Chapter 2: Inoceramid Bivalves from the Coniacian and Basal Santonian (Upper Cretaceous) of the Western Canada Foreland Basin

Abstract

ABSTRACT Inoceramid bivalves are the dominant invertebrate fauna of the Coniacian and basal Santonian of the Western Canada Foreland Basin in western Alberta. In the upper lower Coniacian through to basal Santonian, six successive faunas are recognized, which provide the basis for corresponding, formally defined inoceramid zones. From bottom upward these are the zones of: Cremnoceramus crassus crassus /C. deformis deformis, Inoceramus gibbosus, Volviceramus koeneni, Volviceramus involutus, Sphenoceramus subcardissoides, and Sphenoceramus ex gr. pachti. Particular faunas represent assemblages known widely from the Euramerican biogeographic region, although they characterize mostly its northern, boreal area. The inoceramid-based biostratigraphic scheme allows correlation with other parts of the North American Western Interior and with parts of the Euramerican biogeographic region. The studied succession provides a good record of the Inoceramus gibbosus Zone, which characterizes the topmost lower Coniacian. This zone, first recognized from northern Germany, is usually absent, both in Europe and in North America, due to a stratigraphic gap resulting from a eustatic lowstand. The base of the middle Coniacian is marked by the abrupt appearance of the taxonomically variable Volviceramus fauna (V. koeneni (Müller), V. exogyroides (Meek and Hayden)), with associated Inoceramus undabundus Meek and Hayden and Volviceramus cardinalensis, newly described herein. Scaphites (Scaphites) ventricosus Meek and Hayden, the ammonite marker of the base of the middle Coniacian first appears in the late early Coniacian. The base of the upper Coniacian marks the first appearance of the characteristic northern inoceramid species Sphenoceramus subcardissoides (Schlüter), the appearance of which coincides with Scaphites (Scaphites) depressus Reeside, the ammonite marker of this boundary. Close to this boundary Volviceramus stotti also appears, which is newly described from the Canadian sections. The base of the Santonian corresponds to the abrupt appearance of Sphenoceramus ex gr. pachti (Arkhangelsky). The studied sections demonstrate that the appearance of new inoceramid faunas (lowest occurrence of Cremnoceramus crassus crassus (Petrascheck), of various species of Volviceramus, Sphenoceramus subcardissoides (Schlüter) and of S. ex gr. pachti) takes place immediately above major marine flooding surfaces, suggesting a close correspondence between evolutionary and/or migration events and episodes of relative sea-level rise.