Lithofacies and depositional environments of the Telkwa coal measures central British Columbia, Canada

Abstract

Albian coal measures of the Skeena Group in the Telkwa coalfield comprise more than 500 m of conglomerate, sandstone, siltstone, mudstone and coal that were deposited during two regressive/transgressive cycles. The stratigraphic sequence is divisible into four lithostratigraphic units. The basal unit (Unit 1) is about 100 m thick and comprises conglomerate, sandstone, mudstone, coal and seatearth deposited in a fluvial environment on a deeply eroded volcanic basement. Gravel and sand were deposited in braided channels and bars, and mud accumulated in floodplains. Peat accumulated in poorly drained 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 marine transgression. Unit II consists of up to 140 m of silty mudstone, bioturbated and cross-bedded, chert and muscovite-rich sandstone, and rare thin carbonaceous mudstones deposited in a shallow marine environment. The sands were deposited as distributary channels and mouth-bars. Muds were deposited in bays and thin discontinuous peat accumulated in local salt marshes. Unit III averages 90 m in thickness and comprises bioturbated and rippled, chert and muscovite-rich sandstone, siltstone, carbonaceous mudstone and thick 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 to the east. Peat accumulated in freshwater coastal, domed mires which periodically prograded over tidal flats. All but the lowermost coal seams pinch out eastward into brackish and nearshore marine sediments. The ash yield of many coals increases towards their seaward margin. Locally, the sulphur yield of the coal is high, reflecting occasional inundation by brackish water. The coals were deposited during regressive maximum and are characterized by an essentially vertical stacking pattern comprising an aggradational parasequence set. The major coals are considered to have been deposited at the turnaround between progradation and retrogradation. Unit IV is over 150 m thick and is composed of chloritic sandstone overlain by silty mudstone, deposited in a marine environment. The basal sandstone is a transgressive lag deposit. Silty mudstone is the predominant lithofacies and was deposited in a nearshore, shallow marine environment.