Sediment and Sedimentation

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Sediment and sedimentation

Sediments are loose Earth materials such as sand that accumulate on the land surface, in river and lake beds, and on the ocean floor. Sediments form by weathering of rock. They then erode from the site of weathering and are transported by wind , water , ice , and mass wasting , all operating under the influence of gravity. Eventually sediment settles out and accumulates after transport; this process is known as deposition. Sedimentation is a general term for the processes of erosion , transport, and deposition. Sedimentology is the study of sediments and sedimentation.

There are three basic types of sediment: rock fragments, or clastic sediments; mineral deposits, or chemical sediments; and rock fragments and organic matter , or organic sediments. Dissolved minerals form by weathering rocks exposed at Earth's surface. Organic matter is derived from the decaying remains of plants and animals.

Weathering

Clastic and chemical sediments form during weathering of bedrock or pre-existing sediment by both physical and chemical processes. Organic sediments are also produced by a combination of physical and chemical weathering. Physical (or mechanical) weathering—the disintegration of Earth materials—is generally caused by abrasion or fracturing, such as the striking of one pebble against another in a river or stream bed, or the cracking of a rock by expanding ice. Physical weathering produces clastic and organic sediment.

Chemical weathering, or the decay and dissolution of Earth materials, is caused by a variety of processes. However, it results primarily from various interactions between water and rock material. Chemical weathering may alter the mineral content of a rock by either adding or removing certain chemical components. Some mineral by-products of chemical weathering are dissolved by water and transported below ground or to an ocean or lake in solution . Later, these dissolved minerals may precipitate out, forming deposits on the roof of a cave (as stalactites), or the ocean floor. Chemical weathering produces clastic, chemical, and organic sediments.

Erosion and transport

Erosion and transport of sediments from the site of weathering are caused by one or more of the following agents: gravity, wind, water, or ice. When gravity acts alone to move a body of sediment or rock, this is known as mass wasting. When the forces of wind, water, or ice act to erode sediment, they always do so under the influence of gravity.

Agents of erosion and transport

Gravity

Large volumes of sediment, ranging in size from mud to boulders, can move downslope due to gravity, a process called mass wasting. Rock falls, landslides, and mudflows are common types of mass wasting. If you have ever seen large boulders on a roadway you have seen the results of a rock fall. Rock falls occur when rocks in a cliff face are loosened by weathering, break loose, and roll and bounce downslope. Landslides consist of rapid downslope movement of a mass of rock or soil , and require that little or no water be present. Mud flows occur when a hillside composed of fine grained material becomes nearly saturated by heavy rainfall. The water helps lubricate the sediment, and a lobe of mud quickly moves downslope. Other types of mass wasting include slump, creep, and subsidence .

Water

Water is the most effective agent of transport, even in the desert . When you think of water erosion, you probably think of erosion mainly by stream water, which is channelized. However, water also erodes when it flows over a lawn or down the street, in what is known as sheet flow. Even when water simply falls from the sky and hits the ground in droplets, it erodes the surface. The less vegetation that is present, the more water erodes—as droplets, in sheets, or as channelized flow.

Wind

You may think of wind as a very important agent of erosion, but it is really only significant where little or no vegetation is present. For this reason, deserts are well known for their wind erosion. However, as mentioned above, even in the desert, infrequent, but powerful rain storms are still the most important agent of erosion. This is because relatively few areas of the world have strong prevailing winds with little vegetation, and because wind can rarely move particles larger than sand or small pebbles.

Glacial ice

Ice in glaciers is very effective at eroding and transporting material of all sizes. Glaciers can move boulders as large as a house hundreds of miles.

If you look around, glaciers are not a very common sight these days. However, at times in the geologic past, continent-sized glaciers covered vast areas of the Earth at middle to high latitudes. Today, continental glaciers occur only on Antarctica and Greenland. In addition, many smaller glaciers exist at high altitudes on some mountains . These are called alpine glaciers.

Sediment erosion

Generally, erosive agents remove sediments from the site of weathering in one of three ways: impact of the agent, abrasion (both types of mechanical erosion, or corrasion), or corrosion (chemical erosion). The mere impact of wind, water, and ice erodes sediments; for example, flowing water exerts a force on sediments causing them to be swept away. The eroded sediments may already be loose, or they may be torn away from the rock surface by the force of the water. If the flow is strong enough, clay, silt, sand, and even gravel, can be eroded in this way.

Abrasion is the second mechanism of sediment erosion. Abrasion is simply the removal of one Earth material by the impact of another. Rock hounds smooth stones by "tumbling" them in a container with very hard sand or silt particles known as abrasives . When you use sand paper to smooth a wood surface, you are using the abrasive qualities of the sand embedded in the paper to erode the wood. In nature, when water (or wind or ice) flows over a rocky surface (for example, a stream bed), sedimentary particles that are being transported by the flow strike the surface, and occasionally knock particles loose. Keep in mind that while the bedrock surface is abraded and pieces are knocked loose, the particles in transport are also abraded, becoming rounder and smoother with time.

Corrosion, or chemical erosion, the third erosional mechanism, is the dissolution of rock or sediment by the agent of transport. Wind is not capable of corrosion, and corrosion by ice is a much slower process than by liquid water. Corrosion in streams slowly dissolves the bedrock or sediments, producing mineral solutions (minerals dissolved in water) and aiding in the production of clastic sediments by weakening rock matrix.

Sediment size

Sediments come in all shapes and sizes. Sediment sizes are classified by separating them into a number of groups, based on metric measurements, and naming them using common terms and size modifiers. The terms, in order of decreasing size, are boulder (> 256 mm), cobble (256-64 mm), pebble (64-2 mm), sand (2-1/16 mm), silt (1/16-1/256 mm), and clay (< 1/256 mm). The modifiers in decreasing size order, are very coarse, coarse, medium, fine, and very fine. For example, sand is sediment that ranges in size from 2 millimeters to 1/16 mm. Very coarse sand ranges from 2 mm to 1 mm; coarse from 1 mm to 1/2 mm; medium from 1/2 mm to 1/4 mm; fine from 1/4 mm to 1/8 mm; and very fine from 1/8 mm to 1/16 mm. Unfortunately, the entire classification is not as consistent as the terminology for sand—not every group includes size modifiers.

Sediment load

When particles are eroded and transported by wind, water, or ice, they become part of the transport medium's sediment load. There are three categories of load that may be transported by an erosional agent: dissolved load, suspended load, and bedload. Wind is not capable of dissolving minerals, and so it does not transport any dissolved load. The dissolved load in water and ice is not visible; to be deposited, it must be chemically precipitated.

Sediment can be suspended in wind, water, or ice. Suspended sediment is what makes stream water look dirty after a rainstorm and what makes a wind storm dusty. Suspended sediment is sediment that is not continuously in contact with the underlying surface (a stream bed or the desert floor) and so is suspended within the medium of transport. Generally, the smallest particles of sediment are likely to be suspended; occasionally sand is suspended by powerful winds and pebbles are suspended by flood waters. However, because ice is a solid, virtually any size sediment can be part of the suspended sediment load of a glacier.

Bedload consists of the larger sediment that is only sporadically transported. Bedload remains in almost continuous contact with the bottom, and moves by rolling, skipping, or sliding along the bottom. Pebbles on a river bed or beach are examples of bedload. Wind, water, and ice can all transport bedload, however, the size of sediment in the bedload varies greatly among these three transport agents.

Because of the low density of air, wind only rarely moves bedload coarser than fine sand. Some streams transport pebbles and coarser sediment only during floods, while other streams may transport, on a daily basis, all but boulders with ease.

Flood water greatly increase the power of streams. For example, many streams can move boulders during flooding . Flooding also may cause large sections of a river bank to be washed into the water and become part of its load. Bank erosion during flood events by a combination of abrasion, hydraulic impact, and mass wasting is often a significant source of a stream's load. Ice in glaciers, because it is a solid, can transport virtually any size material, if the ice is sufficiently thick, and the slope is steep.

For a particular agent of transport, its ability to move coarse sediments as either bedload or suspended load is dependant on its velocity . The higher the velocity, the coarser the load.

Rounding and sorting of sediment

Transport of sediments causes them to become rounder as their irregular edges are removed both by abrasion and corrosion. Beach sand becomes highly rounded due to its endless rolling and bouncing in the surf. Of the agents of transport, wind is most effective at mechanically rounding (abrading) clastic sediments, or clasts. Its low density does not provide much of a "cush ion" between the grains as they strike one another.

Sorting, or separation of clasts into similar sizes, also happens during sediment transport. Sorting occurs because the size of grains that a medium of transport can move is limited by the medium's velocity and density. For example, in a stream on a particular day, water flow may only be strong enough to transport grains that are finer than medium-grained sand. So all clasts on the surface of the stream bed that are equal to or larger than medium sand will be left behind. The sediment, therefore, becomes sorted. The easiest place to recognize this phenomenon is at the beach. Beach sand is very well sorted because coarser grains are only rarely transported up the beach face by the approaching waves, and finer material is suspended and carried away by the surf.

Ice is the poorest sorter of sediment. Glaciers can transport almost any size sediment easily, and when ice flow slows down or stops, the sediment is not deposited, due to the density of the ice. As a result, sediments deposited directly by ice when it melts are usually very poorly sorted. Significant sorting only occurs in glacial sediments that are subsequently transported by meltwater from the glacier. Wind, on the other hand, is the best sorter of sediment, because it can usually only transport sediment that ranges in size from sand to clay. Occasional variation in wind speed during transport serves to further sort out these sediment sizes.

Deposition

Mechanical deposition

When the velocity (force) of the transport medium is insufficient to move a clastic (or organic) sediment particle it is deposited. As you might expect, when velocity decreases in wind or water, larger sediments are deposited first. Sediments that were part of the suspended load will drop out and become part of the bed load. If velocity continues to drop, nearly all bedload movement will cease, and only clay and the finest silt will be left suspended. In still water, even the clay will be deposited, over the next day or so, based on size—from largest clay particles to the smallest.

During its trip from outcrop to ocean, a typical sediment grain may be deposited, temporarily, thousands of times. However, when the transport medium's velocity increases again, these deposits will again be eroded and transported. Surprisingly, when compacted fine-grained clay deposits are subjected to stream erosion, they are nearly as difficult to erode as pebbles and boulders. Because the tiny clay particles are electrostatically attracted to one another, they resist erosion as well as much coarser grains. This is significant, for example, when comparing the erodibility of stream bank materials—clay soils in a river bank are fairly resistant to erosion, whereas sandy soils are not.

Eventually the sediment will reach a final resting place where it remains long enough to be buried by other sediments. This is known as the sediment's depositional environment.

Chemical deposition

Unlike clastic and organic sediment, chemical sediment can not simply be deposited by a decrease in water velocity. Chemical sediment must crystallize from the solution, that is, it must be precipitated. A common way for precipitation to occur is by evaporation . As water evaporates from the surface, if it is not replaced by water from another source (rainfall or a stream) any dissolved minerals in the water will become more concentrated until they begin to precipitate out of the water and accumulate on the bottom. This often occurs in the desert in what are known as salt pans or lakes. It may also occur along the sea coast in a salt marsh.

Another mechanism that triggers mineral precipitation is a change in water temperature . When ocean waters with different temperatures mix, the end result may be sea water in which the concentration of dissolved minerals is higher than can be held in solution at that water temperature, and minerals will precipitate. For most minerals, their tendency to precipitate increases with decreasing water temperature. However, for some minerals, calcite (calcium carbonate ) for example, the reverse is true.

Minerals may also be forced to precipitate by the biological activity of certain organisms. For example, when algae remove carbon dioxide from water, this decreases the acidity of the water, promoting the precipitation of calcite. Some marine organisms use this reaction, or similar chemical reactions , to promote mineral precipitation and use the minerals to form their skeletons. Clams, snails , hard corals, sea urchins , and a large variety of other marine organisms form their exoskeletons by manipulating water chemistry in this way.

Depositional environments

Landscapes form and constantly change due to weathering and sedimentation. The area where a sediment accumulates and is later buried by other sediments is known as its depositional environment. There are many large-scale, or regional, environments of deposition, as well as hundreds of smaller subenvironments within these regions. For example, rivers are regional depositional environments. Some span distances of hundreds of miles and contain a large number of subenvironments, such as channels, backswamps, floodplains, abandoned channels, and sand bars. These depositional subenvironments can also be thought of as depositional landforms, that is, landforms produced by deposition rather than erosion.

Depositional environments are often separated into three general types, or settings: terrestrial (on land), marginal marine (coastal), and marine (open ocean). Examples of each of these three regional depositional settings are as follows: terrestrial-alluvial fans, glacial valleys, lakes; marginal marin-beaches, deltas, estuaries, tidal mud and sand flats; marine-coral reefs, abyssal plains, continental slope.

Sedimentary structures

During deposition of sediments physical structures form that are indicative of the conditions that created them. These are known as sedimentary structures. They may provide information about water depth, current speed, environmental setting (for example, marine versus fresh water) or a variety of other factors. Among the more common of these are: bedding planes, beds, channels, cross-beds, ripples, and mud cracks.

Bedding planes are the surfaces separating layers of sediment, or beds, in an outcrop of sediment or rock. The beds represent episodes of sedimentation, while the bedding planes usually represent interruptions in sedimentation, either erosion or simply a lack of deposition. Beds and bedding planes are the most common sedimentary structures.

Rivers flow in elongated depressions called channels. When river deposits are preserved in the sediment record (for example as part of a delta system), channels also are preserved. These channels appear in rock outcrops as narrow to broad, v- or u-shaped, "bellies" or depressions at the base of otherwise flat beds. Preserved channels are sometimes called cut-outs, because they "cut-out" part of the underlying bed.

Submerged bars along a coast or in a river form when water currents or waves transport large volumes of sand or gravel along the bottom. Similarly, wind currents form dunes from sand on a beach or a desert. While these depositional surface features, or bedforms, build up in size, they also migrate in the direction of water or wind flow. This is known as bar or dune migration . Suspended load or bedload material moves up the shallowly inclined, upwind or upcurrent (stoss) side and falls over the crest of the bedform to the steep, downwind or downcurrent (lee) side. If you cut through the bedform perpendicular to its long axis (from the stoss to the lee side) what you would observe are inclined beds of sediment, called cross-beds, that are the preserved leeward faces of the bedform. In an outcrop, these cross-beds can often be seen stacked one atop another; some may be oriented in opposing directions, indicating a change in current or wind direction.

When a current or wave passes over sand or silt in shallow water, it forms ripples on the bottom. Ripples are actually just smaller scale versions of dunes or bars. Rows of ripples form perpendicular to the flow direction of the water. When formed by a current, these ripples are asymmetrical in cross-section and move downstream by erosion of sediment from the stoss side of the ripple, and deposition on the lee side. Wave-formed ripples on the ocean floor have a more symmetrical profile, because waves move sediments back and forth, not just in one direction. In an outcrop, ripples appear as very small cross-beds, known as cross-laminations, or simply as undulating bedding planes.

When water is trapped in a muddy pool that slowly dries up, the slow sedimentation of the clay particles forms a mud layer on the bottom of the pool. As the last of the water evaporates, the moist clay begins to dry up and crack, producing mud cracks as well as variably shaped mud chips known as mud crack polygons . Interpreting the character of any of the sedimentary structures discussed above (for example, ripples) would primarily provide information concerning the nature of the medium of transport. Mud cracks, preserved on the surface of a bed, give some idea of the nature of the depositional environment, specifically that it experienced alternating periods of wet and dry.

The fate of sediments

All clastic and organic sediments suffer one of two fates. Either they accumulate in a depositional environment, then get buried and lithified (turned to rock by compaction and cementation) to produce sedimentary rock , or they are re-exposed by erosion after burial, but before lithification, and go through one or more new cycles of weathering-erosion-transport-deposition-burial.

Chemical sediments, while still in solution, can instead follow a number of different paths, known as geochemical cycles. These pathways include ending up as: chemical sedimentary rocks, cement in clastic rocks, parts of living organisms, gases in the atmosphere, ice at the poles, or water in underground reservoirs. Dissolved minerals may remain in these settings for millions of years or quickly move on to another stage in the cycle.

Whether clastic, chemical, or organic, all sediments are part of what is called the rock cycle, an endless series of interrelated processes and products that includes all Earth materials.

Environmental impacts of sedimentation

Erosion, weathering, and sedimentation constantly work together to reshape the Earth's surface. These are natural processes that sometimes require us to adapt and adjust to changes in our environment. However, too many people and too much disturbance of the land surface can drastically increase sedimentation rates, leading to significant increases in the frequency and severity of certain natural disasters. For example, disturbance by construction and related land development is sometimes a contributing factor in the mudflows and landslides that occur in certain areas of California. The resulting damage can be costly both in terms of money and lives.

It is reported that the world's rivers carry as much as 24 million tons of sediment to the ocean each year. About two-thirds of this may be directly related to human activity, which greatly accelerates the natural rate of erosion. This causes rapid loss of fertile topsoil, which leads to decreased crop productivity.

Increased sedimentation also causes increased size and frequency of flooding. As stream channels are filled in, the capacity of the channel decreases. As a result, streams flood more rapidly during a rainstorm, as well as more often, and they drain less quickly after flooding. Likewise, sedimentation can become a major problem on dammed rivers. Sediment accumulates in the lake created by the dam rather than moving farther downstream and accumulating in a delta. Over time, trapped sediment reduces the size of the lake and the useful life of the dam. In areas that are forested, lakes formed by dams are not as susceptible to this problem. Sedimentation is not as great due to interception of rainfall by the trees and underbrush.

Vegetative cover also prevents soil from washing into streams by holding the soil in place. Without vegetation, erosion rates can increase significantly. Human activity that disturbs the natural landscape and increases sediment loads to streams also disturbs aquatic ecosystems.

Many state and local governments are now developing regulations concerning erosion and sedimentation resulting from private and commercial development. Only by implementing such measures can we hope to curb these and other destructive side effects, thereby preserving the environment as well as our quality of life.

Resources

books

Dixon, Dougal, and Raymond Bernor. The Practical Geologist. New York: Simon and Schuster, 1992.

Hancock P.L. and Skinner B.J., eds. The Oxford Companion to the Earth. Oxford: Oxford University Press, 2000.

Leopold, Luna. A View of the River. Cambridge: Harvard University Press, 1994.

Middleton, Gerard V., and Celestina V. Cotti Ferrero. Encyclopedia of Sediments & Sedimentary Rocks. Boston: Kluwer Academic Publishers, 2003.

Siever, Raymond. Sand. Scientific American Library Series. New York: W.H. Freeman, 1988.

Skinner, Brian J., and Stephen C. Porter. The Dynamic Earth: An Introduction to Physical Geology. 4th ed. John Wiley & Sons, 2000.

Westbroek, Peter. Life as a Geological Force: Dynamics of the Earth. New York: W. W. Norton, 1991.

Clay Harris

KEY TERMS

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Bedload: —The portion of sediment that is transported by rolling, skipping, and hopping along the stream bed at any given time because it is too heavy to be lifted by flowing stream water. It stands in contrast to suspended load.
Bedrock: —The unweathered or partially weathered solid rock layer, which is exposed at the Earth's surface or covered by a thin mantle of soil or sediment.
Clay: —The finest of sediment particles, less than 1/256 of a millimeter in diameter.
Delta: —A landform that develops where a stream deposits sediment at the edge of a standing body of water (lake or sea).
Floodplain: —The flat, low-lying area adjacent to a river or stream that becomes covered with water during flooding; flood waters deposit sand, silt and clay on this surface.
Geochemical cycle: —A number of interrelated environments or settings through which a chemical can move as a result of changes in state or incorporation into different compounds.
Grain size: —The size of a particle of sediment, ranging from clay to boulders; smaller size sediment is called fine grained, larger sediment is coarse grained.
Mass wasting: —Movement of large masses of sediment primarily in response to the force of gravity.
Outcrop: —A natural exposure of rock at the Earth's surface.
Pebbles: —Coarse particles of sediment larger than sand (2 mm) and smaller than boulders (256 mm).
Sand: —Sediment particles smaller than pebbles and larger than silt, ranging in size from 1/16 of a millimeter to 2 millimeters.
Sediment: —Soil and rock particles that wash off land surfaces and flow with water and gravity toward the sea. On the sea floor, sediment can build up into thick layers. When it compresses under its weight, sedimentary rock is formed.
Sedimentation: —The process by which sediment is removed from one place, and transported to another, where it accumulates.
Silt: —Soil particles derived mainly from sedimentary materials that range between 0.0002–0.05 mm in size.