Le systeme fluvio-estuarien Pleistocene moyen-superieur de Penestin (Morbihan); une paleo-Loire?

Abstract

Abstract In France, the basement domains are incised by large fluvial valley networks. Recent studies [Lefebvre et al., 1994; Bonnet, 1998; Lautridou et al., 1999; Antoine et al., 2000; Bonnet et al., 2000] show that these networks were cut during the Pleistocene in response to the uplift of western Europe combined with a fall of the base level [Haq et al., 1987; Shackleton, 1987]. The aim of this article is to study one of the rare Pleistocene sedimentary accumulations preserved in the Armorican Massif, the Penestin fluvial channel system in order to: (1) reconstitute the paleogeomorphological parameters and (2) discuss its relationships with recent tectonic movements. The Penestin fluvial system is localized at the top of the cliff of the Mine d'Or beach (west of Penestin, south of the Vilaine estuary, fig. 1). The paleochannel cuts through the Carboniferous micaschists which pass laterally into Tertiary paleoweathering deposits. The paleochannel is deformed by folds and/or by strike-slip faults. Three sectors are identified (fig. 2). Sector 1: a northern undeformed sub-area; sector 2: an intermediate slightly folded sub-area; sector 3 a southern folded and faulted sub-area. Three sedimentological units and seven associated facies are distinguished. Unit I (facies G), containing local matrix-dominated conglomerates characteristic of distal debris flows, suggests a proximal braided river environment, near the transition with alluvial fans [Blair and MacPherson, 1994]. Paleocurrent trends are directed toward N320 (fig. 3). Unit II (facies Sg, Sgm, Sf, Fb and T) is mainly dominated by sandy facies and corresponds to a more distal braided river flooded at the top by the sea (occurrence of inner estuarine deposits). The 2D and 3D megaripple cross-bedding structures indicate paleocurrents trending N320 (fig. 3). Unit III (facies G, Sg, Sgm and Fr) with multiple erosive channels is filled by braided river deposits (facies G, Sg and Sgm) or temporary lake sediments (facies Fr). The 2D and 3D megaripple cross-bedding structures indicate paleocurrents oriented N135 or N180 (fig. 3). Stratigraphically, these three units record two major base level cycles (fig. 6), in the sense of Wheeler [1958]. The first base level fall corresponds to the incision of the paleochannel. Unit I is amalgamated and was deposited during the beginning of the rise, while unit II corresponds to the maximal facies preservation during the maximum rate of rise. The erosion of unit II by unit III corresponds to a moderate base level fall followed by a small rise (little preservation of unit III). The basement, the weathering deposits and the Quaternary sedimentary units are folded and faulted by N050 to N070 strike-slip faults (fig. 2). Two stages of deformation can be characterized. The first stage, taking place before the incision of the paleochannel, corresponds to a southward tilting of the weathering profile. The second stage took place during the paleochannel infilling, producing thickness variations of the bedsets within unit II. The amount of deformation increased during deposition of unit III and is expressed by N050 to N070 strike-slip faults capped by unit III or by a topmost erosive surface. Fault orientation indicates a NNW-SSE compression (fig. 5) consistent with the stress field in northwestern Europe [Muller et al., 1992; Zoback, 1992]. Paleocurrent directions in units I and II seem to indicate a southerly drainage basin. Today, the Vilaine river flows southward. The units I and II do not represent a paleo-Vilaine river, but since unit III shows paleocurrents similar to those of the present Vilaine river and contains red schists in its basin. It could represent a paleo-Vilaine river. Units I and II might represent a paleo-Loire river, the present Loire river following a curve as it passes over the South Armorican shear zone which reorientates it toward the N320 (fig. 7). Tourenq and Pomerol [1995] show that the Loire river was captured many times ago by the Seine river during the Pleistocene. in this way, when the paleo-Loire river was captured, the paleo-Vilaine river deposited unit III. The Penestin paleochannel resulted from the fluviatile incision/filling of a braided river, which was temporarily overlain by estuarine deposits. The rivers filling this paleochannel first flowed out to the north (flow of a paleo-Loire river: units I and II), and then to the south (flow of a paleo-Vilaine river: unit III). The deformation is attributed to a N110 fault, to a set of N050 and N070 faults and to N050 folds.