Morphology and sedimentology of murtooized terrain on the southern Fraser Plateau, British Columbia, Canada

Abstract

Recent public releases of Light Detection and Ranging (LiDAR) data in south central British Columbia (BC), Canada have revealed several landforms resembling ‘murtoos’, previously identified across portions of Scandinavia. In this study, we investigate the morphology and sedimentology of these landforms, the first report of their kind associated with the Cordilleran Ice Sheet (CIS). From remote observations using LiDAR data, we determined that these landforms do not fit the classification scheme of murtoos used by previous authors, and based on our own observations, we use the term ‘murtooized terrain’, due to their superficial similarities with Scandinavian murtoos. Murtooized terrain consists of 1–8 m high, distinctly steep slopes that can continue laterally up to 2 km, changing in orientation along their lateral lengths, creating angular, zig-zag patterns. It can present as dense, subparallel groupings of slopes giving a washboard appearance. Murtooized terrain occurs in several landscape associations including till plains, ribbed terrain, isolated plateaus and near meltwater corridors. Murtooized terrain in association with till plains and ribbed terrains exhibits distinct hill-hole pairs. It typically consists of diamicton interpreted as a regional till sheet winnowed by persistent subglacial groundwater flow, sometimes overlain by melt-out till and covered with a surface veneer of silt and very fine sand interpreted as loess. Massive sand deposited by postglacial overland flows can fill small troughs at the foot of murtooized terrain slopes. The following event sequence explains the formation of murtooized terrain. The concentration of subglacial groundwater flow through very broad topographic troughs winnowed till and resulted in strong ice-bed coupling through basal ice regelation. Following channelized subglacial floods (underbursts) associated with the formation of large (Chasm and Green Lake) meltwater corridors, the CIS underwent collapse and ice surface slopes were reorganized in the adjacent region. Murtooized terrain formed due to widespread, short-term, glaciotectonism associated with this reorganization where the ice and bed were well coupled. Zig-zag slopes and hill-hole pairs are attributed to spatial variations in coupling. Stagnation followed forming melt-out till and crevasse fill ridges. This hypothesis is distinctly different from those developed for the Scandinavian murtoos, suggesting either that the superficial similarities between BC murtooized terrain and Scandinavian murtoos are the products of formational equifinality or that the BC murtooized terrain is a distinct landform.