Moine Supergroup
Encyclopedia
The Moine Supergroup is a sequence of Neoproterozoic
metamorphic rock
s that form the dominant outcrop of the Scottish Highlands
between the Moine Thrust Belt
to the northwest and the Great Glen Fault
to the southeast. The sequence is metasedimentary in nature and was metamorphosed and deformed in a series of tectonic
events during the Late Proterozoic and Early Paleozoic
. It takes its name from A' Mhòine, a peat bog in northern Sutherland
.
'. A smaller area of similar rocks that are correlated with these, outcrop within the 'Grampian Terrane' to the southeast of the Great Glen. Moinian rocks are also recognised on Mull
, Orkney and Shetland.
terms referring to their position relative to major Caledonian
thrust structures
. The Moine is lithologically monotonous, being dominated by psammite
s (metamorphosed sandstone
s), making lithostratigraphic
correlation extremely difficult.
but below the Sgurr Beag or Naver thrusts. Large slices of Lewisian
-type gneiss
es occur within this group, locally a basal conglomerate
is preserved, confirming the unconformable
nature of the contact between them.
. This together suggests a early Neoproterozoic
sedimentation age. A possible correlation with the lithologically similar Torridon Group
, an unmetamorphosed sequence of similar age found beneath the Moine Thrust, remains unproven.
s from deformed pegmatite
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.
setting.
, while others interpret it to be an orogenic event.
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.
chemistry, consistent with intrusion into a rift setting.
and Sutherland are intruded by a distinctive set of granitic
sheets, which give latest Proterozoic intrusion ages in the range 611-588 Ma. It has been suggested that these granites formed associated with rifting.
Neoproterozoic
The Neoproterozoic Era is the unit of geologic time from 1,000 to 542.0 ± 1.0 million years ago. The terminal Era of the formal Proterozoic Eon , it is further subdivided into the Tonian, Cryogenian, and Ediacaran Periods...
metamorphic rock
Metamorphic rock
Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...
s that form the dominant outcrop of the Scottish Highlands
Scottish Highlands
The Highlands is an historic region of Scotland. The area is sometimes referred to as the "Scottish Highlands". It was culturally distinguishable from the Lowlands from the later Middle Ages into the modern period, when Lowland Scots replaced Scottish Gaelic throughout most of the Lowlands...
between the Moine Thrust Belt
Moine Thrust Belt
The Moine Thrust Belt is a linear geological feature in the Scottish Highlands which runs from Loch Eriboll on the north coast 190 km south-west to the Sleat peninsula on the Isle of Skye...
to the northwest and the Great Glen Fault
Great Glen Fault
The Great Glen Fault is a long strike-slip fault that runs through its namesake the Great Glen in Scotland. However, the fault is actually much longer and over 400 million years old.-Location:...
to the southeast. The sequence is metasedimentary in nature and was metamorphosed and deformed in a series of tectonic
Tectonics
Tectonics is a field of study within geology concerned generally with the structures within the lithosphere of the Earth and particularly with the forces and movements that have operated in a region to create these structures.Tectonics is concerned with the orogenies and tectonic development of...
events during the Late Proterozoic and Early Paleozoic
Paleozoic
The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...
. It takes its name from A' Mhòine, a peat bog in northern Sutherland
Sutherland
Sutherland is a registration county, lieutenancy area and historic administrative county of Scotland. It is now within the Highland local government area. In Gaelic the area is referred to according to its traditional areas: Dùthaich 'IcAoidh , Asainte , and Cataibh...
.
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 Highland terraneTerrane
A terrane in geology is short-hand term for a tectonostratigraphic terrane, which is a fragment of crustal material formed on, or broken off from, one tectonic plate and accreted or "sutured" to crust lying on another plate...
'. A smaller area of similar rocks that are correlated with these, outcrop within the 'Grampian Terrane' to the southeast of the Great Glen. Moinian rocks are also recognised on Mull
Mull
-Places:*Isle of Mull, Scottish island in the Inner Hebrides*Sound of Mull, between the island and the rest of Scotland*Mull , Anglicisation of Gaelic Maol, hill or promontory**Mull of Galloway, Scotland**Mull of Kintyre, Scotland...
, Orkney and Shetland.
Stratigraphy
The Moine Supergroup has been subdivided into different groups or divisions. The relationship between individual groups in terms of age and stratigraphic equivalence is generally unclear, as they are unfossiliferous due to their age and metamorphic state. They are mostly described in tectonostratigraphicTectonostratigraphy
In geology, tectonostratigraphy refers either to rock sequences in which large-scale layering is caused by the stacking of thrust sheets or nappes in areas of thrust tectonics or the effects of tectonics on lithostratigraphy.-Tectonically formed stratigraphy:...
terms referring to their position relative to major Caledonian
Caledonian orogeny
The Caledonian orogeny is a mountain building era recorded in the northern parts of the British Isles, the Scandinavian Mountains, Svalbard, eastern Greenland and parts of north-central Europe. The Caledonian orogeny encompasses events that occurred from the Ordovician to Early Devonian, roughly...
thrust structures
Thrust fault
A thrust fault is a type of fault, or break in the Earth's crust across which there has been relative movement, in which rocks of lower stratigraphic position are pushed up and over higher strata. They are often recognized because they place older rocks above younger...
. The Moine is lithologically monotonous, being dominated by psammite
Psammite
Psammite is a general term for sandstone. It is equivalent to the Latin-derived term arenite. Also, it is commonly used in various publications to describe a metamorphosed sedimentary rock with a dominantly sandstone protolith. In Europe, this term was formerly used for a fine-grained, fissile,...
s (metamorphosed sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
s), making lithostratigraphic
Lithostratigraphy
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...
correlation extremely difficult.
Loch Morar Group
This group lies tectonically above the Moine ThrustMoine Thrust Belt
The Moine Thrust Belt is a linear geological feature in the Scottish Highlands which runs from Loch Eriboll on the north coast 190 km south-west to the Sleat peninsula on the Isle of Skye...
but below the Sgurr Beag or Naver thrusts. Large slices of Lewisian
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. These rocks are of Archaean and Paleoproterozoic age, ranging from 3.0–1.7 Ga. They form the basement on which the...
-type gneiss
Gneiss
Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.-Etymology:...
es occur within this group, locally a basal conglomerate
Conglomerate (geology)
A conglomerate is a rock consisting of individual clasts within a finer-grained matrix that have become cemented together. Conglomerates are sedimentary rocks consisting of rounded fragments and are thus differentiated from breccias, which consist of angular clasts...
is preserved, confirming the unconformable
Unconformity
An unconformity is a buried erosion 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 of time before deposition of the younger, but the term is used to describe...
nature of the contact between them.
East Sutherland Moine
This group lies tectonically above the Naver Thrust. The upper boundary of this group in 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.Dava and Glen Banchor successions
These sequences lie within the Grampian Terrane. 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 of sedimentation
The complete lack of fossils is consistent with a Precambrian age for the sequence. This also shown by the age of the earliest known intrusions 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 orogenyGrenville 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, from Labrador to Mexico, as well as to Scotland...
. This together suggests a early Neoproterozoic
Neoproterozoic
The Neoproterozoic Era is the unit of geologic time from 1,000 to 542.0 ± 1.0 million years ago. The terminal Era of the formal Proterozoic Eon , it is further subdivided into the Tonian, Cryogenian, and Ediacaran Periods...
sedimentation age. A possible correlation with the lithologically similar Torridon Group
Torridonian
In geology, Torridonian describes a series of Neoproterozoic arenaceous sedimentary rocks, extensively developed in the Northwest Highlands of Scotland, and particularly in the district of upper Loch Torridon, a circumstance which suggested the name Torridon Sandstone, first applied to these rocks...
, an unmetamorphosed sequence of similar age found beneath the Moine Thrust, remains unproven.
Knoydartian
Radiometric dating of micaMica
The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...
s from deformed pegmatite
Pegmatite
A pegmatite is a very crystalline, intrusive igneous rock composed of interlocking crystals usually larger than 2.5 cm in size; such rocks are referred to as pegmatitic....
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 age, associated with the Scandian orogeny.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-grabens within an overall riftRift
In geology, a rift or chasm is a place where the Earth's crust and lithosphere are being pulled apart and is an example of extensional tectonics....
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 tectonicsExtensional tectonics
Extensional tectonics is concerned with the structures formed, and the tectonic processes associated with, the stretching of the crust or lithosphere.-Deformation styles:...
, 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 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.
Basic intrusions
The Moine sequence is cut by numerous small metabasic intrusions, particularly in the Glenfinnan and Loch Eil groups. They have a tholeiiticTholeiite
The tholeiitic magma series is one of two main magma series in igneous rocks, the other magma series being the calc–alkaline. A magma series is a series of compositions that describes the evolution of a mafic magma, which is high in magnesium and iron and produces basalt or gabbro, as it...
chemistry, consistent with intrusion into a rift setting.
Early granitic intrusions
The Moinian rocks of Ross-shireRoss-shire
Ross-shire is an area in the Highland Council Area in Scotland. The name is now used as a geographic or cultural term, equivalent to Ross. Until 1889 the term denoted a county of Scotland, also known as the County of Ross...
and Sutherland are intruded by a distinctive set of granitic
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
sheets, which give latest Proterozoic intrusion ages in the range 611-588 Ma. It has been suggested that these granites formed associated with rifting.