Cyclic sediments
Encyclopedia
Cyclic sediments are sequences
of sedimentary rock
s that are characterised by repetitive patterns of different rock types (strata
) within the sequence. Cyclic sediment
s can be identified as either autocyclic or allocyclic, and can be hundreds or even thousands of metres thick. The study of sequence stratigraphy
was developed from controversies over the causes of cyclic sedimentation.
the sediments are deposited in. Changes in the environment of deposition
change the type and amount of sediments that are deposited, resulting in different sedimentary rock types. A series of connected events produces a distinctive and repetitive series of strata that are arranged vertically. At least one rock type, which is regarded as the starting point, must be repeated.
Based on the processes that generate the cyclic deposits, two types of sedimentary cyclic successions — autocycles and allocycles — can be distinguished.
that the sediments are deposited in. Tide
s and storm
s are examples of the types of processes that create autocycles. Autocycles show limited stratigraphic
continuity.
An example of autocyclic sedimentation on a carbonate platform by Septfontaine M. (1985): Depositional environments and associated foraminifera (lituolids) in the middle liasic carbonate platform of Morocco.- Rev. de Micropal., 28/4, 265-289. See also www.palgeo.ch/publications.
Seasonal
changes in weather can create cyclic sediments in the form of alternating bands of clay
and silt
(also known as varve
s). For example, in a glacial
region where sediments are deposited in a lake
, coarse sediments that are trapped in ice are released when the ice melts in the summer. This creates paler, coarser silt bands in the lake deposits. In winter, melting is at a minimum, meaning that only fine material is supplied to the lake, causing thin clay layers. Because the cycles are limited to the depositional basin, the lateral extent of the resultant strata are limited. The time period over which autocycles form are usually less than the time periods of allocyclic deposits.
fluctuations, climate change
s and tectonic activity
are examples of these kinds of processes. Allocyclic successions can extend over great distances.
Changes in sea level can create cyclic sediments of limestone
s, shale
s, coal
s and seat earths
. For these cycles to have been created, the environment at the site of deposition must have changed radically from marine
to deltaic
, then lagoon
al and then continent
al environments. One cause of sea level change is the advance or retreat of continental glaciers caused by climate change. Tectonic movements can affect the environment of deposition by changing water depth. Allocyclic cycles can extend over great distances and are not limited to a single depositional basin.
Another problem with the study of cyclic sediments is that different researchers have different criteria with which they identify cycles and the surfaces that separate the sedimentary layers within the cycles. There is also not a consistent terminology
and classification scheme
to describe the nature of the cycles seen in the stratigraphic record. This is mainly because absolute age dating
is not precise enough at present.
Sequence (geology)
A sequence in geology refers to a sequence of geological events, processes, or rocks, arranged in chronological order.A rock stratigraphical sequence is a geographical, or lithostratigraphic, discrete unit greater than a group or supergroup rank, and traceable over large areas of a continent...
of sedimentary rock
Sedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....
s that are characterised by repetitive patterns of different rock types (strata
Stratum
In geology and related fields, a stratum is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers...
) within the sequence. Cyclic sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
s can be identified as either autocyclic or allocyclic, and can be hundreds or even thousands of metres thick. The study of sequence stratigraphy
Sequence stratigraphy
Sequence stratigraphy is a branch of geology that attempts to subdivide and link sedimentary deposits into unconformity bound units on a variety of scales and explain these stratigraphic units in terms of variations in sediment supply and variations in the rate of change in accommodation space...
was developed from controversies over the causes of cyclic sedimentation.
Processes that create cyclic sediments
Cyclic sedimentation occurs when there is a repetition of a specific series of connected events that affects the environmentNatural environment
The natural environment encompasses all living and non-living things occurring naturally on Earth or some region thereof. It is an environment that encompasses the interaction of all living species....
the sediments are deposited in. Changes in the environment of deposition
Sedimentary depositional environment
In geology, sedimentary depositional environment describes the combination of physical, chemical and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will be formed after lithification, if the sediment is preserved in the rock...
change the type and amount of sediments that are deposited, resulting in different sedimentary rock types. A series of connected events produces a distinctive and repetitive series of strata that are arranged vertically. At least one rock type, which is regarded as the starting point, must be repeated.
Based on the processes that generate the cyclic deposits, two types of sedimentary cyclic successions — autocycles and allocycles — can be distinguished.
Autocycles
Autocycles are cyclic sediments that are created by processes that only take place within the basinSedimentary basin
The term sedimentary basin is used to refer to any geographical feature exhibiting subsidence and consequent infilling by sedimentation. As the sediments are buried, they are subjected to increasing pressure and begin the process of lithification...
that the sediments are deposited in. Tide
Tide
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon and the sun and the rotation of the Earth....
s and storm
Storm
A storm is any disturbed state of an astronomical body's atmosphere, especially affecting its surface, and strongly implying severe weather...
s are examples of the types of processes that create autocycles. Autocycles show limited 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....
continuity.
An example of autocyclic sedimentation on a carbonate platform by Septfontaine M. (1985): Depositional environments and associated foraminifera (lituolids) in the middle liasic carbonate platform of Morocco.- Rev. de Micropal., 28/4, 265-289. See also www.palgeo.ch/publications.
Seasonal
Season
A season is a division of the year, marked by changes in weather, ecology, and hours of daylight.Seasons result from the yearly revolution of the Earth around the Sun and the tilt of the Earth's axis relative to the plane of revolution...
changes in weather can create cyclic sediments in the form of alternating bands of clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
and silt
Silt
Silt is granular material of a size somewhere between sand and clay whose mineral origin is quartz and feldspar. Silt may occur as a soil or as suspended sediment in a surface water body...
(also known as varve
Varve
A varve is an annual layer of sediment or sedimentary rock.The word 'varve' is derived from the Swedish word varv whose meanings and connotations include 'revolution', 'in layers', and 'circle'. The term first appeared as Hvarfig lera on the first map produced by the Geological Survey of Sweden in...
s). For example, in a glacial
Glacier
A glacier is a large persistent body of ice that forms where the accumulation of snow exceeds its ablation over many years, often centuries. At least 0.1 km² in area and 50 m thick, but often much larger, a glacier slowly deforms and flows due to stresses induced by its weight...
region where sediments are deposited in a lake
Lake
A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...
, coarse sediments that are trapped in ice are released when the ice melts in the summer. This creates paler, coarser silt bands in the lake deposits. In winter, melting is at a minimum, meaning that only fine material is supplied to the lake, causing thin clay layers. Because the cycles are limited to the depositional basin, the lateral extent of the resultant strata are limited. The time period over which autocycles form are usually less than the time periods of allocyclic deposits.
Allocycles
Allocycles are cycles of sediments caused by processes that also occur outside the depositional basin. Sea levelSea level
Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...
fluctuations, climate change
Climate change
Climate change is a significant and lasting change in the statistical distribution of weather patterns over periods ranging from decades to millions of years. It may be a change in average weather conditions or the distribution of events around that average...
s and tectonic activity
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
are examples of these kinds of processes. Allocyclic successions can extend over great distances.
Changes in sea level can create cyclic sediments of limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
s, shale
Shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
s, 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...
s and seat earths
Seatearth
Seatearth is a British coal mining term, which is used in the geological literature. As noted by Jackson, a seatearth is the layer of sedimentary rock underlying a coal seam. Seatearths have also been called seat earth, "seat rock", or "seat stone" in the geologic literature. Depending on its...
. For these cycles to have been created, the environment at the site of deposition must have changed radically from 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...
to deltaic
River delta
A delta is a landform that is formed at the mouth of a river where that river flows into an ocean, sea, estuary, lake, reservoir, flat arid area, or another river. Deltas are formed from the deposition of the sediment carried by the river as the flow leaves the mouth of the river...
, then lagoon
Lagoon
A lagoon is a body of shallow sea water or brackish water separated from the sea by some form of barrier. The EU's habitat directive defines lagoons as "expanses of shallow coastal salt water, of varying salinity or water volume, wholly or partially separated from the sea by sand banks or shingle,...
al and then continent
Continent
A continent is one of several very large landmasses on Earth. They are generally identified by convention rather than any strict criteria, with seven regions commonly regarded as continents—they are : Asia, Africa, North America, South America, Antarctica, Europe, and Australia.Plate tectonics is...
al environments. One cause of sea level change is the advance or retreat of continental glaciers caused by climate change. Tectonic movements can affect the environment of deposition by changing water depth. Allocyclic cycles can extend over great distances and are not limited to a single depositional basin.
Problems with the study of cyclic sediments
The debate about the causes of cyclic sedimentation has been contentious in the past, and it remains unresolved. Sequence stratigraphy, the study of sea level change through the examination of sedimentary deposits, was developed from the centuries-old controversy over the origin of cyclic sedimentation and the relative importance of eustatic and tectonic factors on sea level change.Another problem with the study of cyclic sediments is that different researchers have different criteria with which they identify cycles and the surfaces that separate the sedimentary layers within the cycles. There is also not a consistent terminology
Terminology
Terminology is the study of terms and their use. Terms are words and compound words that in specific contexts are given specific meanings, meanings that may deviate from the meaning the same words have in other contexts and in everyday language. The discipline Terminology studies among other...
and classification scheme
Classification scheme
In metadata a classification scheme is a hierarchical arrangement of kinds of things or groups of kinds of things. Typically it is accompanied by descriptive information of the classes or groups. A classification scheme is intended to be used for an arrangement or division of individual objects...
to describe the nature of the cycles seen in the stratigraphic record. This is mainly because absolute age dating
Geochronology
Geochronology is the science of determining the age of rocks, fossils, and sediments, within a certain degree of uncertainty inherent to the method used. A variety of dating methods are used by geologists to achieve this, and schemes of classification and terminology have been proposed...
is not precise enough at present.