The Holocene transgression as recorded by incised-valley infilling in a rocky coast context with low sediment supply (southern Brittany, western France)

Abstract

International audienceA combination of morphobathymetric studies, very high-resolution seismics, core sampling and radiocarbon age data is used to investigate the latest stage of the sedimentary infilling of incised valleys in southern Brittany, related to the Holocene transgression. Owing to the bedrock morphology of this highly irregular rocky coast, two main types of valleys are defined by topographic rocky highs parallel to the coastline: 1) wide and rather shallow incised valleys offshore from a topographic sill, 2) narrow and relatively deep valleys between the sill and the coast (ria-type valley). The sedimentary infilling in both types of valleys becomes highly differentiated as the transgression advances onto the coastal area. In the wide valley seaward of the topographic sill, the infilling consists mainly of offshore heterolithic facies while, in the ria-type valley, most of the infill is composed of brackish mudflat deposits and estuarine tidal muddy sands. As the transgression proceeds, the rocky highs are flooded and the whole area is finally covered by the offshore facies. Radiocarbon dating indicates that: 1) the marine ravinement surface is highly diachronous (a few thousand years cross-shore); 2) the top of the offshore facies, coarser and very shelly, represents an episode of condensed sedimentation from about 3000 to 4000 years ago, amalgamating the maximum flooding surface (MFS) and the highstand systems tract (HST). However, we observe a muddy drape, strongly bioturbated in places, in the most proximal areas, overlying the offshore facies. It is thought to represent the modern and most recent stage of sedimentary infilling. This mud cover is made of fine-grained sediments of fluvial and biological origin, and is interpreted as a prograding HST. It reflects an increased influx, partly due to human activities. Finally, the main features of incised valley sedimentary infilling in a rocky coast context with low sediment supply can be characterized by (i) the very strong control of bedrock morphology, (ii) the diachronous character of the transgression, (iii) the late position of the MFS, and (iv), the highly reduced volume of the HST

Comparison of estuarine sediment record with modelled rates of sediment supply from a western European catchment since 1500

“…1a,b) that lies unconformably on older sand deposits and yielded several consistent 14 C ages younger than 1000 AD. Similar fine-grained deposits settled within historical times have been described at a regional scale in estuaries and inner shelf of the bay of Biscay (Lesueur et al, 1996;Menier et al, 2010). Striking similarities between these silt drapes regarding lithology, sediment sources and timing of deposition strongly support the influence of large scale allogenic factors, rather than fortuitously synchronous decrease in estuarine tidal energy.…”

The paleo-Etel River incised valley on the Southern Brittany inner shelf (Atlantic coast, France): Preservation of Holocene transgression within the remnant of a middle Pleistocene incision?

“…High amplitude oscillations of the mean-sea level during the Quaternary, associated with strong climatic variability, are usually considered as the main cause of shallow incision of river valleys at the top of these Neogene formations (Bouysse and Horn, 1968;Pinot, 1974;Delanoë et al, 1975Delanoë et al, , 1976Vanney, 1977;Menier et al, 2006). On the French Atlantic shelf, the valley network tapers below −50 m in depth (Pinot, 1974;Vanney, 1977;Lericolais et al, 2001;Proust et al, 2001;Chaumillon et al, 2008;Menier et al, 2010). In most cases, the valley infillings correspond to a Holocene transgressive sequence passing from estuarine to marine deposits (Chaumillon et al, 2008;Sorrel et al, 2010).…”

“…The sedimentary infill is attributed to the last post-glacial transgression and is composed of tidal muddy sands deposits passing upwards to marine coarse sand sheets (Chaumillon et al, 2008;. However, a basal unit interpreted as braided river deposits is punctually identified on the valley floor (Menier, 2004;Menier et al, 2006Menier et al, , 2010.…”

“…On the French Atlantic coast, the study of incised valleys preserved in shelf sequences has been mainly based on widely-spaced high resolution Marine Geology 329-331 (2012) [75][76][77][78][79][80][81][82][83][84][85][86][87][88][89][90][91][92] seismic records, supplemented with sparse shallow cores (Bouysse and Horn, 1968;Pinot, 1974;Vanney, 1977;Allen and Posamentier, 1993;Lericolais et al, 2001;Proust et al, 2001;Féniès and Lericolais, 2005;Chaumillon and Weber, 2006;Menier et al, 2006Menier et al, , 2010Chaumillon et al, 2008). Proposed models consist in 2D qualitative models that present the general sedimentary succession and 2D geometrical relations between each seismic/sedimentary unit.…”

Distinguishing natural evolution and human impact on estuarine morpho-sedimentary development: A case study from the Vilaine Estuary, France

“…6.000 years ago. Seawater flooded first the downstream part of the valley, which was rapidly filled up before the stabilization of the dynamic balance between river and marine inputs (Proust et al, 2001;Menier et al, 2010;Sorrel et al, 2010;Traini et al, 2013;Menier et al, 2014). Prior to the construction of the Arzal dam, tidal influence extended to a distance of 80 km onshore (Morzadec-Kerfourn, 1974).…”