Platform (geology)

Geologic provinces of the world (USGS)

  Shield

  Platform

  Orogen

  Basin

  Large igneous province

  Extended crust

Oceanic crust:

  0–20 Ma

  20–65 Ma

  >65 Ma

In geology, a platform is a continental area covered by relatively flat or gently tilted, mainly sedimentary strata, which overlie a basement of consolidated igneous or metamorphic rocks of an earlier deformation. Platforms, shields and the basement rocks together constitute cratons.

It is also common practice to use the term platform as a general term for a sequence of shallow water carbonate platforms. Carbonate platforms can develop in almost any tectonic setting, but they are particularly common along passive margins. Many oilfields produce from sediments deposited along carbonate platforms.

Details of emersive cycles "shallowing upward sequences" in two sections distant of 230km. Note on top the tempestite and tsunami(?) deposits, with displaced, floated foraminifera.

Various types of carbonate platforms are recognised, including (but are not restricted to) rimmed shelf, ramp, epiric platforms, isolated platforms and drowned platforms. An example of carbonate "bahamian type" platform, in a paleotectonic context of continental rifting during the Lias is the high Atlas of Morocco (Septfontaine, 1985). The coastal and lagoonal to supratidal "chotts" deposits of southern Tunisia (Zarzis area) is also a good recent equivalent model (Davaud & Septfontaine, 1995).

High Atlas liassic carbonate platform of Morocco and first order metric (autocyclic)regressive cycles.

Virtual "shallowing upward sequence" observed all along the southern margin of the Tethys during liassic times (over 10.000km).

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  Hurricane deposit (tempestite)cemented (early diagenesis) at the surface of a bed, on top of a metric regressive cycle. Middle liassic, High Atlas.

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  Tempestite breccia in a dolomitic matrix (supratidal environment). Elements are remains of dislocated "algal mats" deposits in an intertidal environment. Middle liassic, High Atlas.

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  Shallowing upward cycle, lagoonal marine deposit at the base, intertidal algal laminations and supratidal hurricane (tempestite) on top.

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  Stalactitic vadose cement on top of sequence,early diagenesis in a supratidal environment. Middle Lias, High Atlas. Thin section. L = 0.5mm.

See also

• List of shields and cratons

References

• Davaud, E. & Septfontaine, M. (1995): Post-mortem onshore transportation of epiphytic foranifera: recent example from the Tunisian coastline.- Jour. Sediment. Research, 65/1A, 136-142. See www.palgeo.ch/publications.
• Parker, Sybil P. (Ed.). 1997. McGraw-Hill Dictionary of Geology and Mineralogy. New York: McGraw-Hill.
• Septfontaine, M. (1985): Depositional environments and associated foraminifera (lituolids) in the middle liassic carbonate platform of Morocco.- Rev. de Micropal,(Paris), 28/4, 265-289. See also www.palgeo.ch/publications.
• Tucker, Maurice E. 1990. Carbonate Sedimentology. Oxford: Blackwell Science.