THE SEDIMENTOLOGY OF THE NEOPROTEROZOIC GRAMPIAN GROUP AND THE SIGNIFICANCE OF THE FORT-WILLIAM SLIDE BETWEEN SPEAN BRIDGE AND RUBHA CUIL-CHEANNA, INVERNESS-SHIRE

The Neoproterozoic metasedimentary rocks between Spean Bridge and Rubh a Cuil-cheanna in the Central Highlands of Scotland form part of the G rampian and Appin Groups of the Caledonides. The oldest strata, the Fo rt William Formation comprise an outer shelf, storm-dominated, muddy ( semi-pelitic) sequence, with minor shallowing events represented by th inner arkosic (psammitic) intercalations. Infrequent dropstone deposit s are interpreted as evidence of glacial influences. The transition in to the overlying Inverlair Formation records a basin shoaling event wh ich resulted in coastal (?estuarine) sedimentation. Deposition of the overlying Spean Viaduct Quartzite (Appin Group) reflects slower subsid ence 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 towards Rubha Cuil-cheann a and represents the culmination of basin shoaling initiated during Gr ampian Group times. There was no significant tectonic movement along t he unconformity due to its unsuitable orientation. Above the unconform ity Ballachulish Subgroup strata on-lap towards the south. To the east of the area (e.g. Glen Spean) continuous sedimentation may have occur red throughout Appin Group times.