Skiddaw Lava Member

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Skiddaw Lava Member (SKDL), Fintry-Touch Block, Carboniferous, Midland Valley of Scotland[edit]

Skiddaw Lava Member is part of the Clyde Plateau Volcanic Formation.

Name[edit]

Previously named the Skiddaw Group (Francis et al., 1970)[1].

Lithology[edit]

The Skiddaw Lava Member consists predominantly of plagioclase-macrophyric basalt (‘Markle’ type) and composite lavas with some plagioclase-microphyric basalt (‘Jedburgh’ type). In the east of the outcrop (NS 6572 9122) the member comprises coarse olivine basalt at the base, above which the sequence comprises: a plagioclase-macrophyric basalt lava, a composite lava composed of massive, rounded bodies mostly of plagioclase-macrophyric basalt, but also including near the centre of the lava some transitional plagioclase-microphyric to olivine-microphyric basalts, in a matrix of ‘slaggy’, highly vesicular microporphyritic basalt; and a massive, transitional plagioclase-microphyric to plagioclase-macrophyric basalt with traces of columnar jointing.

Stratotype[edit]

The type area is in the northern and western Fintry Hills, and the north-western Gargunnock Hills, north-east of Glasgow (NS 6304 8712 to NS 6577 9122) (Francis et al., 1970)[1]. A reference section is at Slackgun, north-western Gargunnock Hills (NS 6572 9122) where a section 34 m thick (apparently the full thickness at this locality) was given by Francis et al. (1970)[1].

Lower and upper boundaries[edit]

The basal basalt lava of the member unconformably overlies sedimentary rocks, including sandstone, siltstone and mudstone of the Clyde Sandstone Formation, of the Inverclyde Group.

The Skiddaw Lava Member is overlain unconformably by the Slackgun Volcaniclastic Member. The lithological change is from transitional plagioclase-macrophyric to plagioclase-microphyric basalt lava or composite basalt lava to a dull red bole reflecting prolonged weathering of the lavas.

Thickness[edit]

Between 0 and 37 m.

Distribution and regional correlation[edit]

The member is restricted to the northern Fintry–Touch Block (Francis et al., 1970)[1] and specifically to the northern and western Fintry Hills and to the north-western Gargunnock Hills, north-east of Glasgow. These rocks crop out from below Double Craigs (NS 6304 8712) north-north-west towards Skiddaw (NS 622 891) and then north-east, to the north-west side of Lees Hill (NS 6577 9122).

Age[edit]

Mid Visean (Arundian to Asbian).

References[edit]

  1. 1.0 1.1 1.2 1.3 Francis, E H, Forsyth, I H, Read, W A, and Armstrong, M. 1970. The geology of the Stirling district. Memoir of the Geological Survey of Great Britain, Sheet 39 (Scotland)