Exchange storage

from Wikipedia, the free encyclopedia
Folded turbiditic, relatively close alternation of limestone and dark claystone, Loughshinny formation, Lower Carboniferous , coastal outcrop on the Irish Sea near Loughshinny (Ireland)

Under alternation is in the geology a layering understood in the alternating layers (usually of sedimentary rocks ) having significantly different petrographic properties ( lithologies ), for example a distinctly different chemical composition, consecutive. A layer sequence constructed in this way is referred to as an alternating sequence . In the case of sequences in which only two different lithologies alternate, one speaks of alternating layer changes .

Terminology

The term alternating position can already be found in Carl Friedrich Naumann's first volume of his textbook on geognosy (1850), in which he ascribes it to Karl von Raumer .

Vortisch (1930) defined the umbrella term “genetic” stratification in general as alternating position. However, a stratification in the strict sense of the word is only considered to be a stratification in which at least two lithologies are approximately equally involved in the formation of the sequence and are each clearly perceptible as strata in the exposed rock structure.

features

Unfolded limestone-mudstone interchange, lower section of the Blue Lias Formation , Lower Jurassic , coastal outcrop at Lavernock Point on the Bristol Channel , Wales

Typical sequences of changes are, for example, sequences of marl and limestone . In the ideal case, such an alternating bedding on a quarry wall can look like a zebra crossing: for example, a twenty centimeter thick dark gray marl layer is followed by a half meter thick light limestone bank, which in turn is overlaid by a marl layer and this again by limestone and so on. Such evenly banded layer packages can consist of hundreds of alternating layers and can be several hundred meters thick.

Alternating storage often reflects a cyclical change in sedimentation conditions . Lime-marl alternations represent the cyclical replacement of high carbonate production due to the stronger influence of terrigenic background sedimentation, which is usually attributed to an increase in sea depth and distance from the coast at the time of deposition at the corresponding deposition site (cf. →  Eustasia ). Sequences of change can also arise from recurring special events, for example from the deposition of coarse-grain turbid sediments , which regularly interrupt the otherwise calm and continuous background sedimentation of clay . Such storage conditions can be found, for example, in the Kulm facies of the Central European Carboniferous .

Individual evidence

  1. ^ Hans Murawski, Wilhelm Meyer: Geological dictionary. 12th edition. Spektrum Akademischer Verlag, Heidelberg 2010, ISBN 978-3-8274-1810-4 , p. 187
  2. ↑ Carl Friedrich Naumann: Textbook of Geognosy. First volume. Verlag von Wilhelm Engelmann, Leipzig 1850, urn : nbn: de: bvb: 12-bsb10284276-7 , pp.  903 , 934
  3. ^ Wilhelm Vortisch: Cause and classification of the stratification. Yearbook of the Federal Geological Institute. Vol. 80, 1930, pp. 455–496 ( PDF 2 MB)