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

Abstract

New 3·5 kHz profiles and a series of piston cores from the north‐west African 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, consisting 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 single large debris flow at around 60 ka. The volcaniclastic flow left a thin deposit less than 5 m thick. This contrasts with the much thicker (over 25 m) deposit left by the pelagic debris flow phase. We suggest that pelagic sediment, sourced and mobilized as debris flow from the African continental margin, loaded and destabilized volcaniclastic material in the vicinity of the western Canaries. When subjected to this loading, the volcaniclastic material appears to have formed a highly fluid sandy debris flow, capable of 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°. It is likely that the 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 phases indicates that the volcaniclastic debris flow stopped within a few tens of kilometres after escaping from beneath the pelagic debris flow, probably because of dissipation of excess pore pressure when the seal of pelagic material was removed.

Occurrence and origin of submarine plunge pools at the base of the US continental slope

“…This latter constraint is justi¢ed by reference to small-scale £ume experiments and may not hold for all submarine debris £ows. For example, Gee et al (1999) document signi¢cant proximal erosion by the extensive Saharan debris £ow o¡shore from West Africa. Deposition by debris £ows within Pratson et al's model can create a depression at the slope break.…”

La erupción submarina de La Restinga en la isla de El Hierro, Canarias: Octubre 2011-Marzo 2012

“…1B), que parece pertenecer a un edificio volcánico más antiguo, al igual que el monte submarino Henry, situado al SE, ambos cubiertos por varios metros de sedimentos (Mitchell, 1993). En campañas oceanográficas en zonas próximas (<3 km de distancia) a la localización de la actual erupción de 2011-12, se dragaron numerosas muestras de lavas (picritas y basaltos alcalinos) e hialoclastitas con escaso grado de alteración, lo que evidencian una edad muy reciente (Mitchell, 1993;Urgeles et al, 1997Urgeles et al, , 1998Schmincke et al, 1998;Gee et al, 1999;Ye et al, 1999;Schmincke & Graf, 2000;Krastel & Schmincke, 2002;Masson et al, 2002).…”

The sedimentary record of the last glaciation in the western Bellingshausen Sea (West Antarctica): Implications for the interpretation of diamictons in a polar-marine setting

“…Elverhbi et al, 1997;Laberg and Vorren, 2000;Nygård et al, 2002). Debris flows can even occur on slopes with gradients as low as 0.058 (Schwab et al, 1996;Gee et al, 1999), but the suggested flow mechanisms on such gentle slopes require very special pre-conditions, such as loading and mobilization of basal volcaniclastic sands.…”