No abstract

The two major “Torridonian” successions, viz . the Stoer Group (~968 Ma) and the Sleat–Torridon Groups (~777 Ma) occupy NNE-trending rifts cutting Archaean and early Proterozoic crust. Palaeocurrents, pebble petrography and age, and the initial 87 Sr/ 86 Sr ratios of clay-rich sediments indicate derivation from flanking areas of this old crust, rather than from late Proterozoic Grenville or Morarian metamorphic rocks. The earlier rift seems to be roughly contemporaneous with Grenville metamorphism but genetically unrelated to it. The rifts perhaps mark initiation of an Iapetus-related ocean in the same way as very similar fault-bounded Triassic and Lower Jurassic sediments mark the start of continental break-up and initiation of the present Atlantic. The western margin of both rifts probably coincides with the Outer Hebrides Thrust and the eastern margin with the Moine Thrust Zone. Reactivation of the old rift-margining faults by Caledonian compression may have generated the lower thrusts of the Moine Thrust Zone, while the Moine nappe itself, by analogy with Lower Devonian events in southern Appalachia, represents a slice of the Grenville microcontinent emplaced after closure of the intervening ocean.

Red sedimentary rocks of the Stoer Group (991 My BP) were resampled through their stratigraphic range (600 m at Stoer Bay). The mean direction of magnetization after thermal cleaning at 500°C is 313,+29 (ag5 = So) with a corresponding palaeomagnetic pole at 35" N, 234" E. This result is not significantly different from that obtained previously from studies of natural remanent magnetization. A small collection from the base of the overlying Torridon Group (796 My BP) confirms the general reliability of the earlier work on these rocks also, and shows that the large change between the Stoer and Torridon Groups coincides with the unconformity between them. The magnetization of these sediments originated at the time they were laid down. Since NW Scotland used to be part of Laurentia, the Stoer and Torridon Group poles can be compared with those from the late Precambrian of the Canadian Shield. In order to quantify these comparisons, a system of grading poles on a seven-point scale is introduced, and a generalized polar path for the Canadian Shield drawn. The poles from NW Scotland do not fit onto this path, indicating either that there has been slight rotation of NW Scotland as a marginal block of Laurentia, or that a hitherto unsuspected polar excursion occurred immediately following the Grenvillian orogeny.

Synopsis An upward-facing succession 2 km thick and comprising ten newly-defined formations is described. The lower half is built of well-sorted, cross-bedded arkoses with interleaved slates and phyllites (deltaic facies). The upper half is built of turbidite greywackes, becoming more distal upwards, with interleaved mudstones and phyllites. The succession is continued upwards in Oronsay and Colonsay where the turbidites revert abruptly to deltaic facies. The sediments are tentatively correlated with the Dalradian. Three deformation phases are recognized, the second accounting for the major folds plunging gently north-east.