Misplaced Pages

#153846

52-420: The origin of greywacke was unknown until turbidity currents and turbidites were understood, since, according to the normal laws of sedimentation , gravel , sand and mud should not be laid down together. Geologists now attribute its formation to submarine avalanches or strong turbidity currents. These actions churn sediment and cause mixed-sediment slurries, in which the resulting deposits may exhibit

104-418: A natural phenomenon whose exact nature is often unclear. The turbulence within a turbidity current is not always the support mechanism that keeps the sediment in suspension; however it is probable that turbulence is the primary or sole grain support mechanism in dilute currents (<3%). Definitions are further complicated by an incomplete understanding of the turbulence structure within turbidity currents, and

156-485: A building material and a sculptural material across many eras and societies. Its oldest known uses date to the early third millennium BCE , in Egypt's early dynastic period . Its wide use in sculpture and vessels is thought to have been due to its fine grain size and resistance to fracturing, making it suitable for fine detail and intricate shapes. Aside from its structural uses, greywacke stone (or molds taken from it)

208-416: A decrease in height. The behaviour of turbidity currents with buoyant fluid (such as currents with warm, fresh or brackish interstitial water entering the sea) has been investigated to find that the front speed decreases more rapidly than that of currents with the same density as the ambient fluid. These turbidity currents ultimately come to a halt as sedimentation results in a reversal of buoyancy, and

260-623: A few to mention. Both Direct numerical simulation (DNS) and Turbulence modeling are used to model these currents. Geological Society of London The Geological Society of London , known commonly as the Geological Society , is a learned society based in the United Kingdom. It is the oldest national geological society in the world and the largest in Europe, with more than 12,000 Fellows . Fellows are entitled to

312-433: A secondary turbidity current on the ocean floor by the process of convective sedimentation. Sediment in the initially buoyant hypopycnal flow accumulates at the base of the surface flow, so that the dense lower boundary become unstable. The resulting convective sedimentation leads to a rapid vertical transfer of material to the sloping lake or ocean bed, potentially forming a secondary turbidity current. The vertical speed of

364-630: A turbidity current following the 1929 Grand Banks earthquake , earthquake triggered turbidites have been investigated and verified along the Cascadia subduction Zone, the Northern San Andreas Fault, a number of European, Chilean and North American lakes, Japanese lacustrine and offshore regions and a variety of other settings. When large turbidity currents flow into canyons they may become self-sustaining, and may entrain sediment that has previously been introduced into

416-409: A variety of sedimentary features. Supporting the turbidity origin theory is the fact that deposits of greywacke are found on the edges of the continental shelves , at the bottoms of oceanic trenches , and at the bases of mountain formational areas. They also occur in association with black shales of deep-sea origin. As a rule, greywackes do not contain fossils , but organic remains may be common in

468-567: A well-defined advance-front, also known as the current's head, and are followed by the current's main body. In terms of the more often observed and more familiar above sea-level phenomenon, they somewhat resemble flash floods. Turbidity currents can sometimes result from submarine seismic instability, which is common with steep underwater slopes, and especially with submarine trench slopes of convergent plate margins, continental slopes and submarine canyons of passive margins. With an increasing continental shelf slope, current velocity increases, as

520-560: A well-established place in petrographical classifications because these peculiar composite arenaceous deposits are very frequent among Silurian and Cambrian rocks, and are less common in Mesozoic or Cenozoic strata. Their essential features are their gritty character and their complex composition. By increasing metamorphism , greywackes frequently pass into mica- schists , chloritic schists and sedimentary gneisses . The term "greywacke" can be confusing, since it can refer to either

572-425: Is 35 to 45 kg/m , depending on the water properties within the coastal zone. Most rivers produce hyperpycnal flows only during exceptional events, such as storms , floods , glacier outbursts, dam breaks, and lahar flows. In fresh water environments, such as lakes , the suspended sediment concentration needed to produce a hyperpycnal plume is quite low (1 kg/m ). The transport and deposition of

SECTION 10

#1732771876154

624-430: Is gravity acting on the high density of the sediments temporarily suspended within a fluid. These semi-suspended solids make the average density of the sediment bearing water greater than that of the surrounding, undisturbed water. As such currents flow, they often have a "snow-balling-effect", as they stir up the ground over which they flow, and gather even more sedimentary particles in their current. Their passage leaves

676-846: Is sometimes calcareous . Greywackes are abundant in Wales , the south of Scotland , the Longford-Down Massif in Ireland and the Lake District National Park of England ; they compose the majority of the main Southern Alps that make up the backbone of New Zealand . Both feldspathic and lithic greywacke have been recognized in Ecca Group in South Africa . Greywackes are also found in parts of

728-453: Is used to reproduce the physical processes which govern turbidity current behaviour and deposits. The so-called depth-averaged or shallow-water models are initially introduced for compositional gravity currents and then later extended to turbidity currents. The typical assumptions used along with the shallow-water models are: hydrostatic pressure field, clear fluid is not entrained (or detrained), and particle concentration does not depend on

780-482: Is valuable to practitioners of traditional motion picture miniature photography , because due to its unusually mixed nature, it remains looking natural when portraying a wide range of miniature scale ratios, from 1:1 to as high as 1:600. Turbidity current A turbidity current is most typically an underwater current of usually rapidly moving, sediment-laden water moving down a slope; although current research (2018) indicates that water-saturated sediment may be

832-1131: The Association of Applied Geochemists ; Journal of Micropalaeontology for The Micropalaeontological Society ; Proceedings of the Yorkshire Geological Society for the Yorkshire Geological Society ; and Scottish Journal of Geology for the Geological Societies of Edinburgh and Glasgow . The society counts many famous geologists amongst its past presidents. These include pioneers of geology William Buckland , Adam Sedgwick , Roderick Impey Murchison , Charles Lyell , Henry Thomas De la Beche , Thomas Henry Huxley , Joseph Prestwich , Archibald Geikie , Jethro Teall , and Charles Lapworth . Later well-known names include Alfred Harker , Arthur Elijah Trueman , Herbert Harold Read , Frederick Shotton , and Janet Watson . In 1831, it began issuing an annual scientific award for geology, known as

884-588: The Eastern Desert east of the Nile . They were an early object of geological study in Britain where the Geological Society was founded in 1807, and excited much public interest in geology. Greywacke was interesting because it was found in many places in Britain and its occurrence in particular places was evidence of the pattern of geological strata that had been laid down. Greywacke stone has been used as

936-585: The Gulf of Cadiz , where the ocean current leaving the Mediterranean Sea (also known as the Mediterranean outflow water) pushes turbidity currents westward. This has changed the shape of submarine valleys and canyons in the region to also curve in that direction. When the energy of a turbidity current lowers, its ability to keep suspended sediment decreases, thus sediment deposition occurs. When

988-504: The geosciences in Britain and abroad, under the auspices of the science writer and palaeontologist Professor Richard Fortey , the president that year. The Society has 24 specialist groups and 15 regional groups which serve as an opportunity for those with specific interests to meet and discuss their subject or region. They are all free for members to join and some are open to non-members. The Regional Groups are: The Specialist Groups are: The society publishes two of its own journals,

1040-443: The interstitial fluid is a liquid (generally water); a pyroclastic current is one in which the interstitial fluid is gas. When the concentration of suspended sediment at the mouth of a river is so large that the density of river water is greater than the density of sea water a particular kind of turbidity current can form called a hyperpycnal plume. The average concentration of suspended sediment for most river water that enters

1092-459: The ocean is much lower than the sediment concentration needed for entry as a hyperpycnal plume. Although some rivers can often have continuously high sediment load that can create a continuous hyperpycnal plume, such as the Haile River (China), which has an average suspended concentration of 40.5 kg/m . The sediment concentration needed to produce a hyperpycnal plume in marine water

SECTION 20

#1732771876154

1144-570: The postnominal FGS (Fellow of the Geological Society), over 2,000 of whom are Chartered Geologists (CGeol). The Society is a registered charity , no. 210161. It is also a member of the Science Council , and is licensed to award Chartered Scientist to qualifying members. The mission of the society is: "Making geologists acquainted with each other, stimulating their zeal, inducing them to adopt one nomenclature, facilitating

1196-458: The sediments in narrow alpine reservoirs is often caused by turbidity currents. They follow the thalweg of the lake to the deepest area near the dam , where the sediments can affect the operation of the bottom outlet and the intake structures. Controlling this sedimentation within the reservoir can be achieved by using solid and permeable obstacles with the right design. Turbidity currents are often triggered by tectonic disturbances of

1248-776: The (formerly Quarterly ) Journal of the Geological Society and the Quarterly Journal of Engineering Geology & Hydrogeology . It also publishes the magazine Geoscientist for Fellows, and has a share in Geology Today , published by Blackwell Science . It also co-publishes journals and publishes on behalf of other organisations. These include Petroleum Geoscience with the European Association of Geoscientists and Engineers ; Geochemistry: Exploration, Environment, Analysis with

1300-592: The Society has been based at Burlington House , Piccadilly , London . This building houses the Society's library, which contains more than 300,000 volumes of books and journals. It is a member of the UK Science Council . In 1907 a decision was made by the Society to admit women as Associates, under the condition they distinguished themselves as geological investigators or submitted their own original research. Women were first allowed to become Fellows of

1352-579: The Society in 1919. Margaret Crosfield became the first, due to alphabetical primacy, of the first eight women to be elected Fellows of the Society, on 21 May 1919. In 1991, the Society merged with the Institution of Geologists, which had been formed in 1977 to represent the geological profession. The Society is a member of the European Federation of Geologists . The Society celebrated its bicentenary in 2007. It ran programmes in

1404-473: The adjacent Venezuela , Guyana and Suriname continental margins. Simple numerical modelling has been enabled to determine turbidity current flow characteristics across the sediment waves to be estimated: internal Froude number = 0.7–1.1, flow thickness = 24–645 m, and flow velocity = 31–82 cm·s . Generally, on lower gradients beyond minor breaks of slope, flow thickness increases and flow velocity decreases, leading to an increase in wavelength and

1456-405: The cabled observatory which provided direct observations, which is rarely achieved. Oil and gas companies are also interested in turbidity currents because the currents deposit organic matter that over geologic time gets buried, compressed and transformed into hydrocarbons . The use of numerical modelling and flumes are commonly used to help understand these questions. Much of the modelling

1508-535: The canyon by littoral drift , storms or smaller turbidity currents. Canyon-flushing associated with surge-type currents initiated by slope failures may produce currents whose final volume may be several times that of the portion of the slope that has failed (e.g. Grand Banks). Sediment that has piled up at the top of the continental slope , particularly at the heads of submarine canyons can create turbidity current due to overloading, thus consequent slumping and sliding. A buoyant sediment-laden river plume can induce

1560-897: The communication of new facts and ascertaining what is known in their science and what remains to be discovered". The Society was founded on 13 November 1807 at the Freemasons' Tavern , Great Queen Street , in the Covent Garden district of London. It was partly the outcome of a previous club known as the Askesian Society . There were 13 founder members: William Babington , James Parkinson , Humphry Davy , George Bellas Greenough , Arthur Aikin , William Allen , Jacques Louis, Comte de Bournon , Richard Knight, James Laird, James Franck, William Haseldine Pepys , Richard Phillips , and William Phillips . It received its royal charter on 23 April 1825 from George IV . Since 1874,

1612-401: The confusion between the terms turbulent (i.e. disturbed by eddies) and turbid (i.e. opaque with sediment). Kneller & Buckee, 2000 define a suspension current as 'flow induced by the action of gravity upon a turbid mixture of fluid and (suspended) sediment, by virtue of the density difference between the mixture and the ambient fluid'. A turbidity current is a suspension current in which

Greywacke - Misplaced Pages Continue

1664-623: The convective plumes can be much greater than the Stokes settling velocity of an individual particle of sediment. Most examples of this process have been made in the laboratory, but possible observational evidence of a secondary turbidity current was made in Howe Sound, British Columbia, where a turbidity current was periodically observed on the delta of the Squamish River. As the vast majority of sediment laden rivers are less dense than

1716-445: The current lifts off, the point of lift-off remaining constant for a constant discharge. The lofted fluid carries fine sediment with it, forming a plume that rises to a level of neutral buoyancy (if in a stratified environment) or to the water surface, and spreads out. Sediment falling from the plume produces a widespread fall-out deposit, termed hemiturbidite. Experimental turbidity currents and field observations suggest that

1768-522: The development of quantitative models of turbidity current behaviour inferred solely from their deposits. Small-scale laboratory experiments therefore offer one of the best means of studying their dynamics. Mathematical models can also provide significant insights into current dynamics. In the long term, numerical techniques are most likely the best hope of understanding and predicting three-dimensional turbidity current processes and deposits. In most cases, there are more variables than governing equations , and

1820-451: The finer beds associated with them. Their component particles are usually not very rounded or polished, and the rocks have often been considerably indurated by recrystallization , such as the introduction of interstitial silica . In some districts, the greywackes are cleaved, but they show phenomena of this kind much less perfectly than the slates. Although the group is so diverse that it is difficult to characterize mineralogically, it has

1872-517: The ground over which they flow scoured and eroded. Once an oceanic turbidity current reaches the calmer waters of the flatter area of the abyssal plain (main oceanic floor), the particles borne by the current settle out of the water column. The sedimentary deposit of a turbidity current is called a turbidite . Seafloor turbidity currents are often the result of sediment-laden river outflows, and can sometimes be initiated by earthquakes , slumping and other soil disturbances. They are characterized by

1924-420: The immature (rock fragment) aspect of the rock or its fine-grained (clay) component. Greywackes are mostly grey, brown, yellow, or black, dull-colored sandy rocks that may occur in thick or thin beds along with shales and limestones . Some varieties include feldspathic greywacke , rich in feldspar , and lithic greywacke , rich in other tiny rock fragments. They can contain a very great variety of minerals ,

1976-553: The material comes to rest, it is the sand and other coarse material which settles first followed by mud and eventually the very fine particulate matter. It is this sequence of deposition that creates the so called Bouma sequences that characterize turbidite deposits. Because turbidity currents occur underwater and happen suddenly, they are rarely seen as they happen in nature, thus turbidites can be used to determine turbidity current characteristics. Some examples: grain size can give indication of current velocity, grain lithology and

2028-415: The models rely upon simplifying assumptions in order to achieve a result. The accuracy of the individual models thus depends upon the validity and choice of the assumptions made. Experimental results provide a means of constraining some of these variables as well as providing a test for such models. Physical data from field observations, or more practical from experiments, are still required in order to test

2080-413: The ocean floor can help to decrease the amount of damage to telecommunication cables by avoiding these areas or reinforcing the cables in vulnerable areas. When turbidity currents interact with regular ocean currents, such as contour currents , they can change their direction. This ultimately shifts submarine canyons and sediment deposition locations. One example of this is located in the western part of

2132-430: The ocean, rivers cannot readily form plunging hyperpycnal flows. Hence convective sedimentation is an important possible initiation mechanism for turbidity currents. Large and fast-moving turbidity currents can carve gulleys and ravines into the ocean floor of continental margins and cause damage to artificial structures such as telecommunication cables on the seafloor . Understanding where turbidity currents flow on

Greywacke - Misplaced Pages Continue

2184-626: The predicted history of turbidity currents in this area was determined, increasing the overall understanding of these currents. Some of the largest antidunes on Earth are formed by turbidity currents. One observed sediment-wave field is located on the lower continental slope off Guyana , South America. This sediment-wave field covers an area of at least 29 000 km at a water depth of 4400–4825 meters. These antidunes have wavelengths of 110–2600 m and wave heights of 1–15 m. Turbidity currents responsible for wave generation are interpreted as originating from slope failures on

2236-532: The primary actor in the process. Turbidity currents can also occur in other fluids besides water. Researchers from the Monterey Bay Aquarium Research Institute found that a layer of water-saturated sediment moved rapidly over the seafloor and mobilized the upper few meters of the preexisting seafloor. Plumes of sediment-laden water were observed during turbidity current events but they believe that these were secondary to

2288-495: The principal ones being quartz, orthoclase and plagioclase feldspars, calcite , iron oxides and graphitic, carbonaceous matters, together with (in the coarser kinds) fragments of such rocks as felsite , chert , slate , gneiss , various schists , and quartzite . Among other minerals found in them are biotite , chlorite , tourmaline , epidote , apatite , garnet , hornblende , augite , sphene and pyrites . The cementing material may be siliceous or argillaceous and

2340-416: The pulse of the seafloor sediment moving during the events. The belief of the researchers is that the water flow is the tail-end of the process that starts at the seafloor. In the most typical case of oceanic turbidity currents, sediment laden waters situated over sloping ground will flow down-hill because they have a higher density than the adjacent waters. The driving force behind a turbidity current

2392-410: The sea floor. The displacement of continental crust in the form of fluidization and physical shaking both contribute to their formation. Earthquakes have been linked to turbidity current deposition in many settings, particularly where physiography favors preservation of the deposits and limits the other sources of turbidity current deposition. Since the famous case of breakage of submarine cables by

2444-485: The shape of the lobe deposit formed by a lofting plume is narrower than for a similar non-lofting plume Prediction of erosion by turbidity currents, and of the distribution of turbidite deposits, such as their extent, thickness and grain size distribution, requires an understanding of the mechanisms of sediment transport and deposition , which in turn depends on the fluid dynamics of the currents. The extreme complexity of most turbidite systems and beds has promoted

2496-409: The simplifying assumptions necessary in mathematical models . Most of what is known about large natural turbidity currents (i.e. those significant in terms of sediment transfer to the deep sea) is inferred from indirect sources, such as submarine cable breaks and heights of deposits above submarine valley floors. Although during the 2003 Tokachi-oki earthquake a large turbidity current was observed by

2548-589: The solution of the Navier-Stokes equations for the fluid phase. With dilute suspension of particles, a Eulerian approach proved to be accurate to describe the evolution of particles in terms of a continuum particle concentration field. Under these models, no such assumptions as shallow-water models are needed and, therefore, accurate calculations and measurements are performed to study these currents. Measurements such as, pressure field, energy budgets, vertical particle concentration and accurate deposit heights are

2600-473: The use of foraminifera for determining origins, grain distribution shows flow dynamics over time and sediment thickness indicates sediment load and longevity. Turbidites are commonly used in the understanding of past turbidity currents, for example, the Peru-Chile Trench off Southern Central Chile (36°S–39°S) contains numerous turbidite layers that were cored and analysed. From these turbidites

2652-407: The velocity of the flow increases, turbulence increases, and the current draws up more sediment. The increase in sediment also adds to the density of the current, and thus increases its velocity even further. Turbidity currents are traditionally defined as those sediment gravity flows in which sediment is suspended by fluid turbulence. However, the term "turbidity current" was adopted to describe

SECTION 50

#1732771876154

2704-410: The vertical location. Considering the ease of implementation, these models can typically predict flow characteristic such as front location or front speed in simplified geometries, e.g. rectangular channels, fairly accurately. With the increase in computational power, depth-resolved models have become a powerful tool to study gravity and turbidity currents. These models, in general, are mainly focused on

#153846