Upper Devonian, Midland Valley of Scotland
|Cameron, I B, and Stephenson, D. 1985. British regional geology: The Midland Valley of Scotland. Third edition. Reprint 2014. Keyworth, Nottingham: British Geological Survey.|
Upper Devonian, introduction
Upper Devonian strata were laid down unconformably on a folded and eroded surface of older rocks. Evidence for strata of Middle Devonian age is lacking and hence the unconformity represents a considerable interval of time during which there was displacement on the boundary faults and folding within the graben about NE-trending axes.
The base of the Upper Devonian as represented in the Midland Valley is marked by an unconformity and the precise position of the top of the Devonian is uncertain. This uncertainty arises because the continental facies of the Devonian, represented by the Upper Old Red Sandstone, probably continued into Carboniferous time so that the top of the Devonian occurs at an horizon within the Upper Old Red Sandstone. The upper limit of the latter division is placed arbitrarily at a locally convenient lithostratigraphical boundary, commonly at the top of a group of strata containing cornstones. This boundary is lithologically convenient but is unlikely to be laterally equivalent throughout the region. In fact interdigitation of the Upper Old Red Sandstone facies with the Cementstone Group facies of the Carboniferous is known to occur in some areas.
In an attempt to solve the problem of the boundary between the Devonian and Carboniferous, a transitional division is used on some recent Geological Survey of Scotland maps. The division is called the Devono–Carboniferous and is defined as consisting of ‘strata which are of indefinite age within the so-called Upper Old Red Sandstone’.
The Upper Devonian rocks differ from those of the Lower Devonian in that they are generally finer-grained and more mature. The sediments consist mainly of fine- and medium-grained red or buff-coloured sandstones, with darker red siltstones and mudstones. Conglomerates and pebbly sandstones are less common than in the Lower Devonian and have a smaller clast-size (P219038).
There is a greater proportion of quartz grains over lithic fragments and in conglomerates there is a tendency for the proportion of vein-quartz and quartzite clasts to increase upwards in the succession. The upper part of the sequence is characterised by the presence of cornstones which are fossil soils thought to be comparable to caliche deposits (P219597). Volcanic rocks are absent.
The outcrop distribution is shown on P915520. In the north-west part of the area the outcrop occupies a strip on the north and west of the Clyde Plateau lavas and dips gently below them to the south-east and east. In Ayrshire the outcrops are generally aligned NW–SE and emerge from below the sedimentary cover of Carboniferous rocks on either side of the Mauchline syncline. The Upper Devonian rocks crop out in the Edinburgh area and on the north-west side of the Pentland Hills where they overstep Lower Devonian and Silurian sediments and lavas. North of the Forth the strata are known in an elongate outcrop from Kinross north-eastwards to the coast, but a parallel outcrop around the Tay estuary is poorly exposed.
The fauna contained in Upper Devonian rocks is inadequate to establish the lateral equivalence between successions in different parts of the region. Correlation is based on lithology and similar successions represent a parallel sequence of depositional environments which were not necessarily synchronous throughout the region.
The succession and correlation of sequences from the Firth of Clyde area to the Firth of Tay is shown in P915522.
Northern part of the Midland Valley
The thickest known sequence of Upper Devonian rocks occurs in the Firth of Clyde area where it may be as much as 3000 m thick. The earliest rocks belong to the Wemyss Bay Formation which consists of cross-bedded sandstones possibly of aeolian and fluvial overbank origin. The base of the formation is not seen.
The Wemyss Bay Formation is succeeded by the Skelmorlie Formation which is a thick conglomerate with interbedded sandstones. The conglomerate consists of clasts of metamorphic and igneous rocks and is thought to have been deposited by proximal braided streams and alluvial fans. The formation is traceable to the north-east where it is considered to be equivalent to the lower part of the Gargunnock Sandstones of the Kippen area and the Burnside Formation of the Kinross area.
The Kelly Burn Formation is predominantly sandstone and pebbly sandstone and it is equated with the upper part of the Gargunnock Sandstones in Stirlingshire which are brick-red, fine- and medium-grained sandstones with thin red siltstones and mudstones. Similar rocks are named the Glenvale Formation in Fife and Kinross and the Clashbenny Formation in Tayside. Both the Glenvale and Clashbenny Formations contain fossil fish.
Adjacent to the Highland Boundary Fault in the Firth of Clyde to Loch Lomond area a breccia and conglomerate development is interbedded with and replaces the Kelly Burn Formation. The breccias and conglomerates are distinguished from the basal conglomerate by their clast content and their palaeocurrent directions. They are thought to be alluvial fans and proximal braided-stream deposits. The sediments were derived from Dalradian and Lower Devonian rocks and were deposited as alluvial fans prograding to the south-east. This development is not represented in the successions father east.
Near Cupar, the Dura Den Formation overlies the Glenvale Formation. It has not been traced either west to the Lomond Hills area or northwards to the Tayside area. The formation is about 40m thick and consists of red, cream and green-coloured siltstones and fine-grained, cream-coloured sandstones. The Dura Den Formation includes a well-known fish-bed containing many well preserved specimens of Holoptychius and Bothriolepis.
In the Fife and Kinross area, the Knox Pulpit Formation follows the Glenvale Formation and Dura Den Formation. It consists of soft, weakly- cemented white feldspathic sandstone and is characterised by a marked variation in grain-size between adjacent laminae. The formation differs from the other arenaceous formations in that, it cannot with certainty be described as fluviatile and there is evidence of alternating directions of current flow. The environment of deposition is possibly shallow marine indicating a marine transgression into the area from the east, but features which suggest a possible aeolian origin are also present. The formation is a good aquifer.
The youngest subdivision is predominantly sandstone characterised by the presence of cornstone. The formation has been variously named the Cornstone Beds, the Leap Moor Formation, or the Kinnesswood Formation, according to area (P915522). The clastic phase of the formation consists of upwards-fining cycles of sandstone, siltstone and silty mudstone. The sandstones are fine- to medium-grained, weakly cemented, and are variously coloured red, brown, yellow or white. They have an erosive base and pass up into more argillaceous beds. The sandstones are commonly from 3 to 6m thick. The finer-grained beds consist of siltstone and silty mudstone and may be greenish or red in colour. They are usually less than 2 m thick.
The cornstones, which characterise the formation, are pedogenic carbonates (fossil soils), pale cream in colour and occurring both as concretions and in conglomerates (P219597). The concretionary cornstones are impersistent rubbly beds or layers of nodules. In conglomerates the cornstones occur as prominent clasts with smaller pebbles of vein quartz and quartzite. Concretionary cornstones occur up to 1 m thick in the north of the region and some can be thicker elsewhere.
The concretionary cornstones tend to occur at particular horizons in argillaceous beds. They commonly have an irregular base and may pass down into nodules.
The beds of cornstone are fossil soils and they represent a prolonged episode of soil-formation in semi-arid conditions during which erosion was minimal and the rate of sedimentation was low. Their occurrence has been used for stratigraphical correlation but they probably have little chronostratigraphic significance. Conditions suitable for cornstone formation also occurred in parts of the Lower Devonian and Carboniferous successions. Similar processes of soil formation occur at the present time in semi-arid climates.
South side of the Midland Valley
The Upper Devonian rocks in the southern part of the Midland Valley, from Dunbar and the Pentland hills to Ayrshire, have not received the same amount of research in recent times as the succession farther north, and they are much less well known.
The thick clastic sequences consisting mainly of conglomerates and breccias seen in the Firth of Clyde area are absent and the sequence, which is undivided, consists mainly of sandstones and mudstones with cornstones.
Basal conglomerates occur only in the Edinburgh area, in parts of the Pentland outcrop and locally in south Ayrshire.
The predominant lithology is sandstone, red or pink in colour, and less commonly purplish, yellow or white. The sandstones are fine- to medium-grained and locally calcareous. They contain thin conglomerate bands and scattered pebbles, both intra-formational and exotic. Bands of siltstone and silty mudstones are also common.
Cornstones are present throughout the sequence except in the Dunbar area where they occur only in the upper part. They are particularly common in Ayrshire where they may be up to 3 m thick and were quarried and mined at one time for agricultural lime. Locally, cornstones were developed on the surface of the underlying folded and eroded Lower Devonian rocks.
The thickness of the strata varies considerably. In the Edinburgh area the maximum is about 600 m but the rocks are apparently absent in places on the north-west side of the Pentlands and at the south-west end of the Midlothian syncline. In Ayrshire, the maximum thickness occurs near Straiton where it is estimated to be between 300 and 400 m. The strata thin to the south-west and north-west.
Upper Devonian rocks are unrepresented or are very thin in the area around Strathhaven, Lanark and Douglas.
Conditions of deposition and palaeogeography
The succession represents an upward-fining and maturing, mainly alluvial sequence. The thickest and coarsest sediments, are present in the Firth of Clyde area and they indicate vigorous erosion and rejuvenation of source areas, both within the Midland Valley and beyond it. The finer-grained sediments were deposited in the more distal parts of the alluvial system and possibly also in temporary lakes. Some features of the Knox Pulpit Formation in Fife and Kinross suggest a shallow marine environment encroaching from the east but an aeolian origin is also possible. The development and preservation of cornstones imply long periods of tectonic inactivity in a mature landscape with erosion and deposition at a minimum.
The strata were laid down in a generally eastward draining fluvial system. The earlier proximal sediments indicate progradation to the north-east but younger sediments give evidence of an easterly dipping palaeoslope. Locally, in the area of the Highland Boundary Fault, provenance from the north-west is indicated.
In Ayrshire, the main sediment supply came from the Southern Uplands, but locally also from the Carrick Hills and the area north of Muirkirk.
The fossils found in the sediments consist principally of plant and fish remains. The fish faunas from Dura Den, near Cupar, and Clashbenny in Perthshire are well known and suggest a Famennian age. The fish from these localities include species of Bothriolepis, Eusthenopteron?, Glyptopomus, Holoptychius, Phaneropleuron and Phyllolepis. Bothriolepis and Holoptychius have also been found at various other localities including Bracken Bay, near Ayr, and near Dumbarton. The presence of Grossolepis brandi in the Berwickshire coast sequence, just beyond the margin of the present region, has been used to suggest the presence of Frasnian strata, but no Frasnian fossils have been recorded from the Midland Valley.
The relationships of the fish-bearing horizons to successions elsewhere are shown on P915523. Plant remains, none of them diagnostic have been found at various localities.
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