Wave-formed ripple

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Wave ripple or symmetric ripple, from Permian rocks in Nomgon, Mongolia. Note "decapatation" of ripple crests due to change in current. WaveRipple.JPG
Wave ripple or symmetric ripple, from Permian rocks in Nomgon, Mongolia. Note "decapatation" of ripple crests due to change in current.

In sedimentology, wave-formed ripples or wave-formed ripple marks are a feature of sediments (sandstones, limestones, siltstones) and dunes. These ripple marks are often characterised (and thus distinguished from current ripples) by symmetric cross sections and long relatively straight crests, which may commonly bifurcate. Commonly, these crests can be truncated by subsequent flows. Their wavelength (periodicity) depends on the sediment grain size, water depth and water-particle orbits in the waves. On tidal flats the pattern of wave-formed ripples may be complicated, as a product of changing depth and wind and tidal runoff directions. [1] Symmetrical ripples are commonly found in shallow waters. Beaches are a good place to find these ripples.

Sedimentology encompasses the study of modern sediments such as sand, silt, and clay, and the processes that result in their formation, transport, deposition and diagenesis. Sedimentologists apply their understanding of modern processes to interpret geologic history through observations of sedimentary rocks and sedimentary structures.

Sandstone A clastic sedimentary rock composed mostly of sand-sized particles

Sandstone is a clastic sedimentary rock composed mainly of sand-sized mineral particles or rock fragments.

Limestone Sedimentary rocks made of calcium carbonate

Limestone is a carbonate sedimentary rock that is often composed of the skeletal fragments of marine organisms such as coral, foraminifera, and molluscs. Its major materials are the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (CaCO3). A closely related rock is dolomite, which contains a high percentage of the mineral dolomite, CaMg(CO3)2. In fact, in old USGS publications, dolomite was referred to as magnesian limestone, a term now reserved for magnesium-deficient dolomites or magnesium-rich limestones.

Contents

While wave-formed ripples are traditionally described as symmetrical, asymmetric wave ripples are common in shallow waters along sandy shores. They are produced by bottom oscillations generated by passing breaker waves, which have unequal intensity in opposite directions. [2]

Wave-formed ripples indicate an environment with weak currents where water motion is dominated by wave oscillations. [3]

Symmetrical ripple Sym ripple.jpg
Symmetrical ripple

Although symmetrical ripples are also called bi-directional ripples there is a difference between them. Bi-directional ripples are rarely symmetrical due to the difference in force of the two directions, where as the wave formed or oscillation ripples form from the circular water movement pattern of water molecules. These ripples form parallel to the shore line. They usually display rounded troughs and rounded crests.

Wave motion-i18n
A=Effect of oscillatory motion in deep water
B=Effect of oscillatory motion in shallow water
1=Progression of wave
2=Wave crest
3=Wave trough Wave motion-i18n.png
Wave motion-i18n
A=Effect of oscillatory motion in deep water
B=Effect of oscillatory motion in shallow water
1=Progression of wave
2=Wave crest
3=Wave trough

Ripples

Ripples are relatively small, elongated ridges that form on bed surfaces perpendicular to current flow. With continuous current flow in one direction, asymmetrical ripples form. Asymmetrical ripples contain a steeper slope downstream. With an alternation in current flow from one direction to the opposite symmetrical ripples form. Symmetrical ripples tend to have the same slope on both sides of the crest. [6]

Formation

Symmetrical ripples form as water molecules oscillate in small circles. A particle of water within a wave does not move with the wave but rather it moves in a small circle between the wave crest and wave trough. This movement of water molecules is the same for all water molecules effected by the wave. The water molecules continue to do this to a depth equal to 1/2 the wavelength. The water molecule traveling in a circular pattern interacts with the sediment on the floor and moves the sediment into symmetrical ripples. These ripples can be either straight crested or sinuous crested ripples. [7]

See also

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Dune A hill of loose sand built by aeolian processes or the flow of water

In physical geography, a dune is a hill of loose sand built by aeolian processes (wind) or the flow of water. Dunes occur in different shapes and sizes, formed by interaction with the flow of air or water. Most kinds of dunes are longer on the stoss (upflow) side, where the sand is pushed up the dune, and have a shorter "slip face" in the lee side. The valley or trough between dunes is called a slack. A "dune field" or erg is an area covered by extensive dunes.

Beach Area of loose particles at the edge of the sea or other body of water

A beach is a landform alongside a body of water which consists of loose particles. The particles composing a beach are typically made from rock, such as sand, gravel, shingle, pebbles. The particles can also be biological in origin, such as mollusc shells or coralline algae.

Sedimentary rock Rock formed by the deposition and subsequent cementation of material

Sedimentary rocks are types of rock that are formed by the accumulation or deposition of small particles and subsequent cementation of mineral or organic particles on the floor of oceans or other bodies of water at the Earth's surface. Sedimentation is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called sediment, and may be composed of geological detritus (minerals) or biological detritus. Before being deposited, the geological detritus was formed by weathering and erosion from the source area, and then transported to the place of deposition by water, wind, ice, mass movement or glaciers, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies. Sedimentation may also occur as dissolved minerals precipitate from water solution.

Transverse wave moving wave that consists of oscillations occurring perpendicular (right angled) to the direction of energy transfer (or the propagation of the wave)

A transverse wave is a moving wave that consists of oscillations that are perpendicular to the direction of propagation of the wave.

Deposition (geology) Geological process in which sediments, soil and rocks are added to a landform or land mass

Deposition is the geological process in which sediments, soil and rocks are added to a landform or land mass. Wind, ice, water, and gravity transport previously weathered surface material, which, at the loss of enough kinetic energy in the fluid, is deposited, building up layers of sediment.

Wind wave Surface waves generated by wind that occur on the free surface of bodies of water

In fluid dynamics, wind waves, or wind-generated waves, are surface waves that occur on the free surface of bodies of water. They result from the wind blowing over an area of fluid surface. Waves in the oceans can travel thousands of miles before reaching land. Wind waves on Earth range in size from small ripples, to waves over 100 ft (30 m) high.

Internal wave Gravity waves that oscillate within a fluid medium with density variation with depth, rather than on the surface

Internal waves are gravity waves that oscillate within a fluid medium, rather than on its surface. To exist, the fluid must be stratified: the density must decrease continuously or discontinuously with depth/height due to changes, for example, in temperature and/or salinity. If the density changes over a small vertical distance, the waves propagate horizontally like surface waves, but do so at slower speeds as determined by the density difference of the fluid below and above the interface. If the density changes continuously, the waves can propagate vertically as well as horizontally through the fluid.

Tempestite

Tempestites are studied throughout sedimentary geology and paleotempestology, they are storm deposits that can be recognized throughout the rock record. The deposit gets its meaning from the word tempest which simply means a violent storm, the tempestite describes the preservation and physical representation of a violent storm event (tempest) within the rock record. Tempestites can be preserved within a multitude of sedimentological environments, including delta systems, estuarian systems, coastal environments, deep sea environments, and even fresh water lacustrine environments. Tempesites most often form in wave dominated delta systems and are preserved within the sedimentological record below fair weather wave base and above storm weather wave base. They are commonly characterized by hummocky cross stratified beds that have an erosive base, and can form under combined flow regimes. This erosive base is often seen in the form of gutter casts.

Diver navigation Underwater navigation by scuba divers

Diver navigation, termed "underwater navigation" by scuba divers, is a set of techniques—including observing natural features, the use of a compass, and surface observations—that divers use to navigate underwater. Free-divers do not spend enough time underwater for navigation to be important, and surface supplied divers are limited in the distance they can travel by the length of their umbilicals and are usually directed from the surface control point. On those occasions when they need to navigate they can use the same methods used by scuba divers.

Cross-bedding

In geology, cross-bedding, also known as cross-stratification, is layering within a stratum and at an angle to the main bedding plane. The sedimentary structures which result are roughly horizontal units composed of inclined layers. The original depositional layering is tilted, such tilting not being the result of post-depositional deformation. Cross-beds or "sets" are the groups of inclined layers, which are known as cross-strata.

Ripple marks sedimentary structures

In geology, ripple marks are sedimentary structures and indicate agitation by water or wind.

Sole markings

Sole marks are sedimentary structures found on the bases of certain strata, that indicate small-scale grooves or irregularities. This usually occurs at the interface of two differing lithologies and/or grain sizes. They are commonly preserved as casts of these indents on the bottom of the overlying bed. This is similar to casts and molds in fossil preservation. Occurring as they do only at the bottom of beds, and their distinctive shapes, they can make useful way up structures and paleocurrent indicators.

Bioclast

Bioclasts are skeletal fossil fragments of once living marine or land organisms that are found in sedimentary rocks laid down in a marine environment—especially limestone varieties around the globe. some of which take on distinct textures and coloration from their predominate bioclasts—that geologists, archaeologists and paleontologists use to date a rock strata to a particular geological era.

Sedimentary structures include all kinds of features 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. These beds range from millimeters to centimeters thick and can even go to meters or multiple meters thick.

Antidune

An antidune is a bedform found in fluvial and other channeled environments. Antidunes occur in 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 upstream (stoss) side and eroded from the downstream (lee) side, opposite lower flow regime bedforms. As a result, antidunes migrate in an upstream direction, counter to the current flow. Antidunes are called in-phase bedforms, meaning that the water surface elevation mimics the bed elevation; this is due to the supercritical flow regime. Antidune bedforms evolve rapidly, growing in amplitude as they migrate upstream. The resultant wave at the water's surface also increases in amplitude. When that wave becomes unstable, breaks and washes downstream, much of the antidune bedform may be destroyed.

Flaser bed

Flaser beds are a sedimentary, bi-directional, bedding pattern created when a sediment is exposed to intermittent flows, leading to alternating sand and mud layers. While flaser beds typically form in tidal environments, they can (rarely) form in fluvial conditions - on point bars or in ephemeral streams. Individual sand ripples are created, which are later infilled by mud during quieter flow periods. These mud drapes are typically a minor constituent of the deposit; they can consolidate within three hours, protecting the underlying layer from erosion. Flaser bedding typically forms in high-energy environments but some have also been described in turbiditic sediments reworked by contour currents.

Hummocky cross-stratification

Hummocky 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 water below fair-weather wave base and above storm-weather wave base. They are not related to "hummocks" except in shape.

Bedform

A bedform is a feature that develops at the interface of fluid and a moveable bed, the result of bed material being moved by fluid flow. Examples include ripples and dunes on the bed of a river. Bedforms are often preserved in the rock record as a result of being present in a depositional setting. Bedforms are often characteristic to the flow parameters, and may be used to infer flow depth and velocity, and therefore the Froude number.

Heterolithic bedding is a sedimentary structure made up of interbedded deposits of sand and mud. It is formed mainly in tidal flats but can also be formed in glacial environments. Examples from fluvial environments have been documented but are rare. Heterolithic bedding forms in response to alternations in sediment supply and tidal velocity. The fluctuations result in the interbedded layers of sand and mud. The rippled sand layer is formed during high tidal currents, while the mud is deposited during slack tide periods .The three main types of heterolithic bedding are flaser, wavy, and lenticular. Starved ripples and cross bedding with flasers can also be considered forms of heterolithic bedding. Differentiating of these various types of heterolithic bedding is based on the relative volume of mud and sand. This key determining factor is controlled by the timing, and duration of both the high tide, and slack tide depositional periods.

Periodic Bedrock Ridges (PBRs) are features of the surface geomorphology of Mars and formerly unknown on Earth, first described in a paper in the Journal of Geological Research - Planets, in March 2012, by Professor David R. Montgomery, Joshua L. Bandfield, and Scott K. Becker of the University of Washington.

References

  1. Maurice E. Tucker (2001) "Sedimentary Petrology",ISBN   0-632-05735-1
  2. O. F. Evans, "The Classification of Wave-formed Ripple Marks", Journal of Sedimentary Research, Volume 11 (1941), doi : 10.1306/D42690DF-2B26-11D7-8648000102C1865D
  3. Monroe, James S., and Reed Wicander. The Changing Earth: Exploring Geology and Evolution, 2nd ed. Belmont: West Publishing Company, 1997. ISBN   0-314-09577-2 pp. 11415, 352.
  4. Hampton, Monty A., Torresan, Michael E., and Barber, Jr., John H., 1997, Sea-floor geology of a part of Mamala Bay, Hawaii: Pacific Science, v. 51, n. 1, p. 54-75. Reproduced by permission of the University of Hawaii Press
  5. Department of Earth Science, Preston Cloud Research Laboratory, University of California, Santa Barbara, CA 93106, United States Precambrian Research; Nov2009, Vol. 174 Issue 3/4, p215-240, 26p
  6. Marshak, Stephen. "Chapter 7: A Surface Veneer: Sediments, Soils and Sedimentary Rocks." Earth Portrait of a Planet. 3rd ed. New York: W.W Norton & company, 2008. 211. Print.
  7. Atlantic coast beaches : a guide to ripples, dunes, and other natural features of the seashore / William J. Neal, Orrin H. Pilkey, Joseph T. Kelley. Publisher Missoula, Mont. : Mountain Press, 2007