Till melange at Amsdorf, central Germany: sediment erosion, transport and deposition in a complex, soft-bedded subglacial system

Glacial melange in the open-cast mine at Amsdorf, central Germany, consists of several square meters of large, sorted sediment blocks embedded in till . The blocks are composed of largely intact to slightly deformed glaciofluv ial and glaciolacustrine sand, silt and clay, initially deposited in a prog lacial lake (2-3 km up-ice) and subsequently overridden by a glacier. The b locks typically have cuboid to subrounded outlines, are randomly distribute d in the till, and the contacts with the surrounding till are distinctly sh arp. Underneath the melange are varved clays which exhibit strong deformati ons occasionally intervening with entirely undisturbed areas. It is suggest ed that the blocks were entrained into debris-rich basal-ice by bulk freeze -on when the glacier sole was lowered onto the bottom of an overridden Lake . After entrainment the blocks were transported englacially and re-deposite d (with far-traveled till matrix) as a melt-out till from stagnant ice. The glacier moved mainly by sliding enhanced by low-permeability varved clays in the substratum. The glacier is believed to have been of a polythermal ty pe. These results show that bulk freeze-on can lead to entrainment of soft sediment blocks at least 20 m(2) in size, and that these blocks can be engl acially transported with little or no deformation for several kilometers an d more. The occurrence of deformed and undeformed clays under the till mela nge indicates a possible mosaic of coupled and decoupled ice, the latter ca used by a thin, transient subglacial water film separating the bed from the glacier. (C) 2001 Elsevier Science B.V. All rights reserved.