Cyclothema

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A cyclotheme ( Greek κύκλος (kýklos) = circle and θέμις (thémis) = order) is a recurring sequence of sediments in geology . The term was coined in 1930 by JM Weller. First, especially in rocks of Carboniferous applied the term was later used generally in many cases repeated rock sequences. The individual cycles are often separated from one another by a discordance .

Examples

Cyclothems occur frequently in the carboniferous region of the Ruhr area . Here there is a regular alternation of claystones , sandstones and coal , some of which were created in the sea and some on land. From top to bottom, i.e. from younger to older, these are the following rocks:

6. Coal seam
5. Mudstone with remains of roots
4. Mudstone with sandy layers and strips
3. sandstone
2. Mudstone with sandy layers and strips
1. Mudstone with fossils from the sea, brackish water or from lakes
6. Coal seam

Cyclothemes have also been described from other carbonic deposits, some of which have a different form than that of the Ruhr area. For example, the North American Carboniferous rock sequences ( Anna Shale , Mound City Shale , Hushpuckney Shale , Heebner Shale and Hughes Creek Shale ) show a cyclothemic sequence that begins with transgressive limestones , continues over phosphate- containing shales and regressive limestones and finally non- marine , differently shows developed and partly sandy slate sequences.

The cyclical sedimentation in the Lofer cyclotheme of the Keuper of the Alps , for example in the Dachstein limestone, is much more recent . Here, from bottom to top, a breccia or a layer of marl can be seen , on top of thinly stratified or banded limestone, which in turn is overlaid by a mighty, thickly banked limestone. The surface of this limestone shows numerous fissures, in which in some cases the breccias of the next cycle have already been preserved. This sequence occurs in many repetitions.

Even more recent cyclothems are also described from the Jura of Portugal .

causes

The cause of the formation of cyclical themes is generally a cyclical sedimentation , which occurs due to the same deposition conditions that are repeated over and over again. Some geologists see the reasons for such sedimentation in endogenous processes such as tectonic uplifts and subsidence, possibly also long-period pulsations , others in turn blame exogenous processes, such as sea level rise, for example as a result of icing . Combinations of exogenous and endogenous processes are also possible causes.

Individual evidence

  1. ^ A b Hans Murawski, Wilhelm Meyer: Geological dictionary . 11th edition. Elsevier / Spektrum, Heidelberg 2004, ISBN 3-8274-1445-8 , pp. 262 .
  2. ↑ Evelyn Christine Suchi: Reconstruction of the evolution of the marine nutrient supply in the course of the Upper Palaeozoic with special consideration of the nitrogen isotopic composition . Dissertation Westfälische Wilhelms-Universität Münster, 165 p., 2005 (pdf; 2.12 MB) ( Memento from July 18, 2013 in the Internet Archive )
  3. ^ A b Alfred G. Fischer: The Lofer Cyclothems of the Alpine Triassic. In: DF Merriam (Ed.): Symposium on cyclic sedimentation. Kansas Geological Survey, Bulletin 169, pp. 107-149, 1964
  4. ^ A b Michael Reyle: Facies and basin development in the Oxfordian (Upper Jurassic) of the central Lusitan basin (Portugal). Dissertation University of Stuttgart, 282 pages, 2004 (pdf; 4.35 MB)