Bedrock Geology UK North: Mostly Devonian - the Old Red Sandstone Supergroup
| This topic provides descriptions of the rock types appearing on the British Geological Survey 1:625 000 scale map of the UK North and gives a brief explanation of their origins.
Author: P Stone (BGS); Contributor: A A Jackson (BGS)
416 to 359 million years ago
From the end of the Silurian Period, and through the Devonian, Britain and Ireland lay south of the Equator towards the southern margin of the large, ‘Laurussian’ land mass (Figure P785799d). The climate varied from humid to arid. Across the north of the region a mountain belt had been uplifted during the Caledonian Orogeny, the closing stages of which were accompanied and followed by the formation, peripheral to the mountains, of largely nonmarine sedimentary basins. Therein was deposited the Old Red Sandstone lithofacies, an assemblage of fluvial, aeolian and lacustrine rocks. Most are of Devonian age but the overall range is from the mid Silurian to the early Carboniferous. Major depositional sequences are seen in the dominantly lacustrine Orcadian Basin in north Scotland and the largely fluvial Strathmore Basin of the northern Midland Valley. Less extensive, fluvial or terrestrial sequences form inliers in the southern Midland Valley (Lanark Basin). Remnants of a formerly more extensive unconformable Old Red Sandstone cover are preserved overlying Dalradian rocks in north-east Scotland, and Lower Palaeozoic rocks in Northern Ireland, in the east of the Scottish Southern Uplands and in the English Lake District and Isle of Man. Several contemporaneous volcanic centres are intimately associated with the Old Red Sandstone.
The Devonian strata of north-east mainland Scotland, Orkney and Shetland accumulated in a vast lake complex. The succession is over 4000 m thick in Caithness and possibly twice that in parts of Shetland, where three distinct sequences have been juxtaposed by strike-slip faulting. During the early Devonian (D1), localised deposition built up sediment on an irregular basal unconformity cut across Proterozoic rocks. Stromatolitic limestone and alluvial-fan breccia were deposited at the lake margins, and then covered either by fluvial and aeolian sands or transgressively by deltaic sediment of the deep-lake facies. The resulting, typical Lower Old Red Sandstone assemblage of fluvial and aeolian sandstone, conglomerate and mudstone is represented in Orkney by the Yesnaby Sandstone Group and on the mainland by, for example, the Sarclet Group. In one exceptional occurrence within the Rhynie Group, an unusual chert, produced as siliceous sinter by hot springs, preserves a unique assemblage of early land plants and arthropods.
Lake transgression and regression occurred repeatedly through the mid Devonian, with deposition of a distinctive Middle Old Red Sandstone succession (D2). Typical is the Caithness Flagstone Group, which comprises alternations of lake-floor laminated mudstone with marginal deltaic, fluvial and some aeolian sandstone. During maximum transgression, laminated fish-bearing mudstone and limestone accumulated with annual cyclicity (Plate P547033). Strata deposited when the lake was shallower are more coarsely laminated with rippled bedding, whilst abundant desiccation-cracked surfaces record intermittent subaerial exposure (Plate P547034). Farther south, the Black Isle and Inverness Sandstone groups around the Moray Firth, are predominantly fluvial red sandstone successions deposited along the lake margins. The sandstones of the Eday Group, seen mostly in Orkney, record both the advance of alluvial fans across the lake basin and rare marine incursions, and here the highest sandstones (components of the Hoy Sandstone Formation) are part of the Upper Old Red Sandstone. At the base of the Hoy Sandstones are up to 60 m of volcaniclastic rock and basaltic lava (Plate P000613). Eruption followed a phase of localised folding (probably related to basin-margin fault movement) so that the volcanic rocks rest on the underlying sandstones with a slight angular unconformity.
In southern Shetland, the Sumburgh Group is broadly equivalent to the Middle Old Red Sandstone Eday Group, comprising fluvial and aeolian sandstone and conglomerate (Plate P547039) with rare laminated fish beds. Some of the western Shetland outcrop is slightly older, equivalent in age to the Caithness Flagstone Group but containing a much higher proportion of sandstone. However, most of the western outcrop is of uncertain, early to mid Devonian age. A northern section comprises fluvial sandstone and conglomerate, with an interbedded interval of mafic lavas; a southern section comprises thick, variable alternations of sandstone and mudstone that were probably deposited in alluvial and shallow lacustrine settings. The strata of this southern section have some similarities with those of the Fair Isle Group, farther to the south on the eponymous island.
Despite localised exceptions, the transition from Middle to Upper Devonian is generally conformable but marked by a change in depositional environment. The mid Devonian lakes were replaced in the basin centre by sabkha plains, whilst the basin margins received the fluvial deposits of meandering and braided rivers. Typically, in the Upper Old Red Sandstone sequences (D3), fluvial pebbly sandstone and aeolian sand dunes interfinger with sabkha deposits of sandstone and mudstone that contain evidence of evaporitic deposition, desiccation and subaerial animal activity.
Along the northern margin of the Midland Valley, late Caledonian sinistral strike-slip movement initiated small basins in which were deposited the mid to late Silurian (Wenlock to Pridoli) Stonehaven Group (S3–4), up to 1500 m of predominantly red sandstone, mudstone and conglomerate (Plate P001255). Many of the sandstones contain a volcaniclastic component. An acceleration of relative uplift and basin subsidence by the early Devonian then led to accumulation of the spectacularly coarse conglomerate beds that characterise the Dunnottar-Crawton Group (D1), which is up to about 2000 m thick. The conglomerates contain rounded boulders, commonly up to 1 m across, and together with interbedded sandstones they were deposited as alluvial fans within a very large, braided river system. Clast lithologies are mostly quartzite or granitoid, but there is an upward increase in the proportion of volcanic rocks present until, at the top of the group, a series of basaltic lava flows interfinger with the sedimentary rocks. The succeeding accumulation of sandstone and conglomerate forms the Arbuthnott-Garvock Group (D1). Apparently 3000 or 4000 m thick, it contains substantial amounts of lava, sills intruded at shallow depth into wet sediment and volcaniclastic strata, though it is unlikely that the complete sequence was ever deposited in any one place. Conglomerate dominates the group in the north-east of the basin but is transitional into red sandstone and mudstone towards the south-west. Hence the character of the group changes from the north to the south limb of the Strathmore Syncline, the large fold structure into which the Strathmore Basin was compressed by Acadian deformation. The axis of the syncline is occupied by the Strathmore Group (D1), a sequence of up to about 1500 m of conglomerate and pebbly sandstone (Plate D05347) that becomes generally finer grained southward until calcareous mudstone becomes the principal rock type.
Deposition of the younger part of the Strathmore Group, in the early Devonian, was influenced and followed by the Acadian folding of the Strathmore Syncline. The Upper Devonian of the Stratheden Group (D3) unconformably overlies the syncline. Up to about 1000 m of strata are present, which become generally finer grained up the sequence: fluvial sandstone dominates (Plate P001125) but there is also some aeolian sandstone and conglomerate. In a few places towards the western end of its outcrop the group oversteps across the Highland Boundary Fault to rest unconformably on Dalradian rocks. Pebbles of Dalradian schist occur in most of the conglomerates, along with abundant quartz pebbles and others derived from the local Lower Devonian strata.
On the south side of the Midland Valley the strata of the Lanark Group, deposited in the early Devonian Lanark Basin, are preserved in a series of inliers (D1). At the base of the Lanark Group is a distinctive conglomerate composed mainly of wacke clasts. This overlies, with local unconformity, a Silurian succession and so could itself be of late Silurian age. Up to about 1200 m of alluvial-fan conglomerates and fluvial, cross-bedded sandstones form the broadly upward-fining sequence that succeeds the basal conglomerate. There is much detrital volcanic material in the sandstones and its abundance increases upward, presaging the appearance of overlying accumulations of lavas, tuffs and volcaniclastic breccias, which are up to about 600 m thick. The lavas range from andesite to rhyolite, and have been dated to around 410 Ma; variations in their characteristics suggest eruption from at least four different centres. Numerous sills and laccolithic bodies, probably intruded at relatively shallow depths, are associated with the extrusive rocks. Thin sandstone and conglomerate beds are present within the lava sequence, which is also overlain by several hundred metres of clastic strata. Similarities between the highest part of the Lanark Group and the Strathmore Group suggest the establishment of a sedimentary link between the Strathmore and Lanark basins.
The Midland Valley tectonic terrane continues to the south-west into Northern Ireland and though its boundaries are there less well-defined, Devonian strata appear to occupy a position analogous to that of the Strathmore Basin. One substantial outcrop extends across parts of counties Fermanagh and Tyrone. This is the Fintona Group (D1), which unconformably overlies the Tyrone Complex and is faulted against both older Dalradian and younger Carboniferous rocks. The Fintona Group comprises over 3000 m of alluvial-fan conglomerate and reddish, fluvial sandstone with subordinate mudstone and calcrete. Early Devonian microfossils have been recovered from the mudstone, whilst a mid to late Devonian radiometric age of about 375 Ma has been determined from andesitic lava interbedded with the sedimentary sequence. A second, and much smaller outcrop of likely Devonian strata, the Cross Slieve Group (D1), lies on the coast of County Antrim. There is no definitive evidence for the age of these rocks but they lie only about 6 km south-west of the Strathmore Basin sequence as exposed on the Mull of Kintyre, and are of broadly similar character. A basal breccia rests unconformably on Dalradian rocks and contains abundant schist clasts. Above the breccia, about 1300 m of conglomerate, composed mainly of quartzite pebbles, is associated with reddish, volcaniclastic sandstone (Plate P225275). Lava flows of porphyritic dacite occur locally. Faulted against the Cross Slieve Group are another 400 m of unfossiliferous conglomerate and red sandstone (Red Arch Formation) that have traditionally been regarded as forming a Devonian, ‘Old Red Sandstone’ sequence but are now thought to be possibly of younger, early Permian age.
Southern Scotland and northern England
Conglomerate and fluvial sandstone of the Reston Group (D1) unconformably overlie Silurian strata in the eastern part of the Southern Uplands. Conglomerate is dominant and consists largely of clasts derived locally from the underlying Lower Palaeozoic sandstone (Plate P001091). Subordinate rock types in the Reston Group include red calcareous mudstone and calcrete. Deposition was localised in small basins but thicknesses of several hundred metres are widely developed. The minimum, early Devonian age of the conglomerate is constrained by cross-cutting Caledonian dykes that have been dated at about 400 Ma, but the group may range down into the late Silurian. Extrusive rocks are also present. About 600 m of andesitic lava and volcaniclastic rocks are associated with pebbly sandstone at the eastern, North Sea coastal end of the Reston Group outcrop, whilst the southern extremity of sedimentary rock interfingers across the England–Scotland border with andesitic lavas erupted from the major volcanic centre at Cheviot at around 395 Ma. On the Isle of Man, a more distant outlier of probable Lower Old Red Sandstone pebbly sandstone (Peel Group) unconformably overlies the Lower Palaeozoic rocks of the Manx Group.
The Lower Old Red Sandstone strata were tilted during the Acadian orogeny with alluvial basins then re-established across southern Scotland. Late Devonian sediments, assigned to the Stratheden Group (D3), accumulated in these basins. Most were fluvial in origin, some aeolian; they consist of red sandstone and siltstone with some conglomerate, and are equivalent to the sequence that overlies the Lower Old Red Sandstone of the Strathmore Basin farther to the north. In the Southern Uplands, the Stratheden Group rests with angular unconformity on both the Reston Group and on Silurian strata (Plate P000818). Farther south, in northern England, likely Upper Old Red Sandstone conglomerate, including that at Mell Fell, unconformably overlies the Ordovician Skiddaw and Borrowdale Volcanic groups in small outliers along the north-east margin of the Lake District.
The major volcanic centres
Apart from the lavas included within some of the mainly sedimentary sequences, there are several major Siluro-Devonian volcanic centres that have generated substantial accumulations of mostly andesitic lava with only a small proportion of volcaniclastic rocks. The apparent age range is 420 to 390 Ma—late Silurian to early Devonian—and in some cases there is a close spatial association with late Caledonian granitic plutons, which may intrude and metamorphose the lavas.
In the western Highlands of Scotland, an extensive lava development is seen at localities peripheral to the Etive Pluton. In the west of this area, the Lorn Plateau lavas are over 800 m thick and show an upward transition from basalt to andesite and trachyte. To the north-east, the Glencoe Volcanic Formation comprises over 1000 m of lavas that have been preserved by subsidence along an arcuate, caldera boundary fault. Andesitic sheets at the base of the succession are mostly sills intruded at very shallow depths into wet, unconsolidated sediment, although some lava flows are probably also present; the overlying sequence comprises volcaniclastic rocks, andesitic lavas and rhyolitic ignimbrites (Plate P000699). A little farther north, at Ben Nevis, about 600 m of mostly trachytic lavas are preserved.
Within the north-east part of the Midland Valley of Scotland, Siluro-Devonian volcanic rocks form the Ochil and Sidlaw Hills and extend eastward to the coast. The sequence is more than 2000 m in thickness, mainly of mafic lavas although the proportion of volcaniclastic rocks and more felsic lavas increases upwards. The volcanic rocks relate to a stratigraphical position within the Arbuthnott–Garvock Group. Farther south, andesitic lavas crop out in association with Lanark Group strata along the southern margin of the Midland Valley, whilst another extensive lava development forms the Cheviot Hills on the England–Scotland border. There, a thickness of about 500 m of volcanic rock, part of the Reston Group, is made up of andesitic to trachytic lava accompanied by minor pyroclastic and volcaniclastic components. At St Abb’s Head, on the North Sea coast, about 600 m of basalt and andesite lavas are associated with volcaniclastic rocks and a subvolcanic vent agglomerate.
Amongst the youngest of the volcanic sequences are those of Orkney (included within the Eday Group) and of Papa Stour and Esha Ness in western Shetland. On the island of Papa Stour, a 200 m sequence commences with andesitic lavas, which are succeeded by rhyolitic lavas and tuffs; farther north, at Esha Ness, a series of andesitic to rhyolitic lavas, volcaniclastic rocks and ignimbrites is several hundred metres thick.