The Wester Ross Supergroup is one of the subdivisions of the

Neoproterozoic The Neoproterozoic Era is the last of the three geologic eras of the Proterozoic geologic eon, eon, spanning from 1 billion to 538.8 million years ago, and is the last era of the Precambrian "supereon". It is preceded by the Mesoproterozoic era an ...

sequence of

sedimentary rock Sedimentary rocks are types of rock (geology), rock formed by the cementation (geology), cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or de ...

s (or their metamorphic equivalents) in the

Scottish Highlands The Highlands (; , ) is a historical region of Scotland. Culturally, the Highlands and the Scottish Lowlands, Lowlands diverged from the Late Middle Ages into the modern period, when Scots language, Lowland Scots language replaced Scottish Gae ...

. It lies

unconformably An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval o ...

on medium to high-grade

metamorphic rock Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock ( protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, caus ...

s and associated

igneous rock Igneous rock ( ), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava. The magma can be derived from partial ...

s of the Archaean and

Paleoproterozoic The Paleoproterozoic Era (also spelled Palaeoproterozoic) is the first of the three sub-divisions ( eras) of the Proterozoic eon, and also the longest era of the Earth's geological history, spanning from (2.5–1.6 Ga). It is further sub ...

age

Lewisian complex The Lewisian complex or Lewisian gneiss is a suite of Precambrian metamorphic rocks that outcrop in the northwestern part of Scotland, forming part of the Hebridean terrane and the North Atlantic craton, North Atlantic Craton. These rocks are of ...

or locally over the

Mesoproterozoic The Mesoproterozoic Era is a geologic era that occurred from . The Mesoproterozoic was the first era of Earth's history for which a fairly definitive geological record survives. Continents existed during the preceding era (the Paleoproterozoic ...

sedimentary rocks of the

Stoer Group The Stoer Group is a sequence of Mesoproterozoic sedimentary rocks that outcrops on the peninsula of Stoer, near Assynt, Sutherland. The dominant lithology is sandstone with breccias and conglomerate (geology), conglomerates developed near the bas ...

. 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.

Stratigraphy

The Wester Ross Supergroup consists of several

group A group is a number of persons or things that are located, gathered, or classed together. Groups of people * Cultural group, a group whose members share the same cultural identity * Ethnic group, a group whose members share the same ethnic iden ...

s 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.

Torridon Group

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.

Sleat Group

The Sleat Group, which outcrops on the

Sleat Sleat ( ) is a peninsula and civil parish on the island of Skye in the Highland council area of Scotland, known as "the garden of Skye". It is the home of the clan '' MacDonald of Sleat''. The name comes from the Scottish Gaelic , which in tur ...

peninsula on

Skye The Isle of Skye, or simply Skye, is the largest and northernmost of the major islands in the Inner Hebrides of Scotland. The island's peninsulas radiate from a mountainous hub dominated by the Cuillin, the rocky slopes of which provide some o ...

, underlies the Torridon Group conformably, but the relationship with the Stoer Group is nowhere exposed. It is metamorphosed to

greenschist facies Greenschists are metamorphic rocks that formed under the lowest temperatures and pressures usually produced by regional metamorphism, typically and 2–10 kilobars (). Greenschists commonly have an abundance of green minerals such as chlorite, ...

and sits within the Kishorn Nappe, part of the Caledonian thrust belt, making its exact relationship to the other outcrops difficult to assess. The sequence consists of mainly coarse

feldspathic Feldspar ( ; sometimes spelled felspar) is a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagiocla ...

sandstones deposited in a fluvial environment with some less common grey

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...

s, probably deposited in a lacustrine environment.

Morar Group

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 The Moinian or just the Moine, formerly the Moine Supergroup, is a sequence of Neoproterozoic metasediments that outcrop in the Northwest Highlands of Scotland between the Moine Thrust Belt to the northwest and the Great Glen Fault to the s ...

. 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 In geology, depositional environment or sedimentary environment describes the combination of physical, chemical, and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will b ...

. This group is now thought to have been deposited as part of the same depositional basin, a

foreland basin A foreland basin is a structural basin that develops adjacent and parallel to a mountain belt. Foreland basins form because the immense mass created by crustal thickening associated with the evolution of a mountain belt causes the lithospher ...

developed in front of the mountain belt formed by the Grenville Orogeny.

Iona Group

The Iona Group is exposed on the island of

Iona Iona (; , sometimes simply ''Ì'') is an island in the Inner Hebrides, off the Ross of Mull on the western coast of Scotland. It is mainly known for Iona Abbey, though there are other buildings on the island. Iona Abbey was a centre of Gaeli ...

and consists of two units separated by a zone of

mylonite Mylonite is a fine-grained, compact metamorphic rock produced by dynamic recrystallization of the constituent minerals resulting in a reduction of the grain size of the rock. Mylonites can have many different mineralogical compositions; it is a ...

. 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.

Tarskavaig Group

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

pelite A pelite () or metapelite is a metamorphism, metamorphosed fine-grained sedimentary rock, i.e. mudstone or siltstone. The term was earlier used by geologists to describe a clay-rich, fine-grained clastic sediment or sedimentary rock, i.e. mud or ...

s, 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.

Shetland

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.

Age

The age of this sequence is constrained by a combination of detrital zircon geochronology and by the dating of later metamorphic events. The youngest detrital zircon and

rutile Rutile is an oxide mineral composed of titanium dioxide (TiO2), the most common natural form of TiO2. Rarer polymorphs of TiO2 are known, including anatase, akaogiite, and brookite. Rutile has one of the highest refractive indices at vis ...

ages are in the range 1070–1000 Ma, providing an upper bound. Metamorphic ages of 950–940 Ma have been calculated for garnets from the lower part of the Morar Group, confirming that these rocks were affected by the

Renlandian Orogeny The Renlandian Orogeny is a Tonian (early Neoproterozoic) tectonic and metamorphic event that is found in East Greenland, on Svalbard, on Ellesmere Island and in Scotland. It takes its name from Renland in East Greenland, where the event was first ...

(960–920 Ma) and providing a lower bound. Together these data give a depositional age range of 1000–960 Ma.

Depositional setting

The Sleat and Torridon groups have been interpreted as deposited in a

rift In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-graben ...

setting. However the scale of the interpreted river systems that deposited the Torridon Group rocks and the consistency in the paleocurrent directions suggest that most of the sequence was deposited in a single basin. This is also true for the Morar Group, which has very similar thickness, sedimentary facies and interpreted paleocurrents to the Torridon Group. Combined with the detrital zircons that indicate a dominantly Grenvillian source, this has led to a reinterpretation that the supergroup was deposited within a foreland basin, in front of the mountains formed by the Grenville Orogeny.

References

{{Reflist Geology of Scotland Geologic supergroups of the United Kingdom Tonian