An evaluation of diagenesis and chemostratigraphy of upper cretaceous molluscs from the Canadian Interior Seaway

Abstract

An Upper Cretaceous fauna of molluscs was collected from Manitoba, Saskatchewan, Alberta and the Northwest Territories, Canada. Most shells of Baculites and Scaphites are preserved as aragonite and show original microstructures. These molluscs have Sr concentrations ranging from 1,500 to 11,000 ppm with corresponding Mn and Mg contents of 10 to 900 and 10 to 1100 ppm, respectively. Sr contents are similar for Baculites and some Scaphites with 1500 to 7000 ppm, whereas the Sr contents of most Scaphites are consistently higher at 3,400 to 11,000 ppm. For Baculites from Saskatchewan, the Sr contents vary with locality, but are closely linked to stratigraphic position. Carbon- and oxygen-isotope values of preserved mollusc samples show a wide range. Their δ 13C-values range from −6.2 to +6.0%o (PDB), and the δ 18O compositions range from -17.0 to +1.0%o (PDB). A vital effect is ascribed only to the most anomalous values representing some specific bivalves, whereas the isotopic values of the ammonoids are interpreted to indicate fluctuating water conditions. Molluscs from the Cretaceous seas that inundated the North American continent during Transgression 8 show increased Sr concentrations, highly variable δ 13C- and δ 18O-values, and calculated water temperatures ranging from 20° to 28°C. After the peak of the transgression, contemporary molluscs contained less Sr, whereas the δ 13C- and δ 18O-values, are relatively constant with calculated water temperatures generally < 20°C, with a minimum of 16°C. Tectono-eustatic cycles of the lower Upper Campanian are responsible for the biogeochemical fluctuations recorded in the fossil molluscs.