Strathgryfe Lava Member
The Strathgryfe Lava Member is part of the Clyde Plateau Volcanic Formation.
The member is named after the valley of the Gryfe Water. It was previously referred to as the Strathgryfe Lavas (with informal lower and upper parts) by Paterson et al. (1990). Part of the ‘Lower Group’ of Richey (1928) and ‘unit 3’ of Johnstone (1965). See also Monro (1999); Stephenson in Stephenson et al. (2003).
Aphyric lava (mugearites) and olivine-free plagioclase-macrophyric hawaiite lava (‘Markle’ type), in roughly equal proportions and commonly in composite flows. Thin olivine-pyroxene-macrophyric basalt lava and olivine-bearing plagioclase-macrophyric basalt (‘Markle’ type) occur in the upper part of the member. Boreholes near Clovenstone (NS 33 16) show individual flows between 3 and 20 m thick, with an average of about 10 m. In the boreholes, the flows are almost invariably separated by boles (dark red weathered horizons, fossil soil), up to 1 m thick, which are rarely seen at outcrop. Most flows contain layers of fresh, massive lava, amygdaloidal lava and hydrothermally altered, autobrecciated, ‘slaggy’ material. The altered and slaggy material is commonly soft, friable and in some cases makes up 90 per cent of the flow.
The type area includes a large number of site investigation boreholes that were drilled around the River Calder (NS 3244 6165). These boreholes penetrated the ‘Markle’ type basalt and mugearite of the Strathgryfe Lava Member down to 60 m. Geological logs of the boreholes are available together with a site investigation report. A reference section is at the Dunrod Hill SSSI and GCR site, Inverclyde (NS 23 72) with excellent exposures of feldspar-phyric hawaiite lavas and aphyric mugearite lavas, often as composite flows. Aphyric basal parts of individual flows grade sharply but uninterrupted into feldspar-phyric upper parts suggesting that they were emplaced in rapid succession as pulses of the same eruption (see Stephenson in Stephenson et al., 2003).
Lower and upper boundaries
From Cloch Point (NS 20 75) and Dunrod Hill (NS 24 72) to Port Glasgow (NS 32 74) the Strathgryfe Lava Member rests disconformably upon sedimentary rock of the underlying Clyde Sandstone Formation. To the south and south-east of there, from Loch Thom (NS 25 71) to Largs (NS 20 59), the Strathgryfe Lava Member is conformably underlain by earlier lavas and tuffs of the Clyde Plateau Volcanic Formation. In that area the underlying Greeto Lava Member consists dominantly of flows of plagioclase-microphyric basalt (‘Jedburgh’ type). To the south, in the Kilbirnie Hills, the Strathgryfe Lava Member extends southwards beyond the limit of the Greeto Lavas and rests unconformably on the Late Devonian Kelly Burn Sandstone Formation, Stratheden Group (Browne et al., 2001).
In the Renfrewshire Hills, the dominantly olivine-free lavas (mugearite and hawaiite) of the upper part of the Strathgryfe Lava Member are overlain by the more-mafic olivine-bearing basalts (‘Dunsapie’ and ‘Craiglockhart’ types) of the Marshall Moor Lava Member. The change is abrupt and could imply a significant time interval between the two volcanic episodes. To the south, on the south-eastern edge of the Kilbirnie Hills, the thinning Strathgryfe Lava Member is overlain directly by the volcaniclastic Kirkwood Formation and the Lower Limestone Formation. To the south of the Muirshiel Fault, upper and lower parts to the Strathgryfe Lava Member have been informally identified (Paterson et al., 1990). In this area the Strathgryfe Lava Member is separated by the lavas and volcaniclastic deposits of the Misty Law Trachytic Member (Paterson et al., 1990).
The member is estimated to be up to 750 m thick in the northern and central part of of the Renfrewshire Hills (Paterson et al., 1990, table 6) and up to 250 m thick in the Kilbirnie Hills (Monro, 1999, p. 32).
Distribution and regional correlation
The member occurs in the northern and central parts of the Renfrewshire Hills from Greenock (NS 28 76) to the Muirshiel Fault (NS 23 65) to east Queenside Muir (NS 31 65) and underlies the length of the valley of the Gryfe Water. South of the Muirshiel Fault, the Strathgryfe Lava Member underlies the River Calder Valley (NS 31 64 to NS 35 60) and Ladyland Moor (NS 30 59). In this region the member has been informally split into upper and lower parts (Paterson et al.,1990) separated by the Misty Law Trachytic Member. The unit extends south into the Kilbirnie Hills but thins markedly south and into the area of the Busbie Muir (NS 24 46) and Munnoch (NS 25 47) reservoirs. It is represented by only a few flows where it is cut out by the extension of the Paisley Ruck fault system (around NS 25 44).
Mid Visean (Arundian to Asbian).
- 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. 1928. The north Ayrshire sequence of Calciferous Sandstone volcanic rocks. Transactions of the Geological Society of Glasgow. Vol. 18, 247–255
- Johnstone, G S. 1965. The volcanic rocks of the Misty Law–Knockside Hills district, Renfrewshire. Bulletin of the Geological Survey of Great Britain, No. 22, 53–64
- Monro, S K. 1999. Geology of the Irvine district. Memoir of the British Geological Survey, Sheet 22W, part 21E (Scotland)
- Stephenson, D, Loughlin, S C, Millward, D, Waters, C N, and Williamson, I T. 2003. Carboniferous and Permian Igneous Rocks of Great Britain North of the Variscan Front. Geological Conservation Review Series, No. 27. (Peterborough: Joint Nature Conservation Committee.)
- Browne, M A E, McKirdy, A, and McAdam, A D. 2001. Fife and Tayside: a landscape fashioned by geology. (Perth: Scottish Natural Heritage.)