Morar Group

The Morar Group outcrops over a large part of the Northwest Highlands, between the Moine Thrust Belt to the northwest to the contact with the Glennfinnan Group, marked by the Sgùrr Beag Thrust or Swordly Thrust, or locally Devonian rocks in southeast Sutherland, to the southeast. It reaches the north coast of Sutherland to the north and extends as far south as the Ross of Mull.^[1]

The group has been subdivided in several parts of its outcrop. In the Knoydart and Morar areas, a fourfold subdivision is recognised starting from the oldest: Basal Pelite Formation, Lower Morar Psammite Formation, Morar Schists Formation and Upper Morar Psammite Formation. Further north in Ross-shire, extending into Sutherland, the Basal Pelite and the lower part of the Lower Psammite is interpreted to be missing but the sequence is otherwise similar. The Lower Psammite is divided into the lower Altnaharra Psammite Formation, with the Glen Achall Semipelite Member at the top, and the upper Glascarnoch Psammite Formation. The Vaich Pelite Formation is the equivalent of the Morar Schists, and the Crom Psammite Formation is matched to the Upper Psammite. Above this is the Diebidale Pelite Formation.^[3] On the Ross of Mull, four formations are recognised: the Lower Shiaba Psammite Formation at the base (no older beds exposed), the Shiaba Pelite Formation, the Upper Shiaba Psammite Formation and the Laggan Mor Formation, which becomes increasingly highly deformed as it approaches its contact with the younger Glennfinnan Group.^[1]

In the lower strain areas there are sufficient sedimentary structures preserved to provide constraints on the depositional environment for the Morar Group. The main challenge has been differentiating between shallow marine shelf and fluvial settings for the deposition of the sequence.^[4] For the Ross succession, the Basal Pelite is interpreted to represent deposition in paleovalleys, similar to the Diabaig Formation of the Torridon Group. The Altnaharra Psammite is interpreted to have been deposited as a prograding braid plain, with fluvial deposition being replaced by shoreline deposits that were tidally influenced, before a deeper marine environment developed during the deposition of the Vaich Pelite Formation. This is interpreted to be a single progradation-retrogradation cycle. A second cycle began with a return to shallow water deposition in the lower part of the Crom Psammite Formation, grading upwards to a distal braid plain setting, before returning to a tidal shoreline and eventually to shallow water marine deposition in the Diebidale Pelite Formation.^[3]

A very similar pattern of cycles has been deduced for the Torridon Group with which they have been correlated within the Wester Ross Supergroup. Due to movement on the Moine Thrust, the depositional areas of the Torridon and Morar groups are likely to have been separated by more than 100 km. These similarities are consistent with a model in which both sequences were deposited in a foreland basin, with the Torridon Group being more proximal to the mountains of the Grenville Orogen and the Morar Group more distal. Previous models of a rift setting are not consistent with the scale of the sedimentary systems observed, nor are there the expected rapid vertical and lateral changes in sedimentary facies typical of rift basins.^[3]

The oldest age of the Morar Group is constrained by the results of detrital zircon geochronology while the youngest age is constrained by the dating of the oldest metamorphic/tectonic event that has affected the group since deposition. The youngest detrital zircon and rutile ages found for samples from the Morar Group are 1070–1000 Ma (million years ago). The oldest metamorphic event known from the Morar Group is that associated with the Renlandian Orogeny, which gives ages of 950–940 Ma, using Lu-Hf and Sm–Nd methods. Together these give an age range of 1000–950 Ma for deposition of the group.^[1]