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Uppermost Albian–lower Cenomanian calcareous nannofossils from the Shishentsi-1 and Makresh-1 boreholes, Kula tectonic unit (NW Bulgaria)

Cenomanian sediments, and Albian/Cenomanian boundary strata, are not largely exposed in NW Bulgaria. Many of the boreholes that have penetrated them have poor core recovery for this stratigraphic interval, and this has greatly hampered investigations. Most of the previously published data are based on sporadic, often poorly preserved, macro- and microfossil finds and some biostratigraphic interpretations are outdated. Sometimes, age assumptions have been made only by means of lithologic comparisons and/or stratigraphic position, without fossil evidence. This paper presents the first calcareous nannofossil study on the available (limited and discontinuous) cores from the Upper Cretaceous of the Shishentsi-1 and Makresh-1 boreholes drilled in the Kula tectonic unit (Vidin District, NW Bulgaria). The Albian–Cenomanian sediments of the Rabisha Formation in both boreholes yielded relatively diverse, generally moderately well-preserved nannofloras; samples from higher stratigraphic levels, which have been assigned to the Kula Formation, proved to be barren. As a result, the presence of the uppermost Albian–lower Cenomanian subzone UC0c (in Shishentsi-1 and Makresh-1) and the lower Cenomanian UC2a (in Makresh-1) has been indicated. The consecutive bases of Gartnerago theta and Prediscosphaera cretacea were both noted within UC0c. The former event has been used to divide UC0c into uppermost Albian–lower Cenomanian and lower Cenomanian intervals. The presence of Gartnerago theta from 1529 m to 1555.15 m in the Shishentsi-1 borehole also suggests that the Albian/Cenomanian boundary lies stratigraphically lower than previously thought.