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Encyclopedia Britannica - Main :: ORC-PAI |
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ORDOVICIAN SYSTEM . Ordovician Rocks: Generalized Correlation Table. England Graptolite Zones. Scotland. Scandinavia. Bohemia. N.-W. France. W. Russia. North American Continent. and Wales. New York
Caradoc Dicellogra plus I Hartlell Shales, Brachiopod beds, D5. Gres de May. Borkholm Richmond beds Lowest or anceps. Ardmillan Series, Trinucleus beds, D4. Calcaire de and and a Anticosti .2 Bala
Pleurograplus Lowther Shales. Leptaena I Wesenberg Lorraine beds. . q and v linearis. I limestone. beds. g '^ Hudson river 3 Dicranogra plus Trinucleus Utica Shale. U q beds clingani. limestone. o e Trenton. limestone. a.5 Coenograptus .,yC Shales. c -ci Llandeilo Coenograptus Glenkiln Shales Middle Da Schistes des Jewe, Itfer, and Trenton beds group. gracilis. and Graptolite beds Dry. Gembloux Kuckers beds. and ( I Didymograptus I Barr Series. and and l Echinosphaerite Galena limestone. Murchisoni. Chasmops ironstone. II Echinosphaerite 3 a limestone. i Black river beds. a Cystidean Lowville limestone. limestone. d A Levis Shales with U Tetragraptus, and Phyllograptus. (Lanvirn) Didymograptus Radiolarian ~ Lower DIP. Gres Vaginatus Chazy limestone q and bifidus. Cherts Graptolite beds Armoricain limestone (part) Arenig group. Tetragraptus and and (part). and and bryonides. Ballantrae Orthoceras Glauconite St Peter's sandstone. Series. I limestone. limestone. U Tremadoc beds, Ceratopyge beds, and beds with Euloma-Niobe fauna here regarded as Cambrian: not invariably present. of the British Isles seem to have been deposited in a North Atlantic sea which embraced also the north of France and Belgium. Confluent with this sea on the east was a rather peculiar basin which included Bohemia, southern France, Spain, Portugal, the eastern Alps, Thuringia, Fichtelgebirge and the Keller Wald. Another European basin, probably separated from the Bohemian or Mediterranean sea in early Ordovician times, lay over the Baltic region, Scandinavia, the Baltic provinces and north Germany, and communicated eastwards by way of Russian Poland and central Russia with far eastern C. Lapworth and his school, and J. E. Marr and the Cambridge school, and in Scandinavia and the Baltic region, through W. C. Brogger, S. A. Tulberg, F. Schmidt and others, the most elaborate subdivision of the Ordovician rocks has been attained.In the Baltic provinces of Russia, F. Schmidt describes the following stages, in descending order: (Stage F) the Lyckholm and Borkholm zones, a highly fossiliferous series, equivalent to the Middle Bala
limestone =Upper Orthoceratite limestone of Sweden. (B) Orthoceratite (Vaginaten) limestone=Orthoceratite limestone of Sweden, Glauconitic limestone, Glauconitic sand (Greensand). The last-mentioned reposes on Cambrian Dictyonema shales. While the Ordovician rocks in Scania, the Baltic provinces and north-central Russia are undisturbed and level-bedded, those. on the western side of the Scandinavian axis and in the Urals have suffered movement
Languedoc
On the North American continent Ordovician rocks cover a very large area in the central, eastern and northern parts (north of lat. 300). As regards the classification and correlation of the strata, which change in character from point to point, as is natural over so large an area, much remains to be done. In the table the divisions of the system that obtain in the New York
In North Africa Ordovician rocks are probably present, and in New Zealand the Arorere series (Wanaka group), and in Australia (Victoria) the graptolitic, gold-bearing shales and slates belong to this period. During this period there appears to have been a general tendency for the sea to transgress on the land, a tendency which increased towards its close, especially in the northern hemisphere (Europe and the Appalachian regions). One of the results of this movement
Volcanic Activity.This period was one of great volcanic activity in several widely separated regions. " In Ayrshire and the south-western districts (of the southern uplands), where the volcanic constituents attain a great development, they consist of basic lavas (diabase, &c.), with intercalated tuffs and agglomerates. A characteristic feature of these lavas is the development of ellipsoidal or pillow -structure in them. This volcanic platform appears to underlie the Silurian region over an area of at least 2000 sq. m., inasmuch as it comes to the surface wherever the crests of the anticlines bring up suffi-ciently deep parts of the formations. It is thus one of the most extensive as well as one of the most ancient volcanic tracts of Europe " (Sir A. Geikie , Text-boos of Geology, 4th ed. vol. ii. p. 951). In the west of England and in Wales there was also a very active volcanic centre. In the Snowdon district thousands of feet of contemporaneous felsitic lavas and tuffs occur in the Bala beds; while in Cader Idris, the Arenig Mountains and the Arans there are similar eruptions of felsitic and rhyolitic lavas, tuffs and agglomeratesprobably many of them submarineinterstratified in the Arenig formation. In the Lake district a great series of lavas and ashesthe Borrowdale serieswas erupted during the middle of the period; the earlier effusions were andesitic, the later ones felsitic and rhyolitic. In Ireland the Arenig lavas of Tyrone resemble some of those in Scotland. Volcanic rocks (porphyrites, syenites and lavas) occur in considerable force in the Ordovician rocks of Nova Scotia and New Brunswick and New Zealand. Tuffs of this age are found in Brittany, and diabase in Bohemia.The economic products obtained from rocks of this period include gold in Australia, New Zealand and Wales; iron ore in France; lead and zinc from the Galena and Trenton horizons in Wisconsin, Iowa and Illinois; manganese in Arkansas; oil and gas from the Trenton stage in Ohio and east Indiana; roofing slates and slate pencils in Wales and the Lake district; limestone in Great Britain and Tennessee; phosphate beds in Wales and Tennessee; marble ip the Appalachian district; graphite (plumbago) in the Lake district; and jasper in Wales and southern Scotland. Ordovician Life.Compared with the preceding Cambrian period, the Ordovician is remarkable for the great expansion in numbers and variety of organisms, apart from the fact that fossils are better preserved in the younger formations. All the great classes of mollusks were represented, the most numerous being the brachiopods, which, in addition to the simple forms of the Cambrian, began at this time to develop spire-bearing genera (Chonetes, Orthis, Orthisina, Strophomena, Crania, Schizotreta, Porambonites, Rafinesquina, Leptaena, Zygospira). The gasteropods now developed all the leading types of shell (Pleurotomaria, Omphalotrochus) ; but both this class and the pelecypods (Lyrodesma, Ctenodonia, Modiolopsis) were subordinate in importance to the cephalopods. These mollusks were probably the most powerful living creatures in the Ordovician seas; straight-shelled, slightly curved, and nautiloid forms predominated (Orthoceras, Cyrtoceras, Gyroceras, Trocholites, Endoceras, Litoceras, Lituites, Actinoceras). Some of the straight shells were of enormous size, 12 to 15 ft. long and as much as 1 ft. in diameter, in the widest part. Trilobites were present in great abundance, and in this period they reached the climax of their development. In the lower stage we find Agnostus, Calymene, Asaphus, Illaenus, Placoparia; on the Llandeilo horizon, Calymene, Asaphus, Megalaspis, Dalmanitis; and, at the summit, Trinucleus and Homalonotus. In the transition zone between Ordovician and Cambrian, Ceratopyge, Euloma, Niobe, flourished. Other important genera are Ogygia, Cheirurus, Harpes, Acidaspis. Ostracods (Leperditia, Beyrichia), cyprids (Bairdia, Macrocypris), phyllocarids (Ceratiocaris, Peltocaris), cirripeds (Lepidocoleus), and, later, eurypterids represented other crustacean groups. The bryozoans, Stomatopore, Monticulipora, Phylloporina, Fenestella and others, were abundant and frequently formed beds of limestone. Among the echinoderms the cystoids were the most prominent (Pleurocystis, Aristocystis) and at this period reached their climax; crinoids (Archaeocrinus, Dendrocrinus) became more important; while opliiuroids, echinoids (Bothriocidaris) and asteroids (Taeniaster, Palaeaster) made their appearance. Corals (Streptelasma, Columnaria) were scarce, and sponges (Aulocopium, Caryospongia, Archaeocyathus) were not particularly important; Receptaculites, Ischadites, are well-known fossils doubtfully referred to this group. Radiolaria assisted in the formation of certain beds of chert, and foraminifera have been observed. The remarkable group, the graptolites, evidently inhabited the seas in countless numbers and have left their remains in the' dark shales of this period all over the world. At this time the diprionidian forms alone were represented by such genera as Tetragraptus, Phyllograptus, Didymograptus, Dicellograptus, Ditilogreptus and others. Of great interest
One of the most striking facts brought out by the study of the distribution of Ordovician fossils is the wide range of the northern or periarctic " faunal assemblage. This periaretic fauna prevails over the whole worldso far as our present knowledge showswith the exception of the peculiar Bohemian or Mediterranean region, which 238 includes north-west and south-west France, Spain, Italy, the Alps, the Fichtelgebirge, east Thuringia, Harz and Rhenish Mountains. End of Article: ORDOVICIAN SYSTEM If you wish, you can link directly to this article.
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