Cyclothems

A Zyklothem (Greek: κύκλος ( Kyklos ) = circle and θέμις ( Thémis ) = order ) is in the geology of a recurring sequence of sediments. The term was coined in 1930 by J. M. Weller. First, mainly applied to rocks of the Carboniferous, the term was commonly used in many later repeated rock sequences. The individual cycles are often separated by a discordance.

Examples

Zyklotheme are common in the Carboniferous of the Ruhr area. Here there is a regular alternation of mudstones, sandstones and coal, which have arisen partly in the sea, partly on land. From top to bottom, ie, from younger to older, these include the following rocks:

6 coal seam

5 mudstone with root fragments

4 mudstone with sandy layers and strips

3 sandstone

2 mudstone with sandy layers and strips

1 mudstone with fossils of sea, brackish water from lakes or

Also from other Carboniferous deposits Zyklotheme have been described, some of which have a different design than that of the Ruhr area. To show rock consequences of the North American Carboniferous (Anna Shale, Mound City Shale, Hushpuckney Shale, Heebner Shale and Hughes Creek Shale ) a Zyklothem sequence that begins with transgressive limestones, continues over phosphatic shales and regressive limestones and finally non-marine, different trained and partly sandy shale sequences shows.

Significantly younger the cyclic sedimentation in Lofer - Zyklothem the Keuper of the Alps, for example in the Dachstein Limestone. Here as seen from bottom to top a breccia or a marl layer over thin layered or banded limestone, which in turn is again overlaid by a powerful, thick banked limestone. The surface of this limestone has many columns, where in some cases the breccia of the next cycle are already received. This sequence is found in many repetitions.

Even younger Zyklotheme also be described from the Jurassic of Portugal.

Causes

The cause of the formation of Zyklothemen is generally a cyclic sedimentation that comes about because of recurring same deposition conditions. The reasons for such a sedimentation are seen by some geologists in endogenous processes, such as tectonic uplift and subsidence, possibly also long-period pulsations, while others make exogenous processes responsible, such as sea-level rise as a result of glaciations. The cause in question are also combinations of exogenous and endogenous processes.