Synopsis

New British Geological Survey mapping has examined the stratigraphy and structure of Dalradian strata in the Gaick region of the Central Grampian Highlands of Scotland. In the north of that area, turbiditic strata in the Creag Dhubh Psammite Formation (Corrieyairack Subgroup) pass, via a well-defined sedimentary transition, into the stratigraphically younger Gaick Psammite Formation (Glen Spean Subgroup). This latter formation dominates the lithostratigraphy of the Gaick region, records shallow-water marine shelf conditions throughout, and was probably 1 to 2 km thick prior to deformation. Ordovician (Grampian) orogenesis affected the sedimentary rocks now arranged in a stack of Caledonian recumbent kilometre-scale F2 folds with gently ENE-plunging axes. Regional facing on these recumbent folds is typically sideways to the south. Southeast of the Gaick region, the folds, and thus facing, become progressively inclined and dip to the SE beneath the outcrop of the Appin Group Dalradian. No significant Fl folds, and hence no related facing changes, have been detected within this D2 fold stack. The structure of the Gaick–Drumochter area is therefore essentially a flat belt formed in the D2 deformation event and here named the Gaick Fold Complex. The D2 recumbent structures of this flat belt were rotated and steepened by D3 deformation into the Tummel Steep Belt to the SE, and adjacent to the Glen Banchor High to the NW. Tectonic transport in D2 in the Gaick Fold Complex is interpreted to be oriented on a north–south azimuth, similar to that in the Tay Nappe farther south. Such an interpretation implies that the NE–SW ‘Caledonian trend’ is a consequence of D3 deformation and reorientation, rather than a primary feature of the Grampian Orogeny.

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