Senkvartär stratigrafi och utveckling i Örseryd, mellersta Blekinge

Sammanfattning: Summary: The aim of this investigation was to study the Holocene history of a river valley near the village of Örseryd, situated in the middle part of the county of Blekinge in South Sweden. Covering 2000 x 600 m the area was investigated by survey mapping. Grain size analysis were made on representative samples and the different sediments were correlated to lithostratigraphic units defined in earlier investigations in Blekinge by Lagerlund and Björck (1979). Threepeat land basins were investigated in detail. Transect corings were made in all areas. Different methods were used to analyse the cores, including mineral magnetic methods and organic content analysis. Radiocarbon dating and pollen analysis were made on one main core to obtain the chronological frame for the valley development. Pollen analysis and 14C dating were also used in adjacent areas to determine when the different areas were overgrown by swamp fen. The southern basin can be interpreted as a kettle hole. An ice block remained in the area after the last deglaciation. When the ice melted a lake was formed. There has been a running through of the river. The bottom sediment consisting of gravel is by pollen analysis correlated to the Younger Dryas zone, wich means that sedimentation started 11,000-10,000 years 14C BP. A more organic clay gyttja, correlated to the Preboreal zone (10,000-9,000 BP), is covering the bottom sediment. Around 9,500 years BP the water level dropped. This is indicated by an erosive gravel layer found in the valley bottom stratigraphy. During the Boreal period the water level rose again. The lake sediments in the central part consist of fine detritus gyttja and further up of coarse-detritus gyttja with branches and roots. Only a small lake existed in the valley 4,000 years BP. This lake was then overgrown by a swamp fen, wich is indicated by wood-rich fen peat. During the late Iron Age a declining tree pollen curve indicates clearings. The river shores have probably since then been used as pasture. An increasing amount of cereal pollen during the Middle Age indicates cultivation of the land around the peat areas. In the uppermost part of the peat layer minerogenic matter increases probably because of cultivation in the area wich caused soil erosion in the valley. The northern part near the Bräkneå river was a shallow bay of the lake in the south. It was overgrown earlier (around 7000 years BP) than the southern part. The western basin was a small lake separated from the river valley. This lake was overgrown about 5000 years BP.