The Loch Ness 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 is found everywhere in tectonic contact above the older

Wester Ross Supergroup 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 medium to high-grade metamorphic rocks and associated ...

. It is thought to be

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 ...

overlain by the

Cryogenian The Cryogenian (from , meaning "cold" and , romanized: , meaning "birth") is a geologic period that lasted from . It is the second of the three periods of the Neoproterozoic era, preceded by the Tonian and followed by the Ediacaran. The Cryoge ...

Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

Dalradian Supergroup.

Stratigraphy

The supergroup is subdivided into three

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 iden ...

Glenfinnan Group

This group, which consists of

amphibolite facies Amphibolite () is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar, but with little or no quartz. It is typically dark-colored and dense, with a weakly foliated or schistose (fl ...

pelitic A pelite () or metapelite is a 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 a mudstone, t ...

gneiss Gneiss (pronounced ) 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. This rock is formed under p ...

and interbanded

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 ...

, semi-pelite,

psammite Psammite (Greek: ''psammitēs'' "(made) from sand", from ''psammos'' "sand") is a general term for sandstone. It is equivalent to the Latin-derived term areniteU.S. Bureau of Mines Staff (1996) ''Dictionary of Mining, Mineral, & Related Terms.'' ...

quartzite Quartzite is a hard, non- foliated metamorphic rock that was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usually related to tecton ...

and

migmatite Migmatite is a composite rock (geology), rock found in medium and high-grade metamorphic environments, commonly within Precambrian craton, cratonic blocks. It consists of two or more constituents often layered repetitively: one layer is an old ...

s, lies tectonically above the

Sgurr Beag Thrust The Sgurr Beag Thrust is an important tectonic structure within the Neoproterozoic metasedimentary rock sequences of the Scottish Highlands. The thrust, or similar structures correlated with it, form the boundary between rocks of the Glennfinnan Gr ...

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, with a highly sheared unconformable contact, incorporated during the Caledonian orogeny. The original stratigraphic thickness of the group is difficult to estimate due to the high level of strain that it experienced but is likely to be several kilometres. The sequence lacks any sedimentary structures due to its strain state.

Loch Eil Group

This group , which consists dominantly of psammite with local developments of quartzite, lies above the Glenfinnan Group in what is interpreted to be a normal stratigraphic contact. The upper boundary of this group is not seen, with

Old Red Sandstone Old Red Sandstone, abbreviated ORS, is an assemblage of rocks in the North Atlantic region largely of Devonian age. It extends in the east across Great Britain, Ireland and Norway, and in the west along the eastern seaboard of North America. It ...

typically found unconformably above it. In contrast to the Glennfinnnan Group, there are well-preserved sedimentary structures. Towards the southwestern end of its outcrop the Group consists of widespread

s 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, Strathglass, River Glass from the point at which it starts at the confluence of the River Affric and Abhainn De ...

districts. The total thickness of the group is thought to lie in the range 2.5–5.0 km.

Badenoch Group

The succession is divided into two subgroups, although the stratigraphic relationship between them remains unclear, the Dava Subgroup (previously referred to as the Dava Succession) and the Glen Banchor Subgroup. The former, named from the locality of Dava between

Inverness Inverness (; ; from the , meaning "Mouth of the River Ness") is a city in the Scottish Highlands, having been granted city status in 2000. It is the administrative centre for The Highland Council and is regarded as the capital of the Highland ...

and

Grantown-on-Spey Grantown-on-Spey () is a town in the Highland Council Area, Counties of Scotland, historically within the county of Moray. It is located on a low plateau at Freuchie beside the river Spey at the northern edge of the Cairngorms, Cairngorm mounta ...

includes the Slochd Psammite and Flichity Semipelite formations. The contact with the Grampian Group is interpreted to be highly sheared unconformity. The latter is named for Glen Banchor, west of

Newtonmore Newtonmore ( ) is a village of approximately 1100 inhabitants in Badenoch, within the Highland (council area), Highland council area of Scotland. The village is only a few miles from a location that is claimed to be the exact geographical centre ...

, the type area being from here to Laggan. The Glen Banchor sequence is believed to be between 1 and 1.5 km thick and unconformably overlain by rocks of the Grampian and Appin groups, though the boundary may be tectonic in nature. The total thickness of the group is estimated to be several kilometres.

Age

The age of this sequence is constrained by a combination of

detrital zircon geochronology Detrital zircon geochronology is the science of geochronology, analyzing the age of zircons deposited within a specific sedimentary rock, sedimentary unit by examining their inherent radioisotopes, most commonly the uranium–lead dating, uranium ...

, the crystallization ages of igneous intrusions that cut the sequence and metamorphic ages for events that later affected the supergroup. The youngest detrital zircon ages are in the range 1000–900 Ma (million years ago), with one zircon from the Badenoch Group giving an age of 900±17 Ma, postdating 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 ...

. Igneous intrusions that cut the Glenfinnan Group give crystallization ages of about 870 Ma and the Badenoch Group was affected by the Knoydartian Orogeny, giving metamorphic ages of about 840 Ma. Taken together these data imply a depositional age range of 900–850 Ma.

Depositional setting

The relatively intense metamorphic and tectonic history of this sequence makes any detailed interpretation of the depositional setting very difficult. The presence of finely interbedded psammite, pelite and quartzite is consistent with a shallow water to shelf setting. The evidence of bimodal magmatism affecting the Loch Eil Group combined with the

MORB A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a div ...

chemistry of the

mafic A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...

intrusions is consistent with a period of rifting. Detrital zircon ages show that some of the sediment was coming from a Renlandian source, suggesting that the sequence was deposited in its hinterland, possibly as part of a foreland basin.

References

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