A diverse assemblage of moderately well-preserved radiolarians occurs in the Ordovician cherts of the Crawford Group in the Southern Uplands terrane of the Scottish borders. A total of 8 genera and 11 species are described. Taxa present correlate to the Middle Ordovician Pygodus anserinus biozone. Key species identified include: Proventocitum procerulum, Inanigutta gansuensis, Inanigutta sp. cf. I. complanata, Inanibigutta sp. cf. I. verrucula, Inanibigutta sp. cf. I. pinglianensis, ?Inanibigutta inconstans, ?Inanihella penrosei, Haplentactinia armillata, ?Oriundogutta ramificans and Syntagentactinia sp. A new species Haplotaeniatum albaensis is introduced here and described on the basis of details of skeletal architecture observed using microcomputed tomography, demonstrating the potential of this technique as a tool for improving the understanding of radiolarian taxonomy.

Previous studies of low-grade metamorphism in the Southern Uplands accretionary terrain indicated prehnite-pumpellyite facies/anchizone conditions developed throughout the area, except for local preservation of trench-slope sediments and an accreted seamount at zeolite facies/advanced diagenetic grade. New graptolite reflectance data are presented that show a general northward increase in temperature in the Southern Uplands. The results from two cross-strike traverses in the southern and central belts in contemporaneous sequences, using illite crystallinity, illite lateral spacing (bo) , and graptolite reflectance, indicate the development of systematic accretion-related low-grade metamorphism. Well-developed and constant anchizone conditions occur throughout the NE (Langholm) traverse, associated with common, F1 accretion-related folding and a regionally penetrative S1 cleavage. In the SW (Kirkcudbright) traverse, however, the youngest, last accreted packets are preserved at a transitional diagenetic stage and lack a penetrative S1 cleavage. Illite crystallinity, graptolite reflectance, and bo increase systematically northward through earlier accreted packets, reaching values of the NE traverse only at the northern end. The concomitant increase of bo with illite crystallinity suggests the relatively high P-low T trajectory characteristic of subduction zones. Integration of metamorphic and structural data relates increasing intensity of aceretion-related F1 folding, developmertt of S1 fabric, and onset of later fold phases to grade of metamorphism and structural level within the accretionary pile.

Stream sediment geochemistry provides an innovative method of assessing the basinal history of the Caledonian slate belts. Despite glaciation, the stream sediment geochemical patterns spatially mimic the outcrop of underlying bedrock lithologies. However, erosion from rock to sediment by fluvial processes may either increase or reduce an element’s abundance depending on the nature of its mineral host. An element held in heavy, resistate minerals will be concentrated, whereas one residing in unstable ferromagnesian minerals, which readily break down to clays during weathering, may be preferentially removed. Examples are provided from the Cr-Ni-V-Mg, base metal and Rb-Sr element suites. Primary and secondary bedrock patterns are recognized in the stream sediments. Primary patterns follow the original composition of the source bedrock, with steep gradients in the element distribution coinciding with lithostratigraphical boundaries. Such patterns also reveal subtle divisions within the established geological units for which the main compositional control was the nature of the ancient sedimentary provenance. Secondary patterns reflect remobilization of elements within the bedrock and so may cut across lithostratigraphical boundaries. These patterns (or their absence) are influenced by the thermal histories of the Caledonian basins, and so are indicative of the geotectonic regime in which the sedimentary sequences were originally deposited.

A study of the décollement zones in the Moffat Shale Group in the Ordovician Northern Belt of the Southern Uplands of Scotland reveals a progressive sequence of deformation and increased channelization of fluid flow. The study concentrates on exposures of imbricated Moffat Shale on the western coast of the Rhins of Galloway. Initial deformation occurred in partially lithified sediments and involved stratal disruption and shearing of the shales. Deformation then became more localized in narrower fault zones characterized by polyphase hydrothermal fluid flow/veining events. Deformation continued after vein formation, resulting in the development of low-temperature crystal plastic microstructures and further veining. Late-stage deformation is recorded as a pressure solution event possibly reflecting the cessation of slip on these faults as the slice became accreted. Most deformation can be ascribed to SE-directed thrusting and incorporation of the individual sheets into the Southern Uplands thrust stack. Later sinistral shear deformation, not observed in overlying turbidites, is also localized in these fault zones. The study reveals the likely structures formed at levels of an accretionary prism deforming under diagenetic to low-grade metamorphic conditions.

Structures deforming Llandovery turbidites of the Gala Group in the Southern Uplands terrane are spectacularly exposed in the Berwickshire coastal section, southeastern Scotland. The upward-facing, upright to NW-vergent folds and associated structures appear to record a single regional phase of subhorizontal NW–SE contractional deformation, with a steeply dipping direction of bulk finite extension. These structures are markedly different from those developed in rocks correlated with the Upper Llandovery Hawick Group exposed some 5 km to the south in the Eyemouth–Burnmouth coastal section. Here a highly domainal system of sinistral transpressional strain occurs, with zones of steeply plunging curvilinear folds, clockwise cleavage transection and bedding-parallel sinistral detachment faults. The markedly different bulk strain patterns in the Berwickshire coastal sections are thought to reflect the regionally diachronous nature of transpressional deformation in the Southern Uplands terrane. There are striking similarities in the structures recognized in the Berwickshire coastal sections and those developed in stratigraphically equivalent units along strike in southwestern Scotland and Northern Ireland. This confirms the lateral structural continuity and correlation of tracts and tract boundaries along the entire length of the Southern Uplands terrane. The regional structure suggests that a phase of top-to-the-NW backthrusting and backfolding associated with the southern margin of the Gala Group outcrop marks the transition from orthogonal contraction to sinistral transpression in the Southern Upland thrust wedge during late Llandovery times.

The Bail Hill Volcanic Group (Caradoc) represents the largest, single volcanic complex exposed within the Ordovician turbidite succession of the Northern Belt in the Southern Uplands of Scotland. The group comprises a heterogeneous sequence of submarine lavas, volcaniclastic and intrusive rocks (up to 2 km thick), and crops out in a small area (c. 4 km2) around Bail Hill, north of Sanquhar. The Cat Cleuch Formation (older) is dominated by a sequence of autobrecciated basaltic lavas which contain large, zoned diopsidic clinopyroxene. The overlying Peat Rig Formation comprises a more mixed sequence of plagioclase-amphibole-phyric lavas, volcaniclastic rocks and contemporaneous volcaniclastic sedimentary rocks. The Cat Cleuch and lower part of the Peat Rig formations are cut by a vent breccia, the Bught Craig vent breccia, which formed part of the feeder to the upper part of the Peat Rig Formation. The Bail Hill volcanic rocks are alkaline in character, ranging from alkali basalt to trachyandesite in composition, possessing trace element characteristics and enrichment patterns typical of oceanic within-plate basalts. The Bail Hill Volcanic Group, although geochemically distinct, forms part of a mixed assemblage of tholeiitic and alkaline withinplate lavas within the Southern Uplands which are of broadly similar age, some of which are intercalated within the greywacke sandstone sequence. This assemblage clearly indicates that a period of extension and within-plate volcanism occurred during the early stages of the development of the Southern Uplands sedimentary basin.

The Ordovician and Silurian successions between Falahill and Galashiels encompass six flysch-dominated formations: the Upper Ordovician Portpatrick and Shinnel Formations representing the Leadhills Group, the Llandovery Mindork, Garheugh, and Buckholm Formations together comprising the Gala Group, and a formation indeterminate of age within the Hawick Group. Southward ensialic andesitic volcanic arc and northward low- to medium-grade sialic sources contributed sediment, whilst ophiolitic and subduction-related sources made minor contributions. Deposition took place firstly, in a SE-migrating back-arc basin bordering the northerly source, the Laurentian continent. Subsequent NW-directed underthrusting led to formation out of the back-arc basin of an imbricate thrust stack which migrated southeastwards. Ultimately a foreland successor basin formed ahead of the rising thrust stack.

Flysch units are typically associated with linear outcrops of Moffat Shales which are the loci of major steep SE-translating reverse faults, two of which participate in a late-stage sinistral strike–slip duplex with large-scale imbrication. The faults divide the succession into a sequence of tectonostratigraphic blocks, successively younger to the SE. At least six of the ten blocks customarily recognised in the Southern Uplands, Blocks 3–8, are represented, some of which coincide with single or complete formations.

The Notre Dame and Exploits subzones of Newfoundland's Dunnage Zone are correlated with the Midland Valley and Southern Uplands of Scotland, using detailed comparisons of two key Lower Palaeozoic successions which record similar histories of extension and compression. It follows that the Baie Verte Line, Red Indian Line and Dover Fault are equivalent to the Highland Boundary Fault, Southern Upland Fault and Solway Line, respectively.

The Betts Cove Complex and overlying Snooks Arm Group of the Notre Dame Subzone are analogous to the Ballantrae Complex of the Midland Valley, both recording the Arenig evolution and subsequent obduction of an arc and back-arc system. The Early Ordovician to Silurian sequence unconformably overlying the Ballantrae Complex is poorly represented in the Notre Dame Subzone but important similarities can still be detected suggesting corresponding histories of continental margin subsidence and marine transgression.

In the Exploits Subzone, Early Ordovician back-arc volcanic rocks are overlain by Llandeilo mudstones and Late Ordovician to Early Silurian turbidites. A similar stratigraphy occurs in the Northern and Central Belts of the Southern Uplands and both areas have matching transpressive structural histories. Deeper erosion in the Exploits Subzone reveals Cambrian and Early Ordovician volcano-sedimentary sequences structurally emplaced on the Gander Zone, and such rocks are probably present beneath the Southern Uplands. Combined data from the Notre Dame Subzone and Midland Valley suggest an Arenig southeast-dipping subduction zone. Early Ordovician volcanic rocks in the Exploits Subzone and Southern Uplands have back-arc basin geochemistry and support the model of the Southern Uplands as a transition from back-arc to foreland basin. Preferential emergence of the Dunnage Zone and contrasts between Exploits Subzone and Southern Uplands turbidite basins are attributed to collision of Newfoundland with a Laurentian promontory and Scotland with a re-entrant. This hypothesis also explains the transpressive structural regime common to both areas.

The Moffat Shale Group is a condensed, variable and partly pelagic sequence of mudrocks of Llandeilo—Llandovery age. The sequence has a five-fold lithological subdivision based mainly on the occurrence of grey mudstones within a succession otherwise dominated by fully euxinic black graptolitic mudrocks. Associated with the black mudrocks, especially in the Llandovery, are metabentonite beds which achieve a climax, both in thickness and in number, within the top quarter of the mudrock sequence. A geochemical and mineralogical study has confirmed a volcanic origin for the metabentonites. Major element data highlight a carbonate-dominated environment above the gregarius—convolutus Zones boundary. Phosphorus levels reach a peak at the same boundary, as well as at the Caradoc—Ashgill boundary where phosphorite horizons are known from Wales and Norway. Immobile trace elements have highlighted regular changes in source magma composition. Prolonged periods of crystal fractionation in magmas of intermediate composition gave rise, on eruption, to large volumes of silicic ash which had a deleterious effect on graptolite species and led to local extinctions. Regular fluctuations in ash composition from silicic to intermediate are ascribed to alternating fractionation and magma mixing cycles.

Sediments, mainly sandstones, conglomerates and shales, accumulated in small turbidite fans along the northern arc–trench margin of the Iapetus Ocean from middle Ordovician to Silurian time. These fans, together with the underlying pelagic facies and part of the oceanic crust, were sliced and accreted northward resulting in the Lower Palaeozoic accretionary prism which forms the Scottish Southern Uplands and the Longford-Down inlier in Ireland. North Down is the continuation of the Northern belt of the Southern Uplands of Scotland into Ireland, bounded to the S by the Orlock Bridge fault. Lithological and petrographical comparison with the rest of the Northern belt indicates closer affinities with the Southern Uplands of Scotland than with the western end of the Longford-Down inlier. Major ENE—WSW-trending Caledonian strike faults define five blocks, in which new formations of Caradoc and ? Ashgill age are defined. Pillowed spilitic rock, interpreted as a fragment of the ocean-floor, is only recognised in the Ballygrot block. Pelagic and hemipelagic black shales and cherts are overlain by arenaceous sediments in all blocks.

The Midland Valley of Scotland was an arc–interarc region during most of Ordovician—Devonian time. This arc terrane extends beneath the allochthonous Southern Uplands and probably beneath the southern Highlands. Models of Caledonian plate tectonism which regard the Midland Valley as a fore-arc basin are rejected principally on the grounds that (i) the Ordovician sequence at Girvan, in the very SW of the Midland Valley, was generated in a proximal fore-arc basin to the immediate S of a contemporaneous plutonic–volcanic arc, and (ii) the source for Silurian sediments in the southern part of the Midland Valley could not have been a rising trench-slope-break, but igneous basement and conglomerates with clasts of metamorphic basement, i.e. the southward extension of the Midland Valley.

The Midland Valley arc first comprised mainly plutonic rocks, some of which may have been basic but most of which were certainly granitic. Little is known of the ages of volcanic clasts in Silurian conglomerates (this time may have been a period of relative volcanic quiescence), but the Silurian–Devonian sequence is considered to have formed in an interarc basin which, like many other basins of this kind, began as marine (Early Silurian) and ended as fluvial (Devonian). At this final Silurian–Devonian stage, the Midland Valley arc was dominated by effusive rocks which made a substantial contribution to the sediments.

In this interpretation, the present Old Red Sandstone volcanic rocks are seen as the final stage of a volcanic arc which occupied the position of the present Midland Valley from at least Llanvirn to Early Devonian time.

Late Devensian ice-sheet movement in the western Southern Uplands of Scotland is demonstrated by the distribution of erratics from four different bedrock sources and by glacial striae and ice-moulded landforms. This evidence shows an almost radial movement of ice from an ice-divide zone. The control of relief over ice flow and the distribution of erratics is emphasised. The distribution of erratics from two of the sources, however, shows that debris in the ice was transported only a very short distance in certain places. Abundant deposits of lodgement till occur in the vicinity of the former ice-divide. Although it is generally considered that glacial erosion, and therefore deposition, are insignificant at the centre of ice-sheets, this study suggests otherwise. It is inferred that till and erratics may be emplaced during ice-sheet build-up when conditions for erosion obtained, these deposits being protected from subsequent removal by the establishment of an ice-divide across the area. The implications for high rates of erosion in ice-sheet source areas are examined.

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.