# Sedimentary structures

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Geologic structures formed during sediment deposition

Megaripple/dune, formed in the upper flow regime, from Utah

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**Sedimentary structures** include all kinds of features in [sediments](/source/Sediment) and [sedimentary rocks](/source/Sedimentary_rock), formed at the time of [deposition](/source/Deposition_(geology)).

Sediments and sedimentary rocks are characterized by [bedding](/source/Bed_(geology)), which occurs when layers of sediment, with different particle sizes are deposited on top of each other.[1] These beds range from millimeters to centimeters thick and can even go to meters or multiple meters thick.

Sedimentary structures such as [cross-bedding](/source/Cross-bedding), [graded bedding](/source/Graded_bedding), and [ripple marks](/source/Ripple_marks) are utilized in [stratigraphic](/source/Stratigraphy) studies. They help indicate the [original position](/source/Way_up_structure) of [strata](/source/Stratum) in geologically complex terrains. They also provide insight into the [depositional environment](/source/Depositional_environment) of the sediment.

## Flow structures

There are two kinds of flow structures: bidirectional (multiple directions, back-and-forth) and unidirectional. Flow regimes in single-direction (typically [fluvial](/source/Fluvial)) flow, which at varying speeds and velocities produce different structures, are called [bedforms](/source/Bedform). In the *lower flow regime*, the natural progression is from a flat bed, to some sediment movement ([saltation](/source/Saltation_(geology)) etc.), to ripples, to slightly larger dunes. Dunes have a vortex in the lee side of the dune. As the *upper flow regime* forms, the dunes become flattened out, and then produce [antidunes](/source/Antidune). At higher still velocity, the antidunes are flattened and most sedimentation stops, as erosion takes over as the dominant process.

## Bedforms vs. flow

Typical unidirectional bedforms represent a specific flow velocity, assuming typical sediments (sands and silts) and water depths, and a chart such as below can be used for interpreting [depositional environments](/source/Depositional_environment), with *increasing* water velocity going down the chart.

Flow regime Bedform Preservation potential Identification tips Lower Lower plane bed High Flat laminae, almost lack of current Ripple marks Relatively Low Small, cm-scale undulations Sand waves Medium to low Rare, longer wavelength than ripples Dunes/Megaripples Low Large, meter-scale ripples Upper Upper plane bed High Flat laminae, ± aligned grains (parting lineations) Antidunes Low Water in phase with bedform, low angle, subtle laminae Pool and chute Very low Mostly erosional features

## Ripple marks

[Wave ripple](/source/Wave_ripple) or symmetric ripple, from [Permian](/source/Permian) rocks in [Nomgon](/source/Nomgon), Mongolia with "decapitation" of ripple crests due to change in current

Ripple marks usually form in conditions with flowing water, in the lower part of the Lower Flow Regime. There are two types of [ripple marks](/source/Ripple_mark):

**Symmetrical ripple marks**
- Often found on beaches, they are created by a two way current, for example the waves on a beach (swash and backwash). This creates ripple marks with pointed crests and rounded troughs, which aren't inclined more to a certain direction. Three common sedimentary structures that are created by these processes are [herringbone cross-stratification](/source/Herringbone_cross-stratification), [flaser bedding](/source/Flaser_bedding), and [interference ripples](/source/Interference_ripples).

**Asymmetrical ripple marks**
- These are created by a one way current, for example in a river, or the wind in a desert. This creates ripple marks with still pointed crests and rounded troughs, but which are inclined more strongly in the direction of the current. For this reason, they can be used as [palaeocurrent](/source/Palaeocurrent) indicators.

## Antidunes

Antidunes are the sediment[2] bedforms created by fast, shallow flows of water with a [Froude number](/source/Froude_number) greater than 1. Antidunes form beneath [standing waves](/source/Stationary_wave) of water that periodically steepen, migrate, and then break upstream. The antidune bedform is characterized by shallow [foresets](/source/Foreset_bed), which dip upstream at an angle of about ten degrees that can be up to five meters in length.[3] They can be identified by their low angle foresets. For the most part, antidunes [bedforms](/source/Bedforms) are destroyed during decreased flow, and therefore cross bedding formed by antidunes will not be preserved.[4][5]

## Biological structures

*[Skolithos](/source/Skolithos)* trace fossil (scale bar is 10 mm)

A number of biologically-created sedimentary structures exist, called [trace fossils](/source/Trace_fossil). Examples include [burrows](/source/Burrow) and various expressions of [bioturbation](/source/Bioturbation). [Ichnofacies](/source/Ichnofacies) are groups of trace fossils that together help give information on the depositional environment. In general, as deeper (into the sediment) burrows become more common, the shallower the water. As (intricate) surface traces become more common, the water becomes deeper.

Microbes may also interact with sediment to form [microbially induced sedimentary structures](/source/Microbially_induced_sedimentary_structure).

## Soft sediment deformation structures

Soft sediment deformation (possibly a [seismite](/source/Seismite)) in [Dead Sea](/source/Dead_Sea) sediments, Israel

[Soft-sediment deformation structures](/source/Soft-sediment_deformation_structures) or SSD, is a consequence of the loading of wet sediment as burial continues after deposition. The heavier sediment "squeezes" the water out of the underlying sediment due to its own weight. There are three common variants of SSD:

- load structures or [load casts](/source/Load_casts) (also a type of [sole marking](/source/Sole_marking)) are blobs that form when a denser, wet sediment slumps down on and into a less dense sediment below.

- [pseudonodules](https://en.wikipedia.org/w/index.php?title=Pseudonodules&action=edit&redlink=1) or [ball-and-pillow structures](/source/Ball-and-pillow_structures), are pinched-off load structures; these may also be formed by earthquake energy and referred to as [seismites](/source/Seismite).

- [flame structures](/source/Flame_structure), "fingers" of mud that protrude into overlying sediments.

- [clastic dikes](/source/Clastic_dike) are seams of sedimentary material that cut across sedimentary strata.

## Bedding plane structures

Flute cast from [Book Cliffs](/source/Book_Cliffs) area, [Utah](/source/Utah)

Mudcracks in rock at [Roundtop Hill](/source/Roundtop_Hill_(Maryland)), Maryland

[Bedding Plane](/source/Bedding_Plane) Structures are commonly used as [paleocurrent](/source/Paleocurrent) indicators. They are formed when sediment has been deposited and then reworked and reshaped. They include:

- [Sole markings](/source/Sole_marking) form when an object gouges the surface of a sedimentary layer; this groove is later preserved as a cast when filled in by the layer above. They include: - Flute casts are scours dug into soft, fine sediment which typically get filled by an overlying bed. Measuring the long axis of the flute cast gives the direction of flow, with the scoop-shaped end pointing in the upcurrent direction and the tapered end pointing downcurrent (paleoflow direction). The convexity of the flute cast also points stratigraphically down. - Tool marks are a type of sole marking formed by grooves left in a bed by objects dragged along by a current. The average direction of these can be assumed to be the axis of flow direction.

- [Mudcracks](/source/Mudcrack) form when mud is dewatered, shrinks, and leaves a crack. This tells you that the mud was saturated with water and then exposed to air. Mudcracks curl upwards, so they can be used as [geopetal](/source/Geopetal) structures. [Syneresis cracks](/source/Syneresis_cracks) form in a similar way, with the exception that they are never exposed to air, instead being caused by changes in the salinity of the surrounding water.

- [Raindrop impressions](/source/Raindrop_impressions) form on exposed sediment by raindrop impacts.

- [Parting lineations](/source/Parting_lineation) are subtly aligned minerals that form in the lower part of the Upper Flow Regime within plane beds.

- **Bomb sag** or **bedding-plane sag** is downwards deformation of tuff beds or other deposits where a [volcanic bomb](/source/Volcanic_bomb) or [volcanic block](/source/Volcanic_block) has fallen.[6]

## Within bedding structures

Cross-bedding and scour in a fine sandstone ([Logan Formation](/source/Logan_Formation), [Mississippian](/source/Mississippian_age), Jackson County, [Ohio](/source/Ohio))

A [teepee structure](/source/Teepee_structure) in modern halite deposits along the western shore of the Dead Sea, Israel

These structures are within sedimentary bedding and can help with the interpretation of depositional environment and [paleocurrent](/source/Paleocurrent) directions. They are formed when the sediment is deposited.

**[Cross-bedding](/source/Cross-bedding)**
- Cross-bedding is the layering of beds deposited by wind or water inclined at an angle as much as 35° from the horizontal.[1] Cross-beds form when sediment particles are deposited on steeper slopes of sand dunes on land or of sandbars in rivers and on the seafloor.[1] Cross-bedding in wind-deposited dunes can be complex as a result of fast changing wind directions.[1]

**[Hummocky cross-stratification](/source/Hummocky_cross-stratification)**
- This stratification is made up of undulating sets of cross-laminae that are concave-up (swales) and convex-up (hummocks). These cross-beds gently cut into each other with curved [erosional surfaces](/source/Erosion_surface). They form in shallow-water, storm-dominated environments. Strong storm-wave action erodes the seabed into low hummocks and swales that lack a specific orientation.

**[Imbrication](/source/Imbrication_(sedimentology))**
- This structure is formed by the stacking of larger clasts in the direction of flow.

**Normal [graded bedding](/source/Graded_bedding)**
- This structure occurs when current velocity changes and grains are progressively dropped out of the current. The most common place to find this is in a [turbidite](/source/Turbidite) deposit. This can also be inverted, called reversed graded bedding, and is common in [debris flows](/source/Debris_flow).

**[Bioturbation](/source/Bioturbation)**
- In many sedimentary rocks, the bedding is broken by cylindrical tubes a few centimeters in diameter that extend vertically through multiple beds.[1] These sedimentary structures are remnants of burrows and tunnels excavated by marine organisms that live on the ocean floor.[1] These organisms churn and burrow through mud and sand a process called bioturbation.[1] They ingest the sediment, digest the organic matter, and leave behind the remnants which fills the burrow.[1]

**[Tidal bundle](/source/Tidal_bundle)**
- Variation in bedding thickness in a tidal environment caused by alternation of spring and neap tides.

## Secondary sedimentary structures

Secondary sedimentary structures form after primary deposition occurs or, in some cases, during the diagenesis of a [sedimentary rock](/source/Sedimentary_rock). Common secondary structures include any form of [bioturbation](/source/Bioturbation), soft-sediment deformation, [teepee structures](/source/Teepee_structure), root-traces, and soil mottling. [Liesegang rings](/source/Liesegang_rings_(geology)), [cone-in-cone structures](/source/Cone-in-cone_structures), [raindrop impressions](/source/Raindrop_impressions), and [vegetation-induced sedimentary structures](/source/Vegetation-induced_sedimentary_structures) would also be considered secondary structures.

Secondary structures include **fluid escape structures**, formed when fluids escape from a sedimentary bed after deposition. Examples of fluid escape structures include [dish structures](/source/Dish_structure), pillar structures,[7] and vertical sheet structures.[8]

## See also

- [Reynolds number](/source/Reynolds_number)

## References

1. ^ [***a***](#cite_ref-:0_1-0) [***b***](#cite_ref-:0_1-1) [***c***](#cite_ref-:0_1-2) [***d***](#cite_ref-:0_1-3) [***e***](#cite_ref-:0_1-4) [***f***](#cite_ref-:0_1-5) [***g***](#cite_ref-:0_1-6) [***h***](#cite_ref-:0_1-7) Jordan, Thomas H.; Grotzinger, John P. (2012). *The Essential Earth* (2nd ed.). New York: W.H. Freeman. [ISBN](/source/ISBN_(identifier)) [9781429255240](https://en.wikipedia.org/wiki/Special:BookSources/9781429255240). [OCLC](/source/OCLC_(identifier)) [798410008](https://search.worldcat.org/oclc/798410008).

1. **[^](#cite_ref-2)** *AILSA ALLABY and MICHAEL ALLABY. "sediment." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. 8 Nov. 2010 <[http://www.encyclopedia.com](http://www.encyclopedia.com)>.

1. **[^](#cite_ref-Boggs_3-0)** Boggs, Sam jr, 2006 Principles of Sedimentology and Stratigraphy, Patrick Lynch, Principles of Sedimentology and Stratigraphy, Pearson Prentice Hall, Upper Saddle River, NJ. Ed 4, p. 83-84

1. **[^](#cite_ref-4)** AILSA ALLABY and MICHAEL ALLABY. "antidune." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. 8 Nov. 2010 <[http://www.encyclopedia.com](http://www.encyclopedia.com)>. B

1. **[^](#cite_ref-5)** [http://jsedres.geoscienceworld.org/cgi/content/abstract/35/4/922](http://jsedres.geoscienceworld.org/cgi/content/abstract/35/4/922) C

1. **[^](#cite_ref-6)** Jackson, Julia A., ed. (1997). "bedding-plane sag". *Glossary of geology* (Fourth ed.). Alexandria, Virginia: American Geological Institute. [ISBN](/source/ISBN_(identifier)) [0922152349](https://en.wikipedia.org/wiki/Special:BookSources/0922152349).

1. **[^](#cite_ref-7)** Tucker, Maurice E. (2011). *Sedimentary rocks in the field : a practical guide* (4th ed.). Chichester, West Sussex: Wiley-Blackwell. p. 160. [ISBN](/source/ISBN_(identifier)) [9780470689165](https://en.wikipedia.org/wiki/Special:BookSources/9780470689165).

1. **[^](#cite_ref-FOOTNOTEJackson1997fluid_escape_structure_8-0)** [Jackson 1997](#CITEREFJackson1997), fluid escape structure.

## Further reading

- Prothero, D. R. and Schwab, F., 1996, *Sedimentary Geology*, pg. 43-64, [ISBN](/source/ISBN_(identifier)) [0-7167-2726-9](https://en.wikipedia.org/wiki/Special:BookSources/0-7167-2726-9)

v t e Rivers, streams and springs Rivers (lists) Alluvial river Braided river Blackwater river Channel Channel pattern Channel types Confluence Distributary Drainage basin Mountain river Subterranean river River bifurcation River ecosystem River source Tributary Streams Arroyo Beck Bourne Burn Chalk stream Coulee Current Stream bed Stream channel Streamflow Stream gradient Stream pool Perennial stream Winterbourne Springs (list) Estavelle/Inversac Geyser Holy well Hot spring list list in the US Karst spring list Mineral spring Ponor Rhythmic spring Spring horizon Sedimentary processes and erosion Abrasion Anabranch Aggradation Armor Bed load Bed material load Granular flow Debris flow Deposition Dissolved load Downcutting Erosion Headward erosion Knickpoint Palaeochannel Progradation Retrogradation Saltation Secondary flow Sediment transport Suspended load Wash load Water gap Fluvial landforms Ait Alluvial fan Antecedent drainage stream Avulsion Bank Bar Bayou Billabong Canyon Chine Cut bank Estuary Floating island Fluvial terrace Gill Gulch Gully Glen Meander scar Mouth bar Oxbow lake Riffle-pool sequence Point bar Ravine Rill River island Rock-cut basin Sedimentary basin Sedimentary structures Strath Thalweg River valley Wadi Fluvial flow Helicoidal flow International scale of river difficulty Log jam Meander Plunge pool Rapids Riffle Shoal Stream capture Waterfall list of waterfalls Whitewater Surface runoff Agricultural wastewater First flush Urban runoff Floods and stormwater 100-year flood Crevasse splay Flash flood Flood Urban flooding Non-water flood Flood barrier Flood control Flood forecasting Flood-meadow Floodplain Flood pulse concept Flooded grasslands and savannas Inundation Storm Water Management Model Return period Point source pollution Effluent Industrial wastewater Sewage River measurement and modelling Baer's law Baseflow Bradshaw model Discharge (hydrology) Drainage density Exner equation Groundwater model Hack's law Hjulström curve Hydrograph Hydrological model Hydrological transport model Infiltration (hydrology) Main stem Playfair's law Relief ratio River Continuum Concept Rouse number Runoff curve number Runoff model (reservoir) Stream gauge WAFLEX Wetted perimeter Volumetric flow rate River engineering Aqueduct Balancing lake Canal Check dam Dam Drop structure Daylighting Detention basin Erosion control Fish ladder Floodplain restoration Flume Infiltration basin Leat Levee River morphology Retention basin Revetment Riparian-zone restoration Stream restoration Weir River sports Canyoning Fly fishing Rafting River surfing Riverboarding Stone skipping Triathlon Whitewater canoeing Whitewater kayaking Whitewater slalom Related Aquifer Aquatic toxicology Body of water Hydraulic civilization Limnology Riparian zone River valley civilization River cruise Sacred waters Surface water Wild river Rivers by length Rivers by discharge rate Drainage basins Whitewater rivers Flash floods River name etymologies Countries without rivers

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Adapted from the Wikipedia article [Sedimentary structures](https://en.wikipedia.org/wiki/Sedimentary_structures) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Sedimentary_structures?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.