EGU2020-10537
https://doi.org/10.5194/egusphere-egu2020-10537
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Geomorphology, distribution and composition of subglacial triangular hummocks (murtoos) in Sweden and Finland

Mark Johnson1, Joni Mäkinen2, Gustaf Peterson1,3, Antti Ojala4, Christian Öhrling3, Elina Ahokangas2, Izabella Remmert1, Karu Kajuutti2, and Jukka-Pekka Palmu4
Mark Johnson et al.
  • 1University of Gothenburg, Earth Sciences, Sweden (markj@gvc.gu.se)
  • 2University of Turku, Geography and Geology
  • 3Geological Survey of Sweden
  • 4Geological Survey of Finland

Triangular hummocks of subglacial origin have been identified in Sweden and Finland due to the increased resolution provided by LiDAR imagery. Their triangular shape is distinctive and recognizable as clearly identifiable landforms. These forms have been previously mapped in some cases as dead-ice hummocks, but geomorphic relationships with eskers, flutes ribbed moraine and De Geer moraines show these to be subglacial. We refer to these new landforms as ‘murtoos.’ Morphometric measurements show murtoos to be 50 to 200 m long and 50 to 100 m wide. The orientation of their apices strongly correlates with local ice-flow orientation. They form preferentially on beds that slope down-ice.  In many cases, they occur in patches in an ice-flow parallel path with eskers, defining corridors we believe to be of subglacial meltwater origin. Murtoos are composed primarily of heterogeneous diamicton with variable amounts of bedded sand and gravel. Murtoos are most common where glacier-melt rates were high during deglaciation (Bølling-Allerød and Holocene), and they are absent where extensive frozen-bed conditions were present. We suggest murtoos are a landform produced as a glacier-bed adjustment to increased delivery of supraglacial meltwater during deglaciation.

How to cite: Johnson, M., Mäkinen, J., Peterson, G., Ojala, A., Öhrling, C., Ahokangas, E., Remmert, I., Kajuutti, K., and Palmu, J.-P.: Geomorphology, distribution and composition of subglacial triangular hummocks (murtoos) in Sweden and Finland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10537, https://doi.org/10.5194/egusphere-egu2020-10537, 2020

Comments on the presentation

AC: Author Comment | CC: Community Comment | Report abuse

Presentation version 1 – uploaded on 01 May 2020
  • CC1: Comment on EGU2020-10537, Matteo Spagnolo, 08 May 2020

    Hi Mark,

    very interesting. If I have got it right, their height is just a couple of metres, very subtle features. But they seem to have a least one or two sharp, linear boundaries. Is there any difference between what you see there, at these boundaries, and elsewhere in and around the landform? Any evidence of erosion?

    • AC1: Reply to CC1, Mark Johnson, 08 May 2020

      Matteo

       

      Thanks for the good question! There seems to be a boulder lag in places that exists on the margins (but also the tops) We also excavated the margin between two murtoos and found (1) no stratigraphic separation of the two and (2) a draping boulder lag. But like other subglacial fluvial erosion,we are a bit mystified where the eroded sediment ould have gone.