Sea chalk

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Alternating storage of lake chalk deposits (light) and cultural layers (dark) in the building pit of the Opera car park in Zurich

Lake marl (also Seekalk , Alm , Wiesenkalk , meadow marl ) is the chalk marl very similar limnic sediment which with up to nine meters thick limestone on reason, lakes , partly below mud, peat and sand deposits found. The lime content of sea chalk is> 95%. For this reason, sea chalk can no longer be assigned to the Mudden , as these are defined by an organic share of> 5%.

The origin is mainly due to biogenic decalcification , in particular also by calcareous algae (compare for example: chandelier algae ), which precipitate calcium carbonate ( Ca C O 3 ) from lime-rich water , which is deposited on the lake floor. In addition, even without such biogenic processes, larger lime precipitates can occur if the chemical equilibrium is shifted when lime-rich groundwater enters the water (see water hardness # lime-carbonic acid balance ). Shell fragments of conchylia also contribute to the composition . A coarse-grained sea chalk caused by such shell remains is known as Schnegglisand in the area of Lake Constance . Alluvial bogs are more often based on sea chalk, which indicates the existence of an open body of water before the bog formed. Deep-sea chalk is the term used to describe the paragraph on the bottom of the oceans, which petrographically belongs to the marls .

From a geotechnical point of view , sea chalk is extremely problematic, as the stability or load-bearing capacity is very low due to the high water content and therefore appears as a sliding horizon. If such unstable layers are built on, the substrate can lose its strength. An example of a catastrophe that can be traced back to the geotechnical properties of the sea chalk is the so-called “ suburban catastrophe in the catastrophe bay ” on July 5, 1887 in the city of Zug (Switzerland).

Individual evidence

  1. Near-natural bank areas and shallow water zones of Lake Constance. Ministry of the Environment Baden-Württemberg; State Institute for the Environment, Measurements and Nature Conservation Baden-Württemberg

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