Loch Ness Supergroup | |
---|---|
Stratigraphic range: 900–850 Ma | |
Type | Geological supergroup |
Sub-units | Loch Eil Group, Glenfinnan Group, Badenoch Group |
Underlies | Dalradian Supergroup |
Overlies | Wester Ross Supergroup with tectonic contact |
Area | Northwestern Scotland |
Thickness | 6–9 km |
Lithology | |
Primary | Semi-pelite, Pelite, Psammite |
Other | Quartzite |
Type section | |
Named for | Loch Ness |
The Loch Ness Supergroup is one of the subdivisions of the Neoproterozoic sequence of sedimentary rocks (or their metamorphic equivalents) in the Scottish Highlands. [1] It is found everywhere in tectonic contact above the older Wester Ross Supergroup. It is thought to be unconformably overlain by the Cryogenian to Cambrian Dalradian Supergroup. [2]
The supergroup is subdivided into three groups.
This group, which consists of amphibolite facies pelitic gneiss and interbanded pelite, semi-pelite, psammite, quartzite and migmatites, [2] [3] 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. [4] 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. [3] [2]
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 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 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. [5] A Tarvie Psammite Formation is recorded in the Strathconon and Strathglass districts. [6] The total thickness of the group is thought to lie in the range 2.5–5.0 km. [7] [2]
The succession is divided into two subgroups, although the stratigraphic relationship between them remains unclear, [2] 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 and Grantown-on-Spey 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, 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. [8] [2]
The age of this sequence is constrained by a combination of detrital zircon geochronology, 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. 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. [2]
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 chemistry of the mafic 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. [2]
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 190 kilometres (120 mi) southwest to the Sleat peninsula on the Isle of Skye. The thrust belt consists of a series of thrust faults that branch off the Moine Thrust itself. Topographically, the belt marks a change from rugged, terraced mountains with steep sides sculptured from weathered igneous, sedimentary and metamorphic rocks in the west to an extensive landscape of rolling hills over a metamorphic rock base to the east. Mountains within the belt display complexly folded and faulted layers and the width of the main part of the zone varies up to ten kilometres, although it is significantly wider on Skye.
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The Dalradian Supergroup is a stratigraphic unit in the lithostratigraphy of the Grampian Highlands of Scotland and in the north and west of Ireland. The diverse assemblage of rocks which constitute the supergroup extend across Scotland from Islay in the west to Fraserburgh in the east and are confined by the Great Glen Fault to the northwest and the Highland Boundary Fault to the southeast. Much of Shetland east of the Walls Boundary Fault is also formed from Dalradian rocks. Dalradian rocks extend across the north of Ireland from County Antrim in the north east to Clifden on the Atlantic coast, although obscured by younger Palaeogene lavas and tuffs or Carboniferous rocks in large sections.
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The Appin Group is a thick sequence of metamorphosed Neoproterozoic sedimentary rocks that outcrop across the Central Highlands of Scotland, east of the Great Glen. It forms a part of the Dalradian Supergroup.
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The Wester Ross Supergroup is one of the subdivisions of the Neoproterozoic sequence of sedimentary rocks in the Scottish Highlands. It lies unconformably on medium 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. 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.
The Knoydartian Orogeny is a Tonian tectonic and metamorphic event, or group of events, that is recognised in the rocks of the Wester Ross and Loch Ness supergroups of the Scottish Highlands. It is dated to about 820–725 Ma, predating the deposition of the Cryogenian to Cambrian Dalradian Supergroup. It is named after Knoydart, one of the localities where the event was first recognised.
The Renlandian Orogeny is a Tonian 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 recognised.
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