Rügen chalk (German: Rügener Kreide or Rügener Schreibkreide) is the common name for a very pure, very fine-grained, white, crumbly and highly porous chalk that forms the highest member of the German Upper Cretaceous, and is of Maastrichtian age. It is found exposed in cliffs on the coast of the Jasmund peninsula in the northeast of the island of Rügen in Mecklenburg-Vorpommern.
In addition, Rügener Kreide is the trade name for the white pigment that is extracted from these limestones.
Stratigraphy
Lithostratigraphically, the Rügen chalk belongs to the Hemmoor Formation of the Chalk Group and is formally defined as the Rügen Member within this formation. Chronostratigraphically, the Rügen Member falls into the late lower to late upper Lower Maastrichtian (approx. 70 million years ago). Chalk widely occurs in the subsurface of Northern Germany, but is largely overlain by several hundred metre thick Cenozoic deposits.
Features
Chalk limestones are often layered. The Rügen chalk, however, is unlayered and massive. The rock is extremely fine grained and very low cementation, which is associated with a high porosity. The low cementation and high porosity mean that the rock can soak up a lot of water. While dry Rügen chalk is relatively crumbly but still brittle, water-saturated Rügen chalk is more plastic, almost like moist clay, and can be cut with a knife. Like many other limestones of the Upper Cretaceous, the Rügen chalk has a very high content of calcium carbonate (CaCO3, "carbonate of lime"). With the Rügen chalk the calcium carbonate content is at least 97%. The vast majority of calcium carbonate is in the form of micrometer-sized low-Mg calcite platelets, so-called coccoliths, which are remnants of unicellular, planktonic calcareous algae, so-called coccolithophores. The calcareous algae lived around 70 million years ago in the light-flooded surface water of a sea that covered northern Germany. After their death, they sank to the bottom of the sea and, over time, formed huge deposits of limestone mud. This lime sludge has come down to us today in the form of the Rügen chalk.
Another characteristic feature of the Rügen chalk is its high content of flint concretions. These are usually formed as nodules and enriched in individual horizons. Sometimes there are also flat formations of different thicknesses. Particularly large, cylindrical flint concretions are known as "Sassnitz flower pots" (see also → Paramoudra). The silicon dioxide (SiO2), from which the flint stones were formed, originally came from single-celled plankton (radiolarians, diatoms), which lived together with the calcareous algae in the Chalk seas. After the deposition of the coccolith sludge containing radiolaria and diatoms, the SiO2 of the diatoms dissolved in the water that circulated in the pore space of the sediment and was again precipitated elsewhere in the sediment. These deposits are seen today as the flint stones. Since the flint is clearly more weathering- and erosion-resistant than the limestone, it collects on the beach of the chalk cliff and is transported to neighboring areas of the coast by waves and currents during storms and deposited there. The Feuersteinfelder at Prora are particularly well-known in this regard.
Occurrence
There are extensive chalk deposits in the northeastern part of the island of Rügen, on the peninsula of Jasmund. The best known are probably the "Rügener chalk cliffs" of the cliffed coast in the vicinity of Sassnitz (see also → Stubbenkammer, → Königsstuhl). These coastal formations are part of the Jasmund National Park and are under strict nature protection.
In the hinterland of the Jasmund, chalk is also found under a 1 to 10 m thickoverburden in the subsoil. There are many quarries and open pit mines, almost all of which have since been abandoned due to inefficiency or for reasons of landscape protection. Overflowing with water, they can be seen in aerial photos of Jasmund as small and large lakes and ponds (the currently largest is the “Kreidesee Wittenfelde”). Even in the urban area of Sassnitz one can still see old chalk quarries, but they date from the beginning of the 19th century. Chalk is currently being mined in the Promoisel open pit mine.