Drumnessie Lava Member
Drumnessie Lava Member (DNLA), Campsie Block, Carboniferous, Midland Valley of Scotland
Drumnessie Lava Member is part of the Clyde Plateau Volcanic Formation.
Name
Previously named the Drumnessie Lavas (see Forsyth et al., 1996)[1].
Lithology
The Drumnessie Lava Member consists of several lavas with intercalations, which make up to half the total thickness of the member, of agglomerate (including breadcrust bombs) and variably stratified lapilli-tuff. The lavas are mostly plagioclase-, plagioclase-olivine- and olivine-microphyric basalts (of 'Jedburgh' transitional to 'Dalmeny' types), but there is also one clinopyroxene-olivine-macrophyric basalt ('ankaramite' of 'Craiglockhart' type). The latter forms the second lava in the Garrel Burn section (NS 701 805), where the tuff and several lavas are well exposed. The lavas exhibit proximal characteristics. Several nearby necks of the South Campsie Linear Vent-swarm are plugged by basalt of similar type (e.g. near St Mirren's Well (NS 721 795)), and are probable sources of the lavas.
Stratotype
The type area is the south-central Kilsyth Hills, north-east of Glasgow (NS 6954 7900 to NS 7414 8092) (Forsyth et al., 1996)[1]. A reference section is the BGS Tak-ma-doon Borehole (BGS Registration Number NS78SW/5) (NS 7291 8053) in the south-central Kilsyth Hills, where a 43 m section penetrated the middle and basal lavas of the member (Craig, 1980)[2]. These notably included (in the upper part of the core) a 12.45 m thick, vescicular, aphyric to plagioclase-microphyric basalt, and a 6.47 m thick, aphyric to olivine-microphyric basalt, separated by more than 6 m of lapilli-tuff. In the Banton Burn, the lower part of the member is exposed, upstream from the waterfall (NS 731 804). Three intensely jointed and locally scoriaceous plagioclase-microphyric basalt lavas pass upwards into lapilli-tuff, before the sequence is truncated by the Drumnessie Fault (Craig, 1980)[2]. To the north-east of the fault, there is a more felsic flow, which forms a prominent feature below the topmost lapilli-tuff unit of the member, which is poorly exposed.
Lower and upper boundaries
The base is unconformable. In the BGS Tak-ma-doon borehole (see above) the basal lava of the Drumnessie Lava Member was seen to overlie the Ballagan Formation, comprising 20 m of dominantly grey mudstone and siltstone, interbedded with many dolomitic cementstones, generally 10-15 cm thick but ranging up to 0.5 m. However, 2 km to the west, in the Bachille Burn (NS 707 793), the Ballagan Formation is absent, and the Drumnessie Lava Member rests directly on the Kinnesswood Formation, which comprises sandstone and nodular limestone (cementstone).
The Drumnessie Lava Member is overlain conformably or disconformably by the Laird’s Loup Lava Member. The lithological change is to plagioclase-macrophyric basalt lava ('Markle' type).
Thickness
Up to 60 m.
Distribution and regional correlation
The member is restricted to the southern part of the Campsie Block (Forsyth et al., 1996)[1] and specifically to the southern side of the central Kilsyth Hills, north-east of Glasgow. These rocks crop out to the north of the Campsie Fault, in faulted ground below the escarpment on the southern side of the Kilsyth Hills, north of Kilsyth, from east of Corrie (NS 6954 7900), eastwards to beyond Drumnessie (NS 7414 8092).
Age
Mid Visean (Arundian to Asbian).
References
- ↑ 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).
- ↑ 2.0 2.1 Craig, P M. 1980. The volcanic geology of the Campsie Fells area, Stirlingshire. Unpublished PhD Thesis, University of Lancaster.