Spout of Ballochleam Lava Member

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Spout of Ballochleam Lava Member (SPBA), Fintry-Touch Block, Carboniferous, Midland Valley of Scotland[edit]

Spout of Ballocheam Lava Member is part of the Clyde Plateau Volcanic Formation.

Name[edit]

Previously named the Spout of Ballochleam Group (see Francis et al., 1970[1]).

Lithology[edit]

The Spout of Ballochleam Lava Member consists predominantly of plagioclase-microphyric basalt lavas (‘Jedburgh’ type) and forms prominent crags. The lavas are comparatively impersistent and hard to correlate between adjacent sections, and some are divided into ‘flow-units’ (Francis et al., 1970)[1]. Many of the basalt lavas have a relatively coarse grain size and are massive. The lavas are also variably slaggy, vesicular with sporadic amygdales, platy, and sporadically columnar jointed, and spheroidally weathered. Boles, some with nodules of decomposed lava, are developed on the tops of some lavas. The member also includes, near the base, rare olivine-microphyric basalt lavas (‘Dalmeny’ type) (NS 7061 9266), and higher within the member isolated plagioclase-macrophyric basalt lavas (‘Markle’ type). North of Scout Head (NS 735 634) in the Touch Hills, the member comprises only three or four lavas, but farther west (NS 7233 9297 to NS 7240 9276) it includes five or more lavas. At the Gargunnock Burn (NS 7061 9266 to NS 7064 9250) there are seven lavas.

Stratotype[edit]

The type area is in the northern and western Fintry Hills, and the northern Gargunnock and Touch hills, north-east of Glasgow (NS 6365 8701 to NS 7437 9374) (Francis et al., 1970)[1]. A reference section is a gully in steep cliffs, east end of Standmilane Craig, north-east Gargunnock Hills (NS 6757 9210 to NS 6762 9200) (Francis et al., 1970)[1]. Here the member is 82 m thick, but contacts between units are generally obscured.

Lower and upper boundaries[edit]

In the northern Touch Hills and the north-eastern Gargunnock Hills, the basal plagioclase- or olivine-microphyric basalt lava of the member lies unconformably on the eroded plagioclase-macrophyric lavas of the Baston Burn Lava Member, progressively overstepping them to the west. In the north-western Gargunnock Hills and in the Fintry Hills, the member overlies, conformably or disconformably, tuffs of the Slackgun Volcaniclastic Member.

In the north and west Fintry Hills, the member is overlain, apparently conformably or disconformably, by the Shelloch Burn Lava Member and the lithological change is to trachybasalt lava. In the northern Gargunnock Hills and north-western Touch Hills, however, the member is overlain, possibly disconformably, by trachybasalt lava of the Lees Hill Lava Member and in the north-eastern Touch Hills, possibly disconformably, by the Gargunnock Hills Lava Member. In the latter case, the lithogical change is to plagioclase-macrophyric basalt lava.

Thickness[edit]

Between 24 and 91 m. The member is of varible thickness, but generally thickens to the west. North of Scout Head (NS 735 634), in the Touch Hills, it is less than 30 m thick, but farther west (NS 7233 9297 to NS 7240 9276) it is more than 50 m thick. In the Gargunnock Burn (NS 7061 9266 to 7064 9250) the member is about 45 m thick. The member is thickest, at more than 80 m, in the headwaters of the Easter Blackspout (NS 6912 9252 to NS 6911 9242).

Distribution and regional correlation[edit]

The member is restricted to the Fintry–Touch Block (Francis et al., 1970)[1] and specifically to the western and northern parts of the Fintry Hills, and the northern parts of the Gargunnock and Touch hills. These rocks crop out from the crest of the crags at Double Craig (NS 6365 8701), westwards to Ballmenoch Burn (NS 6485 8692 to NS 6464 8752), and northwards to the prominent crags below Stronend (NS 6266 8950) which extend eastwards to the Spout of Ballochleam (NS 6526 8998) and on to the north-north-east, below Lees Hill (NS 6587 9106) and east-north-east to Standmilane Craig (NS 6704 9176) and Black Craig (NS 6841 9228). From there, the outcrop continues to the east, passing through more crags and then to Baston Burn (NS 7437 9374).

Age[edit]

Mid Visean (Arundian to Asbian).

References[edit]

  1. 1.0 1.1 1.2 1.3 1.4 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)