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Lower Cretaceous Example of a Shoreface-Attached Marine Bar Complex: The Wabiskaw ‘C’ Sand of Northeastern Alberta
The Wabiskaw Member of the Clearwater Formation consists of three coarsening-upward, vertically stacked, but laterally offset lithic-glauconitic sand bodies. The elongate depositional trend of the lowermost Wabiskaw ‘C’ sand was directly influenced by an underlying, partly filled, narrow valley on the sub-Cretaceous unconformity. Bitumen saturation of some of the cores masks much of the bedding and other physical structures in the sand. However, ichnofossils in the enclosing shales, as well as shale-lined burrows in the sand, provide valuable environmental evidence. In its northern part, the Wabiskaw ‘C’ sand appears to have been deposited in a fully marine, dominantly high-energy environment interpreted as a marine bar complex that developed between storm wave base and normal wave base. The occurrence and relative abundance of specific trace fossil suites are the key to delineating four major facies interpreted as the products of central bar, bar margin, distal bar margin, and interbar shelf environments. In the south of the study area, a Macaronichnus ichnofauna is present, indicating shallower marine conditions, apparently middle to upper shoreface. Correlation of the sand body from well logs is straightforward, except where it is directly overlain by the Wabiskaw ‘B’ sand body; a relationship that produces an amalgamated sand. Its relatively simple three dimensional “geometry”, interpreted from the geophysical logs, strongly suggests that the sand is a continuous body. This morphological evidence, combined with the environmental interpretration, suggest that the Wabiskaw ‘C’ sand body is an example of a shoreface-attached, shallow, marine bar complex.
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