Recognizing bedrock deformation zones through integrated 3D modelling of bedrock topography and sedimentary deposits, Turku, SW Finland

Urban development along the Baltic Sea coast requires high-resolution subsurface information to support effective planning and construction. This study presents a 3D geological model of the subsurface in the City of Turku, SW Finland, based on selected geotechnical investigations, Quaternary superficial deposit maps and LiDAR data. The model comprises three components essential for engineering geological assessments: a digital elevation model of the bedrock surface, the distribution and thickness of coarse grained glacial sediments, and the overlying fine-grained clay deposits. A major part of the analysis focuses on the bedrock surface, which shows a complex and heterogeneous topography shaped by several sets of deformation zones. These structures form linear valley-like depressions that strongly influence the geometry of sedimentary basins and the thickness patterns of both coarse- and fine-grained deposits. The integrated model reveals previously undescribed deformation‑zone geometries, including a continuous NE–SW‑trending system and subsidiary structures, providing new insights into the brittle architecture of the Turku bedrock. The result show how bedrock-controlled depressions have guided sediment accumulation and produced characteristic basin-fills relevant for engineering evaluations. In addition to characterizing the deformation zone framework, the 3D model provides region-wide outputs, including bedrock elevation variations, sediment thickness patterns and the spatial distribution of superficial deposits. These results demonstrate that existing geotechnical investigations, when carefully selected, can effectively support high-resolution subsurface modelling in dense urban environments. When data coverage is limited, additional geotechnical investigations and geophysical surveys are recommended to improve model reliability.