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Radiolarian chert and associated siliceous claystone in the Southern Uplands of Scotland are examined, in order to study the Great Ordovician Biodiversification Event of benthic animals on the pelagic ocean bottom. Trace fossils which are uncommon, but convincing, are found in the grey chert and siliceous claystone of Gripps Cleuch. These observations constitute firm evidence that large benthic animals which could leave visible trace fossils had colonised the Iapetan Ocean by the late Middle Ordovician, confirming previous studies from Australia for Panthalassa, the other huge ocean. Red chert is, however, a poor recorder of trace fossils, probably because the highly oxidising environment breaks down organic matter, both inhibiting high-density activity of large benthic animals and removing clear traces of benthic animal life.
In late Ordovician and early Silurian times, the Girvan district lay in a shelf marinesetting on the margin of Laurentia, on the northern side of the Iapetus Ocean. The Lower Palaeozoic rocks of the Girvan district, and their shelly and graptolitic fossil fauna, were systematically described by Lapworth in 1882 and have formed an important research resource ever since. They provide valuable evidence for the depositional environment and geological setting of Girvan during the early Palaeozoic, in both regional and wider contexts, and demonstrate the long-recognised close affinity with contemporaneous Laurentian faunas. However, by late Ordovician and into Silurian times, the earlier Iapetus oceanic barrier to faunal migration had largely gone and there is good correlation between contemporaneous marine fauna throughout the British Isles and Scandinavia. Despite much recent research in the area, including resurvey work by the British Geological Survey, no comprehensive review of Silurian lithostratigraphy at Girvan has been published since the revision by Cocks and Toghill in 1973. The present review of the Silurian rocks addresses this need and complements the recently published (Fortey et al. 2000) revision of the underlying Ordovician rocks, thus bringing the entire Girvan Lower Palaeozoic succession up to modern standards of nomenclature.
The Southern Uplands is a major Ordovician-Silurian accretionary terrane which developed as a marine basin over a period of c.75 Ma (495-420 Ma). The terrane extends to c. 10,000 km2 in Scotland alone and correlates with the similar-scale Longford–Down terrane in Ireland. Despite tectonic complexity, a detailed lithostratigraphy has been erected. The oldest strata are mudstones, cherts and lavas of mid-Arenig age known only in the Leadhills Imbricate Zone. The next youngest rocks are of similar lithology but of late Llanvirn-early Caradoc age. These oceanic sediments are succeeded by black shales of the Moffat Shale Group which are, in turn, diachronously overlain by huge volumes of turbiditic sandstones, siltstones, mudstones and minor conglomerates (greywackes) of Caradoc to Wenlock age. Overall, the terrane is sandstone-dominated, with other components such as lavas, volcaniclastics and cherts representing only a tiny proportion of the total volume. The conglomerates have a broadly northerly provenance, whereas the sandstones exhibit both marginal (NW and SE), and axial (NE and SW) derivation. During the Ordovician, strongly contrasting sources alternated through time. The youngest sandstones (Hawick and Riccarton groups), are notably rich in detrital biogenic carbonate, a rare component in the Leadhills Supergroup and Gala Group.
Ostracodes have a wide geographical distribution in the Ordovician of Scotland. They are known from the Southern Uplands, the Girvan district, the Highland Border region and the Inner Hebrides. Overall, more than forty species are recorded. They occur in clastic and carbonate rocks indicative of a range of shallow to deeper marine-shelf environments. Though many of the faunas are allochthonous, broad patterns of ostracode palaeoenvironmental distribution can be elucidated, and elements of the shallow marine Leperditella and open marineshelf Anisocyamus associations (previously recorded from N America) are present. Indigenous faunas are absent from the deep marine sediments of the Southern Uplands Northern Belt. Ostracodes are known from the Arenig, Llanvirn, Caradoc and Ashgill series in Scotland; those of the latter two series have widest biostratigraphical value. In the Girvan district the Caradoc species ‘Ctenobolbina’ ventrospinosa, Krausella variata, Balticella deckeri and Monoceratella teres have correlative value with N America, whilst the Ashgill species Kinnekullea comma appears to be a locum for the anceps graptolite Biozone in Britain, Ireland and possibly the eastern Baltic. The ostracodes are of typical Laurentian affinity, but show progressive generic links with the Baltic region during the late Llanvirn–Caradoc interval, and by Ashgill times display species-level links with southern Britain and Ireland. These distributional patterns suggest approaching geographical proximity for the early Palaeozoic continents of Laurentia, Baltica and Avalonia, and the ability of some Ordovician ostracodes to cross the Iapetus Ocean.
Leaching of Ordovician cherts from the Southern Uplands using dilute hydrofluoric acid has yielded numerous radiolaria, sponge spicules and a few conodonts. Although many of the radiolaria have suffered intense recrystallisation, it nevertheless proved possible to extract for SEM examination some examples with exquisitely preserved micro-structure. Despite this, the radiolaria have not proved sufficient, on their own, to answer the outstanding biostratigraphical question of one continuous or two separate episodes (Arenig and Llanvirn/Caradoc) of chert sedimentation in the Southern Uplands. This is partly because the biostratigraphy of Lower Palaeozoic radiolaria is still quite poorly known. A new radiolarian species, Protoceratoiciskum clarksoni Danelian, has been identified and described from cherts in the Crawford area. In addition, the distinctive sponge species Konyrium varium Nazarov & Popov, has also been found for the first time in Southern Uplands cherts. This new occurrence, in deep-water sediments, suggests a much wider habitat for this unusual sponge. Amongst the few conodont elements extracted, one can be identified as probably belonging to the genus Periodon.
A thick Ordovician marine flysch succession including greywackes, conglomer- ates, siltstones and shales occurs in West Nithsdale in the Northern belt of the Southern Uplands. Five new formations of Caradoc and ?Ashgill age are defined based mainly on petrographical and lithological evidence. The proposed correlation with areas up to 80 km distant to the SW, emphasises the persistent nature of the greywacke formations in the Southern Uplands when traced along strike. It is demonstrated that the main inliers of black shale in the Northern belt cannot be simple anticlines but are closely associated with large strike faults. The position of the investigated flysch succession in the accretionary prism model for the development of the Southern Uplands during the evolution of the Caledonides is indicated and its significance assessed.
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