The fluvial style changes during the Upper Pleistocene and Holocene: case stydy of Ploučnice River, Northern Bohemia !" # $ The fluvial style changes during Upper Pleistocene and Holocene: case study od Ploučnice River, Northern Bohemia
Abstract
Sedimentary record of the Ploučnice River system, which is situated in the northern part of the Bohemian Massif, provides remarkable data dor the study of climatic changes since the Upper Pleistocene. Narrow and flat floodplain, which is restricted by the steep wall from Cretaceous quartz sandstones of Jizera Formation was formed. This type of floodplain represents a special case in between the wide deeply incised valley downstream in the Děčín area and narrow deeply incised valley of a mountain river with poorly developed floodplain deposists like in upper part of Ploučnice stream on the Ještěd Ridge. The Ploučnice floodplain is flat, wet and silty and many paleomeanders occur there. Near the floodplain margin we found a system of sandy and gravelly elevations and paleochannels interpreted as relicts of fluvial processes during Upper Pleistocene/Lower Holocene transition. This assessment is supported by local relief, sedimantary record from hand-made boreholes and pollen analysis of paleochannel infill. We recognized four developmental phases of the river:
1. Upper Pleistocene - braided floodplain represented by gravels and medium grained quartz sand.
2. Late Glacial/early Holocene transition - anabranching river environment represented by a succession of silty, fine to medium-grained sand with gravel.
3. Middle Holocene period-abandoned anabranching channles associated with desiccation of channel infill and incision into the previous main terrace infill.
4. Late Holocene (from Middle Ages to present) . fluvial style of meandering river environment with agradation phase represented by sediments consisting of very fine to medium grained sand witch passes downwards to sandy silt with organic matter. According to unpublished palynological data the base of the paleochannel infill indicatess Lower Holocene period followed by Middle Holocene sedimentation hiatus until the Medieval Age and upper part od sedimentary infilling belongs to Middle Ages until the present times.