Large-scale Quaternary movements of the Beinn na Leac Fault Block, SE Raasay, Inner Hebrides

Abstract

The geological map of the southeastern part of Raasay is revised in this paper, with important consequences for the interpretation of the geological structure and history. The strike and dip of the Eyre area to the south are in line with those of the Rudha na Leac–Hallaig area to the north, indicating structural continuity as part of the main southern Raasay tilted fault block. The arcuate fault bounding the Beinn na Leac block extends further south than is shown on previous maps, with a newly recognized part extending from east of the upper part of the Allt Fearns to South Fearns, bounded by the arcuate extension of the main fault. This southern part is extensively overlain by undisturbed Devensian sediments and Pre-Devensian movements are estimated at 173 m. The northern block, forming the hill and cliffs of Beinn na Leac, is not overlain by Devensian deposits and has topographic features, including a detached scree slope, showing clear evidence of later, at least partly post-glacial, movements. The renewed movements, separating it from the southern block, resulted in further movements estimated as at least 165 m, possibly even more further north in the sector, where a total of over 450 m displacement is estimated. The Beinn na Leac Faults are not related to the major NE–SW faults in the Inner Hebrides that define the westerly-tilted fault blocks, including southern Raasay, and dated approximately to the Early Cretaceous. The Beinn na Leac Faults are interpreted as Quaternary listric faults, reflecting the collapse of the scarp of westerly tilted strata into an overdeepened trough east of Raasay formed by repeated glacial erosion. The southern section of the faulted mass was blocked from further movement by thick Devensian deposits offshore filling the southern part of the trough. The Beinn na Leac blocks differ from landslips because they remained coherent and there is no evidence of rotation.