Allostratigraphy

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Allostratigraphy , also known as baffle stratigraphy or lithographic baffle stratigraphy, is a method of stratigraphy in geosciences. It is closely related to lithostratigraphy , which should actually be called more precisely lithofacies stratigraphy, and to sequence stratigraphy .

history

The division of rock units using allostratigraphic methods is already quite old. However, the term allostratigraphy, which is widely used today, has not yet been used. This term was proposed and defined by the North American Commission on Stratigraphic Nomenclature (NACSN) in 1983.

definition

Allostratigraphy or guide surface stratigraphy describes lithological units with the help of almost simultaneous events documented in the rock unit. These can be: large-scale stratified areas, "lead benches", incident situations (e.g. tempestite , tuff and tsunamite ) and (erosion) discordances. These rock units defined in this way, like the purely lithologically defined lithostratigraphic rock units, are subject to the facies rule, ie they are defined by above-below relationships and spatial limitations. In contrast to pure lithostratigraphy, which are used for mapping and description, allostratigraphy uses quasi-isochronous rock boundaries. The Leitbank stratigraphy, which is mainly used in the Germanic Triassic, is also an allostratigraphic method. The structure and correlation of layers with cyclically repeated rock formations (cyclostratigraphy) is another allostratigraphic method. Closely related to allostratigraphy is sequence stratigraphy , which is also not structured purely lithologically, but is based on cyclical changes in lithological features that can be caused by changes in sea level, climatic changes or tectonics.

advantages and disadvantages

Allostratigraphy has several advantages over pure lithostratigraphy. With sedimentological models, a lateral and vertical development of the facies can be predicted to a certain extent. Another advantage can be seen in the fact that the limits of allostratigraphic units can be correlated very well with the log units in the borehole measurements or can be recognized in log curves. A disadvantage is that the allostratigraphic sequences e.g. Some of them are not easy to map because the allostratigraphic boundaries, which are barely detectable in the terrain and under difficult exposure conditions, often go right through lithological units. In the ideal case, allostratigraphic boundaries complement each other very well with the chronostratigraphic methods.

Units of Allostratigraphy

The basic unit of allostratigraphy in German usage is the result. This term was chosen in deliberate contrast to lithostratigraphy. Often, however, the lithostratigraphic formation and the allostratigraphic sequence are identical, e.g. B. in the middle red sandstone, where the discontinuity surfaces are also associated with a significant change in the rock formation. In other cases this is not the case. In English usage it was suggested to add the addition allo- to the lithostratigraphic units (member, formation, group) (NACSN 2005 § 59). However, this nomenclature has not yet caught on.

literature

  • Manfred Lutz: Leitflächen- (Allo-) Stratigraphy and its application in the Keuper. In: Gerhard Beutler (Coord.): Stratigraphy of Germany IV Keuper. Courier Research Institute Senckenberg, 253: 94-105, Frankfurt / M. 2005 ISSN  0341-4116
  • North American Commission on Stratigraphic Nomenclature (NACSn): North American stratigraphic code. American Association of Petroleum Geologists Bulletin, 67: 841-875, Tulsa, Oklahoma 1983 ISSN  0149-1423
  • North American Commission on Stratigraphic Nomenclature (NACSN): North American stratigraphic code. American Association of Petroleum Geologists Bulletin, 89: 1547-1591, Tulsa, Oklahoma 1983 ISSN  0149-1423 PDF