Information AboutOrdovician |
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The Ordovician period is the second of the six (seven in North America ) Periods The Carboniferous in North America is divided in two, the Mississippian and the Pennsylvanian . of the Paleozoic Era , and covers the time roughly between 490 to 440 million years ago. It follows the Cambrian period and is followed by the Silurian period. The Ordovician, named after the Welsh 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 periods respectively. Lapworth, recognizing that the Fossil Fauna in the disputed Strata were different from those of either the Cambrian or the Silurian periods, realized that they should be placed in a period of their own. While recognition of the distinct Ordovician period was slow in the United Kingdom , other areas of the world accepted it quickly. It received international sanction in 1906 , when it was adopted as an official period of the Paleozoic era by the International Geological Congress . ORDOVICIAN DATING The Ordovician period started at a major extinction event called the Cambrian-Ordovician Extinction Events some time about 488.3 ± 1.7 million years ago ( Ma ) and lasted for about 44.6 million years. It ended with another major extinction event about 443.7 ± 1.5 Ma (ICS, 2004) that wiped out 60% of marine Genera . A. Melott et al. (ref. 2006) suggested a ten-second Gamma Ray Burst could have destroyed the Ozone Layer and exposed terrestrial and marine surface-dwelling life to deadly Radiation , but most scientists agree that extinction events are complex with multiple causes. See Below . The dates given are recent Radiometric dates and vary slightly from those used in other sources. This second period of the Paleozoic era created abundant Fossil s and in some regions, major Petroleum and Gas reservoirs. ORDOVICIAN SUBDIVISIONS The Ordovician Period is usually broken into Early ( Tremadoc and Arenig ), Middle ( Llanvirn into Abereiddian and Llandeilian ) and Late ( Caradoc and Ashgill ) epochs. The corresponding rocks of the Ordovician System are referred to as coming from the Lower, Middle, or Upper part of the column. The Faunal Stage s (subdivisions of epochs) from youngest to oldest are:
ORDOVICIAN PALEOGEOGRAPHY Sea levels were high during the Ordovician; in fact during the Tremadocian, Marine Transgressions worldwide were the greatest for which evidence is preserved in the rocks. During the Ordovician, the southern continents were collected into a single continent called Gondwana . Gondwana started the period in Equator ial Latitude s and, as the period progressed, drifted toward the South Pole . Early in the Ordovician, the continents Laurentia , Siberia , and Baltica were still independent continents (since the break-up of the Supercontinent Pannotia earlier), but Baltica began to move towards Laurentia later in the period, causing the Iapetus Ocean to shrink between them. Also, Avalonia broke free from Gondwana and began to head north towards Laurentia. Rheic Ocean was formed as a result of this. Ordovician rocks are chiefly Sedimentary . Because of the restricted area and low elevation of solid land, which set limits to Erosion , marine Sediment s that make up a large part of the Ordovician system consist chiefly of Limestone . Shale and Sandstone are less conspicuous. A major mountain-building episode was the Taconic Orogeny that was well under way in Cambrian times. By the end of the period, Gondwana had neared or approached the pole and was largely Glaciated . CLIMATE The Early Ordovician climate was thought to be quite warm, at least in the tropics. As with North America and Europe , Gondwana was largely covered with shallow seas during the Ordovician. Shallow clear waters over continental shelves encouraged the growth of organisms that deposit calcium carbonates in their shells and hard parts. Panthalassic Ocean covered much of the northern hemisphere, and other minor oceans included Proto-Tethys , Paleo-Tethys , Khanty Ocean which was closed off by the Late Ordovician, Iapetus Ocean , and the new Rheic Ocean . As the Ordovician progressed, we see evidence of Glacier s on the land we now know as Africa and South America . At the time these land masses were sitting at the South Pole , and covered by Ice Caps . ORDOVICIAN LIFE , Batavia, Ohio ]] Ordovician fauna Though less famous than the shell-secreting organisms in the Ordovician compared to the Cambrian.Ibid., 255-6. In North America and Europe, the Ordovician was a time of shallow continental seas rich in life. Trilobites and brachiopods in particular were rich and diverse. The first Bryozoa appeared in the Ordovician as did the first Coral Reef s. Solitary Coral s date back to at least the Cambrian . Mollusc s, which had also appeared during the Cambrian or the Ediacaran , became common and varied, especially Bivalve s, Gastropod s, and Nautiloid cephalopods. It was long thought that the first true Vertebrates (fish - Ostracoderm s) appeared in the Ordovician, but recent discoveries in China reveal that they probably originated in the Early Cambrian . The very first Jawed Fish appeared in the Late Ordovician epoch. Now-extinct marine animals called Graptolites thrived in the oceans. Some Cystoid s and crinoids appeared. During the Middle Ordovician there was a large increase in the intensity and diversity of bioeroding organisms. This is known as the Ordovician Bioerosion Revolution (Wilson & Palmer, 2006). It is marked by a sudden abundance of hard substrate trace fossils such as ''Trypanites'', ''Palaeosabella'' and ''Petroxestes''. |
Image:OrdoutcropINjpgOutcrop Of Upper Ordovician Rubbly Limestone And Shale, Southern Indiana
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Image:OrdOutcropTNJPGOutcrop Of Upper Ordovician Limestone And Minor Shale, Central Tennessee
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Image:OilShaleEstoniaJPGOutcrop Of Ordovician Kukersite Oil Shale, Northern
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Image:OilShaleFossilsEstoniaJPGFossils In Ordovician Kukersite Oil Shale, Northern
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Image:OrdFossilsMNJPG
| "http://wwwinformationdelightinfo/information/entry/Brachiopods" class="copylinks">Brachiopods and Bryozoans in an Ordovician limestone, southern Minnesota |
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