Lawmuir Formation
Lawmuir Formation (LWM), Carboniferous, Midland Valley of Scotland
Lawmuir Formation is part of the Strathclyde Group.
Name
Taken from Lawmuir, Strathclyde Region. The name was introduced by Paterson and Hall (1986)[1]
Lithology
The Lawmuir Formation consists of a sequence of mudstones, siltstones and sandstones with seatrocks, coals and limestones. To the north of Glasgow around Milngavie, the lower part is dominated by fluvial sandstones with a local development of quartz conglomerate. In the south of Glasgow around Paisley, fluvial sandstones are interbedded with thick, poorly bedded siltstones and mudstones with a few thin coals. In one small area these coals coalesce to form a single seam 20 m thick. The upper part of the formation is partly arranged in cyclothems, with several marine incursions represented by marine limestones over a large area. A non-marine limestone (the Baldernock Limestone) also occurs near the top of the sequence.
Genetic interpretation
Fluvial lower part becoming partly cyclothemic upwards with several marine incursions.
Stratotype
The type section of the Lawmuir Formation (complete thickness) is between 11.75 and 266.2 m depth in the Lawmuir Borehole (BGS Registration Number NS57SW/162) (NS 5183 7310) west of Bearsden near Glasgow.
Lower and upper boundaries
The base of the formation is taken at the lithological change from the underlying volcaniclastic strata of the Kirkwood Formation into clastic sedimentary rocks. The boundary is commonly transitional, with interdigitation between sedimentary and volcaniclastic rocks. It can also rest directly on the lavas of the Clyde Plateau Volcanic Formation in the west of the Midland Valley of Scotland (see Paterson and Hall, 1986)[1] (Figure 6, Columns 1, 4A) and on the apparently conformable but probably erosive surface of the characteristic mudstone, siltstone and ferroan dolostone lithologies of the Ballagan Formation (Inverclyde Group) in Ayrshire (see Monro, 1999, p. 39)[2].
The top of the Lawmuir Formation is drawn at the base of the Hurlet Limestone of the Lower Limestone Formation (Clackmannan Group).
Thickness
The maximum thickness of the formation is about 300 m in the Glasgow area. BGS (1987)[3] gave a generalised thickness of 185 m for the formation on the Isle of Arran.
Distribution and regional correlation
Western Midland Valley and the Isle of Arran. The precise relationship between the Lawmuir Formation and the Pathhead Formation in Fife is not known because of a major geographical gap in information between the two areas. It is possible that the marine bands in the upper part of the Lawmuir Formation are equivalent to all the marine bands in the Pathhead Formation, in which case the nonmarine lower part of the Lawmuir Formation would be equivalent to the Sandy Craig Formation in Fife.
Age
Visean (Monro, 1999[2]; Browne, 1999[4]).
References
- ↑ 1.0 1.1 Paterson, I B, and Hall, I H S. 1986. Lithostratigraphy of the late Devonian and early Carboniferous rocks in the Midland Valley of Scotland. Report of the British Geological Survey,Vol. 8, No. 3.
- ↑ 2.0 2.1 Monro, S K. 1999. Geology of the Irvine district. Memoir of the British Geological Survey, Sheet 22W, part 21E (Scotland)
- ↑ British Geological Survey. 1987. Arran. Scotland Special Sheet, 1:50.000 Series. Bedrock. (Southampton: Ordnance Survey for the British Geological Survey.)
- ↑ Browne, M A E, Dean, M T, Hall, I H S, McAdam, A D, Monro, S K, and Chisholm, J I. 1999. A lithostratigraphical framework for the Carboniferous rocks of the Midland Valley of Scotland.British Geological Survey Research Report, RR/99/07.