Facies of supraglacial sedimentation on Icelandic and Alpine temperate glaciers

Abstract

Supraglacial debris transported by temperate valley glaciers is classified as supraglacial morainic till, distinct from lodgement till, and melt-out and flow till species on polar glaciers. In Iceland and the Alps, where annual discharges of supraglacial morainic till vary from 200–2000 m3 (compared with a maximum discharge of 26 000 m3 of lodgement till), till is deposited as three facies.Fades 1 occurs where supraglacial morainic till slows the rate of ice melt such that till is slowly superimposed on the subglacial surface in the form of stagnation or disintegration topography. The rate of deposition shows an initial slowing phase in response to (1) soil formation and (2) melt-out of englacial debris, followed by accelerated deposition accompanying the formation of thaw lakes. Continued mass movement of till on lake margins (backwasting) is more effective in destroying the ice-core than either top or bottom melt and prohibits accumulation of a distinct melt-out sediment. This situation can be contrasted with the style of sedimentation at polar glaciers. Where the depositing glacier is inactive and uncontrolled an unlineated till surface, typical of stagnation or disintegration topography, develops. Significantly, in terms of Pleistocene reconstructions, tracts of stagnation topography are being constructed at the margins of certain active Icelandic glaciers by sequential stagnation of a marginal rim of ice as the active glacier retreats up-valley.Fades 2 occurs where the till cover is too thin or too coarse and ice melt is unretarded and supraglacial morainic till is deposited as a dispersed bouldery veneer by dumping during which gravity sorting occurs. Dump moraine ridges frequently show internal bedding being ice-contact screes at time of formation but are not ice-cored. An active glacier produces a controlled distribution of landforms resulting in a lineated till surface.Facies 3 refers to those stratigraphic sequences where irregular or lensate till horizons alternate repeatedly with ice-contact outwash. The stratigraphic sequence as a whole is defined here as a supraglacial morainic till complex. Well-sorted, clast-supported outwash sediments occur within complexes deposited at inactive low-angled ice margins. At active steep-fronted glaciers, where areas of the till-covered ice front are only episodically scoured, distinct flood-deposited matrix-supported outwash units are found. Their subaerial formation runs counter to recent published interpretation of sediments in certain Pleistocene eskers.