In geology, cyclothems are alternating stratigraphic sequences of

marine Marine is an adjective meaning of or pertaining to the sea or ocean. Marine or marines may refer to: Ocean * Maritime (disambiguation) * Marine art * Marine biology * Marine debris * Marine habitats * Marine life * Marine pollution Military * ...

and non-marine sediments, sometimes interbedded with coal seams. Historically, the term was defined by the European coal geologists who worked in coal basins formed during the

Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...

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. 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. A more general interpretation of sequences invokes Milankovitch 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

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External links

Jacobson, R. J. (2000
''Depositional History of the Pennsylvanian Rocks in Illinois.''
Geonote 2. Illinois State Geological Survey, Champaign, Illinois. Carboniferous Sedimentology Sedimentary rocks