Inoceramid bivalves and calcareous nannofossils from the Coniacian of the Western Srednogorie Unit (Western Bulgaria)

The inoceramid fauna and the calcareous nannoflora of the Coniacian in two sections (Rebro and Kondel Hill) in the Western Srednogorie Unit (Western Bulgaria) have been investigated. Both of the studied sedimentary successions yielded well-preserved inoceramid bivalves. Although the calcareous nannofossil assemblages exhibit poor to very poor preservation, they are reasonably taxonomically diverse and allowed biostratigraphic interpretations to be made. In section Rebro, where the sediments of the Izvor Formation are exposed, the inoceramid fauna indicates the presence of the lower Coniacian Cremnoceramus crassus inconstans and Cremnoceramus crassus crassus/Cremnoceramus deformis deformis inoceramid zones. Based on the consistent occurrence of Broinsonia parca subsp. expansa and the absence of Micula staurophora, the sediments have been assigned to nannofossil subzone UC9c. In section Kondel Hill, the rocks of the Izvor and Melove formations crop out. Only the lowermost ~5 m of the Melove Formation yielded inoceramids, which indicate the presence of the lower Coniacian Cremnoceramus crassus crassus/Cremnoceramus deformis deformis inoceramid Zone. The nannofossil assemblages indicate the presence of nannofossil zones UC9 (in the Izvor and Melove formations) and UC10 (in the Melove Formation), which extend from the upper Turonian (pars.) to the upper Coniacian (pars.). Base Micula staurophora/base UC10 could not be correlated with inoceramid data, however, because it was detected above the inoceramid-bearing strata.