Knowehead Lava Member
Knowehead Lava Member (KNLA), Campsie Block, Carboniferous, Midland Valley of Scotland
Knowehead Lava Member is part of the Clyde Plateau Volcanic Formation.
Name
Previously named the Knowehead Lavas (Forsyth et al., 1996[1]; see also Craig, 1980[2]; Hall et al., 1998[3]).
Lithology
The Knowehead Lava Member consists of a broad range of petrographical types, including plagioclase-macrophyric trachybasalt lavas (hawaiites of ‘Markle’ type), olivine- augite-plagioclase-macrophyric basalt (‘Dunsapie’ type), plagioclase-microphyric trachybasalt (hawaiite), basaltic-trachyandesite (mugearite), and a rhyolite lava of restricted extent of exposure. The rhyolite has fine platy joints ‘with streaky flow-banding picked out in pale and red weathering colours’ (Craig, 1980)[2]. The outcrop is extensively faulted and it has not been possible to erect an internal stratigraphy for the member. The numerous intercalations of coarse tuff and the ‘slaggy’ tongue-like character of some of the lavas, led Craig (1980)[2] to suggest that at least some of the lavas are close to their eruptive source, although no vents have been identified.
Stratotype
The type area is the south side of the central Campsie Fells, north of Glasgow (NS5660 7965 to NS 6238 7940) (Forsyth et al., 1996)[1]. The member is best exposed in the Fin Glen Burn (NS 604 794). Craig (1980)[2] made no attempt to erect a lava flow stratgraphy within the member due to the heavily faulted nature of the exposed section.
Lower and upper boundaries
The Knowehead Lava Member is fault bounded and the lower and upper boundaries are not seen.
Thickness
Greater than 50 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 Campsie Fells, north of Glasgow. These rocks crop out on the south side of the Campsie Fault, in lower ground below the escarpment, bounded by the Campsie Fault to the north, and another west-north-west-trending fault to the south. The crop extends from east of Clachan of Campsie (NS 6238 7940), to the north-east of Lennoxtown, westwards to the Fin Glen Burn (NS 604 794) and beyond to Strathblane (NS 5660 7965). The outcrop was thought by Craig to extend farther west (Craig, 1980)[2], but correlation in that area with the similar Cochno Lava Member of the Kilpatrick Hills succession (Hall et al., 1998)[3], and farther to the south, is uncertain.
Age
Mid Visean (Arundian to Asbian).
References
- ↑ Jump up to: 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)
- ↑ Jump up to: 2.0 2.1 2.2 2.3 2.4 Craig, P M. 1980. The volcanic geology of the Campsie Fells area, Stirlingshire. Unpublished PhD Thesis, University of Lancaster
- ↑ Jump up to: 3.0 3.1 Hall, I H S, Browne, M A E, and Forsyth, I H. 1998. Geology of the Glasgow district. Memoir of the British Geological Survey, Sheet 30E (Scotland)