Blekinge Coast

The cores shown below were sampled on the same day in a coastal area of the south-western Baltic sea situated in Blekinge, south of the town of Karlshamn and between the bight Pukaviksbukten to the west and the archipelago of Hällaryd to the east.

Radiographs of varved clay below an erosional surface, capped by a thin sand layer with bottom water on top in the coring tubes with the cores 1124 and 1126  from a depth of 24 m south of Sternö and from a depth of 35 m southwest of Tärnö respectively. The distance between these two coring sites amounted to 6.7 km.

Time gaps in the sedimentary sequence due to erosion are characteristic of high-energy environments. Examples of this are given by these two radiographs. Observe the graded density distribution within the clay varves.

A rather thin, mainly sandy and partly gravelly top layer, varying in thickness between 1 and 4 cm, was also found in the other cores sampled on the same day in this exposed coastal area. A flow velocity of about 0.5 m/s at a distance of 1 m above the bed would probably be enough to erode this mainly sandy top layer. It is thus probable that, during heavier storms, this coarser top layer will be transported in suspension and/or as bed load, accompanied by the development of bed forms.

The very large time gap recorded close to the sediment surface in this coastal area indicates that erosion has dominated over deposition for a long time, and that newly deposited fine-grained material therefore has a short residence time in the for waves and currents exposed parts of this area. The void ratio is rather low (around 3) and the resistance to erosion therefore high in the varved clay below the thin, coarse-grained top layer. By contrast, recently deposited fine-grained matter often has void ratios higher than 20 and in some areas even higher than 100, e.g. in the East Gotland Deep of the Baltic Sea. Recently deposited fine-grained, more or less organic, and often underconsolidated matter may therefore be eroded and resuspended by rather weak currents.

Stereoradiographs of the upper part of core 1129 from a depth of 37 m south of Tärnö.

The sedimentary structures in the uppermost part of this core reflect periods with high dynamic activity at the sediment-water interface. The deposits are reworked both due to bioturbation and to erosion. This means that there are time-gaps in the sedimentary sequence.

Click here to get a better resolution of the stereoradiographs.

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