The calcareous nannofossil record across the Late Cretaceous Turonian/Coniacian boundary, including new data from Germany, Poland, the Czech Republic and England

Abstract

The proposed definition of the Turonian/Coniacian boundary, at the first occurrence of the inoceramid bivalve Cremnoceramus deformis erectus (Meek) (= Cremnoceramus rotundatus ( sensu Tröger non Fiege)), prompted a rigorous study of the calcareous nannofossil events through this interval, both for calibration of the calcareous nannofossil biostratigraphy, and to provide an assessment of the suitability, in calcareous nannofossil terms, of the proposed stratotype section. New calcareous nannofossil data are presented here, detailing the biostratigraphy of the boundary interval from four locations. These include the candidate boundary-stratotype, the Salzgitter-Salder Quarry section (northern Germany), as well as the Slupia Nadbrzezna outcrop (central Poland), a potential secondary reference section. Also included is the Brezno Pd-1 Borehole and outcrops in the Brezno Formation (= Priesener Schichten) type-area (north-western Czech Republic), which represents an original boundary candidate (Copenhagen Stage Boundaries Meeting, 1983), and the Langdon Stairs coastal section (south-eastern England), part of the British Chalk succession. The calcareous nannofossil events derived from each section provide a sequence across the boundary of (in stratigraphical order): below the boundary, the first occurrence of Lithastrinus septenarius followed by that of Broinsonia parca expansa; above the boundary, the last occurrence of Helicolithus turonicus followed by the first occurrence of Micula staurophora (= Micula decussata of some authors). This places the boundary within Nannofossil Subzone UC9c. A similar sequence of events has previously been determined from sections in north-eastern England and in the south-eastern Indian Ocean. The presented data and correlations suggest that either the Salzgitter-Salder Quarry section or the Slupia Nadbrzezna outcrop section would make a suitable Global Stratotype Section for the Turonian/Coniacian boundary, as far as calcareous nannofossils are concerned. The use of the calcareous nannofossil Marthasterites furcatus, widely quoted as an indicator of this boundary, is discussed and proved to be untenable.