Kirkwood Formation

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Kirkwood Formation (KRW), Carboniferous, Midland Valley of Scotland[edit]

Kirkwood Formation is part of the Strathclyde Group.

Name[edit]

Taken from Kirkwood Farm, Strathclyde Region. The name was introduced by Monro (1982)[1].

Lithology[edit]

The Kirkwood Formation consists of tuffaceous mudstones and tuffs, locally intercalated with non-tuffaceous sedimentary rocks, with some marine horizons developed. The tuffaceous mudstones and tuffs vary in colour from dark reddish brown to greenish grey. Non-tuffaceous sedimentary rocks, sandstones, siltstones and limestones, generally grey in colour, also occur within a restricted area intercalated with volcaniclastic rocks.

Genetic interpretation[edit]

The strata were largely formed by the reworking of materials derived from the underlying volcanic rocks but some primary ash-fall tuffs may be present. The formation shows extensive lateritisation, the product of a period of intense subaerial tropical weathering. There are some marine incursions.

Stratotype[edit]

The type section of the Kirkwood Formation occurs between 38.9 and 75.45 m depth in the Kirkwood Borehole (BGS Registration Number NS34NE/11) (NS 3885 4716), near Kilmarnock (Monro, 1999)[2]. This is the complete thickness of the formation at this locality.

Lower and upper boundaries[edit]

The base of the formation is taken at the lithological change from the underlying lavas of the Clyde Plateau Volcanic Formation into volcaniclastic sedimentary rocks (Figure 6, Column 4A).

The top is generally taken at the base of the overlying mudstones, siltstones and sandstones of the Lawmuir Formation or the cyclical limestones and clastic lithologies of the Lower Limestone Formation (Clackmannan Group). This boundary is commonly irregular and transitional with an interdigitation of lithologies.

Thickness[edit]

The Kirkwood Formation varies in thickness, from about 6 to more than 41 m (Monro, 1999, fig. 9)[2] with thinning generally taking place towards the areas where the Clyde Plateau Volcanic Formation is thick. In the Irvine district, Monro (1999, p. 35, fig. 11)[2] recorded a thickness of 41.39 m encountered in the Coalhill No. 2 Borehole (BGS Registration Number NS24NW/20) (NS 2432 4653) (a minimum thickness since the borehole did not reach the Clyde Plateau Volcanic Formation), and referred to localised thinning in boreholes close to the Dusk Water Fault. In the Kilmarnock district MacPherson et al. (2000, p.9)[3] stated that the thickness of the formation averages around 15 m. BGS (1996)[4] gave a generalised thickness of 18.75 m for the formation in the main coalfield area at Machrihanish.

Distribution and regional correlation[edit]

Western Midland Valley and Machrihanish.

Age[edit]

Visean (Asbian to early Brigantian) (Monro, 1999[2]; Browne et al., 1999[5])

References[edit]

  1. Monro, S K. 1982. Stratigraphy, sedimentation and tectonics in the Dalry Basin, Ayrshire. Unpublished PhD Thesis, University of Edinburgh
  2. 2.0 2.1 2.2 2.3 Monro, S K. 1999. Geology of the Irvine district. Memoir of the British Geological Survey, Sheet 22W, part 21E (Scotland)
  3. MacPherson, K A T, Smith, R A, and Akhurst, M C. 2000. Geology of the Kilmarnock district. Sheet description of the British Geological Survey, Sheet 22E (Scotland)
  4. British Geological Survey. 1996. Campbeltown. Scotland Sheet 12, Provisional Series. Solid and Drift 1:50.000. (Keyworth, Nottingham: British Geological Survey.)
  5. 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