B. J. Bluck

Role of the Midland Valley of Scotland in the Caledonian orogeny

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.

Age, evolution and tectonic history of the Highland Border Complex, Scotland

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.

The Scottish paratectonic Caledonides

Synopsis The southern margin of the Laurasian continent has been destructive or strike-slip from at least 576 Ma to c. 360 Ma. There may have been a pre- or early Arenig obduction of ophiolite onto it, and this ophiolite now provides the base to the Ordovician part of the Highland Border Complex and could have been responsible for regional metamorphism in Dalradian or related rocks. At c. 480 Ma there was certainly final obduction of the Ballantrae Complex, which formed in an oceanic setting some distance south of its present location. This ophiolite was initially obducted over olistostromes and formed the basement to a later fore-arc basin, the proximal part of which is now exposed at Girvan. The Midland Valley, a Palaeozoic plutonic—volcanic arc, founded on a metamorphic basement, supplied sediments to the fore-arc—accretionary prism on its southern margin. This metamorphic basement is unlikely to have been of local Dalradian type. Growth of the paratectonic zone involved thrusting and probably strike-slip accretion of possibly 6 terranes. Ophiolites and the accretionary prism of the Southern Uplands were thrust NW onto Midland Valley basement. The whole of the paratectonic zone was initially juxtaposed against the Moine—Dalradian terrane by either strike-slip or thrust movements, but the latest interaction between the Midland Valley and Dalradian terranes involved thrusting. Future investigations into the timing and extent of thrust and strike-slip activity will greatly increase our understanding of continental growth in this region.

The significance of a reliable Ordovician age for some Highland Border rocks in Central Scotland

A Lower Arenig fauna with North American affinities has been rediscovered at Dounans, Aberfoyle, Central Scotland. This discovery makes part of the Highland Border Complex younger than the adjacent Dalradian rocks, and suggests a provenance from the S.

Structure, generation and preservation of upward fining, braided stream cycles in the Old Red Sandstone of Scotland

A sequence of upward fining cycles contain braid bar structures in the coarse member. Bars, medially placed in the stream are recognised either by radiating large foresets or by identifying bar tail sediments with converging dips to their foresets. Lateral bars are recognised by the growth of major sandstone sheets oblique to the direction of channel trend and over slough mudstone. The fine member contains structures which indicate a direction of flow in the same direction as the channel, suggesting that only the unleveed part of the floodplain is preserved. The direction of transport for the Old Red Sandstone sediments is perpendicular to the direction of bar accretion. This is accounted for by assuming the section dealt with to be on the NNW margin of a cone which builds outward to the NE, and where the river building the cone swings from one side to the other. An upward increase in cycle thickness within the section is possibly the result of an increase in the size of the cone and the consequent amalgamation of the coarse members.

Nature and significance of beerbachites in the Ballantrae ophiolite, SW Scotland

Metamorphosed abyssal ocean-floor tholeiitiic rocks, little affected by crustal contamination and probably formed at a spreading ridge, occur as a dyke complex in banded gabbros with oceanic affinities and as small tectonic lenses in serpentinised peridotite. Igneous textures and mineral assemblages have been partly or completely replaced in both dykes and gabbros. Metamorphic development of clinopyroxene and very Ti-rich hornblende at 900-1000°C was followed by a low temperature actinolite-chlorite assemblage and then by pectolite-bearing veins. Geochemical variations resulting from magmatic and metasomatic processes have been distinguished and plots of major and trace elements used to establish tectonic environment. The very high temperature metamorphic mineral growth associated with beerbachite formation indicates that the dyke rocks were in a high heat-flow environment for a considerable time, presumably in the vicinity of a spreading ridge. Obduction of a very hot slab of oceanic lithosphere accounts for the superimposed mineral assemblages. Spatially associated pillow lavas have been affected by crustal contamination and are not comagmatic with the dykes. There are petrochemical characters indicating that some of the lavas are ocean-floor basalts. For other lavas, features suggesting emplacement in a continental or an island-arc environment could also be consistent with development in a marginal basin. However, there is so much variation in composition of pillows from margin to centre, from small to large and from metasomatic activity, that their compositional fields cannot be used for discrimination of basaltic type with the same confidence as for the dykes. Accordingly, it is suggested that the petrochemistry of sheeted dyke complexes, rather than of pillow lavas, be used for this purpose in the study of ophiolites.

The Highland Boundary Fault and the Highland Border Complex

Synopsis Stratigraphic evidence, petrography and way up criteria are presented to show that the bulk of the Highland Border Complex in Scotland gets younger to the NW, towards the largely Neoproterozoic, Dalradian block. An early ophiolite has fed younger sediments with detritus, which, in the absence of reliable fossils, has provided a control on the stratigraphical order, placing the ‘Margie Series’ younger than the dated Dounans limestone. Tanner & Sutherland (2007) see the complex as divided into two groups one of which overlies, and is a continuation of the Dalradian sequence, and therefore took part in its folding. This view is false. The Highland Border Complex is a sequence of rocks, disposed in phacoids, beginning with an ophiolite in the early Ordovician followed by younger rocks which include shales, pillow lavas and sandstones. This sequence was deposited whilst the Dalradian highlands rose and shed large volumes of sediment, which have yet to be identified south of the Highland Boundary Fault. The boundary between the Dalradian and the Highland Border Complex, the Highland Boundary Fault, is therefore a major fracture bringing unrelated (but not exotic) Highland Border Complex rocks adjacent to the Dalradian block. In terms of its structural control, the terrestrial Old Red Sandstone is a continuation of the Highland Border Complex, being progressively less deformed as time went on. It was probably laid down in transcurrent fault-generated basins which were gradually converged upon by the Dalradian block and in so doing, uplifted some of the more northerly basins, possibly resting on Dalradian sediments, to yield sediment to the younger basins further to the south. The Highland Border Complex forms the basement to the Old Red Sandstone and, in places, forms a plane of detachment upon which the overlying basins formed. This may account for the extreme heterogeneous nature of its deformation from extensively sheared to totally unsheared rocks.

‘Where ignorance is bliss ‘tis a folly to be wise’ (Thomas Gray 1716–1761) – controversy in the basement blocks of Scotland

The Millennium Issue The year 2000 is the Millennium year and it is appropriate that the Scottish Journal of Geology should mark it in some way. The Editorial Board feel that at this time, at the beginning of what is arguably its 4th century, it is important to bring out the unique and varied character of Scottish geology. After some discussion it was decided to invite short essays from scientists for whom the geology of Scotland has had a special importance and who have themselves had a particular influence on Scottish Geology. We asked for personal approaches to what is significant about the geology of Scotland. In what way has it inspired? Is it because of the intrinsic importance of the discoveries made here or more simply the influence of the environment? We hoped that some contributions would take a more geopolitical view. How much influence has Scotland had on the development of the science of geology as a whole and is there anything left here to discover? We have been delighted with the contributions we have received. A recurrent theme has been the influence of place and it is clear that many of our contributors are as aware of the scenery and their feelings about this important legacy as they are about the (in some ways) more mundane geological issues. Many have included a historical viewpoint, and have reminded us how great a part was played in the last century by observers in Scotland in putting the science of geology together from its diverse beginnings. These contributions appear in both issues of this volume of the Journal and are identified by photographs of the authors and brief notes on their careers.

Upward coarsening sedimentation units and facies lineages, Old Red Sandstone, Scotland

Proximal braided stream deposits of the Old Red Sandstone of central Scotland are characterised by a variety of upward coarsening units formed during downstream migration gravel bars. Transitional variations in thickness, grain size and structure of these units, and their associated sediments are used to establish facies and paths of facies change (lineages). Three facies lineages are recognised which are interpreted as recording the downstream changes the structure of sediments of three different types of braided river systems. Lineages differ from each other in proportion of sandstone, channel depth and the structure of upward coarsening units: lineage 1 represents alluvium formed in a river system which produced texturally mature gravels and which increased in depth downstream; lineage 2 comprises sediments deposited in shallow channels which become shallower downstream; lineage 3 represents a downstream change from conglomerate to sandstone and from an unchannelled to achannelled regime. Stream depth and ratio of sand to gravel exert fundamental controls on the structure of alluvium. Thick (thickness >50 m) sections showing vertical changes in clast size often comprise single facies lineage. Thick upward coarsening sequences are found with lineages 2 or 3 and upward fining with lineage 1 or 3.

Geochronology of a granitoid boulder from the Corsewall Formation (Southern Uplands): implications for the evolution of southern Scotland

Synopsis A U–Pb monazite age of 474 ± 2 Ma has been determined for a weakly foliated granite boulder > 1 m in diameter from the Corsewall conglomerate, SW Scotland. The clast contains xenoliths and partly assimilated xenoliths of regionally metamorphosed rock from which the monazites have been derived. The age constrains the original emplacement of the pluton to between 474 ± 2 and c. 458 Ma, the age of the deposit in which the boulder is found. This age confirms an earlier Rb–Sr muscovite Ordovician age for boulders of this type and contradicts a pre-existing age of 1265 Ma used to suggest a source in Newfoundland and a post-Caradoc major lateral displacement of the Southern Uplands. The monazite age reaffirms that a basement-arc complex once existed north of the Southern Uplands and was the principal contributor of sediment to Ordovician–Silurian basins of southern Scotland. A source for the metamorphic detritus in the Dalradian block is unlikely, as the boulders would have been transported for > 150 km through zones of actively accreting terranes and marginal basins. The possibility of Archaean crust within this basement complex is further suggested by the Sm–Nd systematics of the granite boulders at Corsewall Point.