South Wales Coalfield Research Articles

Orogenic fluids and secondary magnetizations: testing the relationship in the South Wales Coalfield foreland basin

Carbonates that rim the South Wales Coalfield contain a pervasive late Paleozoic CRM residing in magnetite. The CRM in the eastern part of the basin was acquired after deformation, and geochemical results are consistent with a connection between remagnetization and orogenic-type fluids. In the western part of the basin, the CRM is synfolding but the geochemical results do not support a connection with orogenic fluids.

The association between tectonic deformation, inorganic composition and coal rank in the bituminous coals from the South Wales coalfield, United Kingdom

Analysis of vertical distribution of coal rank in three collieries and one opencast mine in the SE of the South Wales coalfield has revealed deviations from Hilt's law in both proximate analyses and in vitrinite reflectance. The variations in rank parameters studied are closely correlated with variations in the intensity of tectonic deformation, measured using a numerical deformation index (NDI). Three seams in the sequence were also analysed for major and trace element geochemistry and Rock Eval Pyrolysis. Variations in these parameters are related to thrust inclusion of floor and/or roof mudrocks into the seams. A detailed conceptual model of a major thrust and associated in-seam thrust detachments and lag faults is developed to explain the vertical rank profile by post-coalification faulting. It is concluded that, although the model can explain the vertical pattern of rank, the complexity of the required faults render the model improbable. The alternative mechanisms for enhancement of the rank parameters associated with tectonic deformation, including shear stress control, frictional heating and localised fluid flow are discussed and are thought more likely to be responsible for the deviations from Hilt's law in the vertical rank profile. Rank suppression shown by three seams, two of which are overlain by marine bands, is thought to be related to biochemical activity in the peat environment resulting from the influx of saline waters.

Tree performance on minespoils in the South Wales coalfield

Summary 1. Variable growth of Larix leptolepis (Japanese larch) has been observed on restored opencast coal workings in South Wales. This has implications for the restoration of such sites. A study of the relationship of tree growth with minespoil chemical, nutritional, physical and hydrological factors was carried out. 2. Tree growth was positively related to nitrogen and phosphorus foliar concentrations, but negatively to those of magnesium. Seventy‐three per cent of the variation in tree growth was explained by variation in foliar chemistry. 3. Soil pH and extractable magnesium were negatively correlated with tree growth, with cation exchange capacity positively related to it. 4. Minespoils had bulk densities that commonly exceeded 1·7 g cm–3 below 0·2 m depth. Stone contents were high and typically 25% by volume. 5. Root systems of trees excavated were characterized by a high root density within 0·3 m of the minespoil surface. Restricted rooting was attributed to high bulk density and the incidence of shallow water tables. 6. Waterlogging during the spring and early summer, and the consequent presence of anaerobic soil conditions during periods of active growth, was found to be detrimental to tree growth. 7. The study suggests that landform design, selection of suitable soil or soil‐forming materials, spoil placement technique and appropriate species choice are central to the future success of forestry schemes on restored ground in South Wales.

Impact of sulphur and trace element geochemistry on the utilization of a marine-influenced coal—case study from the South Wales Variscan foreland basin

Incremental channel samples through two high volatile A bituminous coal seams, the Amman Rider and Bute seams, below and above the Vanderbeckei Marine Band at the Westphalian A/B boundary, have been analysed using proximate, petrographic, inductively coupled plasma mass spectrometry (ICP-MS), X-ray fluorescence (XRF) and scanning electron microscopy with energy-dispersive X-ray (SEM-EDX) techniques. The results show that both coals are vitrinite-rich and indicate a similar history of palaeomire development, with a period of falling water tables towards the middle plies of each seam. Mean sulphur contents in the coal with the marine roof, the Amman Rider seam (4.19 wt.%) are almost 4 times higher than in the coal with a non-marine roof, the Bute seam (1.09 wt.%), implying that high sulphur contents are derived from sea water sulphate. In both coals, sulphur is predominantly in the form of pyritic sulphur. Both syngenetic, framboidal and euhedral crystals, and epigenetic vein and cleat-fill pyrite are present. Relatively constant levels of organic sulphur at around 2% have been recorded in the Amman Rider seam. Sulphur levels in the basal and top plies of each seam are at least twice that of the ply with the lowest sulphur content in the seam, reflecting the stages in palaeomire development most strongly influenced by sulphate-containing waters. Thorium/Uranium ratios are below 2.9 in the coal with a marine roof, the Amman Rider seam, indicating an enrichment of U from sea water, most marked at the top and bottom of the seam. The seam without a marine roof shows Th/U ratios between 2.5 and 4.9, indicating mild marine enrichment of U. The concentrations of environmentally significant trace elements (ESEs) are correlated with either the ash or sulphur contents, or both. The correlation between S and Pb, As, Cu, Ni in the coal with a marine roof implies an origin of these ESEs from sea water and their mode of occurrence in sulphide minerals. Most ESEs in this coal are present at levels at least twice that of the global average in coals, the exceptions being Pb and Zn which have concentrations less than half the global average. By contrast, the coal without a marine roof and with lower sulphur contents has most ESEs present at concentrations less than a third of the global average, the exceptions being Ni and Cu. Diagenetic hydrothermal activity is documented in the middle ply of the coal with a marine roof. This hydrothermal system redistributed the sulphur, Pb and As in this ply. The principal utilization of bituminous coals in the South Wales coalfield is as feed coals in power station boilers. The resultant concentration of most ESEs in the solid residues from power plant coal combustion theoretically can be alleviated by careful blending of feed coals with low and high ESE contents.

Pneumoconiosis: a study of its effect on miners' health in South Wales 1900-1980.

Since its discovery in the late 19th century, pneumoconiosis has shaped the lives of thousands of miners. In some ways it became inextricably linked to the South Wales coal field and was a major public health threat to the men who worked in those mines. The disease was caused by the conditions of work and worsened the situation of many who were already living in poverty. This historical research account traces the miners' plight and touches on some of the public health issues raised by the disease.

Was the cirque glaciation of Wales time-transgressive, or not?

AbstractThe lowest glacial cirques in Wales are in the South Wales Coalfield and in western mid-Wales: the highest are in the northeast and on the highest mountains. Floor altitudes show great local variability, but in general rise to the northeast in most of Wales and northward in southern Wales, as does the former glaciation level (from 470–710 m a.s.l.). The pattern is similar for reconstructed Younger Dryas glaciers, which occupied the higher and even some of the lower cirques. If cirque development had spread to lower areas as the ice sheet built up, cirques would be expected in peripheral areas such as the northeast and southwest, away from the main ice-sheet sources. This is not seen and there is no clear evidence of time-transgressive cirque glaciation. Cirques relate to phases of glaciation with an ELA a little below that in the Younger Dryas.

The impact of tectonic deformation upon coal seams in the South Wales coalfield, UK

Observations on the nature and appearance of coal seams, which were deformed during the late Carboniferous Variscan orogeny, have been made over several years within a number of opencast coal sites located across the South Wales coalfield. The deformation of coal seams was dominated by brittle failure and subsequent movement along fracture planes. Examination of hand specimens and smaller-scale samples using a scanning electron microscope reveals the presence of two distinct types of small-scale, tectonic fracture markings within deformed coals. Striated fracture surfaces with prominent ridges and grooves can bear a superficial resemblance to shatter cones in appearance but are unique to coal in their tectonic setting and appear to have formed by static shear failure, whereas strongly polished fracture surfaces result from movement across fracture planes. Deformed coal seams are also affected by distinctive, large-scale, tectonic fracture sets. The extent of fracturing within coal seams can be assessed using different techniques including measuring fracture frequency directly, performing various strength and hardness tests, making relatively simple subjective assessments on the appearance of the coal or monitoring the size distribution of the coal product. By applying some of these methods within working opencast sites, it can be shown that large amounts of abnormally fractured coal were formed along coal-based thrust flats or detachments but that the distribution of fractured coal along detached seams is rarely homogeneous. Enhanced fracturing often occurred in association with the localised thickening of seams along small-scale thrusts or within coal duplexes. Variable seam thicknesses and pockets of structurally thickened coal are seen as the most efficient means by which the presence and location of significant amounts of fractured coal can be predicted from borehole evidence during prospecting.

Evaluation and Control of Employee Exposure to Diesel Particulate at Several Australian Coal Mines

Abstract Employee exposures to diesel particulate (DP) were measured under normal and abnormal mining operations at BHP Steel Collieries Tower Colliery and other underground coal mines in New South Wales coal fields. DP was measured using submicron samplers developed by the U.S. Bureau of Mines, and the results were verified using quantitative scanning electron microscopy. Personal monitoring (n = 480 full-shift samples) conducted at nine underground coal mines has indicated that the exposure of the work force ranges from less than 0.1 to 2.2 mg/m3 of DP, depending on job type and mining operation. Operations such as longwall moves and stonedusting, which generate elevated levels of dust, cause severe interference to submicron DP measurements. Based on previous knowledge that engine tune and condition affect the balance between gaseous and particulate emissions, five technologies for controlling DP were investigated in a combination of studies conducted in an above-ground simulated tunnel and in a special controlled section of underground mine roadway. Potential improvements were validated by application in standard coal mining operations at Tower Colliery. Tests conducted under controlled conditions indicate that, depending on fuel quality, levels of DP in return airways can be reduced by up to 50 percent, and in actual mining situations a reduction of 20 percent can be achieved in exposure of the work force by the use of low sulfur fuels. In addition, subjective responses from the work force indicate that exhaust emissions from low sulfur fuels provide lower irritation and a more pleasant aroma. The use of water-filled scrubber tanks reduces the level of DP emissions by 25 percent. In addition, it was found that the cleanliness of these tanks in general does not affect their scrubbing properties. Chemical decoking of engines resulted in a reduction of 20 percent in DP in return airways. However, the longterm benefits and cost effectiveness of this treatment in mining operations have yet to be established. A nonflammable, disposable dry exhaust filter constructed from synthetic organic fibers with an operational lifetime in excess of 20 hours was found to reduce DP exhaust emissions by 80 percent. Investigations have indicated that the use of increased ventilation to control DP levels does not follow a simple dilution factor, and in some instances compliance with current regulatory requirements may not produce the required reduced exposure levels. The results from single-component control strategies provide considerable reduction in exposure to DP. However, the most efficient and cost-effective control methodology is the use of a combination of individual systems modeled to operations conducted at each mine.

Variscan structures in the Kent Coalfield, southeast England, and their regional significance

The Upper Carboniferous Kent Coalfield lies concealed beneath various Mesozoic formations, its southern areas lying about 20 km north of the commonly accepted position of the main Variscan Deformation Front. However, despite intense intra-coal deformation, the existing literature is ambivalent about compressional Variscan features in Kent, the general view being that coal deformation is largely the product of the depositional environment. The main deformation is interpreted here as the result of Variscan compression, the structural style being imposed by the sandstone-dominated lithology. This conclusion is necessitated by the regularity of deformational structures revealed by mine workings, and supported by coal sequence irregularities suggestive of thrusting, especially in the lower Westphalian strata, all of which is paralleled in parts of the South Wales Coalfield. The Kent data indicate that, as in South Wales, a zone of thrusting many tens of kilometres wide lies in advance of the main deformation front. Structural trends are consistent with an overall swing in the front from east–west across much of central-southern England, to more northwest–southeast across northeastern France. This swing may represent a transpressional transfer zone, within which stress deflection and block rotation produced thrust vergence oblique to the overall direction of maximum compression.

97/02542 The South Wales Coalfield: low grade metamorphism in a foreland basin setting

97/02542 The South Wales Coalfield: low grade metamorphism in a foreland basin setting

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

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.

Coal rank variations with depth related to major thrust detachments in the South Wales coalfield: implications for fluid flow and mineralization

Abstract Coal maturity data in the form of volatile matter (daf and dmmf) and random vitrinite reflectance have been analysed for the South Wales coalfield. They show that in general coals increase in rank with depth, obeying Hilt’s law, and increase in rank laterally from high volatile bituminous coals in the south and east of the coalfield to anthracite in the northwest of the coalfield. The lateral increase in rank does not coincide with the basin depocentre which was located to the southwest of the coal basin during Westphalian times. The rank pattern with depth in the Westphalian A-Lower Westphalian C Coal Measures of the eastern half of the coalfield suggests a palaeogeothermal gradient of approx. 310°C km −1 , equivalent to a basal heatflow of 295 m Wm −2 . Investigation of vitrinite reflectance in a coal sequence repeated by intense Variscan thrusting indicates that coal rank was acquired both pre- and syn-thrusting. Detailed analysis of the volatile matter data reveals the presence of excursions from Hilt’s law present in one or more coal seams close to the boundary between Westphalian A & B. Of the 154 data sets analysed from the coalfield, 94 (61%) show one or more excursion. It is shown that the excursions correlate with thrust detachments within the coal seams, and it is argued that the excursions represent an increase in maturity temperature caused by fluids carrying heat into the coal seam along the seismically active thrusts. The fluids may also have been responsible for the carbonate, oxide and sulphide mineralization of the coalfield. Preliminary comparisons with the Ruhr coal basin in Germany suggest that future studies involving computer generated thermal models are required to understand the thermal evolution of both basins.

The South Wales Coalfield: low grade metamorphism in a foreland basin setting?

The South Wales Coalfield comprises Namurian, Westphalian and Stephanian sequences which developed in the external zone of the Variscan Orogenic Belt. Sedimentation is thought to have been concentrated in a paralic basin related to crustal flexure in a foreland basin setting, linked to Variscan tectonic loading to the south. It has long been recognized that coal rank varies across the Coalfield, from high-volatile bituminous rank in the east and southeast, to anthracite in the northwest. This is confirmed here on the basis of comprehensive volatile matter and vitrinite reflectance data sets, computer contoured to generate isovol and isoreflectance maps. A steady increase in coalification with depth is also recorded. Clay mineral investigations show the presence of illite and kaolinite across the entire coalfield area, while neither pyrophyllite nor mixed clays were detected. Illite crystallinity values from mudstones are in the range 1.20–0.44 Δ°2θ.Vitrinite reflectance data indicate maturation temperatures in the range 123–290 °C; clay mineral and IC values suggest temperatures up to 230°C, while previous descriptions of pyrophyllite from the Coalfield would allow locally for even higher temperatures. Coalification appears to have occurred rapidly (maximum 5 Ma) following deposition, implying high heat flux during Westphalian times. This is also indicated by fluid inclusion evidence from quartz crystals in clay ironstone nodules.The inference is of a high geothermal gradient (c. 50°C km-1) during Westphalian times, contrasting with an earlier gradient estimate of 20–37°C km-1. Relatively high gradients have also been determined previously for the Ruhr Coalfield, which occupies a similar tectonic setting in the external zone of the Variscan Orogenic Belt. Such a gradient is at odds with the proposed foreland basin tectonic setting for the South Wales Coalfield, in which low geothermal gradients would be anticipated.

Landslide research in the South Wales coalfield

The areal density of landslides in the coalfield of South Wales is one of the highest in the U.K. During the past 100 years landsliding has had considerable impact, causing structural damage and loss of life. Most of the landslides were initiated under periglacial conditions but many became reactivated due to the activities of man, particularly, during the late 19th century when widespread urban and industrial development commenced in the Welsh valleys. A number of the areas' larger landslides are first-time slides which occurred during the past 100 years. This paper sets out to chart the history of landslide research in the coalfield, which began through work by mining engineers.

Rapid failures of colliery spoil heaps in the South Wales Coalfield

Abstract The exploitation of the South Wales Coalfield, particularly from the last quarter of the nineteenth century onward, was accompanied by numerous instances of instability within spoil heaps and their foundation materials. Sudden failures which were sufficiently rapid to overwhelm property and services and, in some instances, to threaten life, occurred on at least twenty three occasions, details of which are provided. It is believed that thesel included sixteen flow slides, five debris slides and two failures caused by outbursts of groundwater. At five sites, debris flow was a secondary failure mechanism. The locations of these failures are mostly clustered in those parts of the coalfield with the highest relief and with the highest rainfall, although antecedent rainfall conditions for the failures were variable. Most are shown to be associated with active tipping faces, but one flow slide is believed to have occurred on a tip four years after its abandonment and an outburst failure on a tip fifteen years old. The occurrence of rapid failures is shown to mirror the development of the coalfield but has ceased largely as a result of legislation to improve tipping practice, which was enacted following the Aberfan flow slide.

An historical review of landslide research in the South Wales coalfield

The areal density of landslides in the coalfield of South Wales is one of the highest in the UK. During the past 100 years landsliding has had considerable impact, causing structural damage and loss of life. Most of the landslides were initiated under periglacial conditions but many became reactivated due to the activities of humans, particularly during the late 19 th century when widespread urban and industrial development commenced in the Welsh valleys. A number of the area's larger landslides are first-time slides which occurred during the past 100 years. This paper sets out to chart the history of landslide research in the coalfield, which began through work by mining engineers.

P-T conditions in the South Wales Coalfield: evidence from coexisting hydrocarbon and aqueous fluid inclusions

Siderite nodules in the Carboniferous Coal Measures of South Wales contain cavities which are often infilled with quartz, carbonates, sulphides, and hydrocarbons. The quartz contains a mixture of hydrocarbon and aqueous fluid inclusions. The aqueous fluid inclusions consist of a dilute brine (3 wt % NaCl equivalent) and have homogenization temperatures in the range 97–212 ºC (mean 143 ºC). The hydrocarbon fluid inclusions are dominated by methane with a small component of higher order hydrocarbons; their homogenization temperatures are in the range 35–78 °C (mean 54 °C). It is assumed that the two fluids were trapped simultaneously during growth of the quartz and thus a P–T estimate of entrapment can be obtained by graphical intersection of the hydrocarbon isochores and the aqueous fluid bubble point (homogenization) temperatures assuming hydrocarbon saturation. This method gives temperatures between 130 and 160 ºC, and pressures between 40 and 55 MPa. The timing of mineralization is uncertain, but it is suggested that it took place during burial and low grade metamorphism of the subsiding sedimentary basin (i.e. in the Upper Carboniferous). The hydrothermal fluids were probably derived from evolved meteoric or connate waters expelled during subsidence and sediment compaction.

Thrust-related permeability in the South Wales Coalfield

Abstract The South Wales Coalfield is a Variscan foreland basin extensively deformed by both linked and isolated thrusts in response to regional NW-SE compressive stress. Thrusts normally strike NE-SW, but transpression has caused a variable dextral rotation of the thrusts to strike E-W and even NW-SE. The range in orientation has allowed thrust-related fractures locally to be opened within the neotectonic stress field in which σ 1 is oriented NW-SE. It is argued that similar thrust-related permeability should be developed in other coal-bearing foreland basins both associated with the Late Carboniferous Variscan/Appalachian orogeny and with younger compressional tectonic systems. The dominant meso- to major-scale structures formed during the compression of coal-bearing sequences are thrusts and folds. Strains at leading and trailing tip-lines of isolated thrusts and in the immediate hanging wall and footwall generate tension cracks which may act as methane conduits. Unsealed, these allow permeability parallel to thrust strike. Décollements form as bed-parallel detachments within coals, developing as pervasive shear zones, characterized by cleavage duplexes and C-S fabrics in which a penetrative new hinterland-dipping fabric is formed. This fabric, under changing regional stress conditions, may be opened to form a highly effective gas migration pathway. Coal-bearing strata develop chevron folds with flexural slip in fold limbs and tension gashes in competent strata, generating porosity and permeability parallel to the strike of fold limbs. Incompetent coal seams are strongly sheared, producing cleavage duplexes with contrasting vergence in opposing limbs.

Coal, community, town planning and the management of labour: South Wales, c.1900–1920

Abstract This paper examines the development of the argument within the housing reform societies of Edwardian Wales that the ribbon type of housing development imposed upon the South Wales coalfield by the physical configuration of the region was in some measure responsible for labour problems in the coalfield. Drawing upon the work of Patrick Geddes, H.J. Fleure and Raymond Unwin, Welsh housing reformers held that the geography of the coalfield acted upon the processes of Anglicization and social change in the coalfield and, by limiting the horizons of a mono-industrial society, prevented the development of an appropriate'community sense' or spirit in the mining valleys. This viewpoint, developed after the shock of the Tonypandy riots and before the Addison Act, made a direct connection between housing reform and the improvement of labour relations in the South Wales coalfield. This concept came to frame the approach of investigations into mining life in the mining valleys of South Wales and gained official recognition with the publication by the Ministry of Health of the 1921 Report of the South Wales Regional Survey Committee.

Coal clasts in the upper Westphalian sequence of the South Wales coal basin: implications for the timing of maturation and fracture permeability

Abstract Coal clasts, common within channel lag deposits within the upper Westphalian C and Westphalian D Upper (Pennant) Coal Measures of the South Wales coal basin, represent reworked previously deposited coal-forming material. Analysis of clast shape, coal petrology and palynology at 16 localities within the eastern part of the South Wales coalfield suggests that three types of clast are present: (1) large elongate rafts of coal, with axial ratios up to 90:1, with irregular, often ‘fish-tail’ terminations and showing post-depositional compaction relative to the enclosing sandstone; (2) rafts of similar dimensions to (1), but showing no evidence for differential compaction and terminated by cleat fractures; (3) small near-equidimensional pebbles that either show no evidence for differential compaction or in which the surrounding sandstone shows greater compaction than the coal pebble. Both types 1 and 2 rafts have miospore assemblage ages that are indistinguishable from coal seams in adjacent sediments and show a similar range of vitrinite-dominated, maceral group composition. The pebbles have miospore assemblages suggesting derivation from coal seams ranging from a similar age to those of the rafts to Westphalian A (Lower Coal Measures) and with a wider range of maceral group composition than the rafts. Type 1 rafts were derived by erosion of partially consolidated and lithified peat deposits from the contemporary alluival plain, and cleat fractures were developed in situ during compaction. They formed as extensional cracks parallel to the regional NNW-SSE Variscan compressive stress. In type 2 rafts and pebbles cleat partly controlled the shape of the clasts and was developed before erosion of the clasts from their source coal seams. Vitrinite reflectance ( R m %) indicates that maturation of the rafts to bituminous coal rank developed in situ within the Pennant Measures, but in the pebbles maturation to bituminous rank may have been developed in the source coal seam before erosion. It is suggested that this maturation, and the concomitant compaction and cleat development, occurred within ≈ 1 Ma of deposition after rapid burial to ≈ 1 km in a high heat flow regime, associated with hot thrust-guided fluids in a foreland basin setting. Rapid maturation of type III kerogen into the oil and gas generation windows, together with the development of cleat, forming effective migration pathways, has important implications for coalbed methane exploration.