Étude sédimentologique du Mor Bras, Bretagne

Abstract

A sedimentological study of Mor Bras, Brittany (France) The present contribution belongs to a submarine research project in progress on the nearshore continental shelf of the Bay of Biscay between Quiberon Peninsula and Les Sables d'Olonne, and describes the sedimentary features of Mor Bras, which were studied by dredgings and echo-soundings (Fig.1). Mor Bras is a submarine basin, sheltered by a rocky shoal running from Quiberon to Hoëdic Island. This isolation explains the seasonal diminution of salinity consecutive to fluviatile floods, and is the chief cause of the formation of a calm water mass surrounded by strong tidal currents (Fig.2). The major characteristics and the distribution of sediments are shown on sedimentological sections with figuration of pelite-sand-pebbles ratio variations, bathymetry, calcium carbonate and organic carbon contents (Fig.4), on a map giving the calcium carbonate contents (Fig.5), and on a general map established according to new principles (Fig.6), in which the figuration of the sedimentary supply is detailed. The rocky margins, Pleistocene pebbles, angular stones and gravels are the basic representation; the sandy and pelitic contents are respectively mapped by isopleths and flat tints. Special conventional symbols can express the sorting of sediments (Trask Sorting Index, for instance) or the mineralogic and biologic composition of a few sands. The distribution of sediments is regular, displaying a pattern of concentric areas in which the median diameter becomes finer and finer from the rims to the central part of the basin: this is classic in numerous land-locked seas and bays. Mor Bras includes two kinds of sedimentary environments, the geographical distribution of which is controlled by direction and strength of tidal currents (Fig.7): 1. ( 1) The detrital rim, devoid of muddy sediments (pelitic ration below 10%) due to the relative velocity of currents, where the skeletal fraction plays the predominant part. In this belt (mostly around the Bay of Quiberon), the following areas are described and analysed: ( a) the rocky hard bottom, which is sometimes exposed, and in other places covered by Laminaria or by migrating coarse calcarenites; ( b) the “coralligenous” areas colonized by photophile calcareous Algae, the Lithothamnion or “maërl”; ( c) the infra-littoral beaches composed of fine sand off the tomboles of Quiberon and Le Croisic; ( d) and the main part of the sandy and calcarenite regions, moulded by tidal currents running across the Strait of La Teignouse, such as the Bank of Quiberon, characterised by a complex pattern of sandy ridges (Fig.9, 10). 2. ( 2) In the inner part of the basin, where tidal currents are much weaker (below 1 knot), the finer particles can settle down: the mud deposition shows very different aspects depending on the local environments (Fig.8). In the Bay of Quiberon off the Vilaine estuary (Fig.11) the silt particles (median diameter between 1–10 μ) are restricted to some depressions separated by enbankments of muddy sand: depressions off the entrance of the Gulf of Morbihan and on both sides of the submarine delta formed by deposition of the Vilaine solid discharge. In the Bay of Vilaine, mud with a small pelitic ratio is predominant (Fig.12). The fine part of this mud consists chiefly of particles of silt size and its organic matter content is intermediate between the percentages in the mud on the shelf and those in the tidal marshes. The source of pelitic supply is continental with regard to the Vilaine river and perhaps to the Loire river. More definite conclusions perhaps can be reached by chemical analyses of the sediments. Mor Bras is a sediment trap and an environment mostly controlled by regional or local geographical conditions.