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.