Geology and petrography of the Wellington Seam, Nanaimo Coalfield, Vancouver Island

The Wellington Seam in the Nanaimo Coalfield of Vancouver Island has contributed significantly to British Columbia's coal production since its discovery in 1871. Difficult geological conditions and conflicts between surface land use and mining activities have hampered the efficient development of coal mines in the Wellington Seam. This study contributes to the assessment of the coal resources of the Nanaimo Coalfield by describing the geology, quality, petrographic composition and depositional origin of the Wellington Seam. Surface geological mapping of the western half of the Nanaimo Coalfield was done at scales of 1:6000 to 1:20,000. Detailed mapping of the Wellington Seam was done in the underground workings of Wolf Mountain Colliery. 229 stratigraphic sections were measured, and three columnar samples of coal and associated mudstone partings within the Wellington Seam were collected in the mine. Five coal and three mudstone lithotypes were recognized in the columnar samples. The petrographic composition of the coals and mudstones was determined by point-counting at high magnification under incident light in immersion oil. Moisture and ash content, and free swelling index were determined for the coals and mudstones. Seven formations and nine members, comprising the basal half of the Upper Cretaceous Nanaimo Group, crop out and are present in boreholes and mine shafts within the study area. Formations and members are distinguished on the basis of lithostratigraphy. Coal is present within five lithostratigraphic units: the Northfield and Millstream Members of the Extension Formation, the Cranberry and Newcastle Members of the Pender Formation, and the Reserve Member of the Protection Formation. The Wellington Seam is a composite of up to three closely-associated coal beds near the base of the Northfield Member. Coal of the Wellington Seam is typically bright banded, blocky and very hard. The dry ash content of the coal is seldom less than 5 percent, and is usually over 9 percent. Its sulphur content is usually between 0.5 and 1 percent, and rarely exceeds 1.5 percent. The low sulphur content of the Wellington coal suggests that its precursor peat was generally not in contact with sulphur-rich marine or brackish water. Sandstones of the East Wellington Formation, underlying the Wellington Seam, were deposited on the shoreface of a northwest-prograding strandplain, fed by sand-dominated distributaries. Most of the Wellington coals sampled at Wolf Mountain originated as wet forest swamp peats, and less commonly as fen peats or marsh peats. Mudstone and siltstone partings within the Wellington Seam were deposited as overbank flood deposits, crevasse splays and levee deposits associated with stream channels. Siltstones, mudstones and coals of the Northfield Member, overlying the Wellington Seam, probably originated as overbank flood, crevasse splay and swamp deposits. Isolated sandstones and conglomerates within the Northfield Member may have been deposited by anastomosing streams. Conglomerates and sandstones of the Millstream Member, overlying and locally truncating the Northfield Member, represent deposits of gravel bed rivers, probably part of a coastal braid plain delta. Mining of the Wellington Seam has been hampered by the irregular topography of the seam floor, splits within the seam, and minor faulting and clastic dikes within the seam roof. Most of the topographic irregularities of the seam floor are of sedimentary rather than tectonic origin. Detailed mapping at Wolf Mountain shows that the position and extent of seam splits and perhaps some of the irregularities of the seam floor are controlled by syndepositional faults. Recognition of the controls of seam splits and floor structures affords the possibility of geological forecasting in advance of mining. Petrographic composition of the Wellington Seam can be partially correlated with coal lithotypes. Lustrous coals and dull and bright coals have very high vitrinite contents and low inertinite contents. Bright banded and bright coals have lower vitrinite contents and higher inertinite contents, but still consist predominantly of vitrinite. All of the Wellington coals sampled at Wolf Mountain have very low liptinite contents. Free swelling indices of the Wellington coals at Wolf Mountain are generally less than 3, indicating that the coals are not strongly agglomerating. Very few of the coal samples from Wolf Mountain have free swelling index values greater than 4. There is no clear relationship between ash content and free swelling index of the coals.

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