Garvald Lava Member

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Garvald Lava Member (GVLA), Campsie Block, Carboniferous, Midland Valley of Scotland[edit]

Gravald Lava Member is part of the Clyde Plateau Volcanic Formation.

Name[edit]

Previously named the Garvald Lavas (Forsyth et al., 1996)[1].

Lithology[edit]

The Garvald Lava Member is well exposed in sections in the River Carron (NS 781 838) and the Overton Burn (NS 7789 8350), where the member consists, in downward succession, of plagioclase-microphyric basalts or trachybasalts (basalts or hawaiites of ‘Jedburgh’ type), an olivine-augite-plagioclase-macrophyric trachybasalt (hawaiite of ‘Dunsapie’ type), a prominently exposed microporphyritic basaltic-trachyandesite (mugearite), two plagioclase-macrophyric trachybasalts (hawaiite of ‘Markle’ type), and a plagioclase-microphyric trachybasalt (hawaiite of ‘Jedburgh’ type) with pillow structures, indicating subaqueous emplacement (Craig, 1980)[2].

Stratotype[edit]

The type area is the easternmost Kilsyth Hills, north-east of Glasgow (NS 7720 8210 to NS 7744 8530) (Forsyth et al., 1996)[1]. A reference section is the Overton Burn, eastern Kilsyth Hills (NS 7854 8324 to NS 7750 8355) though, as elsewhere, the base is not seen (Craig, 1980)[2].

Lower and upper boundaries[edit]

The base is not seen, due to faulting. The Garvald Lava Member is overlain with angular unconformity by the Kirkwood Formation of the Strathclyde Group. In the Garvald Burn section (NS 781 838) the lithological change is from plagioclase-microphyric basalt or trachybasalt to volcaniclastic conglomerates and sandstones, which rest on progressively older lavas of the Garvald Lava Member towards the south.

Thickness[edit]

More than 50 m.

Distribution and regional correlation[edit]

The member is restricted to the Campsie Block (Forsyth et al., 1996)[1] and specifically to the easternmost Kilsyth Fells, north-east of Glasgow. These rocks crop out in faulted ground north of the Campsie Fault, from west of Myot Hill (NS 7720 8210), north-eastwards to Garvald (NS 7849 8335), north-north-west along the Carron Glen and then towards Buckleside (NS 7744 8530). The source is thought to be local, either a neck exposed in the River Carron, north-east of Northshields (NS 779 840) or some other obscured source (Craig, 1980)[2].

Age[edit]

Mid Visean (Arundian to Asbian).

References[edit]

  1. 1.0 1.1 1.2 Forsyth, I H, Hall, I H S, and McMillan, A A. 1996. Geology of the Airdrie district. Memoir of the British Geological Survey, Sheet 31W (Scotland)
  2. 2.0 2.1 2.2 Craig, P M. 1980. The volcanic geology of the Campsie Fells area, Stirlingshire. Unpublished PhD Thesis, University of Lancaster