SEDIMENTOLOGY AND 3-D MODELING OF AQUIFER PROPERTIES OF THE SÄKYLÄNHARJU-VIRTTAANKANGAS INTERLOBATE ESKER IN SW FINLAND

Interlobate eskers, like the Säkylänharju-Virttaankangas complex, are often exceptionally large in size compared to other eskers and represent the most important aquifers in recently glaciated areas. Because of thick deposits and complex sedimentology due to formation in varying depositional environments, interlobate eskers are difficult to model for groundwater applications. Therefore, we must employ a set of facies models developed for various depositional environments influenced by glaciofluvial processes. It is also often necessary to evaluate the impact of shore processes that may have considerably altered the morphology and structure of the esker. Unfortunately, representative sedimentological data is often unavailable for extensive deposits, and determination of aquifer properties must rely on synthesis of various data sets including interpretation of sedimentary structures from exposures and GPR surveys supported by drill hole logs. It is thus crucial to interpret the succession of depositional stages with division into glaciofluvial and glaciolacustrine stages. These govern the main grain-size distribution and relate to large-scale hydrogeologic trends affecting the formation and flow of groundwater, and also define the physical limits of the hydrogeological units. The depositional environments are inferred from the grain-size characteristics in relation to large-scale sedimentological key elements like cross-bedded units, thick cosets of cross-bedding, channel-structures, deformation structures and arched architecture of esker cores. Due to high sedimentation rates and rapidly changing depositional conditions, interlobate eskers may also show better preservation of seasonal-to-annual sedimentation patterns. Bedrock topography controls the location of coarse esker cores and related transverse deposits, but also direction of ice-frontal deltaic structures. A bedrock elevation model is therefore needed to understand sedimentation patterns during the depositional stages. Finally, the depositional model is to be tested with hydrological data. The classification of the deposits into 3-dimensional hydrogeological units should be done to a proper scale, which emphasizes the continuity of the units and the major groundwater flow properties between the units.