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. Highlights ► Sedimentary records retrace the landward retreat of tidal processes at the Holocene. ► The stepped bedrock allows unique preservation of depositional environments. ► The bed-rock morphology has a key-role on the conservation of sediment deposition.

Palaeoenvironmental reconstructions during the Meso- to Neolithic transition (9.2–5.3 cal. ka BP) in Northwestern France: Palynological evidences

“…Chenopodiaceae) and a rather weak fluvial dynamic. Considering the current 26 m depth location of core KS-24 and its 1.81 m length, it allows deducing a relative sea-level between 27 and 28 meters under the current sea-level which is very similar to the 26 meters proposed at 9,000 years BP by Gregoire et al (2017), taking into account the macrotidal context of north-western Brittany.…”

“…Palaeo-valleys became flooded according to different steps of large fluvial terraces (Gregoire et al, 2017), considerably decreasing surfaces occupied by intertidal salt-marshes and mudflats (i.e. typical environments submitted to intertidal dynamics); the shallowest parts of the BB being flooded between 9,000 and 7,000 years BP.…”

“…The surrounding continental context may then correspond to small watersheds, while the Aulne River was still flowing to the north of core KS-24 location taking into account the low sea level position at that time (i.e. -26 m; Gregoire et al, 2017;Fig. 11).…”

Long‐term evolution of the Old Rhine estuary: Unravelling effects of changing boundary conditions and inherited landscape

“…Firstly, because of the wide back-barrier plain along the Dutch coast, the Old Rhine could freely move away from former estuaries, connecting to and subsequently capturing a tidal channel outside the formerly active part of the delta, which consequently expanded in size ( Figures 5 and 10). This was possible because sea-level rise had exceeded the palaeovalley shoulders, creating a broad plain of accommodation, unlike the situation in topographically constrained estuaries elsewhere (Allen, 1990;Belknap & Kraft, 1985;Bertin, Chaumillon, Weber, & Tesson, 2004;Clement & Fuller, 2018;Fletcher et al, 1990;Gregoire, Le Roy, Ehrhold, Jouet, & Garlan, 2017;Long, Scaife, & Edwards, 2000;Pye & Blott, 2014;Vis et al, 2008Vis et al, , 2016. Secondly, the Old Rhine entered an infilling tidal basin with an abundant intertidal area and a well-developed tidalchannel network, through which it could rapidly obtain a connection to the open sea ( Figure 10: 6,800 to 5,700 cal.…”

Middle Pleistocene seismically induced clay diapirism in an intraplate zone, western Brittany, France

“…No mud volcanoes have ever been observed on the Holocene mudflats of the Bay of Brest after the recent earthquakes (e.g., October 11, 2013;December 11, 2013;October 30, 2015;December 8, 2016;December 11, 2016; January 24, 2018: M l 3.4 to 3.9, local magnitude according to Comissariat à l'Energie Atomique network) or after the recent, major Hennebont (in Middle Pleistocene seismically induced clay diapirism in an intraplate zone, western Brittany, France 2002; M w 4.3) and Quimper (in 1959; M w 5.4) earthquakes. Nevertheless, Holocene submarine slumps in estuarial clays are present at the entrance of the Elorn estuary (Grégoire et al, 2017;Fig. 1B: S).…”