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.
Alternative abiotic formation mechanism for coal-seam cythlothems:
A rocky meteorite above a certain size and kinetic energy will melt and/or vaporize upon impact, but the target rock of an icy body may survive unmelted due to energy-absorbing endothermic chemical reactions (ECRs) that take place in the highly-compressible ices, such as methane, CO, CO2 and water ices. ECRs may clamp the shock wave pressure below the melting point of rock, preserving terrestrial target rock from melting and obscuring icy-body (comet) impact craters from detection by geologists looking for melt rock signatures.
Most ECRs reaction products releasing oxygen as one of the reaction products immediately recombine as the shock wave pressure begins to relent. This lowers the power of impact, but not the energy. ECRs involving methane ices, however, release hydrocarbons and hydrogen as reaction products, and many of the hydrocarbons may survive, representing sequestered energy that does lower the dissipated energy of impact. These surviving hydrocarbons may be the origin of ‘abiotic oil’ on the planet, and coal as well, despite the plant fossils in coal. The pyroclastic flow created by a large comet impact such as the one that may have brought on the Carboniferous Period settle out to form the primary coal-seam cyclothem. Secondary, tertiary and subsequent coal-seam cyclothems merely represent reworking of the primary cyclothem caused by the original pyroclastic flow that bulldozed the forests ahead of it.