Trace fossils from the Buchak Formation (Lutetian, Eocene) of Luhansk Oblast, Ukraine, and their palaeogeographic significance

Eleven Fodinichnia, Pascichnia, and Domichnia ichnotaxa (Apectoichnus longissimus (Kelly et Bromley, 1984), cf.Conichnus isp., Gastrochaenolites isp., Gnathichnus isp., Macaronichnus segregatis Clifton et Thompson, 1978,Ophiomorpha nodosa Lundgren, 1891,Palaeophycus isp., cf.Rosselia isp.,Skolithos linearis (Haldeman, 1840),Spongeliomorpha isp., and Thalassinoides isp.) were identified from seven sections of shallow marine sediments of the Buchak Formation (Lutetian, middle Eocene) located in Luhansk Oblast, eastern Ukraine. Traces of bioerosion on the contact surface between the Upper Cretaceous marls and the Buchak Formation indicate the development of the Trypanites Ichnofacies in conditions of high water activity with almost absent sedimentation. Most of the sediments of the Buchak Formation in the study area was formed in the conditions of the Skolithos Ichnofacies, namely in littoral environments with significant water activity, to resist which organisms were forced to build burrow systems. Some time intervals were also characterized by a decrease in the activity of the water, during which the sands of the littoral and probably sublittoral zones were colonized by the trace-makers of Macaronichnus segregatis burrows. In places of mass accumulation of wood, the Teredolites Ichnofacies is recognized, which is characterized by the development of teredenid bivalves specializing in feeding on wood. The contact surface of the Buchak and Kyiv formations is a firmground with the Glossifungites Ichnofacies developed on its surface and under it, which was already functioning during the accumulation of Kyiv Formation sediments. This ichnofacies was characterized by high water activity and the presence of organisms capable of bioturbating relatively dense substrates. The origin of massifs and blocks of quartzite-like sandstones among the sands of the Buchak Formation of Luhansk Oblast and many other sections of eastern and north-eastern Ukraine is not fully understood, but their hydrothermal genesis is most likely. The obtained data are of great importance for the reconstruction of the depositional environment of the middle Eocene sediments in eastern Ukraine.