Sediment failures in northern Flemish Pass

Flemish Pass is a north-south trending, mid-slope basin located between Grand Bank and Flemish Cap on the eastern Canadian continental margin. It is an area of active hydrocarbon exploration, with recent significant discoveries at three wells in northern Flemish Pass. A sediment slide complex on the northwest flank of Flemish Cap was mapped by multibeam bathymetry in 2009 and further investigated in 2011 using high resolution seismic reflection systems and piston coring, in order to assess the origins of the slide complex and its geohazard implications for hydrocarbon development in the Mizzen field. This multiple failure complex extends about 65 km along the northeast flank of Flemish Pass and about 20 km downslope with three large arcuate slide scars found at its centre. These arcuate failures are up to 10 km wide and 9 km long downslope, with scarps over 70 m high. Failed sediments have run out as far as 20 km onto the floor of Flemish Pass, forming mass transport deposits (MTDs) typically 50 m thick. Piston cores from the slide complex have sampled the upper part of the MTDs and the overlying drape of undisturbed sediment. The stratigraphy of the undisturbed sediment is correlated with previously reported reference stratigraphic cores elsewhere in northern Flemish Pass and southern Orphan Basin, with the correlation confirmed by new radiocarbon dating. Geotechnical measurements including Atterberg limits, consolidation testing and triaxial shear have been made on the cores. Blocks in the MTDs imply previous burial and consolidation to at least 48 m. Factor of Safety analysis indicates that the sediments are statically stable with the critical slope angle ranging from 3.7 to 13.2°. There is a potential for liquefaction given the high silt content and relatively high water content compared to the liquid limit and lean nature of the sediments. The general style of failure appears to be retrogressive rotational slumping leading to break-up of sediment into a complex slide. The oldest failure involved the upper 30 m of sediment and is dated at 27 ka. Two subsequent failures are identified on the basis of multibeam morphology, but both appear to be of the same age to within the resolution of core stratigraphy at ~20.5 ka and they may be parts of a single failure event. In two cores, a thin blocky MTD involving only very shallow sediment is recognised and dated at 13 ka. A few tens of kilometres to the west, another major sediment failure dated at ~7 ka has been previously reported in southern Orphan Basin. In all these areas, migration of fluids from deep in Orphan Basin along the Cumberland Belt Fault Zone has probably preconditioned the region for failure. The state of excess pore pressure in the region is not known and it is unclear how much pore pressure may have been drained by the observed failures. The widespread occurrence of failures of the same age suggests earthquake triggering of failure by rare passive-margin earthquakes. Geological data reported here from Flemish Pass and previously reported data from Orphan Basin suggest a recurrence interval of approximately 10 ka for large earthquakes capable of triggering landslides in northern Flemish Pass.

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