The Tertiary Geology of Rhum, Inner Hebrides

Abstract

The Tertiary Igneous rocks of Rhum are emplaced in Lewisian gneiss and Torridonian sediments, the latter overlain by a small area of Trias sandstone. In Tertiary times two phases of intrusive activity were separated by the formation of the Main Ring Fault. The earlier phase began with the intrusion of small masses of gabbro, followed by explosion-breccias and tuffisites, felsites and granophyres. The early igneous activity was dominantly acid in character. The members of the early phase, together with the underlying Lewisian and lower members of the Torridonian, were uplifted within the Main Ring Fault. Renewed intrusive activity involved the formation of small ultrabasic masses, and the emplacement of the main body of layered ultrabasic rocks in an essentially solid condition within a ring fracture lubricated by basaltic magma. Basaltic dykes, in a radial system, and cone sheets were intruded at various stages in the development of the complex. Small areas of marble caught in the ring fault system of eastern Rhum may represent highly metamorphosed Jurassic limestones. After the formation of the Main Ring Fault, and probably after the emplacement of the main body of layered ultrabasic rocks there was a period when erosion cut down to almost the present level. During this time basaltic and mugearitic lavas flowed over the irregular land surface, which was mantled in places with coarse conglomerates derived from the earlier rocks. From the gravity survey of Rhum it appears that the Tertiary complex is underlain by a large body of dense basic or ultrabasic rock. This is thought to have been the prime-mover in the Tertiary history of the area, causing central uplift and ring faulting. In addition it probably generated by rheomorphism the acid rocks of the complex: abundant evidence of rheomorphism is found in the rocks at the present level of erosion.