Limestone is a carbonate sedimentary rock, composed of the skeletal fragments of marine organisms such as coral and molluscs. Its major materials are the minerals calcite and aragonite, which are different crystal forms of calcium carbonate. A related rock is dolostone, which contains a high percentage of the mineral dolomite, CaMg2. In fact, in old USGS publications, dolostone was referred to as magnesian limestone, a term now reserved for magnesium-deficient dolostones or magnesium-rich limestones. About 10% of sedimentary rocks are limestones; the solubility of limestone in water and weak acid solutions leads to karst landscapes, in which water erodes the limestone over thousands to millions of years. Most cave systems are through limestone bedrock. Limestone has numerous uses: as a building material, an essential component of concrete, as aggregate for the base of roads, as white pigment or filler in products such as toothpaste or paints, as a chemical feedstock for the production of lime, as a soil conditioner, or as a popular decorative addition to rock gardens.
Like most other sedimentary rocks, most limestone is composed of grains. Most grains in limestone are skeletal fragments of marine organisms such as foraminifera; these organisms secrete shells made of aragonite or calcite, leave these shells behind when they die. Other carbonate grains composing limestones are ooids, peloids and extraclasts. Limestone contains variable amounts of silica in the form of chert or siliceous skeletal fragment, varying amounts of clay and sand carried in by rivers; some limestones do not consist of grains, are formed by the chemical precipitation of calcite or aragonite, i.e. travertine. Secondary calcite may be deposited by supersaturated meteoric waters; this produces speleothems, such as stalactites. Another form taken by calcite is oolitic limestone, which can be recognized by its granular appearance; the primary source of the calcite in limestone is most marine organisms. Some of these organisms can construct mounds of rock building upon past generations. Below about 3,000 meters, water pressure and temperature conditions cause the dissolution of calcite to increase nonlinearly, so limestone does not form in deeper waters.
Limestones may form in lacustrine and evaporite depositional environments. Calcite can be dissolved or precipitated by groundwater, depending on several factors, including the water temperature, pH, dissolved ion concentrations. Calcite exhibits an unusual characteristic called retrograde solubility, in which it becomes less soluble in water as the temperature increases. Impurities will cause limestones to exhibit different colors with weathered surfaces. Limestone may be crystalline, granular, or massive, depending on the method of formation. Crystals of calcite, dolomite or barite may line small cavities in the rock; when conditions are right for precipitation, calcite forms mineral coatings that cement the existing rock grains together, or it can fill fractures. Travertine is a banded, compact variety of limestone formed along streams where there are waterfalls and around hot or cold springs. Calcium carbonate is deposited where evaporation of the water leaves a solution supersaturated with the chemical constituents of calcite.
Tufa, a porous or cellular variety of travertine, is found near waterfalls. Coquina is a poorly consolidated limestone composed of pieces of coral or shells. During regional metamorphism that occurs during the mountain building process, limestone recrystallizes into marble. Limestone is a parent material of Mollisol soil group. Two major classification schemes, the Folk and the 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 and cement; the Folk system uses two-part names. 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; the Dunham scheme focuses on depositional textures. Each name is based upon the texture of the grains. Robert J. Dunham published his system for limestone in 1962.
Dunham divides the rocks into four main groups based on relative proportions of coarser clastic particles. Dunham names are for rock families, his efforts deal with the question of whether or not the grains were in mutual contact, 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 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, it adds some diagenetic patterns and can be summarized as follows: See: Carbonate platform About 10% of all sedimentary rocks are limestones. Limestone is soluble in acid, therefore forms many erosional landforms; these include limestone pavements, pot holes, cenotes and gorges. Such erosion landscapes are known
Brachiopods, phylum Brachiopoda, are a group of lophotrochozoan animals that have hard "valves" on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection. Two major groups are recognized and inarticulate; the word "articulate" is used to describe the tooth-and-groove features of the valve-hinge, present in the articulate group, absent from the inarticulate group. This is the leading diagnostic feature, by which the two main groups can be distinguished. Articulate brachiopods have toothed hinges and simple opening and closing muscles, while inarticulate brachiopods have untoothed hinges and a more complex system of muscles used to keep the two valves aligned. In a typical brachiopod a stalk-like pedicle projects from an opening in one of the valves near the hinges, known as the pedicle valve, keeping the animal anchored to the seabed but clear of silt that would obstruct the opening.
The word "brachiopod" is formed from podos. They are known as "lamp shells", since the curved shells of the class Terebratulida look rather like pottery oil-lamps. Lifespans range from three to over thirty years. Ripe gametes float from the gonads into the main coelom and exit into the mantle cavity; the larvae of inarticulate brachiopods are miniature adults, with lophophores that enable the larvae to feed and swim for months until the animals become heavy enough to settle to the seabed. The planktonic larvae of articulate species do not resemble the adults, but rather look like blobs with yolk sacs, remain among the plankton for only a few days before leaving the water column upon metamorphosing. In addition to the traditional classification of brachiopods into inarticulate and articulate, two approaches appeared in the 1990s: one approach groups the inarticulate Craniida with articulate brachiopods, since both use the same material in the mineral layers of their shell. However, some taxonomists believe it is premature to suggest higher levels of classification such as order and recommend a bottom-up approach that identifies genera and groups these into intermediate groups.
Traditionally, brachiopods have been regarded as members of, or as a sister group to, the deuterostomes, a superphylum that includes chordates and echinoderms. One type of analysis of the evolutionary relationships of brachiopods has always placed brachiopods as protostomes while another type has split between placing brachiopods among the protostomes or the deuterostomes, it was suggested in 2003 that brachiopods had evolved from an ancestor similar to Halkieria, a slug-like Cambrian animal with "chain mail" on its back and a shell at the front and rear end. However, new fossils found in 2007 and 2008 showed that the "chain mail" of tommotiids formed the tube of a sessile animal. Lineages of brachiopods that have both fossil and extant taxa appeared in the early Cambrian and Carboniferous periods, respectively. Other lineages have arisen and become extinct, sometimes during severe mass extinctions. At their peak in the Paleozoic era, the brachiopods were among the most abundant filter-feeders and reef-builders, occupied other ecological niches, including swimming in the jet-propulsion style of scallops.
Brachiopod fossils have been useful indicators of climate changes during the Paleozoic. However, after the Permian–Triassic extinction event, brachiopods recovered only a third of their former diversity. A study in 2007 concluded the brachiopods were vulnerable to the Permian–Triassic extinction, as they built calcareous hard parts and had low metabolic rates and weak respiratory systems, it was thought that brachiopods went into decline after the Permian–Triassic extinction, were out-competed by bivalves, but a study in 1980 found both brachiopod and bivalve species increased from the Paleozoic to modern times, with bivalves increasing faster. Brachiopods live only in the sea, most species avoid locations with strong currents or waves; the larvae of articulate species settle in and form dense populations in well-defined areas while the larvae of inarticulate species swim for up to a month and have wide ranges. Brachiopods now live in cold water and low light. Fish and crustaceans seem to find brachiopod flesh distasteful and attack them.
Among brachiopods, only the lingulids have been fished commercially, on a small scale. One brachiopod species may be a measure of environmental conditions around an oil terminal being built in Russia on the shore of the Sea of Japan. Modern brachiopods range from 1 to 100 millimetres long, most species are about 10 to 30 millimetres; the largest brachiopods known – Gigantoproductus and Titanaria, reaching 30 to 38 centimetres in width – occurred in the upper part of the Lower Carboniferous. Each has two valves which cover the dorsal and ventral surface of the animal, unlike bivalve molluscs whose shells cover the lateral surfaces; the valves are t
Pellets are small spherical to ovoid or rod-shaped grains that are common component of many limestones. They are 0.03 to 0.3 mm long and composed of carbonate mud. Their most common size is 0.04 to 0.08 mm. Pellets lack any internal structure and are remarkably uniform in size and shape in any single limestone sample, they consist either of aggregated carbonate mud, precipitated calcium carbonate, or a mixture of both. They either were composed either of aragonite, calcite, or a mixture of both. Pellets composed of either glauconite or phosphorite are common in marine sedimentary rocks. Pellets occur in Precambrian through Phanerozoic strata, they are an important component in Phanerozoic strata. The consensus among sedimentologists and petrographers is that pellets are the fecal products of invertebrate organisms because of their constant size and extra-high content of organic matter. Pellets differ from oolites and intraclasts, which are found in limestones, they differ from oolites in that pellets lack the radial or concentric structures that characterize oolites.
They differ from intraclasts in that pellets lack the complex internal structure, typical of intraclasts. In addition, quite unlike intraclasts, are characterized by a remarkable uniformity of shape good sorting, small size. By definition, pellets differ from peloids, in that pellets have a specific size and implied origin—while peloids vary in size and origin. Pellets, in the strict sense, are fecal products of invertebrate organisms. Peloids are allochems of structure, or origin; as a result, peloids not only include possible pellets, but include a variety of other distinctly non-pellet grains—such as indistinct intraclasts, micritized ooids, or fossil fragments. In addition, some peloids are microbial or inorganic precipitates. Carbonate geologists consider the vast majority of peloids as secondary allochems created by biological degradation or “micritization” of other primary carbonate grains, i.e. ooids, bioclasts, or pellets. Calcilutite Calcarenite Calcisiltite
The Carboniferous is a geologic period and system that spans 60 million years from the end of the Devonian Period 358.9 million years ago, to the beginning of the Permian Period, 298.9 Mya. The name Carboniferous means "coal-bearing" and derives from the Latin words carbō and ferō, was coined by geologists William Conybeare and William Phillips in 1822. Based on a study of the British rock succession, it was the first of the modern'system' names to be employed, reflects the fact that many coal beds were formed globally during that time; the Carboniferous is treated in North America as two geological periods, the earlier Mississippian and the Pennsylvanian. Terrestrial animal life was well established by the Carboniferous period. Amphibians were the dominant land vertebrates, of which one branch would evolve into amniotes, the first terrestrial vertebrates. Arthropods were very common, many were much larger than those of today. Vast swaths of forest covered the land, which would be laid down and become the coal beds characteristic of the Carboniferous stratigraphy evident today.
The atmospheric content of oxygen reached its highest levels in geological history during the period, 35% compared with 21% today, allowing terrestrial invertebrates to evolve to great size. The half of the period experienced glaciations, low sea level, mountain building as the continents collided to form Pangaea. A minor marine and terrestrial extinction event, the Carboniferous rainforest collapse, occurred at the end of the period, caused by climate change. In the United States the Carboniferous is broken into Mississippian and Pennsylvanian subperiods; the Mississippian is about twice as long as the Pennsylvanian, but due to the large thickness of coal-bearing deposits with Pennsylvanian ages in Europe and North America, the two subperiods were long thought to have been more or less equal in duration. In Europe the Lower Carboniferous sub-system is known as the Dinantian, comprising the Tournaisian and Visean Series, dated at 362.5-332.9 Ma, the Upper Carboniferous sub-system is known as the Silesian, comprising the Namurian and Stephanian Series, dated at 332.9-298.9 Ma.
The Silesian is contemporaneous with the late Mississippian Serpukhovian plus the Pennsylvanian. In Britain the Dinantian is traditionally known as the Carboniferous Limestone, the Namurian as the Millstone Grit, the Westphalian as the Coal Measures and Pennant Sandstone; the International Commission on Stratigraphy faunal stages from youngest to oldest, together with some of their regional subdivisions, are: A global drop in sea level at the end of the Devonian reversed early in the Carboniferous. There was a drop in south polar temperatures; these conditions had little effect in the deep tropics, where lush swamps to become coal, flourished to within 30 degrees of the northernmost glaciers. Mid-Carboniferous, a drop in sea level precipitated a major marine extinction, one that hit crinoids and ammonites hard; this sea level drop and the associated unconformity in North America separate the Mississippian subperiod from the Pennsylvanian subperiod. This happened about 323 million years ago, at the onset of the Permo-Carboniferous Glaciation.
The Carboniferous was a time of active mountain-building as the supercontinent Pangaea came together. The southern continents remained tied together in the supercontinent Gondwana, which collided with North America–Europe along the present line of eastern North America; this continental collision resulted in the Hercynian orogeny in Europe, the Alleghenian orogeny in North America. In the same time frame, much of present eastern Eurasian plate welded itself to Europe along the line of the Ural Mountains. Most of the Mesozoic supercontinent of Pangea was now assembled, although North China, South China continents were still separated from Laurasia; the Late Carboniferous Pangaea was shaped like an "O." There were two major oceans in the Carboniferous—Panthalassa and Paleo-Tethys, inside the "O" in the Carboniferous Pangaea. Other minor oceans were shrinking and closed - Rheic Ocean, the small, shallow Ural Ocean and Proto-Tethys Ocean. Average global temperatures in the Early Carboniferous Period were high: 20 °C.
However, cooling during the Middle Carboniferous reduced average global temperatures to about 12 °C. Lack of growth rings of fossilized trees suggest a lack of seasons of a tropical climate. Glaciations in Gondwana, triggered by Gondwana's southward movement, continued into the Permian and because of the lack of clear markers and breaks, the deposits of this glacial period are referred to as Permo-Carboniferous in age; the cooling and drying of the climate led to the Carboniferous Rainforest Collapse during the late Carboniferous. Tropical rainforests fragmented and were devastated by climate change. Carboniferous rocks in Europe and eastern North America consist of a repeated sequence of limestone, sandstone and coal beds. In North America, the early Carboniferous is marine
Micrite is a limestone constituent formed of calcareous particles ranging in diameter up to four μm formed by the recrystallization of lime mud. Micrite is carbonate of mud grade. In the Folk classification micrite is a carbonate rock dominated by fine-grained calcite. Carbonate rocks that contain fine-grained calcite in addition to allochems are named intramicrite, biomicrite or pelmicrite under the Folk classification depending on the dominant allochem. Micrite as a component of carbonate rocks can occur as a matrix, as micrite envelopes around allochems or as peloids. Micrite can be generated by chemical precipitation, from disaggregation of peloids, or by micritization; the term was coined in 1959 by Robert Folk for his carbonate rock classification system. Micrite is derived from MICRocrystalline calcITE. Folk, R. L. 1959, Practical petrographic classification of limestones: American Association of Petroleum Geologists Bulletin, v. 43, p. 1-38. Https://www2.imperial.ac.uk/earthscienceandengineering/rocklibrary/viewglossrecord.php?
Allochem is a term introduced by Folk to describe the recognisable "grains" in carbonate rocks. Any fragment from around 0.5 mm upwards in size may be considered an allochem. Examples would include ooids, oncolites, fossil or pre-existing carbonate fragments. Fragments are still termed allochems if they have undergone chemical transformations – for example if an aragonite shell were to dissolve and be replaced by calcite, the replacement would still be deemed an allochem; the allochems are embedded in a matrix of micrite or sparry calcite
Intraclasts are irregularly shaped grains that form by syndepositional erosion of lithified sediment. Gravel grade material is composed of whole disarticulated or broken skeletal fragments together with sand grade material of whole and broken skeletal debris; such sediments can contain fragments of early cemented limestones of local origin which are known as intraclasts. The sediments that contain fragments of early cemented limestones of extra-basinal origin are called extraclasts. Examples of intraclasts include mudlumps that are torn up from the bottoms of lagoons during storms, hardened desiccated mudflakes produced in intertidal and supratidal environments and fragments broken from cemented deepsea crusts. Other intraclasts are aggregates of carbonate particles; these include botryoidal grains. Grapestones are composite grains with an irregular shape that resembles a bunch of grapes, whereas botryoidal grains are similar to oolitic coats enveloping the aggregate grains; these types of intraclasts from in shoal water environments with intermediate wave and current activity, where grains that are cemented on the sea floor are broken into aggregate fragments and lumps during storms