Linking bedrock discontinuities to glacial quarrying

J B Woodard,L K Zoet,C Helanow,N R Iverson

doi:10.1017/aog.2019.36

J B Woodard, L K Zoet + Show 2 more

Open Access

https://doi.org/10.1017/aog.2019.36

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Abstract

AbstractQuarrying and abrasion are the two principal processes responsible for glacial erosion of bedrock. The morphologies of glacier hard beds depend on the relative effectiveness of these two processes, as abrasion tends to smooth bedrock surfaces and quarrying tends to roughen them. Here we analyze concentrations of bedrock discontinuities in the Tsanfleuron forefield, Switzerland, to help determine the geologic conditions that favor glacial quarrying over abrasion. Aerial discontinuity concentrations are measured from scaled drone-based photos where fractures and bedding planes in the bedrock are manually mapped. A Tukey honest significant difference test indicates that aerial concentration of bed-normal bedrock discontinuities is not significantly different between quarried and non-quarried areas of the forefield. Thus, an alternative explanation is needed to account for the spatial variability of quarried areas. To investigate the role that bed-parallel discontinuities might play in quarrying, we use a finite element model to simulate bed-normal fracture propagation within a stepped bed with different step heights. Results indicate that higher steps (larger spacing of bed-parallel discontinuities) propagate bed-normal fractures more readily than smaller steps. Thus, the spacing of bed-parallel discontinuities could exert strong control on quarrying by determining the rate that blocks can be loosened from the host rock.

Highlights

Temperate glaciers are highly effective erosional agents resulting in denudation rates approaching 100 mm a−1 (Hallet and others, 1996)
We study fracture propagation dynamics on a subglacial stepped bed by using the COMSOL Multiphysics software to create a finite element model (FEM) to simulate stresses on a fracture propagating normal to the bed
Photos were collected from an unmanned aerial vehicle (UAV) that was fitted with a one-inch, 20-megapixel CMOS camera and F2.8 wide-angle lens with a 9 mm focal length, and was flown at an altitude of ∼50 m above the rock surface

Summary

Introduction

Temperate glaciers are highly effective erosional agents resulting in denudation rates approaching 100 mm a−1 (Hallet and others, 1996). They profoundly affect landscape evolution and sedimentation rates (Hallet and others, 1996; Sternai and others, 2016; Ugelvig and others, 2016; Ugelvig and Egholm, 2018), with ancillary effects on processes as diverse as chemical weathering (e.g., Anderson, 2005) and volcanic eruptions (Sternai and others, 2016). The primary processes of glacial erosion are abrasion, the wearing away of underlying bedrock by debris entrained in basal ice, and quarrying (i.e., plucking), the fracturing and removal of bedrock blocks. The extracted block must be entrained in the ice, thereby completing the erosion process

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