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Cratonic sequence
A cratonic sequence is a very large-scale lithostratographic sequence that covers a complete marine transgressive-regressive cycle across a craton. They are also known as "megasequences", "stratigraphic sequences", "sloss sequence" "supersequence" or simply "sequences." In plain English, it is the geological evidence of the sea level rising and then falling, thereby depositing layers of sediment onto an area of ancient rock called a craton.
Cratonic sequences were first proposed by Lawrence Sloss in 1963;〔(Lawrence Sloss, "Tectonic Cycles of the North American Craton". (Accessed 6/18/06) )〕 each one represents a time when epeiric seas deposited sediments across the craton, while the upper and lower edges of the sequence are bounded by craton-wide unconformities eroded when the seas receded.
==Cause and chronology==
These sequences may in part represent eustatic or global change in sea level; however, when the proper names are used they usually refer to the North American continent. The most likely causes of these cycles is change in mid-ocean ridge volume, which is related to spreading rates.〔Stanley, Steven M. ''Earth System History''. New York: W.H. Freeman and Company, 1999. ISBN 0-7167-2882-6 p. 174.〕 When Earth's mid-ocean ridges spread rapidly, the ridges tend to be longer than usual; also, the greater heat elevates the lithosphere over the ridges.〔Stanley, p. 175〕 This elevated lithosphere reduces ocean-basin volume and displaces water onto the continents; conversely, when spreading rates decline, the ridges subside, and the seas drain from the cratons.〔Stanely, p. 175〕 It is also possible that other mechanisms, such as dynamic topography related to mantle mass anomalies, and intraplate stress related to episodes of contractional and extensional tectonics, play a part by causing significant tectonic uplift and subsidence across the craton.〔Burgess et al. 1997, Bull Geol. Soc. America〕
There have been six cratonic sequences since the beginning of the Cambrian Period. For North America, from oldest to youngest, they are the Sauk, Tippecanoe, Kaskaskia, Absaroka, Zuñi, and the Tejas. Attempts to identify equivalent cratonic sequences on other continents have met with only limited success, suggesting that eustasy is unlikely to be the sole responsible mechanism.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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