Processes of lateral moraine formation at a debris-covered glacier, Suldenferner (Vedretta di Solda), Italy
Sammanfattning: Lateral moraines are key features of glacial landscapes in high-alpine mountain areas. Research on lateral moraine formation is mostly limited to clean-ice glaciers; lateral moraine formation at the margins of debris-covered glaciers is currently poorly understood in an Alpine and sedimentological context. This thesis focuses on the reconstruction of processes of lateral moraine formation at the debris-covered glacier Suldenferner (Vedretta di Solda) in NE Italy. The most important geomorphological elements of the Suldenferner foreland are: (i) the lateral moraines, (ii) former areas of stagnant ice, (iii) a moraine from 1927, (iv) ice-moulded bedrock giving the direction of ice-flow, (v) outwash terraces, (vi) erosional remnants of the lateral moraines, (vii) gullied glaciogenic material and (vii) glacially-overridden outwash. The studied lateral moraines at Suldenferner reach heights between 110 and 130 m above their bases and are up to 3 km wide. The eastern lateral moraine is characterised by a pronounced cross-profile asymmetry, with gentle distal slopes (14°-30°) and steeper proximal slopes (40°-60°), while the western lateral moraine is only slightly asymmetrical, with steep slopes ranging between 60°-80°. Clasts within the lateral moraine along the western margin of the glacier are exclusively dolomite, while the clasts within the lateral moraine along the eastern margin of the glacier contain mainly micaschist, with some dolomite and paragneiss. Sediments within the lateral moraines are divided into three lithofacies associations: lithofacies association 1 (LFA 1) is a silty-sandy, matrix-supported diamicton (DgMm3-2), lithofacies association 2 (LFA 2) comprises a clayey-silty, massive diamicton (DgFm2-2) and a clayey-silty, matrix-supported diamicton (DhMm3-2) and lithofacies association 3 (LFA 3) is a sandy-gravelly, clast-rich diamicton (DmCm1-1, DmCm2-1). These lithofacies associations are interpreted as reworked, glaciofluvial deposits (LFA 1), subaerial debris flows that have been stacked in an ice-marginal position (LFA 2) and debris flows that have incorporated distinctive amounts of micaschist (LFA 3). Clast shape analysis reveals that the sediment delivery towards the lateral moraines contains a signal of mixed sources: the main mode of transport consists of subglacial and englacial sources (active transport), aided by minor contributions of supraglacial/extraglacial sources (passive transport). The presented conceptional model for lateral moraine formation in the Alps highlights the value of active clast delivery, whiles the importance of a supraglacial debris cover, and its final emplacement into laterally forming moraine ridges, exerts a limited influence on the moraine’s sedimentology. The model implies that lateral moraines have a complex formation history with glacier fluctuations, characterised by multiple glacial advance and retreat cycles.
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