Lithofacies types and vertical profile models; an alternative approach to the description and environmental interpretation of glacial diamict and diamictite sequences

ABSTRACT Increased knowledge of modern glacial depositional environments has resulted in rapidly evolving classifications of glacial tills. These are based to a large degree on theoretical considerations of likely depositional processes. The classifications are sophisticated and more advanced than the establishment of simple field criteria whereby individual till facies can be identified in Quaternary and Pre‐Quaternary successions. This situation is compounded in many Quaternary terrains by the continued description of ‘tills’ in terms of laboratory‐derived analytical data only, reflecting a traditional interest in stratigraphic correlation rather than reconstruction of depositional environment. Detailed sedimentological logging of lithofacies is rarely undertaken. There is thus considerable confusion as to what is being described or sampled when analytical data are presented for many Pleistocene ‘tills’. The same remarks apply to Pre‐Pleistocene ‘tillites’. A lithofacies code is presented here for the rapid description and visual appraisal of field sequences or drill cores containing unconsolidated diamicts or lithified diamictites; the term‘till’is not used as it has a strict genetic definition referring to direct aggregation and deposition by glacier ice. Use of a four part code, in conjunction with codes already published for fluvial sediments, allows fundamental field properties to be depicted independent of genetic terminology and provides a firm basis for subsequent environmental interpretation and analytical work. The value of this approach is illustrated by comparing a representative suite of vertical profiles of diamict assemblages deposited by modern grounded glaciers with a classic late Pleistocene glacigenic sequence at Scarborough Bluffs, Ontario.