Middle Cenomanian (Late Cretaceous) molluscs of the Shaftesbury Formation, Birch Mountains, northeastern Alberta, Canada

Abstract

Historical Resources Impact Assessment and Mitigation studies have documented a new middle Cenomanian invertebrate fauna from the Shaftesbury Formation in the Birch Mountains, northwest of Fort McMurray, in northeastern Alberta, Canada. Although older portions (Albian) of the Shaftesbury Formation in Alberta and British Columbia have yielded invertebrate fossils, this is the first fauna of middle Cenomanian age known for the unit. The fauna includes the ammonites Acanthoceras wyomingense and Borissjakoceras orbiculatum, the inoceramid bivalves Inoceramus dunveganensis and Inoceramus prefragilis stephensoni, and the gastropod Pirsila tensa. The occurrence of the late middle Cenomanian zonal fossil A. wyomingense allows correlation between the middle “Fish Scales Formation” of the Shaftesbury Formation and the lower part of the Labiche Formation of Alberta, and with five units farther to the south in the United States, namely the Greenhorn Limestone, Lincoln Formation, and Graneros Shale of Kansas, the Frontier Formation of Wyoming, and the Belle Fourche Shale of Montana.