Sedimentation in glacial environments and the identification of tills and tillites in ancient sedimentary sequences

Abstract

Abstract Tills (and tillites) are defined and subdivided into ‘primary’ tills, comprising lodgement tills, melt-out tills and sublimation tills, and ‘secondary’ tills comprising flow tills and deformation tills. They are defined on the basis of their mode of deposition which endows them with their distinctive properties. Another category of till, that of supra-till, is defined on the basis of its debris source (derived supraglacially from valley sides and nunataks) which gives these tills a coarse-grained character. The processes of till deposition and of outwash sedimentation are associated in several recurrent combinations which allow the definition of three principal glacial sediment associations and land systems which define the character of the sequences within which tills tend to occur. The criteria which allow tills of different genesis to be identified and to be distinguished from non-till diamictons are reviewed. There are several features which are uniquely characteristic of till but, unfortunately, these are few. It has been found generally necessary to employ an array of characteristics which, though not individually unique to till, when taken together can be strong evidence for particular till types. The recognition of several typical and repetitive facies sequences which form in the major glacial sedimentary environments can also be important indicators of till genesis.