Early Palaeozoic Iapetus Ocean, South of Scotland
|Stone, P, McMillan, A A, Floyd, J D, Barnes, R P, and Phillips, E R. 2012. British regional geology: South of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.|
Early Palaeozoic Iapetus Ocean
Scotland’s southern geographical border coincides, more or less, with one of the most fun-Iapetus ocean damental geological boundaries in Britain. This is the Iapetus Suture, the trace of a long-vanished, Early Palaeozoic ocean obliterated by the convergence and ultimate collision of the ancient continents that it once separated. The Iapetus Ocean (as a forerunner of the Atlantic Ocean it was named after the father of the eponymous Atlas) was initiated during late Neoproterozoic times and grew to its maximum width by the beginning of the Ordovician Period (P912314a). Thereafter, subduction at its margins wrought its eventual destruction and drove the series of collisional events that built up the Caledonian Orogen, a major tectonic zone that can be traced from Scandinavia, through Britain and Ireland, and on into Greenland and maritime North America. There are particularly clear geological links from southern Scotland, through Ireland, into Newfoundland, Canada.
Along the northern margin of the Iapetus Ocean at the beginning of Ordovician times, the continent of Laurentia lay in subtropical latitudes (P912314a). The Archaean and Proterozoic crystalline basement rocks of Scotland formed a part of this continent, and subduction of Iapetus oceanic crust beneath its margin led to the sequential accretion of oceanic rock complexes, both volcanic and sedimentary. These now make up much of southern Scotland. Forming the southern margin of the Iapetus Ocean during Ordovician times, and in a latitude of about 60º south, lay the shores of the Gondwanan continent, from which a fragment had broken away early in Palaeozoic times. This continental fragment, known as Avalonia, drifted north, towards Laurentia, as the intervening Iapetus Ocean closed (P912314b). The Lower Palaeozoic inliers of the English Lake District and Cross Fell reveal parts of the northern margin of Avalonia and show how it developed in response to the changing geotectonic regime, as described in the companion volume for Northern England.
Some other parts of the Avalonian continental margin provide evidence from volcanic rocks that southward subduction of the Iapetus Ocean commenced during late Cambrian times, but in the Lake District and Cross Fell inliers the earliest subduction-related volcanic activity occurred late in the Ordovician with eruption of the Borrowdale and Eycott Volcanic groups. The relative brevity but great intensity of the Borrowdale–Eycott volcanic episode may have been the result of the subduction of the Iapetus Ocean spreading ridge at its Avalonian margin. The structural weakness of the ridge zone might then have facilitated detatchment of the ’Avalonian’ oceanic crust to create a break in the descending slab and so allow an upwelling of hot mantle material. This process would also have effectively transferred Avalonia onto the oceanic plate that was being subducted northwards beneath the Laurentian margin of the ocean, creating an asymmetric oceanic plate configuration that would, ultimately, influence the tectonic style of the collision between the Laurentian and Avalonian continents. To consider that phenomenon here is to get a little ahead of the geological story, but the principal stages in the destruction of the ocean are illustrated in the series of cross-sectional sketches shown in P912315.
One important ‘trans-Iapetus’ feature is the variation through time of the fossil assemblages found in the rocks. The Laurentian and Avalonian shelly fossils, remains of animals such as trilobites and brachiopods that lived in shallow, coastal marine environments, are quite different in the Ordovician, so that distinct ‘faunal provinces’ can be identified, but became progressively more cosmopolitan through the Silurian. This neatly illustrates the narrowing of the Iapetus Ocean that, by the late Silurian had changed in character and all but disappeared (P912314c).
Whilst the shelly faunas from the continental margins demonstrate provinciality, the biostratigraphy of the deeper-water, oceanic successions, depends on graptolites. This long-extinct group of colonial animals formed a major part of the oceanic macro-zooplankton during Ordovician and Silurian times, and their fossils are of importance in establishing age and order in the Ballantrae Complex and the Girvan succession (Chapter 2), and crucially so in the Southern Uplands (Chapter 3). Graptolites are commonly slender and delicate and a few centimeters in length, though the length may range from almost microscopic to a metre or so in extreme cases. During the Early Palaeozoic they evolved an extraordinary variety of shapes, and it is this variety that is key to the biostratigraphical zonation now based on their remains. The zonal scheme was established towards the end of the 19th century by Charles Lapworth, who was the first to recognise that the graptolite assemblages changed systematically through time. Lapworth’s most important work on graptolite biostratigraphy was carried out in the Moffatdale area of the Southern Uplands and around Girvan, though his zonal scheme, only slightly modified by subsequent research, has international application. A selection of Ordovician and Silurian graptolites from the south of Scotland is illustrated in P912316 and P912317.
Island arcs and obduction
At its northern, Laurentian margin, the first stage in the closure of the Iapetus Ocean was the Early Ordovician development of oceanic subduction zones and associated volcanic island arcs (P912315a). A modern analogue of this situation is provided by the active volcanic arcs of the south-west Pacific Ocean. One of the Early Ordovician, Iapetus Ocean arcs was to become the Ballantrae Complex of south-west Scotland. This Tremadoc to Arenig assemblage of oceanic, mostly igneous rocks collided with and was tectonically emplaced (obducted) onto the Laurentian continental margin at about 470 Ma (P912315b) to form what is known as an ophiolite complex. Its obduction played a peripheral part in the large-scale collision of a volcanic-arc complex (now forming the buried core of the Midland Valley terrane) with Laurentia that provoked the Grampian event of the polyphase Caledonian Orogeny.
Within the Ballantrae Complex there is a bewildering array of rock types: ultramafic rock of mantle origin, volcanic lavas erupted in contrasting island-arc and within-plate settings, intrusive gabbros and pelagic sedimentary strata. All were tectonically juxtaposed and obducted as the Iapetus Ocean began to close. By early Llanvirn times the complex was in subduction and accretion place and had been deeply eroded. Obduction had been accompanied by a switch in the polarity of subduction and as oceanic crust began to be consumed beneath the continental margin a volcanic arc was generated on what is now the basement to the Midland Valley of Scotland. Relative uplift caused by this ‘Midland Valley’ magmatism was accompanied by extension and relative subsidence of the continental margin to the south (P912315c). There, the Ballantrae Complex was progressively buried by a sedimentary cover sequence of shallow to deep marine strata that systematically overstepped northwards from the late Llanvirn to the early Wenlock (c. 460–428 Ma). This now forms the Girvan succession. Its northwards transgression was controlled by major faults, with downthrow to the south, stepping back sequentially into the Midland Valley arc zone. The eventual, probably late Silurian deformation of the Girvan succession involved the reactivation of those originally normal faults as northward directed thrust planes.
Subduction and accretion
Whilst the Ballantrae Complex was being buried beneath the thick Ordovician to Silurian sedimentary succession now seen around Girvan, a very different process was operating farther south. As the Iapetus oceanic crust was subducted beneath the margin of Laurentia, sections of the oceanic sequence and its sedimentary cover were intermittently stripped from the subducting plate and thrust beneath the stack of similar stripped-off slices to initiate an accretionary complex (P912315c). The Southern Uplands terrane represents the deeply eroded remains of this accretionary complex, which developed along the northern fringe of the Iapetus Ocean sequentially from late Llanvirn to mid Wenlock times.
The sedimentary units incorporated into the accretionary complex originated as sand and mud carried by turbidity currents from the continental shelf, via submarine canyons, and built up into huge depositional fans (now the Southern Uplands sandstone groups, P912382). The clastic turbidite deposits filled the supra-subduction-zone trench and encroached onto the oceanic plate, where they covered the sequence of hemipelagic mud (Moffat Shale Group), radiolarian chert and pillow lava (Crawford Group). As the submarine fans built out, they overstepped progressively younger oceanic sequences that were continually approaching the continental margin as the oceanic plate was subducted. Then, during the subduction process, discrete sections of the oceanic sequence and its cover of turbidite sandstone were sequentially stripped from the subducting oceanic plate and thrust beneath the stack of similar stripped-off slices that made up the growing accretionary complex. These slices, structurally rotated towards the vertical (and in places even beyond it), now give rise to the characteristic pattern of elongated and north-east-trending, fault-bounded tracts that define the Southern Uplands lithostratigraphical outcrop pattern (P912313). Polyphase folding was imposed on the strata as the accretionary complex built up. Early folds were formed in association with thrusting during the subduction process. Subsequent folds developed as accommodation structures when the early-formed part of the accretionary complex adjusted to continued subduction at its leading edge and responded to intervals of strike-slip movement rather than orthogonal compression.
This model of the Southern Uplands as a forearc accretionary complex is now generally accepted, following much discussion of possible alternative models that arose, in part, from the provenance contrasts evident between different sandstone tracts. In particular, the apparent introduction of volcanic detritus from the south, i.e. from the oceanic plate rather than the continental margin, has been cited in support of a backarc origin for all or some of the terrane. There has also been some discussion of the possible extension of the Girvan depositional setting, an extending and subsiding continental margin, into the northern part of the Southern Uplands terrane. Analyses of basin thermal history and radiometric dating of detrital minerals have now ruled out the backarc possibility, and whilst some uncertainties remain, the development of the Southern Uplands terrane as a supra-subduction-zone accretionary complex is now the consensus view.
There is however one important difference between the early and late stages in the development of the accretionary complex. The older tracts were accreted from subducting oceanic crust, but by mid Wenlock times the Iapetus Ocean had effectively closed and the complex, at the leading edge of Laurentia, overrode the margin of Avalonia (P912315d). The meeting of the two continental masses did not produce a climactic deformational event and instead the accretionary complex continued to advance through the foreland basin that formed ahead of it, above Avalonian continental crust depressed by the encroaching mass of Laurentia. In somewhat pedantic terms, the accretionary complex had become a foreland fold and thrust belt. Thereafter, convergence of the two continental plates probably ceased in Ludlow times, to be replaced by intermittent lateral movement between them.
Before leaving the Iapetus Ocean, it is worth looking more closely at turbidity currents, which were an important mechanism in the deposition of both the Girvan and the Southern Uplands successions. Each current was created by the collapse and mobilisation of an unstable pile of sediment, built up on the continental margin, and its consequent flow down into deeper water, via submarine canyons, as a huge mass of turbulent, water-suspended sediment. As each flow spread out across the sea floor it decelerated and began to drop its load of suspended sediment, starting with the largest grains. A single sandstone bed would be deposited from each turbidite flow and may display a complete or partial array of distinctive features. The coarsest sand grade present is seen at the base of the bed and there is an upward grading (‘graded bedding’) to progressively finer sand that, towards to top of the bed, may be paralleland/or cross-laminated (P008471). If substantial turbidity flows closely follow each other in time, a thick sequence may build up in which graded sandstone beds become amalgamated, with only subtle variations in grain size determining the margins of each bed. In more distal parts of the fan, or as a result of small, low-density flows, thin and fine-grained sandstone beds may be pervasively laminated and separated by thicknesses of silty mudstone, the finest-grained part of the submarine fan succession deposited in areas that were spatially or temporally removed from most of the depositional activity. Whilst the deposition of a single turbidite bed was, in geological terms, an instantaneous event, a considerable time interval (perhaps 100–1000 years) might pass between successive flows.
Where the base of a turbidite sandstone bed overlies the fine-grained top of its predecessor, it is usually sharp and clearly defined. Sole marks, common on the base of many sandstone beds, are casts of features formed on the substrate upon which the sand was deposited. They formed when solid objects were carried across the sediment surface, forming linear grooves, or by the brief erosive scouring of the sediment surface by intense vortices carried along in the flow of currents. These erosive hollows were then filled by the subsequent sand deposit to form positive features, protruding from the sandstone bed base, and are known respectively as groove and flute casts. Both types range from a few millimetres to several tens of centimetres in width, and though they may occur individually they are more commonly seen in swarms (P008425 and P008463). The orientation of the palaeocurrent indicators can be used to determine the flow direction of the eroding current. In the Southern Uplands context this is important, since there the marked compositional contrast between different suites of sandstones is most readily explained by their constituent sand having been derived from different source regions. Of even greater importance in the Southern Uplands, where successions of steeply inclined or vertical beds are commonplace, is the proof provided by graded bedding, sole marks and cross-lamination of the stratigraphical top and bottom of a sequence at any one locality; crucial data in any interpretation of the larger-scale structure.
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