A Relative Water-depth Model For The Normandy Chalk (cenomanian-middle Coniacian, Paris Basin, France) Based On Fades Patterns Of Metre-scale Cycles

摘要

A relative water-depth model for the Chalk of the Paris Basin is proposed, based on the lateral variations of the high-frequency metre-scale cycles, which are characteristic features easily identified in the field. The studied outcrops are the Cenomanian-Middle Coniacian cliffs of Normandy. The main result of this study is to highlight the importance of storm activity in the deposition of the Chalk. The relative water-depth model is based on storm-induced shell concentrations observed within the two components of the metre-thick cycles: the depositional interval itself and the top hiatal surface.rnSix types of shell concentrations are defined, along with seven types of depositional facies making up the depositional units, as well as eight types of hiatal surface. Three cycle associations, differing in their thickness and the amount and type of non-carbonate constituents, can be identified in the Lower to Upper Cenomanian, the Upper Cenomanian to Lower Turanian and the Middle Turanian to Middle Coniacian. A relative water-depth profile model for all these cycles is based on the shell concentrations and a "water-depth equivalence" is proposed between the three cycle associations (lateral "facies" substitution diagram). This model is tested using palaeocological data (irregular echinoids) and by correlating field sections in terms of stacking patterns. Most of the studied deposits accumulated above the storm wave base (upper offshore zone or mid ramp).