Cyclic sediments (also called rhythmic sediments) are sequences of sedimentary rocks that are characterised by repetitive patterns of different rock types (strata) or facies within the sequence. Processes that generate sedimentary cyclicity can be either autocyclic or allocyclic, and can result in piles of sedimentary cycles 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 the depositional environments change repeatedly. Changes in the environment of deposition influence the type and amount of sediments that are deposited, producing different sedimentary rock types. 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 can be distinguished.

Allocycles are sedimentary cycles caused by processes outside of the depositional system and that involve forced oscillations of the sedimentary system; in this case the cyclic succession registers some features of the forcing process (Sea level fluctuations, climate oscillations or tectonic activity). Allocyclic successions can extend over great distances and are not limited to a single depositional basin.

Changes in sea level can create cyclic successions of limestones, shales, coals and seat earths. For these cycles to have been formed, the environment at the site of deposition must have been changing radically from marine to deltaic, then lagoonal and then continental. One cause of sea level change is the expansion or shrinking of continental glaciers caused by climate change. Tectonic movements can affect the environment of deposition as well, by changing the local relative sea level. Metric sedimentary cycles could be related to an astronomical (Milankovitch) influence on 20.000 to 400000 years time scales (see Cyclostratigraphy). But these beds are of no use in correlation and should not be taken as a "high resolution" tool for stratigraphy without a severe biostratigraphic control.

Seasonal changes in weather can create cyclic sediments in the form of alternating bands of clay and silt (also known as varves). For example, in a glacial region where sediments are deposited in a lake, coarse sediments that are trapped in ice are released when the ice melts in the summer. This creates paler, coarser silt bands in the lake deposits. In winter, melting is at a minimum, meaning that only fine material is supplied to the lake, causing thin clay layers. Varves form through an allocyclic process, but because the cycles are limited to the depositional basin, the lateral extent of the resultant strata are limited.

Autocycles are sedimentary cycles that are created by processes that only take place within the basin of deposition and that involve free oscillations of the sedimentary system; indeed, the resulting cyclic succession is only function of the geometrical and sedimentary parameters characteristic of the depositional system (e.g.: shelf dimension and shape, carbonate productivity, etc.). Autocycles show limited stratigraphic continuity.

An example of autocyclic sedimentation on a carbonate platform was provided 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.