Hirnantian
Encyclopedia
The Hirnantian is the seventh and final internationally-recognized stage
of the Ordovician Period of the Paleozoic
Era. It was of short duration, lasting about 1.9 million years, from 445.6 ± 1.5 to 443.7 ± 1.5 Ma (million years ago). The early part of the Hirnantian was characterized by cold temperatures, major glaciation, and a severe drop in sea level. In the latter part of the Hirnantian, temperatures rose, the glacier
s melted, and sea level returned to the same or to a slightly higher level than it had been prior to the glaciation.
Most scientists believe that this climatic oscillation caused the major extinction event
that took place during this time. In fact, the Hirnantian (also known as the End Ordovician and the Ordovician-Silurian
) mass extinction event represents the second largest such event in geologic history. Approximately 85 % of marine (sea-dwelling) species died. Only the End Permian
mass extinction event was larger. Unlike many smaller extinction events, however, the long-term consequences of the End Ordovician event were relatively small. Following the climatic oscillation, the climate returned to its previous state, and the species that survived soon (within two or three million years) evolved into species very similar to the ones that existed before.
, in northern Wales
. Cwm Hirnant means the "valley of the long stream" in Welsh
.
The stage was introduced in 1933 by B.B. Bancroft. As proposed by Bancroft, the Hirnantian included the Hirnant Limestone and related sedimentary formations. These formations were located at the very top of Ordovician deposits, and were dominated by a fauna which included brachiopods, trilobites, and other “shelly” or hard-coated animals. In 1966, D. A. Bassett, Harry Blackmore Whittington, and A. Williams, writing in the Journal of the Geological Society of London, proposed a refinement to the Hirnantian stage. This expanded the stage to include all of the Foel-y-Ddinas Mudstones, of which the Hirnant Limestone is a part. This expansion brought the Hirnantian stage to its current scope.
The International Commission on Stratigraphy
(ICS) originally divided the Upper Ordovician into two stages. After considerable research, however, it was determined that no single faunal zone existed that could accommodate the upper stage of this division. Therefore, in 2003, the ICS voted to add an additional stage to its official international time scale. This was named the Hirnantian after Bancroft's stage. The IUGS is currently (mid-2005) nearing a decision on whether to ratify the Wangjiawan North section as the official Global Boundary Stratotype Section and Point
(GSSP) for the Hirnantian stage. This section is located near the village of Wangjiawan, approximately 42 km north of Yichang
city, in the People's Republic of China
. It was approved unanimously in 2004 by the ICS.
As the climate cooled and glaciers formed during the early part of the Hirnantian, sea level dropped. Estimates of the depth of this drop range from more than 50 meters (based on studies in Nevada
and Utah
) to more than 100 meters (from studies in Norway
and the United Kingdom
). More recent research indicates a worldwide reduction in sea level of approximately 80 meters. This drop dried up and exposed the extensive shallow-water continental shelves that existed throughout the world at that time, causing the extinction of large numbers of species who depended on this shallow water environment. Those species that survived were greatly reduced in numbers. The overall result was a major reduction in the biodiversity
of the world's oceans.
During this ice-house period (as scientists sometimes call periods of global cold weather and glaciation), surviving species began to adapt. Cold environment species replaced the warm environment species that had thrived during the previous hot period (hot-house to scientists).
Just as the species were adapting, however, the climate changed again. During the latter part of the Hirnantian, temperatures rose, the glaciers melted, and sea level rose to the same or perhaps an even higher level than before the glaciation. The exposed shelves were flooded, resulting in additional extinctions among the fauna that had survived the first extinction event.
dates for the Hirnantian itself, there are two such dates which bracket the Hirnantian. Both dates are from the Dob's Linn area in the United Kingdom.
The older one comes from the local Hartfell shale formations. Zircon
found in an ash deposit at the site was dated to 445.7 mya, with a margin of error of plus or minus 2.4 million years.
The younger date is from the early Silurian Birkhill shale formations. Radiometric dating placed Zircon samples found in another ash deposit at 438.7 mya, with a margin of error of plus or minus 2.1 million years.
With these dates as a base, scientists were able to use biostratigraphic correlation techniques to determine close approximations for the timing of various events during the Hirnantian.
There are two Graptolite
biozones in the Hirnantian and these are of approximately equal length. The base (beginning) of the Hirnantian stage is defined by the first appearance in the geologic record of the graptolite Normalograptus extraordinarius, and thus, the Normalograptus extraordinarius biozone defines the early (or lower) part of the Hirnantian. The later (or upper) part is defined by the first appearance of the graptolite Normalograptus persulptus, and the Normalograptus persulptus biozone lasts from then until the end of the Hirnantian. This also marks the end of the Ordovician
period.
These biozones allow for additional refinement in dating events occurring within the Hirnantian stage.
During the Hirnantian, much of the world’s land mass was gathered into a supercontinent
called Gondwana
, which occupied extreme southern latitudes and covered the south pole. This included South America
, Africa
, most of Australia
, the bulk of India
, and Antarctica. What is now west Africa was then located at the pole, while South America was close by, joined to Africa along the latter's west coast. Along Africa's east coast were Antarctica and India, while Australia lay just to the north of them, straddling the equator. To the north of Australia was New Guinea
. It may have been the northernmost point of land in the world, located just above 30 degrees north
latitude. To the north of it lay a vast, uninterrupted sea, known today as the Panthalassic ocean
.
Not yet joined with what would become North America; Florida, southern Georgia, and the coastal areas of Mississippi
, Alabama
, and South Carolina
were wedged into a gap between Africa and South America, and were located very near the South pole.
The rest of North America (called Laurentia
by scientists) lay to the north and west of Gondwana, at (like Australia) low and relatively warmer climates. Rotated almost 45 degrees from its modern orientation, the eastern states of today’s U.S. were located along the southeast coast of the continent, while the coastal areas of what are now the southeastern states faced south.
To the east of Laurentia, across a long, narrow sea, was Baltica
. Composed of modern Norway, Sweden
, Finland
, the UK, Belgium
, Denmark
, the Netherlands
, northern Germany
, eastern Ireland
, and Russia
west of the Ural mountains
, this area ranged from the equator in the north to more than 30 degrees south
latitude.
Stretching westward from its southwest tip was an island arch known as Avalonia
by modern scientists. This consisted of what is now the western part of Ireland, and eastern coastal regions of Newfoundland
, Nova Scotia
, New Brunswick
, and New England
.
below the Period level, the following list provides correlations between those regional stages (or epochs in some cases) and the internationally-recognized Hirnantian stage.
Faunal stage
In chronostratigraphy, a stage is a succession of rock strata laid down in a single age on the geologic timescale, which usually represents millions of years of deposition. A given stage of rock and the corresponding age of time will by convention have the same name, and the same boundaries.Rock...
of the Ordovician Period of the Paleozoic
Paleozoic
The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...
Era. It was of short duration, lasting about 1.9 million years, from 445.6 ± 1.5 to 443.7 ± 1.5 Ma (million years ago). The early part of the Hirnantian was characterized by cold temperatures, major glaciation, and a severe drop in sea level. In the latter part of the Hirnantian, temperatures rose, the glacier
Glacier
A glacier is a large persistent body of ice that forms where the accumulation of snow exceeds its ablation over many years, often centuries. At least 0.1 km² in area and 50 m thick, but often much larger, a glacier slowly deforms and flows due to stresses induced by its weight...
s melted, and sea level returned to the same or to a slightly higher level than it had been prior to the glaciation.
Most scientists believe that this climatic oscillation caused the major extinction event
Extinction event
An extinction event is a sharp decrease in the diversity and abundance of macroscopic life. They occur when the rate of extinction increases with respect to the rate of speciation...
that took place during this time. In fact, the Hirnantian (also known as the End Ordovician and the Ordovician-Silurian
Ordovician-Silurian extinction events
The Ordovician–Silurian extinction event, or quite commonly the Ordovician extinction, was the third-largest of the five major extinction events in Earth's history in terms of percentage of genera that went extinct and second largest overall in the overall loss of life. Between about 450 Ma to 440...
) mass extinction event represents the second largest such event in geologic history. Approximately 85 % of marine (sea-dwelling) species died. Only the End Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...
mass extinction event was larger. Unlike many smaller extinction events, however, the long-term consequences of the End Ordovician event were relatively small. Following the climatic oscillation, the climate returned to its previous state, and the species that survived soon (within two or three million years) evolved into species very similar to the ones that existed before.
History
The Hirnantian was named after Cwm Hirnant near BalaBala, Gwynedd
Bala is a market town and community in Gwynedd, Wales, and formerly an urban district of the historic county of Merionethshire. It lies at the north end of Bala Lake , 17 miles north-east of Dolgellau, with a population of 1,980...
, in northern Wales
Wales
Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...
. Cwm Hirnant means the "valley of the long stream" in Welsh
Welsh language
Welsh is a member of the Brythonic branch of the Celtic languages spoken natively in Wales, by some along the Welsh border in England, and in Y Wladfa...
.
The stage was introduced in 1933 by B.B. Bancroft. As proposed by Bancroft, the Hirnantian included the Hirnant Limestone and related sedimentary formations. These formations were located at the very top of Ordovician deposits, and were dominated by a fauna which included brachiopods, trilobites, and other “shelly” or hard-coated animals. In 1966, D. A. Bassett, Harry Blackmore Whittington, and A. Williams, writing in the Journal of the Geological Society of London, proposed a refinement to the Hirnantian stage. This expanded the stage to include all of the Foel-y-Ddinas Mudstones, of which the Hirnant Limestone is a part. This expansion brought the Hirnantian stage to its current scope.
The International Commission on Stratigraphy
International Commission on Stratigraphy
The International Commission on Stratigraphy , sometimes referred to by the unofficial "International Stratigraphic Commission" is a daughter or major subcommittee grade scientific daughter organization that concerns itself with stratigraphy, geological, and geochronological matters on a global...
(ICS) originally divided the Upper Ordovician into two stages. After considerable research, however, it was determined that no single faunal zone existed that could accommodate the upper stage of this division. Therefore, in 2003, the ICS voted to add an additional stage to its official international time scale. This was named the Hirnantian after Bancroft's stage. The IUGS is currently (mid-2005) nearing a decision on whether to ratify the Wangjiawan North section as the official Global Boundary Stratotype Section and Point
Global Boundary Stratotype Section and Point
A Global Boundary Stratotype Section and Point, abbreviated GSSP, is an internationally agreed upon stratigraphic section which serves as the reference section for a particular boundary on the geologic time scale. The effort to define GSSPs is conducted by the International Commission on...
(GSSP) for the Hirnantian stage. This section is located near the village of Wangjiawan, approximately 42 km north of Yichang
Yichang
Yichang is a prefecture-level city located in Hubei province of the People's Republic of China. It is the second largest city in Hubei province after the province capital, Wuhan. The Three Gorges Dam is located within its administrative area, in Yiling District.-History:In ancient times Yichang...
city, in the People's Republic of China
People's Republic of China
China , officially the People's Republic of China , is the most populous country in the world, with over 1.3 billion citizens. Located in East Asia, the country covers approximately 9.6 million square kilometres...
. It was approved unanimously in 2004 by the ICS.
Major events
As mentioned above, there was a major climatic oscillation during the Hirnantian, which is believed to have caused a major mass extinction event. When the Hirnantian stage began, the Earth's climate was hot and sea-levels were substantially higher than today. The seas were filled with a diverse fauna. However, there is considerable evidence to indicate that many of these species were already in trouble when the Hirnantian began, and that overall biological diversity was already in sharp decline.As the climate cooled and glaciers formed during the early part of the Hirnantian, sea level dropped. Estimates of the depth of this drop range from more than 50 meters (based on studies in Nevada
Nevada
Nevada is a state in the western, mountain west, and southwestern regions of the United States. With an area of and a population of about 2.7 million, it is the 7th-largest and 35th-most populous state. Over two-thirds of Nevada's people live in the Las Vegas metropolitan area, which contains its...
and Utah
Utah
Utah is a state in the Western United States. It was the 45th state to join the Union, on January 4, 1896. Approximately 80% of Utah's 2,763,885 people live along the Wasatch Front, centering on Salt Lake City. This leaves vast expanses of the state nearly uninhabited, making the population the...
) to more than 100 meters (from studies in Norway
Norway
Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...
and the United Kingdom
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
). More recent research indicates a worldwide reduction in sea level of approximately 80 meters. This drop dried up and exposed the extensive shallow-water continental shelves that existed throughout the world at that time, causing the extinction of large numbers of species who depended on this shallow water environment. Those species that survived were greatly reduced in numbers. The overall result was a major reduction in the biodiversity
Biodiversity
Biodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...
of the world's oceans.
During this ice-house period (as scientists sometimes call periods of global cold weather and glaciation), surviving species began to adapt. Cold environment species replaced the warm environment species that had thrived during the previous hot period (hot-house to scientists).
Just as the species were adapting, however, the climate changed again. During the latter part of the Hirnantian, temperatures rose, the glaciers melted, and sea level rose to the same or perhaps an even higher level than before the glaciation. The exposed shelves were flooded, resulting in additional extinctions among the fauna that had survived the first extinction event.
Dating
While there are no major radiometricRadiometric dating
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...
dates for the Hirnantian itself, there are two such dates which bracket the Hirnantian. Both dates are from the Dob's Linn area in the United Kingdom.
The older one comes from the local Hartfell shale formations. Zircon
Zircon
Zircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1–x4x–y...
found in an ash deposit at the site was dated to 445.7 mya, with a margin of error of plus or minus 2.4 million years.
The younger date is from the early Silurian Birkhill shale formations. Radiometric dating placed Zircon samples found in another ash deposit at 438.7 mya, with a margin of error of plus or minus 2.1 million years.
With these dates as a base, scientists were able to use biostratigraphic correlation techniques to determine close approximations for the timing of various events during the Hirnantian.
Subdivisions
While it is not usual practice for scientists to formally divide geologic time below the level of a stage (though sub-stages are known in some instances), zones of biological distinction (called Biozones) provide a generally recognized way of defining divisions at smaller levels. Biozones represent a period of time during which a specified species or group of species existed and left a fossil record of their presence.There are two Graptolite
Graptolite
Graptolithina is a class in the animal phylum Hemichordata, the members of which are known as Graptolites. Graptolites are fossil colonial animals known chiefly from the Upper Cambrian through the Lower Carboniferous...
biozones in the Hirnantian and these are of approximately equal length. The base (beginning) of the Hirnantian stage is defined by the first appearance in the geologic record of the graptolite Normalograptus extraordinarius, and thus, the Normalograptus extraordinarius biozone defines the early (or lower) part of the Hirnantian. The later (or upper) part is defined by the first appearance of the graptolite Normalograptus persulptus, and the Normalograptus persulptus biozone lasts from then until the end of the Hirnantian. This also marks the end of the Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
period.
These biozones allow for additional refinement in dating events occurring within the Hirnantian stage.
Paleogeography
Paleogeography is highly debated and often fraught with uncertainties. This is especially true the farther one goes back in the geologic record. The following interpretation represents the current views of many scientists. However, these views are disagreed with and strongly debated by many others.During the Hirnantian, much of the world’s land mass was gathered into a supercontinent
Supercontinent
In geology, a supercontinent is a landmass comprising more than one continental core, or craton. The assembly of cratons and accreted terranes that form Eurasia qualifies as a supercontinent today.-History:...
called Gondwana
Gondwana
In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...
, which occupied extreme southern latitudes and covered the south pole. This included South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
, Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
, most of Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
, the bulk of India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
, and Antarctica. What is now west Africa was then located at the pole, while South America was close by, joined to Africa along the latter's west coast. Along Africa's east coast were Antarctica and India, while Australia lay just to the north of them, straddling the equator. To the north of Australia was New Guinea
New Guinea
New Guinea is the world's second largest island, after Greenland, covering a land area of 786,000 km2. Located in the southwest Pacific Ocean, it lies geographically to the east of the Malay Archipelago, with which it is sometimes included as part of a greater Indo-Australian Archipelago...
. It may have been the northernmost point of land in the world, located just above 30 degrees north
30th parallel north
The 30th parallel north is a circle of latitude that is 30 degrees north of the Earth's equatorial plane. It stands one-third of the way between the equator and the North Pole and crosses Africa, Asia, the Pacific Ocean, North America and the Atlantic Ocean....
latitude. To the north of it lay a vast, uninterrupted sea, known today as the Panthalassic ocean
Panthalassa
Panthalassa , also known as the Panthalassic Ocean, was the vast global ocean that surrounded the supercontinent Pangaea, during the late Paleozoic and the early Mesozoic years. It included the Pacific Ocean to the west and north and the Tethys Ocean to the southeast...
.
Not yet joined with what would become North America; Florida, southern Georgia, and the coastal areas of Mississippi
Mississippi
Mississippi is a U.S. state located in the Southern United States. Jackson is the state capital and largest city. The name of the state derives from the Mississippi River, which flows along its western boundary, whose name comes from the Ojibwe word misi-ziibi...
, Alabama
Alabama
Alabama is a state located in the southeastern region of the United States. It is bordered by Tennessee to the north, Georgia to the east, Florida and the Gulf of Mexico to the south, and Mississippi to the west. Alabama ranks 30th in total land area and ranks second in the size of its inland...
, and South Carolina
South Carolina
South Carolina is a state in the Deep South of the United States that borders Georgia to the south, North Carolina to the north, and the Atlantic Ocean to the east. Originally part of the Province of Carolina, the Province of South Carolina was one of the 13 colonies that declared independence...
were wedged into a gap between Africa and South America, and were located very near the South pole.
The rest of North America (called Laurentia
Laurentia
Laurentia is a large area of continental craton, which forms the ancient geological core of the North American continent...
by scientists) lay to the north and west of Gondwana, at (like Australia) low and relatively warmer climates. Rotated almost 45 degrees from its modern orientation, the eastern states of today’s U.S. were located along the southeast coast of the continent, while the coastal areas of what are now the southeastern states faced south.
To the east of Laurentia, across a long, narrow sea, was Baltica
Baltica
Baltica is a name applied by geologists to a late-Proterozoic, early-Palaeozoic continent that now includes the East European craton of northwestern Eurasia. Baltica was created as an entity not earlier than 1.8 billion years ago. Before this time, the three segments/continents that now comprise...
. Composed of modern Norway, Sweden
Sweden
Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....
, Finland
Finland
Finland , officially the Republic of Finland, is a Nordic country situated in the Fennoscandian region of Northern Europe. It is bordered by Sweden in the west, Norway in the north and Russia in the east, while Estonia lies to its south across the Gulf of Finland.Around 5.4 million people reside...
, the UK, Belgium
Belgium
Belgium , officially the Kingdom of Belgium, is a federal state in Western Europe. It is a founding member of the European Union and hosts the EU's headquarters, and those of several other major international organisations such as NATO.Belgium is also a member of, or affiliated to, many...
, Denmark
Denmark
Denmark is a Scandinavian country in Northern Europe. The countries of Denmark and Greenland, as well as the Faroe Islands, constitute the Kingdom of Denmark . It is the southernmost of the Nordic countries, southwest of Sweden and south of Norway, and bordered to the south by Germany. Denmark...
, the Netherlands
Netherlands
The Netherlands is a constituent country of the Kingdom of the Netherlands, located mainly in North-West Europe and with several islands in the Caribbean. Mainland Netherlands borders the North Sea to the north and west, Belgium to the south, and Germany to the east, and shares maritime borders...
, northern Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
, eastern Ireland
Ireland
Ireland is an island to the northwest of continental Europe. It is the third-largest island in Europe and the twentieth-largest island on Earth...
, and Russia
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...
west of the Ural mountains
Ural Mountains
The Ural Mountains , or simply the Urals, are a mountain range that runs approximately from north to south through western Russia, from the coast of the Arctic Ocean to the Ural River and northwestern Kazakhstan. Their eastern side is usually considered the natural boundary between Europe and Asia...
, this area ranged from the equator in the north to more than 30 degrees south
30th parallel south
The 30th parallel south is a circle of latitude that is 30 degrees south of the Earth's equatorial plane. It crosses the Atlantic Ocean, Africa, the Indian Ocean, Australasia, the Pacific Ocean and South America....
latitude.
Stretching westward from its southwest tip was an island arch known as Avalonia
Avalonia
Avalonia was a microcontinent in the Paleozoic era. Crustal fragments of this former microcontinent underlie south-west Great Britain, and the eastern coast of North America. It is the source of many of the older rocks of Western Europe, Atlantic Canada, and parts of the coastal United States...
by modern scientists. This consisted of what is now the western part of Ireland, and eastern coastal regions of Newfoundland
Newfoundland and Labrador
Newfoundland and Labrador is the easternmost province of Canada. Situated in the country's Atlantic region, it incorporates the island of Newfoundland and mainland Labrador with a combined area of . As of April 2011, the province's estimated population is 508,400...
, Nova Scotia
Nova Scotia
Nova Scotia is one of Canada's three Maritime provinces and is the most populous province in Atlantic Canada. The name of the province is Latin for "New Scotland," but "Nova Scotia" is the recognized, English-language name of the province. The provincial capital is Halifax. Nova Scotia is the...
, New Brunswick
New Brunswick
New Brunswick is one of Canada's three Maritime provinces and is the only province in the federation that is constitutionally bilingual . The provincial capital is Fredericton and Saint John is the most populous city. Greater Moncton is the largest Census Metropolitan Area...
, and New England
New England
New England is a region in the northeastern corner of the United States consisting of the six states of Maine, New Hampshire, Vermont, Massachusetts, Rhode Island, and Connecticut...
.
Correlations to regional stages
The Hirnantian now represents an internationally accepted stage with a carefully specified global beginning and ending. However, it started out in the nineteenth century as a regional stage in the United Kingdom, where it remains as such today. Since various regions of the world have their own local divisions of geologic timeGeologic time scale
The geologic time scale provides a system of chronologic measurement relating stratigraphy to time that is used by geologists, paleontologists and other earth scientists to describe the timing and relationships between events that have occurred during the history of the Earth...
below the Period level, the following list provides correlations between those regional stages (or epochs in some cases) and the internationally-recognized Hirnantian stage.
- Australasia – The Hirnantian is equal to approximately the top 20 % of the regional Bolindian epoch.
- Baltica – The Hirnantian is equivalent to the upper 50 % of the regional Porkuni stage.
- China – The Hirnantian is roughly equal to the upper 25 % of the regional Wufeng stage.
- North America – The Hirnantian is equivalent to all of the regional Gamachian stage.
- United Kingdom – The Hirnantian is nearly equal to the regional Hirnantian stage, which starts slightly (maybe 100,000 years) earlier. The regional Hirnantian constitutes roughly the upper 20 % of the Ashgill epoch.
Sources
- Chen Xu, Rong Jia-yu, Fan Juan-xuan, Zhan Ren-bin, C. E. Mitchell, D. A. T. Harper, M. J. Melchin, and Wang Xiao-feng, "A final report on the GSSP candidate of the Hirnantian Stage" (International Commission on Stratigraphy).
- Gradstein, Felix, James Ogg, and Alan Smith, eds., 2004. A Geologic Time Scale 2004 (Cambridge University Press).
- Salvador, Amos, ed., 1994. International Stratigraphic Guide (The International Union of Geological Sciences and The American Geological Society).
- Sheehan, Peter, "The Late Ordovician Mass Extinction" (Annual Review of Earth and Planetary Sciences, 2001, pp. 331-364).
- Webby, Barry D. and Mary L. Droser, eds., 2004. The Great Ordovician Biodiversification Event (Columbia University Press).
Further reading
- Brenchley, P. J., 1984. "Late Ordovician extinction and their relationship to the Gondwana glaciation", in Brenchley, P. J., ed., Fossils and Climate. pp. 291-316, (John Wiley & Sons).
- Hallam, A. and Paul B. Wignall, 1997. Mass extinctions and their aftermath (Oxford University Press).