Air-fall tuffs in the Southern Highland Group, Dalradian Supergroup, at Birnam, Perthshire

Abstract

Synopsis Ten ‘brown beds’ representing air-fall tuffs in the Southern Highland Group of the Dalradian Supergroup of Scotland are described for the first time from near Birnam, Perthshire, close to the Highland Boundary Fault. Their composition suggests an origin more silicic than the traditional Dalradian ‘green beds’. Expressions of volcanism in the upper Dalradian are preserved as the Tayvallich (Argyll Group) and Loch Avich (Southern Highland Group) basaltic lavas. The Tayvallich lavas are associated with minor quartz keratophyre intrusions and pink felsic tuffs. On the basis of the currently accepted stratigraphic correlation of the upper Dalradian, the ‘brown beds’ at Birnam would have been derived from eruptions of currently unidentified quartz keratophyric magmas in Southern Highland Group times. Alternatively, if the source of the ‘brown beds’ can be traced to the Tayvallich volcanism, then the upper Dalradian stratigraphy of the Birnam area would need to be revised.