Pliocene-Holocene contourite deposition under theAntarctic Circumpolar Current, Western Falkland Trough,South Atlantic Ocean

Abstract

The eastward-flowing Antarctic Circumpolar Current (ACC) has influenced sedimentation on the slope and floorof the western Falkland Trough, where the axis of the current is topographically constrained. Deep-water flow (below3000 m) has produced a symmetrical sediment drift on the trough floor, with non-depositional margins indicatinghigher current velocities at the base of slope. To the southeast of the Falkland Islands there is a gap in the NorthScotia Ridge, north of which the floor of the trough is swept clean of sediment by the ACC. Both echo charactermapping and GLORIA side-scan data indicate that currents follow the bathymetric contours along the slope,redistributing sediment and locally eroding furrows. From six cores on the drift and on the northern slope, two stylesof contourite deposition have been identified. On the drift, Holocene biogenic sandy contourites overlie Last GlacialMaximum muddy contourites and fine-grained diatomaceous hemipelagites. Sedimentation rates here average3-4 cm ka-‘. The sandy contourites present in four of the cores from the sediment drift are sharply underlain by thefiner-grained, diatomaceous hemipelagites. The lack of a coarsening upward sequence, commonly associated with anincrease in current velocity may be indicative of high current activity eroding away the finer (negative) sequence.Pliocene and Mid-Pleistocene glaucony-rich sandy contourites containing radiolaria characterise the Falkland Plateauand the floor of the trough near the gap in the North Scotia Ridge. We suggest that the glaucony is derived froma combination of authigenic formation and erosion of locally outcropping Cretaceous and Tertiary strata;this is supported by dinoflagellate analysis. Sedimentation rates in these current-swept areas average < 1 cm ka-‘.