Petrography and age of agglomeratic vents near Toscaig, Applecross, Scotland

Abstract

The occurrence of two small volcanic vents on the Applecross 1:50,000geological sheet 81(E) was recently noticed during a study (Rock, 1981) of the PermoCarboniferous vents of the Lochaber area, further south. The vents were recorded in 1892 by J. Horne and a few thin sections were made (IGS Scotland sliced collection Nos. S5167-72), but they seem never to have been described. The area was therefore revisited briefly and further thin sections (S64942-6) prepared. The vents occur in the Applecross peninsula (Fig. 1a) to the southeast of Cam nan Uaighean, between the gorge of the River Toscaig and Lochan nan Slochd; the actual locality [NG 737398] (Fig. 1b) is named as 'Bruach Bhuidhe' on 6-inch sheet Ross 109SW. One vent lies on each side of a shallow and partly dry valley, containing a small stream which drains Lochan nan Slochd. The site of the larger vent can be recognised by an almost complete absence of exposure, which contrasts strongly with the craggy ground occupied by the Torridonian sandstone (Applecross Formation) country rocks on all sides; it forms a low grassy knoll surrounded by depressions. Exposures of both vents are extremely poor, and no contacts are seen. The larger vent reaches approximately 200 m by 100 m and its rock-type varies considerably; a tiny exposure on the southwest slope of the knoll [NG 73753890] is formed almost entirely of friableweathering volcanic matrix; Horne recorded a 'decomposed basalt' dyke but this is no longer visible. By contrast, a second exposure at the east end of the northern slope isalmost entirely composed of clasts with only very sparse matrix, while a third, slightly further west, is intermediate in texture between these other two. The clasts are of only one type-a grey gritty sandstone-and the larger ones are often much rounded due to extraordinary spheroidal fracturing (noticeable in both vents); smaller clasts may however be angular. The smaller vent is slightly better seen at [NG 73603910], where there are four small exposures (Fig. 1c): (a) Subsoil outcrop of fractured and weathered igneous rock, carrying numerous well-crystallised pseudomorphs of saponite preserving a good basal cleavage and presumed to be after megacrysts of biotite (XRD identification of saponite by courtesy of Mr. R. I. Lawson). (b) Weathered agglomerate with similar sandstone clasts to those in the larger vent. (c) Variably altered agglomerate with two large masses of this sandstone (2 m and 1 m diameter), showing well-developed spheroidal fracture. (d) Agglomerate dominated by spheroidally fractured clasts, with a rather sparse, though somewhat fresher than usual, matrix. These vents are peculiar for their isolation. Igneous activity in the Applecross peninsula is otherwise represented only by N-S trending Tertiary basalt dykes and a few Caledonian felsites and lamprophyres (see geological sheet 81E). The nearest vents elsewhere on the mainland are the Tertiary ones of Ardnamurchan, and the Permo-Carboniferous ones in the Loch Arkaig area, all some 70-80 km distant. Rather nearer are three vents on Raasay in the area around Brochel Castle [NG 584463], 15 km northwest of Toscaig, which were described by Davidson (1935) and Selley (1966). These are far better exposed but of similar size to the Toscaig vents and, like them, penetrate Torridonian strata.