A Mountain Front Model for the Variscan Deformation of the South Wales Coalfield

Journal ofthe Geological Society, London, Vol. 148, 1991, pp..881-891, 10 figs. Printed in Northern Ireland A mountain front model for the Variscan deformation of the South Wales coalfield JULIE A. JONES Cheltenham and Gloucester College of Higher Education, Shaftesbury Hall, St Georges Place, Cheltenham, Glos. GL50 3PP,UK Abstract: Access to theconsiderable geological database of British Coal, together with seismic evidence of subsurface relationships, has enabled the development of a structural model for the South Wales coalfield. Detailed site investigations are used as key examples. The simple broad synclinal nature of the coalfield masks a more complex structural system. Thrusting of Variscan age is largely confined to the Lower and Middle Coal Measures and throughout the central and northern coalfieldis dominantlya forethrustsystem. However, along the south-crop a major backthrust system is developedwhich in places penetrates the Upper Coal Measures. The south-cropshows features analogous with a mountain front and is interpreted as a triangle zone, in which pre-existing basement faults have facilitated uplift. The lack of thrusting within the Upper Coal Measures, in contrast to the underlying Lower and Middle Coal Measures, suggests they have acted as a passive roof to the thrust system. Geographical variationin the intensity of thedeformation can be related to the proximity of, andbuttressing against, the Caledonian massif. Although broadly analogous with current models of foreland basin development and deformation, the South Wales coalfield demonstrates the importanceof pre-existing massifs and lineaments in determining the precise evolution of an area. TheWestphalian sediments of theSouth Wales coalfield east-cropand reaching values of upto 80" inthe reveal a history of complex deformation, mainly of Variscan Carboniferous Limestone of the south-crop. age although later Mesozoic effects are also recorded. The Currentunderstanding of thestructure of theSouth results of post-depositional deformation are clearly recorded Walescoalfield stems largely from the systematicsurvey in the Coal Measures sequence andcan be studied in British work of Woodland & Evans(1964) and Squirrel1 & Coalopencast sites situated around the perimeter of the Downing (1969) whoestablished amodern correlatable coalfield and numerous deep mines located throughout the stratigraphy and documented the major structural elements. basin. Variousreviews of thestructure and basindevelopment The main South Wales coalfield extends from Pontypool havesince been undertaken (e.g. Owen & Weaver1983; in the east, westwards to Kidwelly, covering an area of 75 by Kelling 1988). An important contribution to understanding 25 km (Fig. l), and preserves a sequence of over 2000 m of the nature of the NNW-SSE faults was made by Gayer et al. Upper Carboniferous strata (Fig. 2). It is the second largest (1973) whodrew attention tothe possibleoblique slip coalfield in Britain. The basin is underlain by Millstone Grit nature and compartmentalizingeffect of the Taff gorge fault. and Carboniferous Limestone now exposed at the margins. More recently, seismic investigations (e.g. Mechie& Brooks To the north of the coalfield Devonian strata are exposed in 1984) have provided important contributions to understand- the Brecon Beacons, and Lower Palaeozoic sequences occur ing the subsurface geology of the area. in thenorth-south inlier of the Usk'anticline and the However, integrated models for basin development and east-west en echelon folds of the Cardiff area (Fig. 1). deformationhave not been presented. This and a Northwest of the coalfield the Lower Palaeozoic rocks of companion paper(Jones 1989b)presents such a model, the Welsh massif, deformed during the Caledonian orogeny, chosing key sites from a study of deep mine and opencast exhibita NE-SW structuralgrain. This area formed the sites (Jones 1989~). Thefollowing account reviews a series stable mass of St George's Land during the Carboniferous. of sites and structures from north to south across the South Atthe eastern end of the coalfield thenorth-south Usk Walescoalfield and also in theeasternmost part of the anticlineand the Forest of Dean synclinemay reflect the coalfield. Key themes and issues are then drawn together in influence of a deep-seated trend, sub-parallel to the Malvern a discussion. axis (Kellaway & Hancock1983). To thesouth, in the Bristol Channelbasin, a thicksequence of Mesozoic Northern coalfield sedimentsextends onshore and onlaps the Palaeozoic sequence in the Vale of Glamorgan. Tower The coalfield is an elongate east-west synclinal basin and At Tower colliery (Fig. 3) the workings are concentrated in thepresent-day margins are delineated doublebya the Middle Coal Measures and extend southwards under the escarpment of CarboniferousLimestone and Upper Coal cover of the Upper Coal Measures (Pennant Measures). A Measures (Pennant sandstone) with the Lower and Middle study, based on surveyor's sections, of the Six Feet seam, CoalMeasures forming the intervening low ground. Dips indicated that the seam was repeatedly overthrust (Fig. 4). are gentle around the northern edge, steepening along the Using these sections the magnitude of thrust development 881 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/148/5/881/4890755/gsjgs.148.5.0881.pdf by guest on 30 September 2021 882 A. J. JONES ............ ..................... Fig. 1. General location map of South Wales and part of South West England. The study area is delineated by the inset box. and the three dimensional geometry of the thrust systems Theimportance of Towerand Ffyndaff sites at the was evaluated. northern margin of the coalfield is that theyclearly Where undeformed, the Six Feet seam at Tower consists demonstratethe penetration of Variscandeformation of alower leaf, 90-120cm thick,separated bya parting northwards throughout the coalfield. (oftena seatearth) from an upper leaf of approximately 60cm. Five thrusts can be recognized from the surveyor's section (Fig. 4 Tl-T5). In many cases thrusting appears only Central coalfield to affect the upper leaf, for example, in the central part of The geographical centre of the coalfield is occupied by a the sections (Tl, T2, T3). Other thrusts cut down laterally, mid-basinsyncline in whichmany of thedeep seams are (e.g. T4) presumably by lateral or oblique ramps, to include unworked due to depth limitations. To thesouth is the the lowerleaf in thedeformation. Displacement varies Maesteg anticline, an east-west-trending fold affected by a laterally andoften decreases onone thrust as movement series of NNW-SSE faults which have a dextral as well as increases onanother (e.g. in Fig. 5 T2 decreases in normal component of displacement (Fig. 3). Eastwards the displacementas T3 increases). The generalstrike and dip Maesteganticline dies outbut is replaced by the slightly direction calculated from the map is065"/35" SE with an moresoutherly east-west-trending Pontypridd Anticline. overall shortening estimate of 31%. These two folds may represent the same structure in which case thepresent day lateral displacement is attributed to Ffyndaff opencast site (Fig. 3) dextralmovement of the NNW-SSE faultsand/or compartmentalization of the deformation. The seams here dip southwards at about 10" and thrusting occurs in severalseams, consisting largely of short ramps andlong flats which do notproduce anylarge scale St John's colliery and the Jubilee Slide repetitions of stratigraphy. At the southern end of the site A series of 16 arcuate extensional faults dissect the crest of there is intense deformation of the Nine Feet and Red Vein the Maesteg anticline. These faults are arranged en echelon, seamswith an estimated shortening of 40-5470 (Jones striking WNW-ESE and downthrow southwards by up to 1989~).All deformation is directednorthwards. Atthe 90 m. They are collectively known as the Jubilee Slide (Fig. northern end of the site bench excavations allowed a three 3). The JubileeSlide is exposedat Mynydd Llangeinor dimensional view of thrusting in theYard seam where a (88.9214.9225) and Pont y Ffald (88.8719.9260) (Woodland normalfault was visible onlybelow thethrust plane, & Evans1964), dipping southwards at 45" andexhibiting suggestinga phase of extensionalfaulting pre-dating the dip-slip slickensides. thrusting. The Jubilee Slide was exposed underground in the now Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/148/5/881/4890755/gsjgs.148.5.0881.pdf by guest on 30 September 2021 VA RISC AN DEFORMATION, SOUTH WALESDEFORMATION,SOUTHVARISCAN 883 important implications for mining since the lateral continuity of seams cannot be predicted. Workings above and below the driveways are restricted and the vertical extent of the upper Coal structures is unknown. Measures (Pennant) The above relationship raises the possibility for a close association of compressional andextensional structures, In perhapsinvolving re-use (inversion) of fracturea by 3 0 successive movements. The spread of fault orientations may K be a consequence of reactivation or inversion of pre-existing structures. There is less variation in the spread of fold axes 150-530 (Jones 1989~). The confinement of the extensional detachment of the Jubilee Slide within the Lower and Middle Coal Measuresis Lower Coal 210.480 Measures nodoubt related tothe ductility of thestrata. These measureshave also been the focus of compressional Milklone Grit deformationas seen in thenumerous boreholes in theSt John'sarea which exhibit no thrusting

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