Sedimentary structures
Encyclopedia
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....
deposition.
Sedimentary structures such as cross bedding, graded bedding
Graded bedding
In geology, a graded bed is one characterized by a systematic change in grain or clast size from the base of the bed to the top. Most commonly this takes the form of normal grading, with coarser sediments at the base, which grade upward into progressively finer ones...
and ripple marks
Ripple marks
In geology, ripple marks are sedimentary structures and indicate agitation by water or wind.- Defining ripple cross-laminae and asymmetric ripples :...
are utilized in 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....
studies to indicate original position
Way up structure
A way up structure, way up criterion, or geopetal indicator is a characteristic relationship observed in a sedimentary or volcanic rock, or sequence of rocks, that makes it possible to determine whether they are the right way up or have been overturned by subsequent deformation...
of 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...
in geologically complex terrains and understand the 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 fluvialFluvial
Fluvial is used in geography and Earth science to refer to the processes associated with rivers and streams and the deposits and landforms created by them...
) flow, which at varying speeds and velocities produce different structures, are called bedform
Bedform
A bedform is a depositional feature on the bed of a river or other body of flowing water that is formed by the movement of the bed material due to the flow. Bedforms are characteristic to the flow parameters, and are particularly to flow depth and velocity, and therefore the Froude...
s. In the lower flow regime, the natural progression is from a flat bed, to some sediment movement (saltation
Saltation
Saltation may refer to:* Saltation , an evolutionary hypothesis emphasizing sudden and drastic change* Saltation , a process of particle transport by fluids* Saltation , the antithesis of Continuous Integration...
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 antidune
Antidune
An antidune is a bedform found in fluvial environments. It occurs where there is supercritical flow, meaning that the Froude number is greater than 1.0 or the flow velocity exceeds the wave velocity; this is also known as upper flow regime. In antidunes, sediment is deposited on the stoss side and...
s. 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 Lower plane bed In geology, lower plane bed refers to the configuration of the bed of a river that is flat and characterized by low rates of sediment transport.... |
High | Flat laminae, almost lack of current | |
| Ripple marks Ripple marks In geology, ripple marks are sedimentary structures and indicate agitation by water or wind.- Defining ripple cross-laminae and asymmetric ripples :... |
High | 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 Upper plane bed In geology, upper plane bed refers to the configuration of the bed of a river that is flat and characterized by a unidirectional flow high with rates of sediment transport as both bed load and suspended load. Upper plane bed conditions can produce current lineations In geology, upper plane bed... |
High | Flat laminae, +/- aligned grains (parting lineation Parting lineation Parting lineation is a subtle sedimentary structure in which sand grains are aligned in parallel lines or grooves on the surface of a body of sand... s) |
|
| Antidunes | Low | Water in phase with bedform, low angle, subtle laminae | |
| Pool and chute | Very low | Mostly erosional features | |
Ripple marks
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-stratificationHerringbone cross-stratificationHerring bone cross-stratification is type of sedimentary structure formed in tidal areas, where the current periodically flows in the opposite direction.-Formation:...
, flaser bedding, and interference ripplesInterference ripplesInterference Ripples are a type of sedimentary structure made up of two sets of ripples formed at right-angles to each other as a result of there being two dominant paleocurrents....
. - 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 bedforms created by fast, shallow flows of water with a Froude numberFroude number
The Froude number is a dimensionless number defined as the ratio of a characteristic velocity to a gravitational wave velocity. It may equivalently be defined as the ratio of a body's inertia to gravitational forces. In fluid mechanics, the Froude number is used to determine the resistance of an...
greater than 0.8. Antidunes form beneath standing waves of water that periodically steepen, migrate, and then break upstream. The antidune bedform is characterized by shallow foresets
Foreset bed
A foreset bed is one of the main parts of a river delta. It is the inclined part of a delta that is found at the end of the stream channel as the delta sediment is deposited along the arcuate delta front. As the sediments are deposited on a sloping surface the resulting bedding is not horizontal,...
, which dip upstream at an angle of about ten degrees that can be up to five meters in length. 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.
Biological structures
Trace fossil
Trace fossils, also called ichnofossils , are geological records of biological activity. Trace fossils may be impressions made on the substrate by an organism: for example, burrows, borings , urolites , footprints and feeding marks, and root cavities...
s. Examples include burrow
Burrow
A burrow is a hole or tunnel dug into the ground by an animal to create a space suitable for habitation, temporary refuge, or as a byproduct of locomotion. Burrows provide a form of shelter against predation and exposure to the elements, so the burrowing way of life is quite popular among the...
s and various expressions of bioturbation
Bioturbation
In oceanography, limnology, pedology, geology , and archaeology, bioturbation is the displacement and mixing of sediment particles and solutes by fauna or flora . The mediators of bioturbation are typically annelid worms , bivalves In oceanography, limnology, pedology, geology (especially...
. Ichnofacies
Ichnofacies
An ichnofacies is an assemblage of trace fossils that provide an indication of the conditions that their formative organisms inhabited.-The concept:...
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
Microbially induced sedimentary structure
Microbially induced sedimentary structures are primary sedimentary structures formed by the interaction of microbes with sediment and physical agents of erosion, deposition, transportation, or deformation traces of microbial activity...
.
Soft sediment deformation structures
Soft-sediment deformation structuresSoft-sediment deformation structures
Soft-sediment deformation structures develop at deposition or shortly after, during the first stages of the sediment's consolidation. This is because the sediments need to be "liquid-like" or unsolidified for the deformation to occur. These formations have been also been put into a category called...
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 structuresBall-and-pillow structuresBall-and-pillow structures are masses of clastic sediment that take the form of isolated pillows or protruding ball structures. These soft-sediment deformations are usually found at the base of sandstone beds that are interbedded with mudstone. It is also possible to find ball-and-pillows in...
, are pinched-off load structures; these may also be formed by earthquake energy and referred to as seismiteSeismiteSeismites are sedimentary beds disturbed by seismic shaking. The German paleontologist Adolf Seilacher first used the term in 1969 to describe a variety of post-depositional effects of seismic shocks on unconsolidated sediments...
s. - flame structureFlame structureA flame structure is a type of soft-sediment deformation that forms in unlithified sediments. The weight of an overlying bed forces an underlying bed to push up through the overlying bed, generally when both strata are saturated with water...
s, "fingers" of mud that protrude into overlying sediments. - clastic dikeClastic dikeA clastic dike is a seam of sedimentary material that fills a crack in and cuts across sedimentary strata. Clastic dikes form rapidly by fluidized injection or passively by water, wind, and gravity . Diagenesis may play a role in the formation of some dikes. Clastic dikes are commonly vertical or...
s are seams of sedimentary material that cut across sedimentary strata.
Bedding plane structures
Bedding Plane Structures are commonly used as paleocurrentPaleocurrent
A paleocurrent or paleocurrent indicator is a geological feature that helps one determine the direction of flowing water in the geologic past...
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:
- 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.
- MudcrackMudcrackMudcracks are sedimentary structures formed as muddy sediment dries and contracts.- Formation :...
s 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 structures. Syneresis cracksSyneresis cracksSyneresis cracks are a sedimentary structure developed by the shrinkage of sediment without desiccation. Syneresis is the expulsion of a liquid from a gel-like substance. Syneresis cracks are formed by the contraction of clay in response to changes in the salinity of a liquid surrounding a...
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 impressionsRaindrop impressionsRaindrop impressions are a geological feature characterized by small craterlike pits with slightly raised edges that are the result of the impact of rain on sediment surfaces...
form on exposed sediment by raindrop impacts. - Parting lineationParting lineationParting lineation is a subtle sedimentary structure in which sand grains are aligned in parallel lines or grooves on the surface of a body of sand...
s are subtly aligned minerals that form in the lower part of the Upper Flow Regime within plane beds.
Within bedding structures
These structures are within sedimentary bedding and can help with the interpretation of depositional environment and paleocurrentPaleocurrent
A paleocurrent or paleocurrent indicator is a geological feature that helps one determine the direction of flowing water in the geologic past...
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. PaleocurrentPaleocurrentA paleocurrent or paleocurrent indicator is a geological feature that helps one determine the direction of flowing water in the geologic past...
s 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-stratificationHummocky cross-stratificationHummocky cross-stratification is a type of sedimentary structure found in sandstones. It is a form of cross-bedding usually formed by the action of large storms, such as hurricanes. It takes the form of a series of "smile"-like shapes, crosscutting each other. It is only formed at a depth of...
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 an specific orientation. - ImbricationImbrication (sedimentology)In sedimentology imbrication refers to a primary depositional fabric consisting of a preferred orientation of clasts such that they overlap one another in a consistent fashion, rather like a run of toppled dominoes...
is the stacking of larger clasts in the direction of flow. - Normal graded beddingGraded beddingIn geology, a graded bed is one characterized by a systematic change in grain or clast size from the base of the bed to the top. Most commonly this takes the form of normal grading, with coarser sediments at the base, which grade upward into progressively finer ones...
occurs when current velocity changes and grains are progressively dropped out of the current. The most common place to find this is in a turbiditeTurbiditeTurbidite geological formations have their origins in turbidity current deposits, which are deposits from a form of underwater avalanche that are responsible for distributing vast amounts of clastic sediment into the deep ocean.-The ideal turbidite sequence:...
deposit. This can also be inverted, called reversed graded bedding, and is common in debris flowDebris flowA debris flow is a fast moving, liquefied landslide of unconsolidated, saturated debris that looks like flowing concrete. It is differentiated from a mudflow in terms of the viscosity and textural properties of the flow. Flows can carry material ranging in size from clay to boulders, and may...
s. - BioturbationBioturbationIn oceanography, limnology, pedology, geology , and archaeology, bioturbation is the displacement and mixing of sediment particles and solutes by fauna or flora . The mediators of bioturbation are typically annelid worms , bivalves In oceanography, limnology, pedology, geology (especially...
- Biological stirring of sediment (i.e. burrowing); typical of shallow water, finer-grained sediment.
Secondary sedimentary structures
Secondary sedimentary structures form during the diagenesis of a sedimentary rock. Common secondary structures include Liesegang ringsLiesegang rings (geology)
Liesegang rings are colored bands of cement observed in sedimentary rocks that typically cut-across bedding. These secondary sedimentary structures exhibit bands of minerals that are arranged in a regular repeating pattern...
, cone-in-cone structures
Cone-in-cone structures
Cone-in-cone structures are secondary sedimentary structures that form in association with deeper burial and diagenesis. They consist of concentric inter-bedded cones of calcite or more rarely gypsum, siderite or pyrite. Although several mechanisms may be responsible for the formation of...
, raindrop impressions
Raindrop impressions
Raindrop impressions are a geological feature characterized by small craterlike pits with slightly raised edges that are the result of the impact of rain on sediment surfaces...
, and vegetation-induced sedimentary structures
Vegetation-induced sedimentary structures
Vegetation-induced sedimentary structures are primary sedimentary structures formed by the interaction of detrital sediment with in situ plants. VISS provide physical evidence of vegetation's fundamental role in mediating sediment accumulation and erosion in clastic depositional environments...
.