Upper intertidal rhythmites in the Mont-Saint-Michel Bay (NW France): Perspectives for paleoreconstruction |
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| Authors: | Bernadette Tessier |
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| Institution: | Laboratoire de Dynamique Sédimentaire et Structurale, URA 719 CNRS, Université de Lille I, 59655, Villeneuve d'Ascq, France |
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| Abstract: | The Mont-Saint-Michel Bay constitutes a good present-day example of sedimentation and facies in an upper intertidal domain. The tangue, a specific sediment of this area, displays an alternated structure of sandy and silty-muddy beds generated by the dynamics of tidal currents. This tidal bedding is frequently characterized by a vertical organization determined by the cyclic evolution of tidal energy during a neap/spring cycle. The organisation mainly consists in thickness variation of the successive sand/mud couplets which represent the deposit of each semi-diurnal tidal cycle. This record of tidal cyclicity can be observed in a wide range of bedding from planar bedding to climbing ripple bedding. The investigations performed allowed to clarify the conditions of deposition and of short-term preservation of these tidal rhythmites. Most well recorded neap/spring cycles develop within protected environments and necessarily require high suspended sediment concentrations. Characteristic features of these modern tidal rhythmites, such as the number of tidal event recorded per neap/spring cycle, are discussed in order to select criteria for the recognition of tidal dynamics in ancient sediments. The use of such criteria should constitute a way to discriminate subtidal to upper intertidal deposits and to propose paleogeographic reconstructions. |
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