Control factors of Holocene sedimentary infilling in a semi-closed tidal estuarine-like system: the bay of Brest (France)

International audienceThis study details the sedimentary infilling of an original tidal-dominated estuary system during the finalstage of the last marine transgression. The Bay of Brest is confined and connects the rivers Elorn andAulne, to the sea of Iroise by a narrow strait encasing a well preserved paleo-channel. The compilationof high- and very-high-resolution bathymetric and seismic data, constrained by sediments datations,allows us to classify the paleo-morphology of the bay into three stepped domains: the paleo-valley floorsurrounded by fluvial terraces, the central plateau, and the shallow embayments. Taking into accountthe main factors controlling the infilling, including sea-level rise, substratum morphology, andhydrodynamics, the stratigraphic scheme of the bay has been reconstructed. The lowstand system track(LST) is assumed to correspond to relict Pleistocene continental deposits associated with the last lowsea level around 21,000 cal yr B.P. The transgressive phase, represented by the transgressive systemtrack (TST), is divided into two parts. The lower part (TST1) constitutes the first stage of transgressivedeposition in the bay. Starting around 9000 cal yr B.P. and currently conserved in the shallowest parts,it is characterized by tidal flats associated with deposits in the inner estuary. This stage ends at about7000 cal yr B.P. and is separated from the upper part (TST2) by a tidal ravinement surface that occursaround 7700 cal yr B.P. at the foot slope. TST2 formed sand bodies in the central part of the baybetween 6800 and 3000 cal yr B.P. These deposits, interpreted as tidal banks, are associated with theouter estuarine environment. The maximum flooding surface (MFS), dated around 3000 and 2000 cal yrB.P., marked the installation of a highstand system track (HST) under the combined influence of tidalcurrents, storms events, and anthropogenic activity. The geometry deposition of each system track iscontrolled at the first order by the combination of the irregular Holocene sea level rise and the inheritedrocky substratum morphology. The retreat of the shoreline, and estuarine environments, is function ofthe successive flooding of stepped domains. The direct evolution of the volume of the assumedaccumulated sea water in the bay favoured the gradual installation of the present-day hydrodynamiccirculation