Anstruther Formation
Anstruther Formation (ARBS), Carboniferous, Midland Valley of Scotland
The Anstruther Formation is part of the Strathclyde Group in the Fife area.
Name
From Anstruther, east Fife. The unit was originally described by Forsyth and Chisholm (1977)[1] and first given formation status by Browne (1986)[2].
Lithology
The Anstruther Formation consists dominantly of mudstone, siltstone and sandstone in thin upward-coarsening cycles. Nonmarine limestone and dolostone are also developed, usually as thin beds, some of which contain oncolites and stromatolites. Minor components include marine mudstone and siltstone and a few algal-rich oil-shale beds. Sandstone, generally off-white and fine- to medium-grained, is subordinate to the argillaceous rocks, but thick, upward-fining, multistorey sandstones are locally developed. Thin beds of coal and ironstone are present. The formation is distinguished from the Fife Ness Formation, below, by the lower proportion of sandstone in it, and from the overlying Pittenweem Formation by the relative paucity of its marine faunas. Locally, basalt lavas and lapilli-tuff of the Charles Hill Volcanic Member are present at the base of the formation.
Genetic interpretation
The pattern of sedimentation is of upward-coarsening lake-delta cycles, with thinner upward-fining fluvial units erosively capping them, and minor marine incursions.
Stratotype
Partial type sections in the Anstruther Formation occur on the Fife coast at Anstruther Wester (NO 5670 0310 to NO 5635 0300), at Billow Ness (NO 5590 0269) and at Cuniger Rock (NO 5598 0265 to NO 5566 0271). A representative section was cored in the Anstruther Borehole (BGS Registration Number NO50SE/5) (NO 5653 0350).
Lower and upper boundaries
The base of the formation is not known at outcrop in east Fife but would be taken at the lithological change from sandstone-dominated sequences of the underlying Fife Ness Formation into mudstone-rich cyclical deposits. The top is drawn at the base of the Cuniger Rock Marine Band at the base of the Pittenweem Formation (see Browne et al., 1999, fig. 2.19)[3].
Thickness
The maximum thickness of the formation exceeds 810 m (Forsyth and Chisholm, 1977, table 1)[1].
Distribution and regional correlation
Fife.
Age and biostratigraphical characterisation
Visean. The marine faunas are usually restricted. The abundant but restricted nonmarine faunas are dominated by Naiadites obesus, with Paracarbonicola in the lower part of the formation.
References
- ↑ 1.0 1.1 Forsyth, I H, and Chisholm, J I. 1977. The geology of east Fife. Memoir of the Geological Survey of Great Britain, Sheets 41 part 49 (Scotland)
- ↑ Browne, M A E. 1986. The classification of the Lower Carboniferous in Fife and Lothian. Scottish Journal of Geology, Vol. 22, 422-425
- ↑ 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