ENTRAINMENT AND UPLIFT OF BASAL SEDIMENT TO THE SURFACE OF A POLYTHERMAL GLACIER MARGIN, STORGLACIÄREN, SWEDEN

High-relief, hummocky glacial topography typically forms by melt-out and redistribution of supraglacial sediment. In ice sheets and some valley glaciers, this sediment may be derived from the glacier bed and subsequently elevated to the ice surface in longitudinally-compressive terminal environments. Supraglacial sediment accumulations originating from englacial debris bands outcrop at the margin of Storglaciären, a non-surge-type polythermal glacier in northern Sweden. Prior work suggests that these debris bands may have been entrained from the bed near the terminus and subsequently uplifted by thrusting. We used a variety of field and numerical methods to investigate further the mechanisms of entrainment and uplift of the Storglaciären debris bands. Stable isotopic and tritium composition of interstitial ice in one long debris band containing ~30% poorly-sorted debris (by volume) is consistent with recent (post-1952) entrainment by basal freeze-on. In other lenticular debris bands containing 30-70% well-sorted sand and gravel, isotopic data similarly indicate freeze-on, but the interstitial ice is largely tritium-free suggesting earlier entrainment. These results, combined with qualitative observations of stratigraphic relationships, suggest that the ice stratigraphy has indeed been disrupted near the glacier margin. However, measurements of surface, englacial and basal velocity and displacement across the debris bands are not consistent with uplift by thrusting. Using simple kinematic and mechanical arguments, we offer alternative mechanisms for uplift of debris-bearing basal ice at Storglaciären. These results are also relevant to supraglacial accumulation of basally-derived sediment in other margins of non-surge-type glaciers, both modern and ancient.