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. 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.
Sedimentary facies and depositional environments pdf
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. 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.
Sedimentary Structures and Environments of Deposition
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 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.