Ayrshire Bauxitic Clay Member

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Ayrshire Bauxitic Clay Member (ABC), Carboniferous, Midland Valley of Scotland[edit]

Ayrshire Bauxitic Clay Member is part of the Passage Formation


Formerly known as the Ayrshire Bauxitic Clay Formation (Monro, 1982[1]). See also Monro (1999)[2]; Browne et al. (1999)[3].


Pale grey to buff clayrock, of massive habit with conchoidal fracture, hard and compact. Texture ranges from fine-grained to oolitic, pisolitic and coarsely clastic. Mineralogically, ‘bauxitic in the chemical sense of containing more Al2O3 than can be accommodated with SiO2 to give kaolinite’. The bauxitic clay may be considered a flint clay. The member may also contain sphaerosiderite, fossil plant fragments and tree trunks and include beds of seatrock, coal and mudstone.


The type section is the Saltcoats shore (NS 2401 4150). Here the section into the bauxitic clay is about 1.2–1.5 m thick and consists of a massive, pale grey, to buff clayrock with steep joints. Dark grey plant scraps are common and ooliths and pisoliths are present. Sphaerosiderite is common and varies in abundance through the bed. There is a gradational passage into an underlying clayrock, which further grades into basaltic lavas of the Troon Volcanic Member (Monro, 1999, p. 65[2]).

Lower and upper boundaries[edit]

The base is taken at the lithological change from olivine-microphyric basalt of the underlying Troon Volcanic Member and is gradational from basalt through clayrock rich in sphaerosiderite to bauxitic clay. The base is considered to be stratiform, although it may be obscured by later lateritic weathering or diagenesis and some in-situ alteration of basalt to bauxitic clay may have occurred.

The top of the member is taken at the base of the Scottish Lower Coal Measures. In Ayrshire this is marked by the Raise Coal seam that lies immediately above the Ayrshire Bauxitic Clay.


Up to about 20 m

Distribution and regional correlation[edit]

Limited to north Ayrshire and parts of central Ayrshire.


Namurian to Westphalian (Langsettian)


  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 Unpublished PhD Thesis, University of Edinburgh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 Monro, S K.1999.Geology of the Irvine district.Memoir of the British Geological Survey, Sheet 22W, part 21E (Scotland).
  3. 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.