The shell is the average of the three lithostratigraphic groups of the Germanic Triassic. This tripartite division, from which the name for the system Triassic of the Alps but is only in the area north formed ( Germanic Triassic supergroup ). Similar to the other two lithostratigraphic units of the Triassic, the subjacent red sandstone and the overlying Keuper, the term limestone is very confusing for non-specialists. He was formerly ( " contains lime shells or composed of shells is " ) on the one hand as a rock term, on the other hand, as a concept of time used ( " Muschelkalk time " ) in the sense of a step. Today the term is usually used in the sense of a rock unit of lithostratigraphy. The international chronostratigraphic stages ( or time intervals ) of the Triassic were defined in the Alpine Triassic.
The name goes back to Georg Christian Füchsel which it first used in 1761. 1834 led Friedrich August von Alberti, the system of a triad. At this time the shell was already a well-established concept that has already been divided by v. Alberti in the Lower Muschelkalk, the Anhydritgruppe and the Upper Muschelkalk. In the further result of research into the Muschelkalk he was always divided in more detail. Since the 1990s, the shell is now divided into lithostratigraphic formations, their lower limits and distribution are defined.
The lower limit of the shell is defined today in Germany with the lower edge of the Grenzgelbkalkes. At the upper limit in southern Germany, the so-called shell - Keuper- Grenzbonebed. Further north about from Thuringia, this bank is no longer available; here the line is drawn with the first Keuper Sandstone. The lower limit of the Muschelkalk is now dated in the lower Anisian. However, it is not yet decided on the floor of the international chronostratigraphic stage of the Anisian. The upper limit is in the Lower Ladinian ( Triassic ).
In Central Europe originated in a shallow marine basin fossils leading, predominantly calcareous deposits (in the sense of a rock unit ) were later solidified and limestone. However, these rocks contain not only shells, but also brachiopods ( Brachiopoda ). In some locations they are even more common than the eponymous shells. Very often, in some parts of the Muschelkalk also remains of echinoderms, which gave their name even individual subdivisions of the Muschelkalk ( " Trochitenkalk " or today Trochitenkalk lineup, according to the Trochiten = crinoid of sea lilies ). The deposition of the sediments of the Muschelkalk found before about 243-235 million years ago (with an uncertainty of about 2 million years) instead. This corresponds to the international stage and the Lower Ladinian Anisian of the Middle Triassic. The rocks of the Muschelkalk are generally characterized by light gray to beige shades.
The shell limestone is divided into three sub-groups:
- Upper Muschelkalk
- Middle Muschelkalk
- Lower Muschelkalk
The Lower Muschelkalk consists mainly of limestones, calcareous clay and Tonmergeln. In Germany it is now divided into six formations: Jena lineup, Ruedersdorf lineup, Udelfangen lineup, Freudenstadt lineup and Eschenbach lineup.
In contrast, the Middle Muschelkalk contains predominantly evaporites (gypsum, anhydrite and halite ). It is subdivided into three formations: Karlstadt Formation, Heilbronn lineup and Diemel lineup. At the margins of the Grafenwoehr formation was deposited, which extends into the Upper Muschelkalk.
The Upper Muschelkalk in turn is rather characterized by limestones, marls and dolomites. It is subdivided into six formations: Trochitenkalk lineup, Meissner Formation, Warburg lineup, Quaderkalk lineup, Rottweil lineup, Schengen formation Irrel lineup, Gilsdorf lineup and Grafenwoehr Formation, which forms the eastern Marginal Facies of the Muschelkalk.
In places, forming today in areas where rocks are pending the Muschelkalk, collapse dolines. These sinkholes occur preferentially where over time the limestone is leached by water ( surface water, groundwater ) underground ( karst ).
Mining in the limestone
The middle shell (mm ) in Baden- Württemberg in a strip of Heilbronn to the Swiss border, a group with evaporite rock salt, under - and overlain by anhydrite ( converted to gypsum near the surface or leached ). During the mostly locally operated gypsum mining plays no role (formerly mostly used as fertilizer gypsum), rock salt is still mined in large quantities in mines near Heilbronn, Bad Frederick Hall and Haigerloch. On the Upper Rhine rock salt from the Middle Muschelkalk still in Rheinfelden ( Aargau, Switzerland ) and Swiss Hall is recovered by leaching and evaporation by covering their need for salt Switzerland ( with the exception of the canton of Vaud: Salt extraction in the Keuper at Bex ). On the German side of the Rhine no more brine is recovered.
In some regions, the layers of the Upper and the Lower Muschelkalk contain ore-bearing inclusions (so-called " calamine ", oxidized zinc and lead ores in some cases ). In the main, it is here to lead, silver and / or zinc ores. These raw materials that were once highly sought after, were, inter alia, silver used for coinage. Numerous archaeological finds former stone mills and lime kilns degradation and exploitation of limestone rock are historically documented. The documents go to in the first century after Christ returns.
Economic importance and concept definition
Even in our days the Muschelkalk rocks are mined and used commercially (eg in road construction, as ballast, manufacture of cement). In the constructed from limestone ridge Elm in Lower Saxony the quality Elmkalkstein was recovered as a building material.
In historical architectural contexts, the term limestone or Quaderkalk is used as a material designation differently. This consists of a fossiliferous limestone with marked stratification. Building stone of this kind were, for example, mined in Lower Franconia, near Würzburg and Jena in Thuringia in the room. Often it involves the economic use of Terebratelbänke. In older literature, these rock names occur in material contexts often and therefore give no primary safe lithostratigraphical information. This is to be judged according to the context. The use of these terms in its double meaning holds up to the present.
Geomorphology and soils
In parts of the wine regions of Württemberg, Franconia, Thuringia and Baden ( Saale Unstrut) limestone forms the bottom base and often emerges rocky. The often extremely steep slopes are difficult to manage and built in terraces of limestone dry stone walls. The weathering layer of the Muschelkalk is small and powerful risk of erosion.