Entrainment and emplacement of englacial debris bands near the margin of Storglaciären, Sweden

Abstract

Internal structure, stable isotope composition and tritium concentration were measured in and around debris‐bearing ice at the margin of Storglaciären, where englacial debris bands have previously been inferred to form by thrusting. Two types of debris bands were distinguished: (i) an unsorted diamicton band that is laterally continuous for more than 200 m, and (ii) well‐sorted sand and gravel bands that are lenticular and discontinuous. Above‐background tritium levels and enrichment of δ18 O and δD in ice from the diamicton band indicate entrainment by basal freeze‐on since 1952. Isotopic enrichment and tritium‐free ice in the sandy debris bands also indicate entrainment in freezing water, but prior to 1952. The lenticular cross‐section, sorting and stratification of the sandy bands suggest that they were deposited englacially. The basally accreted diamicton band has been elevated tens of metres above the bed and presently overlies the englacially deposited sandy bands, suggesting that the stratigraphy has been disrupted. Three interpretations could account for these observations: (i) thrusting of fast‐moving ice over slow, marginal ice uplifting recently accreted basal ice along the fault; (ii) folding near the margin, elevating young basal ice over older basal and englacial ice; and (iii) debris‐band formation by an unknown mechanism and subsequent contamination of ice geochemical properties by meltwater flow through debris bands. Although none of these interpretations is consistent with all measurements, folding is most compatible with observations and local ice‐flow kinematics.