Biostratigraphy and Inoceramus survival across the Cenomanian–Turonian (Cretaceous) boundary in the Ram River section, Alberta, Canada

Abstract

AbstractThe biostratigraphy of the uppermost Cenomanian – Lower Turonian succession in the lower part of the Blackstone Formation exposed in the Ram River (Alberta, Canada), is interpreted in terms of the standard inoceramid/ammonite zonation of the interval. Four successive inoceramid zones are recognized, those ofInoceramus pictus,Mytiloides puebloensis,M. kossmati, andM. mytiloides, as established in the stratotype section at Pueblo, Colorado. Their correlation to Pueblo is confirmed by ammonite data.The mid-Early Turonian zone ofM. kossmatiyielded an assemblage ofInoceramus, with species showing close affinity to the latest Cenomanian lineages. This multi-species sample proves the survival ofInoceramuslineages into the otherwiseMytiloides-dominated Early Turonian, and indicates that their disappearance from the record of the North American Western Interior was not because of their extinction. It is suggested that the apparent lack ofInoceramusin Lower Turonian strata is due to an extremely low population abundance in the Early Turonian sea.