Abstract
The circum-North Atlantic region archives three major late-Mesoproterozoic to Neoproterozoic tectonic episodes, the Grenville–Sveconorwegian and Renlandian orogenies followed by rifting and formation of the Iapetus Ocean, and each is bracketed by sedimentary successions that define three megasequences. In this context, we summarize sedimentological and geochronological data and propose a new stratigraphic framework for the iconic Torridonian–Moine successions and related units in Scotland. The Iona, Sleat, Torridon and Morar groups of the Scottish mainland and Inner Hebrides, and the Westing, Sand Voe and Yell Sound groups in Shetland, form the newly named Wester Ross Supergroup. They were deposited c. 1000–950 Ma within a foreland basin to the Grenville Orogen and, collectively, are in Megasequence 1. Some of these units record Renlandian orogenesis at c. 960–920 Ma. The newly named Loch Ness Supergroup consists of the Glenfinnan, Loch Eil and Badenoch groups of the Scottish mainland, deposited c. 900–870 Ma and are assigned to Megasequence 2. These units record Knoydartian orogenesis c. 820–725 Ma. The regionally extensive Dalradian Supergroup belongs to Megasequence 3; it was deposited c. <725–500 Ma and records the opening of the Iapetus Ocean, ultimately leading to deposition of the passive margin Cambrian–Ordovician Ardvreck and Durness groups.
Highlights
The North Atlantic region contains a number of Neoproterozoicsedimentary successions, each several to many kilometres thick, that record the amalgamation and erosional denudation of the Rodinia supercontinent and its subsequent break-up (e.g. Cawood et al 2007, 2010; Kirkland et al 2007, 2008a, b; Krabbendam et al 2017)
The interval leading up to that boundary is marked a c. 200–300 m wide zone of variably but commonly highly strained platy quartzite, psammite and pelite, culminating in a zone of extremely platy mylonites with paper-thin ‘tramline’ lithons, with the Scoor Pelitic Gneiss juxtaposed on top. In view of this evidence for a ductile shear zone, we suggest that the Morar and Glenfinnan groups are everywhere structurally separate, that no stratigraphic continuity exists across the contact and that previous ideas about basin settings and tectonic evolution relying on stratigraphic continuity within the Moine Supergroup are incorrect
The combined suite of sedimentological and geochronological data indicate that the Torridon and Morar groups are correlative with Megasequence 1 whilst the Glenfinnan and Loch Eil groups, together with Badenoch Group in the Grampian Highlands, are correlative with Megasequence 2
Summary
The Neoproterozoic rocks of the Scottish Highlands have been traditionally consigned to one of three iconic successions: the Torridonian, Moine and Dalradian supergroups (Gibbons and Harris 1994; Trewin 2002; Mendum et al 2009). In the Caledonian foreland (footwall of the Moine Thrust), the basement comprises mostly Archean meta-igneous rocks of the Lewisian Gneiss Complex East of the Moine Thrust (Fig. 3), basement rocks occur as inliers in the cores of major folds and along ductile thrust faults (Ramsay 1957; Tanner 1970; Rathbone and Harris 1979; Strachan and Holdsworth 1988) and those have yielded 2900–2700 Ma and 1800–1710 Ma protolith ages (Friend et al 2008; Strachan et al 2020b). Et al 2020b) and c. 1050 Ma reworking of Archean basement in Shetland (Walker et al 2021)
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