International Geologiical Congress - Oslo 2008

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GDP-04 Palaeozoic-Mesozoic earth geography: Palaeomagnetic, faunal and facies constraints

 

Earth geography in the Ordovician and Silurian

 

L Robin M Cocks, The Natural History Museum (United Kingdom)
Trond H Torsvik, NGU (Norway)
 

 

By combining data from palaeomagnetism, terrane-diagnostic benthic faunas and facies, we have gained new insights into Lower Palaeozoic geography. The supercontinent of Gondwana and the Panthalassic Ocean together occupied most of the globe, but Laurentia, Baltica and Siberia were also very substantial entities independent from Gondwana, and there were numerous other terranes and oceans. The continents were widely dispersed in the Early Ordovician, leading to many endemic local trilobite, brachiopod and ostracod faunas, but many of the oceans gradually became narrow enough for successful dispersal, leading to a high degree of faunal cosmopolitanism by the Mid Silurian, apart from the high-latitude Clarkeia Fauna in the south and the Tuvaella Fauna in the north (with the latter confined to Siberia and peri-Siberia).

The end of the Ordovician was marked by substantial climatic fluctuation between "greenhouse" and "icehouse" regimes, and it terminated in the significant Hirnantian glaciation. The Earth showed considerable dynamism during the 72 My period, including the end phases of the substantial rotation of Baltica, the Caledonide (including Scandian) Orogeny as the Iapetus and Tornquist Oceans closed, and the considerable island arc activity at the edges of many continents, including the Iapetus margins of Laurentia and Avalonia, the Australian and South American margins of Gondwana, the Mongolian Terrane Assemblage margin of Siberia and in and around the many small terranes which later formed Kazakhstania. In contrast, the long Gondwanan margin from Turkey to New Guinea was passive, as was the northern margin of Laurentia.

The "Armorican" area of central and southern Europe, notably France and the Iberian Peninsula, remained part of core Gondwana throughout most of the period, although the nascent Palaeotethys Ocean between Gondwana and Armorica may have started its opening near the end of Silurian time. Global sea levels varied substantially, and were dependant both on ocean-bottom topography and also the amount of water locked up in the polar ice caps. Thus the mid-Ordovician and end-Silurian were eustatic high stands, in contrast to the low-stands in the Hirnantian and earliest Silurian.

 

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