Depositional Environmental for Sedimentary Rocks - Assignment Example

Depositional environmental for sedimentary rocks Introduction The geologic developments on the earth’s surface only create only view changes on the earth’s landscape during the lifetime of human beings. However, over a period of many years, the impact of these geologic processes can be considerable (Tucker 2013, P. 43). Given enough duration, the erosive strength and power of hydrologic can reduce all mountains to become featureless lowlands. In this process, rivers transport all the eroded particles and deposit them as new layers called sedimentary rocks. A series of these rocks can form a thickness of a thousand meters. When these rocks expose on the surface of earth, they provide enough information about the past activities of the earth’s history. For instance, this is case with Moenkopi formation in South Utah. Sedimentary rocks serve as a record of earth’s history, which is remarkable on the layers formed on the sedimentary rocks. Each bedding plane on sedimentary rocks portrays what was once on the surface of earth since these layers are remnants of earth’s surface. Each layer on the sedimentary rocks is a product of the earth’s previous erosion and deposition. Additionally, details of composition, texture, and fossils are essential since they indicate how earth evolved during the past and its potential change in future (Kodama 2013, P. 45). In order to be able to make interpretations about the layers of sedimentary rocks accurately, one must initially have an understanding of the modern sedimentary systems, their sources, transportation pathways, and places where most sedimentary rocks are currently being deposited such as the beaches, rivers, and deltas. Studying on the deposition of modern sedimentary rocks gives one an insight on how past sedimentary rocks formed. Apart from fossils found in sedimentary rocks revealing the deposition environment, they also reveal the course and pace of evolution throughout the earth’s life (Tucker 2009, P. 33). In addition, sedimentary rocks do not only portray a scientific significance, but also serves as a controlling factor that controls the development of society, culture, and industry. Since the Neolithic era, human beings have been using materials from these rocks. For instance, chert and flint were raw materials for manufacturing tools, axes, and arrowheads (Mackenzie 2005, P. 64). Additionally, all the great cathedrals of Europe comprise sedimentary rocks. The statues of ancient Rome and Greek’s artists could be impossible if there were no limestone. In addition, an approximate of 90% of the modern minerals used daily in our society comes from these rocks. Virtually, the current entire stores of natural gas, fertilizer, coal, and petroleum all are products of sedimentary rocks (Kodama 2013, P. 48). Gravel, sand, and limestone, which come from sedimentary rocks, are the primary raw materials for producing cement. Sedimentary rock also act as important groundwater reservoirs and hold essential copper, lead, zinc, uranium, diamonds, and gold deposits. Types of sedimentary rocks There are two classifications of sedimentary rock, whereby this classification depends on the rocks’ composition and texture of their particles. The two main groups of sedimentary rocks include (i) clastic sedimentary rocks, which form from other rock fragments, and (ii) biochemical rocks and chemical rocks (Mackenzie 2005, P. 68). i. Clastic sedimentary rocks One of the most important class of sedimentary rocks contain particles of sand, gravel, or mud, this category of sedimentary rocks is known as clastic rocks. The word clastic is from Greek work “klastos”, which means “broken” describing the worn and broken particles of sedimentary rocks and minerals carried to the deposition sites by wind, streams, marine, and glaciers currents. Generally, clastic rocks categorize into other categories depending on the grain’s size. Starting with the largest size to the smallest size, the subdivisions of clastic rocks are sandstones, mud rocks, and conglomerate rocks (Boggs 2009, P. 87). The size grain of clastic sedimentary rocks is not under control of the progressive growth of the grains as it is the case with igneous rocks. However, the size of clastic sedimentary rocks’ size is under control of the sizes of the clasts available in the source as well as the carrying capacity of their transportation medium such as a glacier, river, or wind. When the transportation mediums lose their carrying capacity, they deposit the grains (Mackenzie 2005, P. 71). Commonly, this occurs when the transporting medium’s velocity decreases. Therefore, sediments, which deposit from a fast-moving river, are coarser than those deposited by a quiet lagoon are. Conglomerate rocks- Conglomerate rocks consist of combined deposits of gravel with some amounts of mud and sand in between the larger grains. These fragments are larger than two millimetres in diameter. The gravels formed are usually well rounded and smooth indicating that, these grains were round and smooth during their transportation (Boggs 2009, P. 90). Most conglomerate rocks are usually crudely-stratified and they include lenses and beds of sandstone. Transportation mediums usually require high energies to enable them transport large sized clasts such as gravel. Therefore, deposition of conglomerate rocks mainly occurs in environments with high energy where water flows rapidly. Currently, deposition of conglomerate rocks tacks place at many mountain basis, beaches, and stream channels (Monroe, Wicander & Hazlett 2006, P. 115). Sandstone-sandstone rocks seem to be the most familiar clastic sedimentary rocks. Although they are not abundant, there well exposition, easy recognition, and general resistant to weathering makes these types of rocks familiar. The diameter of the sand grains ranges from 0.0625 to 2mm and consists of almost all materials (Tucker 2009, P. 52). Therefore, sandstones take any form of colour. The most abundant grain of sandstone rocks is quartz because these grains are the common constituent of a number of other types of rocks. Additionally, quartzes are resistant to chemical breakdown or abrasion during the transportation of the sediment particles. In most sandstone, the particles of sand cement by quartz, calcite, or iron oxide. Other grains may include pieces of chert, feldspar (in arkoses rocks), or small fragments of rocks (Boggs 2009, P. 94). The composition of sandstones provides an essential clue of the sandstone’s history. During long distance transportation, small rocks and some minerals that decompose like feldspar, olivine, and mica breaks down into tiny and finer particles, which are winnowed out, and leaving only quartz, which are stable. Well-sorted, clean sandstone consists of well-rounded grains of quartz indicating prolonged transportation, or several cycles of deposition and erosion (Monroe, Wicander & Hazlett 2006, P. 121). Mudrocks- mudrocks are clastic rocks that have fine grains of less than 1/16 millimetres. These types of rocks are the most plentiful type of sedimentary rocks. They are normally weather rapidly and soft to form slopes. They main deposition environments are river floodplains, deltas, and other shallow water environments (Tucker 2013, P. 43). Most mudrocks show evidence of organisms’ burrowing. There are many varieties of important mudrocks. Siltstone is one of the fine-grained types of clastic rock, which is coarser than claystone. Mudrock’s clasts seem to be angular than clasts of sandstone (Tucker 2009, P. 56). Many these small-sized clasts, especially found in claystones, are mainly flaky minerals such as clay and mica. Another major constituent of mudrocks is small grains of quartz. Shale is a type of mudrock that consists of extremely thin layers. These types of rocks split easily in between these thin layers (laminate) forming small flakes or paper-thin sheets. The particles forming claystone are mainly too small for their identification and one cannot clearly see them even when using a microscope. Most shale is in black colour and constituent organic materials, which accumulate in quiet-water, environments with low oxygen, such as seas and lagoons with poor circulation of oxygen. Red shale has iron oxide colour indicating oxidizing condition of the environments where this deposition occurs (Monroe, Wicander & Hazlett 2006, P. 133). Such deposition environments include river floodplains, lakes, tidal flats, and well-mixed oceans. ii. Biochemical and chemical sedimentary rocks The second category of sedimentary rock is biochemical and chemical sedimentary rocks. Their formation occurs when a chemical process removes ions, which dissolve in water to form solid particles. Some rocks are biochemical since their sediments forms during organisms’ growth such as coral, algae, and swamp vegetation. Other rocks are inorganic chemical rocks formed from shallow seas or lakes (Nations & Eaton 2007, P. 78). Limestone-By far, limestone is one of the most abundant types of sedimentary rock. Principally, it comprises of calcium carbonate (CaCO3). Limestone originates by both biochemical and chemical processes. Indeed, the difference between chemical and biochemical rocks is infrequently clear-cut. Limestone rocks have different types of rock textures, and there are many classifications of limestone rocks. This paper discusses the main examples of limestone rocks: oolitic limestone, skeletal limestone, and microcrystalline limestone. Some of the marine invertebrate animals have body shells or some hard body parts. These animals construct these shells and these hard body parts by extracting carbonate and calcium ions from seawater. Clams, corals, snails, algae, and a number of other marine animals construct their body skeletons using calcium carbonate. After these animals die, their shells gather on the sea floor. After a long duration of time, the accumulated shells, build up a limestone deposit with shells and shell’s fragments texture (Nations & Eaton 2007, P. 79). These particles then cement together since calcite precipitates in the grains. This form of limestone, which forms from skeletal debris is thicker than any other type of limestone and can cover a large geographical area. For instance, chalk is a type of skeletal limestone, whereby the fragments of skeletal are remains of small animals and plants (microscopic organisms). Other forms of limestone comprise of tiny semi spherical particles of calcium carbonate called oolites. Oolite forms in environments where small shell fragments or other small grains coat with successive thin layer of calcium carbonate as they roll to the sea flow by currents and waves. The third type of limestone forms in environments with still and quiet waters where algae precipitate calcium carbonate as needle-like, tiny crystals that decompose on the sea as limy mud. Soon after their decomposition, these grains get modified through recrystallization and compaction process. This modification process forms microcrystalline limestone. Microcrystalline has a dense and a fine-grained texture. One can only see its individual crystals under high magnification. Other forms of carbonate particles may cement together due to microcrystalline limestone (Nations & Eaton 2007, P. 81). Inorganic limestone is another type of limestone, which precipitates from dripping water and spring forming beautiful layered rocks known as travertine. Most of limestone rocks forms on shallow continental shelves with warm water and high rate of organic production. On contrary, carbonate sediments rarely forms in deep water as well as accumulating on abyssal plains. This is because calcite cannot stabilize at high pressures and low temperatures of deep water. Calcite shells, which form on the surface of seawater fall down to the seafloor when the organism dies, but when they form in deep water, they dissolve even before they reach sea bottom. Near the equator line, calcite limestone is unstable in depths, which are below 4,500 meters. However, in places where the floor is sea is shallower than 4,500 meters such as oceanic ridges, there will be an accumulation of carbonate sediment. Depositional environments Vs. Pet physical properties Depositional environments are settings where different types of sedimentary rocks form. Each depositional environment has it own physical properties and geologic characteristics (Owen, Pirie & Draper 2011, P. 60). The type of sediment rock deposited in a certain depositional environment not only depends on the type of sediments transported to that place but also the environment itself. A marine environment entails that the sedimentary rock formed in an ocean or a sea (Nations & Eaton 2007, P. 83). Often, there is always a distinction between shallow and deep marine depositional environment. Deep marine depositional environment refers to those environments, which are below the surface water by 200 meters. On the other hand, shallow marine depositional environment refer to those environments, which are adjacent to the coastline and can sometimes extend their boundaries to the continental shelf (Stow 2005, P. 87). The water in shallow environments always has higher deposition energy than deep environments due to the wave activities. After transportation of available sediments from the continent, deposition of alternating sand, silt, and clay takes place (Owen, Pirie & Draper 2011, P. 63). When there is long distance transportation, the amount of sediment that reaches the depositional environment will always be in small quantity and the biochemical processes will dominate the type of sedimentary rock that forms. For instance, in warm marine depositional environments, only carbonate rocks form. This is because; there are many small organisms that lead to formation of carbonate skeletons in shallow warm waters. The three major sedimentary depositional environments include continental environments, transitional to Shallow marine environments, and deep marine environments (Nations & Eaton 2007, P. 85). 1. Continental environments These are environments, which deposit predominantly siliciclastic sediments such as sandstone, conglomerate, and siltstone. Continental environments are characterised by having no marine fossils and scarce fossils. Continental environments include fluvial, lacustrine, paludal, eolian, and glacial environments. Fluvial (rivers) - the categories of river environments are alluvial, Braded Rivers, and meandering rivers (Stow 2005, P. 90). Alluvial fans contains deposits that deposits at the basis of mountains with rapid flowing streams, which suddenly form from a shallow valley, slow down, spread out, and deposit the large particles in the sediment loads. Secondly, several channels separated by small islands or bars characterize braided rivers. Braiding forms because of rapid, large water volumes, and a coarse sediment abundance (Owen, Pirie & Draper 2011, P. 67). The two major types of braided rivers include rippled and horizontally stratified. The last type of fluvial rivers is meandering rivers. Meandering rivers confine to one, sinuous channel of water containing fine sediments than braided rivers. Although meandering rivers form bars, their bars are in the inside bending of the meander loops (Nations & Eaton 2007, P. 87). The two major types of meandering rivers include rippled, fining-upward, cross-bedded, and fine-grained sediments. The other types of continental sedimentary environments include lacustrine (lakes), paludal (swamps and marshes), eolian (deserts and beaches), and glacial. 2. Transitional to shallow marine environments This type of depositional environment includes deltas, beaches and barrier islands, clastic shelf, and carbonated shelves and platforms. Deltas form in places where rivers meet and form a large stagnant water body. Water in these areas slows down depositing all the sediments forming huge sediments, which currents or waves cannot remove. Although there are some deltas, which form in continental lakes, the large deltas form in oceans (Biju-Duval 2004, P. 64). Beaches are long and narrow places where sand accumulates parallel to shoreline. Barrier islands on the other hand, separate from dry land by a narrow marsh or lagoon. Beach facies primarily compose of fine and medium particles, and well-sorted sand. Coastal environments and continental slopes on the landward and seaward sides respectively surround clastic shelves. The main sediments in these environments mainly consist of mud and sand. Lastly, carbonate shelf and platform occur at lower latitudes in shallow, clear, and tropical seas with small clastic sediments (Kodama 2013, P. 48). The last category of depositional environments is deep marines. Deep marine environments include pelagic and turbidites. In pelagic environments, fine-grained particles deposit far from land with particles accumulating in the water. Turbidites on the other hand, are fine-upward deposits that are carried from seaward to the deep-sea channels. Conclusion Sedimentary rocks serve as a record of earth’s history, which is remarkable on the layers formed on the sedimentary rocks. Sedimentary rocks do not only portray a scientific significance but also serves as a controlling factor that controls the development of society, culture, and industry. In addition, an approximate of 90% of the modern minerals used daily in our societies comes from these rocks (Biju-Duval 2004, P. 67). Virtually, the current entire stores of natural gas, fertilizer, coal, and petroleum all are products of sedimentary rocks. There are different types of sedimentary rocks, which form on different depositional environment characterized by physical properties. Bibliography Biju-Duval, B 2004, Sedimentary geology: sedimentary basins, depositional environments, petroleum formation. Paris, Éd. Technip. Kodama, K. P 2013, Paleomagnetism of sediments and sedimentary rocks process and interpretation. Chichester, West Sussex, John Wiley & Sons. Mackenzie, F. T 2005, Sediments, diagenesis, and sedimentary rocks. Amsterdam, Elsevier. Boggs, S 2009, Petrology of sedimentary rocks. Cambridge, Cambridge University Press. Monroe, J. S., Wicander, R., & Hazlett, R 2006, Physical geology: exploring the earth; [the wrath of Hurricane Katrina ; Could you survive a Tsunami?; catastrophic earthquakes; global warming]. Belmont [u.a.], Thomson. Nations, D., & Eaton, J. G 2007, Stratigraphy, depositional environments, and sedimentary tectonics of the western margin, Cretaceous Western Interior Seaway. Boulder, Colo, Geological Society of America. Owen, C., Pirie, D., & Draper, G 2011, Earth lab: exploring the earth sciences. Belmont, CA, Brooks/Cole, Cengage Learning. Stow, D. A. V 2005, Sedimentary rocks in the field a colour guide. [London], Manson. Tucker, M. E 2009, Sedimentary petrology an introduction to the origin of sedimentary rocks. Oxford, Blackwell. Tucker, M. E 2013, Sedimentary rocks in the field a practical guide. Hoboken, N.J., Wiley. Read More