SUBGLACIAL PROCESS INFERRED FROM BASAL TILLS AND THE RELATIONSHIP BETWEEN MACRO- AND MICRO- SCALES OF OBSERVATION

Inferences regarding the deposition and deformation of sediments beneath ancient ice sheets rely heavily on sedimentological observations of often enigmatic and homogenous-looking diamictons. For this reason, microscale sedimentological observations are now being used alongside macroscale and mesoscale observations to infer subglacial till rheology and strain history. Recent laboratory experiments have shown that microstructure associations (turbate structures, lineations, grain stacks, and crushed grains) are of limited use as a quantitative strain proxy, but clearly more work is required to clarify the relationship between till micromorphology and large scale till deposition and deformation mechanics before inferences regarding till rheology and consequent ice flow dynamics can be made from thin sections.

Past macroscale sedimentological work has identified a range of subglacial diamicton types in a small open-ended mountain valley and adjacent plateau in south central B.C., which seem to represent a continuum between lodgement and deformation processes. Previously identified macroscale sedimentological and structural characteristics of the lodgement tills in this area include unimodal or spread unimodal clast fabrics, and consistently oriented glacigenic shear planes, tension fractures and clast morphological features (stone striae, stoss-lee ends, keels and facets) associated with low pore water pressures during emplacement. Deformation tills include evidence of sub-till erosion and entrainment of bedrock, the preservation of soft sediment clasts, folds, spread unimodal, girdled or multimodal fabrics, and a relatively poor alignment of clast morphological features thought to be associated with high pore water pressures during deformation.

We present a semi-quantitative micromorphological study of two till end-members of this lodgement-deformation continuum to investigate the relationship between micromorphology and the deposition/deformation characteristics inferred from macro-scale sedimentology and structural geology. Additional data will be presented to illustrate the level of spatial variability in preservation of micro and macro-scale structures and fabric within and between exposures of similar diamicton types.