Albian ammonite biostratigraphy in Northwest Bulgaria

The Albian Stage in Northwest Bulgaria is of more restricted occurrence compared to the other Lower Cretaceous Stages. It consists of marls interbedded by clayey limestones and by less common sandstones (Sumer Formation) and glauconitic sandstones, siltstones and marls (Malo Peštene Formation). The Albian sediments are of considerable thickness without any hiatuses and condensations in the sections. The deposits contain diverse macrofaunas dominated by ammonites. This paper describes the ammonite and lithological successions in 15 Albian sections. The Albian is considered here to include the L. (L.) tardefurcata Zone from its base up to the S. dispar Zone. The three Albian substages are characterized and new evidence is reported on the Albian ammonite successions in Northwest Bulgaria. It has made it possible to supplement and to further subdivide the zonations proposed by previous authors. Part of the zones where possible, have been subdivided into subzones. The following biostratigraphic units are characterized in ascending order: L. (L.) tardefurcata Zone with L. (L.) tenuicostata Subzone and L. (N.) regularis Subzone; D. mammillatum Zone with P. (I.) steinmanni Subzone; H. dentatus Zone with L. lyelli Subzone and H. spathi Subzone; E. loricatus Zone with A. intermedius Subzone; E. lautus Zone; M. (M.) inflatum Zone with H. orbignyi Subzone, H. varicosum Subzone and C. auritus Subzone; S . dispar Zone. An attempt is made to correlate them with units already established in other parts of the Mediterranean Region.