Slackgun Volcaniclastic Member
Slackgun Volcaniclastic Member (SLAV), Fintry-Touch Block, Carboniferous, Midland Valley of Scotland
Slackgun Volcaniclastic Member is part of the Clyde Plateau Volcanic Formation.
Previously named the Slackgun Interbasaltic Beds (see Francis et al., 1970).
The Slackgun Volcaniclastic Member consists predominantly of bedded tuff and/or volcaniclastic sedimentary rock, with weathered basalt lava (including bole) and a local olivine-microphyric basalt lava.
The type area is in the northern, western and south-western Fintry Hills, and the north-western Gargunnock Hills, north-east of Glasgow (NS 6224 8636 to 6878 9241) (Francis et al., 1970). A reference section is the corrie-like scar of Slackgun, immediately north-west of Lees Hill, northern Fintry Hills (NS 6576 9118 to NS 6577 9112) where Francis et al. (1970) recorded a partially obscured, 34 m thick, section up the scarp.
Lower and upper boundaries
The Slackgun Volcaniclastic Member unconformably overlies the Skiddaw Lava 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 underlying lavas.
The Slackgun Volcaniclastic Member is overlain, conformably or disconformably, by the Spout of Ballochleam Lava Member. The lithological change is from tuff or volcaniclastic sedimentary rock to plagioclase-microphyric basalt lava (‘Jedburgh’ type).
Between 0 and 80 m.
Distribution and regional correlation
The member is restricted to the northern Fintry–Touch Block (Francis et al., 1970) and specifically to the Fintry Hills and the north-western Gargunnock Hills, north-east of Glasgow. These rocks crop out from north-east of Spittalhill (NS 6521 8669) to the west-north-west to below Double Craigs (NS 631 875), north-north-west to below Stronend (NS 626 895), and east-north-east to Black Craig (NS 6878 9241).
Mid Visean (Arundian to Asbian).
- 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)