Ichnological Assemblages of Wave-Influenced and Mixed River- and Wave Influenced Deltaic Deposits in the Viking Formation, Alberta, Canada

The Late Albian Viking Formation contains a number of coarsening-upward sediment bodies displaying subtle deltaic character. Deltaic deposition is implied through evidence of high sedimentation rates, variable salinity, riverine influx and stressed ichnological assemblages. The sedimentological and ichnological attributes of these deposits reflect riverine, fair-weather basinal (waves and tides), and storm induced processes. The influences of these processes on Viking Formation deposition are illustrated utilizing examples from two study areas in south-central Alberta.

In the Hamilton Lake area, coarsening-upward successions are dominated by wave-formed structures. Subtle evidence of riverine input is present in the form of local synaeresis cracks, soft sediment deformation features, carbonaceous mudstone deposits, and reduced bioturbation intensities. Low diversity and abundance of biogenic structures are observed in prodelta deposits, with the resultant assemblage interpreted to reflect moderately to subtly stressed expressions of the archetypal Cruziana Ichnofacies. The distal delta front is characterized by deposit feeding and grazing traces, with fewer suspension feeding structures. This suite is consistent with a low-diversity, moderately stressed proximal expression of the Cruziana Ichnofacies. Ichnological suites from proximal delta front deposits include elements of both the proximal expression of the Cruziana Ichnofacies and the Skolithos Ichnofacies. This suite is interpreted as a stressed expression of the mixed Skolithos-Cruziana “ichnofacies”. Sedimentological and ichnological characteristics of the Hamilton Lake deposits reflect the dominance of wave-induced processes over the associated riverine influx, and are best described as wave-influenced.

Cores from the Wayne-Rosedale to Chain area comprise thick sandstone units dominated by low-angle laminations and lesser wave-formed structures. Significant river-derived influx and high sedimentation rates are indicated by common synaeresis cracks, sideritized intervals, convolute bedding, carbonaceous mudstone deposits, and coal fragments. Distal and proximal prodelta deposits contain subtly to moderately stressed expressions of the archetypal Cruziana Ichnofacies, respectively. The proximal and distal delta front display strongly and moderately stressed proximal expressions of the Cruziana Ichnofacies, respectively. In general, trace fossil suites display low diversities and abundances of traces, and an impoverishment of structures of inferred suspension-feeding organisms. This ichnological assemblage is a result of harsh environmental stresses, such as heightened water turbidity, rapid sediment influx, and high concentrations of suspended sediment. Based on the sedimentary and ichnological observations, deposits of the Wayne-Rosedale to Chain area are interpreted to reflect deposition in a mixed river- and wave-influenced deltaic system.

In general, deltaic deposits contain trace fossil assemblages impoverished in structures associated with suspension-feeding behaviors. Comparison of the Hamilton Lake and Wayne-Rosedale-Chain deltaic deposits results in a noticeable paucity of suspension-feeding structures in facies of the Wayne-Rosedale-Chain area. This is a result of the stronger wave action in the Hamilton Lake area, which mitigated the effects of environmental stresses related to riverine discharge and facilitated suspension-feeding behaviors.