The Dalradian rocks of the northern Grampian Highlands of Scotland

Abstract

The northern Grampian Highlands are dominated by the outcrop of the Grampian Group, together with infolds and structural outliers of Appin Group strata and inliers of pre-Dalradian ‘basement’, consisting of Badenoch Group metasedimentary rocks. The south-eastern limit of this mountainous region corresponds with the regionally continuous Grampian Group-Appin Group boundary, which in the south is marked by a high-strain zone corresponding to the Boundary Slide of some authors. The more arbitrary southern boundary runs north-west from Blair Atholl along the A9 road and then westwards to Fort William.The Neoproterozoic-age Grampian Group siliciclastic succession accumulated during several transgressive and regressive cycles in multiphase ensialic rift basins. The Badenoch Group constitutes the crystalline floor to those basins and had experienced amphibolite-facies metamorphism, migmatization, gneissification and deformation between c. 840 and 800Ma, prior to deposition of the Dalradian strata. In contrast, evidence for only 470–450Ma Caledonian orogenic events is found at higher structural levels in the Grampian and Appin group successions. Locating and understanding the nature of the contact between the basement gneisses and the Dalradian cover sequence has long been a major challenge of Highland geology. Recent research has argued that not only is a rift-basin architecture evident from the patterns of Neoproterozoic stratigraphy, but also that it played a significant role in influencing the geometry of the superimposed Caledonian deformation, with the basin infill buttressed against its margins or intrabasinal ‘highs’.The GCR sites in this region preserve important evidence of cover-basement relationships, patterns of punctuated deposition, and onlapping sequences. The effects of both pre-Caledonian and Caledonian deformation and metamorphic events are also well represented. Despite the deformation and metamorphism, spectacular sedimentary structures are visible at several of the GCR sites and there is evidence of the earliest recorded glacigenic sediments in the Neoproterozoic rocks of the British Isles.