Baston Burn Lava Member
Baston Burn Lava Member (BAST), Fintry-Touch Block, Carboniferous, Midland Valley of Scotland
Baston Burn Lava Member is part of the Clyde Plateau Volcanic Formation
Name
Previously named the Baston Burn Group (see Francis et al., 1970[1]).
Lithology
The Baston Burn Lava Member consists predominantly of plagioclase-macrophyric basalt lavas (‘Markle’ type) with a few plagioclase-microphyric lavas, or microporphyritic components of composite lavas (‘Jedburgh’ type). The member has eroded to form a prominent and wide step in the cliffs above the Slackdown Lava Member in the northern Touch and Gargunnock hills. In the extreme east (NS 7315 9339 to NS 7318 9331), the member comprises three plagioclase-macrophyric basalt lavas, but farther west (NS 7232 9320 to NS 7234 9297) comprises four such lavas, the lower and upper two being separated by a thin lava composed of microporphyritic basalt (‘Jedburgh’ type) (although this may be a component of a composite lava). In the Gargunnock Burn (NS 7073 9292 to NS 7060 9267), the member lies in a shallow synclinal structure and comprises more than ten lavas. All are of plagioclase-macrophyric basalt except for the highest and lowest, which are transitional between macroporphyritic and microporphyritic basalt. The lowest lava is composite elsewhere (NS 7154 9305).
Stratotype
The type area is in the northern Touch Hills and Gargunnock Hills, north-east of Glasgow (NS 6580 9120 to NS 7351 9352) (Francis et al., 1970)[1]. A reference section is in the Gargunnock Burn, central northern Gargunnock Hills (NS 7073 9292 to 7060 9267), comprising a 67 m thick succession with ten lavas (Francis et al., 1970)[1].
Lower and upper boundaries
In the northern Touch Hills, and north-eastern Gargunnock Hills, the basal macroporphyritic lava of the member is conformable or disconformable on a trachybasalt lava of the Slackdown Lava Member. Farther west, where the trachybasalt is absent, the basal lava of the Baston Burn Lava Member is a transitional plagioclase-macrophyric to plagioclase-microphyric basalt, which overlies conformably or disconformably a microporphyritic basalt of the Slackdown Lava Member.
In the northern Touch Hills and north-eastern Gargunnock Hills, the Baston Burn Lava Member is overlain unconformably by the Spout of Ballochleam Lava Member, and the Baston Burn Lava Member is progressively truncated towards the west. The lithological change is to microporphyritic basalt (‘Jedburgh’ type). In the north-western Gargunnock Hills, the Baston Burn Lava Member is overlain unconformably by the Slackgun Volcaniclastic Member, and the lithological transition is to weathered basalt (bole) and well-bedded tuff.
Thickness
Between 9 and 67 m. In the extreme east (NS 7315 9339 to NS 7234 9297) the member is 15 m thick. It is thickest in the Gargunnock Burn (NS 7073 9292 to NS 7060 9267).
Distribution and regional correlation
The member is restricted to the northern part of the Fintry–Touch Block (Francis et al., 1970)[1] and specifically to the northern Fintry Hills and the north-eastern Gargunnock Hills. These rocks crop out on a wide step in the escarpment above the cliffs formed by the Slackdown Lava Member, extending north-east from the north side of Lees Hill (NS 6580 9120) to Standmilane Craig (NS 670 918) and Black Craig (NS 685 924) and east to the Gargunnock Burn (NS 7073 9292 to NS 7060 9267) and Baston Burn (NS 7351 9352).
Age
Mid Visean (Arundian to Asbian).