Black Mount Lava Member
Black Mount Lava Member (BMLA), Fintry-Touch Block, Carboniferous, Midland Valley of Scotland
Black Mount Lava Member is part of the Clyde Plateau Volcanic Formation.
Name
Previously named the Black Mount Group (see Francis et al., 1970[1]).
Lithology
The Black Mount Lava Member consists predominantly of plagioclase-microphyric basalt lavas (‘Jedburgh’ type) with subordinate basaltic-trachyandesite (mugearite) lavas. The microporphyritic basalt lavas are variably albitised, slaggy, vesicular, and platy-jointed and are locally transitional to a partly plagioclase-macrophyric lithology.
Stratotype
The type area is in the eastern Touch Hills, north-east of Glasgow (NS 7391 9175 to 7600 9327) (Francis et al., 1970)[1]. The member is best exposed in the steep cliff that fringes the northern edge of Black Mount (NS 7436 9226) and forms the south side of the Touch Burn gorge (NS 7438 9244) where, in a reference section, it includes at least three microporphyritic basalt flows and a very fine-grained, platy-jointed, slaggy, basaltic-trachyandesite (mugearite). The section is 27 m thick and includes the base of the member (Francis et al., 1970)[1].
Lower and upper boundaries
The basal plagioclase-microphyric basalt of the Black Mount Lava Member is apparently conformable or disconformable on plagioclase-macrophyric basalt lava of the Gargunnock Hills Lava Member.
The Black Mount Lava Member is overlain by plagioclase-macrophyric basalt lava (‘Markle’ type) of the Touch House Lava Member. The lithological transition is from plagioclase-microphyric basalt to plagioclase-macrophyric basalt.
Thickness
More than 24 m.
Distribution and regional correlation
The member is restricted to the north-eastern part of the Fintry–Touch Block (Francis et al., 1970)[1] and specifically to the eastern Touch Hills, north-east of Glasgow. These rocks crop out from the west side of Black Mount (NS 7391 9175) to near Seton in the north-east (NS 7600 9327) and Garter Wood in the east (NS 7652 9189).
Age
Mid Visean (Arundian to Asbian).