Devonian - the ‘Old Red Sandstone’ and associated volcanic rocks of the Southern Uplands
|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.|
By late Silurian times, the Iapetus Ocean had closed and the more-or-less orthogonal convergence of Avalonia and Laurentia had given way to a sinistral, transtensional tectonic regime across the Iapetus Suture Zone. The intrusion of granitic plutons and dyke swarms spanned the Silurian–Devonian transition, as described in Chapter 4, and by the Early Devonian the south of Scotland region had become an upland area subjected to intensive subaerial erosion. The steeply dipping Lower Palaeozoic strata, underpinned by the granitic plutons, gave rise to a rugged landscape of hills and valleys, the latter controlled by reactivation of Caledonian faults under the influence of a sinistral stress regime orientated north-east to south-west.
The region was subject to an arid climate in low latitudes south of the equator (P912314). Terrestrial vegetation was poorly developed at the time and so erosion was rapid and extensive. Screes and piedmont fans were laid down in the areas of high relief, whilst on the lower ground fluvial and lacustrine sediments were deposited by braided (and most probably seasonal) rivers on floodplains and in ephemeral lakes. These deposits are now seen as coarse conglomerates, sandstones and mudstones and have long been known informally as the ‘Old Red Sandstone’. They are preserved mostly in the eastern part of the region, where they overlie the deeply eroded Lower Palaeozoic rocks with a marked unconformity. Volcanic activity associated with the Caledonian intrusions continued and was widespread at times. In the south of Scotland it gave rise to the series of andesitic lavas and volcaniclastic rocks which are interbedded with sandstones in the area around Eyemouth and St Abbs, the Eyemouth Volcanic Formation, and the less well-exposed but much more extensive outcrop of similar volcanic rocks around Cheviot, the Cheviot Volcanic Formation. Much of the latter, together with an associated pluton and dyke swarm, crops out to the south-east of the English border in Northumberland (P912344).
The historical descriptor ‘Old Red Sandstone’ now has supergroup status, and is largely supported by formal lithostratigraphy applied on a regional basis. In the south of Scotland, the ‘Lower Old Red Sandstone’ is represented in the Girvan area by part of the Lanark Group, and to the south-east of the Southern Upland Fault by the Reston Group. In the latter area, the ‘Upper Old Red Sandstone’ now forms part of the Stratheden Group.
In common with the rest of Scotland south of the Great Glen, the Devonian strata in the Southern Uplands region can be divided into two quite distinct successions separated by an unconformity: the Reston and Lanark groups below, and the Stratheden Group above. The discordance between the upper and lower divisions is markedly angular in places, testifying to some tectonic tilting during the Mid Devonian, perhaps as a peripheral effect of the Caledonian Orogeny’s Acadian event (see Chapter 4). There are no known Middle Devonian rocks in the south of Scotland region, this interval of about 20 million years being a period of erosion both there and in the adjacent parts of northern England and the Midland Valley of Scotland. During this episode of basin inversion and uplift, it is likely that much of the Lower Devonian cover was eroded and recycled towards the north and south. The present distribution of the Lower Devonian strata in several isolated and disconnected outliers, originally sediment-filled valleys in the Devonian landscape, is probably a relic of a more extensive presence prior to Mid Devonian erosion.
The conglomerate, sandstone and siltstone of the Upper Devonian Stratheden Group have a more widespread distribution than its subjacent counterpart, and in places oversteps onto the Lower Palaeozoic rocks. The resulting, sharply angular relationship between subvertical Silurian sandstone and the gently-dipping Stratheden Group strata can be demonstrated at two localities, both known as ‘Hutton’s Unconformity’ after the celebrated 18th century savant who first recognised their significance for the age of the Earth: Siccar Point (NT 813 710) near Cockburnspath on the North Sea coast (P774192), and Allar’s Mill (NT 652 198) on the Jed Water just to the south of Jedburgh. In both these examples, the Stratheden Group strata rest unconformably on steeply inclined, Silurian (Llandovery) sandstones: Ettrick Group at Siccar Point, Hawick Group at Jedburgh. Both of the ‘Hutton’s Unconformities’ have been proposed as ‘Global Geosites’ on the basis of their historical importance, whilst a full-sized casting of part of the Siccar Point unconformity is housed in the American Museum of Natural History, New York.
Lanark Group, Devonian, Southern Uplands
Lanark Group strata were deposited in the Lanark Basin, along the south-east side of the Midland Valley terrane. Part of its outcrop extends into the Girvan district, where it unconformably overlies and is faulted against the Lower Palaeozoic rocks. Conglomerate of the Greywacke Conglomerate Formation, containing clasts of the eponymous sandstone, unconformably overlies steeply dipping Silurian and Ordovician strata in a much-dissected outcrop between Straiton and Girvan, dominated by Hadyard Hill (NX 271 993) and Daljedburgh Hill (NX 313 973). There, the conglomerate is succeeded by red-brown sandstone of the Swanshaw Formation overlain, in turn, by basaltic lavas of the Carrick Volcanic Formation that are correlated with the Duneaton Volcanic Formation farther to the north-east. These examples are peripheral to much more extensive outcrop of the formations elsewhere in the Midland Valley of Scotland that are more fully described in the companion volume for that area in the British Regional Geology series.
Reston Group, Devonian, Southern Uplands
The strata comprising the Reston Group crop out to the south-east of the Southern Upland Fault. They are mostly red-brown conglomerates and sandstones laid down in a fluvial-terrestrial environment wherein large rivers drained broadly towards the south-west from a catchment in what is now Scandinavia. Reston Group strata are mostly preserved as an infill to pre-existing, approximately north–south valleys in the Early Devonian land surface that were instigated by reactivation of the Caledonian, north-east and north-north-west conjugate fault sets.
The most westerly outcrop of the Reston Group is a small isolated patch of poorly exposed red and purple conglomerate, known as the Lamancha Conglomerate Formation, located adjacent to the Southern Upland Fault near Leadburn (NT 200 515). This unit is largely composed of rounded pebbles and boulders of wacke-type sandstone, with some basalt and chert detritus, set in a sandy matrix; it is probably of relatively local origin. Its elevated position on top of a ridge, rather than filling a valley, suggests that it is just the last vestige of a formerly much more extensive deposit.
The westernmost of the three northern outcrops of the Reston Group (P912344) occurs in Lauderdale with an associated but disconnected outlier to the north-west at Carfrae Common, near Soutra (NT 470 590). In the Lauderdale outcrop, red and purple conglomerate of the Great Conglomerate Formation is up to 350 m thick (P912345). It consists of subrounded pebbles and cobbles of wacke-type sandstone, with some chert and jasper, set in a red-brown sandstone matrix. Farther east, another outcrop of very similar Great Conglomerate Formation strata (P005866) extends north–south from the vicinity of Dunbar in the north, to the area between Longformacus and Duns in the south (P912344). Some spectacular exposure is provided in the north of this outcrop by a series of deeply incised glacial meltwater channels near Oldhamstocks (P001091). There is little direct evidence for the age of the Great Conglomerate and it is assigned to the Lower Devonian principally on lithological grounds. One imprecise constraint is provided by the radiometric (K-Ar, biotite) age of 400±9 Ma from a lamprophyre dyke cutting conglomerates that underlie the Eyemouth Volcanic Formation (described below) near St Abb’s Head (P912344 and P912345).
Inland from Eyemouth and St Abbs, the easternmost outcrop of the Reston Group (around Reston itself) contains about 120 m of red feldspathic sandstone and conglomerate with exclusively wacke-type sandstone clasts; the overall lithofacies is very similar to that of the Great Conglomerate Formation as seen elsewhere. A few thin beds of pedogenic nodular limestone (calcrete or ‘cornstone’) are also present. Calcrete is more characteristic of the Upper Devonian and lowermost Carboniferous lithofacies in the south of Scotland, but the Reston–Eyemouth sequence is overlain by about 600 m of Lower Devonian lavas with interbedded volcaniclastic rocks and red sandstones, together known as the eyemouth Volcanic Formation. In addition, as described above, conglomerate underlying the Eyemouth lavas near St Abb’s Head is cut by a lamprophyre dyke dated at about 400 Ma. To the north-west of Eyemouth beach, in the cliffs beneath Eyemouth Fort (NT 943 648) the purple basaltic-andesite lavas are overlain, in turn, by a younger red conglomerate containing clasts of sandstone and andesite. This may correlate with the upper part of the succession around Reston, the Auchencrow Burn Sandstone Formation (P912345), in which the sandstone is largely pebbly and/or volcaniclastic, with plentiful detritus derived from the underlying or possibly coeval andesitic lavas and volcanic breccias. Alternatively, the volcaniclastic succession at Eyemouth Fort could be younger and a part of the Stratheden Group (see below).
The lavas of the Eyemouth Volcanic Formation may possibly have been erupted from volcanic vents now filled with andesitic agglomerate that crop out locally along the coast between Eyemouth and St Abbs; a steep, brecciated and hydrothermally veined vent margin is particularly well exposed to the north-west of St Abbs harbour. The lavas are mostly fine-grained and aphyric or olivine-phyric (up to 12 per cent olivine), or more rarely orthopyroxene-olivine-phyric. They have a compositional range from basalt to basaltic andesite, though pervasive hydrothermal alteration makes analytical data unreliable. Many of the lavas are block flows with a variety of internal features including flow-jointing, vesiculation, auto- and mass-flow brecciation. Most of the associated volcaniclastic beds (P005893) are coarse sandstones that appear to have been derived from the local lavas and to have been deposited under high-energy, flood conditions. In addition, there are local intercalations of breccioconglomerates that consist mostly of large blocks of lava and volcaniclastic rock up to 2 m across. Many of the blocks are moderately well rounded and in places a rough stratification is discernible.
Spanning the border with England in south-east Scotland, the andesitic lavas of the Cheviot Volcanic Formation are poorly exposed over an area of about 600 km2 with a preserved thickness of about 500 m (P912344 and P912345). It is likely that the coeval granite pluton (exposed to the south of the border) once had a substantial cover, and the original thickness of volcanic rocks may have exceeded 2000 m. The volcanic rocks unconformably overlie steeply dipping, tightly folded sandstone and cleaved mudstone of the Wenlock Riccarton Group, though locally the White Hill Sandstone Formation intervenes. This unit is up to 30 m thick and comprises red sandstone and mudstone with sporadic beds of conglomerate.
The Cheviot lavas are overlain by either Upper Devonian, Stratheden Group, conglomerates containing abundant andesite and granite clasts, or by lower Carboniferous beds, only some of which contain andesite fragments. Thin intercalations of red sandstone within the volcanic outcrop have proved to be unfossiliferous, but a radiometric age of about 396±4 Ma (Rb-Sr, biotite) suggests that volcanism occurred at the end of Early Devonian times, late in the Acadian tectonic episode. The Cheviot lava succession comprises stacked sheets of trachyandesite and subordinate trachyte that contain phenocrysts of plagioclase, hypersthene, augite, ilmenite and apatite. There are also a few sheets of biotite-feldspar-phyric trachyte (previously described as ‘micafelsites’) near the base of the succession that may represent eruptions of more fractionated magma. The sheets are massive to amygdaloidal and scoriaceous, with near-horizontal platy jointing a characteristic feature of many. The more readily weathered tops of the units have been eroded in some areas to form a prominent bench and scarp landscape. The benches reflect the gentle eastward dip of the sequence. The sheets have been interpreted as lavas, erupted in a subaerial setting from a cluster of low-profile volcanoes, though the presence of a significant proportion of sills cannot be ruled out.
The basal unit of the volcanic formation, spanning the border in the south-west of the outcrop, comprises up to 60 m of breccia composed of rubbly, angular to subangular clasts of silicic volcanic rock along with some mudstone fragments. This unit is most likely to have been the product of initial phreatomagmatic eruptions, though a sedimentary origin cannot be entirely discounted. Other intercalations of pyroclastic and volcaniclastic sedimentary rocks are present higher in the succession, though they are sparse. In the uppermost parts of some sheets there are enclaves and fissure-fills of green fine-grained sandstone and siltstone. Fragments of these rocks are commonly seen in streams, suggesting that this lithology is more common in the unexposed parts of the succession.
Stratheden Group, Devonian, Southern Uplands
The largely sandstone lithofacies forming the ‘Upper Old Red Sandstone’ in Scotland spans the Devonian–Carboniferous chronostratigraphical boundary. In lithostratigraphical terms, this terrestrial and generally unfossiliferous sequence is divided into a lower unit of red sandstone and conglomerate, the Stratheden Group, and an upper unit characterised by thin beds of pedogenic nodular limestone (calcrete or ‘cornstone’), known as the Kinnesswood Formation and assigned to the Carboniferous Inverclyde Group. The presence of the calcrete beds suggests that the climate, though generally hot and semi-arid, included a substantial seasonal rainfall. A Famennian (latest Devonian) age is suggested for at least part of the Stratheden Group on the basis of a few discoveries of fossil fish (P693051). The Kinnesswood Formation is generally thin and variably developed and its base, though probably diachronous, is commonly taken to be the local base of the Carboniferous succession. However, sparse fish faunas from a few localities show that the lower part of the Kinnesswood Formation may also be of Famennian age. Despite any resulting anachronism, in this account the description of the entire Kinnesswood Formation is reserved until the Carboniferous succession is discussed in Chapter Six.
The main outcrop of the Stratheden Group lies in the east of the Southern Uplands around Gordon and Jedburgh (P912344). There the sequence comprises up to about 200 m of alluvial sandstone and siltstone, with sparse conglomerate at the base, deposited in a fluviatile environment. In general, and ignoring the basal conglomerate, there is an upward trend to coarser lithologies, accompanying a shift from high to low sinuosity fluviatile deposits, and in the east of the outcrop a sediment provenance in the volcanic rocks of the Cheviot Hills is likely. The Famennian age is confirmed by fish faunas. To the west, a thin sequence comprising about 20 m of red sandstone and conglomerate underlies the lowermost calcrete at the base of the Kinnesswood Formation and extends intermittently, following the margin of that formation’s outcrop, past Langholm and Dumfries as far as Kirkbean (NX 975 592) on the Solway Firth. At Kirkbean, the presence of granitic clasts in the conglomerate suggests a provenance in the Galloway Hills. Though palaeocurrent evidence is inconclusive, it is possible that the Late Devonian rivers that laid down the Stratheden Group strata drained to an internal basin in the Jedburgh area.
An aeolian influence is seen particularly in the younger strata, and is most apparent in the north-east, around Siccar Point, site of the more spectacular of Hutton’s unconformities (P774192). Overall, a topographical relief of around 100 m can be established there at the unconformity between the Stratheden Group and the underlying Silurian rocks. Immediately above the unconformity lies the thin basal conglomerate member (up to 6 m thick) of the Redheugh Mudstone Formation. The conglomerate member is mostly made up of angular clasts of the subjacent Silurian sandstone, contained in a red sandstone matrix. Above the conglomerate, the greater part of the Redheugh Mudstone Formation, which is about 200 m thick, comprises red-brown, sandy mudstone interbedded with variable brown, grey-green or cream sandstones that become more numerous and massive upwards. Eventually sandstone dominates the succession and the Redheugh Mudstone Formation passes transitionally into the overlying Greenheugh Sandstone Formation, about 70 to 100 m of mostly red-brown, coarse-grained sandstone with thin interbeds of red mudstone. Famennian fish faunas have been recovered from both the Redheugh and Greenheugh formations.
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