The UK: Palaeoclimates

DateJan 21, 2019
AuthorLucy Speller Bsc (Hons) FGS
Categories

Mainland UK has a varied geology, meaning no two areas of the country have identical underlying ground conditions. This is, in part, due to varying positions of the UK (in terms of latitude) and the resulting palaeoclimates and environments throughout geological time.

Southern Hemisphere

Cambrian lithology in the UK was deposited near the South Pole in cold, shallow seas (BGS, 2017) as part of a marine basin of Gondwana. Submarine turbidites, wackes and slates today seen in Wales would have been deposited as mud slowly accumulated within marine basins, undergoing periodic mudslides due to the proximity to deltas on the mainland Pangaea, of which Gondwana was a part. Volcanism was also heavily prevalent at this time and blueschists in North Wales indicates the UK was located near an active subduction zone (Jackson and Stone, 2015).

During the Ordovician, the UK was still experiencing a cool climate at its latitude south of the Equator (BGS, 2017). Although Scotland had not yet joined England and Wales on the UK’s journey north, the Tethys Ocean separating the countries was beginning to close, causing volcanic activity in the area. This is can be seen in Wales and the Lake District, especially around Scafell (Stone et al., 2010).

Subtropical/Tropical Zone

The Silurian period brought in a tropical/subtropical climate, with the UK still south of the equator. The south of the UK is evidenced to have been covered by shallow seas, leading to deposits of limestone, while deeper marine environments were present further north, leading to sandstone and mudstone deposits (BGS, 2017). Silurian strata can be seen today in Wales, Cumbria, the Lake District and Southern Scotland, however the lithologies vary considerably between areas as England and Wales were part of Avalonia (warm shallow seas), while Scotland was part of Laurentia (deeper marine). By the end Silurian, the Iapetus Ocean separating the two palaeocontinents had closed and what is now mainland Britain was no longer divided (Woodcock, 2000).

During the Devonian, the UK was part of a desert at the base of the Caledonian Mountains (Britannica, 2010); located in the tropics south of the equator (BGS, 2017); and experienced a semi-arid climate (Barclay et al., 2005). This led to the deposition of Old Red Sandstone, currently outcropping in Devon and Dorset, Scotland and the Southern Lake District (BGS, 2017). This iron rich formation is essentially remnants of the Devonian desert. During the same time period, marine limestones, sandstones and mudstones were also formed in a marginal ocean basin (Rheic Ocean), and during the late Devonian/early Carboniferous, this ocean began to close (Shail & Leveridge, 2009; Neace, 2015). This closure likely led to the formation of the Devonian slates seen in the area today. The Rheic Ocean then began to reopen, and as the ancient continents of Gondwana and Laurussia collided during the Variscan Orogeny (Floyd et al, 1993; Neace, 2015), hydrothermal mineralisation and the formation of the Cornubian Batholith occurred (of which Dartmoor and Exmoor granites are a part of), leading to the formation of mineral ores (Neace, 2015); accounting for the South West’s mining history – Cornish Tin for example.

The Lizard Point in Cornwall is unique in that it is a section of Pre-Cambrian oceanic crust thrust up over the continental plate; this occurred during the middle of the Devonian period (Natural Lizard, 2018).

Equatorial Cyclothems

The Carboniferous saw a variety of environments, situated near the equator, the UK was gradually drifting northwards. Shallow tropical seas saw limestone formation, before coastal swamps with forests of giant ferns and deltas dominated the landscape (BGS, 2017). These cyclic fluctuations between environments can be seen today as coal cyclothems, which were deposited by subsiding swamps that flooded and were later infilled with accumulating sediment. In these areas, lithology varies between interbedded bands of sandstone, mudstone and coal. This is generally seen in the UK today as a South West – North East trending band from the South Coast of England across the Pennines. The formation of the Pennines uplifted the Carboniferous strata and subsequent erosion has allowed us to see the underlying geology at the surface (Northern Mine Research Society, 2018).

Northern Tropical Zone

Drifting northwards, the UK had reached the northern tropics by the time of the Permian (Jackson and Stone, 2015). The area would have experienced hot, dry, desert conditions as part of the supercontinent, Pangaea (BGS, 2017), leading to the formation of sandstone in an environment similar to present day Death Valley (Jackson and Stone, 2015). This strata can be found in localised areas around the Pennines and Lake District. There is evidence of an inland sea (Zechstein), in the form of limestones and evaporites (BGS, 2017). The Mendips in Somerset would have been part of a mountain range taller than 1500m during this time period, where flash floods would lead to the accumulation of debris, now seen as deposits of conglomerate (BGS and NERC, 2017).

The evaporates from the Permian continued to form in a similar environment well into the Triassic, where salt marshes within inland seas evaporated due to the surrounding hot desert conditions, to form thick deposits of salt within lime rich marl, as seen in Cheshire (Compass Minerals). Sandstones continued to form in the desert environment, while shallow lakes and rivers led to mudstone formation (BGS, 2017). Triassic red sandstone can be seen in the Dorset and East Devon coast (Ursula's Weekly Wanders, 2014). The first dinosaurs also appeared during the Triassic (BGS, 2017). Mercia Mudstone and limestones found to the South of the Mendips are evidence of rising sea levels in the area towards the end Triassic, when the Mendip Mountains became islands in a sub-tropical shallow sea and eventually developed into an archipelago (BGS and NERC, 2017).

Northern Hemisphere

Jurassic outcrops can be found along a band between Dorset and North Yorkshire in addition to South Wales (Jackson and Stone, 2014). Mainly interbedded limestone, mudstone and sandstone, these strata were formed during a period of varying sea levels and depths within shallow tropical seas and tropical swamps, due to the expanding marine conditions and deepening seas (BGS, 2017). Abundant marine life thrived in latitudes 30 – 40° north. Evidence of coral reefs can be found in Yorkshire as limestones along with finer grained clays such as Kimmeridge Clay, the source rock for hydrocarbons, which was deposited in a deeper marine environment (Jackson and Stone, 2014).

During the Cretaceous, the UK again experienced a warm marine environment at around 35° North. This was a time where warm lagoonal lakes and fluvial areas, where clay deposits formed, was followed by a period of rising sea levels. Warm shallow seas (Tethys) teeming with coccoliths formed, eventually accumulating into the chalk beds of South East England (BGS, 2017). The Alpine Orogeny between 65 and 2.5 Ma brought about the closure of the Tethys Ocean, and as the African and Eurasian plates collided, the undulations of the North and South Downs in Surrey and Sussex were formed (The Geological Society, 2018).

From the Palaeogene, the UK was still drifting north with a terrestrial tropical swamp environment, although the climate was starting to cool. By the Neogene, the climate continued to cool as the UK drifted further north into its present position and was further defined by a shift to a marine environment with cold and temperate seas. During the Quaternary the world was experiencing ice age conditions; glaciation in the north and middle of Britain shaped the topography to the current landscape while first humans appeared on the scene (BGS, 2017).

Today

The Groundsure Geo Insight, designed by geo-environmental specialists, utilises the BGS GeoSure Hazard Ratings to give a susceptibility rating for subsidence based on the underlying geology, in addition to providing information on a variety of other natural and non-natural ground hazards. The Cheshire Salt Search may also be of interested to potential homebuyers in the area https://www.groundsure.com/cheshire-salt-search.

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