CLUSTERED EROSIONAL SUBMARINE FURROWS AT BLAKE OUTER RIDGE ON THE US ATLANTIC MARGIN: IMPLICATIONS FOR SPATIAL VARIATIONS OF THE DEEP WESTERN BOUNDARY CURRENT VELOCITY

Based on multibeam bathymetric and seismic reflection data, morphology and depositional characteristics of the submarine furrows at the Blake Outer Ridge on the U.S. Atlantic margin were described in order to investigate the formation of the bedforms and estimate the spatial variations of the associated flow velocity. The furrows occur in clusters with an area of 83000 km(2) on the continental rise. Most of the furrows run parallel to isobaths at a water depth of 3000-5000 m and a few extend obliquely (similar to 30 degrees) to isobaths at a larger water depth. The furrows show an asymmetric V-shape on cross sections with a steeper wall on the upslope. They are 70-1100 m wide and 0.2-30 m deep. The furrow spacing varies between 145 m and 3045 m and ratios of furrow width to spacing are similar to 0.9 on average. As close to the furrowed field, the depositional layers thinned and truncations below the sea bed became more distinct. The furrows represent an erosional bedform, and the development of them has been governed by the behavior of the along-margin DWBC. During periods (e.g. glacial periods) when the DWBC is vigorous, the stronger bottom current eroded the seafloor by the secondary helical circulation in the bottom boundary layer and formed larger and more erosional furrows. During periods (e.g. interglacial periods) when the DWBC is weak, the weakened bottom current abraded the seafloor and formed smaller and less erosional furrows. It is inferred that the DWBC velocity gradually decreases along its path from 30 cm/s to 10 cm/s at the present. By contrast, the flow velocity had more than doubled through a bend and decreased by 1/3 across a large slump scar.