Diversity of murtoos and murtoo‐related subglacial landforms in the Finnish area of the Fennoscandian Ice Sheet

Self Cite

During the last millennia of the Weichselian glaciation (c. 11 700-14 000 cal BP) and the first millennium of the Holocene interglacial (c. 10 500-11 700 cal BP), melting of the ice sheet and subsequent subglacial drainage resulted in the formation of murtoos and murtoo-related landforms in the Fennoscandian Ice Sheet area in Finland and Sweden. Murtoos are composed of clay-poor diamictons produced by sediment-concentrated creep and material sorting in a subglacial environment under effective pressure close to zero, and potentially represent a transition form from non-channelized to channelized subglacial drainage networks. Here, we explore the geomorphology and substratum characteristics of the Lake Murtoo area, one of the locations where murtoos were initially found and described in SW Finland. Our interpretation suggests an interconnected development from inefficient distributed or semi-distributed cavities (murtoos) to efficient channelized drainage (esker) via erosional escarpments, all displayed in the same region. In addition, our excavation into a murtoo-related erosional escarpment (MRE) revealed subglacial erosional channels alongside MREs that were filled with fine-grained finely laminated sediments. We suggest that these channel fill sediments were deposited subglacially, because they are covered by flow till and show spatial and lithological characteristics that deviate significantly from the proglacial basin fill clay (varve) sediments in the area. The appearance of rhythmically laminated silt and clay sediments indicates sedimentation in a suspension-load-dominated environment in shallow subglacial channels with relatively low (but repetitiously changing) water flow velocities, possibly reflecting disconnected spill-over routes of subglacial drainage in small tributaries that were only periodically used due to water pressure changes in the up-ice direction.

Section: Geological Settingmentioning

confidence: 99%

Section: Geological Settingmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

Subglacial evolution from distributed to channelized drainage: Evidence from the Lake Murtoo area in SW Finland

Ojala

Mäkinen

Kajuutti

et al. 2022

Self Cite

Late erosion pulse triggered by rapid melt in the cold‐based interior of the last Fennoscandian Ice Sheet, an example from Rogen

Boeckel

Boeckel²,

Hall

2022

Ice sheet interiors are conventionally regarded as non-erosive. Yet subglacial conditions may be transformed during deglaciation by the arrival of large volumes of meltwater at the ice sheet bed. The development of a dynamic meltwater drainage system and the onset of basal sliding have potential to increase erosion rates in bedrock and sediment. Here, we examine the impact of late deglacial thawing on the Rogen plateau, located near the former ice divide of the Fennoscandian Ice Sheet.We provide new maps of glacial and glacifluvial landforms which we combine with existing data on Quaternary sediments and landforms. Cross-cutting and overlapping relations allow for an event sequence to be established of the deglaciation period. In the Early Holocene (< 11 ka), an ice lobe onset zone developed at the Rogen plateau.In places where meltwater reached the bed and where pressures rose to overpressure, it caused fracture dilation in horizontally bedded sandstones and rock brecciation. The onset of sliding and application of drag resulted in the mobilization of bedrock sheets. The establishment of meltwater corridors led to fluidization of sediments at the bed, dissection and modification of ribbed moraines and formation of murtoos and hummock corridors. During final stagnation of the ice sheet, meltwater drained through channels forming axial eskers. Bedrock erosion during deglaciation reached depths up to 4 m, and in conjunction with some recycling of till, generated $ 317 km 2 of boulder cover. The average erosion depths by removal and reworking of sediment are $ 0.9-1.1 m across areas below 900 m elevation. This study shows that when the cold-based interiors of ice sheets become briefly activated by large subglacial meltwater delivery late in deglaciation, there can be significant reworking and erosion of rock and sediment.

Origin of glacial curvilineations by subglacial meltwater erosion: Evidence from the Stargard drumlin field, Poland

Hermanowski

Piotrowski

2022

Deciphering the origin of subglacial landforms is crucial for understanding processes operating under ice sheets. Due to the inaccessibility of the ice/bed interface limiting real-time observations, the origin of these landforms is typically interpreted from the morphological and sedimentological record exposed after ice retreat, which makes the conclusions conjectural. Here, we focus on a recently discovered new type of subglacial landform called glacial curvilineations (GCLs), whose formation generates some controversy. We use LiDAR imagery and geological data to constrain the origin of GCLs found in a tunnel valley in the Stargard drumlin field, Poland. The tunnel valley is about 3800 m long, 520 m wide, and it occupies about 2 km 2 . A total of 61 individual GCLs have been mapped there; they are up to 550 m long and typically 0.5-1.5 m high, but some reach a height of up to 5 m. The GCLs are composed of mostly massive till represented by either a single or several diamicton units occasionally separated by thin layers of meltwater sand. Till macrofabrics in the GCLs and in the nearby till plain are distinctly consistent, and have high and uniform clustering strengths. At all locations, the dominant till fabric direction broadly corresponds with the ice movement direction given by the drumlin orientations, but it is oblique to the orientation of the tunnel valley and independent of the orientations of the individual GCLs. The lack of correspondence between the orientation of the GCLs and the till fabrics in them suggests that the GCLs consist of an antecedent till deposited during the drumlinizing ice advance and subsequently carved by subglacial meltwater erosion in the tunnel valley. These results substantiate the hypothesis that GCLs are erosional features postdating the material constituting them, and emphasize the role of subglacial meltwater flows as a landforming agent.