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Some may think that sedimentary rocks is a bit dull since sedimentary rocks isn't created by violent and exciting volcano eruptions from the Earths mantle like the igneous rocks. No, sedimentary rocks have another type of fascinating origin and every single rock tells a story if you just know how to "read" the rock! That is one of the fascinating thing with sedimentary rocks! The other exciting part with sedimentary rocks is that they tell us about Earths history! I will tell you a little about how to do read the rocks and I hope it will help you to see sedimentary rocks in nature in a new way!
Every single particle in a sedimentary rock initially comes from a rock or as soil on land.
Sedimentary rock - Wikipedia
By time, the rock is broken down into small particles by weathering and the small particles are transported away. Sometimes the transportation distance is long and sometimes shorter. And most sedimentary rocks consists of small particles that have a long and fascinating story to tell from their long journey behind them. Read on and you will know why and how! Sediment First we need to make clear what sediment is!
Sediment is material that occurs natural and is broken down by processes like weathering and erosion. This means that sedimentary rocks can consists of all the materials on earth and take a minute to think about the breathtaking fact that every single particle in a sedimentary rock have been transported and shaped by transportation in more than one media, and finally, that particle has settled down upon the deep ocean floor long, long time ago.
It gets even more fascinating to think about that we can actually see and walk on former ocean floor that looks amazing in many places on earth. I have some pictures from such a place further down in the article. And then, when the ocean floor becomes rocks on land the weathering starts again. It is like an ongoing transportation of particles that never ends.
However, in some cases, a certain mineral dissolves and does not precipitate again. This process, called leachingincreases pore space in the rock. Some biochemical processes, like the activity of bacteriacan affect minerals in a rock and are therefore seen as part of diagenesis.
Fungi and plants by their roots and various other organisms that live beneath the surface can also influence diagenesis. Burial of rocks due to ongoing sedimentation leads to increased pressure and temperature, which stimulates certain chemical reactions. An example is the reactions by which organic material becomes lignite or coal.
When temperature and pressure increase still further, the realm of diagenesis makes way for metamorphismthe process that forms metamorphic rock. Properties A piece of a banded iron formationa type of rock that consists of alternating layers with iron III oxide red and iron II oxide grey.
BIFs were mostly formed during the Precambrianwhen the atmosphere was not yet rich in oxygen. Moories GroupBarberton Greenstone BeltSouth Africa Color The color of a sedimentary rock is often mostly determined by ironan element with two major oxides: Iron II oxide FeO only forms under low oxygen anoxic circumstances and gives the rock a grey or greenish colour.
Iron III oxide Fe2O3 in a richer oxygen environment is often found in the form of the mineral hematite and gives the rock a reddish to brownish colour. In arid continental climates rocks are in direct contact with the atmosphere, and oxidation is an important process, giving the rock a red or orange colour.
Thick sequences of red sedimentary rocks formed in arid climates are called red beds. However, a red colour does not necessarily mean the rock formed in a continental environment or arid climate.
Organic material is formed from dead organisms, mostly plants. Normally, such material eventually decays by oxidation or bacterial activity. Under anoxic circumstances, however, organic material cannot decay and leaves a dark sediment, rich in organic material. This can, for example, occur at the bottom of deep seas and lakes. There is little water mixing in such environments; as a result, oxygen from surface water is not brought down, and the deposited sediment is normally a fine dark clay.
Dark rocks, rich in organic material, are therefore often shales. The texture is a small-scale property of a rock, but determines many of its large-scale properties, such as the densityporosity or permeability. Between the clasts, the rock can be composed of a matrix a cement that consists of crystals of one or more precipitated minerals. The size and form of clasts can be used to determine the velocity and direction of current in the sedimentary environment that moved the clasts from their origin; fine, calcareous mud only settles in quiet water while gravel and larger clasts are moved only by rapidly moving water.
The statistical distribution of grain sizes is different for different rock types and is described in a property called the sorting of the rock. When all clasts are more or less of the same size, the rock is called 'well-sorted', and when there is a large spread in grain size, the rock is called 'poorly sorted'. Coquinaa rock composed of clasts of broken shells, can only form in energetic water. The form of a clast can be described by using four parameters: Chemical sedimentary rocks have a non-clastic texture, consisting entirely of crystals.
To describe such a texture, only the average size of the crystals and the fabric are necessary. Mineralogy Most sedimentary rocks contain either quartz especially siliciclastic rocks or calcite especially carbonate rocks. In contrast to igneous and metamorphic rocks, a sedimentary rock usually contains very few different major minerals. However, the origin of the minerals in a sedimentary rock is often more complex than in an igneous rock.
Minerals in a sedimentary rock can have formed by precipitation during sedimentation or by diagenesis. In the second case, the mineral precipitate can have grown over an older generation of cement. Carbonate rocks dominantly consist of carbonate minerals such as calcitearagonite or dolomite. Both the cement and the clasts including fossils and ooids of a carbonate sedimentary rock can consist of carbonate minerals.
The mineralogy of a clastic rock is determined by the material supplied by the source area, the manner of its transport to the place of deposition and the stability of that particular mineral. The resistance of rock-forming minerals to weathering is expressed by Bowen's reaction series.
In this series, quartz is the most stable, followed by feldsparmicasand finally other less stable minerals that are only present when little weathering has occurred.
In most sedimentary rocks, mica, feldspar and less stable minerals have been reduced to clay minerals like kaoliniteillite or smectite. Unlike most igneous and metamorphic rocks, sedimentary rocks form at temperatures and pressures that do not destroy fossil remnants.
Often these fossils may only be visible under magnification. Dead organisms in nature are usually quickly removed by scavengersbacteriarotting and erosion, but sedimentation can contribute to exceptional circumstances where these natural processes are unable to work, causing fossilisation. The chance of fossilisation is higher when the sedimentation rate is high so that a carcass is quickly buriedin anoxic environments where little bacterial activity occurs or when the organism had a particularly hard skeleton.
Larger, well-preserved fossils are relatively rare. Burrows in a turbiditemade by crustaceansSan Vincente Formation early Eocene of the Ainsa Basinsouthern foreland of the Pyrenees Fossils can be both the direct remains or imprints of organisms and their skeletons. Most commonly preserved are the harder parts of organisms such as bones, shells, and the woody tissue of plants. Soft tissue has a much smaller chance of being fossilized, and the preservation of soft tissue of animals older than 40 million years is very rare.
As a part of a sedimentary or metamorphic rock, fossils undergo the same diagenetic processes as does the containing rock. A shell consisting of calcite can, for example, dissolve while a cement of silica then fills the cavity. In the same way, precipitating minerals can fill cavities formerly occupied by blood vesselsvascular tissue or other soft tissues. This preserves the form of the organism but changes the chemical composition, a process called permineralization.
In the case of silica cements, the process is called lithification. At high pressure and temperature, the organic material of a dead organism undergoes chemical reactions in which volatiles such as water and carbon dioxide are expulsed. The fossil, in the end, consists of a thin layer of pure carbon or its mineralized form, graphite. This form of fossilisation is called carbonisation. It is particularly important for plant fossils.
Unlike textures, structures are always large-scale features that can easily be studied in the field. Sedimentary structures can indicate something about the sedimentary environment or can serve to tell which side originally faced up where tectonics have tilted or overturned sedimentary layers.
Sedimentary rocks are laid down in layers called beds or strata. A bed is defined as a layer of rock that has a uniform lithology and texture. Beds form by the deposition of layers of sediment on top of each other. The sequence of beds that characterizes sedimentary rocks is called bedding. Finer, less pronounced layers are called laminae, and the structure a lamina forms in a rock is called lamination.
Laminae are usually less than a few centimetres thick. In some environments, beds are deposited at a usually small angle.
Sedimentary Rocks Formation and Fossils!
Sometimes multiple sets of layers with different orientations exist in the same rock, a structure called cross-bedding. Newer beds then form at an angle to older ones. The opposite of cross-bedding is parallel lamination, where all sedimentary layering is parallel. Laminae that represent seasonal changes similar to tree rings are called varves. Any sedimentary rock composed of millimeter or finer scale layers can be named with the general term laminite.
When sedimentary rocks have no lamination at all, their structural character is called massive bedding.
- Sedimentary rock
Graded bedding is a structure where beds with a smaller grain size occur on top of beds with larger grains. This structure forms when fast flowing water stops flowing. Larger, heavier clasts in suspension settle first, then smaller clasts. Although graded bedding can form in many different environments, it is a characteristic of turbidity currents.
Examples of bed forms include dunes and ripple marks. Sole markings, such as tool marks and flute casts, are groves dug into a sedimentary layer that are preserved.
These are often elongated structures and can be used to establish the direction of the flow during deposition. There are two types of ripples: Environments where the current is in one direction, such as rivers, produce asymmetric ripples. The longer flank of such ripples is on the upstream side of the current.
Mudcracks are a bed form caused by the dehydration of sediment that occasionally comes above the water surface. Such structures are commonly found at tidal flats or point bars along rivers.
Secondary sedimentary structures Halite crystal mold in dolomite, Paadla Formation SilurianSaaremaaEstonia Secondary sedimentary structures are those which formed after deposition. Such structures form by chemical, physical and biological processes within the sediment. They can be indicators of circumstances after deposition.
Some can be used as way up criteria. Organic materials in a sediment can leave more traces than just fossils. Preserved tracks and burrows are examples of trace fossils also called ichnofossils. Most trace fossils are burrows of molluscs or arthropods. This burrowing is called bioturbation by sedimentologists.
It can be a valuable indicator of the biological and ecological environment that existed after the sediment was deposited. On the other hand, the burrowing activity of organisms can destroy other primary structures in the sediment, making a reconstruction more difficult. Chert concretions in chalkMiddle Lefkara Formation upper Paleocene to middle EoceneCyprus Secondary structures can also form by diagenesis or the formation of a soil pedogenesis when a sediment is exposed above the water level.
An example of a diagenetic structure common in carbonate rocks is a stylolite. This can result in the precipitation of a certain chemical species producing colouring and staining of the rock, or the formation of concretions. Concretions are roughly concentric bodies with a different composition from the host rock. Their formation can be the result of localized precipitation due to small differences in composition or porosity of the host rock, such as around fossils, inside burrows or around plant roots.
Calcite concretions in clay are called septarian concretions. After deposition, physical processes can deform the sediment, producing a third class of secondary structures.
Density contrasts between different sedimentary layers, such as between sand and clay, can result in flame structures or load castsformed by inverted diapirism. Sometimes, density contrasts can result or grow when one of the lithologies dehydrates. Clay can be easily compressed as a result of dehydration, while sand retains the same volume and becomes relatively less dense. On the other hand, when the pore fluid pressure in a sand layer surpasses a critical point, the sand can break through overlying clay layers and flow through, forming discordant bodies of sedimentary rock called sedimentary dykes.
The same process can form mud volcanoes on the surface where they broke through upper layers. Sedimentary dykes can also be formed in a cold climate where the soil is permanently frozen during a large part of the year. Frost weathering can form cracks in the soil that fill with rubble from above. Such structures can be used as climate indicators as well as way up structures. Instabilities in such sediments can result in the deposited material to slumpproducing fissures and folding.