Integrated, high-resolution allostratigraphic, biostratigraphic and carbon-isotope correlation of Coniacian strata (Upper Cretaceous), western Alberta and northern Montana.

摘要

Inoceramid bivalves are the dominant invertebrate fauna of the Coniacian and basal Santonian ofudthe Western Canada Foreland Basin in western Alberta. In the upper lower Coniacian through to basaludSantonian, six successive faunas are recognized, which provide the basis for corresponding, formallyuddefined inoceramid zones. From bottom upward these are the zones of: Cremnoceramus crassus crassusud/C. deformis deformis, Inoceramus gibbosus, Volviceramus koeneni, Volviceramus involutus, Sphenoceramusudsubcardissoides, and Sphenoceramus ex gr. pachti. Particular faunas represent assemblages knownudwidely from the Euramerican biogeographic region, although they characterize mostly its northern,udboreal area. The inoceramid-based biostratigraphic scheme allows correlation with other parts of theudNorth American Western Interior and with parts of the Euramerican biogeographic region.udThe studied succession provides a good record of the Inoceramus gibbosus Zone, which characterizesudthe topmost lower Coniacian. This zone, first recognized from northern Germany, is usuallyudabsent, both in Europe and in North America, due to a stratigraphic gap resulting from a eustaticudlowstand. The base of the middle Coniacian is marked by the abrupt appearance of the taxonomicallyudvariable Volviceramus fauna (V. koeneni (Müller), V. exogyroides (Meek and Hayden)), with associatedudInoceramus undabundus Meek and Hayden and Volviceramus cardinalensis, newly describedudherein. Scaphites (Scaphites) ventricosus Meek and Hayden, the ammonite marker of the base of theudmiddle Coniacian first appears in the late early Coniacian. The base of the upper Coniacian marksudthe first appearance of the characteristic northern inoceramid species Sphenoceramus subcardissoidesud(Schlüter), the appearance of which coincides with Scaphites (Scaphites) depressus Reeside, theudammonite marker of this boundary. Close to this boundary Volviceramus stotti also appears, whichudis newly described from the Canadian sections. The base of the Santonian corresponds to the abruptudappearance of Sphenoceramus ex gr. pachti (Arkhangelsky).udThe studied sections demonstrate that the appearance of new inoceramid faunas (lowest occurrenceudof Cremnoceramus crassus crassus (Petrascheck), of various species of Volviceramus, Sphenoceramusudsubcardissoides (Schlüter) and of S. ex gr. pachti) takes place immediately above majorudmarine flooding surfaces, suggesting a close correspondence between evolutionary and/or migrationudevents and episodes of relative sea-level rise.