Sequence stratigraphy of the Coniacian of the Anglo-Paris Basin

The Upper Cretaceous chalk of northwest Europe was deposited in an open epicontinental sea during a period of high global eustatic sea-level – a unique depositional setting requiring modification of the sequence-stratigraphical approach. In this paper, a framework for the sequence-stratigraphical analysis of the Upper Cretaceous chalk is discussed, and an ideal sequence model presented. Analysis of the Coniacian succession has identified eight third-order (short-term) cycles of relative sea-level change (~ 400 Ka duration), superimposed upon a second-order (longer term) cycle (~ 3.2 Ma duration). The third-order cycles represent frequent, basin-wide oceanographic changes for which there are no known tectonic mechanisms. These cycles show a visual correlation with both the δ13C and the δ18C stable isotope curves, indicating a climatic control on sedimentation, probably linked to a Milankovitch eccentricity rhythm. The long-term trend in independently derived δ13C stable isotope values parallels the long-term relative sea-level curve, reflecting increased production as the shelf area expands. By contrast, the δ18C stable isotope values show little change over the long term, suggesting that tectonics, rather than climate, was the controlling factor on second-order relative sea-level change. This is corroborated by the fact that the long-term cycle is coincident with a phase of increased activity at the mid-ocean ridges, which is thought to allow for a 60 m rise in global sea level