Guilhem Estournès (PhD)

Architectures and control factors of the drowned quaternary basins on the armorican inner shelf : the palaeo Etel Valley (southern Brittany) and the Ecrehou Basin examples

Abstract

The shortage of local aggragate ores (more particularily from alluvial origines) leads industrial companies to prospect for new potential ores including marine areas. The inner continental shelf (0-60 m) corresponds to the offshore extension of continental landscapes, reworked by eustatic, climatic and tectonic variations during the Quaternary (2. 6 Myrs-Today). The objective of this thesis is to characterize both the morphology and sedimentary filling of these basins, making the part of eustatic, climatic and tectonic influences on sediments onset. Two areas, chosen because of their good geological context representativeness, have been studied by seismic reflexion, surfacial coring and 3D topographic and sedimlentary reconstitutions. These two areas are 1) the Palaeo Valley of the Etel River (Southern Brittany) and 2) the Ecrehou Basin in the Normand-Breton Gulf. This study consists in an exploration of poorly known areas, carrying interests for both academic research (continental margins knowledge) and industrial sector (new potential material ores). It appaears that Atlantic inner shelf morphology corresponds to the remnant of fluvial topographies (Mid Pleistocene) several times reused since their onset and showing a poor sediments preservation capacity. The Etel Palaeo valley is only filled with transgressive tidal to marine deposits dated between 9 Kyrs BP to present. The onset of these valleys seems to respond to the evolution of quaternary climate dynamic (Mid Pleistocene Transition) and high frenquency eustatic variations since this period. In the Normand-Breton Gulf, the basin sedimentary filling presents a far better preservation capacity (preserving Early and Late Pleistocene deposits) associated to more active tectonic pattern than the Atlantique margin. This filling is dominated by tidal environnements. Basically, at the scale of the whole Armorican Massif, the tectonic forcing seems to modulate the effect of eustatic variations on sediment preservation. This latter is in any case maximum during transgressions and minimum or negative during sea lowstand periodes.

Supervisors
David Menier

Keywords
HR sparker seismic system
Sedimentary basins
Stratigraphy
Quaternary (geological era)
Armorican massif
Etel river
France