- Why are depositional environments important?
- How are depositional environments identified?
- Sedimentary structures
- Sedimentary facies
- Examples of sediments and their particular depositional environments
- Table of Depositional Environments (page will open in new window)
A depositional environment is a specific type of place in which sediments are deposited, such as a stream channel, a lake, or the bottom of the deep ocean. They are sometimes called sedimentary environments. The layers of sediment that accumulate in each type of depositional environment have distinctive characteristics that provide important information regarding the geologic history of an area. The characteristics that can be observed and measured in a sedimentary rock to deduce its depositional environment include its lithology (which is essentially its rock type), its sedimentary structures, and any fossils it may contain.
The schematic diagram below shows different types of depositional environments. It is from Wikipedia (en.wikipedia.org/wiki/File:SedimentaryEnvironment.jpg), uploaded November 8, 2008 by Mikenorton. Click on the thumbnail for a larger version of the image that opens in a new window.
Knowledge of depositional environments is important for reconstructing earth history, understanding earth processes, and helping humans survive and prosper on earth.
Reconstructing earth history -- By analyzing a sedimentary rock, a geologist can deduce what was happening on earth at the place and time the sediment was originally being deposited. What we learn about the the geologic history of a region comes mostly from examining the layers of sedimentary rock from the area and determining their depositional environments. Because sedimentary rocks are stratified in age sequence, as summarized in the principles of relative geologic age, layers of sedimentary rock act as a record of how that area was changing, physically and biologically, over the extent of geologic time spanned by the sedimentary rock layers. Reconstructing depositional environments enables geologists to observe climates of the past, life forms of the past, and geography of the past -- the location of mountains, basins, large rivers, and bays of the ocean. Changes over time in climate, life forms and geography constitute the geologic history of a region. Ultimately, regional geologic histories are compiled into a history of the earth over the whole course of its existence, including the formation, growth, and movements of continents and ocean basins, the growth and erosion of major mountain ranges, and the history of life on earth.
Understanding earth processes -- Sediments are deposited in many environments on the earth's surface, some of which humans have little familiarity with, such as deep ocean environments. Sediments have been deposited in the past in environments that do not exist in the present, such as an atmosphere with no free oxygen, or an environment disturbed catastrophically by a gigantic meteorite impact. Therefore, by examining sedimentary rocks as windows into these environments, we can learn about earth processes that we would otherwise know little about, and deduce details about them such as the chemistry of the air or water with which the sediments were in contact and the physical processes that were occurring in that environment.
Helping humans survive and prosper -- Knowledge of earth processes has many practical applications for human health and survival. To give one example, by reconstructing depositional environments of certain sediments deposited along the coast of the Pacific Northwest, geologists concluded that great subduction earthquakes and tsunamis (giant waves) created by the earthquakes, were the driving forces of the depositional environment of those sedimentary deposits. This has led to re-evaluation of the earthquake hazards in western Washington and Oregon and rewriting of building codes and engineering standards for construction of schools, roads, bridges, and infrastructure in that area. This has affected such things as insurance policies and construction costs.
Geologists use analyses of depositional environments to help locate, inside the earth, sources of oil, coal, natural gas, deposits of valuable metals/minerals/rocks, and aquifers, which are useable sources of groundwater.
The many depositional environments which can be grouped into three major categories - marine, transitional, and continental. See the Basics Table of depositional environments for a more detailed breakdown of each of the categories and the sedimentary rocks, structures and fossils that are common to each environment.
The characteristics of a sedimentary rock that are affected by its depositional environment are its sedimentary lithology (the minerals and texture of the rock), its sedimentary structures, and its fossils. Sedimentary rocks contain sedimentary structures that were formed as the sediments were being deposited. Many sedimentary rocks also contain fossils, which are our main source of information of the history of life on earth. Sedimentary structures, and fossils, are best found and and examined in outcrops, where whole beds of sedimentary rocks are exposed in their undisturbed geological setting. The structures and fossils in sedimentary rocks reveal what was happening on the earth at the place and time the sediments were being deposited.
Sedimentary lithology is a combination of the mineral content and sedimentary texture of the rock. The lithology of a sedimentary rock is largely summarized in the name of the rock. See the Basics page on sedimentary rocks and the sedimentary rock classification table.
Sedimentary structures such as cross-beds, graded beds, and mud cracks are useful for determining which way was up in the original sequence of sediments. It is possible for tectonic forces to deform rocks in the crust to the point that beds of sedimentary rocks have been turned upside down. Therefore, a geologist needs to check the sedimentary structures to be sure which way was up, especially if looking at beds of sediment that have been tilted to high angles, far from their original horizontal position.
Sedimentary beds, or strata, are layers of sediment that can be distinguished from layers above or below by the type, texture, or color of the sediment. Most sediments accumulate under water on the surface of the earth. Some accumulate on the earth's surface at the base of the atmosphere. In either case the deposition of sediment tends to occur in events or pulses of increased sedimentation, such as during high flows or floods of rivers, seasons of strong wind in a desert, certain parts of the tide cycle in shallow marine environments, or yearly freeze and thaw cycles in lakes in sub-arctic environments. The result is sedimentary beds that may be only a few mm thick or may be up to several m thick. Note that the processes that caused the bedding may be inferred, with careful study, from the nature of the bedding itself.
Bedding thickness -- The thickness of sedimentary beds can be measured and described using standardized terminology as follows:
- Massive: No beds apparent over several m of exposure
- Thick beds: greater than 100.0 cm (greater than 1.0 m)
- Moderate beds: 10.0 cm - 100.0 mm
- Thin beds: 1.0 cm - 10.0 cm
- Laminae, or laminated beds: less than 1.0 cm (less than 10 mm)
The fact that in many cases sediments tend to settle from water (or air) and fill in low areas with relatively flat layers is the basis of the principle of original horizontality, one of the important principles of relative geologic age. However, not all sedimentary beds are horizontal to begin with. Cross-beds in particular begin as inclined beds, formed by sediment piling up in layers on the slopes of sediment ripples or dunes, or on slopes that go gradually into deeper water as sediments pile up from a river's mouth into an ocean or lake. Cross-beds formed from sediment ripples being moved at the base of a current of water slope downward in the direction the water was flowing. Wind-blown sediment that were deposited in the form of sand dunes form long cross-beds that represent the migrating, down-wind faces of the sand dunes.
Rhythmic bedding consists of a repeated sequence of beds. Varves are a simple example of rhythmic bedding. Turbidites are a more complex example of rhythmic bedding. Rhythmic beds are sometimes called "rhythmites."
Graded beds have coarser (larger) sediment grains at the bottom, grading up to finer (smaller) sediment grains at the top of the bed; or the grading may occur in a sequence of beds from, at minimum, a bed of coarse sediment overlain by a bed of finer sediment, or several beds of finer and finer sediment on top of each other. Graded bedding results from the fact that larger grains of common rocks or minerals fall out of a body of water faster than the finer grains of sediment do. Once a flow of water slows enough for the sediment grains to settle out, if the sediment grains are in a mixture of sizes, they will form a sedimentary bed, or continuous sequence of sedimentary beds, with the larger sediment grains at the bottom and the larger sediment grains at the top.
Sediment ripples are a structure that forms on the surfaces of beds. They originate similar to the way cross-beds develop, by the migration of sediment in the form of ripples, or larger dunes, at the base of a current of water or air. You have probably seen sediment ripples if you have been to a sandy beach at low tide where the sand has been formed into ripples by the flow of water when the tide was in, or if you have looked at sandy sediment at the base of stream or river channels. Asymmetric sedimentary ripples have steeper faces in the down-flow direction of the current.
Fine-grained sediment, particularly sediment composed at least partly of clay, will form a polygonal pattern of mud cracks on the surface of the bed, if the sediment was covered by water which dried up or receded and left the bed exposed to the air.
Fossils are the remains or traces of biological organisms preserved in rocks. Fossils are commonly found in sedimentary rocks. Besides providing evidence of life forms that have existed in earth's past, and how life on earth has evolved over the course of earth history, fossils provide important information about the depositional environment in which the sediments were deposited. For example, fish fossils imply that the sediments were deposited in a body of water. Fossils of the leaves of trees imply that the sediments were deposited on land, above sea level.
Trace fossils, such as dinosaur footprints preserved on a lithified stratum of mud, or wormholes in silt from the floor of a shallow sea are another important type of fossils. Wormholes or tunnels created by other organisms that lived in the sediment and are preserved as trace fossils are known as bioturbation.
The original organism is likely to be preserved only in terms of its hard parts, such as its shell, skeleton, or teeth. Soft parts of animals are much less likely to be fossilized. Because a certain combination of events and conditions is required for dead organisms to become fossilized, most organisms that live on earth never become fossilized. Many whole species have existed that are not represented in the fossil record. However, there are more fossils in the rocks that have yet to be discovered. Paleontologists - professionals who study fossils scientifically - may occasionally have the pleasure of confirming and reporting the discovery of a previously unknown fossil species.
Fossils are most commonly found in sedimentary rocks. Less commonly, fossils occur in certain types of volcanic rocks, rarely in low-grade metamorphic rocks, and never in plutonic igneous rocks or high-grade metamorphic rocks. Not all sedimentary rocks will contain fossils, but many do, which adds greatly to the information the rock contains about its depositional environment and what it represents in terms of the history of life on earth.
Fossils may not preserve any of the original tissue, bone, or shell that the organism was composed of. Once buried in the earth as part of the rock cycle to become lithified, a fossil may become mineralized completely. For example, petrified wood has had its organic material replaced by quartz as a result of chemical reactions that occurred once it was buried deep enough to be below the water table. Sometimes shells or exoskeletons of marine animals are found to have been completely replaced by the shiny yellow mineral pyrite, forming a detailed mold of the original shell or exoskeleton.
If you are ever investigating stratified rocks yourself, whether layers of sedimentary rock or layers of volcanic rock, keep your eye out for fossils.
Sedimentary facies are bodies of sediment that originate simultaneously in adjacent depositional environments. For example, a beach facies can usually be distinguished from a tide flat facies, both of which were deposited at the same time adjacent to each other. Compared to the beach facies, the tide flat facies will have smaller average sediment grain size, more bioturbation fossils, contain cross-beds and ripples created by tidal currents, and have more mollusk or other shallow-water fossils preserved in their original place, in unbroken form. There will not be a sharp boundary between the two facies preserved in the sedimentary record. Instead the boundary between them will be a zone with beds of sediment that interfinger and grade into each other sideways from one facies to another.
Below is a simplified diagram of three sedimentary facies adjacent to each other: a beach and tide flat facies (combined), a marine or near-shore portion of a continental shelf, and an offshore carbonate platform or reef. The beach and tide flat facies sediments are mostly sand, the bay facies is mostly mud, and the reef facies is mostly shells and corals which are made of carbonate minerals. If these sediments are buried and lithified into sedimentary rocks, the beach sands turn into sandstone, the bay mud turns into shale, and the reef sediments turn into limestone.
The study of sedimentary facies has revealed, among other things, how sea level, relative to the shore of a continent, is constantly changing over the course of geologic time, on time scales that can vary from decades to millions of years. To give a more specific example of how facies changes record sea level change, deep in the Grand Canyon of Arizona is a sequence of three sedimentary rock formations: the Tapeats Sandstone, the Bright Angel Shale, and the Muav Limestone. Those three sedimentary formations are thought to have originated as a continuous series of sediment as sea level gradually rose, relative to the land, over a span of time that took over a million years in that area. As sea level grew deeper, the shore of the ocean moved inland, which means that the beach facies moved inland, the bay facies shifted in the same direction, and so did the reef facies. Along the low-gradient coast of the continent, as sea level rose higher, what had been a beach was covered by deeper water and became the bottom of a bay where mud accumulated on top of the sand. Then, as sea level rose higher still , the area was in deeper water farther from shore where the water was relatively clear and free of clastic sediments. This allowed a coral reef to build on top of the mud. A more geological time passed and the environment of the area changed again, the deposits of sand, mud, and carbonate sediment were buried and lithified into the sequence of sedimentary formations which, from bottom to top, are sandstone, shale, and limestone. The sequence of sediments that record a gradual sideways shift of sedimentary facies during a marine transgression is shown in the diagram below. In the diagram, the part of the continent above sea level would be on the left. As time passed (moving upward in the diagram) the shoreline was moving to the left.
At a given location, such as where the Grand Canyon is now located, evidence of a marine transgression appears as a continuous stratigraphic sequence of sandstone at the bottom, shale above the sandstone, and limestone on top of the shale. The minerals, sedimentary textures, sedimentary structures, and fossils are specifically indicative of beach, tide flat, muddy bay, and offshore reef depositional environments. Such a transgressive sequence is marked as a stratigraphic column in the diagram above. Below is shown a simplified version of the stratigraphic column that represents a marine transgression, with the oldest sedimentary formation at the bottom.
It is also possible for a regressive sequence to occur as sea level goes down relative to the coast of a continent, resulting in the opposite sequence: limestone on the bottom, shale in the middle, and sandstone on top.Regressive sequences are less likely to be preserved in the rock record than are transgressive sequences.This is because, as sea level falls, the exposed parts of the continent, which had previously been below sea level, are exposed above sea level and more subjected to the forces of weathering and erosion. Therefore the sediments are likely to be removed by earth processes rather than kept buried and preserved within the earth.
The oceans receive most of the clastic sediments that erode from the continents. On the edges of the continental shelves, where the submarine slope tilts down into much deeper water, accumulations of mud and sand deposited by rivers build up. Eventually so much sediment builds up on the edge of the steepening slope that it is likely to give way into an underwater landslide. The submarine landslide will flow down the slope into deeper water, mixing with seawater as it goes to form what is called a turbidity current. As the sediments gradually settle out of the turbidity current onto the deeper ocean floor, the coarser-grained sediments (those sediment grains with larger diameters) will settle to the bottom first, followed gradually by finer and finer sediments. This creates a graded sequence of sediments-it grades upward from a bed of sand through a layer of silt to a top layer of fine mud. This graded deposit becomes a rock known as a turbidite. Over the years one turbidite is likely to be deposited on top of another, over and over again thousands of times. This creates repeated beds of coarse sand to fine mud, which may total thousands of feet thick. If parts of the ocean floor end up becoming part of a continent, turbidites are likely to be a major component the accreted terrane.
Varves are annual layers of sediment, layers of sediment that accumulate each year, year after year. Varves are deposited as rhythmic beds, beds laid down in a repeating pattern.
A common depositional environment in which one type of varve is deposited is lakes in cold climates where the surface of the lake freezes every winter and thaws every spring and summer. During the spring-summer thaw, streams discharge at a high rate into the lake, causing the deposition of a layer of silt on the bed of the lake. The silt is usually rich in quartz and feldspar and light-colored. During the winter freeze, when there is little or no stream-borne sediment coming into the lake, only clay-size particles settle to the bottom of the lake, along with any planktonic (floating, mostly microscopic) organisms that flourished in the summer and died as the lake froze. The winter sediment is thus clay, sometimes dark clay due to having a small amount of carbon in it.
The resulting varve is a pair of strata: a light-colored stratum of silt from the spring-summer warm season, and a darker stratum of clay from the winter freeze.
Sequences of varves are especially common in locations that were the beds of lakes near glaciers during ice ages. Ice ages are times when continental glaciers formed and advanced outside of polar regions. The most recent ice age, the Pleistocene epoch (approximately 2.5 million to 12,000 years ago), saw continental glaciers advance several times in northern North America (into what is now the northern-most United States), the Scandinavian Peninsula and nearby parts of Europe including Britain, and parts of northern Asia. During continental glaciations, the glaciers dammed many stream drainages and created temporary lakes in cold climates next to the glaciers, where sequences of varves accumulated.
Limestone, rock made of the calcium carbonate mineral known as calcite, can form in a variety of depositional environments, from hot spring deposits in lakes to coral reefs in the tropical oceans. Most limestone originates in shallow waters of tropical oceans, and may carry fossils of plants and animals that lived in those marine environments. However, limestones made of buried coral reefs are not as common as limestones made simply from lime mud. Lime mud originates from disintegrated organisms that have hard parts made of calcium carbonate. As a result, limestone is commonly massive (lacks obvious beds), fine-grained, and lacks obvious fossils.
When devastating subduction zone earthquakes occur along a coast, extremely large water waves called tsunamis are generated. At the same time, sea level changes relative to land level along the local shore. The combination of a sudden drop in land level and a tsunami washing over coastal lowlands creates several distinctive markers in the sediment layers that remain. These include muddy coastal marsh deposits overlain by gravel or sand deposits that have sedimentary structures indicating high-energy waves flowed inland along the coast. Where the coast is nearly flat rather than steep, these tsunami deposits can extend miles inland. Groves of cedar trees or other evergreen trees that grow adjacent to marshy areas, barely above sea level, may drop down and have their tree roots subjected to salty water. This will kill the trees, though they may stand in place for several hundred years as "ghost forests," silent testimonials to great earthquakes of the recent past.
Coal is a chemical sedimentary rock made mostly of carbon. It forms from the remains of plants that lived in moist environments rich in trees, shrubs, water, and mud. In such swampy settings, the dead plant debris is quickly buried and thus escapes rotting away at the earth's surface. Upon being buried, heated and compressed within the earth's crust, the dead plants will become coal if the right conditions of heat and pressure are achieved.
Sequences of beds of sandstone, conglomerate, siltstone, shale, and plant fossils indicate sediment deposition by a system of meandering rivers. If there were thick woods and densly vegetated swampy areas, there may also be coal. Details in the sedimentary structures, characteristic signatures of particular depositional processes, will confirm if there were meandering river channels, sandbars, stream bank erosion, and occasional floods.
Much sediment is deposited where rivers empty into lakes, or into the ocean. This is because the velocity of the stream current comes to a stop there, and as the flow slows down, the sediments being transported by the stream settle to the bottom and are deposited. Deltas along ocean coasts are transitional environments, where the surface of the earth gradually slopes from on land to beneath the ocean, and where currents of fresh water and sediments eroded from continents meet waves, tides, and marine sediments.
Deltas where large rivers meet the ocean are huge, especially when their submarine parts are taken into account. The southern part of the state of Louisiana is on the Mississippi River delta. Beneath the Gulf of Mexico, there is a much larger volume of the delta sloping down to its base in deep water far from shore. The delta of the Brahmaputra River in Bangladesh is the subaerial part of a large delta that has a submarine component, known as the Bengal fan, which may be the largest body of sediment on earth.
It is common for oil deposits to be found in the sedimentary beds of deltas, including deltas from rivers that have long since disappeared, the sedimentary beds preserved as layers of sedimentary rock. Drilling into delta deposits by oil companies has led to detailed knowledge of the structures, minerals, textures, facies, and fossils that are typically deposited in different parts of a marine delta. Below is a simplified diagram of the major sets of beds that characterize a delta.
The foreset beds of a marine delta are sediments deposited in a continental setting, on the low-gradient parts of the delta above sea level, where there were meandering stream channels and marshy or swampy floodplains. The sediments tend to be fine-grained, thin-bedded, and have certain types of cross-beds, ripples, plant fossils, and in some cases mud cracks.
The foreset beds were deposited on higher-gradient slopes going down into deep water, so foreset beds consist of sediments deposited underwater in relatively high-energy conditions. The coarser sediments of turbidity currents - sand and gravel with cross-beds and graded beds - are are common in the foreset beds.
The bottomset beds formed where turbidity currents, which originated higher on the delta, spread out onto the lower-gradient ocean floor in deeper water and lost their energy, and consist of fine sands, silts and clays, commonly in characteristic sequences of graded beds known as turbidites.
Created by Ralph L. Dawes, Ph.D. and Cheigures unless otherwise noted
updated: 9/11/13 > updated: 7/10/11
Unless otherwise specified, this work by Washington State Colleges is licensed under a Creative Commons Attribution 3.0 United States License.