GCR block - Non-Marine Devonian (NMAR-DEV)

Non-Marine Devonian

Block Description

UK map showing distribution of GCR sites of GCR block NMAR-DEV
Distribution of GCR sites of GCR block Non-Marine Devonian

The GCR sites selected for this GCR Block represent the British geological record of Earth history from about 417 to 354 million years ago (Ma) as represented by non-marine rocks called the ‘Old Red Sandstone’ of Devonian Age. Old Red Sandstone strata of Prídolí (Silurian) age (419–417 Ma) are afforded their own GCR Block (see PRIDOL and discussion below). Strata laid down in the Devonian Period (the fourth Period of the Palaeozoic Era, 540–251 Ma) constitute the Devonian System. Traditionally, the Old Red Sandstone rocks in Britain were regarded as wholly Devonian age, and as the terrestrial equivalents of the marine Devonian rocks of south-west England (see MAR-DEV). The age intervals, or stages, of the Devonian Period, defined in the marine rocks of continental Europe, can be applied to the terrestrial Old Red Sandstone succession, albeit with some difficulty because of its absence of marine fossils.

In the past, the base of the Old Red Sandstone was placed at the base of the Ludlow Bone Bed, a very thin, lenticular, phosphatised lag deposit present marking the top of the Silurian Ludlow Series in South Wales and the Welsh Borderland. However, the modern definition of the base of the Devonian System (in the Czech Republic, at the base of the Monograptus ultimus Biozone) now places the basal parts of the Old Red Sandstone succession in the Silurian System. The strata from the Ludlow Bone Bed up to the base of the Devonian, as yet poorly defined in the Old Red Sandstone, belong to the Pridoli Series, the fourth, uppermost series of the Silurian System see PRIDOL.

The Old Red Sandstone is one of the two major red-bed sedimentary unit in Great Britain, the other being the younger Permo-Triassic rift basin-fill, formerly termed the ‘New Red Sandstone’ (see PER-TRI) to distinguish it from the Old Red Sandstone.

Outcrop pattern

The Old Red Sandstone rocks crop out principally in four main areas in Great Britain, which broadly reflect the original basins in which they were deposited. These are: the Orkney and Shetland islands and north-east Scotland (the Orcadian Basin); the Midland Valley of Scotland (in an amalgamation of several basins of which the largest was the Strathmore Basin); Northumberland (the Northumberland and Solway basins); and south Wales and the Welsh borderland (the Anglo-Welsh Basin).

Palaeogeography and palaeoenvironment

The Old Red Sandstone represents a period when ocean closure and continental collisions resulted in a world geography hugely different to that of much of the early part of the Palaeozoic Era. The Iapetus Ocean, which separated the northern (Laurentian) and southern (Gondwana) continents during the Ordovician Period closed throughout the Silurian Period as the smaller continent of Avalonia fragmented from Gondwana and drifted northwards. As the Iapetus Ocean closed north of Avalonia, the Rheic Ocean opened behind it. To the east, the continent of Baltica also drifted northwards and eastwards and the Tornquist Sea, an arm of the Iapetus between it and Laurentia, slowly closed. Baltica and the eastern part of Avalonia collided first, in Late Ordovician time, moving northwards together to make first contact with Laurentia in the early part of the Silurian Period. By late Silurian (Ludlow) time, the continents had fully docked, with the Iapetus Ocean closed along the line of subduction (the Iapetus Suture) under the Southern Uplands. Thus, the Caledonian–Appalachian Orogen and the newly amalgamated Old Red Sandstone continent (also named Laurussia and Euramerica) was formed (see CAL-STR-LD). Palaeogeographic reconstructions suggest that the continent lay in tropical to sub-tropical latitudes from the equator to about 30ºS. Sedimentological studies of the Old Red Sandstone indicate a warm to hot, tropical setting with rainfall confined to wet seasons. Uplift of the orogen may have caused broad variations in the rainfall pattern, producing periods of wetter and drier climate.

Continuing compression and shortening of the continental crust resulted in the filling to sea level of the Silurian basins, their inversion to upland areas and the establishment of terrestrial conditions in newly developing basins. The oblique angle of impingement of the former Laurentian and East Avalonian continents resulted in sinistral transpressive movement, thrusting along major NE-trending faults, and granitic intrusion, andesitic volcanicity and low-grade metamorphism in northern Britain. The compression in the Laurentian crust, of which the Scottish Highlands were part, was accompanied by the intrusion of large volumes of granitic rocks and the extrusion of predominantly andesitic volcanic rocks (see CAL-IGN, ORS-IGN).

The Orcadian Basin was a large mid-Devonian intramontane lake basin, totally unconnected to the open sea, apart perhaps from a brief period. The Midland Valley of Scotland was not a single discrete basin in the Devonian. Weakened by a long history of igneous activity, internal, transtensional fault movements opened pull-apart basins and transpressive movements subsequently inverted them, resulting in the recycling of the basin-fills and providing weak points for continuing volcanic extrusion. The preserved sequences thus represent the deposits of separate pull-apart basins, formed and brought together in a strike-slip faulted collage. The Stonehaven Basin in the north-east is the earliest, its fill dating perhaps from the Wenlock Epoch. It and its larger successor basins, the Crawton and Strathmore basins, formed by sinistral strike-slip along the Highland Boundary Fault. The southerly Lanark Basin formed along the Southern Uplands Fault. Large volumes of arc-related volcanic rocks were extruded along the central axis of the Midland Valley, on lines weakened by the transtensional stresses. The late Silurian to early Devonian Solway and Northumberland basins formed in the Southern Uplands Terrane and were probably isolated from the basins of the Midland Valley to the north. The late Devonian Scottish Borders Basin formed after Acadian inversion in the mid-Devonian and extended into the Midland Valley.

The Anglo-Welsh Basin is interpreted primarily as the product of load-generated flexural subsidence of the Caledonian foreland. Superimposed on this were transtensional and extensional movement on faults that produced variations in the basin fill in Pembrokeshire and introduced coarse clastic detritus farther north. The isolated succession in Anglesey was probably deposited contiguously with the Přídolí sequences to the south, although the initial coarse conglomerates are unique and of local derivation, and lacustrine deposits suggest internal or impeded drainage; the effect of the nearby Caledonoid faults requires examination.

GCR site selection

The four networks covered by this GCR Block are essentially the four main outcrops of the Old Red Sandstone, arranged geographically from north to south, and which correspond to, or closely reflect, the original depositional basins.

• Orcadian Basin

• Midland valley of Scotland

• Northumberland and Solay basins

• Anglo-Welsh Basin

Although the relatively common invertebrate fossils do not have a separate selection category in the GCR in their own right, the scientific importance of many stratigraphy sites lies in their fossil content. Therefore, some of the GCR sites are selected specifically for their fossil fauna, which facilitates stratal correlation and enables the interpretation of the environments in which the animals lived. Moreover, some sites have international significance because they have yielded fossils that are the ‘type’ material for a taxonomic group.

In contrast to the manner in which most invertebrate fossils are represented in the GCR, fossils of vertebrates, arthropods (except trilobites) and terrestrial plants do have their own dedicated selection categories, owing to the relative rarity of the fossil material. See APD; SIL-DEV-CH; PAZ-PALBOT; PALENT.

Palaeontology, fauna and flora

In the Devonian Period there are biostratigraphical zonal schemes for graptolites, ammonoids, brachiopods, fish, conodonts, microvertebrates and miospores.

The problems of classification and correlation of the Old Red Sandstone of Britain are inherent in its terrestrial origins and the patchy preservation of its non-marine fossils.

The fossils that are present indicate that the Devonian Period saw profound changes in the evolutionary record, with the first significant colonisation of terrestrial habitats by vascular plants, the rapid expansion of the first aquatic vertebrates, and their emergence onto land. However, no direct correlations can be made with the European marine successions and the internationally agreed stages. Because of this, a series of loosely defined local stages (Downtonian, Dittonian, Breconian and Farlovian) were erected for the Anglo-Welsh Basin. Although now largely subsumed into the international stages as a result of increasing refinement in correlation, they continue to be used as a matter of convenience in describing the Anglo-Welsh succession.

The principal macrofossils of stratigraphical use are fish fragments. Miospore classifications and microvertebrate classification also aid correlation and stratigraphical resolution.

Volume Introduction

Non-Marine Devonian

Site List

Your selection found 59 GCR sites. Sites are sorted alphabetically by country, local authority and then by site name -
CodeNameCountryLocal AuthorityGrid RefGCR Site Account
1805GlenthorneEnglandDevon CCSS795499Not available
2820Pooley Bridge SectionEnglandEast CumbriaNY465243Not available
1799LydneyEnglandGloucestershireSO652016Not available
1806Wilderness (Land Grove) QuarryEnglandGloucestershireSO672185Not available
1808Ross-on-Wye, Royal HotelEnglandHerefordshire, County ofSO597240Not available
1807The ScarEnglandHerefordshire, County ofSO354444Not available
1804PortisheadEnglandNorth and North East Somerset, South GloucestershireST461770Not available
1818Crawton BayScotlandAberdeen City and AberdeenshireNO879797Not available
3334Den of Findon, Gamrie Bay and New AberdourScotlandAberdeen City and AberdeenshireNJ839662Not available
3335Dunnottar Coast SectionScotlandAberdeen City and AberdeenshireNO881839Not available
3336Milton NessScotlandAberdeen City and AberdeenshireNO796647Not available
1809North Esk RiverScotlandAberdeen City and AberdeenshireNO595719Not available
3325RhynieScotlandAberdeen City and AberdeenshireNJ494277Not available
3326The TouttiesScotlandAberdeen City and AberdeenshireNO881866Not available
3327Aberlemno QuarryScotlandAngus and Dundee CityNO526551Not available
3328Tillywhandland QuarryScotlandAngus and Dundee CityNO528537Not available
1810Whiting NessScotlandAngus and Dundee CityNO679432Not available
3323Achanarras QuarryScotlandCaithness and Sutherland and Ross and CromartyND150544Not available
3332John o' GroatsScotlandCaithness and Sutherland and Ross and CromartyND390738Not available
1814Pennyland to Castlehill (Thurso-Scrabster)ScotlandCaithness and Sutherland and Ross and CromartyND102696Not available
1813Red PointScotlandCaithness and Sutherland and Ross and CromartyNC932659Not available
3333SarcletScotlandCaithness and Sutherland and Ross and CromartyND352433Not available
1821Tarbat NessScotlandCaithness and Sutherland and Ross and CromartyNH929873Not available
3324Tynet BurnScotlandCaithness and Sutherland and Ross and CromartyNJ383618Not available
1812Wick QuarriesScotlandCaithness and Sutherland and Ross and CromartyND377498Not available
3337Glen ValeScotlandClackmannanshire and FifeNO183062Not available
1815Largs CoastScotlandEast Ayrshire and North Ayrshire MainlandNS191633Not available
1816North Newton ShoreScotlandEast Ayrshire and North Ayrshire MainlandNR933518Not available
1817Dun Chia Hill (Loch Duntelchaig)ScotlandInverness and Nairn and Moray, Badenoch and StrathspeyNH601286Not available
3330Auchensail QuarryScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNS342799Not available
3322Bay of BerstaneScotlandOrkney IslandsHY476111Not available
1794Greenan Nev CoastScotlandOrkney IslandsHY549367Not available
1819Old Man of Hoy CoastScotlandOrkney IslandsHY211052Not available
1795South Fersness BayScotlandOrkney IslandsHY531346Not available
1792South Stromness Coast SectionScotlandOrkney IslandsHY223102Not available
1793Taracliff Bay to Newark BayScotlandOrkney IslandsHY553035Not available
1796Yesnaby and Gaulton Coast SectionScotlandOrkney IslandsHY222144Not available
1820Tay Bank SectionScotlandPerth and Kinross and StirlingNO125328Not available
3329Wolf's Hole QuarryScotlandPerth and Kinross and StirlingNS790981Not available
1797Easter Rova HeadScotlandShetland IslandsHU475454Not available
1798Footabrough to Wick of WatsnessScotlandShetland IslandsHU179502Not available
3331Melby: Matta Taing to Lang RiggScotlandShetland IslandsHU170568Not available
1823The Cletts, ExnaboeScotlandShetland IslandsHU409137Not available
1822Palmers Hill Rail CuttingScotlandThe Scottish BordersNY548985Not available
1811Siccar PointScotlandThe Scottish BordersNT812709Not available
3227Craig-y-cwmWalesGWENT VALLEYSSO282089Not available
3224Cusop DingleWalesHerefordshire, County ofSO233421Not available
1738Porth-y-MorWalesISLE OF ANGLESEYSH492884Not available
1741Abercriban QuarriesWalesPOWYSSO064123Not available
3254Afon y WaenWalesPOWYSSN976147Not available
3226Dyffryn CrawnonWalesPOWYSSO094150Not available
1740Heol Senni QuarryWalesPOWYSSN915221Not available
1731Caeras QuarryWalesSOUTH WEST WALESSN607167Not available
1735Freshwater East to Skrinkle Haven (Tenby Cliffs)WalesSOUTH WEST WALESSS021981Not available
3228Freshwater WestWalesSOUTH WEST WALESSR884996Not available
1737LlansteffanWalesSOUTH WEST WALESSN350100Not available
3225Pantymaes QuarryWalesSOUTH WEST WALESSN913265Not available
1736Sawdde GorgeWalesSOUTH WEST WALESSN727247Not available
1733West Angle Bay (North)WalesSOUTH WEST WALESSM852034Not available
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