Genesis of subglacial triangular‐shaped landforms (murtoos) formed by the Fennoscandian Ice Sheet

Abstract

The purpose of this paper is to describe the internal structure and composition of recently discovered subglacial landforms called “murtoos” in order to interpret their formative processes and depositional environment. So far, murtoos have only been reported from Finland and Sweden, but they probably exist in all areas covered by past ice sheets. Murtoos mainly occur in fields along subglacial meltwater routes or corridors in close relation to eskers and ribbed moraine tracts. Murtoos were excavated perpendicular to the long axes of the triangular murtoo heads in six locations. Murtoos were found to be composed of silt/clay‐poor, sandy and gravelly diamictons interbedded with sorted sediments, and are suggested to be produced by pulsed, highly sediment‐concentrated flows during weak glaciotectonic deformation, indicating effective pressure close to zero. Murtoos can be divided into three main depositional units: 1) the core, 2) the murtoo body and 3) the murtoo mantle. The initial deposition of murtoos took place in a network of low canals and conduits or cavities with fluctuating stream flow likely over 50 km from the ice margin. Murtoos reveal an increasing influence of subglacial meltwater flow in rapidly widening broad and low conduits with increasing sediment transport over short distances. The results of this work suggest that murtoos were formed time‐transgressively over yearly meltwater cycles in a semi‐distributed drainage system not recognised before, in which was high‐pressure porewater conditions with rapid mobilisation of subglacial saturated sediments, a critical factor in the development of semi‐efficient drainage. Murtoos are suggested as missing element between distributed and channelised drainage systems not included in current glaciohydrological models or even in the theoretical basis of glacial hydrology.

Conceptual model for the formation of bedforms along subglacial meltwater corridors (SMCs) by variable ice‐water‐bed interactions

“…Their tip points in the direction of ice flow and they have an asymmetric long-profile characterized by a shorter and steeper down-ice slope (Ojala et al, 2021). Murtoos are composed of sandy diamicton and sorted sediment (Becher & Johnson, 2021;Mäkinen et al, 2023;Ojala et al, 2022).…”

“…Finally, although external (climatic) controls on the large-scale distribution of murtoos have been studied (Ojala et al, 2019), internal factors influencing their formation such as bed lithology and bed topography remain under-explored (Mäkinen et al, 2023).…”

“…Burke et al, 2012b;Campbell et al, 2020;Haiblen, 2017;Rampton, 2000;St-Onge, 1984;Utting et al, 2009), murtoos and related bedforms (e.g. Ahokangas et al, 2021;Becher & Johnson, 2021;Mäkinen et al, 2017Mäkinen et al, , 2023Ojala et al, 2019Ojala et al, , 2021Peterson et al, 2017;Vérité et al, 2022) and ribbed bedforms (e.g. Campbell et al, 2020;Lewington, 2020;McMartin, Campbell, Dredge, LeCheminant, et al, 2015;Peterson et al, 2017).…”

“…[Color figure can be viewed at wileyonlinelibrary.com]4.2.1 | Depositional murtoosBased on sediment sections, the formation of murtoos and related landforms has been interpreted as resulting primarily from the deposition of sediments by meltwater flow in a changing style of subglacial drainage system (from inefficient to efficient) due to the common occurrence of sorted sediments (silt to gravel) interbedded with diamicton in murtoo cores(Mäkinen et al, 2017(Mäkinen et al, , 2023 Peterson Becher & Johnson, 2021;Ojala et al, 2021Ojala et al, , 2022Hovikoski et al, 2023;Mäkinen et al, 2023). Moreover, longitudinal and/or vertical sequences of grain-size coarsening in murtoos (from silt/sand to gravel) suggest episodes of enhanced meltwater delivery and increases in flow velocity and water depth in relation to the enlargement of water conduits or subglacial cavities/ponds(Hovikoski et al, 2023;Mäkinen et al, 2023). The development of upper-flow-regime bedforms, including sigmoidal cross-stratification, humpback dunes, chutes-and-pools and cyclic steps in some murtoos indicate powerful flow and sediment deposition by density flows(Hovikoski et al, 2023).…”