Eocene stratigraphy of key sections of the Dnieper-Donets Depression based on calcareous and siliceous microplankton

Radiolarian, diatom, nannoplankton, and foraminifer assemblages were studied in detail in four key sections (Kantemirovka, Sergeevka, and 9540 Rudaevka, 5-93 Boguchar boreholes) of the south and central parts of the Voronezh anticline area. The widespread middle Eocene sediments lie uncorformably on marls and limestones of Upper Cretaceous age. They are mainly composed by a transgressive-regressive succession of phosphoritic sands, marls, and siliceous clays of the Kiev Formation in the Ukraine (or Sergeevka and Tishki formations in Russia) and by sandy clays and siliceous clays of the lower part of the Khar'kov Formation in the Ukraine (or Kas'anovka Formation in Russia). Lithologically, the coeval formations range from terrigenous-carbonate to siliceous-carbonate. The age of the formations has long remained a point of discussion. Recent studies based on calcareous and, especially, siliceous microplankton allowed a direct correlation of these sections with standard zonal scales.