Lower Cretaceous calcareous nannofossil biostratigraphy of a North Sea borehole: implications for Boreal Cretaceous stratigraphy

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Abstract
SUMMARY: The calcareous nannofossil stratigraphy of a borehole from the southern North Sea has been investigated in detail. The results are used to evaluate the potential of previously proposed nannofossil marker species and to examine proposed correlation of Boreal nannofossil, and other fossil zones, with stage stratotype sections located in southern Europe. The results support the relative order of nannofossil events proposed in previous zonations based on studies of land sections. Certain intervals of continuous fossiliferous section in the borehole, such as the Upper Hauterivian, complement studies of land sections where these intervals are not complete. The observation of certain predominantly Tethyan nannofossil taxa, including Cruciellipsis cuvillieri, Speetonia colligata and Tubodiscus verenae, whose ranges have not been investigated in detail in Northern European sections, calls into question previous correlations of fossil zonations with chronostratigraphic boundaries. In particular, both the Valanginian/Hauterivian and Hauterivian/Barremian boundaries defined by ammonites may have been somewhat inaccurately placed in the North Sea region as a result of indirect correlations and secondary markers. The Hauterivian/Barremian boundary may have been placed some 4m too high in the Speeton section, and the Valanginian/Hauterivian boundary 7 to 11m too low at the same location. The results are by no means conclusive, but illustrate that further detailed studies of Tethyan nannofossil markers in Boreal sections are warranted.

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