LATE QUATERNARY SEDIMENTATION PATTERNS OF THE EBRO TURBIDITE SYSTEMS (NORTHWESTERN MEDITERRANEAN) - 2 STYLES OF DEEP-SEA DEPOSITION

Seven sediment types are identified from the analysis of 60 piston cores in the Ebro turbidite systems (1200-1800 m deep) between the Ebro shelf and the Valencia Trough in the northwestern Mediterranean Sea. These sediments are grouped into five facies associations: channel floor of erosive-depositional channel-levee complex, levee of erosive-depositional channel-levee complex, depositional channel-levee complex, interchannel, and apron facies. Unconfined slides, debris flows, channeled turbidity currents and differential hemipelagic settling are the main depositional processes which develop these facies. Deposition in the Ebro turbidite systems occurred primarily during the Pleistocene when turbidity currents funneled coarse-grained sediments through canyon/channel systems. During the Holocene, most sediment was trapped in deltaic and nearshore environments and turbidite deposition was replaced by hemipelagic settling. In the hemipelagic sequence thus created, grain size, terrigenous components, and chlorite/kaolinite and illite/kaolinite ratios decrease upwards while planktonic and calcite/quartz ratios increase. Two styles of growth patterns are present during periods of low sea level. Most common are base-of-slope channel-levee complexes, each of which is developed alongside an older complex. The complexes were formed by sediment supplied from different canyons as the systems migrated from north to south with time. In addition, in the central sector, base-of-slope aprons have been formed in gullied slope terrains by unconfined slides, and less commonly by unconfined turbidity currents. The major differences between the Ebro and other base-of-slope systems are: (a) the incomplete development of channel-levee complexes as compared to typical deep-sea fans, (b) the coexisting growth of aprons, and (c) multiple canyon sources. Low slope stability, resulting in many sediment supply point sources, is one of the primary factors which encourages development of base-of-slope aprons.