Lees Hill Lava Member

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Lees Hill Lava Member (LEHL), Fintry-Touch Block, Carboniferous, Midland Valley of Scotland[edit]

Lees Hill Lava Member is part of the Clyde Plateau Volcanic Formation

Name[edit]

Previously named the Lees Hill Group (Francis et al., 1970)[1].

Lithology[edit]

The Lees Hill Lava Member consists predominantly of trachybasalt but also includes a plagioclase-macrophyric basalt lava (‘Markle’ type), and the rocks generally form the crest of the escarpment at the top of the cliffs formed by the Spout of Ballochleam Lava Member. The trachybasalt is typically fine grained, massive, locally ‘slaggy’ and highly vesicular, and with local platy jointing. A single trachybasalt lava is present in the east (NS 7184 9247), which occurs as an intercalation within the macroporphyritic basalt lavas of the Gargunnock Hills Lava Member, near to its base. In the Gargunnock Burn (NS 7065 9249 to NS 7059 9222) two trachybasalt lavas are present, separated by a plagioclase-macrophyric basalt lava that is absent farther west, where the member consists entirely of trachybasalt lavas.

Stratotype[edit]

The type area is in the northern Gargunnock Hills and northern Touch Hills, north-east of Glasgow (NS 6616 8971 to NS 7184 9247) (Francis et al., 1970)[1]. A reference section is Gargunnock Burn, central northern Gargunnock Hills (NS 7065 9249 to 7059 9222) (Francis et al., 1970)[1]. Here the entire thickness (about 40 m) of the member exists and is largely exposed, though parts, including the top 1.5 m and basal 0.6 m, are obscured.

Lower and upper boundaries[edit]

The basal trachybasalt of the member lies possibly disconformably on plagioclase-microphyric basalt (‘Jedburgh’ type) of the Spout of Ballochleam Lava Member. In the east of its outcrop, the member is represented by a single trachybasalt lava that overlies, apparently conformably or disconformably, a plagioclase-macrophyric basalt (‘Markle’ type) of the Gargunnock Hills Lava Member.

The Lees Hill Lava Member is overlain, apparently conformably or disconformably, by the Gargunnock Hills Lava Member, and in the east of its outcrop, the Lees Hill Lava Member is represented by a single trachybasalt lava, which is intercalated with lavas of the Gargunnock Hills Lava Member. The lithological change is to plagioclase-macrophyric basalt lava (‘Markle’ type).

Thickness[edit]

Between 0 and about 40 m.

Distribution and regional correlation[edit]

The member is restricted to the northern part of the Fintry–Touch Block (Francis et al., 1970)[1] and specifically to the northern Gargunnock Hills and northern Touch Hills, north-east of Glasgow. These rocks generally form the crest of the escarpment at the top of the cliffs formed by the Spout of Ballochleam Lava Member and crop out northwards from Gourlay’s Burn (NS 6628 8974 to NS 6625 9003) to Lees Hill (NS 660 910), east-north-east to Standmilane Craig (NS 6722 9180) and Black Craig (NS 6829 9219), and then eastwards through crags to the east of Gargunnock Burn (NS 7195 9244).

Age[edit]

Mid Visean (Arundian to Asbian).

References[edit]

  1. 1.0 1.1 1.2 1.3 Francis, E H, Forsyth, I H, Read, W A, and Armstrong, M. 1970. The geology of the Stirling district. Memoir of the Geological Survey of Great Britain, Sheet 39 (Scotland)