CALIBRATING THE MICROPALEONTOLOGY OF THE BLUESKY FORMATION WITH SEDIMENTOLOGICAL AND ICHNOLOGICAL SIGNATURES

The Early Cretaceous Bluesky Formation in Alberta, Canada is an important bitumen reservoir. Much research has focused on the Bluesky’s sedimentology, ichnology, and stratigraphy, but the micropaleontological nature is largely undescribed.

Cored wells containing the Bluesky Formation, the underlying Gething Formation, and the overlying Wilrich Member were logged in the study area. A facies scheme was erected based on the preserved sedimentology and ichnology. To characterize the palynological and foraminiferal nature of those facies, core samples were recovered from several representative wells. The results were calibrated with the sedimentological and ichnological characteristics to provide enhanced paleoenvironmental interpretations.

Sediments of the upper Gething Formation contain an abundance of plant spores and pollen, freshwater and brackish-water dinocysts, and low-salinity tolerant forams. These assemblages 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. Accessory taxa and calcareous forams and microfossils indicate varying degrees of salinity stresses in deltaic and bay subenvironments. These interpretations are consistent with those made from the sedimentological and ichnological features.

There is a discernible shift in the micropaleontological composition of the Bluesky Formation and the overlying Wilrich Member; deposits of the Wilrich suggest an increase in marine influences at the time of deposition. Dinocysts are associated with shallow-water, near-shore conditions, and the presence of agglutinated and calcareous forams indicate meso- to polyhaline marine salinities. These assemblages suggest that overall marine salinity levels of the encroaching Boreal Sea may have been sub-euhaline.

This study underscores the value of integrating sedimentological and ichnological observations with preserved palynological and microfossil evidence. These micropaleontological datasets help establish criterion to distinguish important stratigraphic boundaries between Gething, Bluesky, and Wilrich sediments. They are also useful in constraining the age of the deposits.