Fin Glen Lava Member

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Fin Glen Lava Member (FGLA), Campsie Block, Carboniferous, Midland Valley of Scotland[edit]

Fin Glen Lava Member is part of the Clyde Plateau Volcanic Formation.

Name[edit]

Previously named the Fin Glen Lavas (Forsyth et al., 1996[1]; see also Craig, 1980[2]; Hall et al., 1998[3]).

Lithology[edit]

The Fin Glen Lava Member consists of trachyte and feldspar-microphyric varying to aphyric basaltic-trachyandesite (mugearite) transitional to trachybasalt (hawaiite) to basalt (of ‘Jedburgh’ type). Basaltic pyroclastic rock is present between many of the lavas. In all but the south-west and south-east, the base of the member is a phonolitic trachyte lava, characterised by well-developed planar jointing. This may be derived from a phonolite intrusion in the ‘North Campsie Linear Vent System’ near Fintry (NS 614 863), to the north (Craig, 1980)[2], which is one of very few phonolitic lavas in the Carboniferous and Permian successions of northern Britain. In the southern part of its outcrop, another trachyte lava occurs near the top of the member. The wide variation in characteristics of the lavas and high degree of their interdigitation suggests individual lavas may have been erupted from different sources, some local and others more distant (Craig, 1980)[2]. Between the Fin Glen and the tributary of the Almeel Burn, and near the summit of Owsen Hill (NS 571 823) and Dumbreck (NS 574 815), to the east of Fin Glen, the lavas are highly scoriaceous and rubbly breccias are present. These resemble autobrecciated aa lavas, and have been interpreted (Craig, 1980)[2] as distal lavas. In the Aldessan Burn (NS 608 807) at least five lavas are exposed. The basal trachyte is pink weathering and intensely platy-jointed, with extensive brecciation, some of which appears to be autobrecciation. The remainder of the lavas are thinner and often sparsely feldspar-phyric. Significant thicknesses of basaltic scoriaceous tuff-breccia and lapilli-tuff occur between most of the lavas.

Stratotype[edit]

The type area is western and central Campsie Fells, north of Glasgow (NS 5760 8058 to NS 6210 8405) (Forsyth et al., 1996)[1]. A reference section is Aldessan Burn, central Campsie Fells, north of Glasgow (NS 6079 8066 to NS 6072 8091) (Craig, 1980)[2].

Lower and upper boundaries[edit]

The basal phonolitic trachyte lava is conformable or disconformable on plagioclase-microphyric trachybasalt (hawaiite) lava of the Campsie Lava Member.

The Fin Glen Lava Member is overlain by the Holehead Lava Member. The lithological change is mostly to plagioclase-macrophyric olivine basalts and trachybasalts (basalts to hawaiites of ‘Markle’ type), characterised by the presence of variable amounts of generally small plagioclase macrophenocrysts.

Thickness[edit]

Between 60 and 110 m. The thickest development is on the east side of Fin Glen Burn (NS 581 816), but this area is faulted, and exposure is only sporadic.

Distribution and regional correlation[edit]

The member is restricted to the Campsie Block (Forsyth et al., 1996)[1] and specifically to the western and central Campsie Fells, north of Glasgow. These rocks crop out in high ground north of the Campsie Fault, on the summits of: Dumbreck (NS 5736 8149), and including the ridge to its south (NS 5835 8066); Owsen Hill (NS 5714 8233); Hog Hill (NS 592 822), including the area to the north and the lower flanks to the south-west and south; and Hart Hill (NS 6043 8266), including the flanks to the north and east. The rocks also crop out around Fassis (NS 604 806) and extending both to its north-west (NS 5970 8165) and east (NS 6226 8101); on the lower flanks to the south-east, east and north-east of Holehead (NS 6176 8262); and in crags to the north-east, and on the eastern, northern and western flanks, of Dunbrach (NS 6147 8375).

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 2.3 2.4 Craig, P M. 1980. The volcanic geology of the Campsie Fells area, Stirlingshire. Unpublished PhD Thesis, University of Lancaster
  3. Hall, I H S, Browne, M A E, and Forsyth, I H. 1998. Geology of the Glasgow district. Memoir of the British Geological Survey, Sheet 30E (Scotland)