The age and structural setting of limestone and basalt on the main ring fault of south-east Rhum Inner Hebrides Scotland

Abstract

In south-east Rhum a Mesozoic/Tertiary sequence is preserved as a fault bounded/rotated wedge. This is juxtaposed between Precambrian rocks (Torridonian sediments and Lewisian Gneiss) and caught up in the complex structure of the Main Ring Fault (MRF). The MRF shows three distinct phases of movement, each one along a different sub-parallel component fracture; i) the Outer Ring Fault (ORF), ii) the Centre Ring Fault (CRF), iii) the Inner Ring Fault (IRF), Smith (1985).The Mesozoic rocks comprise fossiliferous limestone, sandstone and shale, which show differing degrees of thermal metamorphism depending on their proximity to the Ultrabasic Complex. On a basis of faunal and lithological evidence the Mesozoic sediments have been correlated with the Lower Liassic, 'Broadford Beds', of Skye. The Rhum. Jurassic sediments are overlain by a sequence of sheared Tertiary amygdaloidal lavas, the contact between them probably representing the original landscape unconformity. These lavas have been successfully correlated by geochemistry with plateau lavas found on the adjacent island of Eigg, and provide evidence for a once extensive lava field extending over Rhum prior to the emplacement of the postulated Rhum Caidera. The presence of these fault bounded lithologies provides crucial evidence for a major phase of central subsidence during the early tectonic history of the Rhum Volcanic Centre.