Yellowknife Geoscience Forum

Sampling and analysis of surficial sediment is an established method of resource exploration in glaciated terrain. For example, sampling of the Exeter Lake Esker led directly to the discovery of the Lac de Gras kimberlite field in the Northwest Territories and establishment of the diamond industry in Canada. During the initial sampling campaign, eskers were thought to act as regional “dipsticks” containing far travelled sediment extending the length of the esker system (100s of km).  By contrast, till dispersal trains, from which esker dispersal trains are thought to be sourced, are typically perceived as being much shorter. To test this hypothesis, 56 paired esker and till samples with an average downflow spacing of 25 km were collected in Summer 2019 along the entire length (~750 km) of the Exeter Lake Esker. Following field work, the lithology of pebbles was determined visually, and the geochemistry of the mud (<63 um) fraction was analyzed using a lithium-borate fusion followed by ICP-MS. Dispersal trains from the Dubawnt Basin, an SiO₂-rich siliciclastic unit at the up-flow end of the esker, are detected in the pebble and mud fractions in both the esker and till samples. Counter to the common perception, on a first order the dispersal trains in the esker mirror those in the till: there is no regional overshoot of esker dispersal trains beyond the till dispersal trains from which they were sourced in either the pebble or the mud fractions. For example, the concentration of Dubawnt pebbles in both the esker and till samples decreases rapidly from 100% to <50% ~25 km down-ice from the bedrock source and approaches 0% within 100 km from the bedrock source. Likewise, the concentration of SiO₂ in the mud fraction in both eskers and till decreases rapidly by 15% ~25 km down-ice from the bedrock source and levels out within 125 km from the bedrock source. If representative, the findings question the common perception that eskers contain regional dispersal trains of gravel, sand and mud that are significantly longer than those in the sub-adjacent till.  Rather, both mediums may typically be composed of sediment of roughly similar provenance. This makes eskers excellent early exploration targets at the local scale, as they are capable of concentrating sand sized indicator minerals into easily traversable ridges which can be sampled much more efficiently when compared to traditional grid-like till transects.