Calcareous nannofossils from the middle Coniacian–Santonian in the Mirkovo Formation, Central Srednogorie tectonic unit (central South Bulgaria)

Three sedimentary sections (Dalgi Rid, Milkova Cheshma and Toplika), containing Mn-bearing nodules, of the Mirkovo Formation in the Central Srednogorie tectonic unit have been investigated for their calcareous nannofossil content. The assemblages are very poorly preserved, depleted and relatively low in taxonomic diversity. The most abundant taxa are Watznaueria barnesiae, Eiffellithus eximius, Helicolithus anceps, Micula staurophora and Lucianorhabdus cayeuxii. Additionally, Lithastrinus grillii was detected in the section Toplika. Based on the nannofossil data, the sediments at Dalgi Rid and Milkova Cheshma have been referred to the middle–late Coniacian, whereas those at Toplika have been dated as late Coniacian–early(?) Santonian. These data indicate that the formation of the Mn-bearing nodules in the studied successions is time-transgressive.