A. W. A. Rushton

A revision of Ordovician series and stages from the historical type area

This paper is a revision of the standard chronostratigraphy of the Ordovician of the historical type area in England and Wales. The revision is a response to the need for more precise definitions of series and stages, especially for practical international correlation. In some cases this has entailed moving horizons for Series bases away from those in the classical sections. The scheme will be used as the standard for a forthcoming revision of the Correlation Chart of the British Ordovician System. The Ordovician is divided into five series: Tremadoc, Arenig, Llanvirn, Caradoc and Ashgill. The Llanvirn is extended to include part of the classical Llandeilo Series, which is included within it as a stage. Where stage subdivisions of standard series have not yet been defined, as in the Tremadoc, they are proposed in this paper. However, Bancroft’s fine stadial divisions of the Caradoc, which have been criticized for their brevity and local utility, are combined into four new stages, which will have wider application. The former Caradoc subdivisions are retained as substages for the purposes of regional correlation.

Palaeogeographical controls on the Cambrian trilobite immigration and evolutionary patterns reported in the western Gondwana margin

Abstract Southward drifting of the western Gondwanan margin during the Cambrian has been demonstrated by means of both palaeomagnetic methods and lithological indicators of climate (such as carbonates and evaporites). Recent improvements in biostratigraphical correlations permit an enhanced understanding of the climatic and palaeobiogeographical constraints that controlled the distribution of Cambrian benthic communities. Palaeogeographical and biogeographical reconstructions based on trilobites are reported in this paper in order to test interaction between migration, speciation and extinction rates. The variability of the documented biogeographical patterns is directly related to species diversity, in which wider distribution coincides with transgressive trends and subsequent connection of neighbouring platforms. Early Cambrian trilobite faunas show a high degree of both substrate control and endemicity, although transgressions led to parallel shifts in faunal compositions. By contrast, Mid-Cambrian trilobite faunas are relatively uniform across western Gondwana, and latest Mid- and Late Cambrian associations document influence of an increased similarity with Asian trilobite faunas.

New uppermost Cambrian U–Pb date from Avalonian Wales and age of the Cambrian–Ordovician boundary

A crystal-rich volcaniclastic sandstone in the lower Peltura scarabaeoidesZone at Ogof-ddu near Criccieth, North Wales, yields a U-Pb zircon age of 491 ± 1 Ma. This late Late Cambrian date indicates a remarkably young age for the Cambrian-Ordovician boundary whose age must be less than 491 Ma. Hence the revised duration of the post-Placentian (trilobite-bearing) Cambrian indicates that local trilobite zonations allow a biostratigraphic resolution comparable to that provided by Ordovician graptolites and Mesozoic ammonites.

The stratigraphy, correlation, provenance and palaeogeography of the Skiddaw Group (Ordovician) in the English Lake District

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 south-east, 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.

Biostratigraphical control of thrust models for the Southern Uplands of Scotland

Graptolite biostratigraphy affords a robust and relatively accurate means of correlating Ordovician and Silurian hemipelagite and turbidite sequences and has been used to establish the structural development of the regional thrust belt in the Southern Uplands of Scotland. The overall structural pattern has long been recognised: greywackes within individual thrust slices, deposited within a relatively short time-interval, become sequentially younger southwards; each overlies the basal Moffat Shale Group which was deposited over a longer time. However, recent refinement of the graptolite biozonal scheme has allowed the better assessment of along-strike variations within the thrust belt which are here illustrated by two transects; one, based on work in the Rhins of Galloway and the Kirkcudbright areas (SW Southern Uplands), and the other in the Peebles-Hawick area (NE Southern Uplands). The SW transect most closely approximates to the regular pattern wherein a southward-propagating thrust-front incorporated sequentially younger greywacke units. The uniform geometry is interrupted only locally, towards the southern margin of the thrust belt, by a system of back-thrusts producing structural pop-ups. The NE transect departs from this regular model: a northern sector shows the orderly initiation of the thrust belt, but towards the SE a more irregular distribution of the thrust-slice agescan be best explained by outof- sequence movement. This transect also shows more repetitive imbrication of the same biostratigraphic interval than is apparent farther SW. In both transects the fundamental changes in thrustbelt geometry took place from mid-Llandovery times onwards, with a reversion to forward-breaking, in-sequence thrusting at the beginning of the Wenlock. The cause is a matter for speculation, but may be linked withthe accommodation of an obstacle to forward-thrust propagation. However it is recognised that such variationsin thrust geometry are a fundamental feature of most thrust belts and do not require a single regionally significant cause.

Late Caradoc graptolitic faunal gradients across the Iapetus Ocean

Late Caradoc graptolite assemblages across the Iapetus Ocean in Wales and Scotland became progressively more disparate despite the narrowing of the ocean. We compare faunal distributions in continuous sections from opposite sides of Iapetus, at Whitland in South Wales and Hartfell Score in the Southern Uplands of Scotland. A comprehensive graptolite range-chart is given for each. The graptolite assemblages from the clingani Biozone are subdivided into a lower Ensigraptus caudatus Subzone and upper Dicellograptus morrisi Subzone at both localities, though the faunas differ in detail. Higher in the sequence, the distinctive Scottish linearis Biozone fauna is not recognizable at Whitland, its presumed equivalent being a fauna dominated by variable morphotypes of the genus Normalograptus . This suggests that environmental gradients (depth and/or temperature) were changing more rapidly than the geography. Significantly, the subsequent inception of limestone deposition at Whitland was approximately contemporaneous with widespread replacement of black mudstones by oxic, bioturbated sediments elsewhere in the Welsh Basin and in Scotland. This was possibly a response to an early phase of cooling prior to the end-Ordovician glaciation. In Wales the claimed hiatus at the Caradoc-Ashgill boundary may rather reflect biofacies variation.

Global Cambrian trilobite palaeobiogeography assessed using parsimony analysis of endemicity

Abstract Palaeobiogeographical data on Cambrian trilobites obtained during the twentieth century are combined in this paper to evaluate palaeoceanographic links through c. 30 myr, once these arthropods biomineralized. Worldwide major tectonostratigraphic units are characterized at series intervals of Cambrian time and datasets of trilobite genera (629 for Cambrian Series 2, 965 for Cambrian Series 3, and 866 for the Furongian Series) are analysed using parsimony analysis of endemicity. Special attention is given to the biogeographical observations made in microcontinents and exotic terranes. The same is done for platform-basinal transects of well-known continental margins. The parsimony analysis of endemicity analysis resulted in distinct palaeogeographical area groupings among the tectonostratigraphic units. With these groupings, several palaeobiogeographical units are distinguished, which do not necessarily fit the previously proposed biogeographical realms and provinces. Their development and spatial distributions are broadly controlled by Cambrian palaeoclimates, palaeogeographical conditions (e.g. carbonate productivity and anoxic conditions) and ocean current circulation. Supplementary material: Global dataset of Cambrian Epoch 2 (A), Cambrian Epoch 3 (B) and the Furongian Epoch (C) trilobite genera are provided at: http://www.geolsoc.org.uk/SUP18669

Cosmopolitan arthropod zooplankton in the Ordovician seas

Abstract Evidence is presented here for a zooplanktonic component in Ordovician marine ecosystems, namely the caryocaridid arthropods, that add to other well-documented midwater organisms such as graptolites, cyclopygid and telephinid trilobites, orthoconic cephalopods and the microphytoplankton (e.g. acritarchs). Although the soft anatomy of caryocaridids is largely hypothetical, their carapace design and ultrastructure, and their phyllocarid-like abdominal morphology (flattened furcal rami, telescopic segments) indicate a swimming lifestyle in midwater niches. Both functional and ecological interpretations are supported by their palaeogeographical and facies distributions and by analogies with modern pelagic ostracods. Caryocaridids occur at numerous localities on the palaeo-plates of Laurentia, Baltica, Avalonia, Perunica, Gondwana and South China and are recurrent faunal components of graptolitic black shales (mainly Tremadoc to Llanvirn). Typical faunal associates are the didymograptid and isograptid graptolites, pelagic cyclopygid and deep-sea benthic atheloptic trilobites. Their depositional environments suggest that the caryocaridids and their pelagic associates (graptolites) most probably thrived in waters above the distal shelf margins, where upwelling-controlled primary productivity possibly reached its maximum. Their exact bathymetrical range within the water column cannot be inferred from fossil evidence. However, their feeding strategies may have led them to exploit food resources across the mesopelagic–epipelagic boundaries as do numerous midwater crustaceans in present-day ecosystems. Caryocaridids represent a significant step in the post-Cambrian colonisation of midwater niches by arthropods and in the construction of complex modern foodwebs.

New age constraints for the Ordovician Tyrone Volcanic Group, Northern Ireland

New biostratigraphical evidence and a high-precision isotope dilution thermal ionization mass spectrometry U–Pb zircon age provide refined age constraint for the Ordovician Tyrone Volcanic Group of the Tyrone Igneous Complex. In a graptolite fauna from Slieve Gallion, the presence of Isograptus victoriae lunatus, the index fossil of the victoriae lunatus graptolite zone, indicates a correlation with the Australasian Castlemainian (Ca1) Stage. The U–Pb zircon age of 473 ± 0.8 Ma dates a volcanic arc-related rhyolite body that is stratigraphically below graptolitic mudstones of Slieve Gallion. The U–Pb isotopic and biostratigraphical age constraints closely match an interpolated age for the base of the Middle Ordovician and indicate a Whitlandian age for the upper Tyrone Volcanic Group, which supports the regional correlation with the Ballantrae Complex, Midland Valley Terrane, Scotland.

Correlation for the Lower Palaeozoic

The global correlation standards now agreed for the Ordovician and Silurian systems, and partly so for the Cambrian System, are summarized. Correlation of the international series and stages with the traditional and revised British series and stages is reviewed, as well as with those in North America (Laurentia).