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93-432: NATM/SEM is generally thought to have helped revolutionise the modern tunneling industry. Many modern tunnels have used this excavation technique. The Sequential Excavation Method is very cost effective, even in karst conditions. The NATM integrates the principles of the behaviour of rock masses under load and monitoring the performance of underground construction during construction. The NATM has often been referred to as

186-455: A Mohs hardness of 2 to 4, dense limestone can have a crushing strength of up to 180 MPa . For comparison, concrete typically has a crushing strength of about 40 MPa. Although limestones show little variability in mineral composition, they show great diversity in texture. However, most limestone consists of sand-sized grains in a carbonate mud matrix. Because limestones are often of biological origin and are usually composed of sediment that

279-463: A bloom of cyanobacteria or microalgae . However, stable isotope ratios in modern carbonate mud appear to be inconsistent with either of these mechanisms, and abrasion of carbonate grains in high-energy environments has been put forward as a third possibility. Formation of limestone has likely been dominated by biological processes throughout the Phanerozoic , the last 540 million years of

372-499: A design philosophy and a construction method . Key features of the NATM design philosophy are: When NATM is seen as a construction method, the key features are: The 1994 Heathrow Airport tunnel collapse led to questions about the safety of the NATM. However, the subsequent trial blamed the collapse on poor workmanship and flaws in construction management, rather than on the NATM. Karst Karst ( / k ɑːr s t / )

465-431: A "design as you go" approach, by providing an optimized support based on observed ground conditions. More correctly it can be described as a "design as you monitor" approach, based on observed convergence and divergence in the lining and mapping of prevailing rock conditions. It is not a set of specific excavation and support techniques. NATM has seven elements: Based on the computation of the optimal cross section , only

558-489: A 1918 publication, Cvijić proposed a cyclical model for karst landscape development. Karst hydrology emerged as a discipline in the late 1950s and the early 1960s in France. Previously, the activities of cave explorers, called speleologists , had been dismissed as more of a sport than a science and so the underground karst caves and their associated watercourses were, from a scientific perspective, understudied. Karst

651-434: A carbonate rock outcrop can be estimated in the field by etching the surface with dilute hydrochloric acid. This etches away the calcite and aragonite, leaving behind any silica or dolomite grains. The latter can be identified by their rhombohedral shape. Crystals of calcite, quartz , dolomite or barite may line small cavities ( vugs ) in the rock. Vugs are a form of secondary porosity, formed in existing limestone by

744-616: A central quartz grain or carbonate mineral fragment. These likely form by direct precipitation of calcium carbonate onto the ooid. Pisoliths are similar to ooids, but they are larger than 2 mm in diameter and tend to be more irregular in shape. Limestone composed mostly of ooids is called an oolite or sometimes an oolitic limestone . Ooids form in high-energy environments, such as the Bahama platform, and oolites typically show crossbedding and other features associated with deposition in strong currents. Oncoliths resemble ooids but show

837-449: A change in environment that increases the solubility of calcite. Dense, massive limestone is sometimes described as "marble". For example, the famous Portoro "marble" of Italy is actually a dense black limestone. True marble is produced by recrystallization of limestone during regional metamorphism that accompanies the mountain building process ( orogeny ). It is distinguished from dense limestone by its coarse crystalline texture and

930-949: A composition reflecting the organisms that produced them and the environment in which they were produced. Low-magnesium calcite skeletal grains are typical of articulate brachiopods , planktonic (free-floating) foraminifera, and coccoliths . High-magnesium calcite skeletal grains are typical of benthic (bottom-dwelling) foraminifera, echinoderms , and coralline algae . Aragonite skeletal grains are typical of molluscs , calcareous green algae , stromatoporoids , corals , and tube worms . The skeletal grains also reflect specific geological periods and environments. For example, coral grains are more common in high-energy environments (characterized by strong currents and turbulence) while bryozoan grains are more common in low-energy environments (characterized by quiet water). Ooids (sometimes called ooliths) are sand-sized grains (less than 2mm in diameter) consisting of one or more layers of calcite or aragonite around

1023-412: A considerable fraction of the limestone bed. At depths greater than 1 km (0.62 miles), burial cementation completes the lithification process. Burial cementation does not produce stylolites. When overlying beds are eroded, bringing limestone closer to the surface, the final stage of diagenesis takes place. This produces secondary porosity as some of the cement is dissolved by rainwater infiltrating

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1116-483: A drop of dilute hydrochloric acid is dropped on it. Dolomite is also soft but reacts only feebly with dilute hydrochloric acid, and it usually weathers to a characteristic dull yellow-brown color due to the presence of ferrous iron. This is released and oxidized as the dolomite weathers. Impurities (such as clay , sand, organic remains, iron oxide , and other materials) will cause limestones to exhibit different colors, especially with weathered surfaces. The makeup of

1209-613: A fellow of the Royal Society , London, introduced the word karst to European scholars in 1689 to describe the phenomenon of underground flows of rivers in his account of Lake Cerknica . Jovan Cvijić greatly advanced the knowledge of karst regions to the point where he became known as the "father of karst geomorphology". Primarily discussing the karst regions of the Balkans, Cvijić's 1893 publication Das Karstphänomen describes landforms such as karren, dolines and poljes . In

1302-637: A few million years, as this is the most stable form of calcium carbonate. Ancient carbonate formations of the Precambrian and Paleozoic contain abundant dolomite, but limestone dominates the carbonate beds of the Mesozoic and Cenozoic . Modern dolomite is quite rare. There is evidence that, while the modern ocean favors precipitation of aragonite, the oceans of the Paleozoic and middle to late Cenozoic favored precipitation of calcite. This may indicate

1395-455: A few thousand years. As rainwater mixes with groundwater, aragonite and high-magnesium calcite are converted to low-calcium calcite. Cementing of thick carbonate deposits by rainwater may commence even before the retreat of the sea, as rainwater can infiltrate over 100 km (60 miles) into sediments beneath the continental shelf. As carbonate sediments are increasingly deeply buried under younger sediments, chemical and mechanical compaction of

1488-435: A limestone sample except in thin section and are less common in ancient limestones, possibly because compaction of carbonate sediments disrupts them. Limeclasts are fragments of existing limestone or partially lithified carbonate sediments. Intraclasts are limeclasts that originate close to where they are deposited in limestone, while extraclasts come from outside the depositional area. Intraclasts include grapestone , which

1581-471: A lower Mg/Ca ratio in the ocean water of those times. This magnesium depletion may be a consequence of more rapid sea floor spreading , which removes magnesium from ocean water. The modern ocean and the ocean of the Mesozoic have been described as "aragonite seas". Most limestone was formed in shallow marine environments, such as continental shelves or platforms . Such environments form only about 5% of

1674-525: A lower diversity of organisms and a greater fraction of silica and clay minerals characteristic of marls . The Green River Formation is an example of a prominent freshwater sedimentary formation containing numerous limestone beds. Freshwater limestone is typically micritic. Fossils of charophyte (stonewort), a form of freshwater green algae, are characteristic of these environments, where the charophytes produce and trap carbonates. Limestones may also form in evaporite depositional environments . Calcite

1767-531: A major role in the formation of ancient Lechuguilla Cave in the US state of New Mexico and is presently active in the Frasassi Caves of Italy. The oxidation of sulfides leading to the formation of sulfuric acid can also be one of the corrosion factors in karst formation. As oxygen (O 2 )-rich surface waters seep into deep anoxic karst systems, they bring oxygen, which reacts with sulfide present in

1860-523: A mechanism for dolomitization, with one 2004 review paper describing it bluntly as "a myth". Ordinary seawater is capable of converting calcite to dolomite, if the seawater is regularly flushed through the rock, as by the ebb and flow of tides (tidal pumping). Once dolomitization begins, it proceeds rapidly, so that there is very little carbonate rock containing mixed calcite and dolomite. Carbonate rock tends to be either almost all calcite/aragonite or almost all dolomite. About 20% to 25% of sedimentary rock

1953-448: A plausible source of mud. Another possibility is direct precipitation from the water. A phenomenon known as whitings occurs in shallow waters, in which white streaks containing dispersed micrite appear on the surface of the water. It is uncertain whether this is freshly precipitated aragonite or simply material stirred up from the bottom, but there is some evidence that whitings are caused by biological precipitation of aragonite as part of

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2046-449: A radial rather than layered internal structure, indicating that they were formed by algae in a normal marine environment. Peloids are structureless grains of microcrystalline carbonate likely produced by a variety of processes. Many are thought to be fecal pellets produced by marine organisms. Others may be produced by endolithic (boring) algae or other microorganisms or through breakdown of mollusc shells. They are difficult to see in

2139-429: A thin shotcrete protection is necessary. It is applied immediately behind the excavated tunnel face to create a natural load-bearing ring and minimize the rock's deformation. Geotechnical instruments are installed to measure the later deformation of excavation . Monitoring of the stress distribution within the rock is possible. This monitoring makes the method very flexible, even if teams encounter unexpected changes in

2232-412: A variety of spatial and temporal scales in karst regions, with the purpose of understanding karst aquifers and ecosystems, and the development of the surface and underground structure, so that the environment can be protected and human activities planned effectively. Limestone Limestone ( calcium carbonate CaCO 3 ) is a type of carbonate sedimentary rock which is the main source of

2325-529: A well in a fracture trace or intersection of fracture traces increases the likelihood to encounter good water production. Voids in karst aquifers can be large enough to cause destructive collapse or subsidence of the ground surface that can initiate a catastrophic release of contaminants. Groundwater flow rate in karst aquifers is much more rapid than in porous aquifers. For example, in the Barton Springs Edwards aquifer, dye traces measured

2418-455: Is a topography formed from the dissolution of soluble carbonate rocks such as limestone and dolomite . It is characterized by features like poljes above and drainage systems with sinkholes and caves underground. There is some evidence that karst may occur in more weathering -resistant rocks such as quartzite given the right conditions. Subterranean drainage may limit surface water, with few to no rivers or lakes. In regions where

2511-505: Is a UNESCO World Heritage Site. Many karst-related terms derive from South Slavic languages , entering scientific vocabulary through early research in the Western Balkan Dinaric Alpine karst. The study of the various aspects of karst regions is called karstology. This includes biological, chemical, ecological, geomorphological, hydrogeological, hydrological, political, socio-economical, and other processes over

2604-562: Is a development of karst observed in geological history and preserved within the rock sequence, effectively a fossil karst. There are for example palaeokarst surfaces exposed within the Clydach Valley Subgroup of the Carboniferous Limestone sequence of South Wales which developed as sub-aerial weathering of recently formed limestones took place during periods of non-deposition within the early part of

2697-538: Is a karst landscape which is developed beneath a cover of insoluble rocks. Typically this will involve a cover of sandstone overlying limestone strata undergoing solution. In the United Kingdom for example extensive doline fields have developed at Cefn yr Ystrad , Mynydd Llangatwg and Mynydd Llangynidr in South Wales across a cover of Twrch Sandstone which overlies concealed Carboniferous Limestone ,

2790-560: Is also favored on the seaward margin of shelves and platforms, where there is upwelling deep ocean water rich in nutrients that increase organic productivity. Reefs are common here, but when lacking, ooid shoals are found instead. Finer sediments are deposited close to shore. The lack of deep sea limestones is due in part to rapid subduction of oceanic crust, but is more a result of dissolution of calcium carbonate at depth. The solubility of calcium carbonate increases with pressure and even more with higher concentrations of carbon dioxide, which

2883-399: Is an obsolete and poorly-defined term used variously for dolomite, for limestone containing significant dolomite ( dolomitic limestone ), or for any other limestone containing a significant percentage of magnesium . Most limestone was formed in shallow marine environments, such as continental shelves or platforms , though smaller amounts were formed in many other environments. Much dolomite

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2976-482: Is an uncommon mineral in limestone, and siderite or other carbonate minerals are rare. However, the calcite in limestone often contains a few percent of magnesium . Calcite in limestone is divided into low-magnesium and high-magnesium calcite, with the dividing line placed at a composition of 4% magnesium. High-magnesium calcite retains the calcite mineral structure, which is distinct from dolomite. Aragonite does not usually contain significant magnesium. Most limestone

3069-402: Is carbonate rock, and most of this is limestone. Limestone is found in sedimentary sequences as old as 2.7 billion years. However, the compositions of carbonate rocks show an uneven distribution in time in the geologic record. About 95% of modern carbonates are composed of high-magnesium calcite and aragonite. The aragonite needles in carbonate mud are converted to low-magnesium calcite within

3162-474: Is clusters of peloids cemented together by organic material or mineral cement. Extraclasts are uncommon, are usually accompanied by other clastic sediments, and indicate deposition in a tectonically active area or as part of a turbidity current . The grains of most limestones are embedded in a matrix of carbonate mud. This is typically the largest fraction of an ancient carbonate rock. Mud consisting of individual crystals less than 5 μm (0.20 mils) in length

3255-416: Is commonly white to gray in color. Limestone that is unusually rich in organic matter can be almost black in color, while traces of iron or manganese can give limestone an off-white to yellow to red color. The density of limestone depends on its porosity, which varies from 0.1% for the densest limestone to 40% for chalk. The density correspondingly ranges from 1.5 to 2.7 g/cm . Although relatively soft, with

3348-420: Is controlled largely by the amount of dissolved carbon dioxide ( CO 2 ) in the water. This is summarized in the reaction: Increases in temperature or decreases in pressure tend to reduce the amount of dissolved CO 2 and precipitate CaCO 3 . Reduction in salinity also reduces the solubility of CaCO 3 , by several orders of magnitude for fresh water versus seawater. Near-surface water of

3441-545: Is converted to low-magnesium calcite. Diagenesis is the likely origin of pisoliths , concentrically layered particles ranging from 1 to 10 mm (0.039 to 0.394 inches) in diameter found in some limestones. Pisoliths superficially resemble ooids but have no nucleus of foreign matter, fit together tightly, and show other signs that they formed after the original deposition of the sediments. Silicification occurs early in diagenesis, at low pH and temperature, and contributes to fossil preservation. Silicification takes place through

3534-503: Is deposited close to where it formed, classification of limestone is usually based on its grain type and mud content. Most grains in limestone are skeletal fragments of marine organisms such as coral or foraminifera . These organisms secrete structures made of aragonite or calcite, and leave these structures behind when they die. Other carbonate grains composing limestones are ooids , peloids , and limeclasts ( intraclasts and extraclasts  [ ca ] ). Skeletal grains have

3627-460: Is described as coquinite . Chalk is a soft, earthy, fine-textured limestone composed of the tests of planktonic microorganisms such as foraminifera, while marl is an earthy mixture of carbonates and silicate sediments. Limestone forms when calcite or aragonite precipitate out of water containing dissolved calcium, which can take place through both biological and nonbiological processes. The solubility of calcium carbonate ( CaCO 3 )

3720-617: Is described as micrite . In fresh carbonate mud, micrite is mostly small aragonite needles, which may precipitate directly from seawater, be secreted by algae, or be produced by abrasion of carbonate grains in a high-energy environment. This is converted to calcite within a few million years of deposition. Further recrystallization of micrite produces microspar , with grains from 5 to 15 μm (0.20 to 0.59 mils) in diameter. Limestone often contains larger crystals of calcite, ranging in size from 0.02 to 0.1 mm (0.79 to 3.94 mils), that are described as sparry calcite or sparite . Sparite

3813-468: Is developed in areas where salt is undergoing solution underground. It can lead to surface depressions and collapses which present a geo-hazard. Karst areas tend to have unique types of forests. The karst terrain is difficult for humans to traverse, so that their ecosystems are often relatively undisturbed. The soil tends to have a high pH, which encourages growth of unusual species of orchids, palms, mangroves, and other plants. Paleokarst or palaeokarst

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3906-462: Is distinguished from micrite by a grain size of over 20 μm (0.79 mils) and because sparite stands out under a hand lens or in thin section as white or transparent crystals. Sparite is distinguished from carbonate grains by its lack of internal structure and its characteristic crystal shapes. Geologists are careful to distinguish between sparite deposited as cement and sparite formed by recrystallization of micrite or carbonate grains. Sparite cement

3999-524: Is found in porous karst systems. The English word karst was borrowed from German Karst in the late 19th century, which entered German usage much earlier, to describe a number of geological, geomorphological, and hydrological features found within the range of the Dinaric Alps , stretching from the northeastern corner of Italy above the city of Trieste , across the Balkan peninsula along

4092-636: Is frequently unseen until the roof of a cavern suddenly collapses. Such events have swallowed homes, cattle, cars, and farm machinery. In the United States, sudden collapse of such a cavern-sinkhole swallowed part of the collection of the National Corvette Museum in Bowling Green, Kentucky in 2014. The world's largest limestone karst is Australia's Nullarbor Plain . Slovenia has the world's highest risk of sinkholes, while

4185-402: Is moderate to heavy. This contributes to rapid downward movement of groundwater, which promotes dissolution of the bedrock, whereas standing groundwater becomes saturated with carbonate minerals and ceases to dissolve the bedrock. The carbonic acid that causes karst features is formed as rain passes through Earth's atmosphere picking up carbon dioxide (CO 2 ), which readily dissolves in

4278-428: Is most strongly developed in dense carbonate rock , such as limestone, that is thinly bedded and highly fractured . Karst is not typically well developed in chalk , because chalk is highly porous rather than dense, so the flow of groundwater is not concentrated along fractures. Karst is also most strongly developed where the water table is relatively low, such as in uplands with entrenched valleys , and where rainfall

4371-560: Is one of the first minerals to precipitate in marine evaporites. Most limestone is formed by the activities of living organisms near reefs, but the organisms responsible for reef formation have changed over geologic time. For example, stromatolites are mound-shaped structures in ancient limestones, interpreted as colonies of cyanobacteria that accumulated carbonate sediments, but stromatolites are rare in younger limestones. Organisms precipitate limestone both directly as part of their skeletons, and indirectly by removing carbon dioxide from

4464-473: Is otherwise chemically fairly pure, with clastic sediments (mainly fine-grained quartz and clay minerals ) making up less than 5% to 10% of the composition. Organic matter typically makes up around 0.2% of a limestone and rarely exceeds 1%. Limestone often contains variable amounts of silica in the form of chert or siliceous skeletal fragments (such as sponge spicules, diatoms , or radiolarians ). Fossils are also common in limestone. Limestone

4557-536: Is partly caused by an increased use of the tunneling method in the United States, particularly in soft ground shallow tunnels. Other designations are seen for this modern tunneling style; Sequential Excavation Method (SEM) or Sprayed Concrete Lining (SCL) are often used in shallower tunnels. In Japan, the terms Centre Dividing Wall NATM or Cross Diaphragm Method (both abbreviated to CDM) and Upper Half Vertical Subdivision method (UHVS) are used. The Austrian Society of Engineers and Architects defines NATM as "a method where

4650-472: Is produced by decaying organic matter settling into the deep ocean that is not removed by photosynthesis in the dark depths. As a result, there is a fairly sharp transition from water saturated with calcium carbonate to water unsaturated with calcium carbonate, the lysocline , which occurs at the calcite compensation depth of 4,000 to 7,000 m (13,000 to 23,000 feet). Below this depth, foraminifera tests and other skeletal particles rapidly dissolve, and

4743-518: Is secondary dolomite, formed by chemical alteration of limestone. Limestone is exposed over large regions of the Earth's surface, and because limestone is slightly soluble in rainwater, these exposures often are eroded to become karst landscapes. Most cave systems are found in limestone bedrock. Limestone has numerous uses: as a chemical feedstock for the production of lime used for cement (an essential component of concrete ), as aggregate for

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4836-575: The geomechanical rock consistency, e.g. by crevices or pit water . Reinforcement is done by wired concrete that can be combined with steel ribs or lug bolts, not with thicker shotcrete. The measured rock properties suggest the appropriate tools for tunnel strengthening, where support requirements can traditionally be estimated using the RMR or Q System. Since the turn of the 21st century, NATM has been used for soft ground excavations and making tunnels in porous sediments . NATM enables immediate adjustments in

4929-496: The Earth's history. Limestone may have been deposited by microorganisms in the Precambrian , prior to 540 million years ago, but inorganic processes were probably more important and likely took place in an ocean more highly oversaturated in calcium carbonate than the modern ocean. Diagenesis is the process in which sediments are compacted and turned into solid rock . During diagenesis of carbonate sediments, significant chemical and textural changes take place. For example, aragonite

5022-434: The base of roads, as white pigment or filler in products such as toothpaste or paint, as a soil conditioner , and as a popular decorative addition to rock gardens . Limestone formations contain about 30% of the world's petroleum reservoirs . Limestone is composed mostly of the minerals calcite and aragonite , which are different crystal forms of calcium carbonate ( CaCO 3 ). Dolomite , CaMg(CO 3 ) 2 ,

5115-657: The beds. This may include the formation of vugs , which are crystal-lined cavities within the limestone. Diagenesis may include conversion of limestone to dolomite by magnesium-rich fluids. There is considerable evidence of replacement of limestone by dolomite, including sharp replacement boundaries that cut across bedding. The process of dolomitization remains an area of active research, but possible mechanisms include exposure to concentrated brines in hot environments ( evaporative reflux ) or exposure to diluted seawater in delta or estuary environments ( Dorag dolomitization ). However, Dorag dolomitization has fallen into disfavor as

5208-427: The coast of the eastern Adriatic to Kosovo and North Macedonia , where the massif of the Šar Mountains begins. The karst zone is at the northwesternmost section, described in early topographical research as a plateau between Italy and Slovenia . Languages preserving this form include Italian : Carso , German : Karst , and Albanian : karsti . In the local South Slavic languages , all variations of

5301-450: The complexity of karst aquifers, and need to be supplemented with dye traces , measurement of spring discharges, and analysis of water chemistry. U.S. Geological Survey dye tracing has determined that conventional groundwater models that assume a uniform distribution of porosity are not applicable for karst aquifers. Linear alignment of surface features such as straight stream segments and sinkholes develop along fracture traces . Locating

5394-525: The construction details, but requires a flexible contractual system to support such changes. NATM was originally developed for use in the Alps, where tunnels are commonly excavated at depth and in high in situ stress conditions. The principles of NATM are fundamental to modern-day tunnelling, and NATM fundamentally involves specifically addressing the specific soil conditions being encountered. Most city tunnels are built at shallow depth and do not need to control

5487-466: The depositional fabric of carbonate rocks. Dunham divides the rocks into four main groups based on relative proportions of coarser clastic particles, based on criteria such as whether the grains were originally in mutual contact, and therefore self-supporting, or whether the rock is characterized by the presence of frame builders and algal mats. Unlike the Folk scheme, Dunham deals with the original porosity of

5580-469: The deposits are highly porous, so that they have a spongelike texture, they are typically described as tufa . Secondary calcite deposited by supersaturated meteoric waters ( groundwater ) in caves is also sometimes described as travertine. This produces speleothems , such as stalagmites and stalactites . Coquina is a poorly consolidated limestone composed of abraded pieces of coral , shells , or other fossil debris. When better consolidated, it

5673-443: The dissolved bedrock is covered (perhaps by debris) or confined by one or more superimposed non-soluble rock strata, distinctive karst features may occur only at subsurface levels and can be totally missing above ground. The study of paleokarst (buried karst in the stratigraphic column ) is important in petroleum geology because as much as 50% of the world's hydrocarbon reserves are hosted in carbonate rock , and much of this

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5766-406: The earth's oceans are oversaturated with CaCO 3 by a factor of more than six. The failure of CaCO 3 to rapidly precipitate out of these waters is likely due to interference by dissolved magnesium ions with nucleation of calcite crystals, the necessary first step in precipitation. Precipitation of aragonite may be suppressed by the presence of naturally occurring organic phosphates in

5859-399: The first refers to the grains and the second to the cement. For example, a limestone consisting mainly of ooids, with a crystalline matrix, would be termed an oosparite. It is helpful to have a petrographic microscope when using the Folk scheme, because it is easier to determine the components present in each sample. Robert J. Dunham published his system for limestone in 1962. It focuses on

5952-557: The fissures. The enlarged fissures allow a larger quantity of water to enter which leads to a progressive enlargement of openings. Abundant small openings store a large quantity of water. The larger openings form a conduit system that drains the aquifer to springs. Characterization of karst aquifers requires field exploration to locate sinkholes, swallets , sinking streams , and springs in addition to studying geologic maps . Conventional hydrogeologic methods such as aquifer tests and potentiometric mapping are insufficient to characterize

6045-572: The formation of distinctive minerals from the silica and clay present in the original limestone. Two major classification schemes, the Folk and Dunham, are used for identifying the types of carbonate rocks collectively known as limestone. Robert L. Folk developed a classification system that places primary emphasis on the detailed composition of grains and interstitial material in carbonate rocks . Based on composition, there are three main components: allochems (grains), matrix (mostly micrite), and cement (sparite). The Folk system uses two-part names;

6138-429: The geologic record are called bioherms . Many are rich in fossils, but most lack any connected organic framework like that seen in modern reefs. The fossil remains are present as separate fragments embedded in ample mud matrix. Much of the sedimentation shows indications of occurring in the intertidal or supratidal zones, suggesting sediments rapidly fill available accommodation space in the shelf or platform. Deposition

6231-466: The global demand for drinkable water. Farming in karst areas must take into account the lack of surface water. The soils may be fertile enough, and rainfall may be adequate, but rainwater quickly moves through the crevices into the ground, sometimes leaving the surface soil parched between rains. The karst topography also poses peculiar difficulties for human inhabitants. Sinkholes can develop gradually as surface openings enlarge, but progressive erosion

6324-456: The karst groundwater flow rates from 0.5 to 7 miles per day (0.8 to 11.3 km/d). The rapid groundwater flow rates make karst aquifers much more sensitive to groundwater contamination than porous aquifers. Groundwater in karst areas is also just as easily polluted as surface streams, because Karst formations are cavernous and highly permeable, resulting in reduced opportunity for contaminant filtration. Well water may also be unsafe as

6417-434: The last 540 million years. Limestone often contains fossils which provide scientists with information on ancient environments and on the evolution of life. About 20% to 25% of sedimentary rock is carbonate rock, and most of this is limestone. The remaining carbonate rock is mostly dolomite , a closely related rock, which contains a high percentage of the mineral dolomite , CaMg(CO 3 ) 2 . Magnesian limestone

6510-562: The last-named locality having been declared a site of special scientific interest in respect of it. Kegelkarst is a type of tropical karst terrain with numerous cone-like hills, formed by cockpits, mogotes , and poljes and without strong fluvial erosion processes. This terrain is found in Cuba, Jamaica, Indonesia, Malaysia, the Philippines, Puerto Rico, southern China, Myanmar, Thailand, Laos and Vietnam. Salt karst (or 'halite karst')

6603-430: The material lime . It is composed mostly of the minerals calcite and aragonite , which are different crystal forms of CaCO 3 . Limestone forms when these minerals precipitate out of water containing dissolved calcium. This can take place through both biological and nonbiological processes, though biological processes, such as the accumulation of corals and shells in the sea, have likely been more important for

6696-647: The most dramatic of these formations can be seen in Thailand 's Phangnga Bay and at Halong Bay in Vietnam . Calcium carbonate dissolved into water may precipitate out where the water discharges some of its dissolved carbon dioxide. Rivers which emerge from springs may produce tufa terraces, consisting of layers of calcite deposited over extended periods of time. In caves, a variety of features collectively called speleothems are formed by deposition of calcium carbonate and other dissolved minerals. Interstratal karst

6789-410: The ocean basins, but limestone is rarely preserved in continental slope and deep sea environments. The best environments for deposition are warm waters, which have both a high organic productivity and increased saturation of calcium carbonate due to lower concentrations of dissolved carbon dioxide. Modern limestone deposits are almost always in areas with very little silica-rich sedimentation, reflected in

6882-709: The period. Sedimentation resumed and further limestone strata were deposited on an irregular karst surface, the cycle recurring several times in connection with fluctuating sea levels over prolonged periods. Pseudokarsts are similar in form or appearance to karst features but are created by different mechanisms. Examples include lava caves and granite tors —for example, Labertouche Cave in Victoria, Australia —and paleocollapse features. Mud Caves are an example of pseudokarst. Karst formations have unique hydrology, resulting in many unusual features. A karst fenster (karst window) occurs when an underground stream emerges onto

6975-426: The reaction: Fossils are often preserved in exquisite detail as chert. Cementing takes place rapidly in carbonate sediments, typically within less than a million years of deposition. Some cementing occurs while the sediments are still under water, forming hardgrounds . Cementing accelerates after the retreat of the sea from the depositional environment, as rainwater infiltrates the sediment beds, often within just

7068-482: The relative purity of most limestones. Reef organisms are destroyed by muddy, brackish river water, and carbonate grains are ground down by much harder silicate grains. Unlike clastic sedimentary rock, limestone is produced almost entirely from sediments originating at or near the place of deposition. Limestone formations tend to show abrupt changes in thickness. Large moundlike features in a limestone formation are interpreted as ancient reefs , which when they appear in

7161-451: The release of in situ stress, as was the case with the original NATM in the Alps. Projects in cities place a higher priority on minimizing settlement and so tend to use different support methods from the original NATM. That has led to a confusion in terminology in that tunnelling engineers use "NATM" to mean different things. New terms have arisen, and alternative names for certain aspects of NATM have been adopted as its use has spread. That

7254-780: The river flows into a cave in a formation known as the Madison Limestone and then rises again 800 m ( 1 ⁄ 2  mi) down the canyon in a placid pool. A turlough is a unique type of seasonal lake found in Irish karst areas which are formed through the annual welling-up of water from the underground water system. Main Article Aquifer#Karst Karst aquifers typically develop in limestone . Surface water containing natural carbonic acid moves down into small fissures in limestone. This carbonic acid gradually dissolves limestone thereby enlarging

7347-460: The rock. The Dunham scheme is more useful for hand samples because it is based on texture, not the grains in the sample. A revised classification was proposed by Wright (1992). It adds some diagenetic patterns to the classification scheme. Travertine is a term applied to calcium carbonate deposits formed in freshwater environments, particularly waterfalls , cascades and hot springs . Such deposits are typically massive, dense, and banded. When

7440-479: The sediments increases. Chemical compaction takes place by pressure solution of the sediments. This process dissolves minerals from points of contact between grains and redeposits it in pore space, reducing the porosity of the limestone from an initial high value of 40% to 80% to less than 10%. Pressure solution produces distinctive stylolites , irregular surfaces within the limestone at which silica-rich sediments accumulate. These may reflect dissolution and loss of

7533-662: The sediments of the ocean floor abruptly transition from carbonate ooze rich in foraminifera and coccolith remains ( Globigerina ooze) to silicic mud lacking carbonates. In rare cases, turbidites or other silica-rich sediments bury and preserve benthic (deep ocean) carbonate deposits. Ancient benthic limestones are microcrystalline and are identified by their tectonic setting. Fossils typically are foraminifera and coccoliths. No pre-Jurassic benthic limestones are known, probably because carbonate-shelled plankton had not yet evolved. Limestones also form in freshwater environments. These limestones are not unlike marine limestone, but have

7626-649: The surface between layers of rock, cascades some distance, and then disappears back down, often into a sinkhole. Rivers in karst areas may disappear underground a number of times and spring up again in different places, even under a different name, like Ljubljanica , the "river of seven names". Another example of this is the Popo Agie River in Fremont County, Wyoming , where, at a site named "The Sinks" in Sinks Canyon State Park ,

7719-415: The surface, complex underground drainage systems (such as karst aquifers ) and extensive caves and cavern systems may form. Erosion along limestone shores, notably in the tropics , produces karst topography that includes a sharp makatea surface above the normal reach of the sea, and undercuts that are mostly the result of biological activity or bioerosion at or a little above mean sea level . Some of

7812-425: The surrounding rock or soil formations of a tunnel are integrated into an overall ring-like support structure. Thus the supporting formations will themselves be part of this supporting structure." Some engineers use NATM whenever they propose shotcrete for initial ground support of an open-face tunnel. The term NATM can be misleading in relation to soft-ground tunnels. As noted by Emit Brown, NATM can refer to both

7905-1002: The system ( pyrite or hydrogen sulfide ) to form sulfuric acid (H 2 SO 4 ). Sulfuric acid then reacts with calcium carbonate, causing increased erosion within the limestone formation. This chain of reactions is: This reaction chain forms gypsum . The karstification of a landscape may result in a variety of large- or small-scale features both on the surface and beneath. On exposed surfaces, small features may include solution flutes (or rillenkarren), runnels , limestone pavement (clints and grikes), kamenitzas collectively called karren or lapiez. Medium-sized surface features may include sinkholes or cenotes (closed basins), vertical shafts, foibe (inverted funnel shaped sinkholes), disappearing streams, and reappearing springs . Large-scale features may include limestone pavements , poljes , and karst valleys. Mature karst landscapes, where more bedrock has been removed than remains, may result in karst towers , or haystack/eggbox landscapes. Beneath

7998-533: The water by photosynthesis and thereby decreasing the solubility of calcium carbonate. Limestone shows the same range of sedimentary structures found in other sedimentary rocks. However, finer structures, such as lamination , are often destroyed by the burrowing activities of organisms ( bioturbation ). Fine lamination is characteristic of limestone formed in playa lakes , which lack the burrowing organisms. Limestones also show distinctive features such as geopetal structures , which form when curved shells settle to

8091-461: The water may have run unimpeded from a sinkhole in a cattle pasture, bypassing the normal filtering that occurs in a porous aquifer . Sinkholes have often been used as farmstead or community trash dumps . Overloaded or malfunctioning septic tanks in karst landscapes may dump raw sewage directly into underground channels. Geologists are concerned with these negative effects of human activity on karst hydrology which, as of 2007 , supplied about 25% of

8184-553: The water. Although ooids likely form through purely inorganic processes, the bulk of CaCO 3 precipitation in the oceans is the result of biological activity. Much of this takes place on carbonate platforms . The origin of carbonate mud, and the processes by which it is converted to micrite, continue to be a subject of research. Modern carbonate mud is composed mostly of aragonite needles around 5 μm (0.20 mils) in length. Needles of this shape and composition are produced by calcareous algae such as Penicillus , making this

8277-417: The water. Once the rain reaches the ground, it may pass through soil that provides additional CO 2 produced by soil respiration . Some of the dissolved carbon dioxide reacts with the water to form a weak carbonic acid solution, which dissolves calcium carbonate . The primary reaction sequence in limestone dissolution is the following: In very rare conditions, oxidation can play a role. Oxidation played

8370-515: The western Highland Rim in the eastern United States is at the second-highest risk of karst sinkholes. In Canada, Wood Buffalo National Park , Northwest Territories contains areas of karst sinkholes. Mexico hosts important karst regions in the Yucatán Peninsula and Chiapas . The West of Ireland is home to The Burren , a karst limestone area. The South China Karst in the provinces of Guizhou , Guangxi , and Yunnan provinces

8463-454: The word are derived from a Romanized Illyrian base (yielding Latin : carsus , Dalmatian : carsus ), later metathesized from the reconstructed form * korsъ into forms such as Slovene : kras and Serbo-Croatian : krš , kras , first attested in the 18th century, and the adjective form kraški in the 16th century. As a proper noun, the Slovene form Grast

8556-589: Was first attested in 1177. Ultimately, the word is of Mediterranean origin. It has also been suggested that the word may derive from the Proto-Indo-European root karra- 'rock'. The name may also be connected to the oronym Kar(u)sádios oros cited by Ptolemy , and perhaps also to Latin Carusardius . Johann Weikhard von Valvasor , a pioneer of the study of karst in Slovenia and

8649-416: Was likely deposited in pore space between grains, suggesting a high-energy depositional environment that removed carbonate mud. Recrystallized sparite is not diagnostic of depositional environment. Limestone outcrops are recognized in the field by their softness (calcite and aragonite both have a Mohs hardness of less than 4, well below common silicate minerals) and because limestone bubbles vigorously when

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