After Iapetus – Devonian-present, South of Scotland

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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.

After Iapetus[edit]

Palaeogeographical reconstructions from the Ordovician to the Cretaceous. P912314.

In the aftermath of the final closure of the Iapetus Ocean a range of igneous rocks were intruded into the Ordovician and Silurian strata of the accretionary complex: a regional swarm of late Caledonian (Silurian to Devonian) calc-alkaline felsic and lamprophyre dykes, several Early Devonian granitic plutons, and a number of smaller diorite-granodiorite-granite intrusions. Radiometric dating has confirmed that the first to be intruded were microdiorite and lamprophyre dykes, with ages ranging from 418 ± 10 Ma to 400 ± 9 Ma. The age of the larger, granitic intrusions varies across the Southern Uplands with the northern plutons (around 410 Ma, i.e. late Silurian) proving to be older than the southern plutons (around 397 Ma, i.e. Early Devonian) that were intruded closer to the Iapetus Suture.

Devonian[edit]

By the Early Devonian, terrestrial conditions were established with a high-relief terrain undergoing rapid erosion in a relatively arid environment. Conglomerate wedges built up against carboniferous fault scarps and alluvial fans carried the finer-grained material out into transtensional strike-slip basins; large rivers traversed the region and reworked the terrestrial deposits into fluvial sequences. This assemblage is now preserved as the red sandstone and conglomerate of the Reston Group, which crops out mostly in the east of the region. Also in the eastern Southern Uplands, extensive flows of andesitic lava were extruded onto, and interfingered with, the fluvioterrestrial Reston Group strata. Some of these lavas were erupted in association with the intrusion of another granitic pluton at about 396 Ma. It is now exposed just south of the Anglo-Scottish border, where it forms the highest ground of the Cheviot Hills.

Broadly coincident with the granitic magmatism in southern Scotland and northern England at about 400 Ma, but focused away to the south, was a major deformation event caused by the collision of another Gondwanan continental microplate at the southern margin of Avalonia. This is commonly described as the Acadian event of the polyphase Caledonian Orogeny, but since it has no connection with closure of the Iapetus Ocean it has also been thought of as a separate orogeny in its own right. In northern Britain the strongest Acadian effects seen are the folding and cleavage developed in the Lower Palaeozoic rocks of the English Lake District. In southern Scotland, uplift in the Mid Devonian was a more distant and less intense Acadian effect of the collision farther south. Lower Devonian strata were tilted and disturbed so that when alluvial basins were re-established, the Upper Devonian strata deposited therein were laid down unconformably on their predecessors, both Lower Palaeozoic and Lower Devonian. Most of the Upper Devonian deposits were fluvial in origin though some also show an aeolian influence; they consist of red sandstone and siltstone with some conglomerate that are now assigned to the Stratheden Group. Across much of southern Scotland there is a conformable transition from these Upper Devonian strata into similar lithologies of early Carboniferous age.

Carboniferous[edit]

Extensional and transtensional tectonics were active for much of Carboniferous time, with the pre-existing Caledonian and Acadian structures exerting strong control on the orientation of the subsiding basins that resulted. Localised volcanic activity also arose from the extensional regime and produced both extrusive lavas and a range of minor intrusions. By this time the region formed part of the southern margin of Laurussia, an aggregation of Avalonia, Laurentia and Baltica (and ultimately Siberia) which formed the northern part of the Pangaea ‘supercontinent’ (P912314d). The British sector drifted slowly north through equatorial latitudes. An initially arid climate became more hot, humid and wet during that northward drift, and then reverted to arid conditions towards the end of the period. Dextral strike slip became progressively more important within the broadly extensional tectonic regime.

Throughout the Carboniferous, the Southern Uplands formed a relatively stable structural block, with subsidence occurring largely at its margins and within an internal series of small rifted grabens that developed by reactivation of Caledonian faults. The most complete sequence, preserved along the southern margin of the Southern Uplands block, is part of the Solway Basin succession, most of which crops out to the south of the border in north-west England. Similarly, the Carboniferous strata that surround the Cheviot block and extend across the Tweed Basin are the extremities of the more extensive succession seen in the Northumberland Basin of north-east England. These sequences contrast stratigraphically with those of the Carboniferous outliers within the Southern Uplands, for example at Sanquhar, Thornhill and Oldhamstocks, which originated as the infill of the internal grabens and have more in common with those seen to the north in the Midland Valley of Scotland.

The Carboniferous sedimentary record is the result of a complex interplay between several factors: subsidence rates, changes in sea level, limestone reef formation and the progradation of major sandstone deltas into the subsiding basins. Early Carboniferous sedimentation was in fluvial and lacustrine to paralic environments, building up the dominantly clastic Inverclyde Group. With continuing subsidence, a greater marine influence is seen in the succeeding strata in the south of the region, where the largely deltaic to shallow marine Border Group built up, whilst the rather more heterolithic but mainly clastic Strathclyde Group accumulated farther north, albeit in a broadly similar depositional environment. Through the middle part of the Carboniferous succession a major delta built out into the Northumberland–Solway basin system, depositing the Yoredale Group, with a similar succession accumulating independently farther north and now forming the Clackmannan Group. At that time cyclic sedimentation was a particular feature, with the sandy, alluvial and deltaic flats subject to intermittent marine incursions that allowed the development of limestone. The colonisation of the delta tops by lush, peat-forming swamp vegetation, and its subsequent burial and conversion to coal, is most evident in the upper Carboniferous successions of the Pennine and Scottish Coal Measures groups, exploited respectively in the Canonbie and Sanquhar coalfields. Towards the end of the Carboniferous Period, in the Canonbie Coalfield, deposition of Warwickshire Group strata occurred across a fluvial to deltaic plain under increasingly arid, oxidising climatic conditions; coal is largely absent and the strata are generally reddened.

By late Carboniferous times, the extensional and thermal subsidence that had largely controlled sedimentation patterns earlier in the period began to wane. Instead, the region experienced the peripheral effects of a major orogenic collision as, far to the south, Laurussia and the huge Gondwana ‘supercontinent’ came together to unite the Earth’s land areas into the single vast expanse of Pangaea (P912314d). The compressive deformation associated with this event, the Variscan Orogeny, was most intense across mainland Europe and the southern parts of England, Wales and Ireland, south of the so-called ‘Variscan Front’. Related deformation farther north was relatively weak, but Carboniferous basins were compressed and their strata folded and faulted, with dextral strike-slip movement imposed in places. The orientation of the resulting structures was much influenced by their interaction with the rigid structural blocks of the Southern Uplands and The Cheviot. Variscan deformation spanned an interval of approximately 15 million years from the late Carboniferous to the early Permian, and was accompanied by the large scale intrusion of basaltic magma to form sill and dyke swarms to both the north and south of southern Scotland. However, within the latter region, magmatism was limited to lava eruption in some of the Southern Uplands extensional grabens, with a more widespread scattering of minor intrusions.

Permian and Triassic[edit]

During Permo-Triassic times, global sea level was relatively low and Scotland was located within the interior of Pangaea and a little to the north of the Equator (P912314e); about 10°N at the beginning of the Permian, drifting to about 30°N by the end of the Triassic. Thus the preserved sequences are largely the result of terrestrial sedimentation in an arid environment. There was rapid erosion of the Carboniferous strata that had been uplifted by the Variscan compression, with several hundred metres likely to have been removed in some places. A renewal of tectonic extension early in Permian times then reactivated the broadly north-trending margins of the Carboniferous outliers in the Southern Uplands, and other pre-existing Caledonian structures, to form the margins of depositional graben and half-graben basins such as those seen at Thornhill, Lochmaben and Dumfries. North-east–south-west faults were also reactivated but mostly with strike-slip displacement. The early Permian extension was accompanied by some local volcanicity.

The lowest Permian strata seen in southern Scotland are fluvial and aeolian sandstones with conglomerates derived locally from the sides of the fault-defined depositional basins. Strata in the Scottish outliers within the Southern Uplands massif (and also to the north in the Midland Valley) are assigned to the Stewartry Group, but to the south, the succession bordering the Solway Firth is a continuation from north-west England of the Appleby Group. Along the Solway coast, the Appleby Group strata form the marginal deposits of the Solway Basin, wherein sedimentation continued from the Permian through into the Jurassic. In southern Scotland, representatives of this succession form the Cumbrian Coast Group (Permian) and the Sherwood Sandstone Group (Permian to Triassic). The former includes evaporite, formed on coastal mudflats and sabkhas, interbedded with fine-grained clastic lithologies deposited by a combination of wind and muddy sheet-flow. The Sherwood Sandstone Group strata were laid down by ephemeral rivers on braided alluvial plains and playa mudflats. The outcrops of the younger Triassic (and Jurassic) components of the Solway Basin succession do not extend into southern Scotland.

Jurassic to Palaeogene[edit]

It was the break-up of Pangaea during Late Triassic times that brought an end to the prolonged period of mainly terrestrial conditions across southern Scotland. Marine transgressions extended across an ever-widening area until, by Early Jurassic times, global sea levels were relatively high and much, possibly all of the region was submerged. Calcareous mudstone from this period, part of the Lias Group, is preserved around Carlisle in the centre of the Solway Basin.

A period of uplift and erosion in Mid Jurassic times is recorded by a widespread unconformity, with the maximum effect seen in North Sea basinal sequences. The commensurate fall in relative sea level continued through the Early Cretaceous and an extensive unconformity developed across the surrounding land areas, until rising sea levels brought renewed marine transgression through the later part of the Cretaceous. Continental drift had carried Britain to the latitude of about 35°N by the end of the Triassic and slow northwards progress continued during the Jurassic and Cretaceous periods (P912314f). The climate was strongly seasonal with warm, relatively dry summers and cool wet winters.

The later Mesozoic geological history is obscure, with no sedimentary rocks from that interval, or the succeeding Cenozoic, preserved in southern Scotland. There is some evidence that normal and possibly strike-slip faulting continued, and in the Solway Basin sedimentation probably continued into Early Cretaceous times. Thereafter, it is likely that continued subsidence allowed deposition of the Upper Cretaceous Chalk Group across much of the region, with the maximum post-Variscan burial probably achieved towards the end of the Cretaceous Period.

At the end of the Cretaceous and into the early Palaeogene Period, regional uplift began as a precursor to the major magmatism associated with the initial opening of the Atlantic Ocean. This was driven by development of the proto-Icelandic mantle plume, which had its maximum impact in what is now the Hebridean province of western Scotland and Northern Ireland and in Greenland, areas that were then adjacent (P912314f). There, from about 60 Ma to 55 Ma, immense volumes of basaltic magma were erupted with the commensurate intrusion of plutons, sill-swarms and swarms of dykes. Some of the latter, emanating from a volcanic centre on Mull, run south-eastward across southern Scotland and extend well into northern England, more than 400 km from their source; examples include the well-known, Cleveland and Acklington dykes.

Additional impetus was given to the regional uplift of northern Britain in Miocene times as a distant effect of the Alpine Orogeny. This resulted from collision between the European and African plates, but its main structural effects are not seen much beyond southern England and Wales. Around the south of Scotland, its influence was likely restricted to uplift of the Solway Basin sequences. Overall, the Palaeogene to Neogene uplift episodes created erosive conditions across southern Scotland that have continued to the present day. A considerable thickness of strata may have been removed in this time, perhaps as much as 2000 m in places, including all of the relatively soft, Jurassic and Cretaceous successions that most probably had once extended across much of the region. Further erosion of the Devonian and Carboniferous rocks followed, revealing the Caledonian, Lower Palaeozoic basement of the Southern Uplands structural block.


Glaciation[edit]

Erosion was locally much more vigorous from about 2.6 million years ago as glacial conditions were established across northern Britain during the Quaternary ‘ice age’. In fact, this was a period of alternating cold and more temperate interludes, with the most recent of the latter commencing only about 12 000 years ago (12 ka BP — where BP stands for Before human InterventIon Present) and continuing to the present day. Ice-sheets repeatedly built up on the higher ground and fed glaciers that eroded deep valleys in places, whilst depositing a thick blanket of glacigenic sediment in others. The multiphase nature of glacial advance and retreat led to complex variations in ice flow direction through time, resulting in a complicated pattern of glacial landforms. This is particularly apparent in coastal areas of south-west Scotland and in the Solway lowlands, where there was interaction of different ice-sheets emanating from the Southern Uplands, the high ground of northern England and the Scottish Highlands, the latter skirting the Rhins of Galloway to enter the Irish Sea area.

As the ice-sheets waxed and waned so relative sea level changed. Short-term, eustatic changes were directly related to the growth of the ice-sheets, but longer term, isostatic changes arose from the depression of the Earth’s crust by the enormous mass of ice, and its slow recovery once the ice had melted. These effects have led to a range of submerged and elevated coastal features, the most prominent of which are raised beaches and clifflines now abandoned several metres above present sea level.

Later glacial events tend to destroy, redistribute and incorporate the deposits of previous glaciations unless the latter are preserved in very special circumstances. The net result is that most of the till and morainic deposits now seen in the south of Scotland were deposited in the early part of the Late Devensian, during the Dimlington Stadial (about 29–14.7 ka BP), when extensive glaciers and ice-sheets grew in the region. A climatic amelioration followed, the Windermere Interstadial (about 15–13 ka BP), before ice returned to the highest parts of the region during the Loch Lomond Stadial (about 12.9–11.7 ka BP). But even when the higher ground was largely free of ice, extended periods of periglacial conditions led to the widespread development of a deep layer of frost-shattered rock, particularly over the more elevated Lower Palaeozoic strata. This material has subsequently been redistributed by solifluction, landslipping, and the development of screes. As deglaciation gathered pace, huge amounts of meltwater were released and deposited extensive sheets of sand and gravel, mostly across the lower ground. Much of this was then reworked into alluvial flood plains as the modern drainage system developed.

Relatively wet climatic conditions were prevalent in the immediately postglacial interval (Holocene, from about 11.7 ka BP) and encouraged the build-up of extensive peat deposits, both in upland areas of poor drainage and across lower but flatter terrain. The relatively lower rainfall of more recent times, coupled with the anthropogenic effects of overgrazing and drainage, has led to the drying-out and extensive erosion of many areas of upland peat.


Human Intervention[edit]

Since the last retreat of the ice from southern Scotland, humans have become a significant geological agent, modifying landscapes and sedimentary patterns through deforestation, reforestation, agriculture, mining and urban development, whilst driving many other species to extinction. Now global warming is once again leading to a rise in sea level, but this time accentuated by an anthropogenic contribution to the cause. Indeed, so profound has been Man’s recent influence on global geological processes that it is becoming common practice to refer to the geological interlude that has followed the Industrial Revolution as the Anthropocene. How these changes will affect the future geological record remains to be seen. In the meantime, across southern Scotland, we continue to rely on geological sources of many raw materials, particularly for construction, road building and water supply. We can also enjoy a landscape of high scenic value created by the interaction of the underlying rocks with eons of geological change, and urban environments visually enriched by the use of local building stone in unique, vernacular architecture.


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