Arthur’s Seat Volcanic Formation
Arthur’s Seat Volcanic Formation is part of the Strathclyde Group in the Lothians.
From Arthur’s Seat, Edinburgh. The formation name was established by Chisholm et al. (1989).
The Arthur’s Seat Volcanic Formation consists of lavas, tuffs and volcaniclastic sedimentary rocks. The lavas are mildly alkaline and show a limited range of composition. The basic rocks are silica-undersaturated. They are macroporphyritic in the range olivine-pyroxene-phyric basalt and basanite–olivine-clinopyroxene-plagioclase-phyric basalt–plagioclase-phyric hawaiite–mugearite.
The formation was produced by one short-lived episode of volcanic activity. The depositional environment was terrestrial, probably on a coastal plain.
The type area is in Holyrood Park, Edinburgh (NT 28 73), but the natural sections here are incomplete.
Lower and upper boundaries
The base of the formation, though nowhere exposed or seen in boreholes, is considered to occur at a lithological change from underlying clastic sedimentary rocks of the Ballagan Formation (Inverclyde Group) to lavas, tuffs or volcaniclastic sedimentary rocks of the Arthur’s Seat Volcanic Formation.
The top of the formation, a presumed transition to the sedimentary lithologies of the Gullane Formation, is likewise concealed (Figure 6, Column 4D).
The maximum thickness may be in excess of 300 m (Mitchell and Mykura, 1962, p. 43).
Distribution and regional correlation
Lothians. The main outcrop is in Holyrood Park, Edinburgh with a faulted block on Calton Hill. The Craiglockhart Hills in Edinburgh may be a faulted outlier of the same rocks. Volcanic rocks at Corston Hill, Torweaving Hill, Black Hill, Harperrig, Buteland Hill and Cock Burn may belong to the same eruptive episode. Other smaller separated outcrops are correlated with the Arthur’s Seat Volcanic Formation (see Browne et al., 1999). A confidential borehole at D’Arcy in the Midlothian Coalfield may have proved an eastward extension of the formation, though this lithostratigraphical unit should now be referred to as the Garleton Hills Volcanic Formation (M A E Browne, verbal communication, 01 March 2007).
Chadian to Arundian.
- Chisholm, J I, McAdam, A D, and Brand, P J. 1989. Litho-stratigraphical classification of Upper Devonian and Lower Carboniferous rocks in the Lothians. British Geological Survey Technical Report, WA/89/26.
- Mitchell, G H, and Mykura, W. 1962. The Geology of the neighbourhood of Edinburgh. Memoir of the Geological Survey of Great Britain, Sheet 32 (Scotland).
- 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