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cyclothems
In geology, cyclothems are alternating stratigraphic sequences of marine and non-marine sediments, sometimes interbedded with coal seams. Historically, the term was defined by the European coal geologists〔Hampson G, Stollhofen H, Flint S (1999) ''A sequence stratigraphic model for the Lower Coal Measures (Upper Carboniferous) of the Ruhr district, north-west Germany.'' Sedimentology vol. 46 (issue 6), pp. 1199-1231〕 that worked in coal basins formed during the Carboniferous and earliest Permian periods. The cyclothems consist of repeated sequences, each typically several meters thick, of sandstone resting upon an erosional surface, passing upwards to pelites (finer-grained than sandstone) and topped by coal.
Depositional sequences have been thoroughly studied by oil geologists using geophysical profiles of continental and marine basins. A general theory of basin-scale deposition has been formalized under the name of sequence stratigraphy.〔Haq BU, Schutter SR (2008) ''A chronology of Paleozoic sea-level changes.'' Science, vol. 322 (issue 5898), pp. 64-68. 〕
Some cyclothems might have formed as a result of marine regressions and transgressions related to growth and decay of ice sheets, respectively, as the Carboniferous was a time of widespread glaciation in the southern hemisphere.〔Stanley, Steven M. ''Earth System History.'' New York: W.H. Freeman and Company, 1999. ISBN 0-7167-2882-6 (p. 426)〕 A more general interpretation of sequences invokes Milankovitch cycles.〔(Milankovic cycles )〕〔Haq BU, Hardenbol J, Vail PR (1987) ''Chronology of fluctuating sea levels since the Triassic.'' Science, vol. 235 (issue 4793), pp. 1156-1167〕
==References==
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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