Characterization and quantifcation of glacial sediment transport in two small valley glaciers

Abstract

This study investigates sediment transport processes and sediment-landform development at two small valley glaciers of contrasting basal thermal-regime. A range of innovative techniques are applied to characterize and quantify glacier structure, glacially transported sediment facies and sediment-landform assemblages at Glacier de Tsaneuron, Switzerland, and Midre Lovénbreen, Svalbard. Sediment transport at Glacier de Tsaneuron is dominated by a rockfall-derived supraglacial debris stripe and abrasion-derived subglacial debris planes. Modeled sediment transport rates are equivalent to a glacier-wide erosion rate of 0.5 mm a -1. Subglacially transported sediment ablates to form a complex network of debris stripes, the texture and morphology of which are consistent with sediment supply from palaeokarst sinkholes. These sinkholes dominate the limestone plateau on which the glacier is situated and also restrict glaciofluvial sediment transport by rerouting much of the glacier's meltwater though the karst system. This results in a landsystem that is atypical of valley glaciers in the Alps. Sediment transport at Midre Lovénbreen is strongly influenced by glacier structure, identified at the glacier surface and in digital optical televiewing borehole logs. Glacier structure is successfully reconstructed in three-dimensions, revealing several visually distinctive englacial ice properties and deformation structures. Basally derived englacial sediment layers are found to be intercalated with primary stratification, elevated into near-vertical planes around a gently upglacier dipping central fold axis by large-scale lateral folding. Sediment analysis also allows supraglacial longitudinal debris ridges to be sub-classified into two types, with a new (Type II) form experiencing secondary deformation by small-scale flow parallel recumbent folding in association with vertical displacements across arcuate shear planes near the glacier terminus. Non-structurally controlled sediment transport at Midre Lovénbreen is dominated by low-intensity deformation of subglacial till, and is also characterized by the formation of supraglacial debris cones and proglacial flutes. Flutes at Midre Lovénbreen are interpreted to form beneath warm-based ice by the squeezing of partially fluidized till into cavities where freeze-on occurs due to a 'heat-pump' effect across subglacial boulders. The results of this investigation demonstrate that, while basal-thermal regime may exert a strong influence on glacial sediment-transport processes that operate at the small-scale, bedrock physiography ultimately defines glacier morphometry and resulting large-scale flow structures. Additionally, although some of the sediment-landform types identified in this study are exclusive to a particular set of basal-thermal conditions, the composition and morphology of these sediment-landform assemblages are such that they are unlikely to be preserved in the geological record.