Deformation of an early Preboreal deposit at Nykvarn (SE Sweden) as a result of the bulldozing effect of a grounding iceberg

Abstract

An early Preboreal glaciomarine deposit near Nykvarn (SE Sweden) consists, from bottom to top, of a submarine outwash fan, a full (glacio)marine deposit, and a near-coast marine unit of poorly sorted ice-rafted debris. The top part of the succession shows soft-sediment deformations that have the same characteristics as material moved forward by a bulldozer. These deformation structures are interpreted as being due to grounding of an eastward-moving iceberg. This caused detachment of the lower part of the succession, in the way that sand is pushed forward by a bulldozer. The iceberg most likely calved off the retreating ice front (which extended into sea) due to a sudden subglacial discharge of meltwater (jökulhlaup), which event seems related to the breakthrough of an ice dam that could no longer withstand the pressure of a growing water volume produced by increased ice melting.