A time-transgressive model for microstructures in subglacial tills - Examples from beneath the Late Wisconsinan (MI 2) Laurentide Ice Sheet

The complexity of subglacial till sedimentology is discussed at the microscale to develop a revised model of microstructure evolution and development in subglacial tills. Mapped thin sections from the Northwest Territories and southern Ontario, Canada reveal a myriad of microstructures. Discussion of the relevance and meaning of these microstructures leads to a new revised model of subglacial soft sediment deformation to account for the development and evolution of these microstructures. The model is time-transgressive such that over time both pervasive and non-pervasive deformation conditions persist repetitively within the subglacial till environment under mixed rheologies. Microstructure types appear to be sequential in development and, during progression, are partially or wholly overprinted, destroyed, rotated, and suffer further subsequent deformation, or remain intact but intercalated with structures of other later or earlier deformation phases. This new revised model helps explain that the vagaries of till microsedimentology can be accounted for and, in most instances, predicted.