Upper-flow regime bedforms in a subglacial triangular-shaped landform (murtoo), Late Pleistocene, SW Finland: Implications for flow dynamics and sediment transport in (semi-)distributed subglacial meltwater drainage systems

Abstract

<p>We know less about subglacial <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/meltwater" title="Learn more about meltwater from ScienceDirect's AI-generated Topic Pages">meltwater</a> flow properties in distributed inefficient and semi-efficient systems in comparison to those of ice marginal <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/glacial-drift" title="Learn more about eskers from ScienceDirect's AI-generated Topic Pages">eskers</a> and <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/proglacial-environment" title="Learn more about proglacial environments from ScienceDirect's AI-generated Topic Pages">proglacial environments</a>. While previous studies have indicated the overall common presence of upper-flow-regime (UFR) <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/bedform" title="Learn more about bedforms from ScienceDirect's AI-generated Topic Pages">bedforms</a> in glacigenic settings, facies expressions of subglacial meltwater flows remain poorly documented. Three ca. 3 m deep and up to 70 m long <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/trench" title="Learn more about trenches from ScienceDirect's AI-generated Topic Pages">trenches</a> excavated across a triangle-shaped subglacial <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/landform" title="Learn more about landform from ScienceDirect's AI-generated Topic Pages">landform</a> called a murtoo in a Lateglacial to <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/holocene" title="Learn more about Holocene from ScienceDirect's AI-generated Topic Pages">Holocene</a> meltwater route in SW Finland provide a detailed window into <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/sedimentary-structure" title="Learn more about sedimentary structures from ScienceDirect's AI-generated Topic Pages">sedimentary structures</a> presumably formed ca. 40–50 km away from the coeval subaqueous margin of the Fennoscandian Ice Sheet (FIS). The aim of this paper is to document small-scale bedforms, which formed subglacially by meltwater flow and to characterize the proximal and central parts of the studied murtoo during its early evolutionary phase. We defined seven main facies types that characterize the <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/depositional-process" title="Learn more about depositional processes from ScienceDirect's AI-generated Topic Pages">depositional processes</a> of the unit. Overall, the studied deposits reflect increasing meltwater delivery through time and are characterized by abrupt lateral changes in sedimentary structures and grain size. While the initial deposits are dominated by massive and horizontally laminated silt with sand lenses interpreted as lower-flow-regime deposits, the latter sediments are characterized by sinusoidal stratification, sigmoidal cross-stratification and scours with backsets or chaotic fill interpreted as deposits of antidunes, humpback dunes, chutes-and-pools and cyclic steps of the upper-flow regime. The upper-flow-regime bedforms developed on a 1 m high and 15 m long bed slope and are associated with the formation of a short-lived enlarged water-filled cavity or pond, where supercritical density flows allowed for the deposition of upper-flow regime bedforms. The final coarse-grained murtoo head-bar development, characterized by planar-cross stratified gravel and pebbly sand, indicates avalanche processes that were controlled by grain size.</p><p>Our results confirm that the core of the murtoo is depositional and meltwater processes played a key role in its deposition. Despite the subglacial setting with a subaqueous ice-sheet margin, the meltwater flow was not permanently characterized by pipe-flow conditions. Overall, the findings contribute to the understanding of semi-distributed subglacial meltwater systems during the retreat of a continental ice sheet (FIS) in a rapidly warming climate.</p>