Massive sand beds attributed to deposition by dense water cascades in the Bourcart canyon head, Gulf of Lions (northwestern Mediterranean Sea)

Abstract

International audienceNowadays, the Gulf of Lions continental shelf and slope are under the influence of dense water cascading, wind-induced bottom currents and the geostrophic Northern Current. In order to characterize sedimentary activity at the shelf break, several interface and piston cores were taken in the Bourcart canyon head and a current meter equipped with temperature, conductivity, pressure and turbidity sensors was moored during the 2003–2004 winter season. Even if the canyon is not connected directly to continental sources since Last Glacial Maximum, detailed grain size, X-ray and sediment facies analysis of interface cores show that down to 350 m water depth, the canyon head is blanketed by up to 1.5 m of structureless muddy medium-grained sand. 210Pbexc activity measurements demonstrate present day sedimentary activity of the canyon head. Time series of currents with peak velocity reaching 0.37 m/s, suspended sediment concentration and temperature indicate that dense water cascading is the main process allowing the reworking, transport and accumulation of sand within the canyon head, even though winter 2003–2004 was characterized by low cascading events compared to other years. These sand beds, called “cascadite” constitute a new type of deposit that differs from other typical slope deposits (turbidites, hyperpycnites, contourites) in terms of flow duration and sedimentological characteristics. They are similar to shallow water contourites. They might be time-equivalent to the recent turbidites described throughout the deep western Gulf of Lions basin

Transient erosion in the Valencia Trough turbidite systems, NW Mediterranean Basin

“…extends more than 150 km downdip on the Algero-Balearic abyssal plain east of the Minorca Island, also records the activity of the Valencia Channel in fresh bedforms and erosional features as well as layers containing pteropod shells of Holocene age (Morris et al, 1998). At this stage the extent to which these fresh bedforms and erosional features are attributable to repeated turbidity currents and not the frequent highstand cascades of dense shelf water is unknown Gaudin et al, 2006). Direct evidence of long-term erosion in the Valencia Channel was observed during Deep Sea Drilling Project Leg 13 site 122 (Ryan et al, 1973), located very close to the middle-course of the current thalweg ( Figs.…”

Post-glacial persistence of turbiditic activity within the Rhône deep-sea turbidite system (Gulf of Lions, Western Mediterranean): Linking the outer shelf and the basin sedimentary records

“…The periodicity of deposition of silt-sand beds, about 500 yr from bed #12 to #8 and about 2000 yr from bed #7 to #1 is beyond the present seasonal variability recorded for sediment transport on the shelf and in the canyons and beyond the pluri-annual variability of the major cascading of dense water that exports large amounts of sediment, including sand, into the canyons Gaudin et al, 2006). This shows that canyons act as a very efficient sediment trap that undergo periodical flushing at centennial and millennial scales.…”

“…Recent investigations have demonstrated that present hydrodynamic processes maintain a mobile reworked carpet, up to few meters thick, on top of these deposits . Transfer into the canyon heads, related to episodic dense water cascading, is currently observed (Gaudin et al, 2006;Palanques et al, 2006). Several canyons, Marti canyon (MC), Herault canyon 2 (HC2), HC3, Bourcart canyon 2 (BC2), Lacaze-Duthier Canyon (LDC) and Cap de Creus Canyon (CCC) (Baztan et al, 2005) and Petit-Rhône canyon (PRC) (Torres et al, 1997) display a marked axial incision that is interpreted as the imprint of erosive turbidity current initiated at the canyon head when it was connected to a river during the last sea-level low stand (Baztan et al, 2005).…”

“…A clay layer is present on top of the silt-sand layer in three beds only, MD99-2345 100-121 cm (80% of thickness), MD99-2345 146-198 cm (50%) and KIGC-07 40-45 cm (75%, bioturbated). The biogenic fraction consists of shell debris, planktonic and benthic foraminifera among which Ammonia and Elphidium are genus indicative of shallow water and nearshore environment (Gaudin et al, 2006). The composition of the silt-sand layers is controlled by grain size.…”

“…4. (Gaudin et al, 2006) and the CCC (Degeest, 2005). Winter dense water cascading events, associated with enhanced bottom sediment transport (velocities up to 80 cm s − 1 ) during eastern storms , provide the energy required to transfer sand from the outer shelf into the canyon heads.…”