What are Cyclothems?
Cyclothems are cyclic stratigraphic sequences that are unique to the Pennsylvanian and earliest Permian periods within the US Midcontinent, that formed as a result of marine transgressions and regressions (rise and fall of sea level) related to the waning and waxing of ice sheets at the South Pole.
The "perfect" Midcontinent cyclothem deposit would consist of 4 basic units:
1) A thin transgressive marine limestone
2) A thin, offshore black shale
3) A thicker, shallowing upward regressive limestone
4) A nearshore to terrestrial non-carbonate unit (e.g. sandstone or paleosol)
See the diagram below for more information --
** The above image comes from Heckel, P.H., 1983, Diagenetic Model for Carbonate Rocks in Midcontinent Pennsylvanian Eustatic Cyclothems, Journal of Sedimentary Petrology, vol.53, p.733-759.
For more information on cyclothems, check out: Heckel, P.H., 1986, Sea-level curve for Pennsylvanian eustatic marine transgressive-regressive depositional cycles along midcontinent outcrop belt, North America, Geology, v.14, p.330-334.