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Slackdown Lava Member (SLLA), Carboniferous, Midland Valley of Scotland

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==Slackdown Lava Member [http://www.bgs.ac.uk/lexicon/lexicon.cfm?pub=SLLA (SLLA)], Fintry-Touch Block, Carboniferous, Midland Valley of Scotland ==
Slackdown Lava Member is part of the [[Clyde Plateau Volcanic Formation]].
=== Name ===
Previously known as the Basal Group (Francis et al., 1970)<ref name="Francis70">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)</ref>.

=== Lithology ===
The Slackdown Lava Member consists of a varied assemblage of dominant trachybasalt lavas, with plagioclase-macrophyric basalt lavas (‘Markle’ type) and plagioclase-microphyric basalt lavas (‘Jedburgh’ type). In crags north of Scout Head (see ''Stratotype'' below) a basaltic-trachyandesite (mugearite), or albitised trachybasalt lava, which is variably massive to platy-jointed, is overlain in succession by a thin and impersistent plagioclase-macrophyric basalt lava, a plagioclase-microphyric basalt lava, and a generally massive trachybasalt, transitional to plagioclase-microphyric basalt lava.

=== Stratotype ===
The type area is in the northern Touch Hills, and the northern and north-eastern Gargunnock Hills, north-east of Glasgow (NS 6576 9125 to NS 7410 9372) (Francis et al., 1970)<ref name="Francis70"></ref>. A reference section is provided by the crags north of Scout Head, northern Touch Hills (NS 7311 9352 to NS 7315 9340). Largely obscured by drift, it is 31 m thick. The lavas form a series of cliffs at the foot of the escarpment and the basal mugearite forms a particularly prominent and laterally extensive crag (Francis et al., 1970)<ref name="Francis70"></ref>.

=== Lower and upper boundaries ===
The basal basaltic-trachyandesite (mugearite) of the member unconformably overlies sedimentary rocks, including sandstone, siltstone and mudstone of the [[Clyde Sandstone Formation]], of the [[Inverclyde Group]].

The Slackdown Lava Member is overlain, conformably or disconformably, by the [[Baston Burn Lava Member]]. The lithological change is from trachybasalt to plagioclase-macrophyric basalt (‘Markle’ type), except in the north-western Gargunnock Hills, where the uppermost Slackdown Lava Member is represented by plagioclase-microphyric basalt (‘Jedburgh’ type).

=== Thickness ===
Between 30 and 79 m.

=== Distribution and regional correlation ===
The member is restricted to the Fintry–Touch Block (Francis et al., 1970)<ref name="Francis70"></ref> and specifically to the northern Touch Hills and the north-eastern Gargunnock Hills. These rocks crop out east-north-eastwards, from the north of Lees Hill (NS 6576 9125) to the north-north-east, to the Baston Burn (NS 7410 9372).

=== Age ===
Mid Visean (Arundian to Asbian).

== References ==
<references/>
[[Category:Carboniferous lithostratigraphical nomenclature]]

Slackdown Lava Member - Revision history

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Geosource>Volunteer 01: /* Slackdown Lava Member (SLLA), Carboniferous, Midland Valley of Scotland */