Geological period

A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place.

These periods form elements of a hierarchy of divisions into which geologists have split the Earth's history.

Eons and eras are larger subdivisions than periods while periods themselves may be divided into epochs and ages.

The rocks formed during a period belong to a stratigraphic unit called a system.

Structure

The twelve currently recognised periods of the present eon – the Phanerozoic – are defined by the International Commission on Stratigraphy (ICS) by reference to the stratigraphy at particular locations around the world.

In 2004 the Ediacaran Period of the latest Precambrian was defined in similar fashion, and was the first such newly designated period in 130 years; but earlier periods are simply defined by age.

A consequence of this approach to the Phanerozoic periods is that the ages of their beginnings and ends can change from time to time as the absolute age of the chosen rock sequences, which define them, is more precisely determined.

The set of rocks (sedimentary, igneous or metamorphic) that formed during a geological period is known as a system, so for example the 'Jurassic System' of rocks was formed during the 'Jurassic Period' (between 201 and 145 million years ago).

The following table includes all currently recognized periods. The table omits the time before 2500 million years ago, which is not divided into periods.

Eon Era Period Extent, Million
Years Ago
Duration, Millions
of Years
Phanerozoic Cenozoic Quaternary (Pleistocene/Holocene) 2.588–0 2.588+
Neogene (Miocene/Pliocene) 23.03–2.588 20.4
Paleogene (Paleocene/Eocene/Oligocene) 66.0–23.03 42.9
Mesozoic Cretaceous 145.5–66.0 79.5
Jurassic 201.3–145.0 56.3
Triassic 252.17–201.3 50.9
Paleozoic Permian 298.9–252.17 46.7
Carboniferous (Mississippian/Pennsylvanian) 358.9–298.9 60
Devonian 419.2–358.9 60.3
Silurian 443.4–419.2 24.2
Ordovician 485.4–443.4 42
Cambrian 541.0–485.4 55.6
Proterozoic Neoproterozoic Ediacaran 635.0–541.0 94
Cryogenian 850–635 215
Tonian 1000–850 150
Mesoproterozoic Stenian 1200–1000 200
Ectasian 1400–1200 200
Calymmian 1600–1400 200
Paleoproterozoic Statherian 1800–1600 200
Orosirian 2050–1800 250
Rhyacian 2300–2050 250
Siderian 2500–2300 200

See Geologic time scale#Proposed Precambrian timeline for another set of periods 4600–541 MYA.

Units in geochronology and stratigraphy[1]
Segments of rock (strata) in chronostratigraphy Time spans in geochronology Notes to
geochronological units
Eonothem Eon 4 total, half a billion years or more
Erathem Era 10 defined, several hundred million years
System Period 22 defined, tens to ~one hundred million years
Series Epoch 34 defined, tens of millions of years
Stage Age 99 defined, millions of years
Chronozone Chron subdivision of an age, not used by the ICS timescale

Correlation issues

In a steady effort ongoing since 1974, the International Commission on Stratigraphy has been working to correlate the world's local stratigraphic record into one uniform planet-wide benchmarked system.

American geologists have long considered the Mississippian and Pennsylvanian to be periods in their own right though the ICS now recognises them both as 'subperiods' of the Carboniferous Period recognised by European geologists. Cases like this in China, Russia and even New Zealand with other geological eras has slowed down the uniform organization of the stratigraphic record.

Notable changes

  • Changes in recent years have included the abandonment of the former Tertiary Period in favour of the Paleogene and succeeding Neogene periods.
  • The abandonment of the Quaternary period was also considered but it has been retained for continuity reasons.
  • Even earlier in the history of the science, the Tertiary was considered to be an 'era' and its subdivisions (Paleocene, Eocene, Oligocene, Miocene and Pliocene) were themselves referred to as 'periods' but they now enjoy the status of 'epochs' within the more recently delineated Paleogene and Neogene periods.

See also

References

  1. ^ Cohen, K.M.; Finney, S.; Gibbard, P.L. (2015), International Chronostratigraphic Chart (PDF), International Commission on Stratigraphy.
Cambrian

The Cambrian Period ( KAM-bree-ən, KAYM-) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 55.6 million years from the end of the preceding Ediacaran Period 541 million years ago (mya) to the beginning of the Ordovician Period 485.4 mya. Its subdivisions, and its base, are somewhat in flux. The period was established (as “Cambrian series”) by Adam Sedgwick, who named it after Cambria, the Latin name of Wales, where Britain's Cambrian rocks are best exposed. The Cambrian is unique in its unusually high proportion of lagerstätte sedimentary deposits, sites of exceptional preservation where "soft" parts of organisms are preserved as well as their more resistant shells. As a result, our understanding of the Cambrian biology surpasses that of some later periods.The Cambrian marked a profound change in life on Earth; prior to the Cambrian, the majority of living organisms on the whole were small, unicellular and simple; the Precambrian Charnia being exceptional. Complex, multicellular organisms gradually became more common in the millions of years immediately preceding the Cambrian, but it was not until this period that mineralized—hence readily fossilized—organisms became common. The rapid diversification of life forms in the Cambrian, known as the Cambrian explosion, produced the first representatives of all modern animal phyla. Phylogenetic analysis has supported the view that during the Cambrian radiation, metazoa (animals) evolved monophyletically from a single common ancestor: flagellated colonial protists similar to modern choanoflagellates.

Although diverse life forms prospered in the oceans, the land is thought to have been comparatively barren—with nothing more complex than a microbial soil crust and a few molluscs that emerged to browse on the microbial biofilm. Most of the continents were probably dry and rocky due to a lack of vegetation. Shallow seas flanked the margins of several continents created during the breakup of the supercontinent Pannotia. The seas were relatively warm, and polar ice was absent for much of the period.

Doushantuo Formation

The Doushantuo Formation (Chinese: 陡山沱; pinyin: dǒu shān tuó) is a fossil Lagerstätte in Weng'an County, Guizhou Province, China that is notable for being one of the oldest beds to contain minutely preserved microfossils, phosphatic fossils that are so characteristic they have given their name to "Doushantuo type preservation". The formation is of particular interest because a part of it appears to cover the boundary between the enigmatic organisms of the Ediacaran geological period and the more familiar fauna of the Cambrian explosion where lifeforms recognizable as ancestors of later and recent lifeforms first emerged.

Taken as a whole, the Doushantuo Formation ranges from about 635 Ma (million years ago) at its base to about 551 Ma at its top, predating by perhaps five Ma the earliest of the 'classical' Ediacaran faunas from Mistaken Point on the Avalon peninsula of Newfoundland, and recording conditions up to a good forty to fifty million years before the Cambrian explosion.

Ediacaran

The Ediacaran Period ( ee-dee-AK-ə-rən), 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.

Flandrian interglacial

The Flandrian interglacial or stage is the name given by geologists and archaeologists in the British Isles to the first, and so far only, stage of the Holocene epoch (the present geological period), covering the period from around 12,000 years ago, at the end of the last glacial period to the present day. As such, it is in practice identical in span to the Holocene. Present climatological theory (based on analysis of Milankovitch cycles) forecasts that the present Flandrian climate should decline in temperature towards a global climate similar to that of the ice age. Less orbital eccentricity may have the effect of moderating this temperature downturn.The Flandrian began as the relatively short-lived Younger Dryas climate downturn came to an end. This formed the last gasp of the Devensian glaciation, the final stage of the Pleistocene epoch and is traditionally seen as the latest warm interglacial in a series that has been occurring throughout the Quaternary geological period.

The first part of the Flandrian, known as the Younger Atlantic, was a period of fairly rapid sea level rise, known as the Flandrian transgression and associated with the melting of the Fenno-Scandian, Scottish, Laurentide and Cordilleran glaciers.

Fjords were formed during the Flandrian transgression when U-shaped glaciated valleys were inundated with water.

Geochronology

Geochronology is the science of determining the age of rocks, fossils, and sediments using signatures inherent in the rocks themselves. Absolute geochronology can be accomplished through radioactive isotopes, whereas relative geochronology is provided by tools such as palaeomagnetism and stable isotope ratios. By combining multiple geochronological (and biostratigraphic) indicators the precision of the recovered age can be improved.

Geochronology is different in application from biostratigraphy, which is the science of assigning sedimentary rocks to a known geological period via describing, cataloging and comparing fossil floral and faunal assemblages. Biostratigraphy does not directly provide an absolute age determination of a rock, but merely places it within an interval of time at which that fossil assemblage is known to have coexisted. Both disciplines work together hand in hand, however, to the point where they share the same system of naming stratum (rock layers) and the time spans utilized to classify sublayers within a stratum.

The science of geochronology is the prime tool used in the discipline of chronostratigraphy, which attempts to derive absolute age dates for all fossil assemblages and determine the geologic history of the Earth and extraterrestrial bodies.

Geological Observations on South America

Geological Observations on South America is a book written by the English naturalist Charles Darwin. The book was published in 1846, and is based on his travels during the second voyage of HMS Beagle, commanded by captain Robert FitzRoy. HMS Beagle arrived in South America to map out the coastlines and islands of the region for the British Navy. On the journey, Darwin collected fossils and plants, and recorded the continent's geological features.It is the third book in a series of geology books written by Darwin, which also includes The Structure and Distribution of Coral Reefs, published in 1842, and Geological Observations on the Volcanic Islands visited during the Voyage of H.M.S. Beagle, published in 1844. It took Darwin four years to write and complete the entire series, from 1842 to 1846. According to his diaries, Geological Observations of South America was written between July 1844 to April 1845.The text contains eight chapters along with appendices on Darwin's Mesozoic and Tertiary fossils. It describes his travels through the regions of modern Chile, Brazil, and Argentina, including the Pampas, Patagonia, and the Andes. With this book Darwin became the first to describe and name Navidad Formation, the reference unit for the marine Neogene in Chile. Darwin established relatives ages for rock units in the high Andes near Potillo. Metamorphic rocks were older than intruding red granites found in the area. By establishing, with aid of fossils, a Cretaceous age for some strata in the high Andes Darwin set time constrains for uplift of the Andes. He did further posited that the western part of the Andes (hinterland) rose before the eastern part, an idea later verified to be correct not only for the part of the Andes he visited but for orogenic mountains in general.Darwin, in a letter to the geologist Charles Lyell, wrote that the book was "dreadfully dull, yet much condensed." He put a great deal of effort into writing the book, but sardonically commented that "geologists never read each other's works, and that the only object in writing a book is a proof of earnestness, and that you do not form your opinions without undergoing labour of some kind."In the book, Darwin voiced sceptical support for the "crater of elevation" theory. The theory proposed that volcanoes were not the product of lavas, but are pushed up from within. Darwin later rejected the theory when sufficient evidence was demonstrated by the geologist Charles Lyell to disprove it.

Francis Darwin, a botanist and the son of Charles Darwin, wrote that the book was significant for the "evidence which it brought forward to prove the slow interrupted elevation of the South American continent during a recent geological period." Darwin's geological work is not considered as notable as his work is biology, but nevertheless was important in advancing "the general reception of Lyell's teaching."

Geosternbergia

Geosternbergia is an extinct pteranodontid reptile from the Late Cretaceous geological period of North America. It was one of the largest pterosaur genera and had a wingspan of up to 7.25 metres (23.8 ft).

Lake Hajikabul

Hajikabul is the sixth largest lake of Azerbaijan.It is located to the south-west of Baku, near Shirvan city, in Hajigabul Rayon, not far from a railway station. Its total area is 1668 ha, maximal length - 6 km, maximal width - 3 km and depth - 5 m. The lake is supplied by waters of the Kura River, with a special channel. Temperature of water hesitates between 5 °C and 28,5 °C. Pellucidity of water is between 0,06 and 2,5 m. Barbel, carp, sheatfish, pike, zander and grass carp are commercial fishes of the lake. Water plants such as reed mace, reed, bulrush, buttercup and hornwort are met in the lake.The lake is located in Kur-Araz Lowland. It formed in the result of withdrawal of the Caspian Sea in a definite geological period. Spring flows of the Kura River increase the area of the lake.It doesn’t freeze in winter and has a great significance for hibernation of migratory birds.

Maastrichtian

The Maastrichtian ( ) is, in the ICS geologic timescale, the latest age (uppermost stage) of the Late Cretaceous epoch or Upper Cretaceous series, the Cretaceous period or system, and of the Mesozoic era or erathem. It spanned the interval from 72.1 to 66 million years ago. The Maastrichtian was preceded by the Campanian and succeeded by the Danian (part of the Paleogene and Paleocene).At the end of this period, there was a mass extinction known as the Cretaceous–Paleogene extinction event (formerly known as the Cretaceous–Tertiary extinction event). In this extinction event, many commonly recognized groups such as non-avian dinosaurs, plesiosaurs and mosasaurs, as well as many other lesser-known groups, died out. The cause of the extinction is most commonly linked to an asteroid about 10 to 15 kilometres (6.2 to 9.3 mi) wide colliding with Earth at the end of the Cretaceous.

Mangala Fossa

Mangala Fossa is a graben in the Memnonia quadrangle of Mars, located near 11.6°S 151.0°W / -11.6; -151.0, which originated in the Hesperian and Amazonian epochs. The graben is located at the head of the outflow channel Mangala Valles, which is thought to have been formed by at least two catastrophic flood events during the same geological period, leading to the release of vast quantities of water from Mangala Fossa onto the Martian surface. The flooding was probably initiated by the emplacement of a dike radiating from the volcano Arsia Mons, resulting in the formation of the graben, Mangala Fossa, at the channels' head. This dike breached a pressurized aquifer trapped beneath a thick "cryosphere" (layer of frozen ground) beneath the surface. As the floor of the graben subsided, water found its way up one or both of the faults in the crust that defined the edges of the graben and spilled into the depression, eventually filling it and overflowing at the lowest point on the rim to erode the Mangala Valles channels."Mangala" is the name for Mars in Jyotish (or Hindu) astrology.

Mayan

Mayan most commonly refers to:

Maya peoples, various indigenous peoples of Mesoamerica and northern Central America

Maya civilization, pre-Columbian culture of Mesoamerica and northern Central America

Mayan languages, language family spoken in Mesoamerica and northern Central America

Yucatec Maya language, language spoken in the Yucatán Peninsula and northern BelizeMayan or Mayan may also refer to:

Mayan, Semnan, Iran

Mayan stage, geological period that occurred during the end of the Middle Cambrian

Mayan (band), a Dutch symphonic death-metal band

Mayan (software)

Neogene

The Neogene ( NEE-ə-jeen, NEE-oh-) (informally Upper Tertiary or Late Tertiary) is a geologic period and system that spans 20.45 million years from the end of the Paleogene Period 23.03 million years ago (Mya) to the beginning of the present Quaternary Period 2.58 Mya. The Neogene is sub-divided into two epochs, the earlier Miocene and the later Pliocene. Some geologists assert that the Neogene cannot be clearly delineated from the modern geological period, the Quaternary. The term "Neogene" was coined in 1853 by the Austrian palaeontologist Moritz Hörnes (1815–1868).During this period, mammals and birds continued to evolve into roughly modern forms, while other groups of life remained relatively unchanged. Early hominids, the ancestors of humans, appeared in Africa near the end of the period. Some continental movement took place, the most significant event being the connection of North and South America at the Isthmus of Panama, late in the Pliocene. This cut off the warm ocean currents from the Pacific to the Atlantic Ocean, leaving only the Gulf Stream to transfer heat to the Arctic Ocean. The global climate cooled considerably over the course of the Neogene, culminating in a series of continental glaciations in the Quaternary Period that follows.

Neotherapsida

The Neotherapsida are a clade of therapsids. The clade includes anomodonts and the more derived theriodonts, which include mammals.

Norian

The Norian is a division of the Triassic geological period. It has the rank of an age (geochronology) or stage (chronostratigraphy). The Norian lasted from ~227 to 208.5 million years ago. It was preceded by the Carnian and succeeded by the Rhaetian.

Ordovices

The Ordovices were one of the Celtic tribes living in Great Britain before the Roman invasion. Their tribal lands were located in present-day North Wales and England between the Silures to the south and the Deceangli to the north-east. The Ordovices were partially conquered by the Roman governor Gnaeus Julius Agricola in the campaign of AD 77–78.

The Celtic name *ordo-wik- could be cognate with the words for "hammer": Irish 'Ord', Welsh 'Gordd' (with a G- prothetic) and Breton 'Horzh' (with a H- prothetic).

The Ordovices farmed and kept sheep, and built fortified strongholds and hill forts. They were among the few British tribes that resisted the Roman invasion. The resistance was mainly organised by the Celtic leader Caratacus, exiled in their lands after the defeat of his tribe in the Battle of the Medway. Caratacus became the warlord of the Ordovices and neighbouring Silures, and a Roman public enemy in the 50s AD. Following the Battle of Caer Caradoc, where governor Publius Ostorius Scapula defeated Caratacus, the Ordovices were no longer a threat to Rome, probably due to heavy losses.

In the 70s, the Ordovices rebelled against Roman occupation and destroyed a cavalry squadron. This act of war provoked an equally strong response from Agricola, who, according to Tacitus, exterminated almost the whole tribe. No other mention of the tribe appears in the historical records, but in view of the mountainous terrain of the lands of the Ordovices, it is questionable whether Agricola could have wiped out the entire population.

The name of this tribe appears to be preserved in the place name Dinorwig ("Fort of the Ordovices") in North Wales.

The Ordovician geological period was first described by Charles Lapworth in 1879, based on rocks located in the lands of the Ordovices.

Ordovician

The Ordovician ( or-də-VISH-ee-ən, -⁠doh-, -⁠VISH-ən) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.2 million years from the end of the Cambrian Period 485.4 million years ago (Mya) to the start of the Silurian Period 443.8 Mya.The Ordovician, named after the Celtic tribe of the Ordovices, was defined by Charles Lapworth in 1879 to resolve a dispute between followers of Adam Sedgwick and Roderick Murchison, who were placing the same rock beds in northern Wales into the Cambrian and Silurian systems, respectively. Lapworth recognized that the fossil fauna in the disputed strata were different from those of either the Cambrian or the Silurian systems, and placed them in a system of their own. The Ordovician received international approval in 1960 (forty years after Lapworth's death), when it was adopted as an official period of the Paleozoic Era by the International Geological Congress.

Life continued to flourish during the Ordovician as it did in the earlier Cambrian period, although the end of the period was marked by the Ordovician–Silurian extinction events. Invertebrates, namely molluscs and arthropods, dominated the oceans. The Great Ordovician Biodiversification Event considerably increased the diversity of life. Fish, the world's first true vertebrates, continued to evolve, and those with jaws may have first appeared late in the period. Life had yet to diversify on land. About 100 times as many meteorites struck the Earth per year during the Ordovician compared with today.

Paralogania

Paralogania is an extinct genus of thelodonti fish that is known from the Upper Silurian geological period (Wenlock / Ludlow); the best fossil deposits are from the Baltic Sea region, but it is widespread in the Northern Hemisphere.

Siluria, Alabama

Siluria is a former town and now a neighborhood in Alabaster, Alabama, located in Shelby County, Alabama in the Birmingham, Alabama, metropolitan area. It was the home of a large cotton mill and company-built mill village which began operations in 1896 and finally closed in 1979. It was incorporated on 25 May 1954, but was later annexed by Alabaster in May 1971. It's named for the Silurian geological period because of rocks found there. A post office was established in 1872, and remained in operation until it was discontinued in 1972.

System (stratigraphy)

A system in stratigraphy is a unit of rock layers that were laid down together within the same corresponding geological period. The associated period is a chronological time unit, a part of the geological time scale, while the system is a unit of chronostratigraphy. Systems are unrelated to lithostratigraphy, which subdivides rock layers on their lithology. Systems are subdivisions of erathems and are themselves divided into series and stages.

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