The Absaroka sequence was a cratonic sequence that extended from the end of the Mississippian through the

Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleo ...

periods. It is the unconformity between this sequence and the preceding

Kaskaskia The Kaskaskia were one of the indigenous peoples of the Northeastern Woodlands. They were one of about a dozen cognate tribes that made up the Illiniwek Confederation, also called the Illinois Confederation. Their longstanding homeland was in ...

that divides the

Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carboniferou ...

into the Mississippian and Pennsylvanian periods in

North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and th ...

. Like the Kaskaskia sequence, Absaroka sedimentary deposits were dominated by

detrital Detritus (; adj. ''detrital'' ) is particles of rock derived from pre-existing rock through weathering and erosion.Essentials of Geology, 3rd Ed, Stephen Marshak, p G-7 A fragment of detritus is called a clast.Essentials of Geology, 3rd Ed, Stephe ...

or siliclastic rocks. The first sediments were deposited near the

continent A continent is any of several large landmasses. Generally identified by convention rather than any strict criteria, up to seven geographical regions are commonly regarded as continents. Ordered from largest in area to smallest, these seven ...

al margins, particularly near the Ouachita and Appalachian highlands. Characteristic of the strata from this time are

cyclothems 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 who worked in coal basins formed during ...

: alternating

marine Marine is an adjective meaning of or pertaining to the sea or ocean. Marine or marines may refer to: Ocean * Maritime (disambiguation) * Marine art * Marine biology * Marine debris * Marine habitats * Marine life * Marine pollution Military ...

and non-marine strata indicative of changes in sea-level, probably due to cyclic glaciation in the Southern Hemisphere. In North America, the waters of the Absaroka sequence regressed westward as the highlands to the east steadily eroded. Restricted oceanic circulation in the west led to extensive

evaporite An evaporite () is a water- soluble sedimentary mineral deposit that results from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporite deposits: marine, which can also be described as ocean ...

formation. By the end of the period, the regression was complete, and the marine strata were superseded by extensive

red bed Red beds (or redbeds) are sedimentary rocks, typically consisting of sandstone, siltstone, and shale, that are predominantly red in color due to the presence of ferric oxides. Frequently, these red-colored sedimentary strata locally contain t ...

deposition.

Transgressive–regressive units

These cycles of

sea level Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datuma standardis ...

change have been divided into at least six magnitudes of order.Busch, R. M., and Rollins, H. B. (1984). Correlation of Carboniferous strata using a hierarchy of transgressive-regressive units. ''Geology'', v. 12, p. 471-474. Each order is a span of years that become smaller. The smallest unit is described as a punctuated aggradational cycle (PAC) and represents between 225,000 and 100,000 years of sediment accumulation. In the field, these units are usually one to five meters thick and contain several different rock units. These units show quick changes in sea level that were controlled by climate change due to glaciers. The fifth order is often called a

cyclothem 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 who worked in coal basins formed during ...

and may contain several PAC sequences and generally represent about 500,000 years. Again, glaciation seems to be the cause of the cyclic nature of the strata. The first four orders, though, are controlled more by continental shifts and orogenic cycles. The first order cycle being the Absaroka, the second being the Carboniferous Period, and third order being rock formations or groups which are generally 8 to 10 million years long. The fourth order might be controlled by large scale glaciation and global ocean patterns over the course of a million years.Chang, K.H., (1975). Unconformity-bounded stratigraphic units. Geological Society of America Bulletin, v. 86, p. 1544-1552.

References

*Monroe, James S., and Reed Wicander. ''The Changing Earth: Exploring Geology and Evolution'', 2nd ed. Belmont: West Publishing Company, 1997. {{ISBN, 0-314-09577-2(pp. 544-9) Geology of the United States Historical geology Carboniferous System of North America Permian System of North America