Clyde Plateau Volcanic Formation

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Clyde Plateau Volcanic Formation (CPV), Carboniferous, Midland Valley of Scotland

Clyde Plateau Volcanic Formation is part of the Strathclyde Group.

Name

Taken from the River Clyde. The name was introduced by Monro (1982)[1] to replace earlier terms such as the Clyde Plateau Lavas.

Lithology

Ardoch Burn, east of Carrot and south of Eaglesham. Lower Carboniferous bedded trachytic agglomerates and tuffs with large ejected lava blocks. Note how the bedding is distorted around the blocks by impact and later differential compaction. Volcanic 'bombs' in bedded, trachyte ashes. Located in the southern part of the Clyde Plateau volcanic outcrop this exposure is part of the c. 900 m. thick succession of salic lavas and associated pyroclastic rocks. P002752.

The Clyde Plateau Volcanic Formation consists of lavas, pyroclastic rocks and volcaniclastic sedimentary rocks. The lavas are transitional to mildly alkaline and show a wide range in composition. The ‘basic’ rocks are mostly hypersthene-normative with a few being silica-undersaturated nepheline-normative. The more-fractionated rocks are almost all quartz-normative in composition, although rare nepheline-normative trachytic lavas have been identified and phonolitic rocks form some intrusions. The range in rock type is olivine+pyroxene-phyric basalt and basanite–olivine+clinopyroxene+plagioclase-phyric basalt–plagioclase-phyric hawaiite–mugearite–benmoreite–trachyte–rhyolite. Benmoreite is rare and the end members of the range are uncommon. In the north-west part of the Midland Valley, in the Kilpatrick–Campsie–Gargunnock blocks, the sequence is dominated by plagioclase-phyric hawaiites. Plagioclase-phyric basalts are less abundant and clinopyroxene-phyric types are uncommon. In the south-western part of the Midland Valley, Renfrewshire Hills and Lanark blocks, a full range of compositions and petrographical variations are present.

Genetic interpretation

The formation was produced by one major episode of subaerial volcanic activity.

Stratotype

The type area is Campsie Glen (NS 6098 8001) where the local base is in exceptionally sharp contact with the underlying Ballagan Formation (rather than the Clyde Sandstone Formation). The Lawmuir 1A Borehole (BGS Registration Number NS57SW/162) (NS 5182 7309), west of Bearsden in Glasgow, provides a partial type section where the top of the formation occurs at 281.12 m, shortly above the bottom of the hole at 286.5 m depth.

Lower and upper boundaries

The base of the formation is taken at the lithological change from the underlying clastic sedimentary rocks into lava, tuff or volcaniclastic sedimentary rocks. This contact, where seen, is normally sharp, representing a gentle but irregular unconformity. Locally, the formation is underlain by the Birgidale Formation on Bute, and by the Laggan Cottage Mudstone Formation on Arran (Figure 6, Columns 3, 1).

The top is marked by a change to the volcaniclastic sedimentary rocks of the Kirkwood Formation (Figure 6, Column 4A).

Thickness

The maximum thickness exceeds 420 m in the Campsie Fells (Forsyth et al., 1996, p. 9)[2]. Paterson and Hall (1986, p.13)[3] suggest up to 800 m. It may be more than 900 m thick in the Renfrewshire Hills (Paterson et al., 1990)[4]. More than 40 m thick in the main coalfield area at Machrihanish (BGS, 1996)[5]. Generally 70–80 m thick on the Isle of Arran (BGS, 1987a)[6]. 150 m and 175 m thick at Bute and Cowal and in the Cumbrae Isles respectively (BGS, 2008)[7].

Distribution and regional correlation

Western Midland Valley of Scotland, on Arran, at Bute and Cowal, in the Cumbrae Isles and at Machrihanish.

Age

Mid Visean (Arundian to Asbian).

Formal subdivisions

Members of the Clyde Plateau Volcanic Formation in approximate stratigraphical order within the following geographical areas include:

Renfrewshire Hills (including the Kilburnie Hills)

Beith-Barrhead Hills

Dunlop-Eaglesham Block

Kilpatrick Hills

Campsie Block (Campsie Fells, Kilsyth Hills, Denny Muir)

Fintry-Touch Block (Fintry Hills, Gargunnock Hills, Touch Hills)

Ayr

References

  1. ↑ Monro, S K. 1982. Stratigraphy, sedimentation and tectonics in the Dalry Basin, Ayrshire. Unpublished PhD Thesis, University of Edinburgh
  2. ↑ Forsyth, I H, Hall, I H S, and McMillan, A A. 1996. Geology of the Airdrie district. Memoir of the British Geological Survey, Sheet 31W (Scotland)
  3. ↑ Paterson, I B, and Hall, I H S. 1986. Lithostratigraphy of the late Devonian and early Carboniferous rocks in the Midland Valley of Scotland. Report of the British Geological Survey, Vol. 8, No. 3
  4. ↑ Paterson, I B, Hall, I H S, and Stephenson, D. 1990. Geology of the Greenock district. Memoir of the British Geological Survey, Sheet 30W part 29E (Scotland)
  5. ↑ British Geological Survey. 1996. Campbeltown. Scotland Sheet 12, Provisional Series. Solid and Drift 1:50.000. (Keyworth, Nottingham: British Geological Survey.)
  6. ↑ British Geological Survey. 1987a. Arran. Scotland Special Sheet, 1:50.000 Series. Bedrock. (Southampton: Ordnance Survey for the British Geological Survey.)
  7. ↑ British Geological Survey. 2008. Dunoon and Millport. Scotland Sheet 29E with part of 21E. Bedrock and Superficial Deposits. 1:50.000. (Keyworth, Nottingham: British Geological Survey.)
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