8th International Nannoplankton Association Conference
Jon R. Ineson, Poul Schiøler, David J. Jutson, Emma Sheldon:
The mid-Cretaceous (Upper Hauterivian-Upper Aptian) of the Danish Central Graben 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 standard sequence stratigraphic principles. The lowstand systems tract (LST) was considered to be characterised by clay-rich facies (marly chalks and marlstones), the result of enhanced basinward transport of siliciclastic fines during low sea-level stand. The two prominent organic-rich marlstone units (the mid-Barremian Munk Marl Bed and the Lower Aptian Fischschiefer) were referred to the transgressive systems tract (TST), by analogy with the condensed section of the siliciclastic sequence stratigraphic model. The cleanest, clay-poor chalks were assigned to the highstands systems tract (HST).
Further investigation of this sequence's stratigraphic subdivision, by means of detailed palynology, nannofossil and microfossil analyses, has resulted in the recognition of a further 'cryptic' sequence boundary, and in significant modification of the preliminary systems tract subdivision. These modifications have primarily resulted from the palynofacies data; in particular, the proportion of land-derived organic matter relative to marine dinoflagellate cysts provides 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. Conclusions pertinent to the general application of sequence stratigraphy to pelagic chalks are: (1) although typically clay-rich, as initially proposed, the LST may be represented by clean, clay-poor chalks. In the Lower Cretaceous Valdemar Field, for example, one of the best reservoir chalk units is referred to the LST. The facies variability of the LST probably reflects the complexity of the relationships between relative sea-level change, basinward dispersal of fines, and pelagic carbonate productivity in such a shallow epicratonic basin; (2) organic-rich marlstones do not necessarily reflect transgressive events. The Munk Marl Bed shows a strong terrestrial signal and is referred to the lowermost LST; in contrast, the affinities of the Fischschiefer are with the TST; (3) the HST typically includes the 'cleanest', clay-poor chalks but may display a rhythmic upward-increase in siliciclastic content (parasequences) reflecting stepwise progradation of marginal siliciclastic systems; such rhythms are mirrored by changes in the organic composition and in shifting nannofossil populations; (4) 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.
Copyright © 2000, most recent revision July 24, 2000Tania Hildebrand-Habel (email@example.com)