Biostratigraphy and environment of the Holocene marine transgression in the Heligoland Channel, North Sea

A 51 m deep borehole in the outer Heligoland Channel, south-eastern North Sea, penetrated 17 m of marine Holocene, laminated, clayey silt and silty fine sand followed by 34 m of Pleistocene, fluvial sand. Samples from the two units were investigated for their content of foraminifers. The fluvial sand yielded only reworked foraminifers and, based on these is divided into two parts that can be related to differences in lithology. Based on the study of the foraminiferal assemblages in 20 samples, the marine unit is divided in five foraminiferal assemblage zones reflecting the marine inundation into this part of the North Sea after the last glaciation. The basal part of the unit is strongly influenced by freshwater and the lowermost part is dated to c. 11.0 cal. ka BP. The freshwater influence diminishes uphole with deposition of beds characteristic of a tidal flat environment in an estuarine setting with slowly increasing sedimentation depth. A subsequent sudden change in the fauna indicating higher salinity and a more open-marine environment is interpreted to reflect a change in the hydrographic situation. The basal part of this zone is dated to c. 8.6 cal. ka BP. A continuation of this trend is observed in the following zones with further increasing sedimentation depth. The fauna in the topmost assemblage zone indicates an environment comparable to the present conditions in this area of the North Sea.