Microsedimentological evidence of vertical fluctuations in subglacial stress from the northwest sector of the Laurentide Ice Sheet, Northwest Territories, Canada

Abstract

The past-producing Pine Point lead–zinc mining district, Northwest Territories, Canada, provides a unique opportunity to study the role of glacial dynamics in a thick, continuous till succession that has not been influenced by the underlying bedrock topography. Parts of the Pine Point mining district are covered by >20 m of subglacial Quaternary sediments (till) associated with the former Laurentide Ice Sheet. Till facies exposed in unreclaimed open-pit K-62 have been classified into four separate units. Micro- and macrosedimentological analyses were undertaken to identify the change in subglacial stress during sediment deposition and across till unit boundaries. An analysis of high- and low-angle microshears (lineations) in thin sections produced from these till units indicate that there is a noticeable decrease in the abundance of low-angle shear features immediately below till unit boundaries. The deformation of low-angle shears in the underlying tills was likely caused by remobilization of the overlying till unit. This remobilization is consistent with aggradation-constant entrainment decay mechanisms for subglacial till emplacement and accretion and subglacial dispersion models.

Microsedimentology of tills near Ainet, Austria - were palaeo-ice streams in the European Alps underlain by soft deforming bed zones?

“…This work stems from and builds upon recent research in northern Austria in the tributary side valleys of the Inn Ice Stream in which soft deformation microstructures and rapid changes in localized stress levels can be distinguished within the subglacial tills. Similarly, these northern Austria tills illustrate the effect of warm based subglacial sediment deformation associated with an array of microstructures that are known to exist in many other subglacial tills in northern Europe, Antarctica, and North America (Cowan et al, 2012(Cowan et al, , 2014Rice et al, 2014Rice et al, , 2018Menzies and van der Meer, 2018). The microstructures present within the tills at Ainet reveal, for the first time, a complex history of microfabric development resulting from ductile shearing and brittle deformation during the emplacement processes under fast ice stream conditions.…”

“…In order to obtain a series of subsamples for thin section analyses, samples were obtained from disturbance-free cleaned vertical exposures using Kubiëna tins or as bulk samples where necessary, then transported, and prepared for resin impregnation using standard micro-morphological techniques (Rice et al, 2014(Rice et al, , 2018Menzies and van der Meer, 2018). Sampling was carried out on a sediment exposure face cleaned parallel to known ice direction (using till macrofabrics to indicate main ice direction, Reitner et al, 2010).…”

“…Such a provenance is also indicated by the occasional occurrence of the prominent tonalite, which appears to have been quarried from 10 km up valley. This deposit is interpreted as characteristic of subglacial traction till (mélange) (Menzies, 1990;Hoffmann and Piotrowski, 2001;Evans et al, 2006;Rice et al, 2018;Menzies et al, 2018) deposited by the Isel Glacier. It is unconformably overlain by a sediment package dominated by sandy gravels partly with boulders (Ainet unit).…”

Glacial microsedimentology—a new lens to investigate glacial sediments—a review

“…Sample PTA-036 of subglacial till from Pine Point, NT, containing microshears (red lines), rotation structures (green), grain stacks (red dots), and multiple domains (light pink). Note the grain stacks parallel many of the microshears within this unit (modified from Rice et al, 2019). Fig.…”