Shallow-water marl-limestone alternations in the Late Jurassic of western France: Cycles, storm event deposits or both?
Résumé
The contribution of event deposits to various basin fills can be very significant, higher than 90% in some cases. Events may lead to the formation of marl–limestone alternations, which can also result from cyclic changes in sea level or climate, for example. The marl–limestone alternations of the Late Jurassic of western France contain abundant coarse-grained accumulations that resemble storm deposits described in other western European successions. The detailed analysis of facies evolution and hierarchical, high-frequency stacking pattern of depositional sequences of the Phare de Chassiron section (Ile d'Oléron, western France) allows the controls on marl–limestone formation to be defined. This section contains nearshore and shallow-marine mud deposits that were exposed to high-energy events. Elementary, small-, and medium-scale depositional sequences are defined. The stacking-pattern and the duration of these sequences suggest an orbital control on sedimentation. Precession (20 ka) cycles notably controlled the formation of elementary sequences that correspond to marl–limestone alternations. The deposition of marly or carbonate mud occurred in this storm-dominated system because of muddy sea beds, the gentle slope of the shelf, and the great amount of particles in suspension, which reduced water energy resulting from storms. Sediment supply was also sufficient to limit bioturbation and favour the preservation of numerous storm deposits. The production of carbonate mud was localised on positive structures and partly controlled by Milankovitch-scale sea-level cycles. Transport by storms of carbonate mud to the adjacent marly depressions during high carbonate production periods led to the formation of calcareous beds. Marl–limestone alternations in the Late Jurassic of western France therefore result from the combined effects of cyclic changes in carbonate production and high-energy, episodic events.