Abstract

Soft-sediment striated surfaces, commonly associated with massive diamictites, have been described from many pre-Quaternary glacial sedimentary sequences around the world. A number of authors interpret these associations as evidence for a subglacial origin. However, soft-sediment striated surfaces are formed by the mechanical scouring action of seabed- (or lakebed-) touching free-floating ice masses over large areas of modern high latitude continental shelves, and in glacial lakes and in lakes subject to seasonal freeze-up. Prolonged reworking of seabed and lakebed sediments by scouring ice keels can cause homogenization of preexisting stratification to form massive diamictons that may be described as iceberg turbates, sea-ice turbates or lake-ice turbates. Evidence from previous work on soft-sediment striated surfaces by other authors is presented and reviewed in the context of possible origin by scouring ice keels.

In the modern East Greenland glacimarine environment massive diamicton has been deposited during the Holocene by the combined action of iceberg-rafting and suspension settling from subglacial and seasonal terrestrial meltwater runoff. During deposition continuous scouring by deep-drafted icebergs (up to 550 m) has resulted in mechanical mixing of the diamicton. The present surface of this iceberg turbate is characterized morphologically by the typical hummocky topography of criss-crossing iceberg scour marks. We show that Quaternary ice keel scour processes have affected an order of magnitude more of the seafloor than have subglacial processes, and suggest that this order of magnitude difference probably can be applied to pre-Quaternary glacial marine and lacustrine environments. Consequently the effects of ice keel scour should be more frequently preserved than features of ice shelf grounding zone processes. Field work directed at reappraising pre-Quaternary soft-sediment striated surfaces associated with diamictites now may be appropriate.