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.
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.
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.
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.
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.
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).
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.
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.
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.
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.
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.
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.
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.