Holehead Lava Member
Holehead Lava Member (HHLA), Campsie Block, Carboniferous, Midland Valley of Scotland
Holehead Lava Member is part of the Clyde Plateau Volcanic Formation.
Name
Previously named the Holehead Lavas (Forsyth et al., 1996[1]; see also Craig, 1980[2]; Hall et al., 1998[3]).
Lithology
The Holehead Lava Member consists mainly of plagioclase-macrophyric olivine basalts and trachybasalts (basalts to hawaiites of ‘Markle’ type), characterised by the presence of variable amounts of relatively small plagioclase phenocrysts. In addition, one lava of transitional trachybasalt to mugearite composition is exposed in the middle of a lava sequence in the Alvain Burn (NS 614 815). In the Aldessan Burn (NS 607 809 to NS 605 816), there is a sequence of seven lavas. The flows tend to be relatively thin with ‘slaggy’ tops and locally rubbly bases.
Stratotype
The type area is central Campsie Fells, north of Glasgow (NS 5894 8166 to NS 6200 8341) (Forsyth et al., 1996)[1]. A reference section is Aldessan Burn, central Campsie Fells, north of Glasgow (NS 6072 8092 to NS 6039 8224) where seven lavas are seen (Craig, 1980)[2].
Lower and upper boundaries
The basal plagioclase-macrophyric basalt, and/or trachybasalt, is conformable or disconformable on basaltic-trachyandesite (mugearite) of the Fin Glen Lava Member.
The Holehead Lava Member occupies summit areas across its outcrop and the top of the member has been eroded.
Thickness
More than 100 m, top eroded.
Distribution and regional correlation
The member is restricted to the Campsie Block (Forsyth et al., 1996)[1] and has a limited geographical extent in the Central Campsie Fells, north of Glasgow. These rocks mostly crop out: in an outlier on the upper, southern flanks of Hog Hill (NS 592 822); around Inner Black Hill (NS 6101 8164); and around the summit of Holehead (NS 6176 8262). The lavas were thought by Craig (1980)[2] to have been derived from the Waterhead Centre (‘Waterhead Central Volcanic Complex’).
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 Unpublished PhD Thesis, University of Lancaster
- ↑ Hall, I H S, Browne, M A E, and Forsyth, I H. 1998. Geology of the Glasgow district. Memoir of the British Geological Survey, Sheet 30E (Scotland)