The BGS Lexicon of Named Rock Units — Result Details

Strathgryfe Lava Member

Computer Code: SGLA Preferred Map Code: notEntered
Status Code: Full
Age range: Arundian Substage (CJ) — Asbian Substage (CR)
Lithological Description: Nonporphyritic lavas (mugearites) and olivine-free plagioclase-macrophyric hawaiitic lavas (Markle type), in roughly equal proportions and commonly in composite flows. Also thin olivine-pyroxene-macrophyric basalt (ankaramitic) lavas and olivine-bearing plagioclase-macrophyric basalts (Markle type) in the upper part of the member. Boreholes near Clovenstone [NS 333 616] show individual flows are between 3 m and 20 m thick, with an average of about 10 m. In the boreholes, the flows are almost invariably separated by boles, up to one metre 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. To the south of the Murishiel Fault, upper and lower parts to the Strathgryfe Lava Member are separated by the lavas and volcaniclastic deposits of the Misty Law Trachytic Member (Paterson et al., 1990).
Definition of Lower Boundary: 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, 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, east of the Largs Fault-zone, 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 Kelly Burn Sandstone Formation.
Definition of Upper Boundary: n the Renfrewshire Hills, the dominantly olivine-free lavas (mugearite and hawaiite) of the upper parts 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 Strathgrye Lava Member is overlain directly by volcaniclastic sedimentary rocks of the Kirkwood Formation and sedimentary rocks of the Lower Limestone Formation.
Thickness: Up to 750m
Geographical Limits: Northern and central parts of the Renfrewshire Hills from Greenock [NS 28 76] to the Muirshiel Fault [NS 23 65 to 31 65]. There, the unit underlies the length of, and is named from, the valley of the Gryfe Water. South of Murishiel Fault, the Strathryfe Lava Member underlies the River Calder Valley [NS 31 64 to 35 60] and Ladyland Moor [NS 30 59], in this region the member has been informally split in to upper and lower parts (Paterson et al.1990) separated by the Misty Law Trachytic Member. The unit also extends south into the Kilbirnie Hills.
Parent Unit: Clyde Plateau Volcanic Formation (CPV)
Previous Name(s): Strathgryfe Lavas, Upper Strathgryfe Lavas, Lower Strathgyrfe Lavas (-4879)
Alternative Name(s): none recorded or not applicable
Reference Section  Dunrod Hill SSSI and GCR site, Inverclyde. Excellent exposures of feldsparphyric hawaiitic lavas and aphyric mugearitic 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. Stephenson 2003 
Type Area  A large number of boreholes were drilled around the River Calder as part of a site investigation. These boreholes penetrate the Markle-type hawaiites and mugearites of the Strathgryfe Lava Member down to 60m. Paterson et al. 1990. Geological logs of the boreholes are available in BGS borehole records together with a site investigation report (SE 1370). 
Paterson, I B, Hall, I H S & Stephenson, D. 1990. Geology of the Greenock district: Memoir for 1:50000 geological sheet 30W and part of sheet 29E (Scotland). 
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. pp 53-64. 
Monro, S K. 1999. Geology of the Irvine District. Memoir of the British Geological Survey, Sheet 22W and part of 21E (Scotland). 
Stephenson, D, 2003. Dunrod Hill, Inverclyde. 94-99 In Stephenson, D, Loughlin, S C, Millward, D, Waters, C N and Williamson, I T. Carboniferous and Permian Igneous Rocks of Great Britain North of Variscan Front. Geological Conservation Review series No 27. Joint Nature Conservation Committee, Peterborough. 
Richey, J E. 1928. The north Ayrshire sequence of Calciferous Sandstone volcanic rocks. Transactions of the Geological Society of Glasgow. 18. pp 247-255. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
S022 S030