Calcareous nannofossil and ammonite integrated biostratigraphy across the Jurassic – Cretaceous boundary strata of the Kopanitsa composite section (West Srednogorie Unit, southwest Bulgaria)

Abstract

Calcareous nannofossil, calpionellid and ammonite occurrences have been directly constrained across the Jurassic–Cretaceous boundary interval in the section of Kopanitsa, SW Bulgaria. This section reveals a continuous and expanded sedimentary record through the Upper Tithonian and Lower Berriasian, besides an excellent calcareous nannofossil and ammonite record. The topmost part of the NJT 16b and the base of NJT 17a nannofossil Subzones correspond to the ammonite Microcanthum / Transitorius Subzone. The major part of the NJT 17a Subzone equates to the Durangites spp. ammonite Zone, whereas the NJT 17b Subzone correlates to the lower part of the B. jacobi ammonite Zone. The NKT nannofossil Zone approximately corresponds to the upper part of the B. jacobi Zone and the NK-1 nannofossil Zone correlates at least to the lowest part of the T. occitanica Zone. The FOs ofNannoconus globulus minor, N. wintereri, N. kamptneri minor, N. steinmannii minor, N. kamptneri kamptneriandN. steinmannii steinmanniiare confirmed as reliable bio-horizons for correlations in the Mediterranean Tethys area. The first occurrence ofNannoconus wintereriis regarded as an almost concomitant event with the first occurrence ofBerriasella jacobi. We suggest it could be the most useful nannofossil proxy for approximating the base of the B. jacobi Zone. Rare, but relatively well preserved calpionellids and calcareous dinoflagellates together with microfacies analysis were used additionally for stratigraphical and palaeoenvironmental interpretations. The investigated sediments are typical for the steep slope of a steepened ramp, with accumulation of hemipelagic and gravitational deposits.

The calcareous nannofossils and magnetostratigraphic results from the Upper Tithonian–Berriasian of Feodosiya region (Eastern Crimea)

“…senoides were recorded approximately 110 m down-section. It is likely that the base of the NJT 17a Subzone is located down the section, within the Upper Tithonian, as well as in the Kopanista section (Stoykova et al 2018a).…”

The Jurassic/Cretaceous boundary and high resolution biostratigraphy of the pelagic sequences of the kurovice section (Outer Western Carpathians, the northern Tethyan margin)

“…7Y) mentioned in the Atlantic Ocean (Bralower et al 1989;. Stoykova et al (2018) mentioned N. compressus from the SW Bulgaria, and Halásová in Bakhmutov et al (2018) from the Crimea (southern Ukraine). The question remains whether these areas were also under marine influence from the north.…”