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Epsilon Cross-Strata in the Athabasca Oil Sands: Abstract
The Lower Cretaceous McMurray Formation oil sands contain solitary sets of cross-strata up to 25 metres thick, with cross-stratal dips of 8 to 12 degrees. In all essential regards, these sets conform to Allen’s definition of epsilon cross-strata.
The principal characteristics of a typical set are as follows: the lower bounding surface is erosional, and on the scale of a single exposure is essentially planar and horizontal: the cross-strata, which discordantly overlie the basal surface, consist of decimetre to metre thick beds of fine sand, separated by thin partings of argillaceous silt; individual beds are remarkably continuous and uniform from the base to the top of the set and, in sections normal to depositional strike, they characteristically exhibit a straight line profile; subtle fining-upwards within the set is manifest largely by an upwards increase in the proportion of silt and clay rather than as a decrease in mean sand size; depositional strike is generally consistent in any given exposure, but adjacent outcrops commonly show wide divergence in attitude.
Single beds within an epsilon set characteristically exhibit the following sequence: on a sharp base lie essentially structureless sands, followed upwards by trough cross-bedded sands (only sporadically developed) overlain by ripple-bedded sands, grading into laminated silt and clay. Preliminary paleocurrent analysis on the trough sets and the ripples indicates unidirectional flow approximately parallel to depositional strike. Common in the silt partings, and characteristically penetrating down into the underlying sands, are tubular walled burrows up to 5 mm in diameter, exhibiting a form very similar to polychaete burrows found in the fresh water portions of some present-day estuaries. Microflora within the silts is also of fresh water origin.
Details of the depositional environment of the epsilon cross-strata have yet to be established, but it is proposed that they originated as lateral accretion deposits, laid down on the sloping inner banks or point bars of deep sinuous channels. The falling flow regime indicated in the sands suggests that sand deposition may have taken place only during waning of the flood stage, with fines accumulating during the remainder of the cycle.
Acknowledgments and Associated Footnotes
1 Alberta Research Council, 11315–87 Avenue, Edmonton, Alberta T6G 2C2
Copyright © 2009 by the Canadian Society of Petroleum Geologists