The Stoer Group
The Group consists of alluvial red sandstones, interspersed with lake sediments, having a maximum exposed thickness of 2 km. The present extent of the Stoer Group is shown in Plate 1. It has survived only as a narrow strip next to the Coigach fault, apparently in a hanging wall roll-over (Stewart 1993a). Figure 3 shows the Stoer Group truncated by the Coigach fault, together with its unconformable relations with the Lewisian gneiss complex beneath and the Torridon Group above. The original extent of the group can only be inferred from the sediments. It has not been identified in the subsurface offshore to the west and it is unlikely that it ever existed at the present level of erosion east of the existing outcrop.
The general outlines of the sedimentary history are clear, but problems lurk in the details. For example, the oldest sediments of the group occupy palaeovalleys eroded in the gneiss complex, some of which were filled exclusively by alluvial deposits whereas others hosted swamps and temporary lakes. Another controversial topic is the origin of the volcaniclastic Stac Fada Member, and the amount of volcanic input to Stoer Group sediments generally.
Figures & Tables
The Torridonian sandstones form one of the principal elements of British stratigraphy. They form the majestic mountains of NW Scotland and also extend westwards under the Minch basin. The sediments were deposited in a Proterozoic rift nearly contemporaneous with the Keweenawan Supergroup of North America.
This book contains the first complete field description of the rocks and the sedimentary environments in which they formed, together with a comprehensive examination of their tectonic and palaeoclimatic significance, geochemistry, palaeomagnetism and diagenesis. It includes the result of over forty years’ work by the author, most of it previously unpublished.