Radiometric dating of ordovician period

By the end of the Late Ordovician the volcanic emissions had stopped.Gondwana had by that time neared the south pole and was largely glaciated.The corresponding rocks of the Ordovician System are referred to as coming from the Lower, Middle, or Upper part of the column, the faunal stages (subdivisions of epochs) from youngest to oldest are: Late Ordovician The Tremadoc corresponds to the (modern) Tremadocian, the Floian corresponds to the lower Arenig; the Arenig continues until the early Darriwilian, subsuming the Dapingian.The Llanvirn occupies the rest of the Darriwilian, and terminates with it at the base of the Late Ordovician, the Sandbian represents the first half of the Caradoc; the Caradoc ends in the mid-Katian, and the Ashgill represents the last half of the Katian, plus the Hirnantian.The dates given are recent radiometric dates and vary slightly from those found in other sources, this second period of the Paleozoic era created abundant fossils that became major petroleum and gas reservoirs.The boundary chosen for the beginning of both the Ordovician Period and the Tremadocian stage is highly significant, it correlates well with the occurrence of widespread graptolite, conodont, and trilobite species.

Shallow clear waters over continental shelves encouraged the growth of organisms that deposit calcium carbonates in their shells and hard parts, the 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.The Ordovician was a time of calcite sea geochemistry in which low-magnesium calcite was the primary inorganic marine precipitate of calcium carbonate.Carbonate hardgrounds were thus very common, along with calcitic ooids, calcitic cements, and invertebrate faunas with dominantly calcitic skeletons.The Taconic orogeny, a major mountain-building episode, was well under way in Cambrian times; in the early and middle Ordovician, temperatures were mild, but at the beginning of the Late Ordovician, from 460 to 450 Ma, volcanoes along the margin of the Iapetus Ocean spewed massive amounts of carbon dioxide, a greenhouse gas, into the atmosphere, turning the planet into a hothouse.Initially, sea levels were high, but as Gondwana moved south, ice accumulated into glaciers and sea levels dropped, at first, low-lying sea beds increased diversity, but later glaciation led to mass extinctions as the seas drained and continental shelves became dry land.

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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.

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