Geomorphic signature of the presence and breakup of large ice-sheet derived multi-keeled tabular icebergs

Abstract

Multi-keeled iceberg ploughmarks have been mapped along 70 km of the mid-Norwegian margin from high-resolution multibeam bathymetric data. A distinctive submarine terrace was observed seaward of a 2 km-wide series of subparallel ploughmarks. The flat and smooth nature of the terrace leads to its interpretation as an ‘iceberg-planed terrace’ formed when a drifting tabular iceberg crossed and planed off the surface of a debris-flow lobe protruding some 10–20 m above the surrounding seafloor. The entire tabular iceberg was probably at least 4 km across. Very large, irregular ploughmarks and a major pit in slightly deeper water northwest of the planed terrace are probably linked to tabular iceberg breakup. More generally, sets of multi-keeled iceberg ploughmarks spanning kilometres in width occurring in relatively deep waters are diagnostic of the former presence of large tabular icebergs and imply a palaeo-glaciological setting in which fast-flowing ice streams draining large ice sheets were active and may also indicate the presence of floating ice shelves or tongues. In addition, the juxtaposition of planed submarine terraces with large chaotic ploughmarks and isolated deep pits provides a suite of landforms representing the geomorphic signature of the fragmentation of a large tabular iceberg after a major grounding event.