A detailed sedimentological and geochemical investigation was performed on slope sediments collected from the Gulf of Lions and Catalonian margins and from the northern Balearic margin. The study was based on cores, about 50<space>cm long. Various physiographic sites (canyons, open slopes) were selected according to the available sediment accumulation rates obtained with the 2 1 0 Pb geochronological method. At a short time scale, various parts of the slope were characterized by muddy hemipelagic sediment. Sedimentological data suggested that deposition was continuous. Total organic carbon and its hydrolysable fraction decreased downcore. Significant increase of C/N ratio downcore resulted from an early diagenetic process in response to the burial effect. However in other parts, the values of the C/N ratio were found to be irregular and maxima could frequently be related to both higher sand content and relict coastal remains derived from erosion and reworking of outcropping deposits of the last glacial maximum (lowstand system tract). Such tracts crop out over much of the shelf slope break seafloor. They provide various biological and lithological indicators: relatively well-preserved Miliolidea, coastal or lagoonal foraminifera (Ammonia beccarii, Elphidium crispum), Characea oogons, oxidized beach-rock debris, gypsum, previously mature glauconite (with cracks) and coarse vegetal debris. Throughout the sections, the degree of diagenetic overprinting of the organic matter frequently showed a positive correlation with the abundance of some coastal indicators supporting the hypothesis of sedimentation induced predominantly by gravity These downslope accumulation may correspond to 10-40<space>% of the total deposit. The gravity induced transfers have been shown to be negligible in the sediment record of the northern Balearic margin. It is suggested that a reduced sedimentation rate (half that observed in the Gulf of Lions) would be expressed by a higher stability of the slope. The consequence of low sedimentation would be early diagenetic solution/reprecipitation of carbonate near the redox transition layer.
Une etude sedimentologique et geochimique detaillee des depots de pente du golfe du Lion, de la Catalogne et du Nord des Baleares a partir de carottes courtes ne depassant pas 50<space>cm a ete effectuee. Les sites presentes (canyons, interfluves) ont ete choisis en fonction de l'existence de taux de sedimentation mesures a l'aide de la methode au 2 1 0 Pb. A cette echelle de temps reduite, plusieurs regions de la pente sont caracterisees par une accumulation de vases hemipelagiques. Les observations sedimentologiques suggerent un processus continu de la sedimentation. Le carbone organique total ainsi que sa fraction hydrolysable tendent a diminuer du haut vers le bas de la carotte<space>; a l'inverse une nette augmentation du rapport C/N est observee. Cette augmentation traduit les effets d'une diagenese precoce consequente de l'enfouissement. Cependant, dans certains sites, le profil vertical du C/N est irregulier et les maxima peuvent etre souvent correles a la fois avec des teneurs en sable plus elevees et avec l'abondance de debris issus de l'erosion et du remaniement des depots littoraux du dernier bas niveau glaciaire. Ces derniers qui tendent a affleurer a proximite de la rupture de pente entre le plateau et la pente sont reconnus grace a plusieurs marqueurs littoraux<space>: Miliolidea assez bien preserves, foraminiferes cotiers ou lagunaires (Ammonia beccarrii, Elphidium crispum), oogones de Characees, fragments oxydes de gres de plage, gypse, glauconie evoluee et debris grossiers de vegetaux. La frequente association de ces indicateurs cotiers avec le degre avance de l'evolution diagenetique correspond a des episodes ou l'accumulation gravitaire joue un role important pouvant representer de 10 a 40<space>% du sediment total. Sur la marge septentrionale des Baleares, ces processus gravitaires paraissent avoir joue un role negligeable. Il est suggere que la vitesse de sedimentation reduite correspond a une plus grande stabilite de la pente. En consequence de cette lenteur de la sedimentation, les carbonates sont l'objet d'une diagenese precoce avec des processus de dissolution et de reprecipitation a proximite de la limite d'oxydoreduction.