The Late Cretaceous phosphatized ichnofauna fossils from the Eocene basal horizon of the Middle Dnieper area

Abstract

The object of our study was phosphated remains of ichnofaunas from the basal horizon of the Kanivska suite (Eocene), which transgressively onlaps here the eroded top of the Burimska suite. The goal of the research was to reconstruct some sedimentation features of the final stages of the Late Cretaceous basin’s existence and during of the Early Eocene transgression on the Middle Dnieper area (Ukraine). The specimens of the basal horizon of the Kanivska suite (Р2kn) were sampled from the outcrops in the Melanchin Potik gulley and in a branch of the Glyadovy gulley. It is a complex of rough-medium grained gravel sandstones with shark teeth, fish bone fragments, detritus of Bivalvia and Brachiopoda. The nature of preservation and arrangement suggests that the fossils were relocated. The sample from the Glyadovy gulley is an agglomeration of phosphatized debris of various types of ichnofauna, cemented with a phosphate and sand substrate. The first type, in our opinion, is a fragment of a burrow nucleus of a crayfishlike decapod Thalassinoides Ehrenberg ichnogenus, as indicated by the surface , absence of a constructed wall and its size. The phosphorite features of the second type are cores of the burrows of the Ophiomorpha Lundgren genus with a characteristic ophiomorphic sculpture in the form of knobbles on the outer surface. The structures of the second type have certain differences in the distribution and nature of phosphate material. The described cores are interpreted as domichnia: dwelling places of fossil fauna. They were not known earlier within the Middle Dnieper area. The animals constructed a system of void ducts in the well ventilated middle ichnofossil layer with depth up to 60 cm (as can be inferred from the diameter of Thalassinoides). Although crayfish burrows can be present in any formation, it is generally thought that they are more specific for the littoral and shallow water environments. Based on the fauna composition and the phosphated debris preservation, we can assume the age of ichnofaunas is Late Cretaceous. An indirect indication of this can be the bones and teeth of Cretaceous vertebrates, found in the same basal layer, in particular, ichthyosaurus genus (Platypterygius sp.) and a phosphate concretion with ammonite remnants. Besides phosphatized ichnofauna debris, the basal level contains black odd-shaped nodules and microconcretions, composed of amorphous phosphate minerals. They contain a considerable amount of microscopic fossils of presumably phytoorganic nature and fragments of undeterrmined groups. From the fact that the phosphate material is not completely decrystallized, and that some microfossils have their initial composition, we infer that they belong to the Early Eocene.