Sedimentary structures

Sedimentary structuresinclude all kinds of features insedimentsandsedimentary rocks,formed at the time ofdeposition.

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

Sediments and sedimentary rocks are characterized bybedding,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 ascross-bedding,graded bedding,andripple marksare utilized instratigraphicstudies to indicateoriginal positionofstratain geologically complex terrains and understand thedepositional environmentof the sediment.

Flow structures

There are two kinds of flow structures: bidirectional (multiple directions, back-and-forth) and unidirectional. Flow regimes in single-direction (typicallyfluvial) flow, which at varying speeds and velocities produce different structures, are calledbedforms.In thelower flow regime,the natural progression is from a flat bed, to some sediment movement (saltationetc.), to ripples, to slightly larger dunes. Dunes have a vortex in the lee side of the dune. As theupper flow regimeforms, the dunes become flattened out, and then produceantidunes.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 interpretingdepositional environments,withincreasingwater 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 rippleor symmetric ripple, fromPermianrocks inNomgon,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 ofripple marks:

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 areherringbone cross-stratification,flaser bedding,andinterference 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 aspalaeocurrentindicators.

Antidunes

Antidunes are the sediment^[2]bedforms created by fast, shallow flows of water with aFroude numbergreater than 1. Antidunes form beneathstanding wavesof water that periodically steepen, migrate, and then break upstream. The antidune bedform is characterized by shallowforesets,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, antidunesbedformsare destroyed during decreased flow, and therefore cross bedding formed by antidunes will not be preserved.^[4]^[5]

Biological structures

Skolithostrace fossil (scale bar is 10 mm)

A number of biologically-created sedimentary structures exist, calledtrace fossils.Examples includeburrowsand various expressions ofbioturbation.Ichnofaciesare 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 formmicrobially induced sedimentary structures.

Soft sediment deformation structures

Soft sediment deformation (possibly aseismite) inDead Seasediments, Israel

Soft-sediment deformation structuresor 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 orload casts(also a type ofsole marking) are blobs that form when a denser, wet sediment slumps down on and into a less dense sediment below.
pseudonodulesorball-and-pillow structures,are pinched-off load structures; these may also be formed by earthquake energy and referred to asseismites.
flame structures,"fingers" of mud that protrude into overlying sediments.
clastic dikesare seams of sedimentary material that cut across sedimentary strata.

Bedding plane structures

Flute cast fromBook Cliffsarea,Utah

Mudcracks in rock atRoundtop Hill,Maryland

Bedding PlaneStructures are commonly used aspaleocurrentindicators. They are formed when sediment has been deposited and then reworked and reshaped. They include:

Sole markingsform 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.
Mudcracksform 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 asgeopetalstructures.Syneresis cracksform 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 impressionsform on exposed sediment by raindrop impacts.
Parting lineationsare subtly aligned minerals that form in the lower part of the Upper Flow Regime within plane beds.

Bomb sagorbedding-plane sagis downwards deformation of tuff beds or other deposits where avolcanic bomborvolcanic blockhas fallen.^[6]

Within bedding structures

Cross-bedding and scour in a fine sandstone (Logan Formation,Mississippian,Jackson County,Ohio)

Ateepee structurein 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 andpaleocurrentdirections. They are formed when the sediment is deposited.

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: 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 curvederosional surfaces.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: This structure is formed by the stacking of larger clasts in the direction of flow.
Normalgraded 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 aturbiditedeposit. This can also be inverted, called reversed graded bedding, and is common indebris flows.
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: 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 asedimentary rock.Common secondary structures include any form ofbioturbation,soft-sediment deformation,teepee structures,root-traces, and soil mottling.Liesegang rings,cone-in-cone structures,raindrop impressions,andvegetation-induced sedimentary structureswould also be considered secondary structures.

Secondary structures includefluid escape structures,formed when fluids escape from a sedimentary bed after deposition. Examples of fluid escape structures includedish structures,pillar structures,^[7]and vertical sheet structures.^[8]

References

^^a ^b ^c ^d ^e ^f ^g ^hJordan, Thomas H.; Grotzinger, John P. (2012).The Essential Earth(2nd ed.). New York: W.H. Freeman.ISBN 9781429255240.OCLC 798410008.
^*AILSA ALLABY and MICHAEL ALLABY. "sediment." A Dictionary of Earth Sciences. 1999. Encyclopedia. 8 Nov. 2010 <http:// encyclopedia>.
^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
^AILSA ALLABY and MICHAEL ALLABY. "antidune." A Dictionary of Earth Sciences. 1999. Encyclopedia. 8 Nov. 2010 <http:// encyclopedia>. B
^http://jsedres.geoscienceworld.org/cgi/content/abstract/35/4/922C
^Jackson, Julia A., ed. (1997). "bedding-plane sag".Glossary of geology(Fourth ed.). Alexandria, Virginia: American Geological Institute.ISBN 0922152349.
^Tucker, Maurice E. (2011).Sedimentary rocks in the field: a practical guide(4th ed.). Chichester, West Sussex: Wiley-Blackwell. p. 160.ISBN 9780470689165.
^Jackson 1997,fluid escape structure.