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53-490: The Winneshiek Shale is a thin and geologically homogeneous package of dark grey to greenish-brown sandy shale . Drill core data has estimated a maximum thickness of 38 meters, though in most areas its thickness is only about 18–27 meters. The shale is replete with pyrite and organic carbon . It lies solely within the Decorah Structure, a 5.6 km (3.5 mile)-wide probable impact crater near Decorah, Iowa. Within

106-646: A 4-meter interval of the shale. Fossils are primarily preserved as biomineralized shells or carbonaceous films, typically representative of hard parts with little soft-tissue preservation. Nevertheless, some Ceratiocaris specimens have phosphatized gut contents, and soft bromalites are preserved as apatite structures. The shale's depositional environment is reconstructed as a calm marine basin or estuary with an anoxic , low- pH seabed. The Winneshiek Shale has no radiometric dating and little overlapping fossil content with nearby formations, making precise age estimates difficult. The overlying St. Peter Sandstone

159-502: A decrease in benthic macroinvertebrate biomass, which lead to the disappearance of food sources into the substrate. Because the benthic system regulates energy in aquatic ecosystems, studies have been made of the mechanisms of the benthic zone in order to better understand the ecosystem. Benthic diatoms have been used by the European Union's Water Framework Directive (WFD) to establish ecological quality ratios that determined

212-414: 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

265-1201: 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

318-555: Is also known from Siberia . The Darriwilian age of the Winneshiek Shale is supported by chemostratigraphy trends tabulated from drill cores. There is a gradual positive trend in organic δ C values going up the shale. This may be correlated with the lower half of the mid-Darriwilian isotope carbon excursion (MDICE), a chemostratigraphic event observed worldwide. The MDICE is preceded by the lower-Darriwilian negative isotope carbon excursion (LDNICE), which likely occurred at

371-420: 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 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

424-638: 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

477-430: 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

530-557: Is firmly late Darriwilian in age based on its conodont fauna. The Winneshiek Shale shares only a few taxa with other formations, namely Multioistodus subdentatus and Archeognathus primus . Both of these conodonts were originally known from the mid- to upper-Darriwilian Dutchtown Formation of Missouri . The mid-Darriwilian global stage corresponds to the late Whiterockian regional stage in North American biostratigraphy . Conodont material similar to Iowagnathus grandis

583-455: 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

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636-651: Is no reason to assume that the Winneshiek record is specifically correlated with the MDICE and LDNICE. The connections between the Decorah impact, other impacts worldwide, and the GOBE have also been criticized for their reliance on imprecise age estimates. Criticisms of the correlation between the MDICE and the Winneshiek excursion have been countered with the argument that alternative explanations have no direct evidence within

689-466: Is not easy to map or observe these organisms and their habitats, and most modern observations are made using remotely operated underwater vehicles (ROVs), and rarely submarines . Benthic macroinvertebrates have many important ecological functions, such as regulating the flow of materials and energy in river ecosystems through their food web linkages. Because of this correlation between flow of energy and nutrients, benthic macroinvertebrates have

742-672: Is recommended in the European Union benthic monitoring program (by Kelly 1998 for the United Kingdom then in the EU and for the EU as a whole by CEN 2003 and CEN 2004) and in some United States programs (by Moulton et al. 2002). Benthic gross primary production (GPP) may be important in maintaining biodiversity hotspots in littoral zones in large lake ecosystems . However, the relative contributions of benthic habitats within specific ecosystems are poorly explored and more research

795-452: 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

848-399: 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

901-497: The hadal zone . For comparison, the pelagic zone is the descriptive term for the ecological region above the benthos, including the water column up to the surface. At the other end of the spectrum, benthos of the deep ocean includes the bottom levels of the oceanic abyssal zone . For information on animals that live in the deeper areas of the oceans see aphotic zone . Generally, these include life forms that tolerate cool temperatures and low oxygen levels, but this depends on

954-466: The water column ; even within the benthic zone variations in such factors as light penetration, temperature and salinity give rise to distinct differences, delineated vertically, in the groups of organisms supported. Many organisms adapted to deep-water pressure cannot survive in the upper parts of the water column: the pressure difference can be very significant (approximately one atmosphere for each 10 meters of water depth). Many have adapted to live on

1007-669: 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

1060-517: The ability to influence food resources on fish and other organisms in aquatic ecosystems . For example, the addition of a moderate amount of nutrients to a river over the course of several years resulted in increases in invertebrate richness, abundance, and biomass . These in turn resulted in increased food resources for native species of fish with insignificant alteration of the macroinvertebrate community structure and trophic pathways. The presence of macroinvertebrates such as Amphipoda also affect

1113-418: The benthic food chain ; most organisms in the benthic zone are scavengers or detritivores . Some microorganisms use chemosynthesis to produce biomass . Benthic organisms can be divided into two categories based on whether they make their home on the ocean floor or a few centimeters into the ocean floor. Those living on the surface of the ocean floor are known as epifauna . Those who live burrowed into

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1166-969: The benthic zone. The microbes found in the benthic zone, specifically dinoflagellates and foraminifera , colonize quite rapidly on detritus matter while forming a symbiotic relationship with each other. In the deep sea, which covers 90–95% of the ocean floor, 90% of the total biomass is made up of prokaryotes. To release all the nutrients locked inside these microbes to the environment, viruses are important in making it available to other organisms. Modern seafloor mapping technologies have revealed linkages between seafloor geomorphology and benthic habitats, in which suites of benthic communities are associated with specific geomorphic settings. Examples include cold-water coral communities associated with seamounts and submarine canyons, kelp forests associated with inner shelf rocky reefs and rockfish associated with rocky escarpments on continental slopes. In oceanic environments, benthic habitats can also be zoned by depth. From

1219-421: 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

1272-416: The bottom. The benthic boundary layer , which includes the bottom layer of water and the uppermost layer of sediment directly influenced by the overlying water, is an integral part of the benthic zone, as it greatly influences the biological activity that takes place there. Examples of contact soil layers include sand bottoms, rocky outcrops, coral , and bay mud . The benthic region of the ocean begins at

1325-669: The break up of the L-chondrite parent asteroid. It may be connected to the Great Ordovician Biodiversification Event (GOBE), a major increase in animal diversity during the mid-Ordovician. The correlation between the MDICE and the positive excursion found in the Winneshiek Shale has been questioned. Small organic carbon isotope excursions may be influenced by local environmental or ecological conditions, rather than worldwide events. Moreover, numerous excursions occur in every time period, so there

1378-598: The bromalites themselves were likely made by eurypterids or larger conodonts. Other fossils include chelicerates , bivalved crustaceans , algae , linguloid brachiopods, a single gastropod specimen, and head shields from armored agnathans (jawless fish). Shale Shale is a fine-grained, clastic sedimentary rock formed from mud that 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

1431-506: 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

1484-408: 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

1537-400: 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 Benthic zone The benthic zone is the ecological region at the lowest level of a body of water such as an ocean , lake , or stream , including

1590-525: The crater fill had been eroded away by the time of the sandstone's deposition. This characteristic was not initially recognized, and the Winneshiek Shale was first believed to be a subunit of the St. Peter Sandstone. Apart from boreholes, the shale is only accessible at a few thin outcrops along the Upper Iowa River . A temporary dam constructed in 2010 allowed for the collection of numerous fossils from

1643-530: The crater, the shale overlies a much thicker unnamed unit mostly composed of impact breccia . The impact which formed the crater occurred after the deposition of the Shakopee Formation and before the deposition of the St. Peter Sandstone , bracketing the Winneshiek Shale between those two formations. The St. Peter Sandstone is separated from the Winneshiek Shale by an unconformity , indicating that most of

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1696-439: The deposition of organic matter, and bacterial communities. The amount of material sinking to the ocean floor can average 307,000 aggregates per m per day. This amount will vary on the depth of the benthos, and the degree of benthic-pelagic coupling. The benthos in a shallow region will have more available food than the benthos in the deep sea. Because of their reliance on it, microbes may become spatially dependent on detritus in

1749-453: The depth of the water. As with oceans, the benthic zone is the floor of the lake, composed of accumulated sunken organic matter . The littoral zone is the zone bordering the shore; light penetrates easily and aquatic plants thrive. The pelagic zone represents the broad mass of water, down as far as the depth to which no light penetrates. Benthos are the organisms that live in the benthic zone, and are different from those elsewhere in

1802-462: The dominance of certain types of algae in Benthic ecosystems as well. In addition, because benthic zones are influenced by the flow of dead organic material , there have been studies conducted on the relationship between stream and river water flows and the resulting effects on the benthic zone. Low flow events show a restriction in nutrient transport from benthic substrates to food webs, and caused

1855-652: The ecological status of lakes in the UK. Beginning research is being made on benthic assemblages to see if they can be used as indicators of healthy aquatic ecosystems. Benthic assemblages in urbanized coastal regions are not functionally equivalent to benthic assemblages in untouched regions. Ecologists are attempting to understand the relationship between heterogeneity and maintaining biodiversity in aquatic ecosystems. Benthic algae has been used as an inherently good subject for studying short term changes and community responses to heterogeneous conditions in streams. Understanding

1908-417: The land mass. At the continental shelf edge, usually about 200 metres (660 ft) deep, the gradient greatly increases and is known as the continental slope. The continental slope drops down to the deep sea floor. The deep-sea floor is called the abyssal plain and is usually about 4,000 metres (13,000 ft) deep. The ocean floor is not all flat but has submarine ridges and deep ocean trenches known as

1961-592: 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

2014-433: 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,

2067-456: The ocean floor are known as infauna . Extremophiles, including piezophiles , which thrive in high pressures, may also live there. An example of benthos organism is Chorismus antarcticus . Sources of food for benthic communities can derive from the water column above these habitats in the form of aggregations of detritus , inorganic matter, and living organisms. These aggregations are commonly referred to as marine snow , and are important for

2120-415: 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

2173-631: 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

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2226-471: The potential mechanisms involving benthic periphyton and the effects on heterogeneity within a stream may provide a better understanding of the structure and function of stream ecosystems. Periphyton populations suffer from high natural spatial variability while difficult accessibility simultaneously limits the practicable number of samples that can be taken. Targeting periphyton locations which are known to provide reliable samples – especially hard surfaces –

2279-817: 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

2332-444: 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

2385-539: The same time as the Decorah impact. Other fossil locales showing a similar negative excursion have an approximate age of around 465-467 Ma. Moreover, the Decorah impact may have occurred at the same time as numerous other mid-Ordovician impact craters in North America and around the Baltic Sea . This brief spike in asteroid impacts and meteorite abundance, the “ Ordovician Meteor Event ”, likely occurred due to

2438-435: The seabed) such as trilobites , echinoderms , brachiopods , bryozoans , or corals . Instead, the Winneshiek biota consists of nektonic animals (which swim in open waters) or nektobenthic animals (which swim close to the seabed). The most common animal remains are conodont elements and bromalites , which make up >50% and >25% of recovered fossils, respectively. Some of the bromalites contained conodont elements, and

2491-503: The sediment surface and some sub-surface layers. The name comes from the Ancient Greek word βένθος ( bénthos ), meaning "the depths". Organisms living in this zone are called benthos and include microorganisms (e.g., bacteria and fungi ) as well as larger invertebrates , such as crustaceans and polychaetes . Organisms here generally live in close relationship with the substrate and many are permanently attached to

2544-433: 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

2597-664: The shallowest to the deepest are: the epipelagic (less than 200 meters), the mesopelagic (200–1,000 meters), the bathyal (1,000–4,000 meters), the abyssal (4,000–6,000 meters) and the deepest, the hadal (below 6,000 meters). The lower zones are in deep, pressurized areas of the ocean. Human impacts have occurred at all ocean depths, but are most significant on shallow continental shelf and slope habitats. Many benthic organisms have retained their historic evolutionary characteristics. Some organisms are significantly larger than their relatives living in shallower zones, largely because of higher oxygen concentration in deep water. It

2650-441: The shore line ( intertidal or littoral zone ) and extends downward along the surface of the continental shelf out to sea. Thus, the region incorporates a great variety of physical conditions differing in: depth, light penetration and pressure. Depending on the water-body, the benthic zone may include areas that are only a few inches below the surface. The continental shelf is a gently sloping benthic region that extends away from

2703-465: The strata. Multioistodus subdentatus was presented as a mid-Darriwilian fossil supporting the Winneshiek Shale's biostratigraphic connection to areas with similar chemostratigraphy, despite the lack of direct biostratigraphic overlap. The fossil content of the Winneshiek Shale, known as the Winneshiek biota, is distinctive when compared to that of most Ordovician fossil sites. There are very few fossils of benthic animals (those which live their lives on

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2756-442: The substrate (bottom). In their habitats they can be considered as dominant creatures, but they are often a source of prey for Carcharhinidae such as the lemon shark . Because light does not penetrate very deep into ocean-water, the energy source for the benthic ecosystem is often marine snow . Marine snow is organic matter from higher up in the water column that drifts down to the depths. This dead and decaying matter sustains

2809-414: 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

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