Transgressive systems tract of a ria-type estuary: The Late Holocene Vilaine River drowned valley (France)
Introduction
Sea-level changes strongly controlled the evolution of coastal environments in northwestern Europe during the late Quaternary. The rapid sea-level rise of the beginning of the Holocene resulted in the flooding of the downstream portion of incised valleys, and the slowing down of the transgression around 7500 years BP led to the building of estuary systems (Nichols and Biggs, 1985).
During the last 20 years, the stratigraphy of late Quaternary coastal sedimentary systems has been extensively investigated (Swift et al., 1991, Allen and Posamentier, 1993, Allen and Posamentier, 1994, Dalrymple et al., 1994, Thomas and Anderson, 1994, Zaitlin, 1994). Models were developed according to the shape of the valley, the tidal range and the wave activity (Boyd et al., 2006). Such models show how these three parameters are decisive in controlling sedimentary facies distribution within three system tracts, which are related to the sea-level evolution.
On the Atlantic coast, recent studies provide a detailed understanding of the sedimentary infill succession of incised valleys during the late Holocene transgression (Lericolais et al., 2001, Proust et al., 2001, Menier, 2004, Weber, 2004, Fénies and Lericolais, 2005, Chaumillon and Weber, 2006, Chaumillon et al., 2006, Menier et al., 2006, Chaumillon et al., 2008, Thinon et al., 2008, Menier et al., 2010, Proust et al., 2010), with recent emphasis on the macrotidal bay of the Vilaine River in South Brittany (Proust et al., 2001, Menier, 2004, Menier et al., 2006, Menier et al., 2010, Sorrel et al., 2010). This Vilaine River estuary occupies a very particular position facing a 200 km wide, passive margin continental shelf (Fig. 1A) affected by a meso- to macro-tidal range. The Vilaine River estuary is the most sheltered estuary of the French Atlantic coast. It is located at the back of the Vilaine Bay bounded by the Quiberon peninsula, the islands of Houat and Hoëdic and the Croisic peninsula (Fig. 1B). This sheltered position preserved a unique and complete sedimentary record of the last marine transgression.
However, while the Holocene sedimentary infill of the Vilaine Bay and the estuary is relatively well documented (Sorrel et al., 2010) we still lack information about the depositional history in the most proximal estuarine part of the system. In particular, the relationships between sedimentary successions preserved in the bay, in the innermost part of the estuary and in the fluvial domain are still poorly understood. This is however of crucial interest for a detailed understanding of the development of the marine flooding of an incised valley within a meso-macrotidal system, in time and space.
The aim of this study is to describe the infilling of Vilaine River estuary and to position the depositional model with respect to other estuarine systems (e.g., the Gironde estuary, the Charente River, the Loire River) (Allen and Posamentier, 1993, Weber et al., 2004, Chaumillon et al., 2010, Proust et al., 2010); rias from Spain (Méndez and Vilas, 2005, Vis et al., 2008) with similar dimensions, structural and geological settings but subjected to stronger wave activity and facing a narrower continental shelf; and English Channel estuaries such as the bays of Mont Saint Michel and the Seine estuary, which are both located behind a bay system comparable to the Vilaine River estuary but which are subject to a stronger tidal range (Tessier et al., 2012).
With the acquisition of new very-high resolution (VHR) seismic data (Chirp) and boreholes in the estuary, it is possible to determine the bedrock morphology of the Vilaine River valley, as well as its geometry and sedimentary architecture. A total of 21 AMS age dating results obtained on sediment samples allow us to evaluate the rate of transgression across the estuary. Analyses of sedimentary facies reveal the variable sedimentary expressions of the hydrodynamic parameters as a function of the transgression rate and the geomorphological context.
Section snippets
Regional setting
Located in the northwestern part of France (Fig. 1A), the Vilaine River is the largest river of Brittany. The river is 227 km long and its catchment drains one third of the area of Brittany (10,530 km2) (Carthage database, http://sandre.eaufrance.fr/). The bedrock of the Vilaine Bay consists of Hercynian magmatic and metamorphic rocks, Bartonian limestones and Tertiary alterites (Proust et al., 2001). The two N110-striking branches of the South Armorican Shear Zone (SASZ, trending N110°–N130°)
Chirp seismic reflection data
To characterise the bedrock morphology and the sediment fill architecture of the Vilaine River estuary (Fig. 3, Fig. 4), a seismic reflection survey (Proust2008-Vilaine) was undertaken onboard the RV Haliotis using a Chirp at a speed of approximately 5 knots. The Chirp was mounted on the hull of the Research Vessel, which runs with a draught of 0.8 m. The Chirp was operated with a 1.7–5.5 kHz pulse bandwidth, enabling a vertical resolution of up to 20 cm. The shot frequency was set at two pulses
Facies analysis
By combining new chirp data (“Proust2008-Vilaine”) with cross-cutting previous boomer data obtained during the “Bingolaine” seismic cruise in 2000 (Menier et al., 2002), we refine the morphology of the estuary valley floor (Fig. 3) and its sediment fill (Fig. 4).
Acoustic facies/units interpretation
Chirp facies correlate with the six units (U0–U5) defined by Menier et al. (2010) based on Boomer data, and interpreted in terms of depositional environments (Fig. 5).
Acoustic signal of Facies Fs0 displaying chaotic reflectors (and which intensity decreases with depth) is interpreted as corresponding to the Hercynian basement and represents the acoustic unit U0. The Ypresian formations described by Proust et al. (2001) at the top of the Hercynian magmatic and metamorphic rocks in the Bay of
Conclusion
The narrow and elongated morphology of the Vilaine River estuary, in addition to its particularly sheltered position for an estuary located on the Atlantic seaboard, has led to the accumulation of a sedimentary record reflecting the regional Holocene history. Indeed, on the contrary to the valleys of south-west England or to the south of Spain (Dabrio et al., 2000), which have recorded several phases of incision and sedimentation corresponding to the various Pleistocene sea-level changes, the
Acknowledgement
This work has been funded by the Conseil Géneral of Morbihan and the Région Bretagne. The authors would like to thank the captain and the crew (Arnaud Gillier and Renaud Cagna) of the “RV Haliotis” for the chirp data acquisition of this study. We would like to thank Philippe Martinez, Isabelle Billy and Joël Saint Paul (University of Bordeaux 1, France) for SCOPIX analyses. We also acknowledge the BRGM for kindly providing on-line descriptions of technical drillings, the Sandre for providing
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