Medial moraines as part of a glacier debris system : their formation and sedimentology

The morphological development of medial moraines on Austerdalsbreen, Norway and Berendon Glacier, British Columbia is dependent upon englacial debris supply. A literature review suggests that this is a general case. An 'ablation-dominant' model of moraine formation is proposed which relates englacial debris supply to the site and manner of englacial incorporation of debris relative to the firn line. On Austerdalsbreen a prominent medial moraine is formed by the confluence of two ice-cap outlet glaciers below the firn line (below firn line subtype of the model). Moraine debris is derived from extraglacial bedrock outcrops between the two ice streams and is englacially entrained via crevasses to which lower depth limits apply. Revelation of crevasse-bound debris generates a distinct ice-cored morphology which is destroyed as crevasse-bottoms are revealed downglacier. In this below firn line subtype, moraines formed by confluence of ice-cap outlet glaciers below the firn line are characterized by a discrete debris supply distinct from subglacial removal of bedrock. -- On Berendon Glacier ice streams coalesce above the firn line (above firn line subtype); debris entrained between ice streams is derived both from extraglacial rock outcrops and from subnival and subglacial bedrock zones. Extraglacially derived debris undergoes seasonal sedimentation with snowfall; debris extends throughout ice depth. As a result a distinct moraine morphology is present in the terminal area in response to continuing englacial debris supply. The bulk of moraine debris is found to be transported at depth near the glacier base following subglacial and subnival derivation in the firn basin. Upper englacial debris (related to extraglacial rockslope activity in the firn basin) is of small quantity. -- In addition, moraines are generated in many cases by confluence below the firn line of large ice-streams carrying a large lateral moraine load. Peculiar ice structures and patterns of ice flow commonly observed from confluence zones have been considered to determine moraine morphology rather than englacial debris supply. However this 'ice-stream interaction' model which was tested on Berendon Glacier where two large ice-streams converge explains moraine morphology in the confluence zone only. Down-glacier, moraine morphology accords with those moraines formed above the firn-line i.e. is dependent upon the nature of englacial debris supply. -- Examination of debris texture, scanning electron microscopy (s.e.m.) of quartz grains and clay mineral investigations form the basis of sedimentological analyses. Medial moraine debris cannot be rigorously distinguished from other elements of the debris system; primary comminution processes produce debris exhibiting a wide particle-size range independent of parent bedrock. A wide spectrum of particle sizes is produced. Limited comminution following initial derivation (i.e. passive glacial transport) is reflected in quartz grain surface textures and a coarser mean size of medial moraine debris compared to subglacial lodgement tills.