Boyd’s Burn Lava Member
Boyd’s Burn Lava Member (BDLA), Campsie Block, Carboniferous, Midland Valley of Scotland
Boyd's Burn Lava Member is part of the Clyde Plateau Volcanic Formation.
Name
Previously named the Boyd’s Burn Lavas (see Forsyth et al., 1996[1]).
Lithology
The Boyd’s Burn Lava Member consists of plagioclase-macrophyric trachybasalt (hawaiite of ‘Markle’ type) lavas. These show a marked variation in the concentration and size of the plagioclase phenocrysts both between and within flows. There are basaltic lapilli-tuffs between the lavas. In the Boyd’s Burn area (NS 650 814), at least four lavas with proximal-facies characteristics occur and tuffs between the lavas are well developed. These lavas have a rather ‘slaggy’ and decomposed nature, and are thought to be derived from the Waterhead Centre (‘Waterhead Central Volcanic Complex’) (see Craig, 1980[2]). In the Goat Burn section (NS 637 793) a lava, 12 m thick, of plagioclase-macrophyric trachybasalt (hawaiite) is overlain by a compound flow with three ‘flow units’ (Craig, 1980)[2] totalling almost 15 m in thickness. These lavas are texturally quite variable with rather sparsely distributed plagioclase phenocrysts in a pale groundmass. Farther to the east, a similar sequence is exposed in the Burniebraes Burn (NS 662 794), although faulting and the coincidence of stratigraphical dip and topographical slope partly obscure the relationships. Here, however, a third lava with numerous large plagioclase phenocrysts overlies the two lavas seen in the Goat’s Burn section.
Stratotype
The type area is the eastern Campsie Fells, and westernmost Kilsyth Hills, north of Glasgow (NS 6186 8004 to NS 6750 8294) (Forsyth et al., 1996)[1]. A reference section is Boyd’s Burn, westernmost Kilsyth Hills (NS 6527 8167 to NS 6487 8126) (Craig, 1980)[2]. The member is also quite well exposed in cliff sections along the southern escarpment of the Campsie Fells, north of the Campsie Fault.
Lower and upper boundaries
The basal plagioclase-macrophyric hawaiite lava of the member is conformable or disconformable on the underlying basaltic-trachyandesite (mugearite) of the Lower Lecket Hill Lava Member.
The Boyd’s Burn Lava Member is overlain conformably or disconformably by the Upper Lecket Hill Lava Member. The lithological change is to mostly more-felsic lavas, including two basal basaltic-trachyandesite (mugearite) lavas, trachybasalt, and trachyte, but also including plagioclase-microphyric basalt to trachybasalt (‘Jedburgh’ type).
Thickness
Between 37 and 50 m. The maxium thickness is developed in the Boyd’s Burn area (NS 650 814).
Distribution and regional correlation
The member is restricted to the Campsie Block (Forsyth et al., 1996)[1] and has a limited geographical extent, (reflecting the proximal character of the lavas) in the eastern Campsie Fells and westernmost Kilsyth Hills, north of Glasgow. These rocks mostly crop out in the escarpment north of the Campsie Fault, to the south-west of Lairs (NS 6424 7994), and on the southern flanks of Cort-ma Law (NS 6515 7995). They also crop out: in a small outlier on Brown Hill (NS 6645 7897); to the south, east and north-east of Box Law (NS 6669 7973); on the eastern, northern and western flanks of Lecket Hill (NS 6445 8121); and in an outlier at Little Bin (NS 6725 8285).
Age
Mid Visean (Arundian to Asbian).
References
- ↑ 1.0 1.1 1.2 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)
- ↑ 2.0 2.1 2.2 Craig, P M. 1980. The volcanic geology of the Campsie Fells area, Stirlingshire. Unpublished PhD Thesis, University of Lancaster