Infilling stratigraphy of macrotidal tide-dominated estuaries. Controlling mechanisms: Sea-level fluctuations, bedrock morphology, sediment supply and climate changes (The examples of the Seine estuary and the Mont-Saint-Michel Bay, English Channel, NW France)

International audienceThis article presents a synthesis of recent works performed on two macrotidal tide-dominated estuaries located along the southern coast of the English Channel, the Seine estuary and the Mont-Saint-Michel Bay (NW France). On the basis of very high resolution seismic data combined with sediment cores and AMS dating, these works allow reconstructing the Holocene infill of such singular estuaries, yet rarely documented in the literature. Rate in sea-level rise, bedrock morphology, sediment supply are the main controlling factors of the general infill pattern. Both infills are composed of two units, the transgressive systems tract (TST) and highstand systems tract (HST). The maximum flooding surface (MFS) is dated around 6500 cal BP when the Holocene sea-level rise slowed down, and corresponds to the main architectural change. The TST is poorly preserved compared with the HST that constitutes the main depositional unit, making this incised valley fill deviating from the classical models. The HST is composed above the axis of the incised valley of a highly wandering estuarine channel belt characterized by deep tidal reworking throughout its building. On the margin of this high-energy estuarine system, the HST is made of wave-dominated barriers, tidal flats and offshore tidal banks.. They constitute dominantly aggradational systems, the aggradation rate of which is in accordance with the rate of sea-level rise from 6500 cal BP onwards. Consequently, the impacts of the rapid climate changesthat characterized mid- to late Holocene times can be preserved in these marginal successions. Various sedimentary and geometrical signatures indicate that periodic enhanced storminess episodes, probably related to the Bond Cold Events (~1500 years periodicity), are responsible for the most drastic environmental changes in these macrotidal tide-dominated settings. The role of long term tidal cycles as well as the impact of human activities are also considered