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III. Structural and sedimentologic patterns of the Wellington coal bed at Wolf Mountain Colliery

3.1 Introduction

Syndepositional faulting in coal-bearing strata has long been recognized, particularly in British and American Carboniferous rocks (Raistrick and Marshall, 1939; Broadhurst and others, 1968; Broadhurst and Simpson, 1983; Weisenfluh and Ferm, 1984), but these studies have largely relied upon drill logs and mine plans as data sources. There is a general lack of detailed information about the sedimentary response to contemporaneous faulting in coal measures. This chapter describes and interprets the interaction between faulting and sedimentation in the Wellington Seam, based upon detailed study of the Wellington Seam at Wolf Mountain Colliery (Maps 1 and 2), where excellent exposures have been created during room-and-pillar mining of the coal.

3.2 Coal bed details

The total thickness of the Wellington Seam at Wolf Mountain ranges from 1.5 to over 5 metres, of which 65 to 95% by thickness is coal and the remainder is partings. Three laterally continuous coal beds (the Wellington Rider bed, the Upper Wellington bed and the Lower Wellington bed) and two major partings (the upper and lower partings) constitute the Wellington Seam at Wolf Mountain (Table 1-2; Figure 3-1). Figures 3-3, 3-4 and 3-5 exemplify the local variability of the Wellington Seam's constituent coal beds and partings.

3.2.1 Coal characteristics

The three coal beds within the Wellington Seam can be readily differentiated on the basis of their characteristic lithotypes. All three coal beds maintain their distinctive characteristics throughout the mined area at Wolf Mountain.

Coal of the Wellington Rider bed is bright banded to bright, hard and blocky, with a very thin (1 to 2 cm) but persistent marker band of black, arenaceous coaly mudstone. Coal of the Upper Wellington bed is dull to dull and bright, thin-bedded and shaly in appearance, with lenses of black coaly mudstone. One or more bands of hard, dull grey coal with a distinctive submetallic lustre are often present within this unit. Coal of the Lower Wellington bed is bright banded and massive, with isolated lenses of dirty coal and coaly and carbonaceous mudstone.

3.2.2 Characteristics of partings

The two major rock partings within the Wellington Seam are distinguished mainly by their stratigraphic positions within the seam. At Wolf Mountain the upper parting, between the Wellington Rider and Upper Wellington coal beds, is 5 centimetres to over 5 metres thick, whereas the lower parting, between the Upper Wellington and Lower Wellington coal beds, is 10 to 40 centimetres thick.

Both partings locally split the Wellington Seam. The lower parting causes a split to the southwest of the mine, as indicated by boreholes beyond the mine workings. The lower parting has had little effect on mine operations thus far, owing to its smaller thickness within the mined area. The upper parting causes a split to the southeast, and thickens in the southeastern workings. Owing to its impact on mining operations and excellent exposure at Wolf Mountain Colliery, the upper parting has been studied in detail.

Both partings are generally structureless where they are thinner than 0.1 metres. Where the partings attain a greater thickness, they consistently display normal fining-upward grading, and become coarser-grained overall.

Throughout most of the mine, both partings consist of fine-grained, more or less carbonaceous clastic sedimentary rock, ranging from soft black coaly mudstone through brown carbonaceous mudstone to hard light brown siltstone.

Within the existing mine workings, the upper parting thickens and coarsens southeastward (Figure 3-6), passing laterally from 1 to 2 centimetres of soft black coaly mudstone, to 30 centimetres of moderately hard brown silty mudstone, to over 5 metres of hard sandstone and sandy siltstone. Figure 3-3 shows the rapid onset of splitting on the upper parting, and the extent to which the parting locally scours down into the underlying coals.

3.3 Structural features at Wolf Mountain Colliery

Wolf Mountain Colliery is situated on the eastern end of a broad, doubly-plunging syncline. Limb dips in the mined area range from 10 to 16 degrees, and the synclinal axis plunges westward at 3 to 5 degrees. Two major faults cross the mine area, and have sufficient downthrow (on the order of a few metres) to constitute barriers to mine development. The roof of the Wellington Seam is displaced by many smaller extensional and compressional faults (Figure 3-7), but is otherwise structurally simple, with only gentle local warps superimposed upon the broad synclinal structure. Besides the broad syncline, more complex structures occur on the floor of the coal bed, as shown by detailed floor elevation surveys (Figure 3-8). Two types of local floor irregularities have been recognized: 'floor rolls', which are local topographic highs, and 'swilleys', which are local depressions.

3.3.1 Faults

Two major faults have been encountered within the workings of Wolf Mountain Colliery. These faults are shown as heavy dark lines of Figures 3-6 through 3-10. Both major faults have extensional displacements of 0.6 to 5 metres down to the north or west. Across the more southerly of the two major faults, substantial changes occur in the thickness of individual coal and dirt bands, along with a consistent local rise in the floor on the immediate northwestern side of the fault (Figures 3-5, 3-6 and 3-9). Across the other, more westerly major fault, the only effect appears to be local shearing and flow of the Wellington coal from the western (downthrown) side to the eastern (upthrown) side of the fault (Figure 3-7).

Minor extension faults are abundant, occurring as two sets, both of which cut the roof and the Wellington Rider coal, and appear to flatten within the upper parting. The dominant set strikes to the northwest, displays consistent downthrow to the northeast, and shows good lateral continuity. This set of faults occurs at intervals of approximately 80 metres. A subordinate set of faults strikes to the northeast, has downthrow to both the northwest or southeast, and shows little lateral continuity. The subordinate faults tend to occur in closely spaced swarms.

Minor compressional faults, ranging from steep thrusts to near-bedding-plane shears, are the next most abundant structures. They strike to the northwest, paralleling the dominant set of minor extensional faults.

A few minor rotational faults are also present in the immediate roof of the Wellington Seam. Their sense of displacement changes along strike from compressional to extensional, passing through an intermediate zone of no apparent offset.

3.3.2 Floor rolls

A floor roll is a local topographic and structural high at the base of a coal bed, over which the coal may either flex upward or thin. The term 'floor roll' is in broad use in Canadian coal mines, although its original origin is obscure; it is geometric rather than geologic in nature, and does not imply any particular mode of origin for the feature.

Floor rolls beneath the Wellington Seam at Wolf Mountain Colliery are 20 to 30 metres wide, and 30 to 90 centimetres high. They usually form gentle swells in the floor, over which the coal thins. Onset of floor rolls is occasionally marked by steep upward slopes in the floor. Dips of these steep faces, upon which a polished coal-floor contact is occasionally present, range from 25 to 35 degrees, and their heights range from 60 to 90 centimetres.

Floor rolls consist of medium-grained, carbonaceous, root-penetrated sandstone which is indistinguishable from the usual floor of the Wellington Seam. The coal abruptly overlies the top of the floor rolls. Along the flanks of the floor rolls, banding in the coal is asymptotic to the floor, and there is no sign of intertonguing of coal with the rock of the rolls.

Floor rolls are present throughout Wolf Mountain Colliery; in the northern workings they occur as a fairly persistent northwest-striking swarms, spaced at 40 to 60 metre intervals, with steep faces on their southwestern sides. Elsewhere in the mine, the distribution of floor rolls is more irregular.

Floor rolls are also present in the Douglas Seam within the Pender Formation of the Nanaimo Coalfield. Clapp (1914) considered these features to be minor folds of the Douglas Seam and its shale floor. He considered the rolls to have formed as a result of lateral sliding of the Douglas Seam over its floor, which was relatively weak in comparison with its sandy shale roof. Clapp noted that the coal above the rolls was dirty and soft, whereas the floor of the Douglas Sea, was nearly always sheared and slickensided at the rolls. Such pervasive shearing within and above floor rolls is not present in the Wellington Seam at Wolf Mountain.

3.3.3 Swilleys

A swilley is a local depression in the floor of a coal bed, which may be filled either with coal or with interbedded coal and fine-grained clastic sedimentary rock. When coal forms the filling of swilleys, it is often bright and high in ash, or may consist of cannel. Banding in the filling of a swilley abuts against the sides of the swilley (Elliott and others, 1984).

The term 'swilley' is geometric rather than geologic in nature, and does not imply any particular mode of origin for the feature.

Swilleys beneath the Wellington Seam at Wolf Mountain are 20 to 40 metres wide, and 0.4 to 2.1 metres in amplitude (Figure 3-8). Their margins are usually gentle slopes of 5 to 10 degrees, although local 'steps' up to a metre high display dips as steep as 60 degrees. The swilleys are sinuous in plan, and at one locality a near right-angle bend in a swilley is accompanied by a rapid 1.5 metre step downward of the swilley's floor. One of the swilleys at Wolf Mountain has two short branches joining it from the north.

The swilleys are filled with bright banded coal, occasionally accompanied by thin bands of coaly mudstone. Banding in the coal and mudstone abuts against the sides of the swilleys, and usually bends upwards in an aysmptotic manner immediately adjacent to the sides of the swilleys. The coal in the swilleys is indistinguishable from that forming the basal part of the Wellington Seam outside the swilleys. The floor and sides of the swilleys consist of rooted sandstone similar to the sandstone beneath the Wellington Seam outside the swilleys.

3.4 Effects of structural features on seam thickness

3.4.1 Thickness variations associated with faults

The Wellington Seam is consistently thinner on the downthrown sides of the two major faults. In the case of the more westerly fault, attenuation of the coal bed is largely due to shearing and squeezing of coal in a narrow belt immediately adjacent to the fault. Coal appears to have flowed cataclastically from the downthrown to the upthrown side of the fault, and has at one point formed a coal dike in the roof.

Thickness and lithologic trends in the upper parting are generally parallel to the more southerly of the two major faults (Figure 3-6). The other major fault appears to have no effect on either thickness or lithology of the parting.

On the northwestern side of the southerly fault, the upper parting consists of 5 to 30 centimetres of soft black coaly mudstone, locally grading at its base to dark brown carbonaceous or brown silty mudstone. Thickness variations appear to be random in this area.

On the southwestern side of the southerly fault, however, the upper parting becomes thicker and coarser-grained. As the parting thickens from 20 to 60 centimetres, it passes from dark brown carbonaceous mudstone through brown silty mudstone to hard siltstone. Isopachs of the upper parting generally parallel the fault, although there is much local variation in both thickness and lithology of the parting (Figures 3-4 and 3-6).

The 60 centimetre isopach of the upper parting (Figure 3-6) marks the limit of mineability and the onset of rapid thickening of the upper parting, and is therefore mapped as the line of split for practical purposes. The parting thickens rapidly to the southeast, doubling in thickness within a distance of 10 to 15 metres. At the southeastern corner of the mine workings, the upper parting is more than 5 metres thick, consisting of a fining-upward sequence of clean rippled sandstone, silty sandstone and sandy siltstone. The basal sandstone of the parting scours down into the underlying Upper Wellington coal bed, and eventually truncates the coal altogether (Figs. 3-3 and 3-11).

3.4.2 Thickness variations associated with floor rolls

The Wellington Seam thins over floor rolls. The magnitude of thinning ranges from 0.3 to 0.9 metres, and is consistently less than the height of the floor rolls, which ranges from 0.4 to 1.2 metres. This difference is probably due to differential compaction of peat over the relatively incompactible sand of the floor roll (Cairncross, 1989). The roof of the coal bed arches gently over floor rolls. Over steep faces of floor rolls, the roof is often broken by minor faults which are subparallel to the steep faces, and have downthrows of 0.15 to 0.45 metres away from the rolls (Figure 3-12).

The total thickness of rock bands within the Wellington Seam decreases over the floor rolls and increases between them (Figure 3-10), with the greatest effects seen in the thickness of the basal partings within the Lower Wellington coal. The overall proportion of coal to rock is slightly lower over the floor rolls, owing to the attenuation of the relatively clean Lower Wellington coal.

3.4.3 Thickness variations associated with swilleys

The Wellington Seam thickens, in places quite dramatically, in swilleys (Figure 3-8). The magnitude of thickening ranges from 0.3 to nearly 2 metres, and is generally slightly less than the depth of the swilleys. The difference in magnitude of thickening is made up by broad, gentle sags of the roof, which conform to the course of the swilleys.

The total thickness of rock bands (Figure 3-10) increases slightly over swilleys, due to the presence of lenses of coaly mudstone in the basal part of the Lower Wellington coal within the swilleys. These mudstone beds pinch out against the margins of the swilleys. The thicknesses of the major rock bands, higher in the coal bed, do not change over swilleys. The overall proportion of coal to rock is, however, slightly higher over swilleys, due to the marked thickening of the Lower Wellington coal.

3.5 Roof characteristics

The immediate roof of the Wellington Seam at Wolf Mountain is a 5 to 6 metres thick unit, which coarsens upward from massive, plant-bearing siltstone through interlaminated siltstone and sandstone to fine-grained silty sandstone (Figure 3-1). The basal contact of the roof with the underlying coal is usually abrupt. In the southwestern corner of the mine, where the two major faults converge, the roof-coal contact is irregular and rough, and the basal 2 to 3 metres of the roof is locally slickensided or brecciated.

3.5.1 Coal and sandstone dikes

Coal and sandstone dikes are infrequent but significant features of the roof of the Wellington Seam at Wolf Mountain. Both types of dikes are narrow, elongate, steep-dipping bodies of material quite different from the usual roof rocks, and occur in close association with faults.

Coal dikes are bodies of intensely sheared and slickensided coal, which occur in the immediate footwall of major faults, and are parallel to them (Figure 3-13). Coal dikes range in width from 0.3 to 2.5 metres, and tend to pinch and swell along strike. Exposure of the upward extent of coal dikes is limited to those localities where they have either collapsed into the mine workings, or have been deliberately excavated during mining. In some areas coal dikes extend at least 4 metres up into the roof. They are bounded by narrow (3 to 5 metres) belts of strongly jointed roof.

Sandstone dikes are bodies of fine-grained, slightly silty, well-indurated sandstone, which project down through the immediate roof and "V" downward into the top of the Wellington Rider coal. Sandstone dikes seldom penetrate more than 10 centimetres into the coal. They parallel, and occasionally pass laterally into, the dominant set of minor extensional faults. The sandstone dike material resembles the sandstone which occurs in the roof, 3.5 to 6 metres above the Wellington Seam.

3.6 Sedimentologic interpretations

3.6.1 Floor rolls

A variety of origins have been suggested for floor rolls in coal mines. Clapp (1914) considered the floor rolls in the Douglas Seam of the Nanaimo Coalfield to be minor folds, related to lateral movement of the Douglas Seam over its relatively incompetent floor. Ward (1984) suggested that floor rolls could also be formed by intrusion of underlying strata into a coal seam following hydration-induced swelling.

A tectonic origin for the floor rolls at Wolf Mountain is unlikely, due to the relatively strength of the sandstone floor as compared with the siltstone roof of the Wellington Seam. If minor folds had formed due to lateral sliding of the Wellington Seam, it is more probable that the folds would occur at the contact of the coal and its relatively incompetent roof. The floor rolls are unlikely to have formed by hydration-induced intrusion, since the floor of the Wellington Seam does not contain moisture-sensitive materials.

Several authors have proposed that floor rolls are sedimentary structures. Diessel and Moelle (1970) and Cairncross and others (1988) have suggested that floor rolls represent the fillings of river channels which either flowed beneath peat deposits (with floating peat roofs) or were incised within and confined by the peat, which later filled the channels following their abandonment. In both theories, the floor rolls have been reported to at least locally have interfingering contacts with the coal; interfingering contacts between floor rolls and coal have not been observed in the case of the Wellington Seam. Macfarlane (1985) has suggested that floor rolls represent scroll bars, part of fluvial point bar complexes which have been buried by peat. Bunnell and others (1984) have interpreted floor rolls as relict beach ridges.

While no shell fossils have been recovered from the sandstones beneath the Wellington Seam, the presence of Macaronichnus segregatus trace fossils approximately 6 metres below the base of the coal together with the coarsening-upward grain-size profile of the sandstone suggests that it was deposited in a shallow marine, shoreface environment. The floor rolls do not display sufficient continuity to be beach ridges, but they may represent sand bars which were formed at a beach surface and then subsequently uplifted and buried by peat.

3.6.2 Swilleys

Elliott (1965) interpreted features which were similar to, but broader than the Wolf Mountain swilleys, as abandoned river channels. Elliott and others (1984) extended this interpretation to include lake basins. They observed that the floor of the swilleys usually lacked evidence of rooting, and suggested that the coal within swilleys might be of allochthonous origin.

The presence of rootlets in the sandstone floor and sides of the swilleys at Wolf Mountain shows that plants grew within the swilleys. The lithotypic similarity of the coal within the swilleys and the basal part of the Wellington Seam outside the swilleys suggests that conditions of peat accumulation and preservation were similar within and without the swilleys, and that it is not necessary to invoke an allochthonous origin for the coal in the swilleys. The branching of one of the swilleys at Wolf Mountain suggests that it originated as some sort of channel rather than as a fold in the floor. The swilleys at Wolf Mountain are therefore tentatively identified as the abandoned channels of small streams which subsequently were occupied by peat-forming mires.

3.6.3 Partings

Thin bands of coaly and carbonaceous mudstone represent the very fine-grained deposits of overbank floods. Thicker, coarser sandy siltstones and silty mudstones represent crevasse splay and possibly levee deposits. The erosive-based, locally-rippled sandstones of the upper parting in the southeastern corner of the mine may represent bars and channel fills of a stream. Local deepenings of the basal scoured contact of the sandstones strike to the northeast. Steep cross-beds within the overlying silty sandstone dip to the north and northeast, suggesting northeastward sediment transport along the channel.

3.7 Relationship Between Tectonic and Depositional Features

The orientation of swilleys may be at least partially fault-related, as suggested by the right- angle bend made by the swilley in the southwestern corner of the mine (Figure 3-7), where the two major faults converge, and the close proximity of this swilley to the more southerly fault.

Thickness changes of the lower parting and individual coal plies within the Wellington Seam adjacent to the southerly fault (Figure 3-5, centre) record scouring and filling of a channel during peat accumulation, and suggest that the fault continued to influence channel position.

The parallelism of thickness and lithology trends in the upper parting to the southern major fault (Figure 3-6) is difficult to explain, particularly as the parting thickens on what is now the upthrown side of the fault. Southward tilting of the upthrown block is suggested by the initially nonerosive wedge-like thickening of the upper parting (Figure 3-3), accompanied by neither appreciable squeezing out of the underlying Wellington Main peat, nor thinning of the overlying Wellington Rider peat.

Close examination of the zone of rapid thickening (Figure 3-4) of the upper parting shows that its basal contact, although still non-erosive, is very irregular in detail. Clastic sediments of the upper parting appear to have filled extensional cracks and sags in the underlying peat and mud, consistent with southeastward downslope sliding of the lower beds. Elliott and others (1984) described similar extensional features in Carboniferous coals of Great Britain, which they believed to be formed by lateral mass movement of unconsolidated sediments and peat into stream channels. Perhaps the extensional cracks and sags in the coal beneath the upper parting at Wolf Mountain were formed by lateral mass movement of the peat towards the northern margin of the washout which cuts out the lower part of the Wellington Seam near the mine portals.

Growth faulting, which has been invoked as the cause of coal bed splits in other coalfields (Weisenfluh and Ferm, 1984) is a possible cause for local syndepositional tilting of the Wellington peats and associated clastic sediments. The most likely position of such a possible growth fault responsible for the tilting is postulated to be somewhere south of the mined area at Wolf Mountain.

The close association of coal and sandstone dikes with minor extensional faults in the roof of the Wellington Seam suggests that the Wellington Seam and its roof were disrupted after burial but before the peat and overlying sediments were consolidated. Shirley (1955) recognized similar disruption features in the roof of a Carboniferous coal bed in Britain, and suggested they were formed by earthquake-induced ground shaking. Although the clastic dikes at Wolf Mountain could have originated as injection features following earthquake-induced liquefaction of sandstone beds, it is also possible that the dikes and associated minor extensional faults were formed by compaction of the sediments overlying the Wellington Seam. It is not possible to clearly establish the origin of the dikes and minor faults, in the absence of further data concerning the vertical and lateral variability of the sedimentary rocks immediately above the coal seam.

3.8 Implications for Resource Assessment

Coal resource assessment is usually done by means of drilling in advance of mining, thus providing detailed information about the coal bed and its bounding strata, at isolated points within a given coal deposit. Some of the geological features of the Wellington Seam, such as splits, washouts and maybe the largest swilleys, can be outlined by closely-spaced drilling. It is obviously impractical, however, to attempt to locate every minor geological structure by means of drilling, because such significant features as floor rolls and sandstone dikes have a very small areal extent. Nevertheless, minor geological structures might upon occasion be intersected by drillholes and recognized in core, as suggested by Cairncross and others (1988) in the case of floor rolls.

Provided that the orientation and spacing of either the controlling structures, such as faults or folds, or their products, such as coal bed splits or channels, can be recognized, predictions may be made as to the location of other such structures and their resultant products. Geologic forecasts would be made easier and more accurate by having either good surface exposures, or well- documented mine workings near the area in question.

Mathew and Ferm (1982) have suggested that the precision of coal reserve estimates is not greatly affected by increasing the spacing between points of observation within the area being investigated. Nevertheless, increased spacing of geologic observations reduces the likelihood that small geological structures, which are responsible for many of the day-to-day operational difficulties in a coal mine (Elliott, 1973), will be observed during exploration. Until a workable means of wide-radius borehole geophysical sounding has been discovered, there will be a need for careful and detailed geological study of coal outcrops, and timely examination of advancing mine workings in order to predict geologic hazards before they stop coal production.

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This document was most recently revised on August 25, 2001.
Copyright to original material included herein is held by Gwyneth Cathyl-Bickford, © 1993.
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