SOURCE AREA, TRIGGER MECHANISMS AND TIMING OF LARGE-SCALE TURBIDITY CURRENTS ON THE NORTHWEST AFRICAN CONTINENTAL MARGIN

The Moroccan Turbidite System, on the Northwest African continental margin, is supplied by large-scale, infrequent turbidity currents. Provenance studies have revealed that these flows are largely derived from the Morocco Shelf and volcanic Canary Islands, and can transport sediments up to 1500 km offshore. Sidescan sonar images indicate that the largest flows excavate giant erosional scours several kilometres across as they exit canyons and channels. Further offshore, sediment cores show that individual turbidites are deposited in three interlinked depositional basins, and can be correlated over distances of up to 1200 km. The multiple source areas, and varying flow volumes, lead to the development of a complex depositional architecture. Individual large-scale flows may deposit a sand sheet in one basin, and transport the majority of the mud fraction further downslope before depositing it in a different basin. Smaller flows tend to deposit both their sand and mud load in the most proximal basin.

Microfossil-based dating indicates that individual turbidity currents can be linked to rapid changes of sea-level at oxygen isotope stage boundaries. However, there is no clear link with either sea-level lowstands or highstands. In addition, the three largest turbidites were deposited during the largest fluctuations in sea-level, suggesting there may be a link between the volume of sediment input and the magnitude of sea-level change. The actual trigger mechanisms for each event are probably different. On the Morocco Shelf, earthquakes and/or instability of rapidly accumulating sediments during late glacial periods are believed to be the main factors. On the slopes of the Canary Islands, turbidity currents are associated with debris avalanches and debris flows, and are linked to large-scale island flank failures.