Carbonate and siliceous rock horizons at the boundary of Eocene and Oligocene deposits in the Ukrainian Carpathians as geotourism sites

Abstract

The outcrops of carbonate and siliceous rock horizons at the boundary of Eocene and Oligocene deposits in the Ukrainian Carpathians represent potential sites for geotourism. They are located in the northern part of the Skyba zone, in settlements (Boryslav, Verkhne Synyovydne, Maydan, Skole) that can be reached by various vehicles. The sites are of interest not only due to the possibility to be shown for tourists in situbut also due to the possibility to tell them about sedimentological history of the Carpathians, the whole Tethys basin and theWorld Ocean being the reason fortheir formation. The article considers global and regional events at the Eocene-Oligocene boundary, exemplified by deposits of the Sheshory horizon and lower siliceous horizon of the Lower Menilite Sub-Formation as well as sedimentologicaland lithogenetic aspects of chertsorigin.In sections of the Carpathian flysch the clay-calcareous and siliceous- clay rocks of the Sheshory (Eocene) and Rybnyk (Oligocene) horizons are overlaid by a pack of carbon-bearing cherts(mainly phthanites) of the Menilite Formationlower siliceous horizon. This was fixed as the «Eocene final event» (climatic cooling, mass extinction of some groups of marine organisms, including certain types of warm-water foraminifera), and had a global significance. At that time at the regional scale separation of the Paratethys Sea from the Tethys Ocean as well as sharp change in biogenic sedimentation (from carbonate to siliceous) in the Carpathian sedimentary basin occurred.At the Eocene and Oligocene boundary and then during the Oligocene the World Ocean demonstrated a series of events that ultimately led to the transformation of a global system of ocean waters circulation from the Cretaceous-Eocene, marked with uniformly warm climate throughout the planet, to contemporary with more contrast climate and distinct climatic zones.The change of warm-water plankton organisms for those who areaccustomed to live in cold seas can be illustrated by the results of study of calcareous and siliceous rocks of the Eocene-Oligocene deposits in the Ukrainian Carpathians.The Sheshory horizon marls are composed of cryptocrystalline clay- carbonate matter with numerous remains of warm-water forms of planctonic foraminifera from a group of globigerina (Globigerinida) sized up to 0.1 mm. The number and size of these organisms significantly reduced in the rocks of Rybnyk horizon. No Their traces are notfound in siliceous formations.Various researchers have justified biogenic, volcanogenic, or chemogenic origin of these cherts. Authors of the article believe that these rocks are the product of sedimentation of biogenic siliceous deposits and their post-sedimentary transformation.