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54-529: This group contains the following formations in descending order: The Ludlow group is essentially shaly in character, except towards the top, where the beds become more sandy and pass gradually into the Old Red Sandstone. The Aymestry limestone , which is irregular in thickness, is sometimes absent, and where the underlying Wenlock limestones are absent the shales of the Ludlow group graduate, downwards into

108-514: A boat trip from Dunglass Burn east along the coast with the geologist Sir James Hall of Dunglass and at Siccar Point found what Hutton called "a beautiful picture of this junction washed bare by the sea", where 345-million-year-old Old Red Sandstone overlies 425-million-year-old Silurian greywacke . In the early 19th century, the paleontology of the formation was studied intensively by Hugh Miller , Henry Thomas De la Beche , Roderick Murchison , and Adam Sedgwick —Sedgwick's interpretation

162-560: A combination of dunes , and sediments that may have been laid down in lakes , river , estuaries, and possibly other coastal environments. The Old Red Sandstone was long thought to have been deposited mostly in freshwater, but more recent studies have discovered marine fossils (such as brachiopods ) in some locations, its vertebrate fauna also occurs in typically marine environments, and an isotopic study also found significant marine influence in mineralised tissues of its vertebrates. Thus, at least some strata appear to have been deposited on

216-414: A hard, fissile, metamorphic rock known as slate . With continued increase in metamorphic grade the sequence is phyllite , then schist and finally gneiss . Shale is the most common source rock for hydrocarbons ( natural gas and petroleum ). The lack of coarse sediments in most shale beds reflects the absence of strong currents in the waters of the depositional basin. These might have oxygenated

270-507: A narrow zone from Dulas Bay on Anglesey's northeast coast, southwards to the town of Llangefni . In 1787 James Hutton noted what is now known as Hutton's Unconformity at Inchbonny, Jedburgh , and in early 1788 he set off with John Playfair to the Berwickshire coast and found more examples of this sequence in the valleys of the Tower and Pease Burns near Cockburnspath . They then took

324-582: A number of distinct sedimentary basins throughout Britain has been established. The Orcadian Basin extends over a wide area of North East Scotland and the neighbouring seas. It encompasses the Moray Firth and adjoining land areas, Caithness, Orkney and parts of Shetland . South of the Moray Firth, two distinct sub-basins are recognized at Turriff and at Rhynie . The sequence is more than 4 kilometres (13,000 ft) thick in parts of Shetland. The main basin

378-583: A very large area. The Tilestones, Downton Castle Sandstone , and Ledbury Formation shales are occasionally grouped together under the term Downtonian. They are in reality passage beds between the Silurian and Old Red Sandstone , and were originally placed in the latter system by Sir Roderick Murchison . They are mostly grey, yellow or red micaceous, shaly sandstones. The common Upper Ludlow fossils include: plants (Actinophyllum, Chondrites), ostracods , phyllocarids , eurypterids ; trilobites (less common than in

432-969: Is a continuous outcrop along the Highland Boundary Fault from Stonehaven on the North Sea coast to Helensburgh and beyond to Arran . A more disconnected series of outcrops occur along the line of the Southern Uplands Fault from Edinburgh to Girvan . Old Red Sandstone often occurs in conjunction with conglomerate formations, one such noteworthy cliffside exposure being the Fowlsheugh Nature Reserve , Kincardineshire . A series of outcrops occur from East Lothian southwards through Berwickshire . Hutton's famous unconformity at Siccar Point occurs within this basin - see History of study below. This relatively large basin extends across much of South Wales from southern Pembrokeshire in

486-446: Is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g., kaolin , Al 2 Si 2 O 5 ( OH ) 4 ) and tiny fragments ( silt -sized particles) of other minerals, especially quartz and calcite . Shale is characterized by its tendency to split into thin layers ( laminae ) less than one centimeter in thickness. This property is called fissility . Shale is the most common sedimentary rock. The term shale

540-1201: Is accompanied by telogenesis , the third and final stage of diagenesis. As erosion reduces the depth of burial, renewed exposure to meteoric water produces additional changes to the shale, such as dissolution of some of the cement to produce secondary porosity . Pyrite may be oxidized to produce gypsum . Black shales are dark, as a result of being especially rich in unoxidized carbon . Common in some Paleozoic and Mesozoic strata , black shales were deposited in anoxic , reducing environments, such as in stagnant water columns. Some black shales contain abundant heavy metals such as molybdenum , uranium , vanadium , and zinc . The enriched values are of controversial origin, having been alternatively attributed to input from hydrothermal fluids during or after sedimentation or to slow accumulation from sea water over long periods of sedimentation. Fossils , animal tracks or burrows and even raindrop impressions are sometimes preserved on shale bedding surfaces. Shales may also contain concretions consisting of pyrite, apatite , or various carbonate minerals. Shales that are subject to heat and pressure of metamorphism alter into

594-638: Is composed of about 58% clay minerals, 28% quartz, 6% feldspar , 5% carbonate minerals, and 2% iron oxides . Most of the quartz is detrital (part of the original sediments that formed the shale) rather than authigenic (crystallized within the shale after deposition). Shales and other mudrocks contain roughly 95 percent of the organic matter in all sedimentary rocks. However, this amounts to less than one percent by mass in an average shale. Black shales, which form in anoxic conditions, contain reduced free carbon along with ferrous iron (Fe ) and sulfur (S ). Amorphous iron sulfide , along with carbon, produce

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648-640: Is considered to be an intramontane basin resulting from crustal rifting associated with post-Caledonian extension , possibly accompanied by strike-slip faulting along the Great Glen Fault system. There are a scatter of exposures of the Old Red Sandstone around Oban and the Isle of Kerrera on the West Highland coast, this unit is sometimes referred to as the Kerrera Sandstone Formation. The unit

702-430: Is evidence that shale acts as a semipermeable medium, allowing water to pass through while retaining dissolved salts. The fine particles that compose shale can remain suspended in water long after the larger particles of sand have been deposited. As a result, shales are typically deposited in very slow moving water and are often found in lakes and lagoonal deposits, in river deltas , on floodplains and offshore below

756-431: Is merely an erosional contact incorporating debris of the slate in a basal conglomerate. The ORS deposits around Oban are considered latest Silurian ( Pridoli ) to earliest Devonian in age. They are interpreted as alluvial fans which filled a depositional basin from the east and northeast. Small outliers occur near Taynuilt and either side of Loch Avich . The deposits are especially obvious on Kerrera where they form

810-455: Is more likely to form nonfissile mudstone than shale. On the other hand, black shales often have very pronounced fissility ( paper shales ) due to binding of hydrocarbon molecules to the faces of the clay particles, which weakens the binding between particles. Lithification follows closely on compaction, as increased temperatures at depth hasten deposition of cement that binds the grains together. Pressure solution contributes to cementing, as

864-452: Is reduced. In addition to this physical compaction, chemical compaction may take place via pressure solution . Points of contact between grains are under the greatest strain, and the strained mineral is more soluble than the rest of the grain. As a result, the contact points are dissolved away, allowing the grains to come into closer contact. It is during compaction that shale develops its fissility, likely through mechanical compaction of

918-698: Is represented by the Gupton and West Angle formations. The Freshwater East Formation, and corresponding Red Cliff Formation of north Pembrokeshire, are both late Silurian in age. A small and separate basin exists here where both alluvial and lacustrine deposits are recorded. Both the middle and upper ORS are missing but the lower ORS is represented, in ascending order, by the Bodafon, Traeth Bach, Porth y Mor and Traeth Lligwy formations. Calcretes are also recorded representing carbonate-rich soils developed between periods of sediment deposition. The present day outcrop occupies

972-839: Is sometimes applied more broadly, as essentially a synonym for mudrock , rather than in the narrower sense of clay-rich fissile mudrock. Shale typically exhibits varying degrees of fissility. Because of the parallel orientation of clay mineral flakes in shale, it breaks into thin layers, often splintery and usually parallel to the otherwise indistinguishable bedding planes . Non-fissile rocks of similar composition and particle size (less than 0.0625 mm) are described as mudstones (1/3 to 2/3 silt particles) or claystones (less than 1/3 silt). Rocks with similar particle sizes but with less clay (greater than 2/3 silt) and therefore grittier are siltstones . Shales are typically gray in color and are composed of clay minerals and quartz grains. The addition of variable amounts of minor constituents alters

1026-475: Is up to 128m thick in its type area and consists of green and red sandstones and conglomerates, typically containing large (10–30 cm or 4–12 in across) elliptical well rounded clasts, accompanied by siltstones, mudstones and limestones. On Kerrera a conglomerate of andesite boulders rests unconformably on Dalradian black, pyritic slates ( Easdale Slate) of the Easdale Subgroup . At Oban there

1080-708: The Plateau Beds Formation is unconformably overlain by the Grey Grits Formation though further east these divisions are replaced by the Quartz Conglomerate Group which is itself subdivided into a variety of different formations. The sequence in Pembrokeshire differs from that of the main part of the basin and falls into two parts. In North Pembrokeshire to the north of the Ritec Fault , both

1134-399: The U.S. Gulf Coast . As sediments continue to accumulate, the older, more deeply buried sediments begin to undergo diagenesis . This mostly consists of compaction and lithification of the clay and silt particles. Early stages of diagenesis, described as eogenesis , take place at shallow depths (a few tens of meters) and are characterized by bioturbation and mineralogical changes in

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1188-456: The science of stratigraphy 's early debates were about the Old Red Sandstone. In older geological works predating theories of plate tectonics , the United States' Catskill Delta formation is sometimes referred to as part of the Old Red Sandstone. In the modern day it is recognized that the two are not stratigraphically continuous but are very similar due to being formed at approximately

1242-651: The wave base . Thick deposits of shale are found near ancient continental margins and foreland basins . Some of the most widespread shale formations were deposited by epicontinental seas . Black shales are common in Cretaceous strata on the margins of the Atlantic Ocean , where they were deposited in fault -bounded silled basins associated with the opening of the Atlantic during the breakup of Pangaea . These basins were anoxic, in part because of restricted circulation in

1296-854: The Bannisdale Slates (5200 ft./1,600 m) and the Kirkby Moor Flags (2,000 ft./615 m). In the Silurian areas of southern Scotland , the Ludlow rocks are represented in the Kirkcudbright Shore and Riccarton districts by the Raeberry Castle Beds and Balmae Grits (500–750 ft.). In the northern belt within Lanarkshire and the Pentland Hills , the Lower Ludlow portion consists of mudstones, flaggy shales, and greywackes; and

1350-707: The Raglan Mudstone is marked by a well-developed calcrete , the Bishop's Frome Limestone . The lowermost Devonian formation is the St Maughans Formation , itself overlain by the Brownstones Formation though with an intervening Senni Formation over much of the area. The Upper Devonian sequence is rather thinner and comprises a series of formations which are more laterally restricted. In the Brecon Beacons ,

1404-690: The Temeside, Raglan Mudstone and St Maughans formations of the central and eastern part of the basin. In south Pembrokeshire to the south of the Ritec Fault, the lower ORS is represented by, in ascending order, the Freshwater East, Moors Cliff and Freshwater West formations. These are unconformably overlain by the Ridgeway Conglomerate Formation. The middle ORS is missing whilst the Upper ORS

1458-541: The Upper Devonian, the contact between the two being unconformable and representing the complete omission of any Middle Devonian sequence. The lowermost formations are of upper Silurian age, these being the Downton Castle Sandstone Formation and the overlying Raglan Mudstone Formation except in Pembrokeshire where a more complex series of formations is recognized. In the east of the basin, the top of

1512-740: The Wenlock shales. In Wales the group is typically developed between namesake Ludlow and Aymestrey , and it occurs also in the detached Silurian areas between Dudley and the mouth of the Severn . In the Lake District the Silurian Coldwell beds, forming the upper part of the Coniston Flags, are the equivalents of the Lower Ludlow. They are succeeded by the Coniston Grits (4,000 ft./1,230 m),

1566-663: The bedrock across half of the island. These are conformably overlain by peperite and the basaltic and andesitic Lorne plateau lavas . The ORS on Kerrera and isolated localities around Oban are known for their fossils, particularly fish. The Midland Valley graben defined by the Highland Boundary Fault in the north and the Southern Uplands Fault in the south harbours not only a considerable amount of Old Red Sandstone sedimentary rocks but also igneous rocks of this age associated with extensive volcanism . There

1620-421: The black coloration. Because amorphous iron sulfide gradually converts to pyrite , which is not an important pigment, young shales may be quite dark from their iron sulfide content, in spite of a modest carbon content (less than 1%), while a black color in an ancient shale indicates a high carbon content. Most shales are marine in origin, and the groundwater in shale formations is often highly saline . There

1674-506: The clumps of clay particles produced by flocculation vary in size from a few tens of microns to over 700 microns in diameter. The floccules start out water-rich, but much of the water is expelled from the floccules as the clay minerals bind more tightly together over time (a process called syneresis ). Clay pelletization by organisms that filter feed is important where flocculation is inhibited. Filter feeders produce an estimated 12 metric tons of clay pellets per square kilometer per year along

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1728-409: The coast, probably in marginal marine environments. The familiar red colour of these rocks arises from the presence of iron oxide , but not all the Old Red Sandstone is red or sandstone – the sequence also includes conglomerates, mudstones , siltstones and thin limestones and colours can range from grey and green through to red and purple. These deposits are closely associated with

1782-408: The color of the rock. Red, brown and green colors are indicative of ferric oxide ( hematite – reds), iron hydroxide ( goethite – browns and limonite – yellow), or micaceous minerals ( chlorite , biotite and illite – greens). The color shifts from reddish to greenish as iron in the oxidized ( ferric ) state is converted to iron in the reduced ( ferrous ) state. Black shale results from

1836-462: The context of underground coal mining , shale was frequently referred to as slate well into the 20th century. Black shale associated with coal seams is called black metal. [REDACTED] Media related to Shale at Wikimedia Commons Old Red Sandstone Old Red Sandstone , abbreviated ORS , is an assemblage of rocks in the North Atlantic region largely of Devonian age. It extends in

1890-470: The east across Great Britain, Ireland and Norway, and in the west along the eastern seaboard of North America . It also extends northwards into Greenland and Svalbard . These areas were a part of the paleocontinent of Euramerica (Laurussia). In Britain it is a lithostratigraphic unit (a sequence of rock strata ) to which stratigraphers accord supergroup status and which is of considerable importance to early paleontology . The presence of Old in

1944-661: The erosion of the Caledonian Mountain chain which was thrown up by the collision of the former continents of Avalonia , Baltica and Laurentia to form the Old Red Sandstone Continent - an event known as the Caledonian Orogeny . Many fossils are found within the rocks, including early fishes, arthropods and plants. As is typical with terrestrial red beds , the vast majority of the rock is not fossil-bearing; however there are isolated, localized beds within

1998-454: The middle and upper ORS are missing with only the lower ORS present; this is divided into an earlier Milford Haven Group comprising in ascending order, the Red Cliff, Sandy Haven and Gelliswick Bay formations and a later Cosheston Group with, again in ascending order, its constituent Llanstadwell, Burton Cliff, Mill Bay, Lawrenny Cliff and New Shipping formations. These respectively equate with

2052-592: The mineral dissolved from strained contact points is redeposited in the unstrained pore spaces. The clay minerals may be altered as well. For example, smectite is altered to illite at temperatures of about 55 to 200 °C (130 to 390 °F), releasing water in the process. Other alteration reactions include the alteration of smectite to chlorite and of kaolinite to illite at temperatures between 120 and 150 °C (250 and 300 °F). Because of these reactions, illite composes 80% of Precambrian shales, versus about 25% of young shales. Unroofing of buried shale

2106-552: The name is to distinguish the sequence from the younger New Red Sandstone which also occurs widely throughout Britain. The Old Red Sandstone describes a group of sedimentary rocks deposited in a variety of environments in the late Silurian , through the Devonian and into the earliest part of the Carboniferous . The body of rock , or facies , is dominated by terrigenous deposits and conglomerates at its base, and progresses to

2160-433: The narrow Atlantic, and in part because the very warm Cretaceous seas lacked the circulation of cold bottom water that oxygenates the deep oceans today. Most clay must be deposited as aggregates and floccules, since the settling rate of individual clay particles is extremely slow. Flocculation is very rapid once the clay encounters highly saline sea water. Whereas individual clay particles are less than 4 microns in size,

2214-401: The older Ludlow sub-groups); numerous brachiopods ( Lingula cornea , Lingula minima , Chonetes striatella ); crustaceans ( ostracods ); gastropods ( Phyllocarida , Platyschisma helicites ); bivalvia ; and cephalopods ( Orthoceras bullatum ); and fish (Cephalaspis, Cyathaspis, Auchenaspis). Shale Shale is a fine-grained, clastic sedimentary rock formed from mud that

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2268-415: The original open framework of clay particles. The particles become strongly oriented into parallel layers that give the shale its distinctive fabric. Fissility likely develops early in the compaction process, at relatively shallow depth, since fissility does not seem to vary with depth in thick formations. Kaolinite flakes have less tendency to align in parallel layers than other clays, so kaolinite-rich clay

2322-817: The presence of greater than one percent carbonaceous material and indicates a reducing environment. Pale blue to blue-green shales typically are rich in carbonate minerals . Clays are the major constituent of shales and other mudrocks. The clay minerals represented are largely kaolinite , montmorillonite and illite. Clay minerals of Late Tertiary mudstones are expandable smectites , whereas in older rocks (especially in mid-to early Paleozoic shales) illites predominate. The transformation of smectite to illite produces silica , sodium , calcium , magnesium , iron and water. These released elements form authigenic quartz , chert , calcite , dolomite , ankerite , hematite and albite , all trace to minor (except quartz) minerals found in shales and other mudrocks. A typical shale

2376-444: The richest source rocks may contain as much as 40% organic matter. The organic matter in shale is converted over time from the original proteins, polysaccharides , lipids , and other organic molecules to kerogen , which at the higher temperatures found at greater depths of burial is further converted to graphite and petroleum. Before the mid-19th century, the terms slate , shale and schist were not sharply distinguished. In

2430-490: The rock that do contain fossils. Rocks of this age were also laid down in South West England (hence the name 'Devonian'; from Devon ) though these are of true marine origin and are not included within the Old Red Sandstone. Since the Old Red Sandstone consists predominantly of rocks of terrestrial origin, it does not generally contain marine fossils which would otherwise prove useful in correlating one occurrence of

2484-519: The rock with another, both between and within individual sedimentary basins . Accordingly, local stage names were devised and these remain in use to some extent today though there is an increasing use of international stage names. Thus in the Anglo-Welsh Basin, there are frequent references to the Downtonian , Dittonian , Breconian and Farlovian stages in the literature. The existence of

2538-522: The same time by the same processes. The Old Red Sandstone has been widely used as a building stone across those regions where it outcrops. Notable examples of its use can be found in the area surrounding Stirling , Stonehaven , Perth and Tayside . The inhabitants of Caithness at the northeastern tip of Scotland also used the stone to a considerable extent. Old Red Sandstone has also frequently been used in buildings in Herefordshire, Monmouthshire and

2592-433: The sediments, with only slight compaction. Pyrite may be formed in anoxic mud at this stage of diagenesis. Deeper burial is accompanied by mesogenesis , during which most of the compaction and lithification takes place. As the sediments come under increasing pressure from overlying sediments, sediment grains move into more compact arrangements, ductile grains (such as clay mineral grains) are deformed, and pore space

2646-889: The upper Downton Castle Sandstone part is made up principally of thick red and yellow sandstones and conglomerates with green mudstones. The Ludlow Group formations of Ireland include the Salrock beds of County Galway , and the Croagmarhin beds of Dingle Peninsula of County Kerry . The Lower Ludlow rocks are mainly grey, greenish and brown mudstones and sandy and calcareous shales. They contain an abundance of fossils . The Lower Ludlow series has been zoned by means of Monograptus species of Graptolites by E. M. R. Wood. The zonal forms in order from older/lower to younger/upper are: Monograptus vulgaris ; Monograptus nilssoni ; Monograptus scanicus ; Monograptus tumescens ; and Monograptus leintwardinensis . In Denbighshire and Merionethshire

2700-1411: The upper portion of the Denbighshire Grits belongs to this horizon. Those from lower to upper are: the Nantglyn Flags; the Upper Grit beds; the Monograptus leintwardinensis beds; and the Dinas Bran beds. Cyathaspis ludensis , the earliest British vertebrate fossil, was found in these rocks at Leintwardine in Herefordshire , a noted fossil locality. Trilobites are numerous ( Phacops caudatus , Lichas anglicus , Homolonotus delphinocephalus , Calymene Blumenbachii ); brachiopods ( Leptaena rhomboidalis , Rhynchonella Wilsoni , Atrypa reticularis ), pelecypods ( Cardiola interrupts , Ctenodonta sulcata ) and gastropods and cephalopods (many species of Orthoceras and also Gomphoceras, Trochoceras) are well represented. Other fossils are Ceratiocaris , Pterygotus , Protaster , Palaeocoma and Palaeodiscus . The Upper Ludlow sub-group rocks are mainly soft mudstones and shales with some harder sandy beds capable of being worked as building-stones. These sandy beds are often found covered with ripple-marks and annelid tracks. One of

2754-399: The uppermost sandy layers is known as the " Fucoid bed " from the abundance of the seaweed-like impressions it bears. At the top of this sub-group a brown layer occurs, from a quarter of an inch to 4 in. (63 mm to 100 mm) in thickness, full of the fragmentary remains of fish associated with those of Pterygotus and mollusca . This layer, known as the "Ludlow Bone bed," has been traced over

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2808-414: The waters and destroyed organic matter before it could accumulate. The absence of carbonate rock in shale beds reflects the absence of organisms that might have secreted carbonate skeletons, also likely due to an anoxic environment. As a result, about 95% of organic matter in sedimentary rocks is found in shales and other mudrocks. Individual shale beds typically have an organic matter content of about 1%, but

2862-509: The west through Carmarthenshire into Powys and Monmouthshire and through the southern Welsh Marches , notably into Herefordshire , Worcestershire and Gloucestershire . Outliers in Somerset and north Devon complete the extent of this basin. With the exception of south Pembrokeshire, all parts of the basin are represented by a range of lithologies assigned to the Lower Devonian and to

2916-597: Was the one that placed it in the Devonian : he coined the name of that period. The term 'Old Red Sandstone' was originally used in 1821 by Scottish naturalist and mineralogist Robert Jameson to refer to the red rocks which underlay the 'Mountain Limestone' i.e. the Carboniferous Limestone. They were thought at that time to be the British version of Germany's Rotliegendes , which is in fact of Permian age. Many of

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