Slackdown Lava Member
Slackdown Lava Member is part of the Clyde Plateau Volcanic Formation.
Previously known as the Basal Group (Francis et al., 1970).
The Slackdown Lava Member consists of a varied assemblage of dominant trachybasalt lavas, with plagioclase-macrophyric basalt lavas (‘Markle’ type) and plagioclase-microphyric basalt lavas (‘Jedburgh’ type). In crags north of Scout Head (see Stratotype below) a basaltic-trachyandesite (mugearite), or albitised trachybasalt lava, which is variably massive to platy-jointed, is overlain in succession by a thin and impersistent plagioclase-macrophyric basalt lava, a plagioclase-microphyric basalt lava, and a generally massive trachybasalt, transitional to plagioclase-microphyric basalt lava.
The type area is in the northern Touch Hills, and the northern and north-eastern Gargunnock Hills, north-east of Glasgow (NS 6576 9125 to NS 7410 9372) (Francis et al., 1970). A reference section is provided by the crags north of Scout Head, northern Touch Hills (NS 7311 9352 to NS 7315 9340). Largely obscured by drift, it is 31 m thick. The lavas form a series of cliffs at the foot of the escarpment and the basal mugearite forms a particularly prominent and laterally extensive crag (Francis et al., 1970).
Lower and upper boundaries
The basal basaltic-trachyandesite (mugearite) of the member unconformably overlies sedimentary rocks, including sandstone, siltstone and mudstone of the Clyde Sandstone Formation, of the Inverclyde Group.
The Slackdown Lava Member is overlain, conformably or disconformably, by the Baston Burn Lava Member. The lithological change is from trachybasalt to plagioclase-macrophyric basalt (‘Markle’ type), except in the north-western Gargunnock Hills, where the uppermost Slackdown Lava Member is represented by plagioclase-microphyric basalt (‘Jedburgh’ type).
Between 30 and 79 m.
Distribution and regional correlation
The member is restricted to the Fintry–Touch Block (Francis et al., 1970) and specifically to the northern Touch Hills and the north-eastern Gargunnock Hills. These rocks crop out east-north-eastwards, from the north of Lees Hill (NS 6576 9125) to the north-north-east, to the Baston Burn (NS 7410 9372).
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)