Precambrian

Abstract

AbstractFor much of Precambrian time, it is not possible to reconstruct any sort of meaningful palaeogeographies. For this Atlas, therefore, the earliest reconstructions are for Proterozoic intervals and are, of necessity, limited geographically.The term 'Torridonian' has long since been used to refer to the entire Upper Proterozoic succession of predominantly fluvial clastic sediments situated along the northwest coast of Scotland. These rocks rest unconformably on Archaean to Lower Proterozoic Lewisian Gneiss, are overlain unconformably by Cambro-Ordovician marine sediments, and constitute the best sedimentary exposures in the British Isles. Stratigraphically, 'Torridonian' deposits are subdivided into the Stoer, Sleat and Torridon groups. The Stoer and Torridon groups are separated by an angular unconformity (Lawson 1965; Stewart 1969) and have been dated at 968 and 777 Ma respectively (Moorbath 1969; Stewart 1982), although these ages may be up to 100 Ma too young (see discussion in Stewart 1988a, p. 98). Sedimentation dates of c. 1050 (Stoer Group) and c. 850 (Torridon Group) are, therefore, likely to be more realistic.The lowest of the three groups consists of a diverse suite of red bed deposits up to 2 km thick representing fluvial, aeolian and ephemeral lacustrine environments, suggesting a semi-arid climate (Stewart 1988a). Deposition occurred at an approximate palaeolatitude of 15°N (Torsvik & Sturt 1987). Stoer Group sediments have been divided by Stewart (1988a) into seven constituent facies. The illustrated palaeogeographical time interval encompasses the end of Bay of Stoer facies deposition, theThe Bay .of Stoer facies consists of roughly 200 m