Ballagan Formation

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Ballagan Formation, (BGN) Carboniferous, Midland Valley of Scotland[edit]

Ballagan Formation is part of the Inverclyde Group

Name[edit]

The name Ballagan Formation was first introduced by Browne (1980)[1] to replace the traditional term Ballagan Beds (or series), which referred, informally, to the mudstone-cementstone sequence at Ballagan Glen (Young, 1867 a[2], b[3]. The name now encompasses strata in the Lothians that were earlier (Chisholm et al., 1989[4]) called the Tyninghame Formation (and in so doing acknowledges the presence of locally thick sandstones) and also part of the Cementstone Group of previous literature in Northern England, the Tweed Basin and the Solway Basin (see Peach and Horne, 1903[5]; Craig, 1956[6]; Craig and Nairn, 1956[7]; Nairn, 1956[8], 1958[9]) and part of the Lower Border Group of Lumsden et al. (1967)[10].

Lithology[edit]

In the Midland Valley of Scotland the Ballagan Formation is characterised by generally grey mudstones and siltstones, with nodules and beds (generally less than 0.3 m thick) of ferroan dolostone (‘cementstone’). Gypsum, and to a much lesser extent anhydrite, and pseudomorphs after halite occur. Desiccation cracks are common and the rocks frequently show evidence of brecciation during diagenesis. Both these features are associated with reddening of the strata. Thin ripple-marked sandstones are present in many areas, and thick localised sandstones are also now included in the formation. in the Northern England province the Ballagan Formation comprises interbedded sandstone, mudstone, limestone (‘cementstone’) and anhydrite, the latter being present in the subsurface only. There are also pseudomorphs after evaporite minerals, some rootlet beds and thin coals.

Genetic interpretation[edit]

In the Midland Valley of Scotland the Ballagan Formation formed in a peritidal environment associated with intermittent emergence. in the Northern England province, deposition of the formation was dominated by the influx of alluvial fans and fluvial and fluviodeltaic sediments from the Southern Uplands, intercalated with lacustrine and arid coastal plain deposits (Deegan, 1973; Leeder, 1974). Hypersaline lacustrine and floodplain facies include pseudomorphs after evaporite minerals, some rootlet beds and thin coals.

Stratotype[edit]

The partial type section, in the Midland Valley of Scotland, is defined south of Perth in the East Dron Borehole (BGS Registration Number NO11NW/24) (NO 1360 1572) from the bedrock surface at 15.26 to 224.80 m depth (see Browne et al., 1999, fig. 1, col. 7). The formation is also well exposed in Ballagan Glen (NS 5731 8022). in the Northern England province (see section 6.3.6 for full locality and stratigraphical details) partial type sections include disused quarries on Whita Hill, the River Esk from Longwood to Broomholm, the River Annan from Rochell to Hoddom Castle, the Tarras Water, Kirkbean Glen and the coastal section from Port Mary to Castle Muir. A reference section occurs in the BGS Hoddom No.2 Borehole (BGS Registration Number NY17SE/3) (NY1641 7285).

Lower and upper boundaries[edit]

In the Midland Valley of Scotland the base of the formation is taken at the lithological boundary between strata characterised by a mudstone–cementstone association and the underlying cornstone bearing sandstone of the Kinnesswood Formation (Figure 6, Column 4B). The top is drawn at the highest unit of mudstone and cementstone in any particular area. The mainly white sandstones of the Clyde Sandstone Formation usually overlie the Ballagan Formation, except in parts of the Lothians where the mainly conformable base of the sandstones, mudstones and siltstones of the Gullane Formation, or volcanic rocks of the Arthur’s Seat or Garleton Hills volcanic formations, overlie the Ballagan Formation.

In the Northern England province the base of the formation is faulted or irregularly conformable upon the Birrenswark volcanic Formation in the Solway Basin (Figure 10, Columns 2, 3), it is conformable on the Kelso volcanic Formation in the Berwick area (Figure 8, Column 12), and elsewhere it is unconformable upon Devonian and older strata. The top of the formation is presumed to be conformable beneath the Lyne Formation of the Northumberland Trough and solway Basin (Figure 6, Column 7; Figure 10, Columns 2, 3), but it is unconformable beneath the Fell Sandstone Formation in the north-east part of the Northumberland Trough (Figure 8, Column 12; Figure 11, Column 1; Figure 12, Columns 1, 2; Figure 13, Column 3).

Thickness[edit]

About 900 m (maximum) in the West Lothian area (based on Mitchell and Mykura, 1962, p. 38), and up to 250 m thick in the Northern England province. Generally, 20 m on Arran (BGS, 1987a), 23 m in the Cumbrae Isles and 38 m at Bute and Cowal (BGS, 2008).

Distribution and regional correlation[edit]

The formation occurs throughout the Midland Valley of Scotland, on Arran, in the Cumbrae Isles and at Bute and Cowal. It also also crops out in the north-eastern margins of the Northumberland Trough and at Kirkbean in the Solway Firth area. in the central part of the Northumberland Trough a lateral facies change sees the Ballagan Formation pass into the Lyne Formation of the Border Group.

Age and biostratigraphical characterisation[edit]

Tournaisian (Courceyan to Chadian). in the Midland Valley of Scotland the Ballagan Formation typically contains miospores indicative of the CM Biozone, although Smith (1996) also recorded a sample from the PC Biozone. where present, the restricted fauna is characterised by the bivalve Modiolus latus, but ostracods are more abundant. in the Northern England province the fauna is also characterised by bivalves (including a modiolid fauna) and the annelid Serpula sp.

Formal subdivisions[edit]

Members of the Ballagan Formation in the Midland Valley of Scotland, in ascending order, include:

Drumwhirn Member (DRWN)

Lindsayston Burn Member (LSBU)

References[edit]

  1. Browne, M A E.1980.The Upper Devonian and Lower Carboniferous (Dinantian) of the Firth of Tay, Scotland.Institute of Geological Sciences Report, No. 80/9.
  2. Young, S W. 1867a. On the Ballagan series of rocks. Transactions of the Geological Society of Glasgow, Vol. 2, 209–212.
  3. Young, S W. 1867b. On the presence of magnesia in rocks.Transactions of the Geological Society of Glasgow, Vol. 2, 64–68.)
  4. Chisholm, J I, McAdam, A D, and Brand, P J. 1989. Lithostratigraphical classification of Upper Devonian and Lower Carboniferous rocks in the Lothians. British Geological Survey Technical Report, WA/89/26.
  5. Peach, B N, and Horne, J. 1903. The Canonbie Coalfield: its geological structure and relations to the Carboniferous rocks of the North of England and Central Scotland. Transactions of the Royal Society of Edinburgh, Vol. 40, 835–877.
  6. Craig, G Y. 1956. The Lower Carboniferous outlier of Kirkbean, Kirkcudbrightshire. Transactions of the Geological Society of Glasgow. Vol. 22, 113–122.
  7. Craig, G Y, and Nairn, A E M. 1956. The Lower Carboniferous outliers of the Colvend and Rerrick shores, Kirkcudbrightshire. Geological Magazine, Vol. 93, 249–256.
  8. Nairn, A E M. 1956. The Lower Carboniferous rocks between the rivers Esk and Annan, Dumfriesshire. Transactions of the Geological Society of Glasgow, Vol. 22, 80–93.
  9. Nairn, A E M. 1958. P etrology of the Whita Sandstone, southern Scotland. Journal of Sedimentary Petrology, Vol. 45, 362–367.
  10. Lumsden, G I, Tulloch, W, Howells, M F, and Davies, A. 1967. The geology of the neighbourhood of Langholm. Memoir of the Geological Survey of Great Britain, Sheet 11 (Scotland).