Wester Ross Supergroup
| Wester Ross Supergroup | |
|---|---|
| Stratigraphic range: 1000–960 Ma | |
 Torridon Group sandstones near Diabaig | |
| Type | Geological supergroup |
| Sub-units | Sleat Group, Torridon Group, Morar Group, Tarskavaig Group |
| Underlies | Ardvreck Group or Loch Ness Supergroup (tectonic contact) |
| Overlies | Lewisian complex or Stoer Group |
| Area | Northwestern Scotland |
| Thickness | 6–9 km |
| Lithology | |
| Primary | Sandstone (psammite where metamorphosed) |
| Other | Conglomerate, mudstone (pelite where metamorphosed) |
| Type section | |
| Named for | Wester Ross |
The Wester Ross Supergroup is one of the subdivisions of the Neoproterozoic sequence of sedimentary rocks (or their metamorphic equivalents) in the Scottish Highlands. It lies unconformably on medieum to high-grade metamorphic rocks and associated igneous rocks of the Archaean and Paleoproterozoic age Lewisian complex or locally over the Mesoproterozoic sedimentary rocks of the Stoer Group.[1] The contact between the Wester Ross Supergroup and the next youngest of the Neoproterozoic sequences in the Scottish Highlands, the Loch Ness Supergroup, is everywhere a tectonic one.[2]
Stratigraphy[edit]
The Wester Ross Supergroup consists of several groups that are geographically or structurally isolated from each other. The certainty of the correlation between the groups is variable, with the Torridon, Sleat and Morar groups considered as very likely to be lateral equivalents of each other, while the Iona and Tarskavaig groups and those on Shetland are likely but not proven.[2]
Torridon Group[edit]
The Torridon Group is the main part of what used to be termed the "Torridonian", although that also included the Stoer Group, which is now known to be a completely separate and older sequence.[2]
Sleat Group[edit]
The Sleat Group, which outcrops on the Sleat peninsula on Skye, underlies the Torridon Group conformably, but the relationship with the Stoer Group is nowhere exposed. It is metamorphosed to greenschist facies and sits within the Kishorn Nappe, part of the Caledonian thrust belt, making its exact relationship to the other outcrops difficult to assess.[3] The sequence consists of mainly coarse feldspathic sandstones deposited in a fluvial environment with some less common grey shales, probably deposited in a lacustrine environment.
Morar Group[edit]
This sequence of variably metamorphosed and deformed sandstones (psammites) was originally considered as separate from the "Torridonian" and formed the lower section of the Moine Supergroup. Comparisons between the Torridon Group with areas of lower strain in the Morar Group have found many similarities in lithology, thickness and interpreted depositional environment. This group is now thought to have been deposited as part of the same depositional basin, a foreland basin developed in front of the mountain belt formed by the Grenville Orogeny.[2]
Iona Group[edit]
The Iona Group is exposed on the island of Iona and consists of two units separated by a zone of mylonite. The group has been assigned to the Wester Ross Supergroup based on its structural position beneath the Moine Thrust Zone and the ages of detrital zircons, which are similar to those found in the Sleat Group.[2]
Tarskavaig Group[edit]
Rocks of the Tarskavaig Group lie above the Tarskavaig Thrust on the southwestern end of the Sleat peninsula. The group consists of a mixture of psammites, semi-pelites and pelites, which are deformed and no sedimentological analysis has been carried out, nor have the group been sampled for detrital zircons. From its structural position it has been tentatively correlated with the lower part of the supergroup.[2]
Shetland[edit]
The Westing, Sand Voe and Yell Sound groups on Shetland are tentatively correlated with other groups within the Wester Ross Supergroup based on lithological similarities, the ages of the youngest detrital zircons and the timing of later metamorphism.[2]
References[edit]
- ^ British Geological Survey. "Wester Ross Supergroup". BGS Lexicon of named rock units. Retrieved 31 May 2024.
- ^ a b c d e f g Krabbendam, M.; Strachan, R.; Prave, T. (2022). "A new stratigraphic framework for the early Neoproterozoic successions of Scotland". Journal of the Geological Society. 179. doi:10.1144/jgs2021-05.
- ^ Park, R.G.; Stewart, A.D.; Wright, D.T. (2003). "3. The Hebridean terrane". In Trewin N.H. (ed.). The Geology of Scotland. London: Geological Society. pp. 45–61. ISBN 978-1-86239-126-0. Retrieved June 23, 2010.