Upper Kimmeridgian to Lower Tithonian foraminifers and ostracods of the Boulonnais region, Northern France. Stratigraphic and palaeoenvironmental implications and inter-regional correlations with the Siberian arctic

Abstract

The well-exposed Upper Kimmeridgian to Lower Tithonian sequences of the Boulonnais Region (Northern France) are revisited with an integrated litho-, bio- and chemostratigraphic study of the Cran du Noirda (CDN) and Pointe de la Crèche (PDC) sections. Three foraminiferal and two ostracods zones are identified and tied to the ammonite zones previously established in the Boulonnais. The Upper Kimmeridgian Pseudolamarckina pseudorjasanensis foraminifera Zone is of particular interest, as we establish that it may be recognized in very distant regions throughout the Subboreal, Boreal, and Arctic provinces. The comprehensive analysis of the TOC content and Rock Eval pyrolysis allows to underline the deposition of a Lower Tithonian Organic-Rich Interval (ORI) associated with the scitulus (p.p.), wheatleyensis ammonite zones. Furthermore, the palaeoecological analysis of the identified foraminiferal assemblages suggests the occurrence of four dysoxic events during the Late Kimmeridgian to Early Tithonian. Foraminiferal assemblages with genera adapted to low-oxygen conditions are present in intervals with high values of δ13CTOC. During the Late Kimmeridgian to Early Tithonian, organic-rich deposits are widespread across the Western European Archipelago and the Laurasian Seaway. The establishment of hypoxic conditions above the sea-floor was favoured by high marine productivity and stratification of the water column. Conversely, periods of low relative sea-level resulted in the redeposition of sediments from the margins into the basin, while water depth was insufficient to allow the development of reduced conditions.