The National Coal Mining Museum in West Yorkshire affords a rare opportunity for the public to visit a former colliery (Caphouse) and experience at first hand the geology of a mine. The geology at the museum can be seen via the public tour, limited surface outcrop and an inclined ventilation drift, which provides the best geological exposure and information. The strata encountered at the site are c . 100 m thick and are of latest Langsettian (Pennsylvanian) age. The ventilation drift intersects several coal seams (Flockton Thick, Flockton Thin, Old Hards, Green Lane and New Hards) and their associated roof rocks and seatearths. In addition to exposures of bedrock, recent mineral precipitates of calcium carbonates, manganese carbonates and oxides, and iron oxyhydroxides can be observed along the drift, and there is a surface exposure of Flockton Thick Coal and overlying roof strata. The coals and interbedded strata were deposited in the Pennine Basin in a fluvio-lacustrine setting in an embayment distant from the open ocean with limited marine influence. A lacustrine origin for mudstone roof rocks of several of the seams is supported by the incidence of non-marine bivalves and fossilized fish remains whilst the upper part of the Flockton Thick Coal consists of subaqueously deposited cannel coal. The mudstones overlying the Flockton Thick containing abundant non-marine bivalves are of great lateral extent, indicating a basin-wide rise of base level following coal deposition that may be compared with a non-marine flooding surface.
Abstract Most deep-mined coal in the UK comes from the Westphalian A and B (Langsettian-Duckmantian) of the coalfields of the Pennine Basin, in which several sedimentary facies have been identified. Deep mining of coal may be divided into four main operations: shaft sinking and driving of access drifts; drivage of underground roadways; excavation of coal faces; and the provision of temporary underground storage. The prediction of the lithological, geometrical and geotechnical properties of the various sedimentary facies likely to be encountered during mining is important for the success of all these operations. Roadway location may be influenced by the geotechnical attributes of the sedimentary facies, which also govern the type of machinery and methods used to cut and support the roads. The thickness, quality and continuity of the coal, and the properties of the roof and floor, are important in selecting the location of longwall faces and in their productivity. Facies composed of strong sedimentary rocks are likely to be the most suitable for the location of staple shafts and bunkers for coal storage. The geotechnical properties of each of the sedimentary facies is described, with particular reference to their excavation and support characteristics, and the discontinuities of sedimentary origin that are likely to be present. Distal lacustrine delta facies appear to provide the optimum roof conditions for coal faces and roadways. A simple scheme for the prediction of the sedimentary facies which may influence mining is presented, and a variety of mapping techniques, including facies maps, isopach maps and palaeocurrent analysis, are outlined. An improved knowledge of the properties and distribution of the various sedimentary facies at a mine site should lead to improved productivity and a reduction in risks and costs.
Abstract The opencast coal industry seeks to identify and extract near-surface coal reserves. Within the UK Pennine Basin, most coal deposits suitable for opencast exploitation range from middle Westphalian A to lower Westphalian C in age. These deposits dominantly accumulated in an upper delta plain setting in which subsidence rates and depositional setting formed the primary controls on the initial distribution, thickness and quality of coal reserves. Hence an appreciation and analysis of the sedimentary setting and controls on sedimentation are important in the delineation of potential areas of thick, persistent coals. The calculations of in situ coal reserves and coal to overburden ratios are important objectives during exploration for prospective opencast coal sites. Coal reserves are detrimentally affected by early, depositionally related factors such as as wash-outs, seam splits, channel bank collapse and compaction faults. The thickness and type of interseam sedimentary rock (overburden) is directly related to the original despositional environment and the nature of the overburden can change substantially across the working area of an opencast site, reflecting sedimentary facies variations. The various sedimentary facies have unique characteristics and adversely influence mining in a variety of ways, including highwall stability problems, seam gradient changes, groundwater problems, interseam interval variations and facies-specific discontinuities. Many of these problems are related to the presence of palaeochannels. Palaeochannels occur at different scales and contain a variety of fills, and their identification is an important aspect of site exploration. Sedimentary facies mapping, incorporating facies analysis, the construction of isopach maps and palaeocurrent studies can play an important part in an opencast exploration programme. Facies maps have many applications which may be invaluable in site planning, including the delineation of areas of sedimentary disturbance, improved understanding of seam thickness and quality variations, and changes in the interseam strata.
Detailed mapping has been carried out in the roof strata of the Threequarters Seam (Carboniferous, Westphalian A) at Markham Colliery, North Derbyshire, England, where 'washouts', 'rock rolls' and sandstone bodies have caused difficult mining conditions. The interpretation of depositional environments and palaeocurrent studies has enabled an understanding of roof conditions, which is of potential value in planning mining activities. The workings are traversed by a major channel, which is accompanied by a belt of seam 'washouts'. The channel banks have undergone crevassing, giving rise to a smaller lateral crevasse channel with 'rock-rolls' at the base. A number of sharp-based crevasse splay sandstones, which thin distally, were fed into a shallow, floodbasin lake through the crevasse channel, which was eventually abandoned and plugged. Floodbasin lake-fill deposits form much of the seam roof and consist of a coarsening-upwards sequence of mudstones and lenticular-bedded siltstones. The interbedded crevasse splay sandstones often show wave- or current-reworked tops. Inundation of indigenous, non-marine bivalves by the crevasse splay sands has resulted in numerous Pelecypodichnus escape structures.
The Silkstone Rock (Westphalian A, Langsettian) is a major multistorey channel sandstone formed in a delta plain environment with minimal marine influence. It occupies up to three interseam intervals in the east Pennine coalfield, England, and exceeds 50 m in thickness and 15 km in width at its maximum development, which is larger than many Westphalian channel sand bodies. The high ash content of coal adjacent to the channel belt indicates that regional peat accumulation took place contemporaneously with the existence of the channel system. The major channel system fed a number of minor channels during different interseam intervals, and conducted sediment and water into nearby delta plain lakes, via lacustrine delta and crevasse splay systems. These minor channels, which show a range of fills, are sub-parallel and show lateral offset stacking, suggesting that differential compaction was an important control on their location. The major channel belt acted as a long-lived distributary system that flowed towards the east, away from the overall basin depocentre, with its position being controlled mainly by deltaic processes.
Abstract Carboniferous geology has developed very rapidly over the past two decades since ideas on sedimentology and new radiometric scales, together with new biostratigraphical schemes based on miospores, conodonts and foraminifera have been synthesized with ideas on plate tectonic processes, climatic changes and oscillations of sea level. The application of the McKenzie model of basin development to the Carboniferous, and the relating of eustatic sea-level changes, which had a profound effect on Carboniferous palaeo-geography, to fluctuations in the size of the Gondwanan ice sheets, have been major factors in the understanding of Carboniferous processes. The Carboniferous evolution of Britain took place to the (present) north of a plate suturing occurring along a line extending from Galicia, through Armorica and the Massif Central to the Vosges, during mid Devonian to early Carboniferous times. In front of this collision zone was the Rheno-Hercynian zone which opened probably during mid Devonian times and which was partly floored by oceanic crust; its closure can be traced through to late Carboniferous times as a series of northwardly directed thrust complexes. To the (present) north of this zone was a foreland on which basins developed on either side of a persistent Wales-London-Brabant High. Studies by Dewey (1982) and Leeder (1982, 1987, 1988) have shown that late Devonian to early Carboniferous extension, involving β factors of up to 2, produced a series of grabens and half-grabens in the Caledonian basement. These developing structures were the dominant control on early Carboniferous sedimentation and produced a sea-bed topography which was
SUMMARY The ‘Dumb Fault’ is a belt of Westphalian A (Langsettian) disturbed strata, that has been re-examined in an opencast mine in Derbyshire. The Deep Hard Seam is absent beneath the ‘Dumb Fault’, and anomalously thick coals occur on either side of the belt. A swilley, consisting of an elongate depression infilled in its lower part with impure coal, is present in the overlying Deep Soft Seam, and anticlinal folds occur in underlying seams. The ‘Dumb Fault’ is not a fault, but a minor channel system, that was subject to episodes of bank collapse which removed and displaced peat masses, forming the anomalously thick coal. Abandonment of the channel left an unplugged reach, which was filled with plant debris forming the swilley. Burial compacted the thickened displaced peat masses, with the anticlines in the underlying seams developing to accommodate the compaction. The potential adverse effects on mining of the features described include loss of coal tonnage, problems of excavation, highwall failure, difficulties in digging clean coal and changes of seam gradient.
An extensive and closely-spaced seismic and well dataset from coal and hydrocarbon exploration in the East Pennine Coalfield and the East Midlands Oilfield has enabled the detailed subsurface correlation and mapping of major Westphalian A to C sandbodies. Maximum flooding surfaces are represented by marine bands which are faunal concentrate condensed horizons. The large fluvial sandbodies do not show characteristics of incized valley fills. The remoteness of the study area from the sea during deposition meant that eustatic changes are less significant than other factors. The interplay of tectonics, climate and autocyclic processes was responsible for exceedingly complex sequences in terms of both lateral and vertical organization, and it is difficult to isolate the relative importance of the various mechanisms controlling sedimentation. Closely spaced onshore borehole data and outcrop studies have been used to model reservoir characteristics and continuity offshore in the Southern North Sea, where well spacing is far greater. Architectural variability and 3D geometry of onshore sandbodies can be used as an analogue for Southern North Sea reservoirs. Geometric data from 20 mapped onshore sandbodies illustrates a relationship between sandbody width and thickness, with 90% of channel sandbodies being less than 25 km in width and less than 40 m in thickness. A remarkable similarity exists between the sand distribution onshore UK and Westphalian reservoir intervals in the Southern North Sea. Sandbodies onshore and offshore show comparable thicknesses (up to 100 m) and widths (up to 30 km). Additionally, the sandbodies do not display incised bases, as is also typical of all the main Westphalian reservoir targets in the Southern North Sea.
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