The sedimentology of the Neoproterozoic Grampian Group and the significance of the Fort William Slide between Spean Bridge and Rubha Cuil-cheanna, Inverness-shire

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Abstract
Synopsis: The Neoproterozoic metasedimentary rocks between Spean Bridge and Rubha Cuil-cheanna in the Central Highlands of Scotland form part of the Grampian and Appin Groups of the Caledonides. The oldest strata, the Fort William Formation comprise an outer shelf, storm-dominated, muddy (semi-pelitic) sequence, with minor shallowing events represented by thinner arkosic (psammitic) intercalations. Infrequent dropstone deposits are interpreted as evidence of glacial influences. The transition into the overlying Inverlair Formation records a basin shoaling event which resulted in coastal (?estuarine) sedimentation. Deposition of the overlying Spean Viaduct Quartzite (Appin Group) reflects slower subsidence rates and greater (tidal?) sediment reworking. The ‘Fort William Slide’, previously interpreted as tectonic in origin, is here regarded as a localised intra-Appin Group unconformity. This cuts out much of the Grampian Group and the Lochaber Subgroup toward s Rubha Cuil-cheanna and represents the culmination of basin shoaling initiated during Grampian Group times. There was no significant tectonic movement along the unconformity due to its unsuitable orientation. Above the unconformity Ballachulish Subgroup strata on-lap towards the south. To the east of the area (e.g. Glen Spean) continuous sedimentation may have occurred throughout Appin Group times.