Late Palaeozoic cyclothems – A review of their stratigraphy and sedimentology

Abstract The term “cyclothem” was coined by Wanless and Weller (1932) to characterize a repetitive stratigraphic motif in the Pennsylvanian succession of the Illinois Basin, central USA, and it subsequently became popular among geologists as a general term for similarly repetitive successions worldwide. Although use of the term has become somewhat indiscriminate in the interim, it is here considered best reserved for the distinctive stratal rhythms characteristic of the late Palaeozoic in the palaeotropical realm of Laurussia, recording repeated alternation of marine and nonmarine depositional conditions. Cyclothems comprise arrays of terrigenous clastic, organic, and chemical/biochemical lithologies that record accumulation mainly on slowly-subsiding platforms and ramps, under limited accommodation and limited sediment supply, often strongly seasonal tropical and subtropical climates, and forcing by repeated, large-magnitude excursions of sea level. A spectrum of variation is recognized, from carbonate-mudrock-dominated variants at one extreme to coarse clastic-dominated styles at the other. Cyclothems can be rationalized as depositional sequences which are thin (metres to a few tens of metres), condensed, incomplete in terms of systems tracts, and top-truncated. Given their demonstrated widespread extent, cyclothems have been attributed by many researchers to forcing by eustatic sea-level fluctuations, and a variety of evidence suggests they correlate to 100 kyr glacial cycles on Gondwana. By analogy with Quaternary 100 kyr glacial cycles, much of the time in a cycle was taken up by a protracted, complex drawdown of sea-level, during which valleys and channels were excavated and interfluves were pedogenically modified. Most of the depositional record of cyclothems probably formed in the relatively short interval following lowstand/glacial maximum when sea-level rose to its interglacial maximum. Analysis of cyclothemic successions can contribute to a fuller understanding of the magnitude of sea-level excursions and palaeoclimatic changes during the late Palaeozoic, and can help to pinpoint the timing of key events such as the main onset of the late Palaeozoic Ice Age.