Tidal Cycles: Annual Versus Semi-Lunar Records
Marsh deposits frequently consist of a succession of packages interpreted to be annual cycle records. Annual sedimentary cycles are usually recognized because of a contrasted seasonal differentiation of sedimentation. In the inner estuary of the Bay of Mont-Saint-Michel, detailed lamination analysis of some marsh packages that are not heavily disturbed by root traces demonstrates that they can be made of planar silty-mud couplets that thicken and thin systematically. In terms of facies and lamina thickness evolution, these sedimentary cycles are similar to neap-spring-neap tidal rhythmites. However, the number of couplets they contain is inconsistent with the number of tides that are able to reach the supratidal domain during a single fortnightly cycle, but it fits the average number of tides that inundate the marshes during one year. The problem with the occurrence of such annual records arises from their possible misinterpretation as neap-spring-neap cycle records in ancient tidal facies. This confusion could lead to erroneous conclusions about sedimentation rate, tidal regime, environmental context, and orbital parameters, conclusions that are usually inferred from the analysis of neap-spring-neap tidal rhythmites.