Holocene to modern fine-grained sedimentation on a macrotidal shoreface-to-inner-shelf setting (eastern Bay of the Seine, France)

This paper presents the results of an investigation on different timescales of the main mechanisms governing fine-grained sedimentation on a macrotidal sandy shoreface-to-inner-shelf setting with a supply of terrigenous sediment: the subtidal area of the southeastern Bay of the Seine (Calvados coast, France). Interpretation and calibration of side-scan sonar imagery clearly shows that compact clays crop out in water depths of 3–6 m. Radiocarbon dating and palynological studies of the material sampled using long cores from this subtidal area show that these relict sediments constitute the infilling of lateral valleys of the palaeo-Seine during the last 10 000 years. The lower parts of these deposits consist of compact clays that accumulated in a floodplain setting, later a salt marsh environment, and are succeeded by sediment with sand/mud couplets which formed in a tidal/estuarine system. The top of this sequence has been truncated by a wave erosion surface formed during the Holocene transgression. Today, the sediment accumulating is composed of fine sand, mixed with fine-grained sediment and sometimes temporarily covered by fresh mud. The more recent sedimentological data, compared with surveys in the 1960s–1970s, demonstrate both an increase in the erosion of the submerged earlier Holocene clays and an increase in the mud (silt+clay) content of the superficial sediments. On a seasonal timescale, the seafloor is affected by high-frequency variations in the nature of the contemporary sedimentary cover. Spatial and temporal observations of the seafloor composition have been undertaken during different seasons for four years (1998–2001) to study these sedimentation events. The sedimentation on the inner-shelf is at its maximum when veneers of fresh mud occur after some particular hydrological periods, i.e. sustained high-river outputs following several dry years (i.e. prolonged weak river flows), when significant volumes of mud have been stored within the Seine estuary. Such mud veneers result from: (1) the direct supply of river-born material, (2) the seaward shifting of the turbidity maximum, and (3) the resuspension of mud from the lower estuary (i.e. fluid mud and intertidal flats) under wind-waves, and have been termed ‘estuarine flood deposits’. On a longer timescale of at least the last decades, the southeastern Bay of the Seine is an area of erosion, but it is subjected to ephemeral fine-grained sedimentation on a seasonal timescale.