Composite biostratigraphy of the Hauterivian-Ryazanian interval in the Iris-1 (5604/30b1) well, Danish sector, North Sea

The Iris-1 well located in the Iris Basin, a subbasin of the Danish Central Graben, is one of the few wells in this area to have been drilled in a basinal setting. The well penetrated a thick interval of claystones and sandy claystones of Ryazanian to Hauterivian age (nannofossil zones BC1-BC11 of BOWN et al. 1998). The interval investigated represents the upper part of the Farsund Formation and the lower part of the Åsgard Formation. An integrated biostratigraphic study (nannofossils, palynology and microfauna) has been undertaken on the interval to produce a composite biostratigraphy which has affinities to schemes from both England and northern Germany. Based on the biostratigraphic results a lower section of Ryazanian to Early Valanginian age was identified. This is succeeded by an upper section of Early to Late Hauterivian age. Condensed or missing sections in the Late Valanginian have been documented from a range of settings in the North Sea region. Recognition of this unconformity in such a basinal setting in the Danish Central Graben supports the regional significance of this event.