STRATIGRAPHIC SIGNIFICANCE OF LOWER MCMURRAY COAL DEPOSITS: ATHABASCA OILSANDS, ALBERTA, CANADA

The Cretaceous-aged McMurray Formation in the northeast quadrant of the McMurray Sub-Basin, Alberta and Saskatchewan, comprises fluvial, tidal-fluvial and inter-channel deposits that occupy topographic lows on the Sub-Cretaceous Unconformity (SCU). The informal lower McMurray member is commonly subdivided into two packages separated by a coal and/or paleosol horizon with a maximum thickness of 10 m. The coal has been interpreted previously to represent initial flooding of the Boreal Sea, but the timeframe for coal accumulation has not been defined. The thickness of the coal/paleosol horizon suggests that it represents a significant time period, and hence, could represent a major hiatus in deposition or a significant base level change.

In order to determine the nature of the coal and the length of time represented by it, the coal horizon and the SCU are evaluated in Petrel using 3500 well logs. These data are used to map the topography of the SCU, and to create an isopach map of the coal / paleosol interval. Sand samples from above and below the coal horizon in three cores are sampled to derive detrital zircon (DZ) signatures from which maximum depositional ages are determined. Calculated maximum depositional ages range between 121.8-127.2 Ma in the underlying sands, placing the lowermost McMurray-aged sediments in the late Barremian-Aptian.

Additionally, multi-dimensional scaling is used to statistically compare relative proportions of DZ populations. Multi-dimensional scaling plots show two distinctly different populations above and below the coal / paleosol horizon. Sands below the coal show a statically higher proportion of zircons derived from the Trans-Hudson Province than sands overlying the coal. This suggests that the coal represents a significant time gap, and there was a reorganization of rivers during or following peat deposition.