Calibrating micropaleontology with sedimentology and ichnology for enhanced paleoenvironmental interpretations in the Bluesky Formation of Alberta, Canada

Research on the Bluesky Formation in Alberta's Peace River oil sands deposit has focused on the sedimentology, ichnology and stratigraphy of the unit, but the micropaleontology of the Bluesky is not well documented in the literature. This study contributes to this under-reported dataset and underscores the value of calibrating sedimentological and ichnological observations with preserved palynological and microfossil assemblages. Cores containing the Bluesky Formation and related strata of the underlying Gething Formation and the overlying Wilrich Member were systematically logged and sampled for their micropaleontological content. The various datasets were then integrated to provide enhanced paleoenvironmental interpretations. At the base of the strata, the upper Gething Formation sediments are characterized by an abundance of plant spores and pollen, the freshwater dinocyst Hurlandsia rugara, faintly brackish-water Nyktericysta spp. dinocysts, and the low-salinity tolerant foraminifera Miliammina spp. These assemblages are interpreted to reflect deposition in lowland swamps, marshes, and brackish bays. The overlying Bluesky Formation palynofloral assemblage reflects encroachment of the transgressing Boreal Sea and contains a unique suite of marginal-marine dinocysts dominated by Circulodinium brevispinosum, Odontochitina operculata, Oligosphaeridium spp. and Palaeoperidinium cretaceum. Accessory taxa, foraminifera, and microfossils indicate varying degrees of salinity stresses in deltaic, shoreface, and bay sub-environments. The palynological and microfossil assemblages associated with the Wilrich Member are markedly different than strata below and point to increasing marine influences. Dinocysts Circulodinium deflandrei and Kiokansium unituberculatum are associated with shallow-water, near-shore conditions, and the presence of agglutinated and calcareous foraminifera indicate meso- to polyhaline marine salinities. These assemblages suggest that overall marine salinity levels of the encroaching Boreal Sea may have been lower than normal (i.e. sub-euhaline) in the Western Canada Sedimentary Basin at that time. These micropaleontological datasets help establish criteria to distinguish important stratigraphic boundaries between Gething, Bluesky, and Wilrich sediments. The palynological framework is also useful for drawing comparison to other coeval deposits, namely the McMurray and Clearwater formations of eastern Alberta's Athabasca oil sands. Furthermore, the abundance of the dinocyst Pseudoceratium pelliferum in the Bluesky palynofloral suite constrains the chronostratigraphic age of that formation to no younger than early Aptian, a significantly older age than previously reported.