Evolution of deep-water climbing dunes in the Rockall Trough — Implications for overflow currents across the Wyville-Thomson Ridge in the (?)late Miocene

Abstract

A (?)late Miocene dune field covering an area of 350 km 2 has been identified at and below the sea bed in the northern part of the Rockall Trough, in 1080–1180 m of water. Seismic reflection records across the area reveal details of the internal bedform arrangement of the dunes. A three-fold subdivision of the dunes can be made, into basal climbing dunes, an intermediate transitional phase and an upper sinusoidal package. These three subdivisions are similar to the phases observed elsewhere in small-scale, sand-ripple drift systems evolving under conditions of decreasing energy. The basal climbing dunes show evidence of southwards migration, along the axis of the Rockall Trough. They were probably deposited primarily from traction currents formed in dense, cold, bottom water overflow moving from the Norwegian Sea, over the Wyville-Thomson Ridge and into the North Atlantic during the late Miocene. As this current energy decreased due to the increasing influence of the Wyville-Thomson Ridge as a barrier to overflow, the depositional style of the dune system changed. The decrease in current strength was accompanied by a decrease in migration rate and an increase in suspension load deposition, with a corresponding change in dune geometry. When the current strength decreased further as the ridge became a more permanent barrier to overflow, the dune system evolved into a sinusoidal phase, with deposition dominated by the fallout of suspension load material. The dune system is erosionally overlain in places by a younger, undated deposit that may represent a slump sheet derived from the eastern side of the Rockall Trough.