Sedimentary structures include all kinds of features in sediments and sedimentary rocks, formed at the time of deposition.

Sediments and sedimentary rocks are characterized by bedding, 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, graded bedding, and ripple marks are utilized in stratigraphic studies to indicate original position of strata in geologically complex terrains and understand the depositional environment of the sediment.

Flow structures

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

There are two kinds of flow structures: bidirectional (multiple directions, back-and-forth) and unidirectional. Flow regimes in single-direction (typically fluvial) flow, which at varying speeds and velocities produce different structures, are called bedforms. In the lower flow regime, the natural progression is from a flat bed, to some sediment movement (saltation 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. 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, 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 or symmetric ripple, from Permian rocks in Nomgon, Mongolia with "decapitation" of ripple crests due to change in current
Wave ripple or symmetric ripple, from Permian rocks in 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:

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, flaser bedding, and 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 indicators.

Antidunes

Antidunes are the sediment[2] bedforms created by fast, shallow flows of water with a Froude number greater than 1. Antidunes form beneath standing waves of water that periodically steepen, migrate, and then break upstream. The antidune bedform is characterized by shallow foresets, 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 are destroyed during decreased flow, and therefore cross bedding formed by antidunes will not be preserved.[4][5]

Biological structures

Skolithos trace fossil (scale bar is 10 mm)
Skolithos trace fossil (scale bar is 10 mm)

A number of biologically-created sedimentary structures exist, called trace fossils. Examples include burrows and various expressions of bioturbation. 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.

Soft sediment deformation structures

Soft sediment deformation (possibly a seismite) in Dead Sea sediments, Israel
Soft sediment deformation (possibly a seismite) in Dead Sea sediments, Israel

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:

Bedding plane structures

Flute cast from Book Cliffs area, Utah
Flute cast from Book Cliffs area, Utah
Mudcracks in rock at Roundtop Hill, Maryland
Mudcracks in rock at Roundtop Hill, Maryland

Bedding Plane Structures are commonly used as paleocurrent indicators. They are formed when sediment has been deposited and then reworked and reshaped. They include:

Within bedding structures

Cross-bedding and scour in a fine sandstone (Logan Formation, Mississippian, Jackson County, Ohio)
Cross-bedding and scour in a fine sandstone (Logan Formation, Mississippian, Jackson County, Ohio)
A teepee structure in modern halite deposits along the western shore of the Dead Sea, Israel
A 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 directions. 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 curved erosional 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.
Normal 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 deposit. This can also be inverted, called reversed graded bedding, and is common in debris 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 a sedimentary rock. Common secondary structures include any form of bioturbation, soft-sediment deformation, teepee structures, root-traces, and soil mottling. Liesegang rings, cone-in-cone structures, raindrop impressions, and 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, pillar structures,[7] and vertical sheet structures.[8]

See also

References

  1. ^ a b c d e f g h Jordan, Thomas H.; Grotzinger, John P. (2012). The Essential Earth (2nd ed.). New York: W.H. Freeman. ISBN 9781429255240. OCLC 798410008.
  2. ^ *AILSA ALLABY and MICHAEL ALLABY. "sediment." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. 8 Nov. 2010 <http://www.encyclopedia.com>.
  3. ^ 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
  4. ^ AILSA ALLABY and MICHAEL ALLABY. "antidune." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. 8 Nov. 2010 <http://www.encyclopedia.com>. B
  5. ^ http://jsedres.geoscienceworld.org/cgi/content/abstract/35/4/922 C
  6. ^ Jackson, Julia A., ed. (1997). "bedding-plane sag". Glossary of geology (Fourth ed.). Alexandria, Viriginia: American Geological Institute. ISBN 0922152349.
  7. ^ Tucker, Maurice E. (2011). Sedimentary rocks in the field : a practical guide (4th ed.). Chichester, West Sussex: Wiley-Blackwell. p. 160. ISBN 9780470689165.
  8. ^ Jackson 1997, fluid escape structure.

Further reading