Geology of the Huntly and Turriff districts : sheet description for the 1:50 000 geological sheets 86W (Huntly) and 86E (Turriff) (Scotland)

This report summarises the results of detailed geological remapping and related studies under-taken by the Geological Survey in the Huntly and Turriff districts of North-east Scotland. It pro-vides 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 tributar-ies, 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 ac-tivities, 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, com-monly 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 linea-ment, which separates stratigraphically discrete packages of Dalradian rocks with different tec-tonometamorphic histories. The shear zone has also facilitated and in part controlled the emplacement of the Huntly and Knock mafic-ultramafic plutons. The Dalradian metasedimen-tary rocks contain both Buchan and Barrovian metamorphic zonal assemblages. Folding and shearing have caused local repetition of lithological units, but overall the Dalradian sequence be-comes 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 Mid-dle 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 meta-morphic aureoles. The limited drilling and more extensive ground magnetic surveys, in combina-tion 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 deline-ated 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

Bedrock geology of the Ben Alder Massif : report of the 2005 field season

This report presents a preliminary interpretation of the bedrock geology of Ben Alder, based on data collected during the 2005 field season. This work contributes to the resurvey of 1:50,000 scale Sheet 54E (Loch Rannoch). The principal aim of this report is to present a lithostratigraphical framework and structural model for testing during the 2006 field season. The Ben Alder massif exposes polydeformed psammite and semipelite belonging to the Neoproterozoic Grampian Group, lower Dalradian Supergroup. It comprises essentially four formations locally termed, the Lethcois Semipelite (lowest), Ben Alder Psammite, Garbh Choire Semipelite and Gaick Psammite Formation. The lithological and stratigraphical character of these formations is compared and contrasted with that of the type-stratigraphies for the Strathtummel and Corrieyairack Basins. The Ben Alder Grampian Group has an unequivocal affinity with the Strathtummel Basin-fill. In the Strathtummel Basin (east of the Geal-charn – Ossian Steep Belt), the resurvey of Sheet 63E (Dalwhinnie) utilised the Corrieyairack Basin’s type-stratigraphy when naming new lithostratigraphical units. Some of these correlations are questioned and new correlations with the Strathtummel Basin stratigraphy are suggested. The stratigraphical pile was progressively deformed by up to four phases of deformation (D1- D4). Early phase (D1) deformation is only represented by fabric development (S1). By contrast, the D2 main phase deformation folded the stratigraphy into a number of tight, almost isoclinal, SE-facing F2-fold structures (e.g. the Ben Alder Anticline) with associated penetrative fabric development (S2). Subsequently, these F2-structures were refolded by the co-axial and co-planar F3-folds, which have a more open nature. Late weak and upright folding (F4) served to ‘kink’ the stratigraphical units without altering their distribution, creating alternating steep and more gently dipping zones. Displacement across major brittle faults (Ericht-Laidon, Inverpattack-Markie Faults) is relatively limited, enabling confidence in stratigraphical and structural correlations made between faultbound blocks. These faults acted as conduits for late- and post-tectonic microdiorite, lamprophyre and pegmatite intrusions