The Upper Morar Psammite of the Moine Supergroup, Ardnamurchan Peninsula, Scotland: depositional setting, tectonic implications

Abstract

Synopsis The Upper Morar Psammite of the Moine Supergroup is well developed along the Ardnamurchan Peninsula where it is c. 2.5 km thick and consists of fine to pebbly arkosic psammite to feldspathic quartzite. Five lithofacies (LF) are recognized. LF1 and LF5 typify the lowermost units and consist of amalgamated and multistorey nested sets of decimetre-scale trough cross-bedding filling channel forms. LF2 and LF3 are distinguished by smaller-scale sets and co-sets of trough and planar cross-bedded units and, in LF3, ripple cross-laminated and planar laminated units. Soft-sediment deformation structures (mostly convolute lamination) are present in both LF2 and LF3. LF4 consists of planar laminated very fine-grained psammite and pelite. There is a progressive decrease in the scale of sedimentary structures upsection concomitant with a greater compositional maturity and textural fining. Palaeo-current data are broadly unimodal but change upward from NNE to NNW directed. No 180° flow reversals are observed nor any other features indicating shallow-marine settings. These observations are used to interpret the Upper Morar Psammite as a retrogradational alluvial braidplain and call into question the commonly held view that the Morar rocks were deposited in a shallow-marine rift basin.