Alan W. Owen

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.

Understanding the Great Ordovician Biodiversification Event (GOBE): Influences of paleogeography, paleoclimate, or paleoecology?

“The Great Ordovician Biodiversification Event” (GOBE) was arguably the most important and sustained increase of marine biodiversity in Earth’s history. During a short time span of 25 Ma, an “explosion” of diversity at the order, family, genus, and species level occurred. The combined effects of several geological and biological processes helped generate the GOBE. The peak of the GOBE correlates with unique paleogeography, featuring the greatest continental dispersal of the Paleozoic. Rapid sea-floor spreading during this time coincided with warm climates, high sea levels, and the largest tropical shelf area of the Phanerozoic. In addition, important ecological evolutionary changes took place, with the “explosion” of both zooplankton and suspension feeding organisms, possibly based on increased phytoplankton availability and high nutrient input to the oceans driven by intense volcanic activity. Extraterrestrial causes, in the form of asteroid impacts, have also been invoked to explain this remarkable event.

Evolution of the Northern Belt, Southern Uplands: implications for the Southern Uplands controversy

Conodont and graptolite dating provide a high resolution biostratigraphical framework for correlation across the Southern Upland Fault. The establishment of an early Caradoc age for the red chert and volcanic sequences within the Northern Belt of the Southern Uplands confounds the accretionary prism model. and our data support the view that following the obduction of the Ballantrae ophiolite, fan complexes prograded southeastwards across a narrow, fault-bounded shelf into the deep basin of the Northern Belt. The fan complexes were initiated by uplift in the hinterland, caused by the intrusion of plutons into the Midland Valley Terrane. Sedimentation was controlled by regional extension, subsidence and eustasy through the Llanvirn and Caradoc. Basic lavas, with attenuated within-plate geochemical signatures, were erupted during the early Caradoc peak of extension and may have just post-dated the initiation of southwards subduction beneath the Lake District.

Terrane evolution of the paratectonic Caledonides of northern Britain

A stratigraphically constrained re-evaluation of terrane amalgamation in the Caledonides of northern Britain allows the development of a new orogenic scenario which accounts for many of the outstanding problems in the paratectonic Caledonides and includes a new terrane template which correlates well with that proposed for Newfoundland. The Arenig Grampian Orogeny resulted from the accretion of two arc terranes to Laurentia: the Midland Valley (=Notre Dame Arc in Newfoundland) and a terrane of probable Avalonian/Gondwanan origin, here termed ‘Novantia’ (= Annieopsquotch Accretionary Tract partim), now hidden beneath the Southern Uplands allochthon. The Tyrone and Ballantrae ophiolites mark the northern boundary of Novantia within the composite Midland Valley Terrane. The Popelogan–Victoria Arc–Grangegeeth Terrane accreted to the amalgamated Midland Valley Terrane during the late Ordovician and initiated the Southern Uplands thrust duplex. A brief period of northward subduction during the Silurian followed, is ascribed to the northerly drift of the amalgamated Avalon–Baltica plate, and a final Wenlock (Scandian) collision and caused underplating of the Midland Valley Terrane. Caledonian deformation had ceased by the Emsian, the age of the undeformed Cheviot lavas that overlie the uplifted and peneplaned Southern Uplands.

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.

Rare earth element geochemistry of upper Ordovician cherts from the Southern Uplands of Scotland

Caradoc and Ashgill radiolarian cherts and siliceous mudstones from the Southern Uplands preserve primary rare earth element (REE) signatures which are comparable to those of more recent deposits from continental margin settings. This is incompatible with the widely held view of these rocks as open ocean deposits incorporated in an accretionary prism and reinforces the model of deposition on an extensional continental margin. The REE signatures can be used as fingerprints to differentiate between some of the fault-bounded formations within the area. They indicate the provenance of the mud grade siliciclastic material in these distal hemipelagites and are comparable with published REE data on grey wackes in the same successions. This detailed analysis of the REE patterns in Lower Palaeozoic cherts demonstrates the usefulness of this approach in ancient orogens.

Rare earth geochemistry of Arenig cherts from the Ballantrae Ophiolite and Leadhills Imbricate Zone, southern Scotland: implications for origin and significance to the Caledonian Orogeny

Rare earth element (REE) data from low to mid-Arenig cherts are used to test competing models for the early Ordovician evolution of the Laurentian margin in the northern British Isles. Cherts from the Ballantrae Ophiolite Complex have chondrite-normalized REE patterns typical of continental margin settings with LREE enrichment, a slight negative Euanom and shale and chondrite-normalized La/Yb values of 0.97–1.41 and 7.78–11.4 respectively. This pattern, together with a large positive chondrite-normalized Ceanom (1.44–1.70), is virtually identical to that found in radiolarian chert of the Gascoyne Abyssal Plain, in the Timor Sea. Cherts from the Raven Gill Formation within the Leadhills Imbricate Zone, Northern Belt, Southern Uplands have typical continental margin REE patterns, chondrite-normalized Ceanom (0.9–1.21) and Euanom (0.61–0.79) values indicating that they formed closer to the continental margin than those from Ballantrae. Shale and chondrite-normalized and La/Yb values of 0.95–1.27 and 4.92–13.88 respectively confirm this interpretation. It is concluded that the Ballantrae ophiolite formed in a rifted–arc basin above a northwards dipping, intra-oceanic subduction zone. The modest depth of burial of the Raven Gill Formation precludes it being part of a marginal basin which was subsequently trapped as the Ballantrae Ophiolite was obducted in the late Arenig. The Arenig rocks of the Leadhills Imbricate Zone represent an allochthonous terrane accreted to the western extension of the Midland Valley in Ireland in pre-Caradoc times. Here it formed the basement to the Southern Uplands basin. Palaeontological evidence places this basin adjacent to Pomeroy, Co. Tyrone in the early Caradoc. Sinistral strike-slip faulting, from the late Ashgill transported the Southern Uplands Terrane to its present location, a distance of less than 250 km.

The Ordovician biogeography of the Grangegeeth terrane and the Iapetus suture zone in eastern Ireland

The Caradoc trilobite and brachiopod faunas of the Grangegeeth terrane show marked Scoto–Appalachian/Laurentian affinities. Contrary to some plate tectonic models, this indicates that the terrane must lie to the north of the Iapetus suture. More limited graptolite evidence from the Llanvirn of Grangegeeth suggests that the terrane may have been closer to the Gondwanan margin or at least at higher latitudes earlier in the Ordovician and migrated to low latitudes prior to its eventual accretion onto the Laurentian margin.

A new Lower Ordovician conodont faunule from the Northern Belt of the Southern Uplands

Synopsis A deep water Periodon—Prioniodus—Protopanderodus assemblage has been isolated from red cherts and mudstones in the Hawkwood Burn, a tributary of the Wandel Burn. Constraints provided by associated graptolites and the absence of Pygodus and Arenig taxa known to occur in the Southern Uplands suggest this part of the sequence is Llanvirn in age.

RARE EARTH ELEMENTS IN CHERT CLASTS AS PROVENANCE INDICATORS IN THE ORDOVICIAN AND SILURIAN OF THE SOUTHERN UPLANDS OF SCOTLAND

Abstract Rare earth element (REE) patterns in chert clasts provide both an indication of the environment of deposition of the source succession and a means of recognising their provenance. The REE compositions of chert clasts in three Ordovician and Silurian conglomerates in the Southern Uplands of Scotland are compared to the REE fingerprints of their possible sources. The REE patterns of the clasts show that they were derived from successions deposited close to a continental margin. The absence of clasts with large positive cerium anomalies suggests that the Ballantrae Complex was not exposed and supplying material to the basin from at least the mid-Caradoc onwards although a single chert clast from the lower Llandovery conglomerate on Pinstane Hill has an almost identical REE pattern to a black chert from the Ballantrae Complex. A chert clast from a monomict chert conglomerate in the mid-Caradoc Kirkcolm Formation has an REE signature identical to that of bedded cherts in that formation, indicating an intrabasinal provenance. The REE patterns of a chert clast from the Caradoc Blackcraig Formation and of some of the clasts from the Pinstane conglomerate lie at the extreme end of those of the bedded Arenig and Caradoc cherts in the Southern Uplands but the REE patterns of other clasts from Pinstane and one from Caradoc-Ashgill Shinnel Formation lie beyond this, suggesting an extrabasinal source.