Clackmannan Group Undivided

Clackmannan Group (undivided)(CKN), Carboniferous, Midland Valley of Scotland

Name

See Section 4.5 for definition. Details relevant to the Sanquhar Basin are provided below:

Lithology

The lower sequence, formerly called the ‘Carboniferous Limestone Series’, comprises basal conglomerate or breccia that passes up into sandstone, which in turn is overlain by fossiliferous, grey, calcareous mudstone and siltstone with local impure limestone beds and thin coals and seatclays. The unit shows marked lateral variations in lithology. The upper sequence, formerly called the ‘Millstone Grit Series’, comprises coarse-grained, greenish sandstone at the base, superseded by greyish-white, cross-bedded sandstones, grey, brown and purple siltstones, and carbonaceous mudstones with marine bands overlain by kaolinitic beds. The unit shows lateral variations in lithology, and local unconformities are present.

Genetic interpretation

The marked lateral variations in lithology in the lower sequence, suggest deposition probably occurred in semi-isolated sub-basins during the period of maximum marine transgression. The lateral variations in lithology and the presence of local unconformities and marine bands in the upper unit, suggest local deposition of condensed sequences in marginal deltaic conditions.

Stratotype

Reference sections are exposed for the lower sequence in Howat’s Burn (NS 828 097) near Sanquhar with conglomerate, sandstone, calcareous mudstone and shelly fossiliferous limestone, and for the upper sequence in Barr Burn (NS 759 089) with sandstones, siltstones and mudstones (including a probable equivalent of the Polhote Marine Band), Polbroc Burn (NS 7293 1029) with conglomerate and sandstone including the Polhote Marine Band, and Kello Water (NS 730 104) with prominent kaolinitic beds.

Lower and upper boundaries

The base of the Clackmannan Group (undivided) is unconformable on the mainly greywacke sandstones of the Ordovician Tappins and Barrhill groups. The top of the lower sequence is unconformably overlain by the cyclical sandstone, siltstone, mudstone, seatearth and coal of the basal Scottish Coal Measures Group (Figure 6, Column 5), whilst the top of the upper sequence is taken at the base of Tait’s Marine Band (TMB) (Davies, 1970) at the base of the Scottish Lower Coal Measures Formation.

Thickness

The lower sequence is up to 10 m thick, whilst the upper sequence is up to 30 m thick.

Distribution and regional correlation

The lower sequence has been proved in the eastern part of the Sanquhar Basin. It has not been encountered in boreholes in the centre and west of the basin. It possibly correlates with Visean strata in the north of the Thornhill Basin namely the Enterkin Mudstone Formation (McMillan, 2002) or the Lower Limestone Formation of the Midland Valley (Browne et al., 1999). Locally, the upper succession is limited to the western part of the Sanquhar Basin. Equivalents extend from the Midland Valley of Scotland into the Thornhill Basin (McMillan and Brand, 1995; Smith, 1999; McMillan, 2002). Davies (1970) concluded that the bulk of the strata could be correlated with the upper part of the Upper Limestone Formation or the lower part of the Passage Formation of the Midland Valley of Scotland. Subsequently, local erosion occurred and then deposition continued up into the Langsettian, when Tait’s Marine Band was deposited.

Age and biostratigraphical characterisation

The lower sequence is late Visean, P1–P2 ammonoid biozones (Wilson in Davies, 1970) or P2 (Greig, 1971). The upper sequence is Arnsbergian to Kinderscoutian, E2–R1 ammonoid biozones, on macrofaunal evidence, and Pendleian (E1) to Langsettian, on microfloral evidence (Davies, 1970).