Cryogenian

The Cryogenian ( /kraɪoʊˈdʒɛniən/, from Greek κρύος (krýos), meaning "cold" and γένεσις (génesis), meaning "birth") is a geologic period that lasted from 720 to 635 million years ago.[5] It forms the second geologic period of the Neoproterozoic Era, preceded by the Tonian Period and followed by the Ediacaran.

The Sturtian and Marinoan glaciations occurred during the Cryogenian period,[6] which are the greatest ice ages known to have occurred on Earth. These events are the subject of much scientific controversy. The main debate contests whether these glaciations covered the entire planet (the so-called "Snowball Earth") or a band of open sea survived near the equator (termed "slushball Earth").

Cryogenian Period
720–635 million years ago
Mean atmospheric O
2
content over period duration
c. 12 vol %
(60 % of modern level)
Mean atmospheric CO
2
content over period duration
c. 1300 ppm
(5 times pre-industrial level)
Mean surface temperature over period duration c. 5 °C
(-9 °C above modern level)
Events of the Cryogenian Period
-760 —
-740 —
-720 —
-700 —
-680 —
-660 —
-640 —
-620 —

Ratification

The Cryogenian period was ratified in 1990 by the International Commission on Stratigraphy.[7] In contrast to most other time periods, the beginning of the Cryogenian is not linked to a globally observable and documented event. Instead, the base of the period is defined by a fixed rock age, that was originally set at 850 million years,[8] but changed in 2015 to 720 million years.[5]

This is problematic because estimates of rock ages are variable and are subject to laboratory error. For instance, the time scale of the Cambrian Period is not reckoned by rock younger than a given age (541 million years), but by the appearance of the worldwide Treptichnus pedum diagnostic trace fossil assemblages. This means that rocks can be recognized as Cambrian when examined in the field and do not require extensive testing to be performed in a lab to find a date.

Currently, there is no consensus on what global event is a suitable candidate to mark the start of the Cryogenian Period, but a global glaciation would be a likely candidate.[8]

Climate

The name of the geologic period refers to the very cold global climate of the Cryogenian.

Characteristic glacial deposits indicate that Earth suffered the most severe ice ages in its history during this period (Sturtian and Marinoan). According to Eyles and Young, "Late Proterozoic glaciogenic deposits are known from all the continents. They provide evidence of the most widespread and long-ranging glaciation on Earth." Several glacial periods are evident, interspersed with periods of relatively warm climate, with glaciers reaching sea level in low paleolatitudes.[9]

Glaciers extended and contracted in a series of rhythmic pulses, possibly reaching as far as the equator.[10]

The Cryogenian is generally considered to be divisible into at least two major worldwide glaciations. The Sturtian glaciation persisted from 720 to 660 million years ago, and the Marinoan glaciation which ended approximately 635 Ma, at the end of the Cryogenian.[11] The deposits of glacial tillite also occur in places that were at low latitudes during the Cryogenian, a phenomenon which led to the hypothesis of deeply frozen planetary oceans called "Snowball Earth".[12]

Paleogeography

Before the start of the Cryogenian, around 750 Ma, the cratons that made up the supercontinent Rodinia started to rift apart. The superocean Mirovia began to close while the superocean Panthalassa began to form. The cratons (possibly) later assembled into another supercontinent called Pannotia, in the Ediacaran.

Eyles and Young state, "Most Neoproterozoic glacial deposits accumulated as glacially influenced marine strata along rifted continental margins or interiors." Worldwide deposition of dolomite might have reduced atmospheric carbon dioxide. The break up along the margins of Laurentia at about 750 Ma occurs at about the same time as the deposition of the Rapitan Group in North America, contemporaneously with the Sturtian in Australia. A similar period of rifting at about 650 Ma occurred with the deposition of the Ice Brook Formation in North America, contemporaneously with the Marinoan in Australia.[9] The Sturtian and Marinoan are local divisions within the Adelaide Rift Complex.

Cryogenian biota and fossils

Fossils of testate amoeba (or Arcellinida) first appear during the Cryogenian period.[13] During the Cryogenian period, the oldest known fossils of sponges (and therefore animals) make an appearance.[14][15][16] The issue of whether or not biology was impacted by this event has not been settled, for example Porter (2000) suggests that new groups of life evolved during this period, including the red algae and green algae, stramenopiles, ciliates, dinoflagellates, and testate amoeba.[17] The end of the period also saw the origin of heterotrophic plankton, which would feed on unicellular algae and prokaryotes, ending the bacterial dominance of the oceans.[18]

In popular culture

See also

References

  1. ^ a b Arnaud, Emmanuelle; Halverson, Galen P.; Shields-Zhou, Graham Anthony (30 November 2011). "Chapter 1 The geological record of Neoproterozoic ice ages". Memoirs. Geological Society of London. 36 (1): 1–16. doi:10.1144/M36.1.
  2. ^ Pu, Judy P.; Bowring, Samuel A.; Ramezani, Jahandar; Myrow, Paul; Raub, Timothy D.; Landing, Ed; Mills, Andrea; Hodgin, Eben; MacDonald, Francis A. (2016). "Dodging snowballs: Geochronology of the Gaskiers glaciation and the first appearance of the Ediacaran biota". Geology. 44 (11): 955. doi:10.1130/G38284.1.
  3. ^ Macdonald, F. A.; Schmitz, M. D.; Crowley, J. L.; Roots, C. F.; Jones, D. S.; Maloof, A. C.; Strauss, J. V.; Cohen, P. A.; Johnston, D. T.; Schrag, D. P. (4 March 2010). "Calibrating the Cryogenian". Science. 327 (5970): 1241–1243. doi:10.1126/science.1183325. PMID 20203045. (Duration and magnitude are enigmatic)
  4. ^ "Press release: Discovery of Possible Earliest Animal Life Pushes Back Fossil Record". National Science Foundation. August 17, 2010.
  5. ^ a b "Chart". International Commission on Stratigraphy. Archived from the original on 13 January 2017. Retrieved 14 February 2017.
  6. ^ These events were formerly considered together as the Varanger glaciations, from their first detection in Norway's Varanger Peninsula.
  7. ^ Plumb, Kenneth A. (1991). "New Precambrian time scale" (PDF). Episode. 2. 14: 134–140. Retrieved 7 September 2013.
  8. ^ a b "GSSP Table - Precambrian". Geologic Timescale Foundation. Retrieved 7 September 2013.
  9. ^ a b Eyles, Nicholas; Young, Grant (1994). Deynoux, M.; Miller, J.M.G.; Domack, E.W.; Eyles, N.; Fairchild, I.J.; Young, G.M. (eds.). Geodynamic controls on glaciation in Earth history, in Earth's Glacial Record. Cambridge: Cambridge University Press. pp. 5–10. ISBN 0521548039.
  10. ^ Dave Lawrence (2003). "Microfossil lineages support sloshy snowball Earth". Geotimes.
  11. ^ Shields, G. A. (2008). "Palaeoclimate: Marinoan meltdown". Nature Geoscience. 1 (6): 351–353. Bibcode:2008NatGe...1..351S. doi:10.1038/ngeo214.
  12. ^ Hoffman, P.F. 2001. Snowball Earth theory
  13. ^ Porter, S.A. & Knoll, A.H. (2000). "Testate amoeba in the Neoproterozoic Era: evidence from vase-shaped microfossils in the Chuar Group, Grand Canyon". Paleobiology. 26 (3): 360–385. doi:10.1666/0094-8373(2000)026<0360:TAITNE>2.0.CO;2. ISSN 0094-8373.
  14. ^ Love; Grosjean, Emmanuelle; Stalvies, Charlotte; Fike, David A.; Grotzinger, John P.; Bradley, Alexander S.; Kelly, Amy E.; Bhatia, Maya; Meredith, William; et al. (2009). "Fossil steroids record the appearance of Demospongiae during the Cryogenian period" (PDF). Nature. 457 (7230): 718–721. Bibcode:2009Natur.457..718L. doi:10.1038/nature07673. PMID 19194449.
  15. ^ Maloof, Adam C.; Rose, Catherine V.; Beach, Robert; Samuels, Bradley M.; Calmet, Claire C.; Erwin, Douglas H.; Poirier, Gerald R.; Yao, Nan; Simons, Frederik J. (17 August 2010). "Possible animal-body fossils in pre-Marinoan limestones from South Australia". Nature Geoscience. 3 (9): 653–659. Bibcode:2010NatGe...3..653M. doi:10.1038/ngeo934.
  16. ^ "Discovery of possible earliest animal life pushes back fossil record". 2010-08-17.
  17. ^ http://palaeos.com/proterozoic/neoproterozoic/cryogenian/cryogenian2.html
  18. ^ Fossil fats reveal how complex life kicked off after Snowball Earth phase

Further reading

Andean-Saharan glaciation

The Andean-Saharan glaciation occurred during the Paleozoic from 450 Ma to 420 Ma, during the late Ordovician and the Silurian period. For the Ordovician/Saharan part, see the more extensive article on the Late Ordovician glaciation.

According to Eyles and Young, "A major glacial episode at c. 440 Ma, is recorded in Late Ordovician strata (predominantly Ashgillian) in West Africa (Tamadjert Formation of the Sahara), in Morocco (Tindouf Basin) and in west-central Saudi Arabia, all areas at polar latitudes at the time. From the Late Ordovician to the Early Silurian the centre of glaciation moved from northern Africa to southwestern South America."During this period glaciation is known from Arabia, Sahara, West Africa, the south Amazon, and the Andes. The center of glaciation migrated from Sahara in the Ordovician (450–440 Ma) to South America in the Silurian (440–420 Ma). The maximum extent of glaciation developed in Africa and eastern Brazil.A minor ice age, the Andean-Saharan was preceded by the Cryogenian ice ages (720–630 Ma, the Sturtian and Marinoan glaciations), often referred to as Snowball Earth, and followed by the Karoo Ice Age (350–260 Ma).

Animal

Animals are multicellular eukaryotic organisms that form the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and grow from a hollow sphere of cells, the blastula, during embryonic development. Over 1.5 million living animal species have been described—of which around 1 million are insects—but it has been estimated there are over 7 million animal species in total. Animals range in length from 8.5 millionths of a metre to 33.6 metres (110 ft). They have complex interactions with each other and their environments, forming intricate food webs. The kingdom Animalia includes humans, but in colloquial use the term animal often refers only to non-human animals. The study of non-human animals is known as zoology.

Most living animal species are in the Bilateria, a clade whose members have a bilaterally symmetric body plan. The Bilateria include the protostomes—in which many groups of invertebrates are found, such as nematodes, arthropods, and molluscs—and the deuterostomes, containing both the echinoderms as well as the chordates, the latter containing the vertebrates. Life forms interpreted as early animals were present in the Ediacaran biota of the late Precambrian. Many modern animal phyla became clearly established in the fossil record as marine species during the Cambrian explosion, which began around 542 million years ago. 6,331 groups of genes common to all living animals have been identified; these may have arisen from a single common ancestor that lived 650 million years ago.

Historically, Aristotle divided animals into those with blood and those without. Carl Linnaeus created the first hierarchical biological classification for animals in 1758 with his Systema Naturae, which Jean-Baptiste Lamarck expanded into 14 phyla by 1809. In 1874, Ernst Haeckel divided the animal kingdom into the multicellular Metazoa (now synonymous with Animalia) and the Protozoa, single-celled organisms no longer considered animals. In modern times, the biological classification of animals relies on advanced techniques, such as molecular phylogenetics, which are effective at demonstrating the evolutionary relationships between animal taxa.

Humans make use of many other animal species for food, including meat, milk, and eggs; for materials, such as leather and wool; as pets; and as working animals for power and transport. Dogs have been used in hunting, while many terrestrial and aquatic animals are hunted for sport. Non-human animals have appeared in art from the earliest times and are featured in mythology and religion.

Brasiliano orogeny

Brasiliano orogeny or Brasiliano cycle (Portuguese: Orogênese Brasiliana and Ciclo Brasiliano) refers to a series of orogenies of Neoproterozoic age exposed chiefly in Brazil but also in other parts of South America. The Brasiliano orogeny is a regional name for the larger Pan-African/Brasiliano orogeny that extended not only in South America but across most of Gondwana. In a wide sense the Brasiliano orogeny includes also the Pampean orogeny. Almeida et al. coined the term Brasiliano Orogenic Cycle in 1973. The orogeny led to the closure of several oceans and aulacogens including the Adamastor Ocean, the Goianides Ocean, the Puncoviscana Ocean and the Peri-Franciscano Ocean.Attempts to correlate the South American Brasiliano belts with the African Pan-African belts on the other side of the Atlantic has in many cases been problematic.

Demosponge

Demosponges (Class Demospongiae) are the most diverse class in the phylum Porifera. They include 76.2% of all species of sponges with nearly 8,800 species worldwide (World Porifera Database). They are sponges with a soft body that covers a hard, often massive skeleton made of calcium carbonate, either aragonite or calcite. They are predominantly leuconoid in structure. Their "skeletons" are made of spicules consisting of fibers of the protein spongin, the mineral silica, or both. Where spicules of silica are present, they have a different shape from those in the otherwise similar glass sponges.The many diverse orders in this class include all of the large sponges. Most are marine dwellers, but one order (Spongillida) live in freshwater environments. Some species are brightly colored, with great variety in body shape; the largest species are over 1 m (3.3 ft) across. They reproduce both sexually and asexually. They are the only extant organisms that methylate sterols at the 26-position, a fact used to identify the presence of demosponges before their first known unambiguous fossils.Because of their long life span (500–1,000 years) it is thought that analysis of the aragonite skeletons of these sponges could extend data regarding ocean temperature, salinity, and other variables farther into the past than has been previously possible. Their dense skeletons are deposited in an organized chronological manner, in concentric layers or bands. The layered skeletons look similar to reef corals. Therefore, demosponges are also called coralline sponges.

Ediacaran

The Ediacaran Period ( ), spans 94 million years from the end of the Cryogenian Period 635 million years ago (Mya), to the beginning of the Cambrian Period 541 Mya. It marks the end of the Proterozoic Eon, and the beginning of the Phanerozoic Eon. It is named after the Ediacara Hills of South Australia.

The Ediacaran Period's status as an official geological period was ratified in 2004 by the International Union of Geological Sciences (IUGS), making it the first new geological period declared in 120 years.

Although the period takes its name from the Ediacara Hills where geologist Reg Sprigg first discovered fossils of the eponymous Ediacara biota in 1946, the type section is located in the bed of the Enorama Creek within Brachina Gorge in the Flinders Ranges of South Australia, at 31°19′53.8″S 138°38′0.1″E.

Kaigas

The Kaigas glaciation was a hypothesized snowball earth event in the Neoproterozoic Era, preceding the Sturtian glaciation. Its occurrence was inferred based on the interpretation of Kaigas Formation conglomerates in the stratigraphy overlying the Kalahari Craton as correlative with pre-Sturtian Numees formation glacial diamictites. However, the Kaigas formation was later determined to be non-glacial, and a Sturtian age was assigned to the Numees diamictites. Thus, there is no longer any evidence for a Neoproterozoic glaciation prior to the Sturtian snowball earth event.

Marinoan glaciation

The Marinoan glaciation was a period of worldwide glaciation that lasted from approximately 650 to 635 Ma (million years ago) during the Cryogenian period. The glaciation may have covered the entire planet, in an event called the Snowball Earth. The end of the glaciation may have been sped by the release of methane from equatorial permafrost.

Neoproterozoic

The Neoproterozoic Era is the unit of geologic time from 1,000 to 541 million years ago.It is the last era of the Precambrian Supereon and the Proterozoic Eon; it is subdivided into the Tonian, Cryogenian, and Ediacaran Periods. It is preceded by the Mesoproterozoic era and succeeded by the Paleozoic era.

The most severe glaciation known in the geologic record occurred during the Cryogenian, when ice sheets reached the equator and formed a possible "Snowball Earth".

The earliest fossils of multicellular life are found in the Ediacaran, including the Ediacarans, which were the earliest animals.

According to Rino and co-workers, the sum of the continental crust formed in the Pan-African orogeny and the Grenville orogeny makes the Neoproterozoic the period of Earth's history that has produced most continental crust.

Nimbia occlusa

Nimbia occlusa is a form of Ediacaran fossil shaped like a circular or oval disk, with a thick rim around the margin. Within the rim the fossil is usually flat, but may have a central nipple or dimple. These fossils were generally believed to be those of cnidarians, but they have since been reinterpreted as structures made by microbial colonies (Grazhdankin, see Ediacaran biota for references and discussion). They can reach up to 6 cm in diameter, with a centimeter-thick rim. Some fossils are distorted.

Nimbia occurs in numerous locations across a large range of time, which lends weight to theories that the fossil does not represent a single animal species. It occurs in the Twitya Formation in the Mackenzie Mountains in Canada dated at 610 million years ago, near the end of the Marinoan glaciation, and in 770 million years ago rocks in Kazakhstan. Aspidella also appears in these areas. Morania and Beltina carbonaceous film fossils in the Twitya Formation are not considered to be animals. Nimbia-like fossils have also been found in the Cambrian period.

Otavia

Otavia antiqua is an early sponge-like fossil found in Namibia in the Etosha National Park. It is claimed to be the oldest animal fossil, being found in rock aged between 760 and 550 million years ago. The genus was named after the Otavi Group in which the fossils were found. The oldest fossils are from the Tonian period, before the Cryogenian glaciations, but the latest found were from the Nama Group rocks, which are from the Ediacaran period.The shape of the fossils is irregular but rounded. The size varies from a third of a millimetre to 5 mm (0.20 in). They are hollow inside, and have many small, osculum-like holes connecting the interior to the outside. The material of the outer wall is predominantly calcium phosphate.The affinities of these fossils, along with other paleontological evidence for precambrian sponges, are disputed.

Proterozoic

The Proterozoic ( ) is a geological eon spanning the time from the appearance of oxygen in Earth's atmosphere to just before the proliferation of complex life (such as trilobites or corals) on the Earth. The name Proterozoic combines the two forms of ultimately Greek origin: protero- meaning "former, earlier", and -zoic, a suffix related to zoe "life". The Proterozoic Eon extended from 2500 mya to 541 mya (million years ago), and is the most recent part of the Precambrian "supereon." The Proterozoic is the longest eon of the Earth's geologic time scale and it is subdivided into three geologic eras (from oldest to youngest): the Paleoproterozoic, Mesoproterozoic, and Neoproterozoic.The well-identified events of this eon were the transition to an oxygenated atmosphere during the Paleoproterozoic; several glaciations, which produced the hypothesized Snowball Earth during the Cryogenian Period in the late Neoproterozoic Era; and the Ediacaran Period (635 to 541 Ma) which is characterized by the evolution of abundant soft-bodied multicellular organisms and provides us with the first obvious fossil evidence of life on earth.

Reusch's Moraine

The Smalfjord diamictite, Bigganjargga Tillite or Reusch's Moraine is a diamictite in Finnmark, northern Norway. The rock was first identified as a tillite by Hans Reusch in 1891, hence its name. The tillite overlies sandstone whose contact surface is striated. Reusch's Moraine belong to the Smalfjord Formation, a geological formation of Neoproterozoic age. The tillite possibly formed during the Sturtian glaciation in connection to a global glaciation.Reusch's Moraine was among the first sites discovered to bear evidence of Precambrian glaciations, being only preceded by findings in Scotland (1871), Australia (1884) and India (1887). The site is remarkably illustrative and Reusch's 1891 sketch has been labeled "iconic" by Paul F. Hoffman. The area has been protected by law at least since the 1960s and it is forbidden to hammer the rocks.

Riphean (stage)

The Riphean is a stage or age of the geologic timescale from 1,600 to 650 million years ago. The name Riphean is used in the Proterozoic stratigraphy of Russia and the Fennoscandian Shield in Finland. It was also used in a number of older international geologic timescales but, in the most recent timescales of the ICS, it is replaced by the Calymmian, Ectasian, Stenian, Tonian and Cryogenian periods of the Neoproterozoic and Mesoproterozoic eras.

The word 'Riphean' comes from the ancient Riphean Mountains, sometimes identified with the Ural Mountains.

The Riphean has been divided by geologists into the Early Riphean (1600–1400 Ma), Middle Riphean (1400–1000 Ma) and Late Riphean (1000–650 Ma) subdivisions.

Rodinia

Rodinia (from the Russian родить, rodít, meaning "to beget, to give birth", or родина, ródina, meaning "motherland, birthplace") was a Neoproterozoic supercontinent that assembled 1.1–0.9 billion years ago and broke up 750–633 million years ago.Valentine & Moores 1970 were probably the first to recognise a Precambrian supercontinent, which they named 'Pangaea I'. It was renamed 'Rodinia' by McMenamin & McMenamin 1990 who also were the first to produce a reconstruction and propose a temporal framework for the supercontinent.Rodinia formed at c. 1.23 Ga by accretion and collision of fragments produced by breakup of an older supercontinent, Columbia, assembled by global-scale 2.0–1.8 Ga collisional events.Rodinia broke up in the Neoproterozoic with its continental fragments reassembled to form Pannotia 633–573 million years ago. In contrast with Pannotia, little is known yet about the exact configuration and geodynamic history of Rodinia. Paleomagnetic evidence provides some clues to the paleolatitude of individual pieces of the Earth's crust, but not to their longitude, which geologists have pieced together by comparing similar geologic features, often now widely dispersed.

The extreme cooling of the global climate around 717–635 million years ago (the so-called Snowball Earth of the Cryogenian Period) and the rapid evolution of primitive life during the subsequent Ediacaran and Cambrian periods are thought to have been triggered by the breaking up of Rodinia or to a slowing down of tectonic processes.

Snowball Earth

The Snowball Earth hypothesis proposes that during one or more of Earth's icehouse climates, Earth's surface became entirely or nearly entirely frozen at least once, sometime earlier than 650 Mya (million years ago). Proponents of the hypothesis argue that it best explains sedimentary deposits generally regarded as of glacial origin at tropical palaeolatitudes and other enigmatic features in the geological record. Opponents of the hypothesis contest the implications of the geological evidence for global glaciation and the geophysical feasibility of an ice- or slush-covered ocean and emphasize the difficulty of escaping an all-frozen condition. A number of unanswered questions remain, including whether the Earth was a full snowball, or a "slushball" with a thin equatorial band of open (or seasonally open) water.

The snowball-Earth episodes are proposed to have occurred before the sudden radiation of multicellular bioforms, known as the Cambrian explosion. The most recent snowball episode may have triggered the evolution of multicellularity. Another, much earlier and longer snowball episode, the Huronian glaciation, which would have occurred 2400 to 2100 Mya, may have been triggered by the first appearance of oxygen in the atmosphere, the "Great Oxygenation Event".

Sturt Gorge Recreation Park

The Sturt Gorge Recreation Park is a protected area in the Australian state of South Australia located in the suburbs of Bellevue Heights, Craigburn Farm and Flagstaff Hill within the Adelaide metropolitan area about 13 kilometres (8.1 mi) south of the Adelaide central business district.The park was established in 1973. and protects an area recognised as an area of great geological significance. It channels the Sturt River down to the Adelaide Plains.

The Sturt Tillite formation was the first area in the world to provide definite evidence of Cryogenian glaciation (the Snowball Earth). It is hypothesised that the landform was created from glacial material that dropped from ice floating in the ocean which covered the area 800 million years ago.All kinds of fires are prohibited in the park.It is classified as an IUCN Category III protected area. In 1980, the recreation park was listed on the former Register of the National Estate.

Sturtian glaciation

The Sturtian glaciation was a glaciation, or perhaps multiple glaciations, during the Cryogenian Period, when the Earth experienced repeated large-scale glaciations. The duration of the Sturtian glaciation has been variously defined, with dates ranging from 717 to 643 Ma. Stern et al. place the period at 715 to 680 Ma.According to Eyles and Young, "Glaciogenic rocks figure prominently in the Neoproterozoic stratigraphy of southeastern Australia and the northern Canadian Cordillera. The Sturtian glaciogenic succession (c. 740 Ma) unconformably overlies rocks of the Burra Group." The Sturtian succession includes two major diamictite-mudstone sequences, which represent glacial advance and retreat cycles. It is stratigraphically correlated with the Rapitan Group of North America.The Sturtian is named after the Sturt River Gorge, near Bellevue Heights, South Australia.

Reusch's Moraine in northern Norway may have been deposited during this period.

Söderfjärden

Söderfjärden is a polder in Ostrobothnia, western Finland, 10 kilometres south of the town of Vaasa. The plain is in an impact crater which was made at least some 640 million years old (Proterozoic, near the end of the Cryogenian). The crater's diameter is 6.6 km and its maximum depth is 300 metres. It is filled with Cambrian sandstones leaving only the outer rim visible. There is also a central uplift, which is buried. After rising from the sea due to post-glacial rebound, Söderfjärden was a wetland but was later drained with help of a pump station. It is currently cultivated and is clearly visible from air as a large circular field. This makes Söderfjärden unique among the other impact structures in Finland which are at least partially below sea level.

Tonian

The Tonian (from Greek τόνος (tónos), meaning "stretch") is the first geologic period of the Neoproterozoic Era. It lasted from 1000 Mya to 720 Mya (million years ago). Instead of being based on stratigraphy, these dates are defined by the ICS based on radiometric chronometry. The Tonian is preceded by the Stenian Period of the Mesoproterozoic era and followed by the Cryogenian.

Rifting leading to the breakup of supercontinent Rodinia, which had formed in the mid-Stenian, occurred during this period, starting from 900 to 850 Mya.

Quaternary /
Late Cenozoic
CarboniferousPermian
OrdovicianSilurian
Ediacaran
Cryogenian
Paleoproterozoic
Mesoarchean
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