Distinctive iceberg ploughmarks on the mid-Norwegian margin: Tidally influenced chains of pits with implications for iceberg drift

Abstract

Curvilinear depressions and chains of asymmetrical to circular pits up to a few meters deep and a few tens of meters wide have been mapped on the upper continental slope beyond the mid-Norwegian shelf. These features are interpreted to represent a continuum of seafloor landforms produced by the grounding of iceberg keels at successive low tides. The features occur along a 50 km long stretch of the upper slope in modern water depths between 400 and 500 m. The average distance between pits varies between about 70 and 175 m, representing one tidal cycle of 12 hr 25 min between successive low tides. This yields average iceberg drift velocities of between 5 and 15 m hr−1. The pits are often asymmetrical, with a downstream surcharge or berm, making it possible to determine the direction of iceberg drift and, therefore, the current direction at the time of ploughmark formation. The c. 300 m thick icebergs producing the ploughmarks are likely to have been sourced mainly from the fast-flowing Norwegian Channel Ice Stream to the south and probably formed during the early part of regional deglaciation of the Norwegian shelf after the Last Glacial Maximum around 20,000 years ago. The icebergs appear to follow a similar northeastward flow direction to that of the present Norwegian Atlantic Current offshore of Norway.

Keywords

• Iceberg ploughmark
• tidal
• ocean current
• iceberg drift velocity