Stratigraphical framework for the Ordovician of Snowdonia and the Lleyn Peninsula : a discussion of the Tremadoc to Caradoc rocks lying between the Menai Straits and the Llanderfel Syncline, and including an appendix on Cambrian rocks. Version 2
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The following description of new genera and species of Lower Ordovician Cryptolithids is the result of a stratigraphical and palaeontological investigation of the rocks in the Llangadock-Llandeilo district, Carmarthenshire.
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Abstract The Drummuck Group of the Girvan district (S. W. Scotland) comprises a varied sequence of marine siliciclastic sedimentary rocks some 350 m thick and ranging in age from early Cautleyan to late Rawtheyan (Ashgill Series). Deposition in an unstable slope environment is envisaged. These upper Ordovician rocks crop out in the centre of the Craighead inlier, north of the Girvan valley and are sporadically exposed and locally very fossiliferous. A detailed revision of the Drummuck rocks has permitted, for the first time, the formal lithostratigraphical division of the group. Four main units, in ascending order, the Auldthorns, Quarrel Hill, Lady Burn and South Threave formations are recognized and, within these, a number of smaller, useful divisions are defined. Detailed geological maps of the Drummuck outcrop are presented. Brachiopods numerically dominate the shelly faunas and all the divisions named are characterized by distinctive brachiopod assemblages. Preliminary correlations are discussed with selected upper Ordovician successions elsewhere but the emphasis of this study is placed upon the establishment of a stratigraphical framework within which future detailed research on the Drummuck faunas may be carried out.
Siliciclastic
Outcrop
Sequence Stratigraphy
Lithostratigraphy
Sequence (biology)
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The difficulties experienced in correlating strata near the Ordovician-Silurian boundary in Britain and North America arise partly from imperfect descriptions of the British Llandovery fauna. Many characteristic Llandovery fossils are attributed in lists to the Bala or Wenlock. In Britain, Ordovician and Silurian rocks occur in three fades: (1) pelitic, (2) psammitic, and (3) calcitic facies. The better-known American Ordovician and Silurian rocks belong mainly to the calcitic facies. A comparison is made between the genera of brachiopods and trilobites recorded from the British and American rocks near the Ordovician-Silurian boundary. In Anti-costi, the English Head, Charleton, and Ellis Bay must be referred to the Bala; the Becsie River is probably Lower Llandovery, while the Gun River, Jupiter River, and Chicotte are Upper Llandovery. The Richmond of the interior states is unmistakably Bala; the Clinton (and probably the Medina) is Upper Llandovery. Evidence that Lower Llandovery rocks occur on the American continent is wanting.
Pelite
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The Ordovician, Silurian and lower Devonian rocks of the
Domes area (near Figueiro dos Vinhos), in central Portugal, form
a continuous sedimentary sequence approximately 1450m thick. Folded
Pre-Ordovician Complexo xisto-grauvaquico (CXG) is unconformably
overlain by a transgressive quartzite, mudstone and greywacke
succession of four Ordovician formations which are locally shelly
and graptolitic. These formations are: Serra do Brejo Formation,
Brejo Fundeiro Formation, Monte da Sombadeira Formation and Monte
do Carvalhal Formation. The Ordovician rocks are conformably overlain
by a Silurian sequence of graptolitic mudstones passing upwards
into sandstones; these beds include the Vale da Ursa Formation,
Foz da Serta Formation, Vale do Serrao Formation, Serra da Mendeira
Formation and part of the Serra do Luacao Formation. The Devonian
rocks are represented by part of the Serra do Luagao Formation and
the Dornes formation, a sandstone-madstone sequence passing up into
Shelly limestones.
Most of the Ordovician faunas are described and discussed
including sixteen species of trilobite, five species of brachiopod
and eleven ichnospecies of trace fossil. Faunal lists of material
identified by specialists in the fields of Ordovician and Silurian
graptolites, Ordovician echinoderms, Devonian brachiopods and
Silurian microfossils are also given. The Ordovician faunas belong
to the Selenopeltis province and show close links with those of Spain,
France, Morocco and Bohemia. The correlation and palaeogeography of the
Dornes area is discussed with emphasis on the Ordovician Period in
the Ibero-Armorican area.
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Devonian
Sequence (biology)
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Conodont species Iapetognathus fluctivagus and Iapetonudus ibexensis are documented for the first time from Australia. The former is the primary marker internationally defining the base of the Ordovician, and the latter is also a distinctive species previously recorded only from the base of the Ordovician in North America. Both species were recovered from a single sample in the Kandie Tank Limestone of the Kayrunnera Group, located about 50 km west of White Cliffs in far western New South Wales. Other species recovered from this sample include Prooneotodus spp., Cordylodus lindstromi, Cordylodus proavus, Hirsutodontus simplex, Teridontus nakamurai and Variabiloconus sp. Recognition of the Iapetognathus fluctivagus Biozone in the Kandie Tank Limestone supports its correlation with the Green Point section (Global Stratigraphic Section and Point for the base of the Ordovician) in western Newfoundland and the Lawson Cove section of Utah (Auxiliary Stratigraphic Section and Point), as well as sections in Asia and South America. Review of other sections in Australia and elsewhere spanning the Cambrian–Ordovician boundary confirms that, in the absence of I. fluctivagus, the presence of C. lindstromi is a good proxy for this level.
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Biozone
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Abstract Limestone outcrops that form a north-south belt approximately 1.4 km long, 15 km east of Manilla and previously referred to as the Uralba Beds, and associated rocks, are shown to be an olistostromal component of the Wisemans Arm Formation. Elongate olistoliths up to 100 m in length are flanked by limestone conglomerate with clasts set in a volcaniclastic matrix derived from a range of volcanic rock types. Volcanic olistoliths, including a mass of ankaramitic basalt, are also present. Conodont faunas of differing ages were obtained from various outcrops. A Late Ordovician (Eastonian, Ea3) fauna of over 400 elements from some outcrops is identical to one recently documented from elsewhere in the Wisemans Arm Formation. A small Early Silurian (late Llandovery-early Wenlock) fauna from four outcrops (including two formerly thought to be Ordovician) comprises the first documented conodont fauna of this age from the New England Fold Belt. Key words: ConodontsOrdovicianLate EastonianSilurianLlandovery - CaradocPeel Fault SystemNew South WalesAustralia
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Preface 1. Introduction 2. Base of the Silurian rocks 3. Lower Silurian rocks 4. Lower Silurian rocks (cont.) 5. Upper Silurian rocks 6. Upper Silurian rocks (cont.) 7. Silurian rocks of Britain 8. Organic remains of the lower Silurian rocks 9. Fossils of the upper Silurian rocks 10. The Devonian rocks, or old red sandstone 11. Carboniferous rocks 12. Permian rocks 13. General view of the Silurian, Devonian, and Carboniferous rocks of Scandinavia and Russia 14. Primeval succession in Germany and Belgium 15. Silurian, Devonian, and Carboniferous rocks of France, Spain, Portugal, and Sardinia 16. Succession of primeval rocks in America 17. On the original formation of gold 18. Conclusion Appendix Index Plates.
Devonian
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Twenty-two graptolite zones are recognized in the Ordovician and Silurian graptolitic shales of northern Yukon and their geographical distribution, faunal assemblages, and correlation with Europe and Arctic Islands are commented upon. All the stages and sub-series in the Ordovician and Silurian of Britain are recognized. A new formational name, Road River, is proposed for the Siluro-Ordovician graptolitic shales and carbonates of the region. At its type locality, the formation is underlain conformably by Cambrian (?) shales and argillites, and overlain disconformably by Devonian Fort Creek shale. The formation is Tremadocian to Ludlovian in age and the Siluro-Ordovician boundary is located on a faunal break.
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Abstract A new lithostratigraphy is presented for the Skiddaw Group (lower Ordovician) of the English Lake District. Two stratigraphical belts are described. Five formations are defined in the Northern Fells Belt, ranging in age from Tremadoc to early Llanvirn. They are all mudstone or sandstone dominated, of turbidite origin; in ascending order they are named the Bitter Beck, Watch Hill, Hope Beck, Loweswater and Kirk Stile formations. Two formations are defined in the Central Fells Belt, ranging in age from late Arenig to Llanvirn. These are the Buttermere Formation – a major olistostrome deposit – overlain by the Tarn Moor Formation, consisting of turbidite mudstones with volcaniclastic turbidite sandstone beds. A revised graptolite and new acritarch biostratigraphy for the Skiddaw Group is presented with eight graptolite biozones and thirteen acritarch assemblages and sub-assemblages. The provenance of the group is assessed from detailed petrographical and geochemical work. This suggests derivation, in the early Ordovician, largely from an old inactive continental arc terrane lying to the southeast, with the appearance of juvenile volcanic material in the Llanvirn. Comparisons and correlations of the Skiddaw Group are made with the Isle of Man and eastern Ireland.
Acritarch
Lithostratigraphy
Biozone
Palaeogeography
Laurentia
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I. ABSTRACT: Research interest in the Highland Border Complex has been pursued sporadically during the past 150 years. The results and conclusions have emphasised the problems of dealing with a lithologically disparate association which crops out in isolated, fault-bounded slivers along the line of the Highland Boundary fault. For much of the present century, the debate has centred on whether the rocks of the complex have affinities with the Dalradian Supergroup to the N, or are a discrete group. Recent fossil discoveries in a wide variety of Highland Border rocks have confirmed that many are of Ordovician age, and hence cannot have been involved in at least the early Grampian deformational events (now accurately dated as pre-Ordovician) which affect the Dalradian Supergroup. Such palaeontological discoveries form the basis for a viable biostratigraphical synthesis. On a regional scale, it is apparent that the geological history of the Highland Border rocks must be viewed in the context of plate boundary tectonism along the entire northwestern margin of Iapetus during Palaeozoic times. II. ABSTRACT: Silicified articulate brachiopods from the Lower Ordovician (Arenig) Dounans Limestone are extremely rare but the stratigraphically diagnostic genera Archaeorthis Schuchert and Cooper, and Orthidium Hall and Clarke, have been identified. In addition, three specimens with characteristic syntrophiid morphology have been recovered. Inarticulate brachiopods are known from Stonehaven and Bofrishlie Burn near Aberfoyle, and have also been previously recorded from Arran. III. ABSTRACT: Micropalaeontological investigation of the Highland Border Complex has produced a range of microfossils including chitinozoans, coleolids, calcispheres and other more enigmatic objects. The stratigraphical ranges of the species lie almost entirely within the Ordovician and reveal a scatter of ages for different lithologies from the Arenig through to the Caradoc or Ashgill, with a pronounced erosional break between the Llandeilo and the Caradoc. IV. ABSTRACT: A Lower Ordovician (Arenig Series) silicified ostracode fauna from the Highland Border Dounans Limestone at Lime Craig Quarry, Aberfoyle, Central Scotland, represents the earliest record of this group of Crustacea from the British part of the early Palaeozoic ‘North American’ plate. V. ABSTRACT: Palaeontological age determinations for a variety of Highland Border rocks are presented. The data are based on the results of recent prospecting which has demonstrated that macro- and microfossils are present in a much greater range of Highland Border lithologies than previously realised. Data from other studies are also incorporated, as are modern taxonomie re-assessments of older palaeontological discoveries, in a comprehensive survey of Highland Border biostratigraphy. These accumulated data demonstrate that all fossiliferous Highland Border rocks so far discovered are of Ordovician age, with the exception of the Lower Cambrian Leny Limestone. VI. ABSTRACT: The Highland Border Complex consists of at least four rock assemblages: a serpentinite and possibly other ophiolitic rocks of Early or pre-Arenig age; a sequence of limestones and conglomerates of Early Arenig age; a succession of dark shales, cherts, quartz wackes, basic lavas and associated volcanogenic sediments of Llanvirn and ? earlier age; and an assemblage of limestones, breccias, conglomerates and arenites with subordinate shales of Caradoc or Ashgill age. At least three assemblages are divided by unconformities and in theirmost general aspect have similarities with coeval rocks in western Ireland. The Highland Border Complex probably formed N of the Midland Valley arc massif in a marginal sea comparable with the Sunda shelf adjacent to Sumatra–Java. Strike-slip and thrust emplacement of the whole Complex in at least four episodes followed the probable generation of all or part of its rocks by pull-apart mechanisms.
Dalradian
Supergroup
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