Any significant lateral or vertical structural change in a coal seam has a direct bearing on its thickness, quality and mine ability. Such changes can be on a small or large scale, affect the internal character of the coal, or simply displace the coal spatially, replacing it with non-coal sediment, or, in certain circumstances, with igneous intrusives. Disruption to coal seam thickness and continuity can lead to the interruption or cessation of mining, which will have economic repercussions, particularly in underground mines where mining flexibility is reduced. Therefore an understanding of the structural character of a coal deposit is essential in order to perform stratigraphic correlation, to calculate coal resource/reserves, and to determine the distribution of coal quality prior to mine planning.
The majority of coal-bearing sediments are deposited in or on the margins of tectonic basins. Such a structural environment has a profound influence on the accumulating sediments both in terms of the nature and the amount of supply of detrital material required to form such sequences, and on the distribution and character of the environments of sedimentation. In addition, diagenetic effects within the accumulating sediments produce structural deformation; this may be due to downward pressure from the overlying strata, and may be combined with water loss from the sediments when still in a non-indurated or plastic state.
Microstructural effects
The combination of thick sediment accumulation and rapid basin subsidence can produce instability particularly along the basin margins. The effects on coal-bearing sediments are frequently seen in the form of slumping and loading structures, and liquifaction effects, with the latter being characterized by the disruption of bedding laminae and the injection of sediment into the layer above and below. Under such loading effects, coal may be squeezed into over lying strata and the original seam structure may be completely disrupted. In addition, coals may be injected by surrounding sediment in the form of sedimentary dykes. Inter-bedded sequences of mudstone, sandstone and coal that have undergone loading deformation exhibit a variety of structures such as accentuated loading on the bases of erosives and stones, flame structures, distorted and dislocated ripples, and folded and contorted bedding. Instability within environments of deposition, whether induced by fault activity or simply by overloading of accumulated sediment, can produce movement of sediments in the form of gravity flows. If a coal is transported in this fashion, the result can be an admixture of coal material and other sediment with no obvious bedding characteristics.
Macrostructural effects
Within sedimentary basins, existing faults in the underlying basement may continue to be active and influence the location, thickness and character of the sedimentary sequence. Many coal-bearing basinal sediments display evidence of growth faulting. In West Virginia and Pennsylvania, United States, broad-scale tectonic features have caused local thickening of the sequence in response to an increased rate of subsidence, as distinct from more stable platform areas (less rapidly subsiding), where sedimentation prograded rapidly over the shelf. In South Wales, United Kingdom, growth faults have again influenced sedimentation in addition to active basement elements, faults are developed that owe their origin to gravity sliding within the sedimentary pile. Over-pressured, non-compacted argillaceous sediments initiate faults on gentle gradients. Such faults tend to have a curved cross-sectional profile, steep at the top and flatten progressively into bedding plane faults, often along the roof of a coal. In many cases such faults are partially eroded before the succeeding sediments are laid down. Seam splitting can also, in certain circumstances, be attributed to growth faulting. Reactivation of faults with changes in the sense of movement can result in the down warping of sections of peat beds, this is then followed by non-peat deposition on the down warped section, and then peat deposition resumed at the original level of the first peat. Periodic changes in base level in deltaic areas through fault activation will result in changes in the development and character of coals. With emergence, coals may become more extensive and, where the influx of detritus is curtailed, have a lower ash content. If submergence occurs, coals may be restricted areally, or receive increased amounts of detritus, which may increase the ash content or even cease to develop at all. Furthermore, submerged coals may be contaminated with marine waters, which could result in a higher sulphur content in the uppermost parts of the seam. Growth folds also influence the deposition patterns in coal basins, local up warping can accelerate the rates of erosion and deposition in some parts of a basin, but can also have the effect of cutting off sediment supply by uplift or by producing a barrier to the influx of detritus. In very thick sedimentary sequences, the continued growth of such folds can result in the production of over steepened fold axes. Where this occurs, over-pressured mudstone at depth may be forced upwards and actually breach the anticlinal axial areas, this can be seen by the breaking up of the surface strata and the intrusion of material from below. Such diapiric intrusion breccia can be found in East Kalimantan, Indonesia, and these are often accompanied by the development of mud volcanoes along the axial region of the anticlines. Development of diapiric structures can disrupt as well as distort coal beds; in the Bełchat´ow opencast coal mine in Poland, a large diapir has intruded into the coal-bearing sequence, dividing the coal reserve into two distinct areas. In the Kutei Basin in East Kalimantan, Indonesia, the established structural pattern continually evolved throughout the Paleogene–Neogene Periods. In this area, the anticlines are tight with steep or overturned dips accompanied by steep reverse and normal faults in the complex axial regions. The synclines are broad and wide with very low dips, the transition between the two structures can be abrupt, now represented by steep reverse faults. These growth folds are thought to have been further accentuated by gravity sliding associated with very thick accumulation of sediment (up to 9000 m) in the Kutei Basin, and rifting in the Makassar Strait to the east. The structural grain and the paleo-strike were roughly parallel in this region, and the resultant sequence is characterized in its upper part by upper delta plain and alluvial plain sedimentation with numerous coals. Penecontemporaneous volcanism can also have a profound effect on the character of coals. Large amounts of airborne ash and dust together with water borne volcanic detritus may result in the deposition of characteristic dark lithic sandstones, possible increases in the ash content in the peat mires, and the formation of tonstein horizons.
Post-depositional effectsAll coal-bearing sequences have undergone some structural change since diagenesis. This can range from gentle warping and jointing up to complex thrusted and folded coalfields usually containing high rank coals. These post-depositional structural elements can be simply summarized as faults, joints (cleat), folds and igneous associations. Mineral precipitation may also produce some changes in the original form and bedding of coal-bearing sequences.
Jointing/cleat in coalCoal, and in particular all ranks of black coal, is noted for the development of its jointing, more commonly referred to as cleat. This regular pattern of cracking in the coal may have originated during coalification, the burial, compaction and continued diagenesis of the organic constituents results in the progressive reduction of porosity and permeability. At this stage micro-fracturing of the coal is thought to be generated. The surfaces and spaces thus created may be coated and filled with mineral precipitates, chiefly carbonates and sulphides. Cleats are fractures that occur in two sets that are, in most cases, mutually perpendicular, and also perpendicular to bedding. Abutting relations between cleats generally show one set pre-dates the other. Through-going cleats formed first and are referred to as face cleats, cleats that end at intersections with through going cleats formed later and are called butt cleats. These fracture sets and partings along bedding planes impart a blocky character to coal. Cleats are subvertical in flat-lying beds and are usually orientated at right angles to the bedding even when strata are folded. In a number of cases, cleats are confined to individual coal beds, or to layers composed of a particular maceral type. These are usually uniform in strike and arranged in sub-parallel sets that have regional trends. Tectonism may obliterate previously formed cleats in anthracites, if this is so, then regular variations in cleat spacing with rank in the range lignite to high volatile bituminous coal might not occur.
Faulting
The development of strong joint and fault patterns in coal bearing sequences is the commonest post-depositional structural expression; principal fault types are described briefly in the following paragraphs. Normal faults, these are produced by dominantly vertical stress resulting in the reduction of horizontal compression, leaving gravity as the active compression, which results in the horizontal extension of the rock sequence. This form of faulting is common, movements can be in the order of a few metres to hundreds of metres. The dip of normal faults ranges widely, in coalfields most are thought to be in the region of 60–70◦. Some normal faults die out along their length by a decrease of throw towards either one or both ends of the fault. Again a fault may pass into a mono-clinal flexure, particularly in overlying softer strata. Such faulting also produces drag along the fault plane, the country rock being pulled along in the direction of movement. Where large faults have moved on more than one occasion, and this applies to all kinds of faulting, a zone of crushed coal and rock may extend along the fault plane and have a width of several metres. Large-scale normal faults are produced by tensional forces pulling apart or spreading the crustal layer; where these faults run parallel, with the down faulted areas in between, they are known as graben structures. Many coalfields are preserved in such structures: the brown coalfields of northern Germany and eastern Europe, and the Gondwana coalfields of India and Bangladesh are examples. Low-angle faults with normal fault displacements are known as lag faults. They originate from retardation of the hanging wall during regional movement. Lag faults are common in the coalfield of South Wales, United Kingdom. Reverse faults are produced by horizontal stress with little vertical compression, which results in the shortening of the rock section in the direction of maximum compression. Very high-angle reverse faults are usually large structures, associated with regional uplift and accompanying igneous activity. In coal geology, those reverse faults with low angles <45◦ are more significant. When the angle is very low, and the lateral displacement is very pronounced, such faults are termed thrust faults. The shape of such low-angle reverse faults is controlled by the nature of the faulted rocks, especially when a thrust plane may prefer to follow the bedding plane rather than to cut across them. In typical sequences of coal, seatearth and mudstone with subordinate sandstone, such low-angle faults often follow the roof and/or the floor of coal seams as these allow ease of movement, the seatearths often acting as a lubricant. One detrimental effect is the contamination of the coal seam with surrounding country rock, thereby reducing its quality and, in some cases, its mine ability. In highly tectonized coal deposits, a great number of coal seam contacts have undergone some movement and shearing, in some cases the whole seam will have been compressed and moved. Thrusting is also accentuated where coal and mudstone sequences are sandwiched between thick sequences of coarse clastic rocks, the upper and lower portions of the sequence reacting to compressive forces quite differently to the incompetent coals and mudstones. Strike-slip faults have maximum and minimum stress in the two horizontal planes normal to one another. This has the effect of producing a horizontal movement either in a clockwise (dextral) or anticlockwise (sinistral) sense. Strike-slip faulting is usually found on a regional scale, and although important, has a lesser influence on the analysis of small coal deposits and mine lease areas. Evidence of faulting on the rock surface can be seen in the form of slicken sides, which are striations on the fault plane parallel to the sense of movement. Some fault planes have a polished appearance, particularly where high rank coal has been compressed along the fault plane. Conical shear surfaces are characteristically developed in coal, which are known as cone-in-cone structures, and are the result of compression between the top and bottom of the coal. Coal responds in a highly brittle manner to increasing deformation by undergoing failure and subsequent displacement along ever increasing numbers of fracture surfaces.
Folding
Coals in coal-bearing sequences may be folded into any number of fold styles. In coalfield evaluation, the axial planes of the folds need to be located and the dips on the limbs of the folds calculated. In poorly exposed country the problem of both true and apparent dips being seen has to be carefully examined. Also in dissected terrain, dips taken at exposures on valley sides may not give a true reflection of the structural attitude of the beds at this locality, many valley sides are unstable areas and mass movement of strata is common, resulting in the recording of over-steepened dips. This is characteristic of areas of thick vegetation cover where a view of the valley side is obscured and any evidence of movement may be concealed. In underground operations, if the dip of the coal seams steepens, it can make the working of the coal difficult, and in the case of long wall mining, prevent further extraction. Therefore it is important to be sure that all readings taken reflect the true nature of the structure in the area of investigation. Compression of coal seams during folding can produce tight anticlinal folds with thrusting along the nose of the fold, these have been termed queue anticlines. Coal seams can be pinched out along the fold limbs and appear to have flowed into the axial areas of the anticlines. Where this occurs from two directions approximately normal to one another, coals can be concentrated in ‘pepperpot’ type structures. Such features are usually found only in highly tectonized coal fields and examples of such intense deformation.
Igneous associationsIn many coal fields associated igneous activity has resulted in dykes and sills being intruded into the coal-bearing sequence. The intrusion of hot molten rock into the coals produces a cindering of the coal and a marked loss in volatile matter content which has been driven off by heat. This can have the effect of locally raising the rank of lower rank coals, and can therefore in certain circumstances make the coal attractive for exploitation. Such ‘amelioration’ of coal seams is a common feature in areas of igneous activity, and good examples are found in Indonesia and the Philippines where Paleogene–Neogene sub-bituminous coals have been ameliorated up to low volatile bituminous and some even to anthracite rank. The majority of dykes and sills are doleritic in composition, as in the case of South African and Indian coal fields, but occasionally other types are found. Igneous sills have a tendency to jump from one coal seam to another so that close spaced drilling is often required to identify precisely the nature and position of such intrusions. Igneous intrusions are found in coal sequences worldwide, but in particular are a common feature of South African coal workings. Where such igneous bodies exist, the coal geologist must identify the areas occupied by igneous material within the mine area, and also those seams affected by igneous activity. In addition, the possibility of methane gas driven off during intrusion may have collected in intervening or overlying porous sandstones. Mine operatives need to investigate this possibility when entering an intruded area of coal.
Mineral precipitatesA common feature of coal-bearing sequences is the formation of ironstone, either as bands or as nodules. They usually consist of siderite (FeCO3) and can be extremely hard. Where iron stone nucleation and development takes place either in, or in close proximity to a coal seam, this can deform the coal, cause mining difficulties, and, because of the difficulty in separating coal and ironstone when mining, will have an effect on the quality of the run-of-mine product. Iron sulphide (FeS) in the form of iron pyrite may be precipitated as disseminated particles, as thin bands, or as is more common, as coatings on cleat and bedding surfaces. Inorganic sulphur held in this form in coal can be removed by crushing and passing the coal through a heavy liquid medium. Organic sulphur held elsewhere in the coal cannot be readily removed, and remains an inherent constituent of the coal. Other mineral precipitates usually are in the form of carbonates, coating cleat surfaces, or occasionally as mineral veins. Where quartz veining occurs, this has the detrimental effects of being hard, liable to produce sparks in an underground environment where gas is a hazard and also when crushed is an industrial respiratory health hazard.
