Cyclic sediments (also called rhythmic sediments
) are
sequences
In mathematics, a sequence is an enumerated collection of objects in which repetitions are allowed and order matters. Like a set, it contains members (also called ''elements'', or ''terms''). The number of elements (possibly infinite) is called ...
of
sedimentary rocks
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particle ...
that are characterised by repetitive patterns of different rock types (
strata
In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as e ...
) or
facies
In geology, a facies ( , ; same pronunciation and spelling in the plural) is a body of rock with specified characteristics, which can be any observable attribute of rocks (such as their overall appearance, composition, or condition of formatio ...
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
Sequence stratigraphy is a branch of geology, specifically a branch of stratigraphy, that attempts to discern and understand historic geology through time by subdividing and linking sedimentary deposits into unconformity bounded units on a variety ...
was developed from controversies over the causes of cyclic sedimentation.
Processes leading to 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
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
Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured.