Variscan Compressional Structures Within the Main Productive Coal-Bearing Strata of South Wales

Journal of the Geological Society, London, Vol. 154, 1997, pp. 195–208, 11 ﬁgs. Printed in Great Britain

Variscan compressional structures within the main productive coal-bearing strata of South Wales

K. FRODSHAM & R. A. GAYER Laboratory for Strain Analysis, Department of Earth Sciences, University of Wales Cardiff, PO Box 914, Cardiff CF1 3YE, UK (email: gayer@cardiff.ac.uk)

Abstract: The structure of the main productive Coal Measures within the anthracite zone in the western part of the South Wales coalfield is re-examined using data from recently operational opencast coal sites. Variscan thrusts and/or folds and bed-parallel shear structures are present within all of the sites studied. At East Pit, unaffected by any large-scale zone of disturbance, bed-parallel shear structures are developed along many of the gently dipping coal seams, which form a series of simultaneously active detachments, representing a process of easy-slip thrusting. A NE–SW-striking, fold dominated disturbance at Gilfach Iago is interpreted to overlie a reactivated basement fault. A more complex disturbance zone at Ffos Las combines both E–W-trending folds and more variably oriented thrusts which together shorten the coal bearing strata by up to 2 km and this is interpreted to be the major Variscan frontal thrust ramp in the extreme west of the coalfield. The Variscan compressional structures within South Wales are not consistent with a simple thin-skinned thrust system. The strain induced by deep-seated Variscan thrusts was distributed throughout much of the coal bearing sequence by easy-slip deformation rather than being localized along a single regional detachment, and additional thrusts present around the northern margin of the coalbasin appear to have formed as isolated autochthonous structures. The inherent weakness of coal seams across South Wales is attributed to the presence of greatly increased fluid pressures, which would have been generated both by the maturing coal and by the influx of fluids along deep-seated disturbance zones.

Keywords: Variscan Orogeny, thrust faults, coal, detachment faults, reactivation.

The structure and tectonic evolution of the external Variscan consequent ease of coal extraction, and is also responsible for margin in South Wales (Fig. 1), UK, has been described in reduced value fractured (or friable) coal, which can contribute detail for both SW Dyfed (Hancock et al. 1983; Dunne 1983; to the life-threatening phenomena of gas outbursts in deep Powell 1989) and the eastern part of the main South Wales mine situations (Dumpleton 1990). coalfield (Gayer & Jones 1989; Jones 1991; Brooks et al. 1994). Despite the complexity of many individual structures within the coal bearing strata of these regions, existing models still view the deformation as a conventional foreland-propagating The tectonic setting of the South Wales coalfield thin-skinned thrust system (Smallwood 1985; Jones 1991). The Upper Carboniferous (Silesian) deposits of South Wales Structures in the western part of the main South Wales from Gwent in the east to SW Dyfed in the west probably coalfield, between these two areas, should provide a suitable formed in a peripheral foreland basin related to the encroach- test for these models. Yet, the area has received little attention ing Variscan deformation from the south (Kelling 1988). The since the pioneering geological surveys of Strahan and col- present-day main South Wales coalfield, a region about 80 km leagues (Strahan 1907a, b; Strahan et al. 1907, 1909), Trotter E–W and 35 km N–S, represents the erosional remnants of the (1947) and, more recently, Archer (1968). Much of this pre- former basin, occupying the axial region of an E–W-trending vious structural work used data from deep coal mines supple- northward-verging Variscan syncline. This structure was prob- mented by rather poor surface exposures. These mines aimed ably laterally continuous westward with the Pembrokeshire to avoid structural complexity so they undersampled the coalfield of SW Dyfed (Fig. 1). Superb coastal exposures in intense Variscan deformation. However, more recent opencast this latter area reveal that the coalbasin formed over the mining operations have redressed this imbalance, because extended northern margin of an earlier Devonian and Early geological structures do not cause insurmountable difficulties Carboniferous rift basin. Inversion of this pre-existing exten- for open pit working and tend to increase quantities of sional template significantly influenced the development of extractable coal. Indeed opencast workings tend to be sited in Variscan contractional structures (Dunne 1983; Powell 1989). structurally complex zones previously avoided by the deep The main South Wales coalfield also overlies a post- mines. In recent years, opencast coal sites have created numer- Caledonian sequence of Devonian Old Red Sandstone and ous excellent exposures (Fig. 1). This paper uses new evidence Carboniferous Limestone. The latter thickens fourfold south- obtained from three of these sites to develop a more complete ward beneath the western coal basin, which suggests that an model for both the Variscan structure of South Wales and the extensional history, similar to SW Dyfed, may have occurred deformation of coal-rich sequences in general. A detailed beneath the region. understanding of the structural style and tectonic evolution The formation of the coalbasin began in the early Namurian within the coalbasin is of economic concern to the local coal following the break-up of the Carboniferous Limestone marine industry because deformation affects both the planning and platform (Kelling 1988). The early basin-fill consisted of

195

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a SVD CCD ND x GI EP x

TD FL x

Swansea MGF

PS MTB x

0 km 10 Cardiff Vale of Glamorgan

Upper Pennant Measures Faults Lower Pennant Measures Antiform b Fig. 1a Lower & Middle Synform Coal Measures SW Dyfed Pre- and Post- x Opencast Coal Site Coal Measures

Fig. 1. (a) Geological map of the main South Wales coalﬁeld. Opencast coal sites: FL, Ffos Las; GI, Gilfach Iago; EP, East Pit; PS, Park Slip. Key structures: CCD, Carreg Cennen disturbance; CF, Carnafon fault; MGF, Moel Gilau fault; ND, Neath disturbance; SVD, Swansea Valley disturbance; TD, Trimsaran disturbance; MTB, Margam thrust belt. (b) Inset showing location of SW Dyfed and the South Wales coalﬁeld.

Namurian marine quartzitic sandstone (Basal Grit) followed smaller scale ‘slides’ (Woodland & Evans 1964) and ‘lag-faults’ by mudstones (Shale Group). Sedimentation continued into (Archer 1968) reported across the coal basin. However, most the Westphalian with the coal- and mudstone-dominated strike-parallel structures are contractional and related to Var- sequences of the Lower and Middle Coal Measures, which iscan compression, producing folds, thrusts and layer-parallel contain most of the productive coal seams in South Wales. shear structures, which are described in the next section. Fluvial sandstones within the Lower and Middle Coal Contractional structures do not occur uniformly across the Measures were derived variously from the north, south and coal basin and are frequently concentrated into narrow zones, east across the coalfield (Bluck & Kelling 1963). During traditionally known as disturbances, which are characterised Westphalian C times, more competent sandstone-dominated by large-scale folds and/or thrusts. Fold and thrust vergence sequences of the Upper Coal or Pennant Measures appeared, on a regional scale is related to the dip of the strata. Hence on derived mainly from the south (Kelling 1974). The change in the north crop, thrusts verge northward and on the south crop source area has been related to major uplift and emergence to southward. However, even where the local dip is horizontal or the south of the coalfield, possibly associated with encroaching gently inclined northward within the axial region of the central Variscan movement along a Bristol Channel thrust zone coalfield, thrust vergence is to the north, suggesting that an (Brooks et al. 1988; Gayer & Jones 1989). element of pervasive north-vergent over-shear accompanied Variscan deformation in this region. Strike-normal faults, known locally as ‘cross-faults’ (Woodland & Evans 1964), The structure of the South Wales coalfield characteristically exhibit extensional geometries and record a The coal basin is mainly affected by structures which are prolonged period of pre-, syn- and even post-Variscan axial oriented either sub-parallel or sub-normal to the local basin extension across the coalbasin (Cole et al. 1991). margin, although the NE–SW-trending (Caledonoid) Neath Stratigraphy controls the distribution of structures within and Swansea Valley disturbances, which traverse the central the coal basin. Whereas the incompetent Lower and Middle coalfield, and the E–W-trending Trimsaran disturbance which Coal Measures frequently exhibit N–S shortening of around occurs in the extreme west of the coalfield (Fig. 1) are major 20–30% from folds and thrusts (Jones 1991), the overlying exceptions. Strike-normal extension occurs across the large- Pennant Measures rarely display evidence of Variscan com- scale Moel Gilau and Carnafon fault systems in the eastern pression, and mainly contain cross-faults with a net E–W and western parts of the coalfield, respectively, and the various extension of around 8%. This problematical variation in bulk

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Fig. 2. Photographic examples of meso-scale structures within the South Wales coalﬁeld. (a) Cleavage duplex exposed in the roof of the Soap seam at the Ffos Las site. North to left. (b) Cross-section across the composite Six Foot seam at Fyndaﬀ Additional opencast coal site, which shows prominent slip planes within the coal and cleavage duplexes developed within both the roof and dirt partings. North to left. (c) Sigmoidal shears developed within the White Four seam at the Nant Helen opencast site. North to right. (d) Rashings band exposed where the Graigog seam was replaced by a ‘tectonic washout’ due to southward directed passive roof shearing at Ffos Las. North to left. (e) Thickening of the Black seam at Nant Helen opencast site along a low angle thrust or ‘lap’. North to right.

shortening between the two units has been interpreted as Cleavage duplexes. Structures similar to small-scale cleavage evidence for a major passive roof thrust close to the base of the duplexes (Nickelsen 1986) are common in the immediate roof Pennant sequence (Jones 1989). of coal seams within the western coalbasin (Fig. 2a). They are narrow (rarely >0.5 m), but laterally extensive zones of deformation that are characterized by very closely spaced The nature of layer-parallel shear structures within the (millimetre-scale) sigmoidally shaped surfaces. At present, we Lower and Middle Coal Measures cannot determine whether the surfaces are crenulation cleav- Structures other than folds and thrusts, that record layer- ages or a series of micro-thrusts. The sigmoidal surfaces parallel shear adjacent to coal seams and/or major thrust suggest that higher shear strains developed closest to the planes are common. They are grouped into three categories boundary faults that separate rocks inside the duplex from described below. relatively undeformed host rocks. Subsidiary fractures, with

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the geometry of ‘c’-bands, and low-angle micro-thrusts occur within cleavage duplexes. In contrast to the cleavage duplexes of Nickelsen (1986), which appear to ramp across bedding as thrust planes, the duplex-like structures in South Wales are characteristically bed-parallel structures, which developed either in the immediate roof to coal seams or in mudstone partings. They never cut up sequence. They represent shear zones, which accommodate strong layer-parallel shear strains adjacent to coal seams.

Slip planes. A common characteristic of coal seams throughout South Wales is the presence of numerous, usually widely spaced (>10 cm) fractures which transect the seam at between 30) and 65) to bedding and are frequently conjugate (Fig. 2b). These fractures, known locally as ‘slip planes’, commonly develop dip-slip, reverse slickenlines and represent planes of high shear stress, consistent with layer-parallel principal stresses acting along the coal seam. Occasionally, more persistent fracture patterns occur which mimic cleavage duplexes (Fig. 2c).

Rashings bands. Complex structures, characterized by multiple repetitions of inter-bedded coal and mudstone, along often sigmoidally shaped microthrusts have traditionally been termed ‘rashings bands’ by workers in the South Wales coalfield (Fig. 2d; Woodland & Evans 1964). These structures are commonly bounded by roof and floor discontinuities, forming duplex geometries of variable thickness. Both roof and floor boundaries may parallel the adjacent strata or cut upsequence parallel to thrust planes. Rashings bands frequently contain small-scale folds and occasionally overprint shear-zone fabrics, such as sheared fish, within the more deformed bands. Rash- ings bands are most commonly developed in the seat earth underlying coal seams, but are recorded in mud rocks between coal seams near major thrusts. The thrust repetition of coal and mudstone within rashings bands indicate a duplex geometry and suggest that, like cleavage duplexes, they record enhanced layer-parallel shear strains. Area balancing of rashings bands, Fig. 3. The structure at East Pit opencast coal site. (a) The broad where the roof has been folded above the duplex, suggest thrust structure and seam sequence. The structure is illustrated by two coal shortening of at least 30%. The contrast between cleavage seam extraction sections, which were constructed by on-site duplex and rashings band development is interpreted to be due surveyors over several years. Evidence for deformation in the upper to the different rheology of the thinly laminated shales of the section is limited to duplexing of the Upper Pencraig seam. In the coal seam roof and the massive seat earth of the floor. lower section there is evidence for thrust thickening of the Big seam and for breaching of higher level slip zones by thrusts. To correlate coal seams between the different opencast sites, the marker (M) Examples of structure revealed in recent opencast coal represents the equivalent position of the Big seam at Ffos Las. (b) sites An example of the structure exposed above the Big seam at East Pit, The three opencast coal sites described below exploit high rank which locally shortened the overlying sequence by up to 38%. anthracite coals in the NW of the South Wales coalfield (Fig. 1). The Gilfach Iago site and the Ffos Las site expose exploited gently south-dipping strata within the Middle Coal structural details across large-scale disturbance zones, which Measures, some 2 km south of the nearest disturbed ground are prominent in the region. The East Pit site, is unaffected by which comprises the Brynamman fold zone (Trotter 1947). The any disturbance zone and so it allows the background Variscan observations recorded here were from the previously exploited deformation within the Lower and Middle Caol Measures to East Pit site, and not from the current East Pit Extension site. be assessed. On a large scale, structure appears relatively simple due to the absence of large-scale folds and thrusts (Fig. 3a). However, the site contains abundant evidence for Variscan compression in Variscan structures observed in the East Pit opencast the form of north-vergent meso-scale Variscan thrusts, folds site and layer-parallel shear structures (Fig. 3b). General structure Coal was extracted by opencast methods from around the East Variscan compressional deformation Pit site for many years and the current site is merely an along The main evidence for Variscan compression at the East Pit strike extension of previous workings (Fig. 1). These sites opencast site is the presence of persistent layer-parallel shear

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structures, such as cleavage duplexes and rashings bands, that sub-Coal Measures sequence of several hundred metres, poss- indicate shear to the NE. The evidence includes southwest ibly even in excess of 1 km. The marked layer-parallel nature dipping thrust ramps (Fig. 3b) and southwest-dipping inclined of the deformation and the scarcity of Variscan structures surfaces in cleavage duplexes and rashings bands (Fig. 3b). transecting bedding, suggest that the slip along coal seams These structures are not homogeneously distributed amongst took place with relative ease. The truncation geometries in the the coal seams but increase irregularly in intensity down duplex roofs suggest a complex timing relationship between sequence towards the base of the site. They are best developed thrusts. In such an environment, relative movement along slip adjacent to the Big, Upper Pencraig and Stwrin seams and are planes would not have been constrained by conventional virtually absent within the high level Soap seams. thrust-related propagation sequences and presumably oc- Thrusts at the East Pit site are discrete ramps within the curred simultaneously along different horizons. This concept siliciclastic sequences between coal seams (Fig. 3b). However, of independently active multiple detachments has previously they rarely have displacements in excess of 5–10 m and only been referred to as easy-slip thrusting (Frodsham et al. 1993). one or two breach overlying coal seams. As a result their Further data are given for the structure at the Ffos Las presence is only rarely recorded on the coal seam extraction opencast site below, and its significance with respect to the plans such as Fig. 3a. As these plans often provide the only deformation of the coal basin as a whole is outlined in source of structural information from worked-out sites, the the discussion section. extent of Variscan deformation is, therefore, likely to have been significantly underrepresented on past mine and opencast plans. Most thrusts at East Pit tend to disappear upward into The structure of a fold dominated disturbance zone at small-scale low-amplitude north-vergent tip folds and down- Gilfach Iago opencast coal site ward into the slip zone developed along an underlying coal The Gilfach Iago opencast site is relatively small and exploits a seam. Consequently they are interpreted as splays arising off stratigraphic sequence around 125 m thick, in the uppermost coal-based detachments (Fig. 3b). Where thrusts displace coal Lower Coal Measures and the lowermost Middle Coal seams along low-angle planes which are sub-parallel to the Measures (Fig. 4). The structure at Gilfach Iago is largely strata (Fig. 2e), seams are locally thickened (e.g., Big seam, consistent with its location within the Ammanford compres- Fig. 3a). Such thrusts are a characteristic of coal measures’ sion belt (Trotter 1947), because it is largely fold dominated deformation across South Wales, and are known as ‘laps’ in with less frequent thrusts and cross-faults. mining terminology because they overlap coals. Hence, variation in coal seam thickness in past extraction sections can be used to provide a rough guide to the deformation intensity enjoyed by a particular seam. Typically, thick seams or com- Detailed structure posite seams containing dirt partings are most prone to this Much of the Gilfach Iago site is affected by a series of thickening. NE–SW-trending relatively open folds, which are slightly Although direct quantification of slip amount is not poss- asymmetric, northwest verging and plunge to the SW (Fig. 4). ible, local measuring of shortening within particular interseam On a regional scale these folds are non-cylindrical, often sequences can be done to obtain minimum values for slip along possess curved fold axes and form a series of discontinuous an underlying slip zone. These shortening estimates range from sub-parallel en echelon periclines (Roberts & James 1986). close to 0% near the top of the sequence to around 35–40% at Many are disharmonic and decrease in amplitude up and down the level of the Upper Pencraig and Big seams. They are sequence from the level of the Stanllyd seam (Trotter 1947). interpreted as the two main slip zones on-site. These shorten- The northwest corner of the site, however, is dominated by a ing strains indicate over 100 m displacement for the two coal fold of much greater magnitude. The strata dip vertically for seams. The spectacular duplexing of the Upper Pencraig seam about 100 m within the steep north-facing limb of a large (shown in Fig. 3a by the overlapping oblique segments of the NE–SW-trending monocline (Fig. 5). This structure is known coal seam), with 36% shortening, does not affect the Upper as the Caerbryn disturbance and has previously been encoun- Black seam, situated only 2 m above. On a small-scale, thrusts tered within the old Glyn Glas site to the northeast (Roberts & and related folds are truncated by the roof fault. Large-scale James 1986). Still further to the northeast, the Caerbryn truncation above the duplex is absent. This demonstrates that disturbance passes into the Caerbryn crush belt where the fold deformation is stratigraphically restricted across site. Indeed, is partially replaced by a series of northward verging thrusts. the extent of layer-parallel deformation within a single seam This tendency for folds to pass laterally and/or vertically into across East Pit does not vary, which precludes a localized thrusts within the compression belts, led Trotter (1947) to origin. By contrast, the duplexes developed above the slip introduce the term ‘fold replacing thrusts’ for such structures. zones are more localized, suggesting that shortening by these Slickenside lineations on bedding surfaces at Gilfach Iago complexes was more heterogeneous, possibly brought about by indicate an element of sinistral slippage across a fold axis (Fig. local sticking along the slip zone. 4e), which may be evidence for sinistral oblique slip movement upon the underlying fault structures which led to fold formation (Cole et al. 1991). Implications of the East Pit structure Thrusts and layer-parallel shear structures are relatively The East Pit structure is dominated by bed-parallel detach- uncommon at the Gilfach Iago site. Thrusts have been detected ments which are developed adjacent to coal seams and have from borehole evidence in two areas to the northeast and south significant northward-vergent displacement, but only a limited of the main workings. In both cases, they are ENE–WSW- amount of thrust thickening (see Fig. 10b). The cumulative trending, SE-dipping ramps that affect the coal seams with effect of this deformation, based on 35–40% shortening at the displacements of up to 20 m (Fig. 4d). The thickness of coal levels of the Upper Pencraig and Big seams, produced a seams remains relatively constant across the site, supporting a northward translation of the Pennant Measures relative to the lack of layer-parallel deformation. A north-vergent cleavage

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Upper Saron Brynlloi --M-- Rock a) general plunge Charcoal b) direction Rock Trichwart x Green A thrust Ddugaled Middle Pumpquart CAERBRYN(zone of DISTURBANCEvertical coal) y 25m

Stanllyd N Hwch syncline Glyn Hir Anticline Stanllyd Middle Pumpquart

Green Upper Trichwart B Charcoal anticline Braslyd Fach Ddugaled old opencast 0 m 300 Cwm Fferws workings Saron Brynlloi Cross Fault

c) A Caerbryn Disturbance Glyn Hir Anticline B Ddugaled Hwch Rock Green

Ddugaled Stanllyd Stanllyd Charcoal Middle Pumpquart 0 m 200 Upper Trichwart

y d) 20 e) 80 30 x Upper Trichwart 20m N 25 10 Charcoal 70 T3 18 Braslyd Fach 84 15 15 10 Saron Brynlloi 8 42 Periclinal antiform with T2 10 T1 36 plunge in degrees Bedding strike & dip 0 m 100 Slickenside lineation

Fig. 4. The structure at Gilfach Iago opencast coal site. (a) Sub-Quaternary seam subcrop map constructed largely from borehole data. (b) Seam sequence encountered at Gilfach Iago, M indicates the corresponding position of the Big seam at Ffos Las opencast coal site. (c) NW–SE-oriented cross-section across Gilfach Iago from on-site observation and borehole data. (d) Thrusts detected from borehole evidence (vertical lines). (e) Detailed structural sketch map of the main Caerbryn fold axis in the ﬂoor of the Stanllyd seam (from Cole et al. 1991).

duplex is developed in the roof of the Stanllyd seam, but much higher levels of bulk shortening recorded across the maintains vergence around fold hinges, which is interpreted to disturbance described from Ffos Las, where Variscan thrusts indicate prefolding initiation. The opencast site is also aﬀected are prominent. by the Cwm Fferws cross fault (Fig. 4a), which displaces strata vertically, but preserves the continuity of folds, suggesting a simple dip-slip displacement of uncertain age. The dominance of folding over thrusting at Gilfach Iago is The structure of a fold and thrust dominated disturbance characteristic of the Ammanford compression belt as a whole, zone at Ffos Las opencast coal site where bulk strain has resulted in approximately 1 km of uplift, The structure of the Ffos Las area, on the western margin of yet only 290 m (25%) shortening. This is in contrast to the the main coalﬁeld (Fig. 1), was relatively unknown prior to the

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above the Big seam in the southeastern corner of the site appears relatively undeformed. Thrusts remain commonplace beneath the Big seam in the north of the site, suggesting that thrust deformation is likely to continue for some distance beneath the base of the current site. One signiﬁcant aspect of the deformation at Ffos Las is that structural complexity decreases up sequence into the more competent strata above the Soap seam. Shortening estimates, based on area balancing from sections constructed with borehole evidence across the most tightly constrained part of the rock sequence between the Big and Graigog seams (such as Fig. 6b) indicate around 54% N–S shortening (Fig. 6c), equivalent to around 2 km across the site. Individual estimates of shortening based on bed-length across the most highly deformed zone of east–west-trending folds and thrusts give values varying between 67% for the sandstone above the Big seam and 52.5% for the Soap seam. Between the overlying Soap and Graigog Rider seams, exposed further to the east, this value is reduced to just 35% (Fig. 6c). Although a small part of the diﬀerence is because some lower-level thrusts must displace the higher seams to the north of the available section line, much is real displacement loss within the Middle Coal Measures.

Detailed structure Folds. Strongly asymmetric north-vergent thrust-related folds are commonly exposed across the Ffos Las site. The majority are small-scale, restricted to individual thrust sheets, and are often truncated by overlying thrusts.We interpret the truncation to indicate a break back origin (Boyer & Elliott 1982). Within the centre of the site, several folds are reoriented around the central fold zone to produce downward-facing structures (Fig. 7). Although, the main folds of the central fold Fig. 5. The Caerbryn Disturbance at Gilfach Iago opencast coal site zone are truncated above and below by thrusts, they are looking toward the west. Note irregular geometry of the fold hinge laterally and vertically more pervasive than most thrust-related in the floor of the Stanllyd seam with local plunge to NE, despite folds encountered at the site (Fig. 8). Indeed, the pronounced overall plunge of the disturbance being to SW. E–W trend of the vertical forelimbs of the central fold zone marks the line of the Trimsaran disturbance of previous opencast site, but was believed to overlie a 090) trending zone workers. The prominent 10–17) easterly plunge of the majority of complex folding and thrusting, which produced a total of folds is caused by the folds deforming originally east- northward downthrow of around 350 m in the local productive dipping bedding close to the northwestern margin of the Measures. This structure, the Trimsaran disturbance, was coalbasin. Occasional local plunge variations may reflect pre- believed to mark the boundary between the so-called ‘Caledo- folding variations in bedding dip induced by early thrust noid (NE–SW-trending)’ and ‘Variscoid (E–W-trending)’ structures. structures of South Wales (Archer 1968), and has also been assumed to mark the position of the enigmatic Variscan front Thrusts. Previous coal extraction from the area around Ffos within the region (Shackelton 1983). However, the disturbance Las has demonstrated that significant thrust-related defor- does not provide a simple boundary between structures with mation within the Lower and Middle Coal Measures is con- Variscoid and Caledonoid trends, as thrusts found in bore- centrated into a broad 2 km wide zone at the present surface. holes immediately to the south of the disturbance show a In contrast to the central fold zone, the strike of this thrust strike-parallel (WSW–ENE) rather than an ideal Variscoid zone more closely follows the local trend of the strata as a (E–W) trend. result of the small angle between thrust surfaces and regional bedding. The majority of thrusts exposed are northward-displacing, General structure but southward-directed displacements are recorded on both The detailed three-dimensional structure of the Ffos Las area local minor backthrusts, and more significantly on passive has been deduced from a combination of borehole (Fig. 6) and roof detachments which have developed along the Soap and on-site evidence (Fig. 7 & Fig. 8). Large-scale meaningful Graigog seams at a higher stratigraphic level on the site (Fig. exposures were effectively above the Big seam in the Middle 8). Evidence for southward displacement on these detachments Coal Measures. At the level of the Big seam, the large-scale includes cleavage duplexes and rashings bands, sigmoidally structure is a network of low angle thrusts deformed by shaped low-strain domains and south-vergent small-scale folds strongly asymmetric east–west-trending folds across the centre within the detachment shear zones (Frodsham et al. 1993). of the opencast site. Only the upper part of the rock sequence Thrusts commonly form long flats along coal seams, causing

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a) Drap Major Fold Trace N Trimsaran Marine Thrust Plane Band Coal 0 m 500 Old Workings Mole Rider Projected Outcrop A Mole 10 Graigog Graigog Big 17 x Rider Graigog Rider j Soap Penny Pieces Graigog F1 Approximate k Position Of Soap T3 F2 Carway Fawr Seam Penny Pieces F3 25 Limit of old workings in y Big 22 1950’s Carway OCCS m T2 20 - -M - - Big Drap Big Green T1 0 B

Old drift workings in Yard Rider the Trimsaran area Yard Two Foot

b) B A Graigog Penny Pieces Big Graigog Rider Soap

F1 T1 250m F2 F3 T3

c) A GRAIGOG SEAM B

BIG SEAM

Estimate of Shortening from AREA BALANCE

Area between Graigog and Big Seams = 96 units 2 Stratigraphic thickness between Graigog and Big Seams = 1.2 units l 0 = 96/1.2 = 80 units l1 = 37 units e = 53.7% Fig. 6. The structure at Ffos Las opencast coal site. (a) Sub-Quaternary seam and subcrop map constructed from borehole data and projection of structural evidence from coal extraction. Thrusts occur in groups T1, T2 & T3 and two laterally persistent folds F1 & F3, with a third F2 being less persistent are present. (b) NNW–SSE cross-section (A–B) constructed largely from borehole evidence and more detailed information obtained during extraction, notably concerning the geometry of thrusts and the distribution of the Big Rock Sandstone (light shading). (c) Estimate of tectonic shortening using an area balance for the stratigraphical interval between the Graigog and Big seams in the cross-section A–B of (b).

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N downward facing S fold Penny Pieces Seam

td Big Rock Sandstone Sequence tc tb Big Seam ta main synclinal 30m F1 j hinge k

Fig. 7. The structure of north-south section (j–k) across fold hinge (F1) within the central fold zone at the Ffos Las site. All thrusts which repeat the Penny Pieces seam truncate structures formed in underlying thrust sheets and are interpreted to have formed in the order of Ta, Tb, Tc & Td.

coals to thicken by duplexing. For example, the thickness of oriented, ramps. The latter are best seen in the SE corner of the Big seam, normally around 2.30 m thick, may vary from site (Fig. 6), and produce considerable structural variation zero, in the case of so-called tectonic washouts, to greater than along strike in many of the thrust sheets. 15 m. within coal duplexes in the seam (Fig. 8). The seat earth beneath the Big seam is replaced by a band of highly polished rashings across much of the site. An unusual geometrical feature of the deformation is that The interrelationship between thrusts and the central the pronounced north vergence of most Variscan structures is fold zone oblique to the local strike of the strata. Thrust geometries The structural relations around the central fold zone, provide are correspondingly complex, usually combining long E- to evidence for the sequence of thrust formation with respect to SE-dipping ﬂats with 060) to 090) striking, often obliquely fold ampliﬁcation. Numerous low-angle thrusts are best seen

Fig. 8. Cross-section (x–y) across the central fold zone at Ffos Las opencast coal site. Thrust Te overlies the break-back thrust system in Fig. 7, which was exposed 300 m to the west. Te is also the lowest thrust to a second break-back system, generated from thrust T2, and formed around fold F3. The interpreted thrust formation sequence was Te, Tf, Tg & Th. Fold, F2, is laterally less persistent than the others and only folded pre-existing thrust segments. The Soap and Graigog seams show southward vergent structures which formed when the seams acted as passive roof detachments to the underlying thrust deformation. See text for explanation. Estimates of tectonic shortening across the section, using bed-lengths of coal seams balance: Soap seam, 52.5%; Penny Pieces seam, 63.4%; Top Big Sandstone, 67% Big seam, 63%.

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beneath the Big seam. They produce multiple repetitions of i) the underlying coal seams, but are folded the same amount as the strata (Fig. 8). By contrast, near the base of extraction t1 of the Big seam, a series of low angle south-dipping thrusts (labelled T3 on Fig. 8) cut across the central fold zone, and must therefore largely post-date fold formation. These late-stage thrusts, when traced up sequence, produce a dis- T placement of around 250 m in the Penny Pieces seam, along thrust planes which clearly cross-cut previously deformed ii) strata. t1 t2

Localized progressive break-back thrust sequences. To our knowledge, the Ffos Las opencast site provides the first documented examples of break-back thrust sequences in the F T South Wales coalfield (Figs 7, 8 & 9). The first occurs over fold F1 and incorporates at least four break-back thrusts iii) which multiply repeated the Penny Pieces seam (Fig. 7). An important clue to the thrust sequence is provided by fault t5 t4 geometry across the syncline adjacent to F1. All thrusts t3 d t2 which repeat the Penny Pieces seam truncate structures t1 formed in underlying thrust sheets and are interpreted to have formed in the order of Ta, Tb, Tc & Td, leading to an upwards diminution of the fold structure. This suggests that the syncline grew during sequential formation of F T thrusts. Another, slightly later but similar, break-back system Fig. 9. Cartoon to illustrate the development of a progressive is also present around F3 (Fig. 8). The recognition of break-back thrust sequence in the manner interpreted at Ffos Las simultaneous growth of folds and thrusts when combined opencast coal site. (a) A single thrust T propagates into the with the observation that both break-back sequences origi- sequence. (b) Before thrust T can completely propagate, nated near fold crests, suggests that the break-back thrusts development of an underlying fold rotates the fault. This rotation

formed simply as a consequence of the progressive folding of leads to locking of the original thrust plane t1 and a break-back

a thrust plane which remained active during fold growth thrust t2 is generated at the fold crest. (c) Continued fold (Fig. 9). amplification causes a progressive break-back thrust sequence t1-t5, Across the central fold zone, only the top thrusts in each as successive thrusts are rotated. Ultimately, thrust sheets are break-back system have propagated into the sequence above vertically stacked and the amplitude of the fold diminishes up the Graigog seam and much of the displacement by inter- sequence to the final break-back thrust. This thrust continues mediate break-back thrusts was accommodated by locally unfolded across the underlying fold with a drastic displacement intense south-directed passive roof shearing along higher decrease at the fold crest. Coeval duplexing is interpreted to occur in seams (Fig. 8). This hinterlandward-shearing contributes sig- a coal adjacent to the fold hinge as with the Big seam at Ffos Las. nificantly to the observed reduction in shortening between the Big and Graigog sequences. Indeed, the only deformation The origin of the Ffos Las structure recorded along strike in the Carway Fawr seam, some 140 m The magnitude of thrusting witnessed at Ffos Las, where higher in the stratigraphic succession, is a large north-vergent individual thrusts have displacements up to at least 250 m and monocline, with shortening below 20% (Fig. 6a). Further the total thrust-related shortening reaches around 2 km, sug- along strike to the east, surface exposures of the thick gests that the thrust zone may be a major north-vergent Pennant cover contain N–NNW-vergent folds (Fig. 1), but Variscan forethrust through the Coal Measures (Fig. 10). The complex thrust geometries are proven in boreholes within the suggested presence of this structure is consistent with recent Lower and Middle Coal Measures, suggesting that the main seismic evidence from beneath the Vale of Glamorgan in the thrust zone remains largely buried within this region (Fig. eastern coalfield (Fig. 1) where a major deep-seated Variscan 10). An important implication of these observations is that forethrust system was interpreted (Brooks et al. 1994) and it an absence of thrusts in the overlying Pennant sequence is would also have a similar magnitude to the main thrusts in the not a reliable indicator for the absence of thrusts and strong coalfield of SW Dyfed (Smallwood 1985). tectonic shortening at lower stratigraphic levels. Also, the The nature of the frontal structure at Ffos Las is compli- large shortening magnitudes at depth but not in the Pennant cated by two additional factors. Firstly, two of the thrusts are sequence necessitates kinematic solutions such as roof back- represented by break-back thrust sequences, interpreted to thrusts (Banks & Warburton 1986; Geiser 1988). We inter- have formed by progressive growth of the main folds of the pret the complexity of deformation to indicate a family of central fold zone. These folds are concentrated within a narrow roof backthrusts rather than a single backthrust at the base but laterally persistent belt and deform the main forethrusts. of the Pennant sequence (Jones 1989). This model has im- The second factor is the nature of the main thrusts. Thrusts of portant implications for other areas of the coalfield. For similar displacement magnitude are only encountered in one example no belt of disturbed ground is known to the south other region of the coalfield, the Margam thrust belt (Fig. 1), of East Pit to account for the thrust shortening recorded at where thrusts verge to the south with a major Variscan the site. However, it is possible that one exists at depth backthrust. When the effects of folding are removed from the beneath the Pennant cover of the central coalfield. thrust geometry at Ffos Las, the faults are geometrically

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a) b) c) d)

Northern Periphery Central North Crop Variscan Front Basin Axis South Crop Gilfach Iago East Pit Ffos Las Park Slip Opencast Coal Site Opencast Coal Site Opencast Coal Site Opencast Coal Site

Pennant Measures

sub-Coal Measures

Fig. 10. Schematic structural cross-section across the South Wales coalbasin, illustrating the relationship between large-scale tectonics and abundant bed-parallel detachments within the main productive coal bearing strata (unshaded). (a) Near to the northern margin, regional detachments are restricted to only a few seams and were folded by uplift upon underlying basement structures. More localized detachments are recognized which have developed in the footwall of autochthonous thrusts. (b) Within the central north crop, detachments are quite pervasive across the main productive Coal Measures, leading to substantial foreland-directed slip. (c) In the vicinity of the main Variscan frontal thrust, a pattern of very intense foreland-directed detachments accompanies Variscan thrusts, but the large-scale thickening of the sequence has also produced a hinterland-directed passive roof thrust system. (d) On the south crop, detachments verge dominantly to the south and become very intense adjacent to a major back-thrust zone.

similar (but with opposite vergence) to thrusts in the Park Slip The role of basement reactivation in the development of group of opencast sites in the Margam area (Jones 1991). fold dominated disturbance zones These thrusts would, in both cases, be concentrated into There is considerable evidence in areas along strike from the narrow zones where thrusts are sub-parallel, and where no South Wales coalfield for the reactivation of basement struc- propagation sequence can be recognized. We interpret these tures during the Variscan. In SW Dyfed, the Variscan reacti- two occurrences to mean that major Variscan thrusts in the vation of the Ritec and Benton faults, shown to be extensional sub-coalbasin sequence become a series of thrusts of lesser growth faults during the Devonian and Early Carboniferous, magnitude within the coal-bearing strata. was demonstrated by Powell (1989). In the eastern South Wales coalfield, Jones (1989) showed that the Pontypridd anticline and the Caerphilly syncline influenced Coal Measures Discussion: a new thrust model for South Wales deposition prior to Variscan intensification. In the western South Wales coalfield, the evidence is not Previous models for the deformation of coalbasins conclusive. Large-scale, asymmetric strongly vergent folds are Thrusts folds and associated structures occur in large numbers not commonly developed outside the anthracite region of the across the whole coalfield as seen, for example, at East Pit coalbasin, even in areas where thrusting becomes very intense opencast coal site. Indeed, fold- and thrust-related defor- as, for example, within the Margam thrust belt on the south mation is a characteristic feature of Carboniferous foreland crop of the coalfield (Fig. 1; Woodland & Evans 1964). That coal basins along the northern Variscan margin from Ireland these fold dominated disturbances possess a relatively steep to the Bohemian massif in the Czech Republic. Two contrast- and deep-seated origin is apparent from Millstone Grit and ing models exist for the origin of the compressional structures Carboniferous Limestone inliers along the north crop, which within these coalbasins. In the more easterly basins, most are all affected by the same large scale structures that deform notably in the Ruhr coalbasin, folds and thrusts are considered the Coal Measures exposed down plunge. The disturbances are to have formed autochthonously in direct response to Variscan believed to represent folds which developed above basement compression (Fig. 11a; Brix et al. 1988). Whereas, in the more structures which became reactivated during Variscan compres- westerly basins, such as southern Ireland (Cooper et al. 1984) sion. The disturbance zones all occur in the region of the and SW Dyfed (Smallwood 1985) they are interpreted as coalfield where the depth to the seismic basement is much classical thin-skinned thrusts arising from a regional detach- reduced (Mechie & Brooks 1984), along strike from the ment. In this latter situation, the basin-fill is considered uplifted basement rocks associated with the Ritec and Benton allochthonous (Fig. 11b). faults.

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The nature of the structures which underlie the compression belts is indicated by the three major bounding faults to the western coalfield; the Carreg–Cennen (Trotter 1948), Swansea (Tawe) Valley (Weaver 1975) and Neath (Owen 1954) disturbances (Fig. 1). At the surface all three lineaments are marked by parallel zones of compressional deformation, which mimic those structures exposed within the compression belts of the coalfield, and which led Trotter (1948) to propose that the Carreg–Cennen disturbance was a major sole thrust to the whole coal basin. However, as all three lineaments affected sedimentation over a long period of time and cut laterally across the surface stratigraphy, they are best interpreted as steeply-dipping long-lived basement faults, which according to Owen & Weaver (1983) have been ‘intermittently active since Fig. 11. Figure illustrating two contrasting models for the origin of the late Pre-Cambrian’. The offset of the Coal Measures along compressional structures within European coal basins. (a) both the Neath and Swansea Valley disturbances includes a Autochthonous model where isolated imbricate thrusts develop significant sinistral strike-slip, in addition to a dip-slip, com- (after Brix et al. 1988). (b) Allochthonous model where structures ponent, suggesting that Variscan reactivation may have been propagate from a detachment which underlies much of the partially oblique. coalbasin. The fold-dominated structure of the Gilfach Iago site is part of the Caerbryn disturbance. This forms a narrow northeast-trending zone of folding, parallel to the Carreg– Variscan front, and the contrast in deformation between the Cennen disturbance, and is interpreted to have a similar thrust-deformed Lower and Middle Coal Measures versus the deep-seated origin. lack of thrusts in over- and underlying sequences. The central fold belt of the Ffos Las opencast site is unlikely The thin-skinned thrust model of Jones (1991) is critically to have formed in response to movement upon the kind of low dependent on the presence of a regional detachment with a angle thrusts which are present within the thrust zone. The displacement sufficient to produce the average shortening of geometrical similarity of these folds with those present within around 20–30% which is observed within the Lower and the fold dominated compression belts, such as the Caerbryn Middle Coal Measures (Fig. 11b). No such detachment has disturbance, favours a similar origin, forming in response to been identified, and, as the magnitude of individual thrust the reverse sense reactivation of an appropriately aligned displacements, as at Ffos Las and Park Slip opencast sites, is underlying fault zone. Although, the depth to crystalline constrained, a discrete regional detachment is unlikely to be basement appears to be locally high in this region when present. In reality, as demonstrated from East Pit, bulk compared to the compression belts (Mechie & Brooks 1984), displacement is distributed throughout the Lower and Middle the presence of Upper Palaeozoic extensional faults similar to Coal Measure sequence, and is accommodated along a series those in SW Dyfed, and which exerted a profound influence of detachments provided by coal seams. Their cumulative upon Variscan compression, is to be expected (e.g. Jones 1989), slippage may be substantial (Fig. 10). In this situation, small- and may, indeed, account for variations in the stratigraphic scale thrust systems can be generated along any slip horizon, sequence which have been recorded beneath the Big seam but the individual thrusts and folds which develop will ulti- across the disturbance zone. Reactivation of this fault zone mately be smaller than if they had formed above a single would have been responsible for folding higher level Variscan regional detachment. Also, shortening will vary in an irregular thrusts with the consequent formation of the observed break- manner across the coal bearing sequence. Many examples of back thrust sequences. Variscan deformation described from the Lower and Middle The structures observed within both Gilfach Iago and Ffos Coal Measures of both the main South Wales coalfield and SW Las opencast sites have demonstrated the roles that deep- Dyfed (Smallwood 1985) suggest that detachments are wide- seated structures played in the formation of disturbance zones spread and that the role of conventional linked thrust systems within the Coal Measures of South Wales. However, thrusts, with a single regional sole thrust is overstated. These coal- folds and associated structures are not just restricted to the parallel detachments also activated as a network of roof immediate vicinity of disturbance zones backthrusts beneath the Pennant Measures. Thus, the Pennant was passively translated up-dip on both sides of the main coal basin (Fig. 10). The role of easy-slip deformation in South Wales The style of thrust deformation within the coal-bearing The South Wales coalfield is located between the two proposed sequence of the South Wales coalfield, with the development of regimes of autochthonous and allochthonous behaviour. sets of coal-parallel detachments, is likely to be due to the Meso-scale thrusts and folds in South Wales were originally mechanical strength of the coals. Cross-cutting relationships at viewed as ‘incompetence structures’ (Woodland & Evans 1964) the East Pit site suggest a complex displacement history for which arose from the overall weakness and high mechanical individual detachments. At Ffos Las, the break-back sequence anisotropy of the Lower and Middle Coal Measures sequence of detachments, with successively activated detachments lying (i.e. akin to an autochthonous model). However, Jones (1991) in the Penny Pieces coal, suggests that detachments are guided recently invoked a thin-skinned model for the tectonic evol- by coal seams. This style of deformation, with a lithological ution of the eastern South Wales coalfield. She cited as control on slip horizons, is termed easy-slip deformation. evidence the relatively constant vergence of thrusts and folds, Evidence of easy-slip along coals is widespread throughout the the northward persistence of these structures with significant coalfield, but is spectacularly developed in the Ffos Las levels of shortening to some 15 km ahead of the presumed opencast site.

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The role of fluids in controlling Variscan deformation lying Pennant Measures is explained by a network of roof Much of the structural evidence presented in this paper shows backthrusts which passively translated the Pennant up-dip on that coal seams acted as weak, easy-slip, horizons during both sides of the basin. The formation of multiple detachments Variscan deformation. This lithological control may have instead of a single regional detachment is believed to have been resulted from fluid overpressures in the coals, generated in- controlled by the presence of high pore fluid pressures within itially during the compaction and devolatilization of the coals the coal seams. A number of steeply inclined thrusts near the and later by fluids arising from depth along the deep-seated northern margin of the coalbasin probably formed as initially disturbance zones (Gayer et al. 1991). Recent studies of isolated structures in a largely autochthonous foreland setting. anthracite vitrinite reflectance in relation to thrusting at Ffos The pattern of disturbance zones within the western coalfield Las (Davies 1995; Gayer et al. in press) showed that coal directly reflects the presence of deep-seated structures. Large- maturation developed both before and during the formation of scale monclinal folds developed where underlying fault zones the major thrusts. This relationship confirms the close depen- became reactivated during Variscan compression. Narrow dence between thrusting, folding and orientation of vitrinite zones of thrusting, within which displacement may exceed reflectance anisotropy demonstrated in the Ffos Las site (Salih 1 km, are major Variscan thrusts which produced multiple coal & Lisle 1988). That coal seams can act as conduits for the detachments. Simultaneous movement upon different struc- migration of hydrothermal fluids within a deforming basin has tures during Variscan compression produced crosscutting re- been demonstrated for the eastern Pennsylvanian coalfield by lationships within the Coal Measures, ranging from the small Daniels et al. (1990). Preliminary work in South Wales, scale breaching of overlying coal detachments to the super- suggests that coal seams in the anthracite belt record higher imposition of large scale fold- and thrust-related disturbances temperatures of metamorphism than surrounding mudstones, along the Variscan front. a fact that could be explained by the flow of high-temperature fluids along coal seams, perhaps driven by increasing Variscan K.F. was funded by British Coal Opencast who also provided borehole compression. Thus, easy-slip thrust detachments occur within data and surveyors’ coal extraction sections and plans for the opencast coal basins in which deformation occurs at the time of coal coal sites. More recently Celtic Energy Ltd have allowed access to the maturation, when the pore-fluid pressures are at a maximum. sites. The work formed part of an NERC Special Topic—Basin Dynamics Research Grant (GST/02/350) to R.A.G. We are grateful to the staff of British Coal Opencast and Celtic Energy for valuable Thrusts of autochthonous origin assistance on site. An early version of the paper was greatly improved by the critical reviews of D. Wilson and W. Dunne. Correspondence to In addition to thrusts linked to Variscan structures outside the R. Gayer (email: gayer@cardiff.ac.uk). coalfield and those arising directly from easy-slip detachments, thrusts which have developed autochthonously are also present. These thrusts are isolated, planar and show no affinity References to coal seams. 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Received 13 June 1996; revised typescript accepted 8 September 1996. Scientiﬁc editing by Alex Maltman.

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