Mid-Cretaceous sequence stratigraphy in the Danish Central Trough - a contribution to the EFP-93 project: Lower and Upper Cretaceous stratigraphy in the Central Trough

The mid-Cretaceous (Upper Hauterivian - Upper Aptian) of the Danish Central Trough comprises a heterogeneous succession of interbedded chalks, argillaceous chalks and marlstones, punctuated by two distinctive organic-rich marlstones that are recognised regionally in the North Sea basin. Preliminary sequence stratigraphic analysis of this basinal succession based on seismic, petrophysical, sedimentological and biostratigraphic data resulted in the recognition of three sequences. These were further subdivided into systems tracts on the basis primarily of lithofacies and a simplistic model derived from classical Exxon sequence stratigraphic principles. Further investigation of this scheme, by means of detailed palynology and nannofossil analysis has resulted in the recognition of a further “cryptic” sequence boundary and in significant modification of the preliminary systems tract subdivision. These modifications have resulted, to a large degree, from the palynofacies data, the proportion of land-derived organic matter relative to marine dinoflagellate cysts providing an unambiguous sea-level signal. Detailed analysis of the relative abundances of key dinoflagellates and nannofossils has provided supportive data but these data, considered alone, are typically ambiguous. The study demonstrates the necessity of an integrated approach in attempting to identify depositional sequences and systems tracts in pelagic chalk successions and illustrates the potential pitfalls of a lithofacies-based approach utilising an over-simplistic sequence stratigraphic model.