L. R. M. Cocks

Earth geography from 500 to 400 million years ago: a faunal and palaeomagnetic review

Very different palaeogeographical reconstructions have been produced by a combination of palaeomagnetic and faunal data, which are re-evaluated on a global basis for the period from 500 to 400 Ma, and are presented with appropriate confidence (or lack of it) on six maps at 20 Ma intervals. The palaeomagnetic results are the most reliable for establishing the changing palaeolatitudes of Baltica, Laurentia and Siberia. However, global palaeomagnetic reliability dwindles over the 100 Ma, and more evidence for relative continental positioning can be gleaned from study of the distribution of the faunas in the later parts of the interval. The new maps were generated initially from palaeomagnetic data when available, but sometimes modified, and terranes were positioned in longitude to take account of key faunal data derived from the occurrences of selected trilobites, brachiopods and fish. Kinematic continuity over the long period is maintained. The many terranes without reliable palaeomagnetic data are placed according to the affinities of their contained fauna. The changing positions of the vast palaeocontinent of Gondwana (which has hitherto been poorly constrained) as it drifted over the South Pole during the interval have been revised and are now more confidently shown following analysis of both faunal and palaeomagnetic data in combination, as well as by the glacial and periglacial sediments in the latest Ordovician. In contrast, the peri-Gondwanan and other terranes of the Middle and Far East, Central Asia and Central America are poorly constrained.

Faunal evidence for oceanic separations in the Palaeozoic of Britain

The faunal criteria for the recognition of the relative separation of old continents are: 1, the contrast between shallow- (not deep-) water faunas off opposite shorelines; 2, the recognition of deeper-water facies surrounding the edges of continents; and 3, the recognition of the disposition of ancient climatic belts. These criteria are defined and used to assess the relative position of the various parts of what is now Britain in the Palaeozoic. The Iapetus Ocean is well characterized by faunal differences from the Cambrian until the lower Ordovician, but from Caradoc times onwards these differences dwindle, until by the latest Silurian only the distribution of the ostracods separates the two sides of the ocean on faunal grounds. In the early Ordovician, S British faunas are comparable with those from Bohemia, France and elsewhere to the S, indicating connection with Gondwanaland, and these faunas differ from those in the Baltic area, a distribution which we attribute to a true ocean, Tornquists Sea, between N and S Europe. Tornquists Sea probably closed in the upper Ordovician, and from Ashgill times onwards the S British and Baltic faunas remained similar. During the middle and late Silurian there is evidence of increasing faunal distinction between Britain, the Baltic area and North America on the one hand and southern Europe and Gondwanaland on the other, indicating an opening Rheic Ocean which appears to have persisted until late Carboniferous times.

Biogeography of Ordovician and Silurian faunas

Abstract The new reconstructions generated for this volume fit well with the majority of Ordovician and Silurian faunal data and are a great improvement on previous attempts. The distribution of selected trilobites, brachiopods and graptolites are plotted on the new maps and confirm the importance of palaeolatitude in controlling the faunal distributions, particularly of the old cratons, which are shown for the Early Ordovician and Late Silurian. Two contrasting patterns of cratonic faunas are (a) disjunct or (b) gradational across a large palaeocontinent, e.g. Gondwana. Marginal and deeper-water biofacies show different patterns, which help to define the edges of palaeocontinents, but which are not so constrained in their palaeolatitudinal distributions. In contrast pelagic trilobites do not help to define palaeocontinents, but were sensitive to palaeo temperature and palaeolatitude. Specific case histories are considered, in particular the closing of Iapetus and the contemporary widening of the Rheic Ocean and the positioning of Avalonia, and the ancestry of the various associated trilobite and brachiopod genera during the later Ordovician. The Ashgill deeper-water Foliomena fauna is also plotted.

The Early palaeozoic geography of Europe

In the early Ordovician three main continents are recognized by faunas in Europe, Laurentia (northwest of the closed suture of the Iapetus Ocean), which was tropical; Baltica (north of the general area of the Trans‐European Suture Zone), which was temperate; and Gondwana (south of the TESZ), which was high latitude. As the Ordovician progressed, various terranes separated and drifted away from the Gondwana supercontinent at different times, namely Avalonia (which was probably originally part of Gondwana near South America), Iberia–Armorica, Perunica (Bohemia) and various Alpine fragments. Each terrane developed progressively different faunas as time went by. Avalonia collided with Baltica near the end of Ordovician, confirmed by faunas, tectonics and palaeomagnetism, and subsequently Avalonia–Baltica collided with Laurentia to form Laurussia from the mid‐Silurian to the early Devonian. The Ibero‐Armorica, Perunica and Alpine terranes did not join the European collage until the Devonian. The modern sites of the terrane boundaries bear little direct relationships to the original palaeogeographical boundaries in the Early Palaeozoic.

Ordovician and Silurian global geography

The main palaeocontinents during the early Ordovician were Gondwana, Laurentia, Baltica, and Siberia, and a brief survey is made of their limits in the Ordovician and Silurian. In particular Gondwana, by far the largest continent, is analysed as including a core of South America, Antarctica, Africa, Australia and peninsular India, and also the marginal terranes, all of which show faunal links with, but which may not have been attached, to the core, of Avalonia, Ibero-Armorica and other European fragments, Turkey, Arabia, and terranes from the Far East including South China, Sibumasu and parts of Australasia. Changes within the benthic faunas reveal that Baltica, whilst isolated in the early Ordovician, became united with Avalonia by the end Ordovician and the two with Laurentia by the mid-Silurian to form the new supercontinent of Laurussia. Island arcs had distinctive faunas during the Ordovician, in particular those in the Iapetus between Laurentia, Baltica and Gondwana and the huge Kipchak Arc which ran from Baltica to Siberia. As the period progressed, the Iapetus arcs became subducted beneath or accreted to their neighbouring cratons, and the Kipchak Arc gradually collapsed to form the core of the new Kazakhstania terrane. Gondwana drifted over the South Pole and this movement is reflected in the cratonic benthic faunas, particularly brachiopods and trilobites, which in the early Ordovician had formed a cline between the high-latitude faunas of North Africa and the equatorial faunas of the Far East and Australia, but by the Devonian lived in much warmer seas in southern Europe and cooler waters in Antarctica. The faunas also reflect the global palaeoclimates, which were warm in the early to mid Ordovician and mid to late Silurian but which were much colder in the half million year period of the late Ordovician and early Silurian, particularly in the latest Ordovician Hirnantian ice age with its attendant widespread Hirnantia brachiopod Fauna. The relative closeness of the chief palaeocontinents by the early Silurian enabled brachiopod and trilobite larvae to cross the narrower oceans, enabling a relatively cosmopolitan benthic fauna to be established over much of the globe, apart from the cooler-water higher latitude Clarkeia Fauna to the south and the Tuvaella Fauna to the north.

Avalonia: a long-lived terrane in the Lower Palaeozoic?

Abstract Avalonia was undoubtedly an independent terrane throughout the Ordovician, merging with Baltica at about Ordovician–Silurian boundary times (443 Ma). We consider it to have been internally unified throughout the Lower Palaeozoic, and not the two independent ‘East’ and ‘West’ Avalonian terranes of some authors. In the early Ordovician its faunas were certainly Gondwanan. However, its earlier history is controversial: we discuss whether Avalonia split off from Gondwana in the early Ordovician, or whether it left Gondwana in the late Neoproterozoic. We conclude that the varied basement terranes underlying Avalonia were aggregated to the margin of Gondwana before 650 Ma. Some substantial transform movements occurred along the Gondwanan margin between 610 and 530 Ma, but the Avalonian area remained part of core Gondwana until about the end of the Cambrian (490 Ma), when the rift–drift initiation of the opening Rheic Ocean between Gondwana and Avalonia began.

Triassic pebbles, derived fossils and the Ordovician to Devonian palaeogeography of Europe

Papers published by Salter (1864) and Davidson (1870) on the faunas from pebbles in a Triassic conglomerate at Budleigh Salterton, Devon, are reviewed. After modern reassessment, these pebbles, although of apparently similar quartzites, have been found to be of four different ages, two Ordovician (mid-Arenig and late Llandeilo) and two Devonian (Lochkovian-Pragian and Frasnian). By comparing these four faunas with those contemporary in adjacent palaeocontinents, it can be shown that, apart from the earliest one, they have affinities closer to those of the Armorican peninsula of Brittany and Normandy than to the rest of Britain and that these Armorican faunas are in clasts which were transported northward by Triassic rivers. Consideration of all the various faunas in the whole of northwest Europe reflect the earliest Ordovician of southern Britain as part of the vast Gondwanan continent, from which it became detached by the mid-Ordovician, with a widening Rheic Ocean between the two palaeocontinents; and the subsequent merging of Avalonia with Baltica and Laurentia to form Laurussia by Mid-Devonian times. New palaeogeographical maps depicting phases from the Ordovician to the Devonian are presented.