049] and Blaengwrach [SN 872 053]. Much of the area was worked as part of an opencast coal-extraction site, but the land and rock 0016-7878/03 $15.00

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2003 Geologists' Association

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Fig. 1. Simplified geological map of area south of Glyn-neath, showing the Cwm Gwrelych-Nant Llyn Fach stream section, and the location of the principal marine bands and coal seams. Topographic details after Unitary Development and Minerals Planniing Map, Neath Port Talbot County Borough Council, used with permission of Neath Port Talbot County Borough Council and Celtic Energy Ltd.: geological detail after independent mapping by the authors.

exposures immediately adjacent to the streams were retained to allow continued drainage of the area. The worked land has subsequently been backfilled,

re-profiled and forested. Access to some parts of the site, especially in the upper parts of the sequence, can be difficult due to the dense forestation.

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TH E WESTP HALIAN OF THE GLY N - N E A TI-I A REA. S . WAL E S

Th e South Wales Coalfield. which covers some 500 krrr', is part of a struct urally elongate forelandbasin formed between the Wales-Braban t Massif and the north ward-mi gratin g Va riscan Fr ont (G ayer & Jones, 1989: Jones, 1989, 1991: Hartley & Warr, 1990; Gayer et al., 1993). There is an essentially continuous sequence here th rou gh the entire Westph alian Series, up into the basal Stephanian (Thomas, 1974; Cleal, 1978; Cleal & Thomas, 1996). T he geology of the Gl yn-n eath area is compl ex. A narrow belt of intense fold ing and faulting (the Nea th D istu rbance) runs through Pon t Walby (Owen, 1953, 1974; Owen & Weaver. 1983). Th e Mississippian Carboniferous Limestone to the north of the Disturbance is highly folded, e.g. at Craig-y-Dinas (Owen & Weaver, 1983). To the south of the Disturbance, where the sections described in this paper lie, the effects are less immediately dramatic and the Westphalian coalbearing strata dip gently southward. Several small th rust fault s have been identified on older Geological Survey maps and there is histo rical mine documentat ion to suggest that some of the thru sts are assoc iated with co mplex folding. Evidence of such deformati on occurs to the west of the main section, in the Blaengwrac h area, and in the westerly cutti ng at the Selar opencast site. Similar Vari scan deformat ion has also been repor ted elsewhere in Sou th Wales (e.g. Cole et al., 1990; Jones, 1991).

3. STRATI GRAPHI CAL NO MENCLATURE Lithostratigraphy The coa l-bearing sequence in the South Wales Co alfield was first classified lithostrat igraph ically in earl y Geological Survey memoirs (see Cleal & Th omas. 1998 for details). Th ese divided the Pennsylvan ian succession in South Wales int o four unit s: the Millstone Grit. Lower Coa l 'Series', Pennant ' Series' and Upper Coal 'Series'. Stubblefield & Trotter (1957) suggested a fund amentally different approach in which the succession is divided at marine bands ident ifiable throughout most of the British coalfields (many are also found in coalfields throughout the Variscan Foreland). Alth ough their divisions have since been used on the geological maps of South Wales and, thu s, appear to be lithostrat igraphical, the boundaries are located at horizons that are effectively isoch ronous over very wide areas. The division s are conseq uently more chro nos tra tigraphical than lith ostrat igraph ical, as they do no t really reflect the lithological variatio n within the coa lfields. In order to revert to a more str ictly lithostratigraphical approac h, Cleal & Th om as (1998) introduced a refined version of the lith ostratigraph y used in South Wales prior to Stubblefield & Trott er (1957), with forma lly defined formation s. In this scheme, the Gl ynneath succession includes the top part of the Middle Sha les Fo rmation, all of the Fa rewell Rock and Productive Coa l forma tions and the lower part of the

Table I. Lithost rat igraph ical classification of the Pennsylva nian successio n in the main part of the So uth Wales Coalfield (i.e. exclud ing Pemb rokeshire). Groups

Forma tions

Members

G rovesend

South Wa les Penn an t

Swansea H ughes Brithd ir Llynfi

Productive Coal

Uppe r Ma rine Midd le Coa l Amman Coa l Lower Coa l Iro nstone Lower Marine

Coa l Measures

Millstone G rit

Fa rewell Rock Middl e Shales Basal Grit

The group s and formations are as proposed by C1ea l & Thomas (1998). The membe rs of the Sout h Wales Pennant For mation essentially follow Wood land et a!' (1957). The members for the Product ive Coa l Formation are as defined in the present pa per.

South Wales Penn ant F ormation. As it is the only essentially complete and well-exposed section thro ugh the Produ ctive Coa l Formation, Cleal & T homas (1998) proposed the Cwm G wrelych- Nant Llyn Fach section as its stra totype. Cleal & Th omas (1998) recognized members within the South Wales Penn ant Form ation based on ' Beds' defined by Woodland et al. (1957). Th ey also suggested that members could be recognized within the Productive Coa l Forma tion based on the ' Lower Coa l Measures' and 'Middle Coal Measures' division s of Stub blefield & Trotter (1957). Th is pape r now proposes a new set of members, of more clearl y lithostrat igraphical character, for which the Cwm GwrelychNant Llyn Fac h section is proposed as the stra trotype. Table Isumma rizes the lithostratigraphy used here. Marine band s have, historically, proved important for Westph alian correlations over much of the Variscan Forel and (Calver, 1968). Ramsbottom et al. (1978) prop osed a unified nomencl ature for these marine band s for use throughout the British Isles, based largely on the correlations suggested by Ca lver (1968). Whilst these correlations have not been seriously cha llenged, they are based on assumptions that are difficult to prove unequivocally. Th e traditional names used for these marine band s in South Wales have, therefore, been retained (Table 2 shows the correlation between this and the Ram sbottom et al. (1978) scheme). Chronostratigraphy There is continuing debate concerning the chro nostra tigra phical nomencl ature for the Pennsylvani an

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Table 2. Likely equivalence between the names used for the Westphalian marine bands in this paper and those of Ramsbottom et al. (1978).

South Wales names Upper Cwm Gorse Lower Cwm Gorse Five Roads Foraminifera Cefn Coed Brittanic Havod Heulog Amman

Ms M4 M3 M2 M, Subcrenatum

Ramsbottom et al. (1978) names Cambriense Shafton

4. DESCRIPTION

The Cwm Gwrelych-Nant Llyn Fach site provides an almost completely exposed section from the upper Yeadonian Stage (upper Namurian Series) to the upper Bolsovian Stage (middle Westphalian Series) (Figs 2-4).

} Edmondia Aegiranum Sutton Haughton Vanderbeckei Amaliae Parkhouse Listeri Honley Springwood Subcrenatum

Subsystem (e.g. Menning et al., 2000, 2001). More than at almost any other time before the Pleistocene, there was major latitudinal provincialism in both faunas and floras and this has significantly hampered attempts at making global biostratigraphical correlations. The main floral and faunal belts (Gondwanan or southern boreal, palaeoequatorial, and Angaran or northern temperate) tend to contain mutually exclusive floras and faunas (Cleal, 1991). Attempts to identify 'mixed' faunas and floras to facilitate correlation between the belts have generally failed or, at best, have produced very generalized results. Consequently, although there continue to be attempts to produce globally-applicable chronostratigraphical schemes (e.g. Heckel & Villa, 1999), most practical stratigraphical work is still mainly based around regional chronostratigraphies. In western and central Europe, the Heerlen Classification is generally used (named after the location of the early International Congresses on Carboniferous Stratigraphy where the scheme was first developed; Jongmans, 1928; Jongmans & Gothan, 1937). This divided Pennsylvanian strata into three stages, later reclassified as series: the Namurian, Westphalian and Stephanian. The subdivisions of these units are now referred to as stages. The strata seen in the Glyn-neath sections belong to the lower three divisions of the Westphalian, now referred to as the Langsettian, Duckmantian and Bolsovian stages (formerly known as the Westphalian A, Westphalian B and Westphalian C substages). Wagner (1974) and Wagner & Winkler Prins (1994) give a more complete review of the Heerlen Chronostratigraphy. Ramsbottom et al. (1978) introduced a set of chronozones for the Westphalian stages based on non-marine bivalves biozones. but these have not been widely adopted and are not used here. We use the term 'zone' here to refer exclusively to biozones.

Middle Shales Formation

The lowest part of the sequence comprises 6 m of medium to dark grey shales of the Subcrenatum Marine Band (Fig. 2), which is relatively well exposed alongside the left-hand bank of the pathway, downstream from the viaduct [SN 8908 0645]. This band marks the base of the Langsettian Stage and, consequently, of the Westphalian Series. In the scheme adopted by the British Geological Survey (Stubblefield & Trotter, 1957) it is included within the Coal Measures. According to Cleal & Thomas (1996), this marine band is an integral part of the Middle Shales Formation. The mudstone here represents only the lower transgressive phase of the marine incursion, the upper regressive phase having been cut out by the overlying Farewell Rock Formation. Consequently, the fossil content of the marine band is here relatively low compared with many other localities, where the Farewell Rock has not cut down as far in to the underlying shales (Bloxham & Thomas, 1969; Bloxham, 1974). Nevertheless, Leitch et al. (1958) reported a diverse fauna from here, including Lingula, Derbvia, Productus, Edmondia, Nuculopsis, Schizodus. Euphemites and the goniatites Gastrioceras subcreantum (Frech), G. aff. listeri (Sowerby), and Anthracoceras arcuatilobum (Ludwig). Many of these names are clearly out of date and the fauna is in need of revision, but this is beyond the scope of the present paper. Farewell Rock Formation

The Farewell Rock Formation, which immediately overlies the Subcrenatum Marine Band, is 52 m thick in this section (Fig. 2). The formation is generally composed of grey coarse-grained massive sandstones, which weather to an orange colour, good exposures of which occur to the left and right of the pathway along Cwm Gwrelych [SN 8919 0636]. These massive sandstones are occasionally interrupted by thin, finely bedded muddy siltstones that tend to be less well exposed due to coverage by scree and vegetation. The sandstones themselves display a range of internal features, ranging from trough cross-bedding and climbing ripple lamination to slump structures. Palaeocurrent measurements indicate that the sandy detritus was derived from the northeast. The lowest bed is a coarsening-upwards unit, similar to the distributary mouth-bar deposit seen at the nearby

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Productive Coal Formation (i) Lower Marine Member Immediately above the Farewell Rock are plantbearing shales referred to by Leitch et at. (1958) as the P z plant bed (Fig. 2). They can be seen on the southeast bank of the river [SN 8911 6333] and in a steep tributary that falls precipitously from the old British Rhondda Colliery site [SN 8917 0588]. Between the base of this plant bed and the C 1 non-marine bivalve band are 59 m of mainly argillaceous deposits. This part of the sequence includes significant evidence of marine influence, including five marine bands, and so is referred to as the Lower Marine Member (base defined at the base of the P z plant bed). The Lower Marine Member is well developed in the Cwm Gwrelych tributary section where it is exceptionally well exposed [SN 8913 0629]. The fivemarine bands (M ,-Ms. see Fig. 2) are regularly spaced and each is recognized by a distinctive horizon.

• M, includes a thin grit (known as the One Foot Sandstone) and has yielded Lingula. Orbiculoidea, Productus. Schizodus, Bucanopsis and fish fragments.

• M, is underlain by a thin coal, seatearth and a siliceous grit and has yielded Lingula, Marginifera and Productus. • M 3 has a pyritic layer of bioturbation and has yielded large Lingula, Productus, A viculopecten and Euphemus. • M 4 is underlain by coal and abundant siderite nodules and has yielded Lingula. • M, is overlain by a thin layer of granular quartz rubble (light in colour) and has yielded fish fragments.

The faunas listed (based on Leitch et al., 1958) are clearly in need of revision. The thickest band, M 3 , is equivalent to the Alton, Hard Bed and Bullion Mine Marine Bands in central and northern England (Listeri Marine Band sensu Ramsbottom et al., 1978; see Table 2). This represents the most extensive marine incursion during the early Langsettian other than that which produced the Subcrenatum Marine Band (Calver, 1968). Leitch et al. (1958) reported a diverse fossil flora from the P 2 plant bed at the base of the Lower Marine Member at Cwm Gwrelych, although none of the specimens were figured. The flora includes Lyginopteris hoeninghausii (Brongniart) Gothan, Paripteris gigantea (Brongniart) Gothan, Karinopteris acuta (Brongniart)

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B. G. EVANS ET AL.

Boersma and Neuralethopteris neuropteroides (Susta) Josten (current morphogeneric names used). Dix (1934) also gives a general review of this flora. It is a typical macroflora of the Neuralethopteris jongmansii Subzone (lower L. hoeninghausii Zone) sensu Cleal (1991) and is evidence of an early Langsettian age (Cleal & Thomas, 1996). The roof shales of the Cnapiog Coal form the topmost bed of the Lower Marine Member. They are a distinctive metallic blue in colour and often contain abundant fish remains. Unfortunately, a landslide has recently obscured this part of the succession in the main stream section, but an alternative exposure occurs in the roof of an adit in a nearby old drift mine. (ii) Ironstone Member Above the Lower Marine Member are 142 m of mainly lacustrine deposits that are referred to as the Ironstone Member (Fig. 2). Two non-marine bivalve bands, C, and C2 , occur in the lower part of this interval (Leitch et al., 1958; Eagar, 1962; Fig. 2) and the base of C, defines the base of the member. The rest of the member is dominated by highly laminated mudstones and shales, with numerous bands of nodular and tabular siderite ironstone, which often leach iron-mineralized waters. The growth of the siderite nodules often produced deformation fabric in the surrounding mudstones. Some of the thicker seams of ironstone worked commercially in the area have previously been named but, in the sections studied here, individual seams could not be recognized. Exposures of this member occur in the numerous abandoned mines and quarries in the area, the best of which occur to the northwest of the main footpath, 1 km upstream from the viaduct and just west of the old, re-profiled Rhigos opencast site [SN 9024 0620]. The member is disturbed midsequence by several faults, one of which runs NNESSW and causes tight anticlinal folding but no loss of section. Fossils are not particularly abundant in this member except for two non-marine bivalve bands in the lower part of the member (C, and C 2 ; see Fig. 2). The fauna at Cwm Gwrelych from C, has been studied by Eagar (1962), who reported species indicating the Carbonicola proxima Subzone (uppermost C lenisulcata Zone), including Carbonicola pontifex Eagar, for which this is the type locality. The C 2 species listed by Leitch et al. (1958) indicate the Carbonicola torus Subzone (basal C communis Zone). Hence, the C lenisulcata-C communis zonal boundary must lie between these two bands. Robertson (1932) listed a fauna from Cwm Gwrelych (in siderite from the middle of the member) indicating the C communis Zone, but not a particular subzone, and a more diagnostic fauna from Nant Hir (from shales above the Cnapiog Coal) indicating the C pseudorobusta Subzone (upper C communis Zone). Several bands with wel1-preserved and extensive stigmarian rooting-structures, along with broken plant fragments of unidentifiable origin, occur, especial1y in

the upper part of the member. Ironstone nodules centred on these smal1 fragments are common. (iii) Lower Coal Member The base of the Lower Coal Member is defined at the base of the lowest seam of the so-called 'Bluers Group' of coals, i.e. 12 m below the Grey Coal (Fig. 2). It corresponds to the upper part of the Lower Coal Measures sensu Stubblefield & Trotter (1957) and includes the stratigraphical1y lowest set of economical1y significant coals in the coalfield (e.g. the Grey, Rhyd, Upper and Lower Bluers, and Peacock Coals). This member is best observed in the Rhyd-y-glo tributary and in the main Nant Gwrelych riverbed. Exposure is relatively uninterrupted and accessible, except for the cascades formed by the prominent sandstones during periods of rainfal1 and high river-levels. There is some evidence of residual lacustrine conditions in the lower part of the member, including some sideritic ironstone above the Grey Coal and a non-marine bivalve band below the Upper Bluers Coal. The member largely represents a change to a fluvial-dominated depositional regime. There are 152 m of typical coarsening-upward cycles, each cycle being terminated by a coal. The coals here are wel1developed and clearly defined units, regularly spaced and unaffected by the localized complex folding and faulting that affect other parts of the succession. There are also numerous seatearths without coals in the middle part of the member, whilst in the higher beds there are numerous thin, but relatively persistent, coals with very thin seatearths. The only non-marine bivalves found are from a bed first reported by Robertson (1932) between the Upper and Lower Bluers Coals. Matthews (1955) recorded Carbonicola cf. venusta Trueman & Weir from here, indicating the basal Anthracosia modilaris Zone. Matthews also listed plant remains from several beds within this member. Unfortunately, the whereabouts of his col1ection is unknown and none of the specimens were figured. Clearly, some of the identifications are in error (e.g. Alethopteris serlii (Brongniart) Goppert, which does not occur below the upper Westphalian D), but his lists seem to indicate the Laveineopteris loshii Subzone (upper Lyginopteris hoeninghausii Zone). This would support a late-Langsettian age for these horizons. Dix (1934) listed a similar flora from below what she cal1ed the Lower Yard Coal (probably the Peacock) in a stream-section on the opposite side of the Neath Val1ey. (iv) Amman Marine Member The Amman Marine Band occurs throughout the district and marks the boundary between the Langsettian Stage and Duckmantian Stage. The Amman Rider coal occurs immediately below the marine band and the base of the member is defined at the base of that coal. The Amman Marine Band represents the only marine incursion over the Variscan Foreland during the middle Langsettian to middle Duckmantian. In

THE WESTPHALIAN OF THE GLYN-NEATH AREA, S. WALES

many places, it yields a diverse fauna (Calver, 1968), but only inarticulate brachiopods were found in the current study. Bioturbation is apparent, with many pyritized bifurcating-traces in both vertical and horizontal orientation. Marine muds in the upper part of the band are highly laminated and pyrite rich, giving them a distinctive grey/yellow appearance. This member is well exposed in Nant Llyn Fach just above the confluence with the Nant Gwrelych. It is best accessed from the Selar opencast access road [SN 9051 0529], where the Llyn Fach stream passes through a culvert. (v) Middle Coal Member This member is equivalent to most of the Middle Coal Measures sensu Stubblefield & Trotter (1957) and contains many of the economically important coals in South Wales, including the Brass, Nine Feet, Cornish, Six Feet, Four Feet and Two Feet Nine Coals. The member represents a return to deposition in relatively stable fluvio-deltaic conditions without marine transgressive phases. The sediments between the coals are mainly crevasse-splay and overbank deposits formed in a floodbasin-lake environment. There are also some retrograde fining channel-fill sandstones with an erosive base, especially above the Nine Feet and Two Feet Nine Coals. The base of the member is defined at the base of the shale immediately underlying the Brass Coal. This represents the lowest fully non-marine beds above the Amman Marine Band. It includes most of the Duckmantian Stage within the sequence and, apart from the small amount of faulting in the section directly above the Amman Marine Band, it is relatively complete. Obscured exposures make it difficult to give an exact thickness here, but it is probably about 260 m. The best exposures of this member are in the Nant Llyn Fach above the Selar opencast access road. Passage can be made upstream (i.e. southwards) but should not be attempted during high rainfall when the river levels may rise suddenly. Exposure is intermittent with mining-spoil and occasional culverts obscuring the exposure. An alternative equivalent temporary section is available immediately to the west in the Selar opencast site. Non-marine bivalves are relatively uncommon in this member. The most distinctive band is of shales immediately overlying the Amman Marine Band, from which Matthews (1955) reported Anthracosia ovum Trueman & Weir, A. planitudima Trueman, A. aquilinoides (Tchernyshev) and A. cf. concinna (Wright). This assemblage indicates the A. ovum Subzone (upper A. modiolaris Zone). Similar faunas have been reported from this level in many parts of South Wales (Jenkins, 1962; Woodland & Evans, 1964; Archer, 1968). A second non-marine bivalve fauna was reported by Robertson (1932) from the roof shales of the Cornish Coal in Cwm Ceffyl, but the nomenclature used is clearly out of date and the assemblage needs revising. The bed yielding this fauna could not be located in the current study.

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Plant fossils are particularly abundant between the Brass and Nine Feet Coals, and we found six distinct plant-beds. Dix (1934) reviewed the floras from above the Brass Coal throughout South Wales but, unfortunately, she did not separately list the species found in each locality. Those listed include Laveineopteris tenuifolia (Sternberg) Cleal et al., Paripteris gigantea (Potonie) Gothan, Sphenophyllum cuneifolium (Sternberg) Zeiller and Mariopteris muricata (Brongniart) Zeiller, which can only be described as probably early Westphalian in age. However, Dix also mentions rare specimens of Renaultia rotundifolia (Andra) Zeiller, which is unknown below the Duckmantian Laveineopteris hollandica Subzone (lower Lonchopteris rugosa). The roof shales of the Nine Feet Coal also yield plant remains. According to Dix (1934) these shales generally yield one of the most diverse floras in the middle Westphalian of South Wales. She noted that this flora occurs in the Tawe and Amman valleys and the areas near Cross Hands and Maesteg, as well as the Neath valley, but (as with the Brass Coal flora) she did not separately list the species from the different localities. The assemblages are broadly similar to those found above the Brass Coal but significantly also include Paripteris pseudogigantea (Potonie) Gothan and Sphenophyllum majus (Bronn) Bronn), whose lowest occurrence mark the base of the S. majus Subzone (upper L. rugosa Zone), indicating the upper Duckmantian Stage. Plant beds associated with the Cornish, Six Feet and Four Feet Coals were also found, but they will need extensive excavation before they can be properly sampled. (vi) Upper Marine Member The base of this member is taken at the base of a major sandstone unit known as the Lower Cockshot Rock (Fig. 3). It represents a return to lower delta-plain conditions and, although there are some coals, the most characteristic deposits are marine bands and thick channel-sandstones. Five marine horizons have been identified in this part of the succession. The stratigraphically lowest marine band, the Havod Heulog, lies 13 m above the Lower Cockshot Rock and consists of about I m of dark grey laminated mudstone with a greasy appearance [SN 9055 0454]. The deposits are pyritic and bioturbated and contain occasional bivalves. The Cefn Coed band is less well exposed here, partly because of faulting and partly because of conifer cover. All that can be seen here is 1.5 m of blue/grey marine shales exposed above a fault [SN 9053 0454] but, in the neighbouring Selar opencast mine and a small private mine to the southeast, the Cefn Coed Marine Band consists of about 0.5 m of dark grey pyritized mudstones containing plant, crinoid and inarticulate brachipod remains. This is an important horizon in the succession because it marks the boundary between the Duckmantian Stage and Bolsovian Stage.

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Four further marine bands were found above the Cefn Coed Marine Band. Although none yielded diagnostic faunas, by comparing their relative positions with the successions of marine bands in other British Coalfields (e.g. Ramsbottom et al., 1978) it may be assumed that they are the Foraminifera, Five Roads, Lower Cwmgorse and Upper Cwmgorse Marine bands. In the Cwmgwrach section [SN 87370330] the Foraminifera Marine Band occurs about 42 m above the Cefn Coed Marine Band and consists of 2 m of irregularly bedded, probably bioturbated, mudstone with much weathered pyrite but no fossils. About 14 m higher in the section, another similar band of pyritic shale occurs, which probably correlates with the Five Roads Marine Band recognized by Archer (1968) in the Gwendraeth Valley. If correctly identified, this will be the first record of this marine band in South Wales. The top surface is uneven and has been subjected to erosion by the overlying fluvial cross-bedded sandstone. Above this sandstone is the Lower Cwmgorse Marine Band, which is a 0.4 m thick interval of mudstone containing bivalves and crinoid ossicles [SN 9050 0427]. The highest marine band in the sequence, the Upper Cwmgorse, is at least 6 m thick (the upper part not being found) and consists of alternating marine mudstone/shales (laminated and light grey in colour) and occasional siltstones (dark grey) [SN 9050 0417]. The fossil content is low and what little material is found comprises broken shell material and poorly preserved burrows. The channel-fill deposits consist of both finingupwards and coarsening-upwards sandstones, often with cross trough-bedding and/or ripple marks. These sandstone bodies usually have erosive bases, with load casts where they overlie mudstones. The most significant channel-fills are the Lower Cockshot Rock (c. 20 m thick), the Upper Cockshot Rock (c. 10 m thick) and an unnamed sandstone above the Cefn Coed Marine Band (c. 25 m thick). This latter sandstone has casts of large lycopsid trunks preserved in a vertical orientation. Shales with thin coals «50 mm thick) are mainly found in the interval between the Cefn Coed and Foraminifera marine bands. The stratigraphically lowest seam, the Gorllwyn Coal, is only represented in the section by its seatearth, visible in the brick-lined adit to the west of the stream [SN 9051 0469], but it is also exposed in the adjoining Selar opencast site [SN 88440492] where it is worked as a three-leaf coal. Other, relatively thick, seams can be tentatively identified by their position in the sequence. A coal between the Lower and Upper Cwmgorse Marine Band is 0.35 m thick and, based on this thickness and its position in the sequence, it has been tentatively correlated with the Welsh Seam. The predominance of marine conditions in this part of the sequence means that non-marine bivalve and plant remains are rare (Robertson, 1932; Dix, 1934).

South Wales Pennant Formation

(0 Llynfi Member Above the Upper Cwmgorse Marine band, there is a significant change in the succession at the base of the South Wales Pennant Formation. The lowest part belongs to the Llynfi Member, whose base is defined at the lowest thick sandstone above the Upper Cwmgorse Marine Band (Fig. 4). This member is only accessible from the forest road that begins above Tower Colliery [SN 9286 0434] and runs westward through the forest where the Nant Llyn Fach [SN 9054 0407] can be traced downstream. Exposure here is intermittent due to glacial drift and comprises massive sandstone crags and cascades. The Llynfi Member is only c.90 m thick, which is a little low and there may have been some fault shortening. No significant faults (throw 30-40 m or more) were seen, but they are known to exist in several of the nearby workings (e.g. the Selar and Parnell opencast sites) and in the underground workings at the old British Rhondda and Empire collieries. The Llynfi Member mainly consists of immature coarse-grained sandstones with no marine intervals. Several coals occur within the member, but these have only occasionally been worked, as highly competent sandstones roof them. Such sandstones can provide extensive unsupported roofs, but can also cause expensive problems if faults or coal washouts are encountered. Previous studies have indicated a northwesterly flowing depositional system (e.g. Kelling, 1964, 1968; Jones, 1989) but this could not be verified. The sandstones are not particularly fossil rich, although they do contain much charcoal and occasional coal smuts; their lithology tends to be finer nearer to the coal seams and the seatearths are sphaerosidic. No fossil-bearing horizons were located and none have been reported in the literature. Elsewhere in South Wales, the Llynfi Member has yielded fossil macrofloras of the upper Paripteris linguaefolia Zone (Cleal, 1978). (ii} Rhondda Member The stratigraphically highest beds in this section belong to the Rhondda Member (Fig. 4), the base of which is defined at the base of the No.2 Rhondda Coal. At Craig-y-Pant [SN 8890 0368] good exposures occur in the cliffs and crags that run alongside the forest trackway, where more than 25 m of sandstones occur above the Rhondda No.2 Coal. The latter is thick and widespread in this area and has been exploited at numerous private opencast and drift mine sites in the area. The sequence comprises coarsening-upward cyclic deposition of sandstone and, commonly, quartzitic-conglomerate. Occasionally persistent mudstones, siltstones and uneconomic coals divide up the massive cross-bedded sandstone units. The fossil content of this formation is, again, very poor, comprising occasional plant material from

THE WESTPHALIAN OF THE GLYN-NEATH AREA. S. WALES

the mudstones and much charcoal. Another 80 m or more of the Rhondda Member occurs in the inaccessible vertical crags of nearby Craig Isaf, where the Rhondda No. I seam should crop out [SN 8845 0353]. No fossil-bearing horizons were located here and none have been reported in the published literature. Elsewhere in South Wales, the lower Rhondda Member has yielded fossil macrofloras of the upper Paripteris linguaefolia Zone (Cleal, 1978). 5. GEOLOGICAL INTERPRETAnON

The broad pattern of the geological evolution of South Wales during the early and middle Westphalian Epoch can be followed through the sequence exposed at Cwm Gwrelych-Nant Llyn Fach. As pointed out by Kelling (1974), the geological evolution of the South Wales Coalfield broadly falls into two phases, which are characterized by the Productive Coal Formation and South Wales Pennant Formation. During earliest Westphalian times, this area was part of a lower delta-plain that was regularly subjected to marine incursions (Leitch et al., 1958), resulting in the Middle Shales Formation and the Lower Marine Member. These marine incursions could have been caused by eustatic sea-level rise or to increased intrabasinal subsidence. In one example (represented by the Subcrenatum Marine Band) the rate of subsidence was such that marginal deep-water conditions prevailed, allowing free-swimming goniatites to pervade. In others, only brackish conditions developed. A fluvio-deltaic complex prograded southwards from the Wales-Brabant Massif during this time and produced the Farewell Rock Formation (Bluck, 1961). The rest of the Productive Coal Formation shows a change to middle and upper delta-plain conditions. There is no evidence of decreased subsidence at this time; if anything, subsidence rates were increasing (Burgess & Gayer, 2000), as a result of hinterland uplift as the Variscan Front moved northwards. The resultant succession of middle Langsettian to middle Bolsovian age exhibits predominantly non-marine fluvio-Iacustrine coal-bearing deposits with only rare marine bands (Kelling, 1974; Hartley, 1993a). Initially, conditions were relatively high energy, with welloxygenated bottom conditions (Barclay et al., 1988), which produced siltstones often containing bivalves, and the sideritic ironstone layers that characterize the Ironstone Member. There then followed a period of quiescence represented by the Lower Coal Member, which is dominated by lacustrine mudstones, siltstones and fine sandstones (Hartley, 1993a, b). They were the sediment infill of inter-distributary lakes from crevasse-splays and overbank flooding. Provided that subsidence kept pace with sedimentation, the lake would remain, but sediment-fill often outpaced subsidence of the lake, resulting in a period of emergence. If the emergence was short-lived, there may be little

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preserved other than a palaeosol marking the establishment of vegetation. Where the emergence was longer, the build-up of vegetation debris resulted in peat deposits that ultimately became converted to coal. Continuing subsidence eventually resulted in the area being flooded again, thus killing the vegetation and causing in the reintroduction of clastic sedimentation. This apparent cyclicity is typical of much of the Productive Coal Formation and is well-represented in the Cwm Gwrelych-Nant Llyn Fach section. Also present, however, are some units of coarser-grained sandstones. These mainly represent deposits from a range of high- and low-sinuosity river-channels that migrated over the contemporaneous floodplain and were the main distributary channels for this fluviodeltaic system. The Amman Marine Member signifies a major change in conditions with a widespread marine transgression. At Cwm Gwrelych it is only represented by brackish-water deposits, although elsewhere in South Wales more fully marine conditions developed (Barclay et al., 1988). This interval of marine conditions was short-lived and fluvio-Iacustrine conditions (including peat formation) soon returned, resulting in the Middle Coal Member. This consists of a series of clastic units culminating in a palaeosol and/or coal (essentially similar to the Lower Coal Member but with generally thicker coals). These thicker coals could be interpreted in terms of reduced basin subsidence, although the basin analysis of Burgess & Gayer (2000) gives no indication of this. Notably, many of the lacustrine sequences are cut by distributary channel sands. and coals often have washouts, indicating that uplift and erosion was still actively occurring in the hinterlands to the north. Late Duckmantian and early Bolsovian times brought a return of more marine-influenced conditions, represented by the Upper Marine Member. Six marine incursions are represented in this part of the Nant Llyn Fach section, albeit mainly as brackishwater deposits (elsewhere in Britain, another three marine bands have been identified in strata of this age; Ramsbottom et 01., 1978). The Cefn Coed Marine Band represents the most-extensive marine incursion, having been identified throughout much of the Variscan Foreland (Bless & Winkler Prins, 1972). The others are mostly only known from Britain (Bless et al., 1972). Other marine bands have been reported from contemporaneous strata in the Ruhr (Rabitz, 1967), but it has so far proved impossible to correlate them with the British marine bands. In certain other parts of the Variscan Foreland (e.g. the Ruhr), marine influence continued until the Westphalian D, but in South Wales it is terminated by a sudden change in facies represented by the South Wales Pennant Formation (as seen in the upper part of the Nant Llyn Fach section). This was the result of the progressive northwards migration of the Variscan Front, causing uplift of the area to the south of Wales.

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Erosion of this rising hinterland caused an influx of southerly-derived coarse-grained sediment that was deposited under proximal braidplain conditions (Kelling, 1974, 1988; Jones, 1989; Hartley & Warr, 1990; Gayer & Pesek, 1992; Hartley, 1993b). 6. COMPARISONS WITH OTHER AREAS

natural exposures show a lower Langsettian (or upper Namurian) to middle Bolsovian succession (e.g. Cobb et al., 1981; Blake et al., 1998,2002). Nevertheless, the succession is different from that seen in South Wales, in that the lower parts (Pocahontas and New River formations) are exclusively non-marine with relatively low-diversity floras. This contrasts with the contemporaneous (early and middle Langsettian age) strata seen at Cwm Gwrelych, which has significant marine influence and yields more diverse floras. The overlying Kanawah Formation (late Langsettian to early Bolsovian in age), on the other hand, contains numerous marine bands, whereas the contemporaneous strata in South Wales has a much more restricted marine influence (as represented by the Amman and Upper Marine members).

Research has so far failed to identify a comparable locality elsewhere in Europe where an essentially continuous succession can be seen through the lower and middle parts of the Westphalian Stage in natural exposure. Because of cover by post-Carboniferous rocks or by Quaternary drift there are no surface exposures of Westphalian rocks in Poland, northern Germany, the Netherlands, Belgium or northern France. When members of the lUGS Subcommission 7. DISCUSSION on Carboniferous Stratigraphy were seeking candidate stratotypes for the lower three Westphalian stages in the 'classic' areas of the Ruhr and Nord-Pas-de-Calais There is continuing interest in the geology of the Coalfields, the only available sections were in under- Pennsylvanian of Europe, especially of the Westphalground workings (e.g. Bouroz et al., 1969; Bless et al., ian Series. This is partly for economic reasons, since a better understanding of the conditions of formation 1972; Rabitz, 1977). In Britain, there are a few other documented natural of the coals should improve the efficiency of their exposures of Westphalian rocks (Cleal & Thomas, exploration and exploitation (Scott, 1987, 1991; 1996). Most of those found inland only show a short Whatley & Spears, 1995). In recent years, it has also stratigraphical interval, although coastal exposures been attracting the attention of palaeoecologists and show longer sections. For instance, the Pembrokeshire palaeoclimatologists. The Pennsylvanian was the only coast exposes a more-or-less complete Langsettian other time in the geological past when there was the and lower Duckmantian succession (Jenkins, 1962; combination of both extensive polar ice and wideWilliams, 1968; Kelling & George, 1971; George & spread tropical vegetation (e.g. Gastaldo et al., 1996). Kelling, 1982). It is possible to identify many of the The study of changes in global climates and environmarine bands in the Lower Marine Member of the ments during this time may help to improve underProductive Coal Formation. The Ironstone Member in standing of the underlying dynamics of comparable the main part of the South Wales Coalfield may be changes happening today. For instance, did declining equivalent to the Amroth Limestone and associated tropical vegetation in the early Stephanian cause meltstrata in Pembrokeshire (it yields abundant non- ing of polar ice, as suggested by Cleal & Thomas marine bivalves; Jenkins, 1960). The Lower Coal, the (1999)? What is the relationship between global temAmman Marine and part of the Middle Coal Members perature changes, eustatic sea-level changes and floodcan also all be recognized in Pembrokeshire but, since ing of coastal plains, as discussed by Ramsbottom Pembrokeshire is much nearer the Variscan Front than (1979)? Is there any relationship between tectonic is Cwm Gwrelych-Nant Llyn Fach, the strata have activity, erosion and atmospheric CO 2 levels, as consequently suffered more severe tectonic disruption. suggested by Dorn & Brady (1995)? The economic importance of Westphalian strata has The middle Duckmantian and Bolsovian parts of the succession are not preserved in Pembrokeshire, nor is resulted in a long history of research, especially in the Farewell Rock Member fully developed (equivalent Europe and has generated an enormous scientific strata are less sandy deposits with slump structures; literature and extensive museum collections. Such historical sources of data are of great value in developing Kuenen, 1949). There are spectacular exposures of Westphalian geological studies on these rocks but, as argued by strata along the coast of northeastern England, such as Cleal & Thomas (2001), they are not enough: it is the Tynemouth-Seaton Sluice section (Land, 1974; essential that we continue to be able to examine the Haszeldine & Anderton, 1980; Haszeldine, 1983a, b, rocks in the field, both for research and for educational 1984) and the Creswell and Newbiggin section purposes. The principles of geology are only fully (Fowler, 1936), both reviewed by Cleal & Thomas appreciated by studying natural exposures. (1996). However, these only show Duckmantian strata and they are in a rather higher-energy facies than is ACKNOWLEDGEMENTS present in South Wales. The best comparisons are found in the Appalachi- We are grateful to the Countryside Council for Wales ans, where extensive road cuttings and numerous for funding this project as part of their International

THE WESTPHALIAN OF THE GLYN-NEATH AREA, S. WALES

Resource Assessment Programme. Special thanks also go to Celtic Energy and the Selar opencast site management for their co-operation throughout the project, in particular for allowing the collection of palaeobotanical specimens and study of mining records. The

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authors also thank Neath Port Talbot County Borough Council for providing them with the Unitary Development and Minerals Planning Map used in Fig. I. National Grid coordinates are used with kind permission of the Ordnance Survey.

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Memoirs of the British Geological Survey. HMSO, London. Barclay, WJ., Taylor, K, & Thomas, L.P. 1988. Geology of the South Wales Coalfield. Part V. The country around Merthyr Tydfil (3rd edn). Memoirs of the British Geologi

cal Survey. HMSO, London. Blake, B.M., Martino, R.L, Grady, W.C & Eble, CF. 1998. Coal geology, paleobotany, and regional stratigraphy of the middle part of the Kanawah Formation, southern West Virginia. West Virginia Geological and Economic Survey,

Morgantown. Blake, B.M., Cross, AT, Eble, C.E, Gillespie, W.H. & Pfefferkorn, H.W. 2002. Selected plant megafossils from the Carboniferous of the Appalachian Region, eastern United States: geographic and stratigraphic distribution. In (Hills, L.V" Henderson, CM. & Bamber, W.: eds) Carboniferous and Permian of the World. Canadian Society of Petroleum Geologists Memoir, 19, 259-335. Bless, MJ.M. & Winkler-Prins, CF. 1972. Paleoecology and paleogeography of the Aegir Marine Band and its equivalents in north-western Europe. Compte rendu 7e Congres International de Stratigraphie et de Geologie du Carbonifere ( Krefeld 197 I), 1, 231-239. Bless, MJ.M., Calver, M.A. & Josten, K.-H. 1972. Report of the working group on the Westphalian C in N.W. Europe. Compte rendu 7e Congres International de Stratigraphie et de Geologie du Carbonifere (Krefeld 1971), 1,223-230. Bloxham, T.W. 1974. Geochemical parameters for distinguishing palaeoenvironments in some Carboniferous shales from the South Wales Coalfield. In (Owen, T.R.; ed.) The Upper Palaeozoic and post-Palaeozoic rocks of Wales. University of Wales Press, Cardiff, 263-284. Bloxham, T.W. & Thomas, R.L 1969. Palaeontological and geochemical facies in the Gastrioceras subcrenatum marine-band and associated rocks from the North Crop of the South Wales Coalfield. Quarterly Journal of the Geological Society, London, 124, 239-281. Bluck, B. J. 1961. The sedimentary history of the rocks between the horizon of G. subcrenatum and Garw Coal in the South Wales Coalfield. PhD thesis, University of Wales (Swansea). Bouroz, A., Chalard, J., Corson, P. & Laveine, J.-P. 1969. Le stratotype du Westphalien C dans Ie bassin houiller du Nord et du Pas-de-Calais: limites et contenu paleontologiqucs, Compte rendu oe Congres International de Stratigraphie et de Geologie du Carbonifere (Sheffield, 1(67), 1, 99-105. Burgess, P.M. & Gayer, R.A. 2000. Late Carboniferous tectonic subsidence in South Wales: implications for Variscan basin evolution and tectonic history in SW Britain. Journal of the Geological Society, London, 157, 93-104. Calver, M.A. 1968. Distribution of Westphalian marine faunas in northern England and adjoining areas. Proceedings of the Yorkshire Geological Society, 37, 1-72.

Cleal, CJ. 1978. Floral biostratigraphy of the upper Silesian Pennant Measures of South Wales. Geological Journal, 13, 165-194.

Cleal, CJ. 1991. Carboniferous and Permian biostratigraphy. In (Cleal, CJ.; ed.) Plant fossils in geological investigation: the Palaeozoic. Ellis Horwood, Chichester, 182-215. Cleal, C.J. & Thomas, B.A. 1996. British Upper Carboniferous stratigraphy. Chapman & Hall, London. Cleal, CJ. & Thomas, B.A. 1998. European frameworks for the selection of Palaeozoic palaeobotany sites. Geologica Balcanica, 28, 59-66. Cleal, CJ. & Thomas, B.A. 1999. Tectonics, tropical forest destruction and global warming in the Late Palaeozoic. Acta Palaeobotanica Supplement, 2,17-19. Cleal, CJ. & Thomas, B.A. 2001. The conservation of ancient tropical rain forests. In (Hackaney, P. & Tyrie, C.R.; eds) Biological collections and biodiversity. Linnean Society, London, 9-26. Cobb, J.C, Chesnut, D.R., Hester, N.C & Hower, J.C 1981. Coal and coal-bearing rocks of eastern Kentucky. Kentucky Geological Survey, Lexington. Cole, JE, Miliorizos, M" Frodsham, K., Gayer, R.A" Gillespie, P.A" Hartley, A.J. & White, S.C 1990. Variscan structures in the opencast coal sites of the South Wales Coalfield. Proceedings of the Ussher Society, 7, 375-379. Dix, E. 1934. The sequence of floras in the Upper Carboniferous, with special reference to South Wales. Transactions of the Royal Society of Edinburgh, 57, 789-838. Dorn, R.T. & Brady, P.V. 1995. Rock-based measurement of temperature-dependent plagioclase weathering. Geochimica et Cosmochimica Acta, 59, 2847-2852. Eagar, R.M.C 1962. New Upper Carboniferous non-marine lamellibranchs. Palaeontology, 5, 307-339. Evans, B., Cleal, CJ. & Thomas, B.A. 2000. Exposures of the Westphalian Series in the upper Neath and Swansea Valleys, South Wales - a geoconservation project. Newsletter on Carboniferous Stratigraphy, 18, 23-25. Evans, B., Cleal, CJ. & Thomas, B.A. 2001. Exposures of the Westphalian Series in the upper Neath Valley, South Wales. Newsletter on Carboniferous Stratigraphy, 19, 45-47. Fowler, A. 1936. The geology of the country around Rothbury, Amble and Ashington. Memoirs of the British Geological Survey. HMSO, London. Gastaldo, R.A., DiMichele, W.A. & Pfefferkorn, H.W. 1996. Out of the Icehouse into the Greenhouse: a Late Paleozoic analog for modern global vegetational change. GSA Today, 6, 1-7. Gayer, R.A. & Jones, J.A. 1989. The Variscan foreland in South Wales. Proceedings of the Ussher Society, 9, 177-179. Gayer, R.A. & Pesek, J. 1992. Cannibalisation of coal measures in the South Wales Coalfield - significance for foreland basin evolution. Proceedings of the Ussher Society, 8, 44-49. Gayer, R.A., Cole, J.E., Greiling, R.O., Hecht, C & Jones, J.A. 1993. Comparative evolution of coal-bearing foreland basins along the Variscan northern margin in Europe.

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Manuscript received 23 April 2002; revised typescript accepted 9 May 2003