Hellenica World

Sedimentary structures

Sedimentary structures are those structures formed during sediment deposition.

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.

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 plane bed High Flat laminae, almost lack of current
Ripple marks High Small, cm-scale undulations
Sand waves Medium to low Rare, longer wavelength than ripples
Dunes/Megaripples Low Large, meter-scale ripples
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. Note "decapatation" 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.

Biological structures
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 in etched section of carbonaceous sandstone bed of Reedsville Formation from along PA Rt. 36 near Loysburg, Bedford Co., PA.

Soft sediment deformation 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 (also a type of sole marking) are blobs that form when a denser, wet sediment slumps down on and into a less dense sediment below.
* pseudonodules or "ball and pillow" structures, are pinched-off load structures; these may also be formed by earthquake energy and referred to as seismites.
* flame structures, "fingers" of mud that protrude into overlying sediments.

Bedding Plane Structures
Flute cast from Book Cliffs area, Utah.
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:

* Sole markings 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:
o 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.
o 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 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 or hypersaline water. Mudcracks curl upwards, so they can be used as geopetal structures.
* Raindrop impressions form on exposed sediment by raindrop impacts.
* Parting lineations are subtly aligned minerals that form in the lower part of the Upper Flow Regime within plane beds.

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

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 - This can include ripples and dunes, or any cross stratification caused by currents. The "cross" refers to the angle between flat bedding and the inclined bedding of the cross bed, typically about 34 degrees. Paleocurrents are best found from cross beds that have 3D architecture exposed so you can measure the axis of the trough of the cross bed.
* Hummocky cross-stratification is formed by storm waves, but is not a good paleocurrent indicator.
* Imbrication is the stacking of larger clasts in the direction of flow.
* Normal graded bedding 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 - Biological stirring of sediment (i.e. burrowing); typical of shallow water, finer-grained sediment.

See also

* Reynolds number
* Froude number


Prothero, D. R. and Schwab, F., 1996, Sedimentary Geology, pg. 43-64, ISBN 0-7167-2726-9

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