The Stoer Group

Abstract

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 1993 a ). 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. Stratigraphy The regional stratigraphy of the Stoer Group is shown in Figure 4. The stratotypes of the three constituent formations, originally defined at Stoer (Stewart 1991 a ), are described on pp. 57-70. The oldest is the Clachtoll Formation, overlying the Lewisian gneiss complex and identifiable by its clasts, virtually all of which can be traced to local basement lithologies.