Ichnological Aspects of Incised-Valley Fill Systems from the Viking Formation of the Western Canada Sedimentary Basin, Alberta, Canada

The Upper Albian Viking Formation produces hydrocarbons from anomalous deposits in the Crystal, Willesden Green, Cyn-Pem, Sundance, and Edson fields of west-central Alberta. These anomalous deposits are interpreted to represent lowstand incised-valley systems, and are erosionally juxtaposed against regionally extensive offshore to lower shoreface parasequences. These parasequences are stacked into a progradational parasequence set of the preceding highstand systems tract. The stratigraphically underlying parasequences possess trace fossil assemblages characterized by abundant, uniform burrowing with a high diversity of elements from the Cruziana ichnofacies. These characteristics, coupled with abundant specialized and complex feeding and grazing structures, attest to an equilibrium community under fully marine conditions.

The underlying parasequences contrast markedly with the anomalous incised-valley deposits. The valley fill deposits overlie an erosional discontinuity typically corresponding to a co-planar surface of lowstand erosion and transgressive erosion or marine flooding (FS/SB), that is locally demarcated by the substrate-controlled Glossifungites ichnofacies. The incised-valley deposits best fit a wave-dominated estuarine model. The succession displays a tripartite zonation of facies associations corresponding to the main depositional zones of the estuary: the sandy Bay Head Delta, the muddy Central Basin, and the sandy Estuary Mouth. A fourth depositional complex corresponds to Channel-fill and reflects probable stages of re-incision into previous estuarine deposits during renewed sea-level fall, although locally, distributary channels, tidal channels, and tidal inlets may be indicated. The trace fossil suite of the estuarine system is characterized by a variable and sporadic distribution of burrowing, variability in ichnogenera distribution, and dominance by simple structures of trophic generalists. The suite is dominated by opportunistic suites characteristic of stressed environments, particularly those subjected to fluctuations in salinity, episodic deposition, variable aggradation rates, and variability in substrate consistency. Specialized feeding and grazing structures are of secondary importance in the trace fossil suite, but record periods of fully marine, unstressed conditions. The estuarine system appears to have ranged repeatedly from brackish to fully marine. Although the character of the ichnological record in valley fills is complex, this complexity is, itself, distinctive and differentiates it from the underlying highstand parasequences of the regional succession.