Middle Miocene Subseries in part of Northeastern Bulgaria – chronostratigraphy and correlations

Franz Toula (1890, 1892) expressed his opinion that the rocks in the vicinity of Varna and southernly of the town are similar to those from the Kerch Peninsula. Therefore all the rest investigators accepted these rocks as formed in the Euxinian basin during the Middle Miocene subage.
   The Middle Miocene Subseries in Northeastern Bulgaria is comparatively well studied. The Regional Stages according to the scheme of the Eastern Paratethys, and namely the Tarkhanian, Chokrakian, Karaganian, Konkian and Sarmatian (s.l.) Regional Stage are distinguished.
   In the last years extensive research has been done in a part of Northeastern Bulgaria (Fig. 1). The presence of some species in the gathered collection of molluscs, and some referred to in the publications, request a revision of the concepts concerning the presence and volume of some stages and substages as well as their correlation.