Architectural element analysis applied to glacial deposits: Internal geometry of a late Pleistocene till sheet, Ontario, Canada

Deposits left by continental ice sheets are characterized by sedimentological complexity and stratigraphic heterogeneity, but stratigraphic descriptions of such deposits, and resulting “first-generation” facies models, are still based primarily on one- or two-dimensional borehole or outcrop data. Reconstruction of depositional environments, hydrogeological investigations of Pleistocene glacial deposits, and hydrocarbon exploration in pre-Pleistocene glaciated basin fills require a more detailed understanding of the form and heterogeneity of lithofacies sequences in three dimensions. Architectural element analysis is used widely by sedimentologists for categorizing internal stratigraphic heterogeneity in sandstones, particularly those of fluvial origin. This paper demonstrates the first application of architectural element analysis to glacial deposits such as tills. Outcrop, borehole, and a broad range of subsurface geophysical data were collected from a thick (60 m) till sheet present across an 80 km2 study area near Toronto, Canada. The till sheet is not homogeneous, but is composed of three distinct architectural elements and associated lithofacies, viz, diamict elements, interbeds of subglaciofluvial sediments, and glaciotectonically deformed zones. Application of architectural element analysis to these subglacial strata provides insights into the origin of drumlin bedforms and subglacial processes below the Laurentide Ice Sheet and creates a framework for understanding ground-water and contaminant movement in underlying aquifers.