| Dalradian Supergroup | |
|---|---|
| Stratigraphic range: 725–470 Ma | |
 Folded Dalradianphyllites of theEdiacaran to lowerCambrian Ben Ledi Grit Formation, part of theSouthern 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) |
TheDalradian Supergroup (informally and traditionally theDalradian) is astratigraphic unit (a sequence ofrock strata) in thelithostratigraphy of theGrampian Highlands ofScotland and in the north and west ofIreland. The diverse assemblage of rocks which constitute the supergroup extend across Scotland fromIslay in the west toFraserburgh in the east and are confined by theGreat Glen Fault to the northwest and theHighland Boundary Fault to the southeast. Much ofShetland east of theWalls Boundary Fault is also formed from Dalradian rocks. Dalradian rocks extend across the north ofIreland fromCounty Antrim in the north east toClifden on theAtlantic coast, although obscured by youngerPalaeogene lavas andtuffs orCarboniferous rocks in large sections.
The land to the southeast of theGreat Glen was the oldCeltic region ofDál Riata (Dalriada), and in 1891Archibald 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 thestratigraphical sequence.[1]
In Archibald Geikie's words, "they consist in large proportion of alteredsedimentary strata, now found in the form ofmica-schist,graphite-schist,andalusite-schist,phyllite,schistose grit,greywacke andconglomerate,quartzite,limestone and other rocks, together withepidiorites,chlorite-schists,hornblende schists and other allied varieties, which probably marksills,lava-sheets or beds oftuff, 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 easternSutherland,Ross-shire andInverness-shire, theMoinegneiss, as well as the metamorphosedigneous andsedimentary rocks of the central, eastern and southwesternScottish Highlands.[1]
The Dalradian Supergroup spans the late Tonian through to early Ordovician, however, there is an unfortunate lack of direct geochronological age constraints throughout the succession. There is some debate as to the age of the base of the Dalradian with some suggesting that sedimentation must be younger than ca. 806 Ma, which dates the deformation of the underlying basement Badenoch Group. However, the relationship between the Badenoch Group and the lowermost Dalradian is unclear.[2] A definitive depositional age of 601 ± 4 Ma[3] is derived from the Tayvallich Volcanic Formation at the top of the Argyll Group and provides a useful age constrain for the uppermost Dalradian. A detrital zircon study provides useful maximum depositional age constraints for the lowermost Argyll group 1.1 km thick glaciogenic Port Askaig Formation.[4] The age constraints suggest that the Port Askaig Formation correlates with the global ‘Sturtian’ Snowball Earth interval and spans ca. 717 – 660 Ma. At present, no conclusive ‘Marinoan’ aged glacial deposit has been found.
The supergroup is composed of fourgroups 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 nowmetamorphosed, the Dalradian sequence was originally deposited as marinesands andmuds,silt andlimestone. Metamorphism has been low to medium grade for the most part and resulted in the formation ofslates,phyllites,psammites,pelites and semipelites. The Tayvallich Subgroup containsvolcanics within aturbidite basin and there arelavas within the overlying Southern Highland Group.[5]
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.[6]
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 Cambriantrilobite fossils have been recovered. Apparently higher in the succession is the Margie Formation, which includes the Margie Limestone Member, which has yieldedconodont fossils of early Ordovician age. Rocks of this group are interpreted to be everywhere in tectonic contact structurally below theHighland 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).[7]
The Southern Highland Group is found along the entire southeastern margin of the Grampian Highlands fromKintyre toStonehaven and also in the northeast along the coastal strip betweenFraserburgh andPortsoy, extending south to theDon valley. In Shetland, the Clift Hills Division extends from north ofLerwick south toFitful Head. In Ireland the most extensive outcrop of the Southern Highland Group is to the north and south ofLough Foyle and west toLough Swilly. Smaller exposures occur as far to the southwest asInishbofin, County Galway.
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 ofMainland whilst the Scatsta ‘Division’ forms the western halves ofUnst andFetlar, all ofWhalsay and much of the southeastern part of Mainland, east of theNesting Fault. These rocks are also present in Ireland acrossCounty Londonderry andCounty Donegal and appear again in theOx Mountains, theNephin Beg Range and theTwelve Pins of Connemara.[8]
The Grampian Group rocks occupy the ground south west fromElgin and extending down the Great Glen as far asCorran onLoch Linnhe and, further east, as far south asTyndrum. It is divided into three subgroups:
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, thePort Askaig Tillite Formation (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.[9]
TheGrampian orogeny folded the sequence in mainland Scotland into a series of major tightfolds with NE-SW aligned fold axes. Much of the southeastern part of the outcrop forms a part of theTay 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.[10]
One or more of the preceding sentences incorporates text from a publication now in thepublic domain: Chisholm, Hugh, ed. (1911). "Dalradian".Encyclopædia Britannica. Vol. 7 (11th ed.). Cambridge University Press. p. 777.
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