Depositional cycle

Cyclic sediments (also called rhythmic sediments) are sequences of sedimentary rocks that are characterised by repetitive patterns of different rock types (strata) within the sequence. Cyclic sediments can be identified as either autocyclic or allocyclic, and can be hundreds or even thousands of metres thick. The study of sequence stratigraphy was developed from controversies over the causes of cyclic sedimentation.

Cyclic sedimentation occurs when there is a repetition of a specific series of connected events that affects the environment the sediments are deposited in. Changes in the environment of deposition change the type and amount of sediments that are deposited, resulting in different sedimentary rock types. A series of connected events produces a distinctive and repetitive series of strata that are arranged vertically. At least one rock type, which is regarded as the starting point, must be repeated.

Based on the processes that generate the cyclic deposits, two types of sedimentary cyclic successions — autocycles and allocycles — can be distinguished.

Autocycles are cyclic sediments that are created by processes that only take place within the basin that the sediments are deposited in. Tides and storms are examples of the types of processes that create autocycles. Autocycles show limited stratigraphic continuity.

An example of autocyclic sedimentation on a carbonate platform by Septfontaine M. (1985): Depositional environments and associated foraminifera (lituolids) in the middle liasic carbonate platform of Morocco.- Rev. de Micropal., 28/4, 265-289. See also www.palgeo.ch/publications.

"Shallowing upward" metric first order cycle in the Middle Liassic platform of the High Atlas, Morocco. Algal dolomitized laminations on top.

Recent equivalent of an emersive autocyclic cycle in a lagoonal to supratidal environment. Algal mats in yellow. Tunisia.

Stalactitic cement from a sample in the supratidal zone in vadose environment (air within the sediment), top of regressive cycle, middle Lias, High Atlas. Thin section, L = 0,5mm.

...
Wikipedia