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Stratigraphy, sedimentology and coal quality of the Lower Skeena Group, Telkwa Coalfield, Central British Columbia Palsgrove, Regan Jane


The Albian Lower Skeena Group in the Telkwa coalfield comprises more than 500 metres of conglomerate, sandstone, siltstone, mudstone and coal deposited during two regressive/transgressive cycles. The stratigraphic sequence is divisible into four lithostratigraphic units. The basal unit, Unit I, may be more than 100 metres thick and comprises conglomerate, sandstone, mudstone, coal, and seat earth. Conglomerate and sandstone are composed dominantly of chert and volcanic rock fragments, and mudstones are kaolinitic. Unit I was deposited in a fluvial environment on an eroded volcanic basement. Gravel and sand were deposited in braided channels and bars, and mudstone accumulated in floodplains. Coal formed in poorly drained, peat-forming backswamps. In the northern part of the study area, coal seams thin and split, a result of periodic flooding of peat swamps with sediment-laden water from nearby streams. Deposition of Unit I ended with a marine transgression and deposition of Unit II. Unit II consists of up to 140 metres of silty mudstone, bioturbated or cross-bedded, chert and muscovite-rich sandstone, and rare thin coaly mudstones deposited in a deltaic/shallow marine environment. Sand was deposited in distributary channels and mouth-bars, mud accumulated in bays, and thin discontinuous peat beds accumulated in local salt marshes. There is structural evidence for the presence of an unconformity within Unit II, but palynological and paleontological data suggest that the strata are all similarly aged. Unit III averages 90 metres thick, and comprises bioturbated or rippled, chert and muscovite-rich sandstone, siltstone, carbonaceous mudstone and thick, laterally extensive coal seams deposited in a variety of low-energy, paralic environments. Sand and mud were deposited and biogenically reworked in tidal flats, and siltstone accumulated in a restricted, nearshore marine environment in the eastern edge of the study area. Peat accumulated in freshwater coastal marshes which periodically prograded over tidal flats. All but the lowermost coal seams pinch out eastward into restricted, nearshore marine sediments, and the ash content of the coal increases toward the margin of the seam. Locally, the sulphur content of the coal is high, reflecting occasional inundation of the fresh-water swamps by brackish water. High sulphur coal contains relatively more pyritic sulphur and less organic sulphur, compared to low-sulphur coal. Unit IV is at least 150 metres thick and is composed of chloritic, green sandstone overlain by silty mudstone, deposited in a marine environment. The basal sandstone is a transgressive lag deposit, and silty mudstone, the predominant lithofacies, was deposited in a nearshore, shallow marine environment. The provenance of the sediments in the Telkwa coalfield changes from the base to the top of the stratigraphic section. Conglomerate and sandstone of Unit I contain an abundance of volcanic clasts and grains, locally derived from underlying and surrounding volcanic rocks of the Jurassic Hazelton Group, which were uplifted as part of the Skeena Arch and subsequently eroded and reworked. Sandstones of Units II, III and IV, which contain much less volcanic-derived material and an abundance of mica flakes, were derived from high-grade metamorphic rocks in the Omineca Belt. Chert grains are abundant throughout, reflecting continued clastic influx from the Pinchi Belt-Columbian Orogen.

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