Sergeantlaw Lava Member
Sergeantlaw Lava Member, Beith-Barrhead Hills, Carboniferous, Midland Valley of Scotland
The Sergeantlaw Lava Member is part of the Clyde Plateau Volcanic Formation.
Olivine-microphyric basalt (‘Dalmeny’ type) and olivine-clinopyroxene-plagioclase-macrophyric basalt (‘Dunsapie’ type). In the closure of the Beith Anticline the basal flow of the member is a distinctive clinopyroxene-olivine-macrophyric basalt (‘ankaramite’ or ‘Craiglockhart’ type). In Boylestone Quarry (NS 495 597) hydrothermal veins and cavities contain a wide variety of minerals including calcite, analcime, prehnite, heulandite, bowlingite, thomsonite and natrolite. Native copper, cuprite and malachite are also found.
The Sergeantlaw Lava Member is well seen in its type area between Sergeantlaw (NS 455 596) and Greenfieldmuir (NS 456 583) (Hall et al., 1998). Also between Townhead of Threepwood (NS 39 55) and Gillies Hill (NS 38 54) (see Monro, 1999).
Lower and upper boundaries
The Sergeantlaw Lava Member overlies the plagioclase-macrophyric basalts (‘Markle’ type) of the Gleniffer Lava Member. South of Wester Gavin (NS 380 589) the mafic basalts of the Sergeantlaw Lava Member appears to rest upon volcaniclastic sedimentary rocks and it is possible that in this area the underlying Gleniffer Lava Member has been overlapped. To the north of Harelaw Reservoir (NS 485 590) the Sergeantlaw Lava Member appears to cut out or overlap most of the Gleniffer Lava Member.
Resting upon the Sergeantlaw Lava Member and forming a broad dip slope outcrop south of Muirdykes (NS 395 591) are the plagioclase-macrophyric basalts (‘Markle’ type) and olivine-clinopyroxene-plagioclase macrophyric basalts (‘Dunsapie’ type) of the Fereneze Lava Member.
Distribution and regional correlation
The Renfrewshire and Beith–Barrhead hills: the Sergeantlaw Lava Member crops out to the north and south of Barcraigs Reservoir (NS 39 57). It is well seen between Townhead of Threepwood (NS 39 55) and Gillies Hill (NS 38 54) and forms scarp features around Walls Hill (NS 412 589). It is also present in the area between Sergeant Law (NS 455 596) and Greenfieldmuir (NS 456 583) and in the Lochliboside Hills (NS 450 575).
Mid Visean (Arundian to Asbian).
- 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)
- Monro, S K. 1999. Geology of the Irvine district. Memoir of the British Geological Survey, Sheet 22W, part 21E (Scotland)
- Paterson, I B, Hall, I H S, and Stephenson, D. 1990. Geology of the Greenock district. Memoir of the British Geological Survey, Sheet 30W part 29E (Scotland)
- Richey, J E, Anderson, E M, and MacGregor, A G. 1930. The geology of north Ayrshire. Memoir of the Geological Survey of Great Britain, Sheet 22 (Scotland)