Sedimentary characteristics of Finnish MURTOOs - triangular-shaped subglacial landforms produced during rapid retreat of continental ice sheets

Finnish and Swedish researchers, using national LiDAR DEMs, have recently discovered a triangular-shaped subglacial landform (murtoo) that is new to science. In Finland murtoos mainly occur along subglacial meltwater corridors, indicate local ice flow direction, and are associated with rapid melting and retreat of the continental ice-sheet after the Younger Dryas. We present common subsurface stratigraphy and sedimentology of Finnish murtoos based on five trenches excavated across their triangular heads. Murtoos show an arched and stratified core of sorted sediments including silt beds, laminated silty sands, current ripples and crude gravel beds. The core is covered by loose, sandy and gravelly, crudely stratified to massive/matrix-supported diamicton with mostly subangular to subrounded clasts. Proportion of clay is minimal and internal boulders exceeding 1 m in diameter are missing. Large boulders of up to 2-3 m are found on the murtoo surfaces. Diamicton is intercalated with poorly preserved disturbed and laminated silty sands with some current ripples. Contacts are diffuse and clasts pushed into sand beds are common. Diamicton reveals open-work gravel patches, crude stone layers, and poorly defined troughs. A common feature is a "raisin-cake" like structure where granules and smaller pebbles are scattered in sand beds or in sandy diamicton. In conclusion, murtoos are primarily depositional landforms with erosional triangular heads, their material is intermediate between till and glaciofluvial sediment, and they indicate cyclic deposition with short transport distances. The stratified cores are interpreted to represent deposition in wide and shallow subglacial canals and cavities followed by their rapid enlargement associated with sandy and gravelly diamictons. Cyclic deposition might be due to fluctuating subglacial water pressure and clasts truncating sand beds indicate push by ice. Limited boulder size in diamicton might indicate that maximum space for meltwater flow was about 1 meter. Murtoos represent efficient drainage in high-pressure subglacial conditions providing a link to channelized drainage system. Therefore, murtoos offer high potential to improve understanding of subglacial hydrology and related modelling of rapidly melting ice sheets.