Bedrock Topographic and Till Thickness Controls on Contrasting Till Dispersal Patterns from Kimberlites, Southeast of Lac de Gras, Northwest TerritoriesWednesday, November 20, 2019 - 9:00am to 9:20am Theatre One
Till dispersal patterns may appear as a consistent train of indicators extending in the direction of the latest ice-flow phase from a source, or along a direction defined by an older ice-flow phase. However, other dispersal patterns, sometimes even in the same area, may have poorly-defined, discontinuous trains, or even lack dispersal trains all together. This research investigates dispersal patterns from two sites southeast of Lac de Gras that were affected by the same ice-flow history, but show important differences in bedrock topography, till thickness, and subglacial landform assemblages. The goal is to improve our understanding of bedrock and till thickness effects on dispersal trains.
New local ice-flow indicators (n=16) constrain local ice-flow history. Digital elevation models and a surficial map are used to identify surficial landforms and to loosely constrain bedrock topography. We also use a subset of KIM results from a large industry-donated RC-drilling database (n=502 from 185 boreholes) which includes information on subsurface sediment characteristics and depth-to-bedrock data, which further constrain bedrock topography. In addition, we use texture, matrix geochemistry, KIMs, and clast lithology from a smaller set of 51 surface samples to compare dispersal patterns at surface and at depth. Part of the eastern study area is characterized by a well-defined drumlin field associated with the young NW ice-flow phase, variable till thickness (0-18m), and relatively flat bedrock topography (<20m elevation change). Kimberlites WO-17/WO-20 exhibit a short, but well-defined KIM dispersal train in the direction of the last dominant flow phase (NW); the dispersal area is also characterized by thin discontinuous till. A second KIM dispersal train is also recognized in the thicker till of the drumlin field SW of WO-17/20. Based on its location relative to WO-17/20, and till geochemistry and lithology counts, this pattern is interpreted to be a palimpsest train associated to the oldest SW ice flow. The western study area, located 20km from the eastern area, is characterized by a similar ice-flow history, but its bedrock topography varies more (~70m), with thin till, generally under 4m. A known kimberlite within the western area (Big Blue) is nestled within a bedrock topographic high ~20m above the surrounding terrain. Fragmented and more elusive till anomalies occur down-ice from this source. The lack of a well-developed dispersal train associated with the kimberlite is noteworthy, and may be due to the evolution of subglacial conditions around the bedrock hill. Our current model involves initial basal sliding and erosion of the top of the kimberlite and englacial entrainment. This phase was followed by reduced local abrasion and erosion rates within the kimberlite depression, possibly related to the development of low-pressure cavities over several local depressions: an idea supported by evidence of late-stage meltwater activity. This research highlights the important role of bedrock topography and related subglacial conditions both in the source area and dispersal area, as well as the potential for enhanced preservation of palimpsest trains in drumlinized till blankets.