The Saharan debris flow: an insight into the mechanics of long runout submarine debris flows

New 3.5 kHz profiles and a series of piston cores from the north-west Afric an margin provide evidence that the Saharan debris flow travelled for more than 400 km on a highly fluid, low-friction layer of poorly sorted sediment . Data suggest that the Saharan debris flow is a two-phase event, consistin g of a basal, volcaniclastic debris flow phase overlain by a pelagic debris flow phase. Both phases were emplaced on the lower continental rise by a s ingle large debris flow at around 60 ka. The volcaniclastic flow left a thi n deposit less than 5 m thick. This contrasts with the much thicker (over 2 5 m) deposit left by the pelagic debris flow phase. We suggest that pelagic sediment, sourced and mobilized as debris flow from the African continenta l margin, loaded and destabilized volcaniclastic material in the vicinity o f the western Canaries. When subjected to this loading, the volcaniclastic material appears to have formed a highly fluid sandy debris flow, capable o f transporting with it the huge volumes of pelagic debris, and contributing to a runout distance extending over 400 km downslope of the Canary Islands on slopes that decrease to as little as 0.05 degrees. It is likely that th e pelagic debris formed a thick impermeable slab above the volcanic debris, thus maintaining high pore pressures generated by loading and giving rise to low apparent friction conditions. The distribution of the two debris pha ses indicates that the volcaniclastic debris flow stopped wit:hin a few ten s of kilometres after escaping from beneath the pelagic debris flow, probab ly because of dissipation of excess pore pressure when the seal of pelagic material was removed.