A new stratigraphic framework for the early Neoproterozoic successions of Scotland

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 summarise sedimentological and geochronological data and propose a new stratigraphic framework for the iconic Torridonian-Moine-Dalradian 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.

Tectonic Transport Directions, Shear Senses and Deformation Temperatures Indicated by Quartz c-Axis Fabrics and Microstructures in a NW-SE Transect across the Moine and Sgurr Beag Thrust Sheets, Caledonian Orogen of Northern Scotland

“…However, rocks assigned to the Morar Group in Sutherland were metamorphosed at c. 950-940 Ma [37], whereas the Glenfinnan and Loch Eil groups were deposited after c. 910 Ma [38]. Recently, it has been proposed that the Morar Group, together with the Torridon and Sleat groups below the Moine thrust, belongs to an older Wester Ross Supergroup, and that the Glenfinnan and Loch Eil groups belong to a younger Loch Ness Supergroup [39].…”

“…Note much smaller opening angle for sample MT-9 (yellow filled circle) from deformed quartzite with c-axis fabric (Figure 10) indicating constrictional strain. 40 Ar/ 39 Ar 'ages' derived from muscovite are generally accepted as providing information on cooling through c. 400 • C [97][98][99] and thus may provide additional thermochronological information on shearing in the area. In order to investigate the suitability of our suite of samples in Area 1 for future 40 Ar/ 39 Ar geochronology, a muscovite separate of sample MT-10, collected at 1220 m above the Moine thrust (Figures 3, 4a and 14a), was prepared for 40 Ar/ 39 Ar geochronology following the method of Spencer et al, [100].…”

“…40 Ar/ 39 Ar 'ages' derived from muscovite are generally accepted as providing information on cooling through c. 400 • C [97][98][99] and thus may provide additional thermochronological information on shearing in the area. In order to investigate the suitability of our suite of samples in Area 1 for future 40 Ar/ 39 Ar geochronology, a muscovite separate of sample MT-10, collected at 1220 m above the Moine thrust (Figures 3, 4a and 14a), was prepared for 40 Ar/ 39 Ar geochronology following the method of Spencer et al, [100]. A full description of analytical methods [101][102][103][104] is given in Supplementary Files S3 and S4.…”

“…Two, two-to three-grain aliquots were processed and analyzed. Two aliquots of sample MT-10 yielded plateau dates of 431.6 ± 2.6 Ma (aliquot 1; MSWD = 0.033, includes 87.1% of released 39 Ar) and 432.2 ± 1.8 Ma (aliquot 2; MSWD = 0.14, includes 99.7% of released 39 Ar), which are statistically indistinguishable from one another and very similar to the white mica Rb-Sr dates of Freeman et al, [51] reported in the underlying basal Moine mylonites.…”

“…If the feldspar clasts in the mylonites had formed by mechanical fracturing and breakdown of these higher tempera-Figure 15. 40 Ar/ 39 Ar apparent age spectra and Ca/K plot for muscovite in sample MT-10 collected at 1220 m above the Moine thrust in Area 1. Plateau date of 432.2 ± 1.8 Ma is reported for thirteen incremental heating steps (steps 1-13/14) represented by the gray boxes, which comprise 99.7% of radiogenic argon released during the experiment.…”