TECTONOSTRATIGRAPHY OF THE MOINE SUPERGROUP - A SYNTHESIS

The Neoproterozoic (early Riphean) Moine Supergroup crops out extensiv ely in the Scottish Caledonides north of the Great Glen and is charact erized by structural complexity and monotonous siliciclastic lithology with a lack of biostratigraphic control. Despite these problems, it h as become possible to combine locally defined successions into a regio nal stratigraphic framework. This permits an evaluation of the tectoni c setting of Moine deposition, in a subsiding tract that became the Sc ottish part of the Laurentian margin when Iapetus opened in the Vendia n. Two major rift-basins are inferred, each of half-graben type, contr olled by east-facing normal faults. The earlier Morar Group basin rece ived a thick fill of dominantly shallow-marine arkosic sandstones, the Upper Morar Psammite forming a major regressive sequence. The Glenfin nan Group consists of mixed and muddy deposits that are interpreted pa rtly as distal equivalents of the Morar Group, partly as a post-Morar transgressive thermal re-equilibration sequence. The Loch Eil Group, d ominated by shallow marine arkosic and siliceous psammites, was deposi ted in a second, more easterly rift-basin that was subsequently juxtap osed with the Morar basin during Caledonian thrusting. Sediment disper sal in the major sandy sequences was northwards, and there is some evi dence that provenance was, at least in part, from a mid-Proterozoic ba sement terrain to the south. The extension-dominated depositional sett ing of the Moine Supergroup, and basin geometry, are similar to those inferred for the other major Riphean elastic sequences of Scotland, th e 'Torridonian' of the foreland and the Grampian Group to the southeas t. The latter is followed by Upper Riphean transgressive strata of the Appin Group. Scottish Riphean stratigraphy records a major pre-Iapeta n cycle of lithospheric extension and thermal recovery, with no region al scale unconformities yet identified that might constitute evidence for contemporary orogenesis.