Geological/geophysical observations and inferred bottom-current flow: South flank Iceland—Faeroe Ridge

Abstract

The surface morphology of the Iceland—Faeroe Ridge and Hatton Drift, as shown by seismic reflection (3.5 kHz) and bottom photographic data, is predominantly shaped by vigorous bottom currents but also influenced, in the case of the ridge, by a generally smooth acoustic basement of probable basaltic composition. Redistribution by the currents has sculpted the sediment on the south flank of the ridge into lens and wedge-like drift deposits having remarkably smooth surfaces and uniform internal stratification. In other areas, notably over large areas of the ridge crest, acoustic basement appears to be exposed. Estimated current velocities range from a few centimeters per second on off-ridge areas up to and perhaps greater than 30 cm s −1 on the ridge itself. Three distinct current flows can be differentiated: an intense mid-flank flow representing the outflow of Norwegian Sea Deep Water from the Faeroe Channel; a moderately strong flow on the upper flank produced by water passing over the ridge crest; and a weak to moderate flow on the lower flank which is a continuation of the north-flowing Hatton Drift current. Discontinuous, channel-like features on the eastern part of the ridge flank are attributed to the erosive action of the currents as they accelerate around topographic highs. In contrast, the extreme western end of the ridge and Icelandic slope is cut by several turbidity current channels which transport basaltic debris from Iceland to the Maury Fan. Turbidite deposits account for the characteristic prolonged echo type exhibited in the 3.5-kHz profiles across the upper fan; the south flank and Hatton Drift contourites produce sharp bottom echoes with continuous, parallel subbottom reflectors, whereas sharp bottom echoes with no subbottom reflectors typify the ridge crest and outcrop areas. Sediment thicknesses on the ridge range from virtually zero on the crest to usually 0.3 s on the lower flank. Near Iceland and over the Maury Fan the sediment accumulation may exceed 1.0 s.