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| Old Red Sandstone | |
|---|---|
| Stratigraphic range:Late Silurian – earliestCarboniferous419–358 Ma | |
 Folded Old Red Sandstone rock formation at St Ann's Head inPembrokeshire, Wales | |
| Type | Supergroup |
| Sub-units | See text |
| Thickness | More than 4 km (2.5 mi) (Shetland) |
| Lithology | |
| Primary | Sandstone |
| Other | Conglomerate,shale,mudstone,siltstone,limestone |
| Location | |
| Region | North Atlantic |
| Country | Canada, Greenland, Ireland, Norway, United Kingdom |
| Extent | 700 km (430 mi)[1] |
Old Red Sandstone, abbreviatedORS, is an assemblage of rocks in the North Atlantic region largely ofDevonian age. It extends in the east across Great Britain, Ireland and Norway, and in the west along the eastern seaboard ofNorth America. It also extends northwards intoGreenland andSvalbard.[3] These areas were a part of thepaleocontinent ofEuramerica (Laurussia). In Britain it is alithostratigraphic unit (a sequence ofrock strata) to whichstratigraphers accordsupergroup status[4] and which is of considerable importance to earlypaleontology. The presence ofOld in the name is to distinguish the sequence from the youngerNew Red Sandstone which also occurs widely throughout Britain.
The Old Red Sandstone describes a group ofsedimentary rocks deposited in a variety of environments in the lateSilurian, through theDevonian and into the earliest part of theCarboniferous. The body ofrock, orfacies, is dominated byterrigenous deposits andconglomerates at its base, and progresses to a combination ofdunes, and sediments that may have been laid down inlakes,river, estuaries, and possibly other coastal environments. The Old Red Sandstone was long thought to have been deposited mostly in freshwater, but more recent studies have discovered marine fossils (such asbrachiopods)[5] in some locations. Its vertebrate fauna also occurs in typically marine environments,[6] and anisotopic study also found significant marine influence indicating abrackish environment in mineralised tissues of its vertebrates,[7] even in taxa that had long been thought to have been freshwater inhabitants.[8][9] Thus, at least some strata appear to have been deposited on the coast, probably in marginal marine environments.
The familiar red colour of these rocks arises from the presence ofiron oxide, but not all the Old Red Sandstone is red or sandstone – the sequence also includes conglomerates,mudstones,siltstones and thinlimestones and colours can range from grey and green through to red and purple. These deposits are closely associated with the erosion of the Caledonian Mountain chain which was thrown up by the collision of the former continents ofAvalonia,Baltica andLaurentia to form theOld Red Sandstone Continent – an event known as theCaledonian Orogeny.
Many fossils are found within the rocks, including early fishes, arthropods and plants. As is typical with terrestrialred beds, the vast majority of the rock is not fossil-bearing; however there are isolated, localized beds within the rock that do contain fossils. Rocks of this age were also laid down inSouth West England (hence the name 'Devonian', fromDevon) though these are of true marine origin and are not included within the Old Red Sandstone.[1]
Since the Old Red Sandstone consists predominantly of rocks ofterrestrial origin, it does not generally contain marinefossils which would otherwise prove useful in correlating one occurrence of the rock with another, both between and within individualsedimentary basins. Accordingly, localstage names were devised and these remain in use to some extent today though there is an increasing use of international stage names. Thus in the Anglo-Welsh Basin, there are frequent references to theDowntonian,Dittonian,Breconian andFarlovian stages in the literature. The existence of a number of distinct sedimentary basins throughout Britain has been established.[1]
TheOrcadian Basin extends over a wide area of North East Scotland and the neighbouring seas. It encompasses theMoray Firth and adjoining land areas, Caithness, Orkney and parts ofShetland. South of the Moray Firth, two distinct sub-basins are recognized atTurriff and atRhynie. The sequence is more than 4 kilometres (13,000 ft) thick in parts of Shetland. The main basin is considered to be an intramontane basin resulting from crustal rifting associated with post-Caledonianextension, possibly accompanied by strike-slip faulting along theGreat Glen Fault system.[10]
There are a scatter of exposures of the Old Red Sandstone aroundOban and theIsle of Kerrera on theWest Highland coast, this unit is sometimes referred to as the Kerrera Sandstone Formation. The unit is up to 128m thick in its type area and consists of green and red sandstones and conglomerates, typically containing large (10–30 cm or 4–12 in across) elliptical well rounded clasts, accompanied by siltstones, mudstones and limestones.[11] On Kerrera a conglomerate ofandesite boulders rests unconformably onDalradian black,pyriticslates (Easdale Slate) of theEasdale Subgroup. At Oban there is merely an erosional contact incorporating debris of the slate in a basal conglomerate. The ORS deposits around Oban are considered latest Silurian (Pridoli) to earliest Devonian in age. They are interpreted asalluvial fans which filled a depositional basin from the east and northeast.[12] Smalloutliers occur nearTaynuilt and either side ofLoch Avich.[13] The deposits are especially obvious on Kerrera where they form the bedrock across half of the island.[14] These are conformably overlain bypeperite and thebasaltic and andesiticLorne plateau lavas. The ORS on Kerrera and isolated localities around Oban are known for their fossils, particularly fish.[15]
TheMidland Valleygraben defined by theHighland Boundary Fault in the north and theSouthern Uplands Fault in the south harbours not only a considerable amount of Old Red Sandstone sedimentary rocks but also igneous rocks of this age associated with extensivevolcanism. There is a continuous outcrop along the Highland Boundary Fault fromStonehaven on theNorth Sea coast toHelensburgh and beyond toArran. A more disconnected series of outcrops occur along the line of the Southern Uplands Fault fromEdinburgh toGirvan. Old Red Sandstone often occurs in conjunction with conglomerate formations, one such noteworthycliffside exposure being theFowlsheugh Nature Reserve,Kincardineshire.
A series of outcrops occur fromEast Lothian southwards throughBerwickshire. Hutton's famous unconformity at Siccar Point occurs within this basin – seeHistory of study below.
This relatively large basin extends across much of South Wales from southernPembrokeshire in the west throughCarmarthenshire intoPowys andMonmouthshire and through the southernWelsh Marches, intoHerefordshire,Worcestershire andGloucestershire. Outliers inSomerset and northDevon complete its extent.
With the exception of south Pembrokeshire, all parts of the basin are represented by a range of lithologies assigned to the Lower Devonian and to the Upper Devonian, the contact between the two being unconformable and representing the complete omission of any Middle Devonian sequence.
The lowermost formations are of upper Silurian age, these being the Downton Castle Sandstone Formation and the overlying Moor Cliffs Formation (formerly the Raglan Mudstone Formation). The top of this formation is marked by a well-developedcalcrete, theChapel Point Limestone.[16] The lowermost Devonian formation is the Freshwater West Formation (formerly the St Maughans Formation), itself overlain by the Senni Formation (formerly the Senni Beds) which is in turn overlain by the Brownstones Formation. In the east, a further calcrete, the Ffynnon Limestone (sometimes pluralised) is developed at the interface between the Freshwater West and Senni formations. The Senni Formation is not recorded further east.
The Upper Devonian sequence is rather thinner and comprises a series of formations which are more laterally restricted. In theBrecon Beacons, the Plateau Beds Formation is unconformably overlain by the Grey Grits Formation though further east these divisions are replaced by the Quartz Conglomerate Group which is itself subdivided into a variety of different formations.
The sequence in Pembrokeshire differs from that of the main part of the basin to the east, and falls into two parts.[17]
In North Pembrokeshire to the north of theRitec Fault, both the middle and upper ORS are missing with only the lower ORS present; this is divided into an earlierMilford Haven Group comprising in ascending order, the Red Cliff, Sandy Haven and Gelliswick Bay formations and a laterCosheston Group with, again in ascending order, its constituent Llanstadwell, Burton Cliff, Mill Bay, Lawrenny Cliff and New Shipping formations. These respectively equate with the Temeside, Raglan Mudstone and St Maughans formations of the central and eastern part of the basin.
In south Pembrokeshire to the south of the Ritec Fault, the lower ORS is represented by, in ascending order, the Freshwater East, Moors Cliff and Freshwater West formations. These areunconformably overlain by the Ridgeway Conglomerate Formation. The middle ORS is missing whilst the Upper ORS is represented by the Gupton and West Angle formations.
The Freshwater East Formation, and corresponding Red Cliff Formation of north Pembrokeshire, are both late Silurian in age.[18]
A small and separate basin exists here where bothalluvial andlacustrine deposits are recorded. Both the middle and upper ORS are missing but the lower ORS is represented, in ascending order, by the Bodafon, Traeth Bach, Porth y Mor and Traeth Lligwy formations. Calcretes are also recorded representing carbonate-rich soils developed between periods of sediment deposition. The present day outcrop occupies a narrow zone fromDulas Bay on Anglesey's northeast coast, southwards to the town ofLlangefni.[1][19]
In 1787James Hutton noted what is now known asHutton's Unconformity at Inchbonny,Jedburgh, and in early 1788 he set off withJohn Playfair to theBerwickshire coast and found more examples of this sequence in the valleys of the Tower and Pease Burns nearCockburnspath.[20] They then took a boat trip from Dunglass Burn east along the coast with the geologistSir James Hall ofDunglass and atSiccar Point found what Hutton called "a beautiful picture of this junction washed bare by the sea",[21] where 345-million-year-old Old Red Sandstone overlies 425-million-year-oldSiluriangreywacke.[2][22]
In the early 19th century, the paleontology of the formation was studied intensively byHugh Miller,Henry Thomas De la Beche,Roderick Murchison, andAdam Sedgwick—Sedgwick's interpretation was the one that placed it in theDevonian: he coined the name of that period. The term 'Old Red Sandstone' was originally used in 1821 by Scottishnaturalist andmineralogistRobert Jameson to refer to the red rocks which underlay the 'Mountain Limestone' i.e. the Carboniferous Limestone. They were thought at that time to be the British version of Germany'sRotliegendes, which is in fact ofPermian age.[1] Many of thescience ofstratigraphy's early debates were about the Old Red Sandstone.
In oldergeological works[by whom?] predating theories ofplate tectonics, the United States'Catskill Delta formation is sometimes[when?] referred to[by whom?] as part of the Old Red Sandstone.[citation needed] In the modern day it is recognized that the two are not stratigraphically continuous but are very similar due to being formed at approximately the same time by the same processes.
The Old Red Sandstone has been widely used as a building stone across those regions where it outcrops. Notable examples of its use can be found in the area surroundingStirling,Stonehaven,Perth andTayside. The inhabitants ofCaithness at the northeastern tip of Scotland also used the stone to a considerable extent. Old Red Sandstone has also frequently been used in buildings in Herefordshire, Monmouthshire and the formerBrecknockshire (now south Powys) of south Wales.
