The calcareous nannofossil biostratigraphic framework of the Late Maastrichtian-Danian North Sea chalk

The calcareous nannofossil biostratigraphy of cored Maastrichtian (Tor Form ation) and Danian (Ekofisk Fm.) chalk sections from 10 selected wells of th e Greater Ekofisk area (Central North Sea) was studied. Wells come from the Edda, Tor, SE Tor, Ekofisk and Eldfisk fields among others. More than 1000 m of cores were examined and about 500 samples were selected and analysed, most of them quantitatively. The main emphasis was placed on the Danian se quence and on the WT boundary. Biostratigraphy provided detailed subdivision for the Danian sediments. Int ra-Danian hiatuses were detected and consistently recorded at the regional scale. Extensive mass-movement was observed highlighting the complex deposi tional history during this time interval. Intervals of Cretaceous chalk wer e found embedded in the Danian sequence (Well 2/5-7), redeposited by slides as discrete chalk packages. Similarly Lower Danian intervals were recognis ed as transported into younger Danian sediments (Well 2/4-B19A). An attempt to reconstruct a regional stratigraphic picture, for the area no rth of Edda field, including Ekofisk, SE Tor and Tor fields, has been made. This suggests the following salient features. The K/T boundary of three wells of the northern sector of the studied area (Tor field), is preceded by a Prediscosphaera steveri acme, suggesting the presence of a rather complete Maastrichtian sequence below the boundary. Above the WT boundary a few tens of centimetres of generally clay-enriched pelagic chalk, of the basal Danian (D1/Biantholithus sparsus Zone) are pres ent in most of the studied wells. Above this a condensed section, often marked by hard ground levels or clay- rich intervals, was detected in almost all the wells. The sedimentary expre ssion of upper D1 (above Cruciplacolithus primus FAD), D2 and D3 time inter val is lacking or extremely reduced. In some cases sediments whose age is near the Early/Late Paleocene boundary were found unconformably overlying pelagic chalk referred to D5-D6 Biozone s. (C) 2000 Elsevier Science B.V. All rights reserved.