Transgressive systems tract of a ria-type estuary: The Late Holocene Vilaine River drowned valley (France)
Résumé
The understanding of control parameters of estuarine-infilling at different scales in time and space is important for near-future projections and better management of these areas facing sea-level change. During the last 20 years, the stratigraphy of late Quaternary estuaries has been extensively investigated leading to the development of depositional models which describe the sedimentary facies distribution according to changes in sea level, tidal range and wave activity but few of them describe in detail the influence of the estuarine geomorphology. On the Atlantic coast, recent studies provide a detailed understanding of the sedimentary infill succession of incised valleys during the late Holocene transgression with recent emphasis on the macrotidal bay of the Vilaine River in South Brittany. The latter has shown the importance of the inherited morphology in controlling the preservation of the sedimentary deposits. However, until now, there has been little information available on the geometry of the underlying bedrock of the estuary and its immediate approaches including the Holocene history of the most proximal part of the Vilaine River estuary. The recent acquisition of very-high resolution seismic reflection (Chirp) and sediment core data in the Vilaine Estuary provides the opportunity to address several of these issues leading to the identification of the controlling factors that played a role in the sedimentary filling of the estuary. The estuary of the Vilaine River is one of the most sheltered estuaries of the Atlantic coast and thus shows an excellent record of the successive transgressive phases since the last glacial period. The valley of this ria-type estuary is incised in the Hercynian magmatic and metamorphic rocks of the Armorican Massif along the striking branches of the South Armorican Shear Zone and filled with well-stratified sediments. The study of the morphology of the substratum in the estuary reveals narrow paleovalleys (1 km wide) with a maximum depth of 20 m. The shapes and dimensions of these paleovalleys are comparable with those observed upstream in the present-day Vilaine River valley. Facies analysis of seismic and sedimentary data, complemented by 21 AMS radiocarbon age dating, enables the reconstruction of the infilling of the estuarine valley. The valley is filled by Pleistocene to early Holocene alluvial, coarse-grained, pebbly sands overlain by mid- to late Holocene marine sands, silts and muds. From 10,000 to 5000 cal. BP, the marine transgression shows a rapid initial phase of transgression followed by a progressive slowing down which can be recognised over more than 90 km in an upstream direction. The rapid transgression developed homogeneously over the entire length of the estuary with a widespread expression of tidal currents (tide-dominated estuary). The slow phase of the transgression gives rise to a differentiated expression of the hydrodynamic factors according to the geomorphological setting. The sedimentary deposits of the innermost and narrow part of the estuary reflect a strong control of tidal and river currents when the influence of tidal currents increases at the expense of fluvial control in a downstream direction. The sedimentary filling of the outer reaches of estuary shows a strong influence of tidal currents modulated by changes in sea level where, from 5700 to 3000 cal. BP, the progressive decrease in accommodation space promotes the expression of wave action (wave-dominated estuary). Finally, from 5000 years BP to present day, the stabilisation of sea level gives birth to scour and lag sedimentary morphologies (scour and lag estuary). The sedimentary record appears thus strongly marked by the Holocene transgression and the geomorphological setting where hydrodynamic parameters superimposed on the geomorphological and eustatic contexts expressed in different ways in time and space govern the evolution of the estuarine system.