A VALLEY-FILL SEQUENCE IN OLD BEDROCK VALLEYS PROVIDE EVIDENCE FOR COMPLEX CYCLICAL INTERLOBATE AND ICE-MARGINAL FLUCTUATIONS DURING THE LAST DEGLACIATION IN WEST-CENTRAL FINLAND

Next to the town of Kurikka in western Finland, drilling, seismic and gravity surveys for groundwater research have revealed a 50–110m deep, and at least 20-km-long valley system cut in Paleoproterozoic gneisses of the Southern Finland craton, which underwent multi-stage uplift, erosion, relief reduction and burial after the Svecofennian orogeny (1.76 Ga). These old valleys are now filled by highly heterogenous Pleistocene sediments, which provide useful stratigraphic records to reconstruct glacial events in the region. Here, we present an analysis of the buried bedrock valley sequence.

The bedrock surface is derived from gravity surveys. The valley-fill Pleistocene deposits were modeled in 2D and 3D based on surficial geology maps, ground penetrating radar, seismic surveys, and numerous boreholes strengthened by downhole geophysics. The new subsurface datasets have revealed a sequence consisting of ~15 different sedimentary beds, which consist mainly of diamictons of variable texture, especially in the upper portion of the sequence. Chaotic gravel patches occur proximal to a ridge-shaped sediment accumulation. At more distal locations, coarsening-upward successions of variably sorted material (silt, sand and gravel) occur. Several borehole logs appear to show cyclical successions. Results also show important lateral discontinuities and facies transitions. The bottom of the sequence, which include over-consolidated till beds overlain by the lowermost gravel unit are most likely from older glaciations, whereas the rest of the overlying sequence is assigned to the Late Weichselian glaciation. This part of the valley-fill sequence is interpreted to record the dynamic evolution of an interlobate system that formed between the Western Finland ice stream lobe and an isolated less active part of retreating ice that existed in the Southern Ostrobothnia region during deglaciation. Later in the deglaciation, the valley became an ice frontal margin where more sediments were deposited in a proglacial basin.