Stratigraphic sections show repetitions of similar or identical conditions on all scales from millimeters to many hundreds of meters, representing time intervals from possibly seconds to many millions of years. Any such repetitions have been called cycles. To understand the reason why cycles occur, a time scale is essential. Beds can be with or without time information: the latter are known as event stratifications. The event, which triggers the formation of beds or any succession of sediments, could be a random effect, or it could be part of a mechanism in which cyclic behaviour is due to an oscillating physical system. Such systems are dynamic systems, which means that they involve the transport of masses over distances and in definite time intervals. Cycles generated by such systems can be used as time units. A special example is the earth solar system which can generate cycles from daily periods to orbital variations of thousands and millions of years. Orbital variations are known as Milankovitch cycles and they are the subject of cyclostratigraphy proper. The recognition of Milankovitch cycles or of any other cycle associated with oscillating systems, rests on the regularity or time periodicity of such patterns. The correct ways of analysing such cycles are the methods of time series analysis. Repetition patterns representing time intervals of millions and possibly many hundreds of millions of years, are known from sequence stratigraphy. We know as yet very little about the mechanisms which generate such long periods of repetition. It has been suggested that the earth's crust has reached a state of self organized criticality (Bak, P., 1996. How nature works. Springer, New York, 212 pp.), in which case sequence formation could be part of the complexity which undoubtedly is an attribute of the earth's crust and history.
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