Introduction

Abstract

Torridonian is an informal stratigraphic name for the Proterozoic reddish-brown sandstones overlying the Lewisian gneiss complex of the NW Scottish mainland. These sandstones form one of the principal elements of British stratigraphy, comparable in volume (over 150000 km 3 ) to the Lower Old Red Sandstone of eastern Scotland, or the Triassic of England. They form the majestic mountains of NW Scotland, but also extend westwards under the Minch basin (Fig. 1). The subcrop has been identified 20 km north of Cape Wrath on the MOIST seismic reflection profile (Blundell et al . 1985), and beneath Devonian strata in the west Orkney basin (Cheadle et al . 1987). It extends south for 330 km to the latitude of Iona (Binns et al . 1974; Evans et al . 1982). The Torridonian was deposited on the edge of the Laurentian shield, near the roughly contemporaneous Grenville orogenic belt. It lies just outside the Caledonian orogen and has consequently escaped appreciable deformation, except in the Moine Thrust zone. Dips are generally low and the thermal history reflects little more than burial, giving ample scope for studies of the sedimentology, geochemistry, palaeoclimate and palaeomagnetism. Combined investigations of the sedimentology and chemistry of the rocks by several workers over the last ten years, using a total of nearly 600 whole rock analyses, have been particularly fruitful despite the relative neglect of the petrography. The most surprising lacuna in Torridonian studies is the paucity of published work on the micropalaeontology. The main objects of this memoir are to provide a comprehensive