Greeto Lava Member

From Earthwise
Jump to navigation Jump to search

Greeto Lava Member (GRLA), Renfrewshire Hills, Carboniferous, Midland Valley of Scotland[edit]

The Greeto Lava Member is part of the Clyde Plateau Volcanic Formation.

Name[edit]

The name is derived from exposures in the valley of the Greeto Water. Previously named the Greeto Lavas (Paterson et al. 1990[1], Monro 1999[2]). Part of the ‘Lower Group’ of Richey (1928)[3] and part of ‘unit 2’ of Johnstone (1965)[4].

Lithology[edit]

Plagioclase-microphyric basaltic to hawaiite lava (‘Jedburgh’ type) with a few flows of plagioclase-macrophyric basaltic to hawaiite lava (‘Markle’ type). At the base of the member in the Kilbirnie Hills thin beds of purple nodular tuff with intercalated fine-grained red ‘marls’ are developed locally, for example east and south-east of Bradshaw (NS 240 530). A single flow of more-mafic olivine-pyroxene-macrophyric basalt or basanite occurs within the member on the north slope of Castle Hill at Largs (NS 220 589).

Stratotype[edit]

A reference section is well exposed in the valleys of the Greeto and Gogo Waters, close to their confluence (NS 230 593).

Lower and upper boundaries[edit]

The member is underlain by the more-mafic olivine-microphyric basalts (‘Dalmeny’ type) of the Largs Lava Member.

It is overlain by the aphyric mugearites and olivine-free plagioclase-macrophyric hawaiites (‘Markle’ type) of the Strathgryfe Lava Member.

Thickness[edit]

Up to 110 m in the Largs area.

Distribution and regional correlation[edit]

Renfrewshire and Kilbirnie hills: crops out extensively along the top of the scarp slope east of Noddsdale Water from Largs (NS 210 590) north to the Muirshiel Fault. Exposed in the valley of the Greeto Water (NS 230 600). To the south of Largs the Greeto Lava Member forms a scarp feature at the top of the hills behind Largs and Fairlie (NS 230 550). The scarp extends south-south-east on the east side of the Caaf Water valley (NS 230 530). The member is about 100 m thick in the Largs area but thins southwards and lenses out at Wardlaw (NS 250 510). An outlier caps Kaim Hill (NS 220 530) to the west of the Caaf Water valley.

Age[edit]

Mid Visean (Arundian to Asbian).

References[edit]

  1. 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)
  2. Monro, S K. 1999. Geology of the Irvine district. Memoir of the British Geological Survey, Sheet 22W, part 21E (Scotland)
  3. Richey, J E. 1928. The north Ayrshire sequence of Calciferous Sandstone volcanic rocks. Transactions of the Geological Society of Glasgow. Vol. 18, 247–255
  4. 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