New geological aspects for freshwater seepage and formation in Eckernförde Bay, western Baltic

The subsurface geology relevant to the submarine freshwater seepage in Eckernförde Bay has been investigated using shallow seismic instruments and vibrocoring. Detailed surveying revealed that the pockmarks are aligned like pearls on a string or densely clustered in furrow-like structures depending on the glacial and postglacial setting of the underlying strata. Two possible aquifers have been verified: The older Miocene sand aquifer is partly sealed by a till unit forming the central part of the Mittelgrund. The younger aquifer consists of a mixture of glacial till and meltwater sediments partly sealed by till and partly by lateglacial galciolacustrine silt and clay sediments. The investigations imply that connections exist between the aquifers and that groundwater leakage takes place in the marginal zones of the bay due to thinning and coarsening of the sediment composition of the lateglacial seal. Within the seepage areas, the pockmarks are restricted to areas covered by unconsolidated Holocene mud of low thickness' that are easy to penetrate by artesian groundwater. Macrofossil studies and AMS ¹⁴C dating of the lateglacial and Holocene units reveal that the Mittelgrund shoal of glacial origin has been modified by coastal processes and formation of cuspate foreland deposits during the subsequent palaeo-lake phases of 15-20m below the present sea level (b.s.l.). The lake phases correlate in time with the regional Baltic Ice Lake highstand about 10,000 ¹⁴C years BP and the Ancylus Lake highstand about 9200 ¹⁴C years BP. This means that local contemporary lakes existed or the western margin of the regional lakes can be moved considerably further west than expected hitherto. In the earliest phase of the Littorina Sea transgression, the Mittelgrund shoal was exposed to coastal erosion once more before the final drowning and the initiation of mud sedimentation in the surrounding basins took place.