02425nas a2200301   4500000000100000000000100001008004100002260002800043100001700071700001200088700001600100700001300116700001400129700001600143700002000159700002500179700001500204700001800219700002100237700001800258700001100276245008000287856004200367300001200409490000800421520168000429020001402109       2007                            d  bNature Publishing Group1 aP.J. Talling1 aR. Wynn1 aD.G. Masson1 aM. Frenz1 aBT Cronin1 aR. Schiebel1 aA. Akhmetzhanov1 aS. Dallmeier-Tiessen1 aS. Benetti1 aP.P.E. Weaver1 aA. Georgiopoulou1 aC. Zuhlsdorff1 aL. Amy00aOnset of submarine debris flow deposition far from original giant landslide  uhttp://dx.doi.org/10.1038/nature06313  a541-5440 v4503 aSubmarine landslides can generate sediment-laden flows whose scale is impressive. Individual flow deposits have been mapped that extend for 1,500 km offshore from northwest Africa. These are the longest run-out sediment density flow deposits yet documented on Earth. This contribution analyses one of these deposits, which contains ten times the mass of sediment transported annually by all of the worlds rivers. Understanding how this type of submarine flow evolves is a significant problem, because they are extremely difficult to monitor directly. Previous work has shown how progressive disintegration of landslide blocks can generate debris flow, the deposit of which extends downslope from the original landslide. We provide evidence that submarine flows can produce giant debris flow deposits that start several hundred kilometres from the original landslide, encased within deposits of a more dilute flow type called turbidity current. Very little sediment was deposited across the intervening large expanse of sea floor, where the flow was locally very erosive. Sediment deposition was finally triggered by a remarkably small but abrupt decrease in sea-floor gradient from 0.05\textdegree to 0.01\textdegree. This debris flow was probably generated by flow transformation from the decelerating turbidity current. The alternative is that non-channelized debris flow left almost no trace of its passage across one hundred kilometres of flat (0.2\textdegree to 0.05\textdegree) sea floor. Our work shows that initially well-mixed and highly erosive submarine flows can produce extensive debris flow deposits beyond subtle slope breaks located far out in the deep ocean.  a0028-0836