OR/15/026 Foreword

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Gunn, A G, Mendum, J R and Thomas, C W. 2015. Geology of the Huntly and Turriff Districts. Sheet description for the 1:50 000 geological sheets 86W (Huntly) and 86E (Turriff) (Scotland). British Geological Survey Internal Report, OR/15/026.

These articles summarises the results of detailed geological remapping and related studies undertaken by the Geological Survey in the Huntly and Turriff districts of North-east Scotland. It provides an account of the geology to accompany the published 1:50 000 geological maps sheets 86W (Huntly) and 86E (Turriff). The districts occupy a tract of generally rolling country, which ranges from about 50 to 200 m above sea level in its eastern part, to more elevated hilly country, commonly reaching over 300 m and 400 m above OD, in its western and southern parts. The drainage is dominated by the sinuous incised valley of the River Deveron and its main tributaries, the rivers Isla and Bogie, except for the southern part of the Turriff district, which is drained by the headwaters of the River Ythan. Agriculture and forestry are the dominant economic activities, with tourism and whisky distilling also significant.

The Huntly and Turriff districts are underlain mainly by Dalradian metasedimentary rocks of Neoproterozoic age that have been strongly deformed and metamorphosed during the Grampian event of the Caledonian Orogeny. The Dalradian rocks have been intruded by igneous intrusions that range from large plutons to small pods and dykes. Granite sheets and pods were emplaced at about 600 million years (Ma), followed later by small ultramafic and mafic bodies. However, the main intrusive igneous event occurred in the Early to Mid Ordovician at 474 to 470 Ma, coeval with the Grampian event. It resulted in the emplacement of mafic and ultramafic plutons, commonly zoned; the Insch Pluton includes monzonites and syenites in its upper zone. Dioritic and granitic bodies are associated with the plutons.

The major structural feature is the north-north-east-trending Portsoy Shear Zone, which traverses the Huntly district. This steeply easterly dipping zone is coincident with a major regional lineament, which separates stratigraphically discrete packages of Dalradian rocks with different tectonometamorphic histories. The shear zone has also facilitated and in part controlled the emplacement of the Huntly and Knock mafic-ultramafic plutons. The Dalradian metasedimentary rocks contain both Buchan and Barrovian metamorphic zonal assemblages. Folding and shearing have caused local repetition of lithological units, but overall the Dalradian sequence becomes younger eastwards until the broad hinge of the regional Turriff Syncline is reached in the central part of that district. Here, the youngest Dalradian rocks are exposed and metamorphic grade is low (biotite grade). Farther east older Dalradian rocks again crop out and the Buchan metamorphic isograds are re-crossed such that the rocks contain andalusite and sillimanite.

Following the Grampian orogenic event the area was uplifted, and during the Early Devonian fluvial and lacustrine Old Red Sandstone rocks were deposited in the northerly trending fault-bounded basins, namely the Turriff and Rhynie basins. In Strath Bogie a basaltic andesite lava unit linked to the Rhynie chert occurs within the Rhynie Basin succession. Subsequently, a Middle Devonian conglomerate-dominated sequence, linked to the Orcadian Lake farther north, was deposited unconformably on the older succession in the Turriff Basin.

In the Turriff district deep Tertiary weathering profiles and local fluvial sands and gravels are preserved, testifying to only limited erosion by the later Quaternary glaciations. The Devensian glacial and postglacial history of the districts was dominated by an ice sheet derived from the highland areas farther to the south-west. The related glacial and postglacial superficial deposits form a pervasive if generally thin cover over much of the bedrock. Till derived from the Moray Firth is present in the north-eastern part of the Turriff district. Eastward migration of glacial meltwater gave rise to channels that in places have significantly influenced development of the Holocene fluvial drainage pattern.

The Huntly district has been the focus of several periods of exploration since 1970 for platinum group elements, copper and nickel linked mainly to the mafic-ultramafic plutons and their metamorphic aureoles. The limited drilling and more extensive ground magnetic surveys, in combination with geochemical studies and gravity modelling, have provided detailed information as to the nature, distribution and origin of prospective areas. No economic deposits have been delineated to date. However, this BGS report and the geological maps provide a sound foundation for existing and any future commercial and/or conservation-related developments.