Large dead-ice depressions in flat-topped eskers: evidence of a Preboreal jökulhlaup in the Stockholm area, Sweden

Sedimentological investigations in Pålamalm, one of the few elongated, flat-topped, raised glaciofluvial deposits of the Stockholm area, show that the deposit was formed in a subglacial tunnel environment during the early Preboreal. The study provides evidence for dynamic links between the morphology of a subglacial conduit, the regional subglacial discharge, and the regional ice-sheet dynamics. The general morphology of the deposit and the lateral esker displacement are parts of a regional pattern. The development and interrelations of the eight distinguished lithofacies at Pålamalm provides evidence for the triggering mechanism responsible for the deposition of this 3-km-long glaciofluvial deposit. Strongly deformed gravels occur close to large dead-ice structures. The occurrence of another elongated and flat-topped glaciofluvial deposit, Jordbromalm, further to the east suggests a sudden regional subglacial outburst (jökulhlaup) in the area. The sudden, intensive enhancement of water discharge in Pålamalm is probably due to the same outburst. This is assumed to have caused the ice roof of the conduit to collapse. The high meltwater-pressure gradient caused the diameter of the conduit to increase rapidly. In addition, the subglacial tunnel took a new route because the original course became blocked by large ice blocks that had fallen from the roof. The steep flanks of the deposit, the presence of large dead-ice depressions along the central part of the deposit and the appearance of two different tunnel-core facies in the main cross-section of the Pålamalm deposit support the hypothesis of a course change after the jökulhlaup. A probable late-glacial crustal rebound in response to the rapid deglaciation in the area may have been the triggering mechanism for the abrupt discharge of the subglacial lake