May 11, 2011 (Coal Geology): Cyclothems are alternating stratigraphic sequences of marine and non-marine sediments representing the transgression and regression of water level or the submergence and emergence of land surface. Cyclothems are sometimes interbedded with coal seams. Cyclothem represent a series of sedimentary beds deposited during a SINGLE sedimentary cycle. Many of coal basins of the world have multiple cyclothems. In such basins, similar environments repeated and formed coal in each cyclic period. According to Britannica, cyclothem ia a complex, repetitive stratigraphic succession of marine and nonmarine strata that are indicative of cyclic depositional regimes. Ideal cyclothem successions are rare, and reconstructions of generalized sequences result from the study of examples in which typical beds of limestone, clastic sediments, or coal seams may be missing.
Historically, the term was defined by the European coal geologists that worked in coal basins formed during the Carboniferous and earliest Permian periods.
The cyclothems consist in 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. A typical top down cyclothem sequence would consist of shale, limestone, shale, coal, underclay, limestone, shale and sandstone. Appalachian cyclothems in the USA show dominant presence of clastics and coals with lesser amount of marine beds.
Marine clyclothem could be easily identified by alternating shale and limestone beds.