Submarine 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 world's 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 degrees to 0.01 degrees. 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 degrees to 0.05 degrees) 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.

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Citation for published item:etersD tFvF nd fenettiD F nd hunlopD F nd ¡ y gofighD gF nd woretonD FqF nd heelerD eFtF nd glrkD gFhF @PHITA 9edimentology nd hronology of the dvne nd retret of the lst fritishEsrish se heet on the ontinentl shelf west of srelndF9D uternry siene reviewsFD IRH F ppF IHIEIPRF Further information on publisher's website: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. Abstract 12The last British-Irish Ice Sheet (BIIS) had extensive marine-terminating margins and was 13 drained by multiple large ice streams and is thus a useful analogue for marine-based areas of modern 14 ice sheets. However, despite recent advances from investigating the offshore record of the BIIS, the 15 dynamic history of its marine margins, which would have been sensitive to external forcing(s), remain 16 inadequately understood. This study is the first reconstruction of the retreat dynamics and chronology 17 of the western, marine-terminating, margin of the last (Late Midlandian) BIIS. Analyses of shelf 18 geomorphology and core sedimentology and chronology enable a reconstruction of the Late 19Midlandian history of the BIIS west of Ireland, from initial advance to final retreat onshore. Five 20 AMS radiocarbon dates from marine cores constrain the timing of retreat and associated readvances 21 during deglaciation. The BIIS advanced without streaming or surging, depositing a bed of highly 22 consolidated subglacial traction till, and reached to within ~20 km of the shelf break by ~24,000 Cal 23 BP. Ice margin retreat was likely preceded by thinning, grounding zone retreat and ice shelf 24 does not allow us to determine conclusively whether these readvances were a glaciodynamic 38 (internally-driven) response of the ice sheet during deglaciation or were climatically-driven. 39Following the <18,500 Cal BP readvance, the Galway Lobe experienced accelerated eastward retreat 40 at an estimated rate of ~113 m/yr. 41

Multibeam swath bathymetry data collected through the Irish National Seabed Survey provides direct evidence for extensive glaciation of the continental shelf off NW Ireland. Streamlined subglacial bedforms on the inner shelf record former offshore-directed ice flow. The major glacial features, however, consist of well developed, nested arcuate moraines which mark the retreat of a former ice sheet margin(s) across the shelf. Distal to these moraines on the outermost shelf prominent zones of iceberg-ploughmarks give way into a well developed system of gullies and canyons which incise the continental slope. The large-scale, nested, arcuate moraines record the episodic retreat, probably punctuated by minor readvances or oscillations, of a grounded ice sheet lobe across this sector of the continental shelf during regional deglaciation. Initial retreat from the outer shelf was associated with an episode of ice sheet break-up and calving as recorded by extensive zones of iceberg ploughmarks distal to the outermost moraine. It is conceivable that this could have been driven by rising sea level. The data indicate a major reorganisation of the Irish Ice Sheet on the NW shelf during deglaciation; an initial elongate ice sheet configuration extending along the shelf edge changed to a pronounced lobate form during retreat. Consideration of dated, marine stratigraphic records from the wider NW margin suggests that ice sheet advance to the shelf edge likely occurred at about 29-27 ka BP, but that retreat from this shelf edge position did not take place until after 24 cal ka BP. Large-scale contrasts in continental margin morphology west of Ireland, from trough mouth fans in the north to gully/canyon systems further to south, reflects a combination of factors including spatial variations in sediment flux related to palaeo-glaciology.