Autocycles[edit]Autocycles are cyclic sediments that are created by processes that only take place within thebasinthat the sediments are deposited in.Tidesandstormsare examples of the types of processes that create autocycles. Autocycles show limitedstratigraphiccontinuity.[3]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.

High Atlas middle liassic carbonate platform of Morocco and succession of regressive, autocyclic, "shallowing upward" metric sequences.

"shallowing upward" sequences from two sections distant of 230 km; note the hurricane (tempestites and tsunami?)levels with abundant displaced foraminifera on supratidal flat. Middle Liassic, Morocco.

Model of a virtual "shallowing upward" metric sequence observed on carbonate platforms all along the south tethyan margin (about 10.000km) during the Liassic. (Micro)fossils are identical from the Maghreb till Oman and furtheron. "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. Hurricane breccia cemented on the surface of a bed, top of a "shallowing upward" cycle. Middle Lias, High Atlas. Top of a metric regressive cycle withdesiccations, surface of bed dolomitised. Middle liassic of the High Atlas, Morocco. Washover, ammonites and belemnites (by storms) on top of a regressive cycle, supratidal environment. Middle liassic of the High Atlas, Morocco. Giant dinosaur tracks, in a muddy sediment on top of a metric regressive cycle. Middle liassic, High Atlas, Morocco. Metric to hectometric regressive cycles (related to an acceleration of the rate of subsidence); South of the High Atlas, Morocco.Seasonalchanges in weather can create cyclic sediments in the form of alternating bands ofclayandsilt(also known asvarves). For example, in aglacialregion where sediments are deposited in alake,coarsesediments 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.[4]Because the cycles are limited to the depositional basin, the lateral extent of the resultant strata are limited. The time period over which autocycles form are usually less than the time periods of allocyclic deposits.[1]Allocycles[edit]Allocycles are cycles of sediments caused by processes that also occur outside the depositional basin.Sea levelfluctuations,climate changesandtectonic activityare examples of these kinds of processes. Allocyclic successions can extend over great distances.[3] "Shallowing upward" cycles in the lagoonal Liassic platform of the Musandam Peninsula(Oman). Possible allocyclic origin. Possible allocyclic metric cycles, arranged in decametric sequences, limited by yellow algal, dolomitized levels, below depositional hiatus. "Shallowing upward" cycles in the Middle Jurassic (Saghtan form.) of the jbel Laghdar Range (Oman).Changes in sea level can create cyclic sediments oflimestones,shales,coalsandseat earths. For these cycles to have been created, the environment at the site of deposition must have changed radically frommarinetodeltaic, thenlagoonaland thencontinentalenvironments.[4]One cause of sea level change is the advance or retreat of continental glaciers caused by climate change. Tectonic movements can affect the environment of deposition by changing water depth. Allocyclic cycles can extend over great distances and are not limited to a single depositional basin.[1]Metric sedimentary carbonate cycles could be related to an astronomical (Milankovitch) precession influence (through thermic dilatation of oceans?) on an estimated 20.000 years scale, according to numerous authors interpretations. But these beds are of no use in correlation, even at hectometric to kilometric scale, and certainly not a "high resolution" tool for stratigraphy without a severe biostratigraphic control!Problems with the study of cyclic sediments[edit]The debate about the causes of cyclic sedimentation has been contentious in the past, and it remains unresolved. Sequence stratigraphy, the study of sea level change through the examination of sedimentary deposits, was developed from the centuries-old controversy over the origin of cyclic sedimentation and the relative importance of eustatic and tectonic factors on sea level change.[2]Another problem with the study of cyclic sediments is that different researchers have different criteria with which they identify cycles and the surfaces that separate the sedimentary layers within the cycles. There is also not a consistentterminologyandclassification schemeto describe the nature of the cycles seen in the stratigraphic record. This is mainly becauseabsolute age datingis not precise enough at present.[1]