The preservation of englacial structure : A comparative analysis of basal ice and till
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Englacial structure (sedimentary and glaciotectonic) found in the basal zone of many glaciers can, in some circumstances, be preserved in till deposits. This structure provides evidence that can be used to infer the basal zone characteristics of former glaciers, and may therefore contribute to the reconstruction of past glacial environments. The preservation of englacial structure results from passive deposition, which occurs under restricted conditions. Some of these conditions relate to the characteristics of the basal ice, and others to the nature of the proglacial environment. Sublimation is an inherently passive process that occurs when ice is transformed directly to water vapour, without an intermediate liquid phase. This process formed the basis of an arid polar model of deposition (Shaw 1977a). Laboratory experiments conducted on basal ice samples from the Taylor Valley, Antarctica, show that slow melt may also deposit till passively. Melt-out till was produced by allowing the debris to consolidate vertically, with little lateral dislocation during deposition. A comparative analysis of basal ice and laboratory-generated till deposits show that lamination, folding and banding were preserved. Two conceptual models of passive deposition illustrate that sublimation and slow melt may operate concurrently and in close association in arid polar environments. Both processes may lead to the preservation of englacial structure in arid polar environments, and it may be difficult to distinguish between sublimation and melt-out tills.