Zechstein

The Zechstein (German either from "mine stone" or "tough stone") is a unit of sedimentary rock layers of Middle to Late Permian (Guadalupian to Lopingian) age located in the European Permian Basin which stretches from the east coast of England to northern Poland. The name Zechstein was formerly also used as a unit of time in the geologic timescale, but nowadays it is only used for the corresponding sedimentary deposits in Europe.

The Zechstein lies on top of the Rotliegend; on top of the Zechstein is the Buntsandstein or Bunter.

Formation

The evaporite rocks of the Zechstein formation were laid down by the Zechstein Sea, an epicontinental or epeiric sea that existed in the Guadalupian and Lopingian epochs of the Permian period. The Zechstein Sea occupied the region of what is now the North Sea, plus lowland areas of Britain and the north European plain through Germany and Poland. At times the Zechstein Sea may have connected with the Paleotethys Ocean through southeastern Poland; the point is disputed by researchers.

Though located, in its own era, near the Earth's equator (where high temperatures and arid conditions facilitated evaporation), the sea's inception likely stemmed from a marine transgression rooted in a phase of de-glaciation; the southern portion of Pangaea, the former (and future) Gondwanaland, supported ice sheets in the early Permian. The eventual disappearance of the Zechstein Sea was part of a general marine regression that preceded and accompanied the Permian-Triassic extinction. [Glennie, Kenneth William, ed. "Petroleum Geology of the North Sea: Basic Concepts and Recent Advances." London, Blackwell, 1998; pp. 160-75.] [Moores, Eldridge M., and Rhodes Whitmore Fairbridge, eds. "Encyclopedia of European and Asian Regional Geology." London, Chapman & Hall, 1997; pp. 97, 263.]

tratigraphy

The Zechstein is usually given the status of a lithostratigraphic group and as such encompasses a number of geologic formations.

The Zechstein consists of at least five depostional cycles of evaporite rocks, which are labelled Z1 to Z5 respectively. The lithologies found are halite ("rock salt"), anhydrite, dolostone and shale.

Economic importance

The Zechstein has significant economic importance in the North Sea Oil province. In the southern gas basin it forms the main cap rock to the gas fields with Rotliegend reservoirs. It also forms a reservoir in the Auk oilfield in the central part of the North Sea. Further north the Zechstein salt becomes diapiric, forming salt domes which form the structure for several oil fields, such as Machar. Zechstein dolomites outcrop near the coast of County Durham, England where they are known as the Magnesian Limestone.

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