Ordovician period dating

18-Mar-2020 08:43

In 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 into the atmosphere, turning the planet into a hothouse.These volcanic island arcs eventually collided with proto North America to form the Appalachian mountains.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.Unlike Cambrian times, when calcite production was dominated by microbial and non-biological processes, animals (and macroalgae) became a dominant source of calcareous material in Ordovician deposits.The Ordovician saw the highest sea levels of the Paleozoic, and the low relief of the continents led to many shelf deposits being formed under hundreds of metres of water.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.

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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.