Sequence stratigraphy of the Coniacian succession of the Anglo-Paris Basin

The Upper Cretaceous chalk of northwest Europe was deposited in an open epi continental sea during a period of high global eustatic sea-level - a uniqu e depositional setting requiring modification of the sequence-stratigraphic al approach. In this paper, a framework for the sequence-stratigraphical an alysis of the Upper Cretaceous chalk is discussed, and an ideal sequence mo del presented. Analysis of the Coniacian succession has identified eight third-order (shor t-term) cycles of relative sea-level change (similar to 400 Ka duration), s uperimposed upon a second-order (longer term) cycle (similar to 3.2 Ma dura tion). 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 delta(13)C and the delta(18)C Stable is otope curves, indicating a climatic control on sedimentation, probably link ed to a Milankovitch eccentricity rhythm. The long-term trend in independently derived delta(13)C stable isotope valu es parallels the long-term relative sea-level curve, reflecting increased p roduction as the shelf area expands. By contrast, the delta(18)C stable iso tope values show little change over the long term, suggesting that tectonic s, rather than climate, was the controlling factor on second-order relative sea-level change. This is corroborated by the fact that the long-term cycl e 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.