دورة ترسيب

دورة الترسيب Cyclic sediments، هي تتابع رأسي من الصخور الرسوبية، تكون أثناء دورة طغيان - انحسار البحر transgressive - regressive cycle.

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 العمليات التي تنشئ دورة الترسيب

 دورات تلقائية

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.

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.

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

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

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  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.

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  Hurricane breccia cemented on the surface of a bed, top of a "shallowing upward" cycle. Middle Lias, High Atlas.

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  Top of a metric regressive cycle with desiccations, surface of bed dolomitised. Middle liassic of the High Atlas, Morocco.

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  Washover, ammonites and belemnites (by storms) on top of a regressive cycle, supratidal environment. Middle liassic of the High Atlas, Morocco.

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  Giant dinosaur tracks, in a muddy sediment on top of a metric regressive cycle. Middle liassic, High Atlas, Morocco.

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  Metric to hectometric regressive cycles (related to an acceleration of the rate of subsidence); South of the High Atlas, Morocco.

 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 fluctuations, climate oscillations or tectonic activity). Allocyclic successions can extend over great distances and are not limited to a single depositional basin.

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  "Shallowing upward" cycles in the lagoonal Liassic platform of the Musandam Peninsula(Oman). Possible allocyclic origin.

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  Possible allocyclic metric cycles, arranged in decametric sequences, limited by yellow algal, dolomitized levels, below depositional hiatus.

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  "Shallowing upward" cycles in the Middle Jurassic (Saghtan form.) of the jbel Laghdar Range (Oman).

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.

 المصادر

• Septfontaine, M. (1985): Milieux de dépôts et foraminifères (Lituolidae) de la plate-forme carbonatée du Lias moyen au Maroc.- Rev. Micropaléont., 28/4, 265-289. (Le modèle ancien proposé ci-dessous a son équivalent actuel au fond du golfe de Gabès et dans les chotts associés, voir Davaud & Septfontaine, 1995.

• Davaud, E. & Septfontaine, M. (1995): Post-Mortem onshore transportation of epiphytic foraminifera: recent example from the Tunisian coastline.- Jour. Sediment. Research, 65/1A, 136-142.

• Septfontaine, M. & De Matos, E. (1998): Pseudodictyopsella jurassica nov. gen., nov. sp., a new foraminifera from the Earlay Middle Jurassic of the Musandam Peninsula. Sedimentological and stratigraphical context.- Rev. Micropaléont., 41/1,71-87. (Dans cet article, on note l'absence du genre Orbitammina en Oman, souvent confondu avec Timidonella par les auteurs anglo-saxons).