Integrating Sedimentology and Ichnology to Shed Light on the System Dynamics and Paleogeography of an Ancient Riverine Estuary

Ichnology has proven to be one of the most useful criteria for the recognition of brackish-water deposits in the Phanerozoic sedimentary record. The study of modern brackish settings has demonstrated organized patterns of physical sedimentary processes and faunal distributions. Through comparison with modern systems, it is possible to work out the spatial relations of ancient marginal marine deposits with increased precision.

Within the Lower Cretaceous McMurray Formation of northeast Alberta, deposits with inclined heterolithic stratification (IHS) have come to be interpreted as the accretionary bank deposits of estuarine channels. While ichnologically and sedimentologically diverse, many of these IHS packages exhibit characteristics that are consistent with longitudinal variations in a river-fed, channelized estuary. Within such a scheme, sediment character exhibits a tripartite distribution. Towards the fluvial end of the system, poorly sorted sands with decimeter-scale cross-stratification dominate. Bioturbation is exceedingly rare and is restricted to sporadic tidally influenced fine-grained beds. The central zone is characterized by clay- and silt-rich deposits, with mudstone beds grading seaward from finely interlaminated to structureless in character. The marine end of the channelized system is dominated by well-sorted, ripple cross-laminated, very fine-grained sand, displaying comparatively abundant and diverse bioturbation within both the sand and mudstone beds.

The combination of physical sedimentology and ichnology also provides insight into the processes responsible for the heterolithic character of the IHS, as well as the temporal scale upon which they operated. The observed features can be accounted for through seasonal variations in fluvial discharge, and resulting changes in water circulation and sedimentation patterns within the estuary. An understanding of the primary depositional relationships of IHS deposits should facilitate the refinement of the stratigraphic architecture within the McMurray Formation, and add to the efficiency of bitumen exploration