Dinantian and Silesian volcanic activity, Carboniferous, Midland Valley of Scotland
From: Cameron, I B, and Stephenson, D. 1985. British regional geology: The Midland Valley of Scotland. Third edition. Reprint 2014. Keyworth, Nottingham: British Geological Survey.
Dinantian volcanic activity
The earliest widespread activity in the Midland Valley is of early Viséan age. Thick lava sequences are developed within the lower Calciferous Sandstone Measures of East Lothian and Edinburgh City and thinner developments may be traced south-westwards towards Carstairs. The thickest and most extensive development is in the west where the ‘Clyde Plateau Lavas’ form a continuous outcrop to the north, west and south of Glasgow (P915542). East of Glasgow, the lavas are known to be present beneath younger sediments and thinner sequences occur further west on the islands of Bute and Little Cumbrae. In the Bathgate Hills of West Lothian and in the Burntisland area of Fife, the main volcanic accumulations commenced in the Upper Viséan and continued into the Namurian.
Many of the Viséan volcanic centres were initiated in a shallow marine or coastal plain environment and sequences often commence with bedded pyroclastic material and/or volcaniclastic sediments. However, in most areas a rapid accumulation of volcanic deposits created subaerial lava plateaux, which then remained above sea level throughout the volcanic episode, so that intercalated sediments are rare or non-existent. Tropical weathering between eruptions in some areas has produced reddened flow tops or persistent red-brown lateritic boles. Individual flows vary in thickness between 5 and 30m and are usually of a type, although rare pahoehoe features have been reported.
Eruptions are thought to have been from relatively small and short-lived central volcanoes, the remains of which are preserved in some areas as vents and plugs, usually of less than 500m diameter, surrounded by proximal lava flows and pyroclastic features. Away from such centres, pyroclastic rocks are rare and lava plateaux occur which may have been fed, at least in part, from fissure eruptions. In some areas vents, plugs and local dyke swarms are grouped together in linear zones with a predominant NE–SW trend, which may reflect deep-seated fractures along Caledonian structures in the underlying basement. Several such fractures exerted a major structural control during the Viséan and marked changes in thickness of both sedimentary and volcanic successions occur across NE–SW faults such as the Dusk Water Fault. Other, less well defined volcanic lineaments have been postulated on a NW–SE trend in the western Midland Valley where such trends become more important during later igneous activity.
In East Lothian the volcanic sequence of the Garleton Hills is up to 520 m thick. Basal basaltic tuffs are interbedded with thin, lagoonal sediments (200 m) and are succeeded by ankaramite, basalt, hawaiite and mugearite flows (160m) and an upper group of thick trachyte flows and tuffs (160m). Similar but thinner sequences are observed in the Spilmersford Borehole (250m) and in the D’Arcy Borehole (75m) (P915542). The basic lavas are mainly of Craiglockhart, Dunsapie and Markle type and intermediate types include kulaites (trachybasalts with hornblende phenocrysts and analcime, possibly as pseudomorphs after leucite). The more salic rocks are mostly quartz-trachytes but locally include quartz-banakites (quartz-bearing trachyandesites with phenocrysts of green sodic augite), tuffs and rare welded tuffs. Agglomerate-filled vents are particularly abundant and well exposed in coastal sections where the basal pyroclastic unit reaches its thickest development. Some contain basaltic minor intrusions and larger sills, laccoliths and plugs of trachyte, phonolitic trachyte and phonolite form prominent landmarks such as the Bass Rock, North Berwick Law and Traprain Law. Such intrusions are believed to belong to the same eruptive phase as the lavas and this is supported by K-Ar dating.
In Midlothian the sequence of lava flows and tuffs which form Arthur’s Seat and Calton Hill in the centre of Edinburgh occur at a similar horizon to the East Lothian volcanic rocks (P915543). The 400 to 500 m-thick lava and pyroclastic sequence at Arthur’s Seat comprises some 13 flows, with several well defined tuff bands. Lower flows are basalts and ankaramites of Dunsapie and Craiglockhart type with Markle and Jedburgh types and mugearites above. At Calton Hill, a similar sequence is only 200 m thick. The area was a major centre of activity with three large, agglomerate-filled vents, two on Arthur’s Seat (the Lion’s Head and Lion’s Haunch Vents) and one on Salisbury Craigs (the Western Vent). Blocks of lava within the vents have been matched with specific local flows and comparable rock types also occur in numerous plugs, sills and dyke-like masses. The basalt plug of Edinburgh Castle Rock, 3 km to the west-north-west, may be part of the same activity.
A 60 m-thick sequence of tuffs and basalt at Craiglockhart Hill, 6 km to the south-west of Arthur’s Seat, rests upon red sandstones similar to those which occur at various levels in the Cementstone Group. The volcanic rocks have traditionally been placed near the base of the local Carboniferous succession but could equally be near-contemporaneous with the Arthur’s Seat activity. Further to the south-west thin sequences of basalt, mugearite and tuff occur between Corston Hill and Crosswood Reservoir. These outcrops are almost continuous with wider outcrops resting upon rocks of Devono–Carboniferous facies or directly upon Lower Devonian rocks, in poorly exposed ground around the south-western end of the Pentland Hills anticline and around Carstairs. Thin basaltic lava flows occur at a higher stratigraphical level in the Midlothian syncline at Carlops.
In the western Midland Valley the Clyde Plateau volcanic rocks occur in several fault-bounded blocks, each with its own characteristics and between which only tentative correlations may be made. Limited stratigraphical evidence suggests that lava plateaux, up to 900 m thick in places, accumulated within a relatively short space of time (P915543).
The volcanic sequences overlie rocks ranging from the Upper Devonian to high in the Cementstone Group of the basal Carboniferous and hence it seems likely that a period of uplift and erosion preceded the initial volcanic outpourings. Following the termination of eruptive activity, continuing subsidence allowed upper Dinantian sediments to encroach upon the newly-created volcanic terrains and volcaniclastic detritus eroded from such areas commonly overlies the lavas.
Volcanic sequences in the north-east from the Touch Hills to the Campsie Hills are 400 to 500 m thick and are composed almost entirely of feldspar-phyric hawaiites (Jedburgh and Markle type) with subordinate mugearites and trachybasalts at higher stratigraphic levels in some areas. To the south-west a similar succession may be recognised in the Kilpatrick Hills, with additional, more-basic lower and upper units of basalts (Dalmeny and Dunsapie type) and ankaramites (Craiglockhart type) in a total thickness of about 400m.
Evidence for the existence of eruptive centres is abundant throughout the northern Clyde Plateau. Volcanic rocks and underlying sediments are pierced by numerous agglomerate-filled vents, with or without vent intrusions, and by small cylindrical plugs of basalt which form prominent landmarks (P001074 and P000980). Close to the vents, lava flows are often subordinate to bedded pyroclastic rocks, some of which may be the degraded remains of lines of ash cones. Many of the vents lie on prominent NE–SW lines within a 2 to 3 km wide zone extending 27 km from Fintry to Dumbarton (including Dumbarton Rock, Dumgoyne and Dunmore). Others occur along an 8 km length of the Campsie Fault to the south-east of the Kilsyth Hills. Less well defined lines of vents trend NW–SE, one through Bowling, east of Dumbarton, and one in the Campsie Hills from Dunmore to Meikle Bin. The Meikle Bin vent is thought to lie on the margin of a caldera within which the rocks are brecciated and heavily metasomatised and an underlying major basic intrusion is indicated by geophysical evidence. The presence of acid intrusions and trachytic agglomerate within the vent indicates that more-salic lavas were erupted, although none are now preserved. A small intrusion of phonolitic trachyte occurs at Fintry.
To the south-west of Dumbarton, vents and plugs are rare in the northern part of the Renfrewshire Hills, where the general form is of a widespread lava plateau, probably fed from fissure eruptions. However, where the lower part of the lava pile is exposed in deep valleys, numerous dykes with a predominant NE–SW trend occur, particularly along the projected continuation of the Dumbarton–Fintry line. Comparable dyke swarms, vents and plugs are also seen along a further projection of the line, cutting Upper Devonian and basal Carboniferous sediments on Great Cumbrae, and the lava succession of South Bute.
The succession in the Renfrewshire Hills may be as much as 800 m thick, consisting largely of an alternating sequence of Markle-type hawaiites and mugearites. Basaltic tuffs, ankaramites and microporphyritic basalts form a basal unit and Dunsapie and Dalmeny type basalts predominate in a well-defined, upper series. In the middle of the main series a thick pile of trachyte and rhyolite lavas, resting upon trachytic agglomerate and cut by trachytic plugs form the Misty Law Centre. Large trachytic vents occur at Irish Law and Knockside Hill and trachytic dykes are common in the underlying dyke swarm.
The Renfrewshire Hills volcanic succession thins southwards towards Ardrossan where numerous sills, small bosses and agglomerate-filled vents also occur. Some of these are contemporaneous with the lavas, others may be of Namurian age and one group is believed to be Permian. The Heads of Ayr vent some 25 km further south, cuts bedded agglomerates at the top of the Cementstone Group, and may be an isolated occurrence of Dinantian activity, although vent intrusions have more in common with the Permian vents of the area. Westwards, 180 m of Markle and Jedburgh type hawaiites with subordinate mugearites occur on Little Cumbrae and a sequence of comparable thickness on south Bute ranges in composition from basalt (Dunsapie type) to trachyte.
The southern part of the Clyde Plateau volcanic outcrop extends eastsouth-east from the Renfrewshire Hills to Strathaven and is divided into several blocks by major NE–SW faults such as the Dusk Water Fault. Geophysical investigations have suggested that the volcanic succession is thickest, possibly up to 900m, along the WNW–ESE axis of this outcrop, but that marked changes in thickness occur across the NE–SW faults (Hall, 1974). The lavas thin abruptly in the south-east at the Inchgotrick Fault, south of which only a few thin flows are known from the Sorn area. A wide range of basic and intermediate lavas are represented, but microporphyritic basalts of Dalmeny type are particularly common. The Dunlop–Eaglesham–Darvel Hills are characterised by the presence of a wide range of salic lavas including hornblende-trachyandesites, trachytes, quartz-trachytes and rhyolites, together with associated bedded pyroclastic material (P002752). Over 20 plugs and vents, mostly of trachyte and phonolite are broadly distributed along an ESE axis between Irish Law and Loudoun Hill.
The Clyde Plateau volcanic rocks are assumed to be continuous beneath the Central Coalfield Syncline in the Glasgow area and probably extend eastwards as far as the Rashiehill Borehole at Slamannan and the Salsburgh No 1A oil well. Further east they are thought to thin rapidly and are replaced by the thick sedimentary succession of the West Lothian oil-shale basin. The original extent of the Clyde Plateau may therefore have been some 3000 km2.
In West Lothian most of the volcanic activity is younger than the Clyde Plateau and Arthur’s Seat sequences. The Crosswood Ash (100 m thick), the Seafield–Deans Ash (250 m thick) and numerous thin, widespread tuffs occur in the middle of the Calciferous Sandstone Measures. Bedded tuffs and agglomerates become more persistent in the Bathgate Hills in the upper Calciferous Sandstone Measures. The tuffs are succeeded by basaltic and basanitic, microporphyritic lavas of Dalmeny and Hillhouse type, with subordinate pyroclastic rocks, which constitute most of the Lower Limestone Group. Intercalated sedimentary sequences show that, in contrast to the Clyde Plateau, the balance between eruption, erosion and deposition was periodically reversed. Many of the volcanic episodes formed ephemeral islands in a shallow sea and in some cases fringing limestone reefs have been identified. Numerous necks and plugs which cut the West Lothian oil-shale field suggest a former eastwards extension of the volcanic field, which may also be traced westwards, beneath the Central Coalfield, at least as far as the Rashiehill Borehole. The volcanic activity continued into the Namurian resulting in a total of up to 500 m of volcanic rocks.
In Fife a volcanic pile, similar in petrography and overall character to that of the Bathgate Hills, constitutes much of the upper Calciferous Sandstone Measures in the closure of the Burntisland Anticline. Thin, sedimentary intercalations are common in a sequence of Dalmeny and Hillhouse type basalt lavas with subordinate tuffs. The sequence is 450 m thick at Burntisland and Seafield Colliery and extends eastwards to Inchkeith Island where it is at least 150m thick. Several necks and plugs, consisting of similar petrographic types to the lavas, are assumed to be contemporaneous, the most conspicuous being the Binn of Burntisland. Small outcrops of volcanic rocks, including some rhyolites, in the Cleish Hills of west Fife are of Dinantian age.
Silesian volcanic activity
Volcanism continued almost continuously throughout the Silesian, but at a less productive level than in the Dinantian. Activity was concentrated in relatively short-lived, local centres where, in most cases, phreatic explosive eruptions from a multitude of complex vents produced an abundance of bedded pyroclastic deposits within shallow-water sedimentary sequences.
Surface lava flows are predominant only in Ayrshire, but large volumes of magma solidified at depth in most areas as sill-complexes. A restricted compositional range of basalts and basaltic hawaiites, with some basanites, is in marked contrast to the wide range of differentiates present in most Dinantian sequences. The more explosive nature of the volcanism may be caused by changes in magma composition and tectonics, but could also be related to the increasing thicknesses of geotechnically weak sediments in the rapidly-developing Silesian basins. In general, such sequences would be of too low density to support a column of magma, which would spread laterally in the form of sills. Magmas which did have sufficient energy to rise to shallower levels would react with wet sediments to produce violent phreatic eruptions.
The Bathgate Hills sequence of basalt lava flows with subordinate tuffs is continuous from the Dinantian, into the Limestone Coal Group, which is almost completely replaced by lavas around Linlithgow. In the Upper Limestone Group, between 40 and 180 m of lavas occur, mainly between the Index and Orchard limestones, but they extend almost up to the Calmy Limestone in their thickest development. In the northern part of the Central Coalfield, 15 m of lavas and tuffs have been proved in boreholes above the Calmy Limestone and tuff bands occur at several horizons up to the lower Passage Group.
In Fife, evidence of Silesian volcanicity in the form of interbedded tuffs, tuffaceous siltstones and small diatremes is widespread. The tuffs consist of a mixture of basaltic and comminuted sedimentary debris and are often well-graded, indicative of ash-fall into shallow water. Diagenetic alteration produces kaolinised tuffs or tonsteins which are especially common in or near to coal seams. Volcanic activity is particularly concentrated in three areas. In the Saline Hills of west Fife thick tuffs and rare, thin basalt flows in the Limestone Coal Group and parts of the Upper Limestone Group are cut by several necks and plugs. In central Fife, five flows of basaltic pillow lava with associated tuffs and hyaloclastites are interbedded with the top of the Upper Limestone Group and basal Passage Group around Westfield opencast coal pit, near Kinglassie. In east Fife, bedded tuffs and a few lavas are associated with the Upper Limestone Group and Passage Group around the large complex vents of Largo Law and Rires. Over 100 smaller necks, many with plugs of Hillhouse-type olivine-basalt, cut almost the full local age range of Dinantian and Namurian strata. It is therefore inferred that at least some of these necks are contemporaneous with the Namurian bedded tuffs. However, a few small necks cut Lower and Middle Coal Measures strata and one, the Viewforth neck, contains sediments with Westphalian A spores. Some necks are thus of Westphalian age or younger and may be contemporaneous with a sequence of over 170m of tuff, agglomerate and rare basalt encountered in offshore boreholes near Leven which range from topmost Passage Group to Middle Coal Measures (Westphalian B).
Some of the vent agglomerates, including those at Largo Law and Rires, are cut by plugs and minor intrusions of basanite, olivine-analcimite or olivine-nephelinite (formerly termed monchiquites) and hence resemble the Permian age vents of Ayrshire. Indirect evidence suggests that these vents post-date the late Westphalian/early Stephanian quartz-dolerite suite. Whole-rock K-Ar dates obtained from the freshest basanites and monchiquites fall within a narrow range of 290 to 280 Ma suggesting that the vent intrusions are of late Stephanian or early Permian age.
Monchiquitic and basanitic rock-types also occur in vent intrusions, stocks and sills of the south side of the Firth of Forth, where they cut Viséan lavas and overlying sediments. In the absence of any evidence to the contrary it seems reasonable to suppose that these intrusions are a continuation of the east Fife late Stephanian/early Permian province, from which they are separated by only 15 km. Notable examples of intrusions occur at Kidlaw, Limplum and around North Berwick. It is probable that some of the numerous agglomerate- filled necks in the area belong to the same phase of activity.
The Fife vents have been the subject of much detailed work on their internal structure, mode of emplacement and relationships to surrounding bedded tuffs and contemporaneous sedimentation. Most appear to be funnel-shaped tuff-pipes, giving rise to Surtseyan eruptions and modified by post-eruptive subsidence and inward collapse of the area surrounding the initial pipe. Structural control of vent sites is well illustrated by the NE–SW Ardross Fault, along which ten or more vents are sited in a distance of 4 km from Elie to St Monance. A number of vents contain megacrysts and/or rock clasts of deep-seated igneous material or metamorphic basement (pp. 7–8, 112).
In Ayrshire, tuffs and agglomerates occur throughout the Limestone Coal Group in the area around Dalry, replacing locally the Dalry Blackband Ironstone and large parts of the coal-bearing succession. Less extensive tuffs occur in the Upper Limestone Group and all may have been derived from necks which cut older strata to the west.
In the Passage Group, widespread volcanism produced a series of basalt lavas, which underlie much of the Ayrshire Coalfield Basin (P915542). The sequence of lavas with minor intercalated sediments thickens from 10m on the basin margins to 160 m in the Barassie inlier, near Troon. It occupies a stratigraphical position at the junction of the Passage Group and Lower Coal Measures and hence is perhaps partly Westphalian in age. The lavas are almost exclusively microporphyritic olivine-basalts of Dalmeny type but have been shown to have some tholeiitic to transitional chemical tendencies.
These lavas were erupted on to a flat, subsiding land surface, which was frequently submerged, allowing shallow-water lagoonal sediments and coals to be deposited between flows. Other flows show many indications of long periods of weathering between eruptions under hot and humid climatic conditions which led to the formation of ferruginous or aluminous clays, either as in situ lateritic residual deposits, or by a combination of sedimentary reworking and chemical precipitation from solutions charged with aluminium hydroxide. Following the last eruptions, a prolonged period of emergence and deep weathering gave rise to the Ayrshire Bauxitic Clay which reaches thicknesses of up to 9m at the top of the lava pile in the northern outcrops.
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