Misty Law Trachytic Member

From Earthwise
Jump to navigation Jump to search

Misty Law Trachytic Member (MLLA), Renfrewshire Hills, Carboniferous, Midland Valley of Scotland[edit]

The Misty Law Trachytic Member is part of the Clyde Plateau Volcanic Formation.

Name[edit]

Previously named the Misty Law Trachytic Centre (Paterson et al., 1990)[1]. Part of the ‘Lower Group’ of Richey (1928)[2] and part of ‘unit 3’ of Johnstone (1965)[3].

Lithology[edit]

The member comprises trachyte and rhyolite lavas with associated volcaniclastic rocks, including pyroclastic breccia and tuff, minor amounts of trachyandesite and trachybasalt (capping Totterie Law (NS 293 621)) and one major basaltic intercalation.

Stratotype[edit]

Reference sections or type areas are described for six main phases of the extrusive activity:

1. Trachyte lavas exposed in streams south-east of Waterhead Moor (NS 263 621) and around the headwaters of the River Calder;

2. Pyroclastic rocks in the scarp to the east of Waterhead Moor (NS 269 624), also in Gogo Burn, Routdane Burn, Murchan Burn (NS 290 609) (including lenses of carbonaceous mudstone and plant fragments) and Surge Burn;

3. Trachytic lavas and welded tuffs forming high land that extends north and north-east from the Gogo Water headwaters (about NS 264 596);

4. Basaltic and intermediate lavas forming outliers in The Tongue (NS 280 610) and more continuous outcrops around East Girt Hill (NS 279 627), Burnt Hill (NS 270 634) and Queenside Loch (NS 292 642);

5. Massive flows of trachyte and rhyolite overlying the basic lavas on and west of Queenside Hill (NS 293 638), on Hill of Stake (NS 274 630) and south-east from East Girt Hill (NS 279 627) to Little Misty Law (NS 299 621);

6. Trachyandesite and trachybasalt capping Totterie Law (NS 293 621) and Misty Law (NS 295 620).

Lower and upper boundaries[edit]

Along the western margin of the ‘centre’, the contact of the Misty Law Trachytic Member with the underlying Strathgryfe Lava Member is not exposed but appears to be conformable and to dip gently to the south-east.

The more steeply dipping upper boundary of the Misty Law Trachytic Member is believed to be overstepped by the overlying lavas of the Strathgryfe Lava Member but, where exposed, the contacts are generally faulted. Unfaulted junctions are seen only in the River Garnock (NS 293 593) and in the Rough Burn (NS 315 617).

The distal parts of the cone, where it interdigitates with more mafic rocks, are mainly obscured by faulting but may be represented by thin developments of trachyte and trachytic tuff at South Burnt Hill (NS 255 651), Feuside Hill (NS 252 597) and at Lang Hill (NS 250 612).

Thickness[edit]

Up to 300 m.

Distribution and regional correlation[edit]

In the Renfrewshire Hills, the member’s outcrop is 8 km wide in the area around Misty Law (NS 295 620), extending from the headwaters of the River Calder (NS 258 646) in the north-west, to Muirshiel Country Park (NS 315 630) in the south-east, to the Knockside Hills (NS 256 582) in the south.

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. Richey, J E. 1928. The north Ayrshire sequence of Calciferous Sandstone volcanic rocks. Transactions of the Geological Society of Glasgow. Vol. 18, 247–255
  3. 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