Calcareous nannofossil and U-Pb zircon geochronological data on the Coniacian–Santonian in a part of the Central Srednogorie Zone (central South Bulgaria)

The calcareous nannofossil assemblages in three sections (Dalgi Rid, Milkova Cheshma and Toplika) of the clayey-carbonate Mirkovo Formation in the Central Srednogorie Zone have been investigated. These sections were specifically chosen because, at their basal parts, they contain silica-manganese (± iron) mineralisations, which, according to recent studies, may prove useful for investigations into their potential as a source of ore and critical elements. The nannofloras exhibit very poor preservation and comparatively-low taxonomic diversity. Nevertheless, the presence of some biostratigraphically-significant taxa has been detected. Micula staurophora occurs consistently in sections Dalgi Rid and Milkova Cheshma, which, in the absence of Lithastrinus grillii, indicates the middle to late Coniacian UC10. The latter species was recognised, with a relatively-consistent stratigraphical range, in section Toplika. Although Lithastrinus septenarius and Arkhangelskiella cymbiformis are absent at Toplika, the studied sediments could not confidently be assigned solely to UC12, and thus a tentative late Coniacian (pars.)–Santonian (pars.) age was adopted for the strata in this section. This is consistent with the obtained LA-ICP-MS U-Pb age (86.2±1.4 Ma) of the youngest zircons from the volcaniclastics in the uppermost part of the Chelopech Formation at the same locality, immediately below the boundary with the overlying Mirkovo Formation.