Control factors of Holocene sedimentary infilling in a semi-closed tidal estuarine-like system: the bay of Brest (France)
Abstract
This study details the sedimentary infilling of an original tidal-dominated estuary system during the final
stage of the last marine transgression. The Bay of Brest is confined and connects the rivers Elorn and
Aulne, to the sea of Iroise by a narrow strait encasing a well preserved paleo-channel. The compilation
of 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 floor
surrounded by fluvial terraces, the central plateau, and the shallow embayments. Taking into account
the main factors controlling the infilling, including sea-level rise, substratum morphology, and
hydrodynamics, 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 low
sea level around 21,000 cal yr B.P. The transgressive phase, represented by the transgressive system
track (TST), is divided into two parts. The lower part (TST1) constitutes the first stage of transgressive
deposition 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 about
7000 cal yr B.P. and is separated from the upper part (TST2) by a tidal ravinement surface that occurs
around 7700 cal yr B.P. at the foot slope. TST2 formed sand bodies in the central part of the bay
between 6800 and 3000 cal yr B.P. These deposits, interpreted as tidal banks, are associated with the
outer estuarine environment. The maximum flooding surface (MFS), dated around 3000 and 2000 cal yr
B.P., marked the installation of a highstand system track (HST) under the combined influence of tidal
currents, storms events, and anthropogenic activity. The geometry deposition of each system track is
controlled at the first order by the combination of the irregular Holocene sea level rise and the inherited
rocky substratum morphology. The retreat of the shoreline, and estuarine environments, is function of
the successive flooding of stepped domains. The direct evolution of the volume of the assumed
accumulated sea water in the bay favoured the gradual installation of the present-day hydrodynamic
circulation.
Origin | Files produced by the author(s) |
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