Dalradian

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Dalradian Supergroup
Stratigraphic range: 725–470 Ma
Folded Dalradian phyllites.JPG
Folded Dalradian phyllites of the Ediacaran to lower Cambrian Ben Ledi Grit Formation, part of the Southern Highland Group
Type Geological supergroup
Sub-units Grampian Group, Appin Group, Argyll Group, Southern Highland Group, Trossachs Group
Underlies Old Red Sandstone, Highland Border Ophiolite (tectonic contact)
Overlies Loch Ness Supergroup
Thickness>20 km (summing the individual groups)
Lithology
Primary Psammite, quartzite, semipelite, pelite, limestone
Other diamictite
Location
Region Grampian Highlands, Shetland, County Antrim, County Tyrone, County Londonderry, Northern and Western Region
Country Scotland, Northern Ireland, Republic of Ireland
Type section
Named for Dál Riata (Dalriada)
Andalusite crystals in Dalradian (Southern Highland Group) metamorphic rock at Boyndie Bay, north-east Scotland Andalusite Crystals Whitehills Scotland.jpg
Andalusite crystals in Dalradian (Southern Highland Group) metamorphic rock at Boyndie Bay, north-east Scotland

The Dalradian Supergroup (informally and traditionally the Dalradian) is a stratigraphic unit (a sequence of rock strata) 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.

Contents

Historical

The land to the southeast of the Great Glen was the old Celtic region of Dál Riata (Dalriada), and in 1891 Archibald Geikie proposed the name Dalradian as a convenient provisional designation for the complicated set of rocks to which it was then difficult to assign a definite position in the stratigraphical sequence. [1]

In Archibald Geikie's words, "they consist in large proportion of altered sedimentary strata, now found in the form of mica-schist, graphite-schist, andalusite-schist, phyllite, schistose grit, greywacke and conglomerate, quartzite, limestone and other rocks, together with epidiorites, chlorite-schists, hornblende schists and other allied varieties, which probably mark sills, lava-sheets or beds of tuff, intercalated among the sediments. The total thickness of this assemblage of rocks must be many thousand feet." The Dalradian Series (as then defined) included the "Eastern or Younger schists" of eastern Sutherland, Ross-shire and Inverness-shire, the Moine gneiss, as well as the metamorphosed igneous and sedimentary rocks of the central, eastern and southwestern Scottish Highlands. [1]

Age

The Dalradian is largely of Neoproterozoic (Sturtian to Vendian) age but likely extending into the Cambrian and possibly the Ordovician with the youngest rocks being at least 470 million years old. The oldest part of the sequence may be around 800 million years old, [2] although the lack of any evidence of Knoydartian metamorphism suggests that deposition did not start until after the youngest part of this orogeny at about 725 Ma (million years ago). [3]

Sub-units

The supergroup is composed of four groups which in stratigraphic order i.e. youngest at top, are:

The upper three groups are applied to Ireland too; the Grampian Group is not recorded here nor in Shetland where the Dalradian is divided into a Clift Hills ‘Division’ which equates to the Southern Highland Group, a Whiteness ‘Division’ which equates to the Argyll Group and a Scatsta ‘Division’ which equates to the Appin Group. Though now metamorphosed, the Dalradian sequence was originally deposited as marine sands and muds, silt and limestone. Metamorphism has been low to medium grade for the most part and resulted in the formation of slates, phyllites, psammites, pelites and semipelites. The Tayvallich Subgroup contains volcanics within a turbidite basin and there are lavas within the overlying Southern Highland Group. [4]

An additional group is recognised by some workers, the Trossachs Group, forming the uppermost part of the succession, which outcrops close to the Highland Boundary Fault. [5]

Trossachs Group

The Trossachs Group as defined lies conformably above rocks of the Southern Highlands Group just northwest of the Highland Boundary Fault. Most of the outcrops of the Trossachs Group are fault-bounded, making overall correlation difficult. The oldest part of the group is interpreted to be the Keltie Water Formation, which includes the Leny Limestone and Slate Member from which lower Cambrian trilobite fossils have ben recovered. Apparently higher in the succession is the Margie Formation, which includes the Margie Limestone Member, which has yielded conodont fossils of early Ordovician age. Rocks of this group are interpreted to be everywhere in tectonic contact structurally below the Highland Border Ophiolite. The sedimentary rocks that lie unconformably above the ophiolite include the Dounans Limestone Formation that contains a fossil fauna of mid-Arenig age (near the boundary between the lower and middle Ordovician). [6]

Southern Highland Group

Folded rocks of the Southern Highland Group at Whitehills Folded Rocks - geograph.org.uk - 224051.jpg
Folded rocks of the Southern Highland Group at Whitehills

The Southern Highland Group is found along the entire southeastern margin of the Grampian Highlands from Kintyre to Stonehaven and also in the northeast along the coastal strip between Fraserburgh and Portsoy, extending south to the Don valley. In Shetland, the Clift Hills Division extends from north of Lerwick south to Fitful Head. In Ireland the most extensive outcrop of the Southern Highland Group is to the north and south of Lough Foyle and west to Lough Swilly. Smaller exposures occur as far to the southwest as Inishbofin, County Galway.

Argyll and Appin groups

Dalradian metasediments of the Easdale Subgroup of the Argyll Group near Ormsary Dalradian Metasediments near Ormsary in Scotland 1.jpg
Dalradian metasediments of the Easdale Subgroup of the Argyll Group near Ormsary

In mainland Scotland, the Appin and Argyll group sequences occupy the intermediate ground between the Southern Highland and Grampian groups. The Argyll Group is divided into four subgroups, thus:

whilst the Appin Group is divided into three subgroups:

In Shetland the Whiteness ‘Division’ forms the core of Mainland whilst the Scatsta ‘Division’ forms the western halves of Unst and Fetlar, all of Whalsay and much of the southeastern part of Mainland, east of the Nesting Fault. These rocks are also present in Ireland across County Londonderry and County Donegal and appear again in the Ox Mountains, the Nephin Beg Range and the Twelve Pins of Connemara. [7]

Grampian Group

The Grampian Group rocks occupy the ground south west from Elgin and extending down the Great Glen as far as Corran on Loch Linnhe and, further east, as far south as Tyndrum. It is divided into three subgroups:

Shetland sequence

Dalradian tilted metasedimentary rocks of the Hevda Phyllite Formation at Skaw, on Unst, Shetland Armeria maritima and Metasediments at Skaw in Shetland.jpg
Dalradian tilted metasedimentary rocks of the Hevda Phyllite Formation at Skaw, on Unst, Shetland

The stratigraphic position of the sequence identified as Dalradian in the Shetland Islands is uncertain, because the main marker within the sequence in Scotland and Ireland, the Port Askaig Formation tillite (of the Islay Subgroup), is not present. Carbon isotope data from four metamorphosed limestones within this mainly siliclastic sequence, suggest that the entire Shetland sequence probably lies stratigraphically above the tillite marker, explaining its absence. [8]

Structure

The Grampian orogeny folded the sequence in mainland Scotland into a series of major tight folds with NE-SW aligned fold axes. Much of the southeastern part of the outcrop forms a part of the Tay Nappe and involves the inversion of a large part of the succession. Caledonoid faulting on NE-SW lines affects the sequence across the entire area. [9]

Related Research Articles

<span class="mw-page-title-main">Moine Thrust Belt</span> Fault in Highland, Scotland, UK

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) south-west 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 10 kilometres (6.2 mi), although it is significantly wider on Skye.

<span class="mw-page-title-main">Highland Boundary Fault</span> Geological fault zone crossing Scotland

The Highland Boundary Fault is a major fault zone that traverses Scotland from Arran and Helensburgh on the west coast to Stonehaven in the east. It separates two different geological terranes which give rise to two distinct physiographic terrains: the Highlands and the Lowlands, and in most places it is recognisable as a change in topography. Where rivers cross the fault, they often pass through gorges, and the associated waterfalls can be a barrier to salmon migration.

<span class="mw-page-title-main">Argyll Group</span>

The Argyll Group is a thick sequence of metamorphosed Neoproterozoic sedimentary rocks that outcrop across the Central Highlands of Scotland, east of the Great Glen, as well as appearing in the north of Ireland. It is a subdivision of the Dalradian Supergroup and is itself divided into four units; from oldest to youngest these are the Islay, Easdale, Crinan and Tayvallich subgroups.

<span class="mw-page-title-main">Colonsay Group</span>

The Colonsay Group is an estimated 5,000 m thick sequence of mildly metamorphosed Neoproterozoic sedimentary rocks that outcrop on the islands of Colonsay, Islay and Oronsay and the surrounding seabed. They have been correlated with the Grampian Group, the oldest part of the Dalradian Supergroup.

<span class="mw-page-title-main">Bowmore Sandstone Group</span>

The Bowmore Sandstone Group is a sequence of metasedimentary rocks, dominantly sandstones, of probable Neoproterozoic age. Their outcrop on the island of Islay in the Inner Hebrides is entirely fault-bounded, between the Loch Gruinart Fault to the west and the Loch Skerrols Shear Zone to the east.

The Grampian Group is an estimated 9,000 m 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. The sequence thickness at any one place is limited to 5km.

<span class="mw-page-title-main">Appin Group</span>

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.

<span class="mw-page-title-main">Southern Highland Group</span>

The Southern Highland Group is a sequence of metamorphosed Neoproterozoic sedimentary rocks that outcrop across the Central Highlands of Scotland, east of the Great Glen. It forms the uppermost/youngest part of the Dalradian Supergroup and is divided into two formations. Volcanism is recorded by the Loch Avich Lavas Formation which divides the Loch Avich Grits Formation into lower and upper parts – the grits are turbidite and submarine fan deposits. In Perthshire and on Deeside, the Leny Limestone, which has been dated to 514 Ma, is present. A tillite, the Macduff Boulder Bed is known from the upper part of the group in northeast Scotland.

The Badenoch Group is a sequence of metamorphosed Tonian age sedimentary rocks that outcrop across the Central Highlands of Scotland, east of the Great Glen. This rock sequence has formerly been referred to as the Central Highland Migmatite Complex and the Central Highland Division.

This article describes the geology of the Cairngorms National Park, an area in the Highlands of Scotland designated as a national park in 2003 and extended in 2010. The Cairngorms National Park extends across a much wider area than the Cairngorms massif itself and hence displays rather more varied geology.

The geology of national parks in Britain strongly influences the landscape character of each of the fifteen such areas which have been designated. There are ten national parks in England, three in Wales and two in Scotland. Ten of these were established in England and Wales in the 1950s under the provisions of the National Parks and Access to the Countryside Act 1949. With one exception, all of these first ten, together with the two Scottish parks were centred on upland or coastal areas formed from Palaeozoic rocks. The exception is the North York Moors National Park which is formed from sedimentary rocks of Jurassic age.

The geology of Loch Lomond and The Trossachs National Park in the southwestern part of the Scottish Highlands consists largely of Neoproterozoic and Palaeozoic bedrock faulted and folded and subjected to low grade metamorphism during the Caledonian orogeny. These older rocks, assigned to the Dalradian Supergroup, lie to the northwest of the northeast – southwest aligned Highland Boundary Fault which defines the southern edge of the Highlands. A part of this mountainous park extends south of this major geological divide into an area characterised by younger Devonian rocks which are assigned to the Old Red Sandstone.

<span class="mw-page-title-main">Torridonian</span> Sequence of rocks in Scotland

The Torridonian is the informal name given to a sequence of Mesoproterozoic to Neoproterozoic sedimentary rocks that outcrop in a strip along the northwestern coast of Scotland and some parts of the Inner Hebrides from the Isle of Mull in the southwest to Cape Wrath in the northeast. They lie unconformably on the Archaean to Paleoproterozoic basement rocks of the Lewisian complex and unconformably beneath the Cambrian to Lower Ordovician rocks of the Ardvreck Group.

<span class="mw-page-title-main">Loch Ness Supergroup</span>

The Loch Ness Supergroup is one of the subdivisions of the Neoproterozoic sequence of sedimentary rocks in the Scottish Highlands. 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.

<span class="mw-page-title-main">Wester Ross Supergroup</span>

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.

<span class="mw-page-title-main">Highland Border Complex</span> Rocks in Scotland

The Highland Border Complex is an assemblage of rocks of probable early Cambrian to late Ordovician age, found as fault-bounded blocks of variable lithology exposed immediately to the southeast of the Highland Boundary Fault at the edge of the Grampian Highlands, Scotland.

<span class="mw-page-title-main">Morar Group</span>

The Morar Group is a sequence of Tonian sedimentary rocks that have been subjected to a series of tectonic and metamorphic events since their deposition. Originally interpreted to be lowest (oldest) part of a "Moine Supergroup", this sequence now forms part of the Wester Ross Supergroup. They lie unconformably on Archean to Paleoproterozoic basement of the Lewisian complex. The contact with the overlying Glenfinnan Group of the Loch Ness Supergroup is everywhere a tectonic one, formed by the Sgurr Beag Thrust or related structures.

<span class="mw-page-title-main">Moinian</span> Geological rock formation in Scotland

The Moinian or just the Moine, formerly the Moine Supergroup, is a sequence of Neoproterozoic metasediments that outcrop in the Northwest Highlands between the Moine Thrust Belt to the northwest and the Great Glen Fault to the southeast and one part of the Grampian Highlands to the southeast of the fault. It takes its name from A' Mhòine, a peat bog in northern Sutherland.

The Highland Border Ophiolite (HBO) is a set of rocks that are ophiolitic in character found within the Highland Border Complex. They are exposed in a series of fault-bounded outcrops along the line of the Highland Boundary Fault that forms the southeastern boundary to the Grampian Highlands in Scotland. They represent fragments of a piece of oceanic crust or exhumed mantle that has been obducted onto continental crust. These rocks provide an important constraint on models of how the current geometry arose during the Caledonian Orogeny.

References

  1. 1 2 Wikisource-logo.svg One or more of the preceding sentences incorporates text from a publication now in the public domain :  Chisholm, Hugh, ed. (1911). "Dalradian". Encyclopædia Britannica . Vol. 7 (11th ed.). Cambridge University Press. p. 777.
  2. Trewin, Nigel H., ed. (2002). The Geology of Scotland (Fourth ed.). London: The Geological Society. p. 8. ISBN   1-86239-126-2.
  3. Krabbendam, M.; Strachan, R.; Prave, T. (2022). "A new stratigraphic framework for the early Neoproterozoic successions of Scotland". Journal of the Geological Society. 179. doi:10.1144/jgs2021-054. hdl: 10023/24155 .
  4. Trewin, Nigel H., ed. (2002). The Geology of Scotland (Fourth ed.). London: The Geological Society. p. 96. ISBN   1-86239-126-2.
  5. Leslie, A.G.; Krabbendam, M.; Thomas, C.W.; Banks, C.J.; Clarke, S.M. (2024). "Geometrical restoration of a late Neoproterozoic depositional framework and an intrabasinal unconformity in the Laurentian margin Dalradian Supergroup, Grampian Highlands, Scotland". Journal of the Geological Society. 181. doi: 10.1144/jgs2023-208 .
  6. Cawood, P.A.; Merale, R.E.; Strachan, R.A.; Tanner, P.W.G. (2012). "Provenance of the Highland Border Complex: constraints on Laurentian margin accretion in the Scottish Caledonides". Journal of the Geological Society. 169 (5). doi:10.1144/0016-76492011-076.
  7. Bedrock Geology of the United Kingdom and Ireland (1:1,250,000 scale map) (First ed.). British Geological Survey. 2017. ISBN   978-0-7518-3793-3.
  8. Prave, A.R.; Strachan, R.A.; Fallick, A.E. (2009). "Global C cycle perturbations recorded in marbles: a record of Neoproterozoic Earth history within the Dalradian succession of the Shetland Islands, Scotland". Journal of the Geological Society. 166. London: Geological Society: 129–135. doi:10.1144/0016-76492007-126.
  9. Trewin, Nigel H., ed. (2002). The Geology of Scotland (Fourth ed.). London: The Geological Society. pp. 9, 92. ISBN   1-86239-126-2.

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