Characteristics of the Middle Cenomanian rhythmic sequence from Mount Selbukhra in southwest Crimea

New lithological, ichnological, and isotope data are given for the rhythm from the section of the Middle Cenomanian rhythmic sequence at Mount Selbukhra. The intense rock bioturbation has distinct multistage structure that is typical of writing chalk sections and corresponds to the Cruziana ichnofacies that are characteristic of the soft grounds of the open shelf. The values of the δ 13 C and δ 18 O ratios in ichnofossils contradicts the model of the farming structures for the Chondrites and Zoophycos and indicate the passive infilling of Thalassinoides burrows by sediments. It was established that organic-rich marlstones accumulated under higher bioproductivity and the bottom anoxia were augmented by dissolution cycles. Based on our data, it is not possible to relate the rhythmicity to the precession cycles with confidence.