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

Pages: 
pp. 25-38
Geological Institute, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Bl. 24, 1113 Sofia, Bulgaria
Abstract: 

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based on sporadic, often poorly preserved, macro- and microfossil finds and some biostratigraphic interpretations are outdated.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.

Keywords: 

calcareous nannofossils, biostratigraphy, uppermost Albian–lower Cenomanian, Kula Unit, NW Bulgaria

VOLUME 44 (1-3)/December 2015

University of Mining and Geology, Department of Geology and Geo-Information, Studentski Grad,“Prof. Boyan Kamenov” St., Sofia 1700; Geological Institute, Bulgarian Academy of Sciences, Acad. G. Bonchev St., Bl. 24, 1113 Sofia, Bulgaria

Geological Institute, Bulgarian Academy of Sciences, Acad. G. Bonchev St., Bl. 24, 1113 Sofia, Bulgaria
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University of Mining and Geology, Department of Geology and Geo-Information, Sofia; Geological Institute, Bulgarian Academy of Sciences, Acad. G. Bonchev St., Bl. 24, 1113 Sofia, Bulgaria
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Westphälische Wilhelms-Universität, Institut für Geologie und Paläontologie, Corrensstr. 24, D-48149 Münster, Germany

Geological Institute, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Bl. 24, 1113 Sofia, Bulgaria

Geological Institute, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Bl. 24, 1113 Sofia, Bulgaria
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Hungarian Natural History Museum, Department of Botany, 1476 Budapest, P.O. Box 222, Hungary

Geological Institute, Bulgarian Academy of Sciences, Acad. G. Bonchev, Bl.24, 1113 Sofia; Institute of Geochemistry and Petrology, ETH-Zurich
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Institute of Mineralogy and Crystallography, Bulgarian Academy of Sciences, Acad. G. Bonchev, Bl.7, 1113 Sofia
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Institute of Geochemistry and Petrology, ETH-Zurich
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Institute of Mineralogy and Crystallography, Bulgarian Academy of Sciences, Acad. G. Bonchev, Bl.7, 1113 Sofia

Geological Institute, Bulgarian Academy of Sciences, Acad. G. Bonchev, Bl.24, 1113 Sofia, Bulgaria
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Geological Institute, Bulgarian Academy of Sciences, Acad. G. Bonchev, Bl.24, 1113 Sofia, Bulgaria