A fluctuating ice front over an esker near Ryssjön (S Sweden) as a cause of a giant load cast

Abstract

A well-exposed section in an esker near Ryssjön (S Sweden), strongly affected by several phases of glacitectonism, showed two phenomena that are uncommon and that seem to have a causal relationship. The first phenomenon is the occurrence of a load cast consisting predominantly of gravel in deposits formed just inside or outside a tunnel mouth. The load cast is over 3m high and 8m wide, a size that has previously not been described from unconsolidated sediments. Two nearby, somewhat smaller load casts consisting mainly of sand also have dimensions of over 2m, indicating that the formation of huge load casts was apparently a feature related to the special conditions under which loading could develop at this site.The second remarkable feature is that not only is a till present in the succession above the load cast, thus indicating re-advance of the ice, but deposits overlying this till are also deformed by glacitectonism, proving that at least a second phase of ice re-advance must have occurred. This situation has not previously been documented. It is concluded that the varying differential overburden of sediment and particularly the retreating and re-advancing ice in this area were responsible for the formation of the extremely large load casts, possibly triggered by seismic shocks related to isostatic adjustment caused by oscillations of the ice front.