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The Moine Supergroup is a sequence of
Neoproterozoic The Neoproterozoic Era is the unit of geologic time from 1 billion to 538.8 million years ago. It is the last era of the Precambrian Supereon and the Proterozoic Eon; it is subdivided into the Tonian, Cryogenian, and Ediacaran periods. It is prec ...
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 that form the dominant
outcrop An outcrop or rocky outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth. Features Outcrops do not cover the majority of the Earth's land surface because in most places the bedrock or superficia ...
of the
Scottish Highlands The Highlands ( sco, the Hielands; gd, a’ Ghàidhealtachd , 'the place of the Gaels') is a historical region of Scotland. Culturally, the Highlands and the Lowlands diverged from the Late Middle Ages into the modern period, when Lowland S ...
between the Moine Thrust Belt to the northwest and the
Great Glen Fault The Great Glen Fault is a strike-slip fault that runs through the Great Glen in Scotland. The fault is mostly inactive today, but occasional moderate tremors have been recorded over the past 150 years. Location Aligned northeast to southwest, t ...
to the southeast. The sequence is
metasediment In geology, metasedimentary rock is a type of metamorphic rock. Such a rock was first formed through the deposition and solidification of sediment Sediment is a naturally occurring material that is broken down by processes of weathering and e ...
ary in nature and was
metamorphosed 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, causi ...
and deformed in a series of
tectonic Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents ...
events during the Late
Proterozoic The Proterozoic () is a geological eon spanning the time interval from 2500 to 538.8million years ago. It is the most recent part of the Precambrian "supereon". It is also the longest eon of the Earth's geologic time scale, and it is subdivided i ...
and Early
Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ...
. It takes its name from ''
A' Mhòine A' Mhòine () is a peninsula in the Highlands, Scotland. The peninsula is bounded to the west by Loch Eriboll, and to the east by the Kyle of Tongue. The A838 road crosses the peninsula on an east–west axis. Much of the peninsula is owned by ...
'', a
peat bog A bog or bogland is a wetland that accumulates peat as a deposit of dead plant materials often mosses, typically sphagnum moss. It is one of the four main types of wetlands. Other names for bogs include mire, mosses, quagmire, and muskeg; a ...
in northern
Sutherland Sutherland ( gd, Cataibh) is a historic county, registration county and lieutenancy area in the Highlands of Scotland. Its county town is Dornoch. Sutherland borders Caithness and Moray Firth to the east, Ross-shire and Cromartyshire (later c ...
.


Distribution

The main outcrop of the Moine series lies northwest of the Great Glen Fault, structurally above the Moine Thrust to the west forming what is known as the Northern Highlands Terrane.Strachan, R.A., Smith, M., Harris, A.L.& Fettes, D.J. 2002. Chapter 4: The Northern Highland and Grampian terranes in: Trewin, N.H (Ed) The Geology of Scotland, Geological Society, London, 550pp.
/ref> A smaller area of similar rocks that are correlated with these, outcrop within the 'Grampian
Terrane In geology, a terrane (; in full, a tectonostratigraphic terrane) is a crust fragment formed on a tectonic plate (or broken off from it) and accreted or " sutured" to crust lying on another plate. The crustal block or fragment preserves its ow ...
' to the southeast of the Great Glen. Moinian rocks are also recognised on Mull,
Orkney Orkney (; sco, Orkney; on, Orkneyjar; nrn, Orknøjar), also known as the Orkney Islands, is an archipelago in the Northern Isles of Scotland, situated off the north coast of the island of Great Britain. Orkney is 10 miles (16 km) north ...
and
Shetland Shetland, also called the Shetland Islands and formerly Zetland, is a subarctic archipelago in Scotland lying between Orkney, the Faroe Islands and Norway. It is the northernmost region of the United Kingdom. The islands lie about to the n ...
.


Stratigraphy

The Moine Supergroup has been subdivided into different
groups 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 ide ...
or divisions. The relationship between individual groups in terms of age and
stratigraphic Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithostra ...
equivalence is generally unclear, as they are unfossiliferous due to their age and metamorphic state. They are mostly described in tectonostratigraphic terms referring to their position relative to major Caledonian thrust structures. The Moine is lithologically monotonous, being dominated by psammites (metamorphosed
sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicat ...
s), making
lithostratigraphic Lithostratigraphy is a sub-discipline of stratigraphy, the geological science associated with the study of strata or rock layers. Major focuses include geochronology, comparative geology, and petrology. In general, strata are primarily igneou ...
correlation extremely difficult.


Morar Group

This group lies tectonically above the
Moine Thrust The Moine Thrust Belt or Moine Thrust Zone is a linear tectonic feature in the Scottish Highlands which runs from Loch Eriboll on the north coast south-west to the Sleat peninsula on the Isle of Skye. The thrust belt consists of a series of ...
but below the Sgurr Beag or Naver thrusts. Large slices of Lewisian-type
gneiss Gneiss ( ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures a ...
es occur within this group, locally a basal
conglomerate Conglomerate or conglomeration may refer to: * Conglomerate (company) * Conglomerate (geology) * Conglomerate (mathematics) In popular culture: * The Conglomerate (American group), a production crew and musical group founded by Busta Rhymes ** ...
is preserved, confirming the unconformable nature of the contact between them.


East Sutherland Moine

This group lies tectonically above the Naver Thrust. The upper boundary of this group is not seen.


Glenfinnan Group

This group lies tectonically above the Sgurr Beag Thrust and below the Loch Eil Group. There is evidence of a true stratigraphic transition between these two groups in some areas. Slices of Lewisian-type gneisses are found above the Sgurr Beag thrust and are interpreted to represent pieces of basement to the group incorporated during the Caledonian orogeny.


Loch Eil Group

This group lies above the Glenfinnan Group. The upper boundary of this group is not seen. Towards the southwestern end of its outcrop the Group consists of widespread psammites within which are identified a lower Kinlocheil Quartzite Formation which is anything up to 1.5 km in thickness, an overlying Glen Gour Quartzite and Pelite Formation of 500-800m thickness and above this, the 100-650m thick Stronchreggan Formation. A Tarvie Psammite Formation is recorded in the Strathconon and
Strathglass Strathglass is a strath or wide and shallow valley in the Northwest Highlands of Scotland down which runs the meandering River Glass from the point at which it starts at the confluence of the River Affric and Abhainn Deabhag to the point where ...
districts.


Dava and Glen Banchor successions

These sequences lie within the Grampian Terrane and are classed as subgroups within a newly established Badenoch Group. They have been tentatively correlated with the Moine, which they resemble lithologically and with which they share a metamorphic history. These successions may have a basement-cover relationship with the Grampian Group, the lowermost part of a younger metamorphic sequence, the Dalradian Supergroup.


Age


Age of sedimentation

The complete lack of fossils is consistent with a Precambrian age for the sequence. This is also shown by the age of the earliest known
intrusion In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...
s that cut the sequence, dated at around 870 Ma. There is also no evidence of the sequence being affected by the 1300-1000 Ma
Grenville orogeny The Grenville orogeny was a long-lived Mesoproterozoic mountain-building event associated with the assembly of the supercontinent Rodinia. Its record is a prominent orogenic belt which spans a significant portion of the North American continent, f ...
. This together suggests an early
Neoproterozoic The Neoproterozoic Era is the unit of geologic time from 1 billion to 538.8 million years ago. It is the last era of the Precambrian Supereon and the Proterozoic Eon; it is subdivided into the Tonian, Cryogenian, and Ediacaran periods. It is prec ...
sedimentation age. A possible correlation with the lithologically similar
Torridon Group In geology, the term Torridonian is the informal name for the Torridonian Group, a series of Mesoproterozoic to Neoproterozoic arenaceous and argillaceous sedimentary rocks, which occur extensively in the Northwest Highlands of Scotland. Th ...
, an unmetamorphosed sequence of similar age found beneath the Moine Thrust, remains unproven.


Age of metamorphism


Knoydartian

Radiometric dating of
mica Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect basal cleavage. Mica is ...
s from deformed
pegmatite A pegmatite is an igneous rock showing a very coarse texture, with large interlocking crystals usually greater in size than and sometimes greater than . Most pegmatites are composed of quartz, feldspar, and mica, having a similar silici ...
s provided the first evidence of a Precambrian metamorphic event with ages of 690-750 Ma. With improved techniques, the timing of this event has been constrained to ca. 820-790 Ma, with later ages relating to cooling.


Caledonian

It was originally thought that the Highland and Grampian terranes had distinct geological histories as it appeared that only the latter had been affected by the Ordovician age Grampian metamorphic event. Although it is now known that Moine was also affected by the Grampian event, the dominant metamorphic event is of late
Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleoz ...
age, associated with the Scandian orogeny.


Structure


Pre-Knoydartian

Despite the degree of both metamorphism and deformation in the Moine sequence, it has been possible to identify variations in the original depositional thickness of the various groups. These thickness variations have been interpreted in terms of
half-graben A half-graben is a geological structure bounded by a fault along one side of its boundaries, unlike a full graben where a depressed block of land is bordered by parallel faults. Rift and fault structure A rift is a region where the lithosphere ...
s within an overall
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-grabe ...
setting.


Knoydartian

The cause of the Knoydartian metamorphic event has yet to be established. Some authors interpret this to be a result of a high heatflow due to
extensional tectonics Extensional tectonics is concerned with the structures formed by, and the tectonic processes associated with, the stretching of a planetary body's crust or lithosphere. Deformation styles The types of structure and the geometries formed depend ...
, while others interpret it to be an orogenic event.


Caledonian

The main tectonic event recorded in the Moine rocks is the Late Silurian Scandian event. The main structures formed are regionally significant thrusts, the Moine Thrust, the Naver Thrust and the Sgurr Beag Thrust. The thrusts that lie structurally above the Moine Thrust Zone have all been folded and deformed in a
ductile Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile stres ...
fashion. The final stages of the Caledonian orogeny involved sinistral strike-slip faulting. The Great Glen Fault is the largest of the structures formed at this time, but other major faults occur within the Moine outcrop including the Strathconon and Strathglass faults.


Igneous rocks


Basic intrusions

The Moine sequence is cut by numerous small metabasic intrusions, particularly in the Glenfinnan and Loch Eil groups. They have a tholeiitic chemistry, consistent with intrusion into a rift setting.


Early granitic intrusions

The Moinian rocks of
Ross-shire Ross-shire (; gd, Siorrachd Rois) is a historic county in the Scottish Highlands. The county borders Sutherland to the north and Inverness-shire to the south, as well as having a complex border with Cromartyshire – a county consisting ...
and Sutherland are intruded by a distinctive set of
granitic A granitoid is a generic term for a diverse category of coarse-grained igneous rocks that consist predominantly of quartz, plagioclase, and alkali feldspar. Granitoids range from plagioclase-rich tonalites to alkali-rich syenites and from quartz- ...
sheets, which give latest Proterozoic intrusion ages in the range 611-588 Ma. It has been suggested that these granites formed associated with rifting.


'Newer' granites

The main suite of granites intruded into the Moine are the Late Silurian to Early Devonian age 'newer granites' that occur throughout the Scottish Caledonides.


References


External links

{{Commons category, Moine Supergroup Stratigraphy of Scotland Geological supergroups Neoproterozoic Europe Geography of Highland (council area)