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