The Sgurr Beag nappe: A reassessment of the stratigraphy and structure of the Northern Highland Moine

Abstract

The rocks of the Northern Highland Moine have suffered two phases of intense, mid-crustal orogenesis. Elucidation of original stratigraphic relationships within these rocks requires the application of techniques that can be applied to other terranes of this type, in which the regional lithostratigraphy has, on first inspection, been obscured by a complex deformation history. Mapping along the boundary between the Glenfinnan and Loch Eil divisions of the Moine has shown that the rocks of both divisions have a common structural, metamorphic, and igneous history. The Glenfinnan division passes upward by stratigraphic passage into the Loch Eil division. The usefulness of assigning rocks rigorously to one division or the other is therefore called into question. It is suggested that the Moine rocks in the Northern Highlands which lie above the Sgurr Beag thrust, that is, those previously assigned to the Glenfinnan or Loch Eil divisions, should be referred to a single tectonostratigraphic unit, the Sgurr Beag nappe. The Glenfinnan and Loch Eil divisions have the status of lithostratigraphic groups within the nappe. It is further possible to subdivide the stratigraphy of the nappe into a number of mappable formations, some of which are laterally continuous, and some of which die out laterally. The base of the Loch Eil Group may be diachronous. Stratigraphic considerations suggest that the Sgurr Beag thrust cuts down-section northward, exposing deeper levels of the nappe from Loch Quoich northward than those present above the slide farther south. The Morar division is also thought to have the status of a lithostratigraphic group, but its original relationship to the Glenfinnan and Loch Eil groups is normally obscured by the Sgurr Beag thrust.