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114-813: Banded-iron formations are iron formations which formed about 2,000 million years ago and were first described in the Lake Superior region . Sediments associated with the last stage of the Great Lakes tectonic zone contain banded-iron formations. These sediments were deposited for two hundred million years and extend intermittently along roughly the same trend as the Great Lakes tectonic zone, from Minnesota into eastern Ontario , Canada, and through upper Wisconsin and Michigan . They are characterized by bands of iron compounds and chert . Enough oxygen had accumulated in seawater so that dissolved iron

228-538: A continental shelf . This classification has been more widely accepted, but the failure to appreciate that it is strictly based on the characteristics of the depositional basin and not the lithology of the BIF itself has led to confusion, and some geologists have advocated for its abandonment. However, the classification into Algoma versus Lake Superior types continues to be used. Banded iron formations are almost exclusively Precambrian in age, with most deposits dating to

342-455: A peneplain is a low-relief plain formed by protracted erosion . This is the definition in the broadest of terms, albeit with frequency the usage of peneplain is meant to imply the representation of a near-final (or penultimate) stage of fluvial erosion during times of extended tectonic stability. Peneplains are sometimes associated with the cycle of erosion theory of William Morris Davis , but Davis and other workers have also used

456-445: A photic zone inhabited by cyanobacteria that had evolved the capacity to carry out oxygen-producing photosynthesis, but which had not yet evolved enzymes (such as superoxide dismutase ) for living in an oxygenated environment. Such organisms would have been protected from their own oxygen waste through its rapid removal via the reservoir of reduced ferrous iron, Fe(II), in the early ocean. The oxygen released by photosynthesis oxidized

570-434: A tsunami at least 1,000 m (3,300 ft) high at the point of impact, and 100 m (330 ft) high about 3,000 km (1,900 mi) away. It has been suggested that the immense waves and large underwater landslides triggered by the impact caused the mixing of a previously stratified ocean, oxygenated the deep ocean, and ended BIF deposition shortly after the impact. Although Cloud argued that microbial activity

684-514: A Snowball Earth state the continents, and possibly seas at low latitudes, were subject to a severe ice age circa 750 to 580 Ma that nearly or totally depleted free oxygen. Dissolved iron then accumulated in the oxygen-poor oceans (possibly from seafloor hydrothermal vents). Following the thawing of the Earth, the seas became oxygenated once more causing the precipitation of the iron. Banded iron formations of this period are predominantly associated with

798-525: A condition is ignored. Geomorphologist Karna Lidmar-Bergström and co-workers consider the base level criterion crucial and above the precise mechanism of formation of peneplains, including this way some pediplains among peneplains. While peneplains are usually assumed to form near sea level it has also been posited that peneplains can form at height if extensive sedimentation raises the local base level sufficiently or if river networks are continuously obstructed by tectonic deformation . The peneplains of

912-509: A factor of 50 under conditions of low oxygen. Oxygenic photosynthesis is not the only biogenic mechanism for deposition of banded iron formations. Some geochemists have suggested that banded iron formations could form by direct oxidation of iron by microbial anoxygenic phototrophs . The concentrations of phosphorus and trace metals in BIFs are consistent with precipitation through the activities of iron-oxidizing bacteria. Iron isotope ratios in

1026-657: A few centimeters thick. Many of the chert mesobands contain microbands of iron oxides that are less than a millimeter thick, while the iron mesobands are relatively featureless. BIFs tend to be extremely hard, tough, and dense, making them highly resistant to erosion, and they show fine details of stratification over great distances, suggesting they were deposited in a very low-energy environment; that is, in relatively deep water, undisturbed by wave motion or currents. BIFs only rarely interfinger with other rock types, tending to form sharply bounded discrete units that never grade laterally into other rock types. Banded iron formations of

1140-447: A higher energy depositional environment , in shallower water disturbed by wave motions. However, they otherwise resemble other banded iron formations. The great majority of banded iron formations are Archean or Paleoproterozoic in age. However, a small number of BIFs are Neoproterozoic in age, and are frequently, if not universally, associated with glacial deposits, often containing glacial dropstones . They also tend to show

1254-674: A higher level of oxidation, with hematite prevailing over magnetite, and they typically contain a small amount of phosphate, about 1% by mass. Mesobanding is often poor to nonexistent and soft-sediment deformation structures are common. This suggests very rapid deposition. However, like the granular iron formations of the Great Lakes, the Neoproterozoic occurrences are widely described as banded iron formations. Banded iron formations are distinct from most Phanerozoic ironstones . Ironstones are relatively rare and are thought to have been deposited in marine anoxic events , in which

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1368-541: A hydrous silica gel. The conversion of iron hydroxide and silica gels to banded iron formation is an example of diagenesis , the conversion of sediments into solid rock. There is evidence that banded iron formations formed from sediments with nearly the same chemical composition as is found in the BIFs today. The BIFs of the Hamersley Range show great chemical homogeneity and lateral uniformity, with no indication of any precursor rock that might have been altered to

1482-430: A key element of most theories of deposition. The few formations deposited after 1,800  Ma may point to intermittent low levels of free atmospheric oxygen, while the small peak at 750  million years ago may be associated with the hypothetical Snowball Earth. The microbands within chert layers are most likely varves produced by annual variations in oxygen production. Diurnal microbanding would require

1596-462: A lower member, which lacks iron oxide components, and an upper member dominated by beds of iron oxide argillite and lean iron-formation interlayered with non-iron oxide argillite, siltstone and quartzose sandstone. The Trommald Formation – the principal iron formation of the North Range – is a chemically precipitated unit. This formation is 14 to 150 m (46 to 492 ft) thick and

1710-625: A mode of formation does not require a global anoxic ocean, but is consistent with either a Snowball Earth or Slushball Earth model. Banded iron formations provide most of the iron ore presently mined. More than 60% of global iron reserves are in the form of banded iron formation, most of which can be found in Australia, Brazil, Canada, India, Russia, South Africa, Ukraine, and the United States. Different mining districts coined their own names for BIFs. The term "banded iron formation"

1824-647: A northeasterly-trending belt; most of it lies in Ontario. The upper sedimentary layer is the 1,800- to 1,600-million-year-old Rove Formation. The seas and laid down the shales, slates and mudstones of the Rove Formation . Because the formation is on the northern part of the Animikie Basin these rocks escaped the crustal deformation from the Penokean orogeny that characterizes the equivalent strata of

1938-472: A peculiar kind of Precambrian evaporite . Other proposed abiogenic processes include radiolysis by the radioactive isotope of potassium , K, or annual turnover of basin water combined with upwelling of iron-rich water in a stratified ocean. Another abiogenic mechanism is photooxidation of iron by sunlight. Laboratory experiments suggest that this could produce a sufficiently high deposition rate under likely conditions of pH and sunlight. However, if

2052-408: A process that did not produce great quantities of biomass, so that little carbon was present to reduce hematite to magnetite. However, it is possible that BIF was altered from carbonate rock or from hydrothermal mud during late stages of diagenesis. A 2018 study found no evidence that magnetite in BIF formed by decarbonization, and suggests that it formed from thermal decomposition of siderite via

2166-496: A region is known as a primary peneplain. An example of a primary peneplain is the Sub-Cambrian peneplain in southern Sweden. The peneplain concept is often juxtaposed to that of pediplain . However authors like Karna Lidmar-Bergström classify pediplains as a type of peneplain. On the contrary Lester Charles King held them as incompatible landforms arguing that peneplains do not exist. King wrote: According to King

2280-1075: A sub-set of peneplains or partially overlap with the term. The last is the case of planation surfaces that may be peneplains or not, while some peneplains are not planation surfaces. In their 2013 work Green, Lidmar-Bergström and co-workers provide the following classification scheme for peneplains: Rhodes Fairbridge and Charles Finkl argue that peneplains are often of mixed origin (polygenetic), as they may have been shaped by etchplanation during periods of humid climate and pediplanation during periods of arid and semi-arid climate. The long time spans under which some peneplains evolve ensures varied climatic influences . The same authors do also list marine abrasion and glacial erosion among processes that can contribute in shaping peneplains. In addition, epigene peneplains can be distinguished from exhumed peneplains. Epigene peneplains are those that have never been buried or covered by sedimentary rock. Exhumed peneplains are those that are re-exposed after having been buried in sediments. The oldest identifiable peneplain in

2394-827: A thickness of 60 meters (200 feet). Other examples of early Archean BIFs are found in the Abitibi greenstone belts , the greenstone belts of the Yilgarn and Pilbara cratons , the Baltic shield , and the cratons of the Amazon , north China , and south and west Africa. The most extensive banded iron formations belong to what A.F. Trendall calls the Great Gondwana BIFs. These are late Archean in age and are not associated with greenstone belts. They are relatively undeformed and form extensive topographic plateaus, such as

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2508-497: A thin layer on the ocean floor. Each band is similar to a varve , resulting from cyclic variations in oxygen production. Banded iron formations were first discovered in northern Michigan in 1844. Banded iron formations account for more than 60% of global iron reserves and provide most of the iron ore presently mined. Most formations can be found in Australia , Brazil , Canada , India , Russia , South Africa , Ukraine , and

2622-485: A twofold division of BIFs into an Algoma type and a Lake Superior type, based on the character of the depositional basin. Algoma BIFs are found in relatively small basins in association with greywackes and other volcanic rocks and are assumed to be associated with volcanic centers. Lake Superior BIFs are found in larger basins in association with black shales, quartzites , and dolomites , with relatively minor tuffs or other volcanic rocks, and are assumed to have formed on

2736-429: A very high rate of deposition of 2 meters per year or 5 km/Ma. Estimates of deposition rate based on various models of deposition and sensitive high-resolution ion microprobe (SHRIMP) estimates of the age of associated tuff beds suggest a deposition rate in typical BIFs of 19 to 270 m/Ma, which are consistent either with annual varves or rhythmites produced by tidal cycles. Preston Cloud proposed that mesobanding

2850-411: Is 4 km (2.5 mi) – and 110 to 240 m (360 to 790 ft) thick. Its natural ore is hematite- or geothite-rich leached iron formation; natural ores contain up to 50% iron and less than 10% silica. These thick sedimentary layers contain millions of tons of iron and minor ores which have been mined in the Great Lakes region since before the turn of the 20th century. Sedimentation ended when

2964-671: Is a shallow-marine layer which was deposited on the Archean basement; deposition in the Chocolay Group began 2,207 ± 5 million years ago and ended 2,115 ± 5 million years ago. The Menominee Group is a foredeep deposit whose layers were deposited in second-order basins created by oblique subduction of the continental margin, rather than in basins formed on a rifting margin. The upper Baraga Group represents deeper marine basins resulting from increased subsidence and continued collision. Deposition continued until 1,850 million years ago when

3078-416: Is composed of carbonate-silicate iron formations and associated manganese oxide deposits. The iron oxidised to form hematite and goethite . The uppermost Rabbit Lake Formation has a lower member of black mudstone inserted with beds of iron formation and units of volcanogenic origin; and an upper member of slate, carbonaceous mudstone, greywacke and thin units of iron-rich strata. The top sedimentary layer

3192-536: Is inclined 70-80° to the north. The volcanic rocks of the Ely Greenstone are divided into a lower and upper sequence; the upper and lower volcanic sequences are separated by the Soudan Iron Formation – a 50 to 3,000 m (160 to 9,840 ft) thick unit that is transitional with the Ely Greenstone – which consists chiefly of banded iron-formation. The Soudan Iron Formation is in

3306-432: Is irrelevant and that the term peneplain has been used and can be used in a purely descriptive manner. Further, alternation of processes with varying climate, relative sea level and biota make old surfaces unlikely to be of a single origin. Peneplains that are detached from their base level are identified by either hosting an accumulation of sediments that buries it or by being in an uplifted position. Burial preserves

3420-411: Is more precisely defined as chemically precipitated sedimentary rock containing greater than 15% iron . However, most BIFs have a higher content of iron, typically around 30% by mass, so that roughly half the rock is iron oxides and the other half is silica. The iron in BIFs is divided roughly equally between the more oxidized ferric form, Fe(III), and the more reduced ferrous form, Fe(II), so that

3534-425: Is oxidation by anaerobic denitrifying bacteria . This requires that nitrogen fixation by microorganisms is also active. The lack of organic carbon in banded iron formation argues against microbial control of BIF deposition. On the other hand, there is fossil evidence for abundant photosynthesizing cyanobacteria at the start of BIF deposition and of hydrocarbon markers in shales within banded iron formation of

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3648-599: Is southwest of the Mesabi Range in east-central Minnesota; it is 110 km (68 mi) by 32 km (20 mi) of tightly folded iron formations. Its thickness ranges from 0 to 135 m (0 to 443 ft). Two sequences – the Mille Lacs and North ranges – underlie the southern part of the Animike Group. The Mille Lacs Group is more than 2,197 ± 39 million years old. The North Range Group

3762-407: Is structurally controlled, for example, drainage divides in peneplain can follow more resistant rock. In the view of Davis large streams do became insensitive to lithology and structure, which they were not during the valley phase of erosion cycle. This may explain the existence of superimposed streams . There are various terms for landforms that are either alternatives to classical peneplains,

3876-610: Is the Ely Greenstone layer. Ely Greenstone consists of igneous rocks which were metamorphosed by the gabbro of the Duluth Complex. The Ely Greenstone is a belt consisting chiefly of metamorphosed volcanic rocks which have been deformed so that original bedding stands nearly vertical. In the Soudan area the Ely Greenstone has been tightly folded and slightly overturned southward into the Tower-Soudan anticline , and bedding

3990-467: Is the Thomson Formation which was deposited 1,880 to 1,870 million years ago and deformed by the Penokean orogeny 1,850 million years ago. The formation contains folded and metamorphosed greywacke, siltstone, mudstone and slate which were originally deposited in the sea as horizontal beds of mud and sand; the Penokean orogeny subjected the rocks to intense compression from the south. This folded

4104-664: Is the basal unit for the Cuyuna Range. It is divided into three structural units: South Range (The rocks of the South Range are assigned to the Mille Lacs group.), North Range and the Emily District each with its own characteristic stratigraphy and structure. The rocks of the South and North ranges are separated by a major north-verging thrust fault , and both are overlain unconformably by the Emily District. The rocks of

4218-632: Is the sedimentary layer on top of the Biwabik Iron Range and forms the footwall of the 1,100-million-year-old Duluth Complex in the Ely – Hoyt Lakes region. The Virginia formation consists of black to dark gray argillite , which does not crop out in natural exposures. The Vermilion Range is north of the Mesabi Iron Range; it is 154 km (96 mi) long and ranges from 3 to 30 km (1.9 to 18.6 mi) wide. Its basal unit

4332-585: The Animikie Series , it was proposed to be renamed in 1970 to avoid confusion with the Animikie Group in Ontario and Minnesota. On the map it is the dark grey area south of Lake Superior with four iron ranges shown. This supergroup consists of the Chocolay, Menominee, Baraga and Paint River groups, in descending order of age. The Chocolay Group – up to 160 m (520 ft) thick –

4446-671: The Archean basement formed the Animikie Group. The next tectonic event was the Great Lakes tectonic zone which began with compression caused by the collision of the Superior province and the Minnesota River Valley subprovince during the Algoman orogeny about 2,700 million years ago; it continued as a pulling apart (extensional) rift from 2,450 to 2,100 million years ago, followed by a second compression which deformed

4560-762: The Biwabik Iron Formation with the Virginia Formation deposited on top. The Vermilion Range consists of the basal Ely Greenstone , then the Soudan Iron formation with various granites on top. The Cuyuna Range consists of the basal North Range group, then the Trommald Formation with the Thomson Formation deposited on top. The Animikie Group sediments were deposited between 2,500 and 1,800 million years ago, in

4674-775: The Great Lakes region and the Frere Formation of western Australia are somewhat different in character and are sometimes described as granular iron formations or GIFs . Their iron sediments are granular to oolitic in character, forming discrete grains about a millimeter in diameter, and they lack microbanding in their chert mesobands. They also show more irregular mesobanding, with indications of ripples and other sedimentary structures , and their mesobands cannot be traced out any great distance. Though they form well-defined, discrete units, these are commonly interbedded with coarse to medium-grained epiclastic sediments (sediments formed by weathering of rock). These features suggest

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4788-797: The Hamersley Range . The banded iron formations here were deposited from 2470 to 2450 Ma and are the thickest and most extensive in the world, with a maximum thickness in excess of 900 meters (3,000 feet). Similar BIFs are found in the Carajás Formation of the Amazon craton, the Cauê Itabirite of the São Francisco craton , the Kuruman Iron Formation and Penge Iron Formation of South Africa, and

4902-984: The Mulaingiri Formation of India . Paleoproterozoic banded iron formations are found in the Iron Range and other parts of the Canadian Shield . The Iron Range is a group of four major deposits: the Mesabi Range , the Vermilion Range , the Gunflint Range , and the Cuyuna Range . All are part of the Animikie Group and were deposited between 2500 and 1800 Ma. These BIFs are predominantly granular iron formations. Neoproterozoic banded iron formations include

5016-549: The Pyrenees and Tibetan Plateau may exemplify these two cases respectively. A common misconception about peneplains is that they ought to be so plain they are featureless. In fact, some peneplains may be hilly as they reflect irregular deep weathering , forming a plain grading to a base level only at a grand-scale. At the grand-scale peneplains are characterized by appearing to be sculpted in rock with disregard of rock structure and lithology , but in detail, their shape

5130-575: The Sturtian glaciation . An alternative mechanism for banded iron formations in the Snowball Earth era suggests the iron was deposited from metal-rich brines in the vicinity of hydrothermally active rift zones due to glacially-driven thermal overturn. The limited extent of these BIFs compared with the associated glacial deposits, their association with volcanic formations, and variation in thickness and facies favor this hypothesis. Such

5244-553: The United States . A typical banded iron formation consists of repeated, thin layers (a few millimeters to a few centimeters in thickness) of silver to black iron oxides , either magnetite (Fe 3 O 4 ) or hematite (Fe 2 O 3 ), alternating with bands of iron-poor chert , often red in color, of similar thickness. A single banded iron formation can be up to several hundred meters in thickness and extend laterally for several hundred kilometers. Banded iron formation

5358-452: The oxygenation of the Earth's oceans . Some of the Earth's oldest rock formations, which formed about 3,700  million years ago ( Ma ), are associated with banded iron formations. Banded iron formations are thought to have formed in sea water as the result of oxygen production by photosynthetic cyanobacteria . The oxygen combined with dissolved iron in Earth's oceans to form insoluble iron oxides, which precipitated out, forming

5472-469: The shale and greywacke of the southernmost unit – the Thomson Formation – and metamorphosed the shale into slate . The Animikie strata on the Gunflint and Mesabi ranges were far enough away so they escaped this deformation and metamorphism; they contain some of the oldest unmetamorphosed sedimentary deposits in the world. About 1,100 million years ago a fourth tectonic event occurred in

5586-643: The Animikie Basin. Deposition of sediments began after the Algoman orogeny and continued through the Great Lakes tectonic zone rupture from 2,200 to 1,850 million years ago. The Animikie Group formations are in east-central and northeastern Minnesota, and the Thunder Bay District of Northern Ontario; they are geographically divided into the Gunflint Range, the Mesabi and Vermilion ranges, and

5700-537: The Archean. These older BIFs tend to show a positive europium anomaly consistent with a hydrothermal source of iron. By contrast, Lake Superior-type banded iron formations primarily formed during the Paleoproterozoic era, and lack the europium anomalies of the older Algoma-type BIFs, suggesting a much greater input of iron weathered from continents. The absence of hydrogen sulfide in anoxic ocean water can be explained either by reduced sulfur flux into

5814-559: The Cuyuna Range. The Animikie Basin was an extensional basin which developed over a basement consisting of the 2,750- to 2,600-million-year-old Superior province to the north and the 3,600-million-year-old Minnesota River Valley subprovince to the south. The extension was caused by the east-northeast–trending Great Lakes tectonic zone; it separates the Superior province from the Minnesota River Valley subprovince . The sediments were deformed, metamorphosed and intruded by

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5928-458: The Fe(II) to ferric iron, Fe(III), which precipitated out of the sea water as insoluble iron oxides that settled to the ocean floor. Cloud suggested that banding resulted from fluctuations in the population of cyanobacteria due to free radical damage by oxygen. This also explained the relatively limited extent of early Archean deposits. The great peak in BIF deposition at the end of the Archean

6042-620: The Great Lakes tectonic zone. Twenty-seven hundred million years ago the Algoman orogeny formed mountains; these bare mountains eroded for several hundred million years to a broad level peneplain . A sea invaded central Minnesota and extended eastward through northern Wisconsin and the Upper Peninsula of Michigan. Sediments composed of quartz -rich sand were deposited along the shoreline of this sea; these were succeeded by thick iron-rich layers and eventually kilometers of mud and muddy sand. The deposition of sedimentary strata on top of

6156-528: The Great Oxygenation Event. Prior to 2.45 billion years ago, the high degree of mass-independent fractionation of sulfur (MIF-S) indicates an extremely oxygen-poor atmosphere. The peak of banded iron formation deposition coincides with the disappearance of the MIF-S signal, which is interpreted as the permanent appearance of oxygen in the atmosphere between 2.41 and 2.35 billion years ago. This

6270-687: The Lake Superior region. A hotspot of magma from the Earth's mantle beneath present-day Lake Superior rose, causing the crust to dome and break apart. This zone of crustal thinning and fracturing is the Midcontinent Rift System ; it extends in a boomerang shape for over 2,200 km (1,400 mi) from northeastern Kansas northward through Iowa, under the Twin Cities of Minnesota, beneath Lake Superior, and then south through

6384-934: The Lower Cherty (which was deposited upon the Pokegama Quartzite), the Lower Slatey, the Upper Cherty and the Upper Slaty (which the Virginia Formation rests upon). The two ore-producing layers are the Upper and Lower Cherty subdivisions; cherts make up the bulk of the formation. The east end of the Biwabik Iron Formation was metamorphosed by the heat of the Duluth Complex. The 1,850-million-year-old Virginia Formation

6498-611: The North Range – assigned to the North Range Group, – are divided into three formations, the Mahnomen, Trommald and Rabbit Lake. The North Range of the Cuyuna Range was regionally metamorphosed during the Penokean orogeny, which peaked between 1,870 and 1,850 million years ago. The iron ore of the Cuyuna is a Lake Superior-type iron-formation similar to other iron formations in the region. The Mahnomen Formation has

6612-581: The Penokean orogeny began 1,850 million years ago . The three different formations exposed along the Mesabi Iron Range were deposited along the leading edge of a foredeep basin – the Animikie Basin – which transgressed north over the Archean craton during the Penokean orogeny. Deposition of the basal Pokegama Quartzite, the medial Biwabik Iron Formation and the upper Virginia Formation's sediments represent near-shore, shelf and slope environments, respectively. These three layers were formed 2,500 to 1,600 million years ago. Pokegama Quartzite occupies

6726-451: The Penokean orogeny began. Banded iron formation Banded iron formations ( BIFs ; also called banded ironstone formations ) are distinctive units of sedimentary rock consisting of alternating layers of iron oxides and iron-poor chert . They can be up to several hundred meters in thickness and extend laterally for several hundred kilometers. Almost all of these formations are of Precambrian age and are thought to record

6840-486: The Pilbara craton. The carbon that is present in banded iron formations is enriched in the light isotope, C, an indicator of a biological origin. If a substantial part of the original iron oxides was in the form of hematite, then any carbon in the sediments might have been oxidized by the decarbonization reaction: Trendall and J.G. Blockley proposed, but later rejected, the hypothesis that banded iron formation might be

6954-585: The Precambrian world, they have been intensively studied by geologists. Banded iron formations are found worldwide, in every continental shield of every continent. The oldest BIFs are associated with greenstone belts and include the BIFs of the Isua Greenstone Belt , the oldest known, which have an estimated age of 3700 to 3800 Ma. The Temagami banded iron deposits formed over a 50-million-year period, from 2736 to 2687 Ma, and reached

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7068-611: The Richter scale. The results of this impact caused the worldwide extinction of many species (including dinosaurs). The Sudbury Impact would have also had global ramifications; it is conjectured that this caused the end of the banded-iron deposits. The results of the impact fundamentally affected concentrations of dissolved oxygen in the sea; the accumulation of marine sediments (the banded-iron formations) were almost instantaneously shut down and banded-iron formation buildups suddenly ended about 1,850 million years ago . In northeastern Minnesota these iron-banded formations lie immediately under

7182-404: The Thomson Formation; this left the Rove Formation unmetamorphosed and lying flat. These are some of the oldest undeformed and unmetamorphosed sedimentary rocks in North America. The dikes and sills within the Rove Formation were intruded during the Midcontinent Rift. The Mesabi Range is over 320 km (200 mi) long and less than 16 km (9.9 mi) wide – its typical width

7296-414: The Urucum in Brazil, Rapitan in the Yukon , and the Damara Belt in southern Africa. They are relatively limited in size, with horizontal extents not more than a few tens of kilometers and thicknesses not more than about 10 meters (33 feet). These are widely thought to have been deposited under unusual anoxic oceanic conditions associated with the " Snowball Earth ." Banded iron formation provided some of

7410-424: The availability of reduced iron on time scales of decades. In the case of granular iron formations, the mesobands are attributed to winnowing of sediments in shallow water, in which wave action tended to segregate particles of different size and composition. For banded iron formations to be deposited, several preconditions must be met. There must be an ample source of reduced iron that can circulate freely into

7524-604: The change in oxygen levels is that the sediments of the earlier Archean are dark brown and black caused by unoxidized carbon, iron sulfide , and other elements and compounds. As oxygen levels increased in the atmosphere and oceans, the sediments changed. In the late Archean, sediments went through a transitional stage with the banded-iron formations; after this transition they demonstrate an oxygen-rich environment – shown by iron oxide-stained siltstones or mudstones called red beds . Enough oxygen had accumulated in seawater so that dissolved iron – which had earlier eroded from

7638-417: The coalesced pediments of the pediplains form a series of very gentle concave slopes, a difference with Davis' understanding of peneplains may lie in the fact that his idealized peneplains had very gentle convex slopes instead. However, Davis' views on the subject are not fully clear. Contrary to this view Rhodes Fairbridge and Charles Finkl argue that the precise mechanism of formation (pediplanation, etc.)

7752-444: The crust. The Huronian and Marquette Range supergroups are similar sedimentary groups to the Animikie Group; all three are in the Great Lakes region. Rifting of continental plates create sedimentary basins; the largest of these basins in the Great Lakes area are the Animikie Group in Minnesota, the Marquette Range Supergroup in northern Michigan and Wisconsin, and the Huronian Supergroup in eastern Ontario. The Huronian Supergroup on

7866-471: The current composition. This suggests that, other than dehydration and decarbonization of the original ferric hydroxide and silica gels, diagenesis likely left the composition unaltered and consisted of crystallization of the original gels. Decarbonization may account for the lack of carbon and preponderance of magnetite in older banded iron formations. The relatively high content of hematite in Neoproterozoic BIFs suggests they were deposited very quickly and via

7980-417: The deep ocean became sufficiently oxygenated at that time to end transport of reduced iron. Heinrich Holland argues that the absence of manganese deposits during the pause between Paleoproterozoic and Neoproterozoic BIFs is evidence that the deep ocean had become at least slightly oxygenated. The "Canfield ocean" model proposes that, to the contrary, the deep ocean became euxinic and transport of reduced iron

8094-472: The deep ocean or a lack of dissimilatory sulfate reduction (DSR), the process by which microorganisms use sulfate in place of oxygen for respiration. The product of DSR is hydrogen sulfide, which readily precipitates iron out of solution as pyrite. The requirement of an anoxic, but not euxinic, deep ocean for deposition of banded iron formation suggests two models to explain the end of BIF deposition 1.8 billion years ago. The "Holland ocean" model proposes that

8208-412: The deposition basin. Plausible sources of iron include hydrothermal vents along mid-ocean ridges, windblown dust, rivers, glacial ice, and seepage from continental margins. The importance of various sources of reduced iron has likely changed dramatically across geologic time. This is reflected in the division of BIFs into Algoma and Lake Superior-type deposits. Algoma-type BIFs formed primarily in

8322-551: The depositional basin became depleted in free oxygen . They are composed of iron silicates and oxides without appreciable chert but with significant phosphorus content, which is lacking in BIFs. No classification scheme for banded iron formations has gained complete acceptance. In 1954, Harold Lloyd James advocated a classification based on four lithological facies (oxide, carbonate, silicate, and sulfide) assumed to represent different depths of deposition, but this speculative model did not hold up. In 1980, Gordon A. Gross advocated

8436-458: The difference between pediplains and Davis’ peneplains is in the history and processes behind their formation, and less so in the final shape. A difference in form that may be present is that of residual hills, which in Davis’ peneplains are to have gentle slopes, while in pediplains they ought to have the same steepness as the slopes in the early stages of erosion leading to pediplanation. Given that

8550-538: The east 1,850 ± 1 million years ago – is the likely ejecta source, making these the oldest ejecta linked to a specific impact. Additional evidence indicates a 16 km (10 mi) diameter meteorite collided with Earth in the current-day vicinity of Sudbury, Ontario, Canada. The meteorite vaporized and created a 240 km (150 mi) wide crater. Earthquakes shattered the ground hundreds of kilometers away and within seconds ejecta (cloud of ash, rock fragments, gases and droplets of molten rock) began to spread around

8664-402: The eastern Upper Peninsula of Michigan and beneath the central Lower Peninsula of Michigan. As the crust was being stretched thin and more magma flowed out from below, the center of the rift was continuously subsiding . The vast quantities of rising magma created a vacuum under the crust, the weight of the solidified magma on the surface caused the crust to subside into that vacuum so the edges of

8778-499: The ejecta layer. One use of the impact layer is as a precise timeline that ties together well-known stratigraphic sequences of the various geographically separated iron ranges. The Sudbury Impact layer lies at a horizon that records a significant change in the character of sediments across the region. The layer marks the end of a major period of banded-iron formation deposition that was succeeded by deposition of fine clastic rocks – commonly black shales. Sedimentation styles of

8892-423: The elevation was now above sea level. During later stages, the spreading center was adding oceanic crust – which is heavier than continental crust – so the area subsided, seas returned, and the second layer of sediments were deposited unconformably on the basin fill. The third tectonic event was the Penokean orogeny which is dated 1,850 million years ago. The intense, northward-directed compression folded

9006-563: The end of the banded-iron deposits. The results of the impact affected concentrations of dissolved oxygen in the sea; the accumulation of banded-iron formations suddenly ended 1,850 million years ago . The Gunflint Range consists of a basal conglomerate , then the Gunflint Iron formation and the Gunflint Chert with the Rove Formation deposited on top. The Mesabi Range consists of the basal Pokegama Quartzite layer, then

9120-501: The first evidence for the timing of the Great Oxidation Event , 2,400 Ma. With his 1968 paper on the early atmosphere and oceans of the Earth, Preston Cloud established the general framework that has been widely, if not universally, accepted for understanding the deposition of BIFs. Cloud postulated that banded iron formations were a consequence of anoxic, iron-rich waters from the deep ocean welling up into

9234-641: The globe. It is estimated that at ground zero the earthquake would have registered 10.2 on the Richter magnitude scale . To put the Sudbury meteorite impact in perspective, the Chicxulub impact on the Yucatán Peninsula occurred 66 million years ago with the impact of a 16.5 km (10.3 mi) diameter comet. The kinetic energy from this impact probably generated earthquakes registering 13 on

9348-442: The iron came from a shallow hydrothermal source, other laboratory experiments suggest that precipitation of ferrous iron as carbonates or silicates could seriously compete with photooxidation. Regardless of the precise mechanism of oxidation, the oxidation of ferrous to ferric iron likely caused the iron to precipitate out as a ferric hydroxide gel. Similarly, the silica component of the banded iron formations likely precipitated as

9462-416: The island arc settled on top of the previously deposited sequences. The Gunflint Range is a mountain range in northeastern Minnesota, U.S., and western Ontario, Canada. The Gunflint and Mesabi ranges form a belt extending from the upper Mississippi River to the extreme northeast part of Minnesota and into Canada to Thunder Bay . The two ranges are separated by the 1,099-million-year-old Duluth Complex which

9576-421: The last stage of the Great Lakes tectonic zone contain the banded-iron formations. Banded-iron formations are iron formations which formed about 2,000 million years ago and were first described in the Lake Superior region. They are characterized by interlayers – bands – of iron minerals and chert (quartz). These sediments were laid down for two hundred million years and extend intermittently along roughly

9690-566: The late Archean (2800–2500 Ma) with a secondary peak of deposition in the Orosirian period of the Paleoproterozoic (1850 Ma). Minor amounts were deposited in the early Archean and in the Neoproterozoic (750 Ma). The youngest known banded iron formation is an Early Cambrian formation in western China. Because the processes by which BIFs are formed appear to be restricted to early geologic time, and may reflect unique conditions of

9804-441: The late Archean peak of BIF deposition was spread out over tens of millions of years, rather than taking place in a very short interval of time following the evolution of oxygen-coping mechanisms. However, his general concepts continue to shape thinking about the origins of banded iron formations. In particular, the concept of the upwelling of deep ocean water, rich in reduced iron, into an oxygenated surface layer poor in iron remains

9918-506: The layers into east–west trending anticlines and synclines , and compressed the muddy beds into slate, a metamorphic rock. The Thomson Formation has steeply dipping beds of greywacke, siltstone and slate. Several basaltic dikes , from the lava of the Midcontinent Rift period, cut across the Thomson Formation slate and greywackes. Most of these dikes trend in a northeasterly direction; they represent magma that rose in fissures in

10032-626: The lowest level of the Mesabi Range sequence and is younger than 2,500 million years old. It contains shale, siltstone and sandstone , which were deposited in a flat environment of the sea that covered the Archean surface. It is 0 to 153 m (0 to 502 ft) thick, with an average of 60 m (200 ft). The 1,900- to 1,850-million-year-old Biwabik Iron Formation is a narrow belt of iron-rich strata that extends east-northeast for 200 km (120 mi); its thickness varies from 60 to 600 m (200 to 1,970 ft), its average may be 305 m (1,001 ft). It has four primary subdivisions:

10146-497: The main basin. A 25 to 70 cm (9.8 to 27.6 in) thick lateral layer between the metasedimentary Gunflint Iron Formation and overlying Rove Formation, and between the Biwabik Iron Formation and overlying Virginia Formation has evidence that the layer contains hypervelocity impact ejecta . Radiometric dating reveals that this layer was deposited between 1,878 and 1,836 million years ago. The Sudbury Impact event – which occurred 650 to 875 km (404 to 544 mi) to

10260-468: The margins of this basin, five of which contained sufficient concentrations of iron to be economically mined. These banded-iron formations have been one of the world's greatest sources of iron ore since mining began in the area during the late 19th century. Major iron formations in different parts of the basin represent either nearly contemporaneous shelf sedimentation on either side of the main basin, or deposits formed simultaneously in isolated sub-basins of

10374-526: The north shore of Lake Huron in Ontario overlies an Archean basement. On the map it is the formation north of both Lake Huron and the Grenville Front Tectonic Zone . Huronian sedimentary rocks form a 300 km (190 mi) east–west fold belt and reach a thickness of 12 km (7.5 mi) near Lake Huron. Deposition of sediments began 2,450 to 2,219 million years ago and continued until 1,850 to 1,800 million years ago when

10488-592: The oldest banded iron formations (3700-3800 Ma), at Isua, Greenland, are best explained by assuming extremely low oxygen levels (<0.001% of modern O 2 levels in the photic zone) and anoxygenic photosynthetic oxidation of Fe(II): This requires that dissimilatory iron reduction, the biological process in which microorganisms substitute Fe(III) for oxygen in respiration, was not yet widespread. By contrast, Lake Superior-type banded iron formations show iron isotope ratios that suggest that dissimilatory iron reduction expanded greatly during this period. An alternate route

10602-477: The passive margin changed as deposition came to a close. The sedimentary environment recorded near the end changed from deep water shales derived from Archean rocks to coarser clastic rocks derived from a younger Proterozoic source. This change is interpreted to be from the Pembine-Wausau island arc as it closed in from the south just before its collision during the Penokean orogeny. Sediments shedding off

10716-402: The peneplain. Any exposed peneplain detached from its baselevel can be considered a paleosurface or paleoplain . Uplift of a peneplain commonly results in renewed erosion. As Davis put it in 1885: Uplifted peneplains can be preserved as fossil landforms in conditions of extreme aridity or under non-eroding cold-based glacier ice. Erosion of peneplains by glaciers in shield regions

10830-434: The plutonic rocks of the 1860 ± 50-million-year-old Penokean orogeny . The rocks of the Animikie Basin form a sequence up to 10 km (6.2 mi) thick and show a complete transition from a stable shelf environment to deep-water conditions. Irregularities in the basement influenced the thickness of the sequence. The 700 km (430 mi) by 400 km (250 mi) basin is an elongated oval parallel to and straddling

10944-587: The ratio Fe(III)/Fe(II+III) typically varies from 0.3 to 0.6. This indicates a predominance of magnetite, in which the ratio is 0.67, over hematite, for which the ratio is 1. In addition to the iron oxides (hematite and magnetite), the iron sediment may contain the iron-rich carbonates siderite and ankerite , or the iron-rich silicates minnesotaite and greenalite . Most BIFs are chemically simple, containing little but iron oxides, silica, and minor carbonate, though some contain significant calcium and magnesium, up to 9% and 6.7% as oxides respectively. When used in

11058-505: The reaction The iron may have originally precipitated as greenalite and other iron silicates. Macrobanding is then interpreted as a product of compaction of the original iron silicate mud. This produced siderite-rich bands that served as pathways for fluid flow and formation of magnetite. The peak of deposition of banded iron formations in the late Archean, and the end of deposition in the Orosirian, have been interpreted as markers for

11172-677: The rift axis. As much as 8 km (5.0 mi) of sedimentary rocks accumulated in the center before the sinking stopped and the region stabilized. A north-northeast trending branch of the Midcontinent Rift System separated the Animikie Basin into two distinct segments; the present-day Animikie Group and the Marquette Range Supergroup; the historical name for the Marquette Range Supergroup is the Animikie Series. Oceanic sediments associated with

11286-459: The rift tilted toward the center. The rifting stopped after a few million years; one reason could be that the Grenville orogeny stopped the rift process when that collision occurred. Subsidence continued for several million years after the lava flows had ceased; immense volumes of sediments – sand, gravel and mud – were eroded off the barren landscape into the still-sinking basin along

11400-407: The rocks in the Lake Superior region during the Penokean orogeny which lasted from 1,900 to 1,850 million years ago. The first deposits occurred during the initial stages of extension of the Great Lakes tectonic zone in the continental crust. As the crust expanded it thinned, and magma was intruded through fissures in the thinned crust. Sedimentation stopped during this transitional period because

11514-619: The rocks were deformed and metamorphosed during the Penokean orogeny. The supergroup's sedimentary layers are divided into lower and upper sequences. The lower sequence is subdivided into the Elliot Lake, Hough Lake and Quirke Lake groups; the upper sequence is the Cobalt Group. The lower sequences were deposited in a continental rift basin and the upper sequence was deposited in a stable passive margin. The Marquette Range Supergroup also overlies an Archean basement. Originally termed

11628-482: The same trend as the Great Lakes tectonic zone, from Minnesota into eastern Canada, and through upper Wisconsin and Michigan. Banded-iron sediments record the introduction of abundant free oxygen into earth's atmosphere. Microbial life played an important role in changing atmospheric conditions by releasing free oxygen as a waste product of photosynthesis. Free oxygen was taken up by elements with strong affinities for it – hydrogen , carbon and iron. Evidence for

11742-528: The singular, the term banded iron formation refers to the sedimentary lithology just described. The plural form, banded iron formations, is used informally to refer to stratigraphic units that consist primarily of banded iron formation. A well-preserved banded iron formation typically consists of macrobands several meters thick that are separated by thin shale beds. The macrobands in turn are composed of characteristic alternating layers of chert and iron oxides, called mesobands , that are several millimeters to

11856-454: The surrounding land – was oxidized. Oxygenated water has low levels of dissolved iron because iron reacts with oxygen to form compounds that precipitate out; the compounds include hematite (Fe 2 O 3 ), limonite (Fe 2 O 3 ·2H 2 O) and siderite (FeCO 3 ). These iron compounds precipitated from the sea water in varying proportions with chert, producing banded-iron formations. Banded-iron formations occur in several ranges around

11970-404: The term in a purely descriptive manner without any theory or particular genesis attached. The existence of some peneplains, and peneplanation as a process in nature, is not without controversy, due to a lack of contemporary examples and uncertainty in identifying relic examples. By some definitions, peneplains grade down to a base level represented by sea level , yet in other definitions such

12084-614: The western part of the Vermilion Range. It is in narrow belts, and consists of cherts, hematite, magnetite and small amounts of pyrite . The narrow belts trend east–northeast with the widest part to the southwest. These iron-bearing rocks are of sedimentary origin which overlie an igneous series. The iron formation is tightly folded with greenstone . and is overlain by granites in the Vermilion , Trout, Burntside , Basswood and Saganaga lake areas. The Cuyuna Iron Range

12198-529: Was a key process in the deposition of banded iron formation, the role of oxygenic versus anoxygenic photosynthesis continues to be debated, and nonbiogenic processes have also been proposed. Cloud's original hypothesis was that ferrous iron was oxidized in a straightforward manner by molecular oxygen present in the water: The oxygen comes from the photosynthetic activities of cyanobacteria. Oxidation of ferrous iron may have been hastened by aerobic iron-oxidizing bacteria, which can increase rates of oxidation by

12312-406: Was a result of self-poisoning by early cyanobacteria as the supply of reduced iron was periodically depleted. Mesobanding has also been interpreted as a secondary structure, not present in the sediments as originally laid down, but produced during compaction of the sediments. Another theory is that mesobands are primary structures resulting from pulses of activity along mid-ocean ridges that change

12426-565: Was accompanied by the development of a stratified ocean with a deep anoxic layer and a shallow oxidized layer. The end of deposition of BIF at 1.85 billion years ago is attributed to the oxidation of the deep ocean. Until 1992 it was assumed that the rare, later (younger) banded iron deposits represented unusual conditions where oxygen was depleted locally. Iron-rich waters would then form in isolation and subsequently come into contact with oxygenated water. The Snowball Earth hypothesis provided an alternative explanation for these younger deposits. In

12540-479: Was blocked by precipitation as pyrite. Banded iron formations in northern Minnesota are overlain by a thick layer of ejecta from the Sudbury Basin impact. An asteroid (estimated at 10 km (6.2 mi) across) impacted into waters about 1,000 m (3,300 ft) deep 1.849 billion years ago, coincident with the pause in BIF deposition. Computer models suggest that the impact would have generated

12654-660: Was coined in the iron districts of Lake Superior , where the ore deposits of the Mesabi, Marquette , Cuyuna, Gogebic , and Menominee iron ranges were also variously known as "jasper", "jaspilite", "iron-bearing formation", or taconite . Banded iron formations were described as "itabarite" in Brazil, as "ironstone" in South Africa, and as "BHQ" (banded hematite quartzite) in India. Peneplain In geomorphology and geology ,

12768-640: Was formed during the Midcontinent Rift. The Gunflint Iron Formation is 1,878 ± 2 million years old. It lies on top of a basal conglomerate, unlike the Biwabik Iron Formation which was deposited on top of the Pokegama Quartzite in the Mesabi Range, and the Cuyuna Iron Formation which was deposited on top of the Mille Lacs and North ranges. It is 150 km (93 mi) long, less than 8 km (5.0 mi) wide, and 135 to 170 m (443 to 558 ft) thick. This iron formation lies in

12882-414: Was oxidized; iron reacts with oxygen to form compounds that precipitate out – including hematite , limonite and siderite . These iron compounds precipitated from the seawater in varying proportions with chert, producing banded-iron formations. These iron formations are abundant in the Lake Superior region. The Sudbury Impact event occurred 1,850 million years ago; it is theorized that this caused

12996-432: Was thought to be the result of the evolution of mechanisms for living with oxygen. This ended self-poisoning and produced a population explosion in the cyanobacteria that rapidly depleted the remaining supply of reduced iron and ended most BIF deposition. Oxygen then began to accumulate in the atmosphere. Some details of Cloud's original model were abandoned. For example, improved dating of Precambrian strata has shown that

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