Cross-bedding
Encyclopedia
In geology
, the sedimentary structures known as cross-bedding refer to (near-) horizontal units that are internally composed of inclined layers. This is a case in geology in which the original depositional layering is tilted, and the tilting is not a result of post-depositional deformation. Cross-beds or "sets" are the groups of inclined layers, and the inclined layers are known as cross strata.
Cross bedding forms during deposition on the inclined surfaces of bedforms such as ripples and dune
s, and indicates that the depositional environment
contained a flowing fluid (typically, water or wind). Examples of these bedforms are ripples, dunes, anti-dunes, sand waves, hummocks, bars, and delta slopes. Cross-bedding is widespread in many environments. Environments in which water movement is fast enough and deep enough to develop large-scale bed forms fall into three natural groupings: rivers, tide-dominated coastal and marine settings.
The direction of motion of the cross-beds can show ancient flow or wind directions (called paleocurrent
s). The foresets are deposited at the angle of repose (~34 degrees from the horizontal), so geologists are able to measure dip
direction of the cross-bedded sediments and calculate the paleoflow direction. However, most cross-beds are not tabular, they are troughs. Since troughs can give a 180 degree variation of the dip of foresets, false paleocurrents can be taken by blindly measureing foresets. In this case, true paleocurrent direction is determined by the axis of the trough. Paleocurrent direction is important in reconstructing past climate and drainage patterns: sand dunes preserve the prevalent wind directions, and current ripples show the direction rivers were moving.
from its upstream ("stoss") side onto its downstream ("lee") side. Before and during the deposition of the sediment on the lee slope, the grain sizes of the arriving sediment are generally separated. This grain-size sorting is what allows cross strata to be recognized in rocks and sediment deposits.
The angle and direction of cross-beds are generally fairly consistent. Individual cross-beds can range in thickness from just a few tens of centimeters, up to hundreds of feet or more depending upon the depositional environment and the size of the bedform. Cross-bedding can form in any environment in which a fluid flows over a bed with mobile material. It is most common in stream
deposits (consisting of sand and gravel), tidal areas, and in aeolian dunes.
s", which are the series of layers that form on the lee side of the bedform (ripple or dune). These foresets are individually differentiable because of small-scale separation between layers of material of different sizes and densities.
Cross-bedding can also be recognized by truncations in sets of ripple foresets, where previously-existing stream deposits are eroded by a later flood, and new bedforms are deposited in the scoured area.
Tabular cross-bedding is formed mainly by migration of large scale, straight crested ripples and dunes. It forms during lower flow regime conditions and its individual beds range in thickness from a few tens of centimeters to a meter or more, but bed thickness down to 10 centimeters has been observed. Where the set height is less than 6 centimeters and the cross-stratification layers are only a few millimeters thick, the term cross-lamination is used. For larger features, the term cross-bedding is used. They occur typically in granular sediments, especially sandstone, and are indication of sediments deposited in deltas, sand dunes and glaciers.
Bed forms are relatively dynamic sediment storage bodies with response times that are short relative to major changes in flow characteristics. Large scale bed forms are periodic and occur in the channel (scaled to depth). Their presence and morphologic variability have been related to flow strength expressed as mean velocity or shear stress.
In a fluvial environment, the water in a stream loses energy and its ability transport sediment. The sediment “falls” out of the water and is deposited along a point bar. Over time the river may dry up or avulse and the point bar may be preserved as cross bedding.
Although the flow direction reverses regularly, the flow patterns of flood on ebb currents commonly do not coincide. Consequently the water and transport sediment may follow a roundabout route in and out of the estuary. This leads to spatially varied systems where some parts of the estuary are flood dominated and other parts are ebb dominated. The temporal and spatial variability of flow and sediment transport, coupled with regular fluctuating water levels creates a variety of bed form morphology.
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
, the sedimentary structures known as cross-bedding refer to (near-) horizontal units that are internally composed of inclined layers. This is a case in geology in which the original depositional layering is tilted, and the tilting is not a result of post-depositional deformation. Cross-beds or "sets" are the groups of inclined layers, and the inclined layers are known as cross strata.
Cross bedding forms during deposition on the inclined surfaces of bedforms such as ripples and dune
Dune
In physical geography, a dune is a hill of sand built by wind. Dunes occur in different forms and sizes, formed by interaction with the wind. Most kinds of dunes are longer on the windward side where the sand is pushed up the dune and have a shorter "slip face" in the lee of the wind...
s, and indicates that the depositional environment
Sedimentary depositional environment
In geology, sedimentary depositional environment describes the combination of physical, chemical and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will be formed after lithification, if the sediment is preserved in the rock...
contained a flowing fluid (typically, water or wind). Examples of these bedforms are ripples, dunes, anti-dunes, sand waves, hummocks, bars, and delta slopes. Cross-bedding is widespread in many environments. Environments in which water movement is fast enough and deep enough to develop large-scale bed forms fall into three natural groupings: rivers, tide-dominated coastal and marine settings.
Significance
Cross beds can tell geologists much about what an area was like in ancient times. The direction the beds are dipping indicates paleocurrent. The type and condition of sediments can tell geologists the type of environment (rounding, sorting, composition…). Studying modern analogs allows geologists to draw conclusions about ancient environments. Paleocurrent- The rough direction of sediment transport can be determined by seeing a cross section of a set of cross beds. However, to get a true reading, the axis of the beds must be visible. It is also difficult to distinguish between the cross beds of a dune and the cross beds of an antidune. This could lead to misinterpretation since dunes dip downstream while antidunes dip upstream.The direction of motion of the cross-beds can show ancient flow or wind directions (called paleocurrent
Paleocurrent
A paleocurrent or paleocurrent indicator is a geological feature that helps one determine the direction of flowing water in the geologic past...
s). The foresets are deposited at the angle of repose (~34 degrees from the horizontal), so geologists are able to measure dip
Strike and dip
Strike and dip refer to the orientation or attitude of a geologic feature. The strike line of a bed, fault, or other planar feature is a line representing the intersection of that feature with a horizontal plane. On a geologic map, this is represented with a short straight line segment oriented...
direction of the cross-bedded sediments and calculate the paleoflow direction. However, most cross-beds are not tabular, they are troughs. Since troughs can give a 180 degree variation of the dip of foresets, false paleocurrents can be taken by blindly measureing foresets. In this case, true paleocurrent direction is determined by the axis of the trough. Paleocurrent direction is important in reconstructing past climate and drainage patterns: sand dunes preserve the prevalent wind directions, and current ripples show the direction rivers were moving.
Formation
Cross-bedding is formed by the migration of bedforms, such as ripples or dunes. The fluid flow causes bedforms to migrate by moving sedimentSediment
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....
from its upstream ("stoss") side onto its downstream ("lee") side. Before and during the deposition of the sediment on the lee slope, the grain sizes of the arriving sediment are generally separated. This grain-size sorting is what allows cross strata to be recognized in rocks and sediment deposits.
The angle and direction of cross-beds are generally fairly consistent. Individual cross-beds can range in thickness from just a few tens of centimeters, up to hundreds of feet or more depending upon the depositional environment and the size of the bedform. Cross-bedding can form in any environment in which a fluid flows over a bed with mobile material. It is most common in stream
River
A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...
deposits (consisting of sand and gravel), tidal areas, and in aeolian dunes.
Internal sorting patterns
Cross-bedded sediments are recognized in the field by the many layers of "foresetForeset 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,...
s", which are the series of layers that form on the lee side of the bedform (ripple or dune). These foresets are individually differentiable because of small-scale separation between layers of material of different sizes and densities.
Cross-bedding can also be recognized by truncations in sets of ripple foresets, where previously-existing stream deposits are eroded by a later flood, and new bedforms are deposited in the scoured area.
Geometries
Cross-bedding can be subdivided according to the geometry of the sets and cross strata into subcategories. The most commonly described types are tabular cross-bedding and trough cross-bedding. Tabular cross-bedding, or planar bedding consists of cross-bedded units that are large horizontal wise with reverence to set thickness and that have essentially planar bounding surfaces. Trough cross-bedding, on the other hand, consists of cross-bedding units in which the bounding surfaces are bowed .Tabular (planar) cross-beds
Tabular (planar) cross-beds consist of cross-bedded units that have large horizontal extent relative to set thickness and that have essentially planar bounding surfaces. The foreset laminae of tabular cross-beds have curved laminae that have a tangential relationship to the basal surface .Tabular cross-bedding is formed mainly by migration of large scale, straight crested ripples and dunes. It forms during lower flow regime conditions and its individual beds range in thickness from a few tens of centimeters to a meter or more, but bed thickness down to 10 centimeters has been observed. Where the set height is less than 6 centimeters and the cross-stratification layers are only a few millimeters thick, the term cross-lamination is used. For larger features, the term cross-bedding is used. They occur typically in granular sediments, especially sandstone, and are indication of sediments deposited in deltas, sand dunes and glaciers.
Trough cross-beds
Cross beds are layers of sediment that are inclined relative to the base and top of the bed they are associated with. Cross beds can tell modern geologists many things about ancient environments such as- depositional environment, the direction of sediment transport (paleocurrent) and even environmental conditions at the time of deposition. Typically, units in the rock record are referred to as beds, while the constituent layers that make up the bed are referred to as laminae, when they are less than 1 cm thick and strata when they are greater than 1 cm in thickness. . Cross beds are angled relative to either the base or the top of the surrounding beds. As opposed to angled beds, cross beds are deposited at an angle rather than deposited horizontally and deformed later on. Trough cross-beds have lower surfaces which are curved or scoop shaped and truncate the underlying beds. The foreset beds are also curved and merge tangentially with the lower surface. They are associated with sand dune migration.Sediment
The shape of the grains and the sorting and composition of sediment can provide additional infomration on the history of cross beds. Roundness of the grains, limited variation in grain size, and high quartz contents are generally attributed to longer histories of weathering and sediment transport. For example: well-rounded, and well-sorted sand that is mostly composed of quartz grains is commonly found in beach environments, far from the source of the sediment. Poorly sorted and angular that is composed of a diversity of minerals is more commonly found in rivers, near the source of the sediment. However, older sedimentary deposits are frequently eroded and re-mobilized. Thus, a river may well erode an older formation of well-rounded, well-sorted beach sands of nearly pure quartz...Rivers
Flows are characterized by climate (snows, rain, and ice melting) and gradient. Discharge variations measured on a variety of time scales can change water depth, and speed. Some rivers can be characterized by a predictable seasonably controlled hydrograph (reflecting snow melt or rainy season). Others are dominated by durational variations characteristic of alpine glaciers run-off or random storm events, which produce flashy discharge. Few rivers have a long term record of steady flow in the rock record.Bed forms are relatively dynamic sediment storage bodies with response times that are short relative to major changes in flow characteristics. Large scale bed forms are periodic and occur in the channel (scaled to depth). Their presence and morphologic variability have been related to flow strength expressed as mean velocity or shear stress.
In a fluvial environment, the water in a stream loses energy and its ability transport sediment. The sediment “falls” out of the water and is deposited along a point bar. Over time the river may dry up or avulse and the point bar may be preserved as cross bedding.
Tide-dominated
Coastal water bodies that are partially enclosed by topography yet have a free connection to the sea are usually tide-dominated or coast lines that are greater than one meter tidal range or where fresh water run-off volume is low relative to the tidal volume. In general, the greater the tide range the greater the maximum flow strength.Although the flow direction reverses regularly, the flow patterns of flood on ebb currents commonly do not coincide. Consequently the water and transport sediment may follow a roundabout route in and out of the estuary. This leads to spatially varied systems where some parts of the estuary are flood dominated and other parts are ebb dominated. The temporal and spatial variability of flow and sediment transport, coupled with regular fluctuating water levels creates a variety of bed form morphology.