Weald Basin
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The Weald Basin () is a major topographic feature of the area that is now southern England and northern France from the
Triassic The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...
to the
Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the more recent of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''cre ...
. Its uplift in the Late Cretaceous marked the formation of the Wealden Anticline. The rock strata contain
hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and Hydrophobe, hydrophobic; their odor is usually fain ...
deposits which have yielded coal, oil and gas.


Formation

The Weald Basin's formation commenced during the
Carboniferous The Carboniferous ( ) is a Geologic time scale, geologic period and System (stratigraphy), system of the Paleozoic era (geology), era that spans 60 million years, from the end of the Devonian Period Ma (million years ago) to the beginning of the ...
, with the rocks which are today basement deposited within a low swamp providing coals which were exploited to the north and east in
Kent Kent is a Ceremonial counties of England, ceremonial county in South East England. It is bordered by Essex across the Thames Estuary to the north, the Strait of Dover to the south-east, East Sussex to the south-west, Surrey to the west, and Gr ...
, but boreholes drilled in the 19th century failed to find this deposit in the area of the Weald. The Carboniferous coals may be overlain by early Triassic sediments. The sediments were uplifted and faulted within the
Variscan Orogeny The Variscan orogeny, or Hercynian orogeny, was a geologic mountain-building event caused by Late Paleozoic continental collision between Euramerica (Laurussia) and Gondwana to form the supercontinent of Pangaea. Nomenclature The name ''Varis ...
, with the land now occupied by the Weald Basin being a low external fold belt to the main orogeny, which was located within the present day English Channel. The remnants of the mountain belt can be seen today in Devon and Cornwall in what is known as the Cornubian Massif. Unlike in Devon and Cornwall the deformation caused little or no metamorphism. The mountain belt collapsed soon after the orogeny, leading to the former northward thrusts to be reactivated as normal faults, and led to the formation of the Weald basin, which developed as an extension of the considerably larger Wessex Basin . Reconstructions of the geometry of the early fault systems in the Weald Basin reveal that for the early history of the basin a series of steep normal faults to the north were active against the London-Brabant Massif, but it is not clear whether this reflects a
syn-rift In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-graben ...
origin for these rocks. The Weald basin gently subsided throughout the Jurassic, Cretaceous and Early Tertiary leading to a thick succession of sedimentary rocks being deposited. During the
Early Jurassic The Early Jurassic Epoch (geology), Epoch (in chronostratigraphy corresponding to the Lower Jurassic series (stratigraphy), Series) is the earliest of three epochs of the Jurassic Period. The Early Jurassic starts immediately after the Triassicâ ...
a north and east shallowing mud-dominated shelf developed.


Basin reactivation

As a result of the Alpine orogeny the basin was squeezed between the basement to the north and the south, this resulted in the reactivation of the formerly normal faults into minor thrusts (as they had been during the
Variscan Orogeny The Variscan orogeny, or Hercynian orogeny, was a geologic mountain-building event caused by Late Paleozoic continental collision between Euramerica (Laurussia) and Gondwana to form the supercontinent of Pangaea. Nomenclature The name ''Varis ...
) and the formation of the Wealden Anticline. The region's two surface structural highs (areas of crust and rocks which are uplifted), the Wealden Anticline and the Channel High, are superimposed upon earlier, Mesozoic basins, (the Weald and Channel basins). This nearly exact superimposition of compressional features upon underlying formerly extensional features exemplifies perfectly the principles of structural inversion across a large and well defined geologic feature. The overall uplift produced by the
Tertiary Tertiary (from Latin, meaning 'third' or 'of the third degree/order..') may refer to: * Tertiary period, an obsolete geologic period spanning from 66 to 2.6 million years ago * Tertiary (chemistry), a term describing bonding patterns in organic ch ...
inversion in the eastern Wealden basin has been estimated to be as much as 1525 m; which as a result of the large amount of
Mesozoic The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...
sediments does not reveal the underlying
Paleozoic The Paleozoic ( , , ; or Palaeozoic) Era is the first of three Era (geology), geological eras of the Phanerozoic Eon. Beginning 538.8 million years ago (Ma), it succeeds the Neoproterozoic (the last era of the Proterozoic Eon) and ends 251.9 Ma a ...
basement. Utilising estimations of the original thickness of
Chalk Chalk is a soft, white, porous, sedimentary carbonate rock. It is a form of limestone composed of the mineral calcite and originally formed deep under the sea by the compression of microscopic plankton that had settled to the sea floor. Ch ...
(400–460 m) and other Mesozoic strata indicate a complex fold structure which, in the event it was not eroded during uplift, attained a crestal elevation of 1400 m over what is today
Ashdown Forest Ashdown Forest is an ancient area of open heathland occupying the highest sandy ridge-top of the High Weald National Landscape. It is situated south of London in the county East Sussex, England. Rising to an elevation of above sea level, its ...
. However it is likely that erosion kept pace with uplift, resulting in large quantities of sediment supply to the North Sea and the English Channel


Economic resources

Coal seams were discovered when test boring for an early proposal for a
Channel Tunnel The Channel Tunnel (), sometimes referred to by the Portmanteau, portmanteau Chunnel, is a undersea railway tunnel, opened in 1994, that connects Folkestone (Kent, England) with Coquelles (Pas-de-Calais, France) beneath the English Channel at ...
at
Dover Dover ( ) is a town and major ferry port in Kent, southeast England. It faces France across the Strait of Dover, the narrowest part of the English Channel at from Cap Gris Nez in France. It lies southeast of Canterbury and east of Maidstone. ...
in 1890. This led to the development of four deep mines in the Kent Coalfield in the early 20th century. The Weald Basin has yielded significant quantities of
gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate Hydrate, dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, drywall and blackboard or sidewalk ...
from
Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...
Purbeck beds The Purbeck Group is an Upper Jurassic to Lower Cretaceous lithostratigraphic group (a sequence of rock strata) in south-east England. The name is derived from the district known as the Isle of Purbeck in Dorset where the strata are exposed in th ...
and a number of brickworks exploit the lower Cretaceous clays. The inversion of the Weald Basin throughout the late Cretaceous and early Tertiary resulted in the formation of the Wealden Anticline and a number of smaller anticlines within the larger structure. The discovery in 1897 of
natural gas Natural gas (also fossil gas, methane gas, and gas) is a naturally occurring compound of gaseous hydrocarbons, primarily methane (95%), small amounts of higher alkanes, and traces of carbon dioxide and nitrogen, hydrogen sulfide and helium ...
while drilling for water at Heathfield railway station provided fuel for the first natural gas lighting in the United Kingdom. The existence of the same strata within the Weald basin which are the source rocks for the
Wytch Farm Wytch Farm is an oil field and processing facility in Dorset, England. It is the largest onshore oil field in Western Europe. The facility, taken over by Perenco in 2011, was previously operated by BP. It is located in a coniferous forest on Wytc ...
oilfield in
Dorset Dorset ( ; Archaism, archaically: Dorsetshire , ) is a Ceremonial counties of England, ceremonial county in South West England. It is bordered by Somerset to the north-west, Wiltshire to the north and the north-east, Hampshire to the east, t ...
led to an interest in the petroleum potential of the Wealden anticline, with exploration taking place on Ashdown forest examining the Ashdown Anticline, a large structure over 30 km long x 7 km wide, located in the centre of the Weald Basin in north
Sussex Sussex (Help:IPA/English, /ˈsʌsÉȘks/; from the Old English ''SĆ«ĂŸseaxe''; lit. 'South Saxons'; 'Sussex') is an area within South East England that was historically a kingdom of Sussex, kingdom and, later, a Historic counties of England, ...
; significant quantities of natural gas were found but oil was absent. Oil and gas have subsequently been found at a number of sites in the Weald including Singleton and Storrington in West Sussex,
Godstone Godstone is a village and civil parishes in England, civil parish in the Tandridge District of Surrey, England. It is east of Reigate, west of Oxted, east of Guildford and south of London. Close to the North Downs, both the North Downs Way ...
and Lingfied in Surrey, and Cowden in Kent. In 2009 remaining recoverable oil reserves in the Weald Basin were estimated at one and a half million tonnes. In 2010 the Weald Basin contributed 18% of onshore gas and less than 5% of onshore oil production in the UK. As of August 2013 there was significant opposition to
hydraulic fracturing Fracking (also known as hydraulic fracturing, fracing, hydrofracturing, or hydrofracking) is a well stimulation technique involving the fracturing of Formation (geology), formations in bedrock by a pressurized liquid. The process involves the ...
developing in southeast England centred on Balcombe where an exploratory well was planned and the Balcombe drilling protest was in progress. A BGS/ Department for Energy and Climate Change (DECC) report from May 2014 suggest that there is the possibility for the extraction of
light tight oil Tight oil (also known as shale oil, shale-hosted oil or light tight oil, abbreviated LTO) is light crude oil contained in unconventional petroleum-bearing formations of low permeability, often shale or tight sandstone. Economic production from ...
(LTO) in Weald Basin and the average figure of is suggested. The overall range of estimations is from . The data is said to have a "high degree of uncertainty", and the amount that could be produced is unknown, and could be zero.


See also

* The
Weald The Weald () is an area of South East England between the parallel chalk escarpments of the North and the South Downs. It crosses the counties of Hampshire, Surrey, West Sussex, East Sussex, and Kent. It has three parts, the sandstone "High W ...
region in
South East England South East England is one of the nine official regions of England, regions of England that are in the ITL 1 statistical regions of England, top level category for Statistics, statistical purposes. It consists of the nine counties of england, ...
* Geology of East Sussex *
Wessex Basin The Wessex Basin is a petroleum-bearing geological area located along the southern coast of England and extending into the English Channel. The onshore part of the basin covers approximately 20,000 km2 and the area that encompasses the Englis ...
, and its oil fields


References

{{Reflist Geology of England Hydraulic fracturing in the United Kingdom Oil fields of England Sedimentary basins of Europe Structural geology