Discordance

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Angular discordance in the Geological Garden Bochum : weakly solidified sands of the Upper Cretaceous (Essen green sand) on tilted sand, silt and clay stones of the Upper Carboniferous (Bochum Formation). The corresponding layer gap covers a period of around 200 million years.

As unconformity (after latin discordans "non-compliant") is referred to in the geology , the angular or irregular are superposed rock layers. Discordance is the opposite of concordance in stratigraphy .

Various processes can be the cause of discordance:

  • An angular discordance , or tectonic discordance , usually forms in sedimentary rocks that are tilted by tectonic movements such as expansion, compression, folding , uplift and depression, or by singular volcanic events. The raised rocks are then eroded , for example when the sea level falls ( regression ). Later sediments, which can be deposited when the sea level rises again ( transgression ), are again horizontal and consequently form an angle with the older rock layers . There is now a so-called layer gap between the two rock packages (the hanging wall “above” and the horizontal wall “below”). The exact reconstruction of the relationships between deposits and discordances is of great importance for understanding the temporal development of a mountain range ( orogeny ).
  • If the gap ( hiatus ) was not created by tectonic movements, but only by erosion or sedimentation interruption, for example when a river cuts into the subsoil and the self-created valley fills with sediments, one speaks of erosion discordance .
  • The so-called cross stratification, or oblique stratification, which occurs, for example, during sedimentation in running water, is sometimes referred to as sham discordance .
  • A dislocation discordance arises from the shearing ( dislocation ) of rock packages along faults . In ceiling tectonics, this is also referred to as angled cutting .
  • Magma passages and salt domes often also penetrate the secondary rocks with an angle or with irregular contact surfaces.

See also

Web links

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