Gravity survey of Colonsay and Oronsay

Abstract

Most of Colonsay and Oronsay consists of low-grade metasedimentary rocks which are usually assumed to be of Torridonian age ([Stewart 1962][1]), but a small area of Lewisian gneiss occurs in northeast Colonsay and plutonic rocks cut the metasedimentary sequence in northern and east-central Colonsay. The geology of the islands is shown in Figure 1. The character of the boundary between the Lewisian and the metasedimentary sequence is uncertain. Although the evidence for structural juxtaposition is reasonably clear for the southern margin, epidotic sandstones and conglomerates occur along the northern outcrop which give the impression of a basal sequence above an unconformity. On Oronsay the metasedimentary sequence comprises the oldest part of the succession, the age of the formations decreasing towards the east and north of Colonsay. The metasedimentary sequence is considered to have a total thickness of about 3000 m ([Stewart 1962][1]), but considerable variation in the thickness of individual formations reflects both the structural style and lateral changes of sedimentary facies. Three phases of deformation have been identified by [Stewart (1962)][1]. The oldest deformation caused the formation of a penetrative cleavage sub-parallel to bedding. This was subsequently folded by upright structures with the development of a steep axial-plane cleavage, and finally by the local formation of folds with crenulation cleavage. It is the folds of the second phase of deformation which govern the essentially right-way-up outcrop pattern. Cutting the metasedimentary rocks are small plutonic intrusions near Balnahard, Kiloran Bay and Scalasaig, and a number of minor dykes and sills . . . [1]: #ref-6