Cyclothems
Encyclopedia
In geology
, cyclothems are alternating stratigraphic
sequences of marine
and non-marine sediments, sometimes interbedded with coal
seams. 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.
The term is somewhat outdated. 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 might have formed as a result of marine transgressions
and regressions related to decay and growth 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
.
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
, cyclothems are alternating stratigraphic
Stratigraphy
Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....
sequences of marine
Marine (ocean)
Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...
and non-marine sediments, sometimes interbedded with coal
Coal
Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...
seams. Historically, the term was defined by the European coal geologists that worked in coal basins formed during the Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
and earliest Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...
periods. The cyclothems consist in repeated sequences, each typically several meters thick, of sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
resting upon an erosional surface, passing upwards to pelites (finer-grained than sandstone) and topped by coal.
The term is somewhat outdated. 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 might have formed as a result of marine transgressions
Transgression (geology)
A marine transgression is a geologic event during which sea level rises relative to the land and the shoreline moves toward higher ground, resulting in flooding. Transgressions can be caused either by the land sinking or the ocean basins filling with water...
and regressions related to decay and growth of ice sheets, respectively, as the Carboniferous was a time of widespread glaciation
Ice age
An ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...
in the Southern hemisphere. A more general interpretation of sequences invokes Milankovitch cycles
Milankovitch cycles
Milankovitch theory describes the collective effects of changes in the Earth's movements upon its climate, named after Serbian civil engineer and mathematician Milutin Milanković, who worked on it during First World War internment...
.