A comparative analysis of the ichnology of wave- and river- dominated allomembers of the Upper Cretaceous Dunvegan Formation

The Upper Cretaceous (Cenomanian) Dunvegan Formation is a siliciclastic unit, which lies in the subsurface of western Alberta and crops out in the west-central portion of the province. The Dunvegan is deltaic in origin. Previous workers have subdivided the Dunvegan into 7 allocyclic members (A through G) in the subsurface, based on detailed sedimentological and stratigraphic analyses. Each allomember corresponds to a deltaic lobe and exhibits varying degrees of wave and river dominance. Allomembers D and E were chosen to test ichnological variations in deltaic systems because D is strongly wave-dominated and E is strongly river-dominated. Tidally influenced end members were not included in the scope of this study. The study area comprises Townships 59 to 67, and Range 20 W5M to 8 W6M. Fifteen cores, totaling 269 m in length, were analysed sedimentologically and ichnologically in order to compare the ichnological characteristics of river- and wave-dominated deltas. Allomember E is characterized by an overall lower intensity of bioturbation, a lower diversity of ichnogenera within proximal facies, and a general suppression of the Skolithos ichnofacies. In contrast, Allomember D shows a greater intensity of burrowing, a generally higher ichnogeneric diversity, and a diverse Skolithos ichnofacies. The contrasting ichnology provides insights into the ethology exhibited by infauna in the different depositional settings. More importantly, the differences observed are independent of bathymetry and emphasize the influences that environmental parameters such as variability in salinity, temperature, sedimentation rates, water turbidity, and substrate consistency play in an organism's selection of an ethological survival strategy. As such, ichnological data can be successfully integrated with physical sedimentological data in order to form predictive models that may be used to better comprehend and ultimately differentiate between these two depositional systems.