Foraminiferal evidence for Cenomanian sequence stratigraphy and palaeoceanography of the Boulonnais (Paris Basin, northern France)

Abstract

A middle to upper Cenomanian chalk succession in the northwestern European Paris Basin (Escalles section) was investigated with respect to its sediment composition, biofacies, and foraminiferal content. Most planktic foraminifers exhibit a high-frequency cyclicity in their distribution pattern and show little relationship to the sequence stratigraphic subdivision. A few characteristic species of benthic foraminifers ( Tritaxia pyramidata, Ataxophragmium compactum, Dorothia levis, and Gavelinella cenomana) are related in their frequency distribution to certain systems tracts. Other benthic foraminiferal species are more evenly distributed through the systems tracts. Cluster analysis and factor analysis allow the separation of three benthic foraminiferal biofacies domains, which exhibit a characteristic frequency distribution pattern with respect to sequence stratigraphy. Biofacies 1 is dominant in the highstand systems tract (HST), and biofacies 3 is characteristic for the transgressive systems tract (TST). Biofacies 2 is most abundant around the maximum flooding–downlap surface (mfs) and less abundant around the sequence boundary. Factor analysis shows that most of the variance in the data set is explained by variables related to plankton productivity. Variables related to changes in water depth and changes in sea-level have minor influence on the variance of the whole data set, but have some influence on the variance of the benthic foraminiferal data set. The different reaction of benthic and planktic foraminifers to relative sea-level changes is also recorded in the total number of foraminifers per gram sediment (foraminiferal number) and in the ratio between planktic and benthic foraminifers (p/b-ratio). The foraminiferal number is highest at the mfs and lowest at the sequence boundaries of the investigated depositional sequences. The p/b-ratio has no correlation with the sequence stratigraphic subdivision, but shows a high-frequency cyclicity. The abundance pattern of planktic foraminifers is controlled by changes in the productivity of calcareous plankton. The regular variation in calcareous plankton productivity leads to marl (low productivity)–chalk (high productivity) cycles that form conspicuous couplets in the section. The productivity cycles are also recorded in gamma-ray logs in the environmental setting of the Escalles section. In contrast, the abundance patterns of benthic foraminifers are more influenced by relative sea-level changes, changes in water depth, and variations in substrate composition. The stacking of probably precession-controlled development of couplets lead to a distinct 100,000-year cyclicity that does not match very well with published sequence stratigraphic subdivisions.