Abstract

The Neoproterozoic (early Riphean) Moine Supergroup crops out extensively in the Scottish Caledonides north of the Great Glen and is characterized by structural complexity and monotonous siliciclastic lithology with a lack of biostratigraphic control. Despite these problems, it has become possible to combine locally defined successions into a regional stratigraphic framework. This permits an evaluation of the tectonic setting of Moine deposition, in a subsiding tract that became the Scottish part of the Laurentian margin when Iapetus opened in the Vendian.

Two major rift-basins are inferred, each of half-graben type, controlled by east-facing normal faults. The earlier Morar Group basin received a thick fill of dominantly shallow-marine arkosic sandstones, the Upper Morar Psammite forming a major regressive sequence. The Glenfinnan Group consists of mixed and muddy deposits that are interpreted partly as distal equivalents of the Morar Group, partly as a post-Morar transgressive thermal re-equilibration sequence. The Loch Eil Group, dominated by shallow marine arkosic and siliceous psammites, was deposited in a second, more easterly rift-basin that was subsequently juxtaposed with the Morar basin during Caledonian thrusting. Sediment dispersal in the major sandy sequences was northwards, and there is some evidence that provenance was, at least in part, from a mid-Proterozoic basement terrain to the south.

The extension-dominated depositional setting of the Moine Supergroup, and basin geometry, are similar to those inferred for the other major Riphean clastic sequences of Scotland, the ‘Torridonian’ of the foreland and the Grampian Group to the southeast. The latter is followed by Upper Riphean transgressive strata of the Appin Group. Scottish Riphean stratigraphy records a major pre-Iapetan cycle of lithospheric extension and thermal recovery, with no regional scale unconformities yet identified that might constitute evidence for contemporary orogenesis.

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