Difficult geological conditions, including seam splits, washouts and faults, have hampered development of underground coal mines at Nanaimo. Nevertheless, the Wellington Seam was extensively mined, yielding about 30 megatonnes of coal from 96 mine entries within the western half of the Nanaimo Coalfield. Mining of the Wellington Seam began in 1871, and continued, with some interruptions, until 1987. Although all of the mines are now closed, they continue to present hazards to urban development of the city of Nanaimo, through subsidence, collapse and mine fires.
Basement beneath the Nanaimo Group in the study area consists mainly of Triassic volcanic rocks of the Karmutsen Formation, locally intruded by Jurassic granodiorite of the Nanaimo River Batholith. The basement paleosurface formed isolated hills during deposition of the basal Nanaimo Group within the study area. At least one of the basement hills remained emergent while the precursory peats of the Wellington Seam were being formed, resulting in local absence of the Wellington Seam.
Formations of the basal half of the Nanaimo Group are defined on the basis of lithostratigraphy. From the base upwards, they are the Comox Formation, the Haslam Formation, the East Wellington Formation, the Extension Formation, the Pender Formation, the Protection Formation, and the Cedar District Formation.
The Comox Formation consists of two members within the study area: the basal conglomeratic Benson Member, nil to 120 metres thick, and the overlying sandy Dunsmuir Member, nil to 100 metres thick. The coal-bearing Cumberland Member of the formation, recognised in the Comox Coalfield, is absent within the study area. The Benson Member unconformably overlies pre-Cretaceous basement and is best developed along the flanks of basement paleohighs. The Benson Member intertongues basinwards with the Dunsmuir Member and the Haslam Formation. Where the Benson Member is absent, the Dunsmuir Member directly overlies the basement.
The Haslam Formation consists of 100 to 150 metres of mudstone and siltstone with minor sandstone. The upper 15 to 40 metres of the Haslam Formation locally contain more abundant sandstone beds. In the southern part of the Nanaimo Coalfield, thrust faults and associated folds have increased the apparent thickness of the Haslam Formation to as much as 500 metres.
The East Wellington Formation consists of a single coarsening-upward unit of 5 to 47 metres of sandstone with minor pebble-conglomerate and grit. The East Wellington is thickest along its outcrop edge, and thins gradually downdip to the south and east. The contact of the East Wellington sandstone with the overlying coal of the Wellington Seam is almost always abrupt, marked by small-scale undulations ranging in amplitude from 15 to 120 centimetres. In the northwestern corner of the Nanaimo Coalfield, the uppermost East Wellington sandstone grades upwards and southeastwards into dirty, sandy coal, which in turn grades into clean coal of the Wellington Seam. The East Wellington Formation was probably deposited on the shoreface of northwest-prograding wave-dominated strandplain, fed by sand-dominated distributaries.
The Extension Formation consists of two members within the study area: the basal, dominantly fine-grained Northfield Member, nil to 45 metres thick, and the overlying conglomeratic Millstream Member, 90 to 230 metres thick. The Millstream Member thickens to the south and east at the expense of the Northfield Member, which is locally altogether absent due to erosion prior to deposition of the Millstream conglomerates. Coal beds are present within both members of the Extension Formation, but coal of mineable thickness is confined to the Northfield Member. The thickest and most laterally continuous coal is the Wellington Seam.
The Wellington Seam is a composite of up to three closely-associated coal beds: the Wellington Rider, Upper Wellington and Lower Wellington, which have been mined together as a unit in localities where the intervening clastic rock partings are thin. Mudstones are the most common clastic rocks found in close association with the Wellington Seam. A very thin (1 to 2 cm) but laterally persistent band of black arenaceous or 'granular' mudstone occurs in the middle of the Wellington Rider coal. This band may represent an altered tuff, an eolian deposit or a fire splay. Fining-upward siltstones, silty mudstones and carbonaceous mudstones associated with the Wellington Seam probably represent crevasse-splay or levee deposits. Cross-bedded, rippled, locally erosive-based sandstones, which occasionally form very thick partings within the Wellington Seam, may represent point bar or proximal crevasse splay deposits.
The Wellington Seam consists mainly of bright banded coal and coaly mudstone. Individual coals within the Wellington Seam are more continuous than the associated rock bands. The Wellington coals contain abundant vitrinite, modest amounts of inertinite and mineral matter, and minimal amounts of liptinite.
Five coal lithotypes were recognised in the Wellington Seam: bright, bright banded, dull and bright, dull, and lustrous. The distinction between bright coals and bright banded coals, which was made during fieldwork in Wolf Mountain Colliery, cannot be upheld on the basis of their petrographic composition. Likewise, the distinction between dull and bright coals and lustrous coals is not supported by differences in petrographic composition.
Lustrous coals and dull and bright coals have very high vitrinite contents and low inertinite contents. Bright banded coals and bright coals have lower vitrinite contents and higher inertinite contents, but they still consist predominantly of vitrinite. Dull coal, which was only sampled once, appears to be intermediate in composition between these two groups.
Bright banded coals and bright coals of the Wellington Seam probably originated mainly as wet forest swamp peats, and less commonly as fen peats or marsh peats. Dull and bright coals and dull coals of the Wellington Seam probably also originated mainly as wet forest swamp peats. However, these peats were either more rapidly buried or accumulated under wetter, more oxic conditions than the peats that formed bright coals and bright banded coals. Lustrous coals probably originated mainly as wet forest swamp peats. Some samples of lustrous coals, which contain very high amounts of macerals with preserved cellular structure, probably originated as individual logs.
Bright banded coals and bright coals have ash contents of 5 to 15 percent (by weight, dry basis). FSI values of these coals are strongly depressed by increased ash content; when ash exceeds 10% (by weight, dry basis), FSI values drop below 3. Dull and bright coals, lustrous coals and dull coals have ash contents of 10 to 34 percent (by weight, dry basis). There is no clear relationship between ash content and FSI for these coals. The maximum FSI observed in the coal samples examined in this study was 4.7. Very few of the coal samples from Wolf Mountain Colliery had FSI values over 4, and it appears doubtful that the Wellington Seam at Wolf Mountain could be readily sold as a soft coking coal.
Two types of small-scale topographic and structural irregularities, floor rolls and swilleys, occur at the floor of the Wellington Seam. Floor rolls are gentle swells in the floor, over which the coal thins. Where examined in detail at Wolf Mountain, they are 20 to 30 metres wide, and 30 to 90 centimetres high. The floor rolls beneath the Wellington Seam may represent sand bars which were formed at a beach surface and then subsequently uplifted and buried by peat.
Swilleys are local depressions in the floor of the Wellington Seam, over which the coal thickens. In the workings of Wolf Mountain Colliery, where they were examined in detail, they are 20 to 40 metres wide, and 0.4 to 2.1 metres deep. They are sinuous in plan, and one swilley has two short branches joining it from the north. Swilleys beneath the Wellington Seam may represent abandoned channels of small streams, which were subsequently occupied by peat-forming mires.
The orientation and position of swilleys, splits and erosional channels in the Wellington Seam may be partially fault-related. Some of the extensional faults in the Wellington Seam may have been formed by lateral mass movement of unconsolidated sediments and peat. Lateral mass movement was probably initiated by erosion or undercutting of the peat alongside stream channels, perhaps enhanced by growth fault-induced syndepositional tilting of the peats.
Clastic dikes of coal and sandstone are locally abundant within the roof of the Wellington Seam, and often parallel minor extensional faults. The clastic dikes may have originated as injection features following earthquake-induced liquefaction of sediments overlying the Wellington Seam, or they may have formed, along with the minor faults, in the course of compaction of the sediments.
The general scarcity of coarse clastic deposits within the Northfield Member suggests that widespread meander belts are absent within the Northfield Member of the study area. The isolated sandstone and conglomerate bodies of the Northfield Member may therefore have been deposited by anastomosing streams. In contrast, the more widespread conglomerates and sandstones of the Millstream Member represent deposits of gravel bed rivers, probably part of a coastal braidplain delta. Fine-grained deposits within the Millstream Member probably represent floodplain and backswamp deposits which formed in areas between active river channels.
The Pender Formation consists of two members within the study area: the basal Cranberry Member, 130 to 160 metres thick, and the overlying Newcastle Member, 30 to 60 metres thick. Both members consist mainly of mudstone and siltstone. The Cranberry Member contains interbeds of coarse-grained gritty sandstone, with occasional thin to medium interbeds of coal, including the Cranberry bed, 0.2 to 0.6 metres thick, which occurs 10 to 15 metres below the top of the member. The Newcastle Member contains several thick coal beds: the basal Newcastle Seam, 0.8 to 1.2 metres thick, the Douglas Seam, 0.6 to 4.5 metres thick, and the uppermost Douglas Rider Seam, 0.8 to 1.2 metres thick. The Newcastle and Douglas Seams coalesce southeast of Nanaimo Harbour, forming the Douglas Main Seam, which is locally up to 21 metres thick.
The Protection Formation consists of three members within the study area: the basal sandy Cassidy Member, 80 to 105 metres thick, the medial, fine-grained, coal-bearing Reserve Member, 40 to 60 metres thick, and the overlying sandy McMillan Member, 60 to 90 metres thick. Coals of the Reserve Member are generally less than 30 centimetres thick.
The Cedar District Formation consists of 330 to 600 metres of shale and siltstone. Thin sandstone bands and sandstone dykes are locally abundant in the middle of the formation. A ridge-forming sandstone unit, similar to the sandstone of the McMillan Member, lies near the base of the Cedar District Formation.
Framework compositions of sandstones and conglomerates of the lower half of the Nanaimo Group within the study area vary with stratigraphic position. The basal conglomerate of the Benson Member consists almost exclusively of volcanic rock clasts, while the Dunsmuir sandstones consist of subequal amounts of quartz, feldspar and volcanic rock fragments. Sandstones of the East Wellington Formation consist mainly of quartz and feldspar with minor volcanic rock fragments, while the sandstones and conglomerates of the Extension Formation consist mainly of quartz and dark grey chert with minor red chert. Sandstones of the Cassidy Member consist of subequal amounts of quartz and feldspar with minor hornblende. The overall upward decrease in volcanic content and increase in plutonic content probably reflects the gradual uplift and unroofing of plutonic rocks within the Coast Range.
Overall intensity of deformation increases from northwest to southeast across the Nanaimo Coalfield, and northwest-striking thrust faults and folds are increasingly abundant towards the southern boundary of the coalfield. Extensional faults are common throughout the coalfield. The central part of the Nanaimo Coalfield is dislocated by a major northeast-striking cross fault, the Chase River Fault. Downthrow across the Chase River Fault is 50 to 100 metres to the southeast; offsets of northwest-striking faults and geological contacts across the Chase River Fault suggest its displacement also has a substantial dextral strike-slip component, on the order of 1 to 2 kilometres.