Sedimentation and tectonism (?Permo-Triassic) on the margin of the North Minch Basin, Lewis

Abstract

The (?) Permo-Triassic Stornoway Formation is c . 4000 m thick and consists mainly of conglomerates. The sediments were deposited on alluvial fans as mudflow, streamflood and braided stream deposits, and on floodplains as channel and overbank sediments. In each of the lower, middle and upper units of the Formation there is evidence of two major phases of alluvial fan-building. It is suggested that these phases were tectonically controlled and that within each alluvial fan sequence the time-trend of sedimentation reflects the rate of basin subsidence. Fining-upwards fan sequences suggest basin-margin faulting of gradually decreasing intensity; coarsening-upwards sequences suggest a history of increasing fault intensity. Floodplain deposits, overlying either type of sequence, indicate a cessation of tectonism and a gradual overlap of the old fault lines by fine-grained sediment. The Stornoway Formation is seen as the sedimentary fill within the deep, western margin of an asymmetrical North Minch (Permo-Triassic) Basin. Palaeogeographic reconstructions through time suggest that this western margin shifted westwards as the locus of faulting and fault-generated sedimentation migrated by 15 km from the Minch Fault.