A cratonic sequence in

geology Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Ea ...

is a very large-scale

lithostratigraphic Lithostratigraphy is a sub-discipline of stratigraphy, the geological science associated with the study of strata or rock layers. Major focuses include geochronology, comparative geology, and petrology. In general, strata are primarily igneou ...

sequence in the rock record that represents a complete cycle of

marine transgression A marine transgression is a geologic event during which sea level rises relative to the land and the shoreline moves toward higher ground, which results in flooding. Transgressions can be caused by the land sinking or by the ocean basins filling ...

and regression on a

craton A craton (, , or ; from grc-gre, κράτος "strength") is an old and stable part of the continental lithosphere, which consists of Earth's two topmost layers, the crust and the uppermost mantle. Having often survived cycles of merging an ...

(block of

continental crust Continental crust is the layer of igneous, sedimentary, and metamorphic rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called '' sial'' be ...

) over geologic time. Cratonic sequences are also known as "megasequences", "stratigraphic sequences", "Sloss sequences", "supersequences" or simply "sequences". They are geologic evidence of

relative sea level Relative sea level (abbreviated as RSL) is defined as the sea level that is observed with respect to a land-based reference frame. It is often contrasted with eustatic sea level, which is a measure of the total mass or volume of the oceans. Relative ...

rising and then falling (transgressing and regressing), thereby depositing varying layers of

sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sa ...

onto the craton, now expressed as

sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

. Places such as the Grand Canyon are a good visual example of this process, demonstrating the changes between layers deposited over time as the ancient environment changed. Cratonic sequences were first proposed by

Laurence L. Sloss Laurence L. Sloss (August 26, 1913 – November 2, 1996) was an American geologist. He taught geology at Northwestern University from 1947 until his retirement in 1981. He was president, Geological Society of America (GSA), with his tenure beginni ...

in 1963. Each one represents a time when

inland seas An inland sea (also known as an epeiric sea or an epicontinental sea) is a continental body of water which is very large and is either completely surrounded by dry land or connected to an ocean by a river, strait, or "arm of the sea". An inland se ...

deposited sediments across the craton. The top and bottom edges of a sequence are each bounded by craton-wide

unconformities An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval ...

(time gaps in the rock record). The unconformities indicate when the seas receded and sediment was

eroded Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is disti ...

rather than deposited.

Cause and chronology

These sequences may in part represent

eustatic 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 standardised g ...

(global) change in sea level; however, when the proper names are used they usually refer to relative sea level changes on the

North American continent 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 the Car ...

. The most likely causes of these cycles is change in

mid-ocean ridge A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a diver ...

volume, which is related to

seafloor spreading Seafloor spreading or Seafloor spread is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. History of study Earlier theories by Alfred Wegener a ...

rates. When Earth's mid-ocean ridges spread rapidly, the ridges tend to be longer than usual; also, the greater heat elevates the lithosphere over the ridges.Stanley, p. 175. This elevated lithosphere displaces seawater onto the continents; conversely, when spreading rates decline, the ridges subside, and the seas drain from the cratons. It is also possible that other mechanisms, such as

dynamic topography The term dynamic topography is used in geodynamics to refer the elevation differences caused by the flow within Earth's mantle. Definition In geodynamics, ''dynamic topography'' refers to topography generated by the motion of zones of differing ...

related to mantle mass anomalies, and intraplate stress related to episodes of contractional and extensional tectonics, play a part by causing significant

tectonic uplift Tectonic uplift is the geologic uplift of Earth's surface that is attributed to plate tectonics. While isostatic response is important, an increase in the mean elevation of a region can only occur in response to tectonic processes of crustal th ...

and subsidence across the craton. There have been six cratonic sequences since the beginning of the Cambrian Period. For North America, from oldest to youngest, they are the Sauk, Tippecanoe,

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 ...

, Absaroka, Zuñi, and Tejas sequences. Attempts to identify equivalent cratonic sequences on other continents have met with only limited success, suggesting that eustasy (total global sea-level change) is unlikely to be the sole responsible mechanism.

References

External links

General information on Paleozoic sequences
- Department of Earth and Environmental Sciences, University of Michigan

- GEOL 342 Sedimentation and Stratigraphy, University of Maryland Historical geology Regional geology Stratigraphy {{geology-stub

Cause and chronology

See also

References

External links