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Kristalina Stoykova, M. Ivanov:
Integrated ammonite and nannofossil biostratigraphy of the Barremian-Albian interval in Bulgaria
(Poster)

We present an integrated nannofossil and ammonite biostratigraphy of the Barremian-Albian interval based on direct correlation between the bioevents of the two groups. The study is grounded on the combined logging of 25 sections, cropping out in the territory of N Bulgaria.

Ammonite stratigraphy of the Barremian-Albian interval in Bulgaria has received much study, and different biozonations have been developed (e.g. Nikolov, 1987; Breskovski, 1975; Stoykova, 1990; Ivanov, 1991). Calcareous nannofossil studies are recently in progress. They are based on direct correlation and calibration with ammonite finds.

Barremian stage There exist different controversial opinions about the placement of the base of the Barremian in Bulgaria. We accept its placement at the first occurrence (FO) of Barremites (Barremites) and the disappearance (LO) of P. angulicostata and a major part of the species of Pseudothurmannia (i.e. above the Pseudothurmannia Beds). Calcareous nannofossil assemblages are of little help in precisely determining this boundary. The upper part of the Lower Barremian corresponds to the H. perezianus Ammonite Biozone. In the Upper Barremian, four successive ammonite zones are identified: E. barremense, H. feraudianus, I. giraudi and C. sarasini.

Calcareous nannofossils Chiastozygus litterarius has a rare and discontinuous occurrence in Bulgarian sections. Its FO is somewhere in the Imerites giraudi Ammonite Biozone, but this event could not be fixed precisely due to the low abundance of the species. The FO of Flabellites oblongus is recorded in the lower portion of the Colchidites sarasini Ammonite Biozone, its first representatives being of small size. Our data are fairly consistent with the observations of Aguado et al. (1997) in S Spain. The subsequent FO of Hayesites irregularis in the middle of the sarasini Biozone is clearly detected, and the most significant nannofossil event. It is recognised in all sections studied, from the W to E of N Bulgaria. This event is of great stratigraphic value because it is reported in many sections of the Mediterranean region: Spain, SE France, Italy (Erba, 1996; Aguado et al., 1997).

Aptian stage The Barremian/Aptian boundary in Bulgaria, in terms of ammonites, is drawn by the FO of Procheloniceras pachystephanum or P. tenuicostatus. The Lower Aptian includes the following ammonite zones: P. pachystephanum, D. deshayesi (with subzone D. forbesi) and D. furcata. Its upper boundary is marked by the disappearance of Dufrenoya, T. bowerbanki and the appearance of the first Parahoplitidae and Epicheloniceras. In the Middle Aptian (Gargasian), two ammonite zones are distinguished: A. martinioides and E. subnodosocostatum. The base of the Upper Aptian (Clansayesian) is placed at the FO of A. nolani, and it is subdivided into the zones A. nolani and H. jacobi.

Calcareous nannofossils No significant nannofossil bioevent, coinciding with the conventional Barremian/Aptian boundary, is detected in Bulgarian sections. The FO of typical Braarudosphaera africana is documented at the middle part of the Pachystephanum Ammonite Biozone, immediately above the Barremian/Aptian boundary. Above this bioevent, another reference level is recorded: abundance of Nannoconus cf. N. truittii. The FOs of the marker-species, Rhagodiscus angustus and Eprolithus floralis are reported from the upper portion of the deshayesi Ammonite Biozone (Lower Aptian). The FO of Prediscosphaera aff. P. columnata is located within the upper part of the jacobi Ammonite Biozone (Upper Aptian).

Albian stage In terms of ammonites, the Albian Stage is interpreted as covering an interval from the base of the L. (L.) tardefurcata Biozone to the top of the S. dispar Biozone. The Aptian/Albian boundary is drawn at the extinction of H. jacobi, Neosilesites, Nodosohoplites and the FO of Leymeriella. A refined ammonite zonal scheme, including seven zones and nine subzones, is accepted for the Albian Stage in Bulgaria (Ivanov, 1991) (Figure 1). The stage is traditionally subdivided into three substages: Lower (L. tardefurcata and D. mammilatum Biozones), Middle (H. dentatus, E. loricatus and E. lautus) and Upper (M. inflatum and S. dispar). The Albian/Cenomanian boundary is placed at the disappearance of S. dispar, O. puzosianum and the FO of M. martimprey and M. oehlerti.

Calcareous nannofossils In the Albian, the following reliable nannofossil bioevents are directly correlated to the ammonite zones: Lower Albian: the FO of Cribrosphaerella ehrenbergii in the upper part of the L. (L.) tardefurcata Biozone (L. (N.) regularis Subzone); the FO of Tranolithus phacelosus within the topmost part of the D. mammillatum Biozone (P. (I.) eodentatus Subzone). At the base of the Upper Albian, the FO of Eiffellithus turriseiffelii is documented. No reliable nannofossil event occurs around the Albian/Cenomanian boundary. The FO of Corollithion kennedyi is recorded some 8m above the boundary in ammonite terms. The nannofossil succession of bioevents has been tested for correlation of several sections lacking of ammonites.

The proposed integrated parallel biozonation includes widely-accepted biozones, as well as specific (local) ones. It is opened for discussion and confirmation or rejection.

References

Aguado, R., M. Company, J. Sandoval & J. M. Tavera. 1997. Biostratigraphic events at the Barremian/Aptian boundary in the Betic Cordillera, southern Spain. Cretaceous Research, 18: 309-329.

Breskovski, S. 1975. Les zones et sous-zones ammonitiques dans Barremien en Bulgarie du Nord-Est. Geologica Balc., 5(2): 47-66.

Erba, E. 1996. The Aptian Stage. Bull. Inst. Royal des Sci. Nat. Belgique, Sci. de la Terre, 66: 31-43.

Ivanov, M. 1991. Albian ammonite biostratigraphy in Northwest Bulgaria. Geologica Balc., 21(4): 17-53.

Nikolov, T. 1987. The Mediterranean Lower Cretaceous. Ed. Acad. Bulg. Sci.: 264pp.

Stoykova, K. 1990. Certain stratigraphy and correlation problems of the Aptian in Bulgaria. C. R. Acad. Bulg. Sci., 43(1): 95-96.

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