In geology, cyclothems are alternating stratigraphic sequences of marine and non-marine sediments, sometimes interbedded with coal seams. The cyclothems consist of repeated sequences, each typically several meters thick, of sandstone resting upon an erosion surface, passing upwards to pelites (finer-grained than sandstone) and topped by coal.
Historically, the term was defined by the European coal geologists who worked in coal basins formed during the Carboniferous and earliest Permian periods. 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 may 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.
References
- Wanless, H.R.; Weller, J.M. (1932). "Correlation and extent of Pennsylvanian cyclothems". Geological Society of America Bulletin. 43 (4): 1003–1016. Bibcode:1932GSAB...43.1003W. doi:10.1130/gsab-43-1003.
- 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
- Haq BU, Schutter SR (2008) A chronology of Paleozoic sea-level changes. Science, vol. 322 (issue 5898), pp. 64-68. doi:10.1126/science.116164
- Stanley, Steven M. Earth System History. New York: W.H. Freeman and Company, 1999. ISBN 0-7167-2882-6 (p. 426)
- 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
External links
- Jacobson, R. J. (2000) Depositional History of the Pennsylvanian Rocks in Illinois. Geonote 2. Illinois State Geological Survey, Champaign, Illinois.