Bedrock structures controlling the spatial occurrence and geometry of 1.8Â Ga younger glacifluvial deposits - Example from First Salpausselkä, southern Finland

The glacifluvial deposits within formerly glaciated areas of southern Finland comprise the predominance of well-sorted subglacial and ice marginal sediments. The deposits are less than 100 m thick and form significant aquifers utilized by the respective areas. The spatial correlation of subglacial deposits with bedrock structures, particularly the deformation zones, has been long recognized, but most often not systematically investigated. The purpose of this study was to understand how specific bedrock structures control the position and processes of formation of glacifluvial deposits, using the First Salpausselkä area of southern Finland as a model area. We apply a means of structural analysis to compile structural interpretations (form lines and 3D-surfaces) of the bedrock and correlate the results with the patterns of the glacifluvial deposits and the topography of the underlying bedrock surface. Two major E-W striking shear zones defining abrupt breaks at the bedrock surface along with secondary SW-NE striking splays, originating from the horsetail-like termination of the Somero shear zone, control the deposition of eskers and ice marginal deposits. Based on correlations between the bedrock topography, glacial erosion and sedimentation, we infer that laterally extensive shear zones may have indirectly affected the glacial dynamics within the areas of areal scour more than previously considered. Recognized deformation zones are important for modelling the internal stratigraphy of glacifluvial deposits, their hydrogeological properties and for mapping fresh water supplies within the Nordic countries and other glaciated areas which have undergone substantial tectonic deformation. The development of 3D geologic models is essential for understanding regional-scale correlations between Quaternary sediments and bedrock structures.