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 also extends northwards into Greenland and Svalbard. These areas were a part of the paleocontinent of Euramerica (Laurussia). In Britain it is a lithostratigraphic unit (a sequence of rock strata) to which stratigraphers accord supergroup status and which is of considerable importance to early paleontology. The presence of Old in the name is to distinguish the sequence from the younger New Red Sandstone which also occurs widely throughout Britain.

Sedimentology

The Old Red Sandstone describes a group of sedimentary rocks deposited in a variety of environments in the late Silurian, through the Devonian and into the earliest part of the Carboniferous. The body of rock, or facies, is dominated by terrigenous deposits and conglomerates at its base, and progresses to a combination of dunes, and sediments that may have been laid down in lakes, 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 as brachiopods) in some locations. Its vertebrate fauna also occurs in typically marine environments, and an isotopic study also found significant marine influence indicating a brackish environment in mineralised tissues of its vertebrates, even in taxa that had long been thought to have been freshwater inhabitants. 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 of iron oxide, but not all the Old Red Sandstone is red or sandstone – the sequence also includes conglomerates, mudstones, siltstones and thin limestones 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 of Avalonia, Baltica and Laurentia to form the Old Red Sandstone Continent – an event known as the Caledonian Orogeny. Many fossils are found within the rocks, including early fishes, arthropods and plants. As is typical with terrestrial red 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 in South West England (hence the name 'Devonian', from Devon) though these are of true marine origin and are not included within the Old Red Sandstone.

Stratigraphy Since the Old Red Sandstone consists predominantly of rocks of terrestrial origin, it does not generally contain marine fossils which would otherwise prove useful in correlating one occurrence of the rock with another, both between and within individual sedimentary basins. Accordingly, local stage 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 the Downtonian, Dittonian, Breconian and Farlovian stages in the literature. The existence of a number of distinct sedimentary basins throughout Britain has been established.

Orcadian Basin The Orcadian Basin extends over a wide area of North East Scotland and the neighbouring seas. It encompasses the Moray Firth and adjoining land areas, Caithness, Orkney and parts of Shetland. South of the Moray Firth, two distinct sub-basins are recognized at Turriff and at Rhynie. The sequence is more than four 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-Caledonian extension, possibly accompanied by strike-slip faulting along the Great Glen Fault system.

Argyll There are a scatter of exposures of the Old Red Sandstone around Oban and the Isle of Kerrera on the West Highland coast, this unit is sometimes referred to as the Kerrera Sandstone Formation. The unit is up to 128 m 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. On Kerrera a conglomerate of andesite boulders rests unconformably on Dalradian black, pyritic slates (Easdale Slate) of the Easdale 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 as alluvial fans which filled a depositional basin from the east and northeast. Small outliers occur near Taynuilt and either side of Loch Avich. The deposits are especially obvious on Kerrera where they form the bedrock across half of the island. These are conformably overlain by peperite and the basaltic and andesitic Lorne plateau lavas. The ORS on Kerrera and isolated localities around Oban are known for their fossils, particularly fish.

Midland Valley of Scotland The Midland Valley graben defined by the Highland Boundary Fault in the north and the Southern 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 extensive volcanism. There is a continuous outcrop along the Highland Boundary Fault from Stonehaven on the North Sea coast to Helensburgh and beyond to Arran. A more disconnected series of outcrops occur along the line of the Southern Uplands Fault from Edinburgh to Girvan. Old Red Sandstone often occurs in conjunction with conglomerate formations, one such noteworthy cliffside exposure being the Fowlsheugh Nature Reserve, Kincardineshire.

Scottish borders A series of outcrops occur from East Lothian southwards through Berwickshire. Hutton's famous unconformity at Siccar Point occurs within this basin – see History of study below.

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