BIOSTRATIGRAPHY OF THE TURONIAN-CONIACIAN BOUNDARY INTERVAL IN WESTERN SASKATCHEWAN: MICROPALEONTOLOGICAL ANALYSIS OF THE CONTACT BETWEEN THE NIOBRARA AND CARLILE FORMATIONS

Three cores located in western Saskatchewan were examined for foraminifera, radiolarians and diatoms in order to evaluate the paleoecological and paleogeographical changes that took place during the Turonian-Coniacian boundary interval in the Canadian Western Interior Basin. The upper part of the Carlile Formation in this location is composed of slightly calcareous to non-calcareous gray mudstones with high proportion of agglutinated foraminifera and very low diversity but high abundance of calcareous benthic and planktic species. The contact between both formations seems to be transitional and marked by the presence of more than twenty bentonite beds in the lowermost part of the Niobrara Formation. Although this bentonitic interval is barren of foraminifera it has a diverse association of radiolarians, both Spumellaria and Nasellaria. Above the bentonite beds the Govenlock Member is composed of fine-laminated calcareous siltstones and mudstones with few agglutinated foraminifera and moderate to low abundance of calcareous benthic species. Diatoms have not been recovered in any sample analysed so far.

The paleoecological conditions of the upper Carlile Formation were optimum for organisms to thrive as indicated by the abundance of foraminifera, fish remains, bivalves, ammonites and gastropods. The first stages of the Niobrara cycle in this part of the Interior Seaway were characterized by the disappearance of most of the marine fauna with exception of the radiolarians. This phenomenon is likely related to the spreading of volcanic ash into the seaway that changed the oxygen content of the bottom water and/or the nutrient availability, while provided siliceous material for the radiolarians to build up their skeletons. The absence of diatoms in a paleoecological setting were other siliceous microfossils flourished is hard to explain, although it could be due to lack of preservation for dissolution, slow accumulation rates and/or turbidity of the water that inhibited photosynthesis. The content of foraminifera above the bentonitic interval is very low, although it increases considerably upwards as the content of some macrofossils such as fish and bivalves also increases. The high abundance of the benthic calcareous species Neobulimina albertensis in both units is indicative of low-oxygen conditions.