70 / 2022-01-23 17:30:47

Sandy patches in a silty Holocene lagoon (central Netherlands): the role of Pleistocene palaeogeography

待补充

The brackish lagoon in the central part of The Netherlands was closed by a dike in 1932 and gradually changed into a lake. Parts of this lake were reclaimed and the surficial sediments of one of the polders (Noordoostpolder), reclaimed in 1942, has been investigated in detail by mapping of the walls of some 1000 km of drainage ditches of approx. 1.5-2 m deep. It appeared that the sediments consist of an uncommonly large amount of silt, to different degrees mixed with reworked peat that had developed during the Holocene transgression. Some sandy deposits occur locally, at places that are at first site distributed in a haphazard way. The sands have different characteristics (grain-size distribution, rounding of the grains, mineral composition). These sands cannot have been introduced from the sea, nor can they have been supplied by the rivers that discharged into the lagoon, so they must have a local origin. It is found that several types of Pleistocene sandy or diamictic deposits below the Holocene peat and lagoonal sediments had an irregular topography and became eroded during the Holocene by wave activity. This resulted in sandy deposits around these Pleistocene highs, which consisted of glacial tills (boulder clays), river dunes (formed by aeolian activity along rivers under dry permafrost conditions) and coversand ridges, formed by aeolian activity in a belt between the ice margin and the more distal loess belt.