Wessex Basin

The Wessex Basin is a petroliferous geological area located along the southern coast of England and extending into the English Channel.[1] The onshore part of the basin covers approximately 20,000 km2 and the area that encompasses the English Channel is of similar size.[2] The basin is a rift basin that was created during the Permian to early Cretaceous in response to movement of the African plate relative to the Eurasian plate.[3] In the late Cretaceous, and again in the Cenozoic, the basin was inverted as a distant effect of the Alpine orogeny.[3] The basin is usually divided into 4 main sub-basins including the Winterborne-Kingston Trough, Channel Basin, Weald Basin, and Vale of Pewsey Basin.[3] The area is also rich in hydrocarbons with several offshore wells in the area.[4] With the large interest in the hydrocarbon exploration of the area, data became more readily available and therefore came a better understanding of the type of inversion tectonics that characterize this basin.[3][5]

Wessex basin lithostratigraphy
Lithostratigraphy of the Wessex Basin

Tectonic mechanisms

The basin was first created in response to a divergent boundary between the African and Eurasian plates.[3] Lithospheric thinning and crustal extension led to the subsidence and consequently the creation of asymmetrical graben.[6] These events happened in pulses starting in the Permian and concluding in the early Cretaceous period.[6] By mid Cretaceous the active crustal extension ceased and the region underwent a period of unfaulted subsidence in part due to the thermal relaxation effects from the earlier lithospheric action.[3] Inversion began starting in the late Cretaceous and carried on through Tertiary times coinciding with the Alpine orogeny happening at the now convergent boundary of the African and Eurasian plates.[3]

Basement structure and stratigraphy

Starting at the basement of the structure, the area of interest overlies Variscan externides consisting of Devonian and Carboniferous sediments in imbricated thrust sheets.[3] The Hercynian basement was rather thin in origin with the thrust emplacement in a north-northwest trend developed most likely with a strike-slip fault.[3] After the conclusion of the initial Hercynian deformation, normal reactivation of these same faults occurred along with differential subsidence beginning in the late Carboniferous as a direct result from the northwest/southeast trending wrench movements.[3] The sands created within this period lie in an unconformed manner atop the Devonian and Carboniferous basement.[3] The asymmetrical grabens are found within these sands and through the use of dating techniques mark the point in history of basin initiation during the late Carboniferous.[3] Following this stage the basin underwent a period of erosion removing approximately 10 km of sediment from the area with the following Permian sediment deposition being dictated by the former Hercynian structure.[3] This semi-arid, desert sedimentation occurred on the western portion of the basin with the clasts eventually thinning as they progress towards the east side of the basin.[3] Triassic sediments sit atop of the Permian layer again in an unconformed manner with the makeup being mainly sand and silts.[3] Like the former stage, the Triassic sediments were first concentrated on the west side of the basin but also overwhelmingly appeared in the southwest as a direct result from marine transgression.[3] The Jurassic period followed with the same marine sedimentation, but by the end of the period, sea-level began to shallow leaving behind shallow marine sediments.[3] The Cretaceous period marks a major transition as the area became unstable with the basement faults reactivating and additional deposition of 1000m of brackish non-marine/freshwater sediments along with major uplift of the basin margins.[3] The later Cretaceous consisted of deposition almost entirely of chalk with a basin wide unconformity delineating the period between the Tertiary and Late Cretaceous.[3] These sediments are important as they represent the marker bed that the inversion of the basin begins.[4]

Basin inversion

One of the most obvious signs of the beginning of basin inversion is that the timing of the Wessex Basin inversion correlates directly with other adjacent inversion basins.[3] The basin's inversion mechanism can be traced back to the movements of the alpine orogeny that resulted in a series of monoclines along the boundary faults and the uplift of pre-tertiary sediments.[3] Inversion can happen along reactivated fault lines and in this case the faults involved are the Mesozoic tensional growth faults.[3] The reverse movements of the tensional faults in response reactivated the basement faults but this time as thrust faults.[3] Former structural highs underwent inversions to become areas where large amounts of sediments began being deposited, while also slowly deepening to the basement faults.[3] The transpression of the whole are is the most probable cause for the subsidence of the former basin high points.[3] The amount of deepening of a former high structure correlates directly with the amount of over thrusting.[3] Simply, the faults have to completely reverse the previous movements and then some in order to see a reversal of structure changes.

In addition, there are several sediment depositional environments that also help catalog when and where initial basin inversion was happening. Basin erosion during the late Cretaceous and early Tertiary is one of the first indicators along with a regional change in the chalk composition to non-marine fluvial sediments.[3] Major chalk fissures also became infilled with Paleocene sediments.[3] There are east-west trending styolites specifically in the Pubreck chalk formation that date the onset of the compression, the start of the inversion.[3] Additionally, Eocene paleosols created during the late Cretaceous south of the Purbeck- Isle of Wright fault line is a direct contrast of the marine sediments of the same age to the north.[3]

Presently the basin is described with a series of east-west trending monoclonal and antiforms that lie above the underlying Mesozoic growth faults.[3] The Purbreck-Isle of Wright fault is one of the areas where inversion of the basin is distinctly seen with pre and post inversion structures.[3]

Hydrocarbons

Hydrocarbon exploration led to immense seismic profiling in the areas of South Dorset, Hampshire, and the Isle of Wright.[4] The petroleum geology is primarily confided in the Mesozoic strata being first drilled in 1937.[7] The Kimmeridge oilfield is an offshore field that was first discovered in the 1950s is the largest in the area and is still in production to this day.[7] Wytch Farm is the major onshore field that entered production during the 1970s.[7] This basin stems from three major source rocks known as the Blue Lias formation with Liassic clays, the Oxford Clay formation, and the Kimmeridge clay formation.[7]

See also

References

  1. ^ Stoneley, R (January 1982). "The Structural Development of the Wessex Basin". Journal of the Geological Society. 139: 543–554 – via Science Direct.
  2. ^ Chadwick, R.A (January 1993). "Aspects of Basin Inversion in Southern Britain". Journal of the Geological Society. 150: 311–322 – via Science Direct.
  3. ^ a b c d e f g h i j k l m n o p q r s t u v w x y z aa ab ac ad ae af Lake, Stuart; Karner, Garry D. (1987). "The Structure and evolution of the Wessex Basin, Southern England: An Example of Inversion Tectonics". Tectonophysics. 137: 347–378 – via Science Direct.
  4. ^ a b c Underhill, J.R; Paterson, S. (January 1998). "Genesis of Tectonic Inversion Structures: Seismic Evidence for the Development of Key Structures along the Purbeck-Isle of Wight Disturbance". Journal of the Geological Society. 155: 975–992 – via Science DIrect.
  5. ^ Worden, R.H; Benshatwan, M.S; Potts, G.J; Elgarmadi, S.M (2016). "Basin Scale Fluid Movement Patterns Revealed by Veins Wessex Basin U.K". Geofluids. 16: 149–174 – via Science Direct.
  6. ^ a b Chadwick, R.A (1986). "Extension Tectonics in the Wessex Basin, Southern England". Journal of the Geological Society. 143: 465–488.
  7. ^ a b c d Zanella, A (2015). "Natural Hydraulic Fractures in the Wessex Basin, SW England: Widespread Distribution, Composition and History". Marine and Petroleum Geology. 68: 438–448 – via Science Direct.

External links

Coordinates: 50°40′N 2°0′W / 50.667°N 2.000°W

Beacon Limestone Formation

The Beacon Limestone Formation is a formation of early Jurassic age (Pliensbachian–Toarcian). It lies above the Dyrham Formation and below the Bridport Sand Formation. It forms part of the Lias Group. It is found within the Wessex Basin and parts of Somerset, in England.

Bridport Sand Formation

The Bridport Sand Formation is a formation of Toarcian (Early Jurassic) age found in the Worcester and Wessex Basins of central and southern England. It forms one of the reservoir units in the Wytch Farm oilfield in Dorset. The sandstone is very-fine grained to fine-grained and contains regular narrow bands that are calcite-cemented and more resistant to weathering, giving it a characteristic banded appearance at outcrop, such as in the cliffs between Bridport and Burton Bradstock in Dorset. It is named for Bridport and has previously been known as the Midford Sand(s), Cotteswold Sands, Yeovil Sands and Upper Lias Sand(s). It forms a locally important aquifer, particularly around Yeovil.

Charmouth Mudstone Formation

The Charmouth Mudstone Formation is a geological formation in England. It preserves fossils dating back to the early part of the Jurassic period (Sinemurian–Pliensbachian).

Corallian Limestone

Corallian Limestone or Corallian Group is a coralliferous sedimentary rock, laid down in the Oxfordian stage of the Jurassic. It is a hard variety of "coral rag". Building stones from this geological structure tend to be irregular in shape. It is often found close to seams of Portland Limestone (e.g. Abbotsbury in Dorset, England). It is a younger limestone than its near-neighbour, the Oolitic, as found in the Cotswolds, in Gloucestershire.

A ridge of Corallian Limestone rises above the Vale of Avon and the Thames Valley in its Oxfordshire stretch. The Oxfordshire Corallian ridge is an escarpment holding back the hanging valley that is the Vale of White Horse and its hardness forced the River Thames to take a wide northern detour, to cut through the low ridge at Oxford. High points along the ridge are Cumnor Hurst and Wytham Hill. The outcrop known as Headington stone was quarried at Headington Quarry on the outskirts of Oxford and used for many of the historic University buildings there.

Hilly outcrops above this corallian ridge, composed of Lower Greensand, occur at Badbury Hill, Faringdon (Folly Hill) and Boars Hill.

Softer sandy deposits occur within the Corallian, found for example at Faringdon, Shellingford and Hatford in Oxfordshire, where the sands and gravels are extensively quarried.

The Corallian Limestone aquifer is present at outcrop in Yorkshire and in the Cotswolds. In Yorkshire it consists of limestones and grits up to about 110 m thick, thinning to about 20 m towards the south of the region, where the limestones are progressively replaced by clay. It is typically well jointed and gives rise to numerous springs. Here it yields up to 15 l/sec. In the Cotswolds the aquifer is up to 40 m thick, give yields of 5 to 10 l/sec, with water quality becoming increasingly saline down dip as the aquifer becomes confined in the Wessex Basin.

In England, Corallian Limestone is to be found in Dorset, Wiltshire, Oxfordshire, Buckinghamshire, Cambridgeshire, Lincolnshire and Yorkshire.

The most noted scholar of the Corallian strata of England was the geologist W.J. Arkell (1904–1958).

Dyrham Formation

The Dyrham Formation is a geologic formation in England. It preserves fossils dating back to the early part of the Jurassic period (Pliensbachian).

Eotyrannus

Eotyrannus (meaning "dawn tyrant") is a genus of tyrannosauroid theropod dinosaur hailing from the Early Cretaceous Wessex Formation beds, included in Wealden Group, located in the southwest coast of the Isle of Wight, United Kingdom. The remains (MIWG1997.550), consisting of assorted skull, axial skeleton and appendicular skeleton elements, from a juvenile or subadult, found in a plant debris clay bed, were described by Hutt et al. in early 2001. The etymology of the generic name refers to the animals classification as an early tyrannosaur or "tyrant lizard", while the specific name honors the discoverer of the fossil.

Fuller's Earth Formation

The Fullers Earth Formation is a geological formation that outcrops in southern England. It is also mostly present in the subsurface of the Wessex Basin and offshore in the English Channel Basin, Celtic Sea Basin and St George's Channel Basin. It preserves fossils dating back to the Bathonian stage of the Middle Jurassic series. It is the lateral equivalent of the Rutland Formation and the Sharp's Hill Formation

Geology of Dorset

Dorset (or archaically, Dorsetshire) is a county in South West England on the English Channel coast. Covering an area of 2,653 square kilometres (1,024 sq mi); it borders Devon to the west, Somerset to the north-west, Wiltshire to the north-east, and Hampshire to the east. The great variation in its landscape owes much to the underlying geology which includes an almost unbroken sequence of rocks from 200 Ma to 40 Ma and superficial deposits from 2 Ma to the present. In general the oldest rocks (Early Jurassic) appear in the far west of the county, with the most recent (Eocene) in the far east. Jurassic rocks also underlie the Blackmore Vale and comprise much of the coastal cliff in the west and south of the county; and although younger Cretaceous rocks crown some of the highpoints in the west, they are mainly to be found in the centre and east of the county.Dorset's coastline is one of the most visited and studied coastlines in the world because it shows, along the course of 95 miles (153 km) (including some of east Devon), rocks from the beginning of Triassic, through the Jurassic and up to the end of the Cretaceous, documenting the entire Mesozoic era with well-preserved fossils. Throughout Dorset there are a number of limestone ridges. The largest and most notable is the band of Cretaceous chalk that runs from the south-west to the north-east of the county and forms part of the Chalk Group that underlies much of the south of England, including Salisbury Plain, the Isle of Wight and the South Downs. Between the bands of limestone and chalk are wide clay vales with flood plains.

South-east Dorset, around Poole, Bournemouth and the New Forest, lies on younger and less resistant beds: Eocene clays (mainly London Clay), sands and gravels. These rocks produce thin soils that historically have supported a heathland habitat. The chalk and limestone hills of the Purbecks lie atop Britain's largest onshore oil field. The field, operated from Wytch Farm, produces a high-quality oil and has the world's oldest continuously pumping well at Kimmeridge, which has been in use since the early 1960s. The source of this oil is the organic-rich shales found in the lower lias. Landslides along the coast have been known to ignite these shales causing cliff fires, the most recent of which occurred in 2000.

Geothermal power in the United Kingdom

The potential for exploiting geothermal energy in the United Kingdom on a commercial basis was initially examined by the Department of Energy in the wake of the 1973 oil crisis. Several regions of the country were identified, but interest in developing them was lost as petroleum prices fell. Although the UK is not actively volcanic, a large heat resource is potentially available via shallow geothermal ground source heat pumps, shallow aquifers and deep saline aquifers in the mesozoic basins of the UK. Geothermal energy is plentiful beneath the UK, although it is not readily accessible currently except in specific locations.

Gravity anomalies of Britain and Ireland

Variations in the strength of gravity occur from place to place according to the density distribution of the rocks beneath the surface. Such gravity anomalies have been mapped across the British Isles and adjacent areas and they reveal aspects of these islands’ geological structure.

Istiodactylus

Istiodactylus is a genus of pterosaur that lived during the Early Cretaceous period, about 120 million years ago. The first fossil was discovered on the English Isle of Wight in 1887, and in 1901 became the holotype specimen of a new species, O. latidens (Latin for "wide tooth"), in the genus Ornithodesmus. This species was moved to its own genus, Istiodactylus, in 2001; this name is Greek for "sail finger". More specimens were described in 1913, and Istiodactylus was the only pterosaur known from three-dimensionally preserved fossils for much of the 20th century. In 2006, a species from China, I. sinensis, was assigned to Istiodactylus, but it has since been found to belong to a different genus.

Istiodactylus was a large pterosaur; estimates of its wingspan range from 4.3 to 5 metres (14 to 16 ft). Its skull was about 45 centimetres (18 in) long, and was relatively short and broad for a pterosaur. The front of the snout was low and blunt, and bore a semicircle of 48 teeth. The triangular teeth were closely spaced, interlocked, and formed a "razor-edged" outline. The lower jaw also had a tooth-like projection that occluded with the teeth. The skull had a very large naso-antorbital opening (which combined the antorbital fenestra and the opening for the bony nostril) and a slender eye socket. Some of the vertebrae were fused into a notarium, to which the shoulder blades connected. It had very large forelimbs, with a wing-membrane distended by a long wing-finger, but the hindlimbs were very short.

Until the 21st century, Istiodactylus was the only known pterosaur of its kind, and was placed in its own family, Istiodactylidae, within the group Ornithocheiroidea. Istiodactylus differed from other istiodactylids in having a proportionally shorter skull. The distinctive teeth of Istiodactylus indicate that it was a scavenger that may have used its teeth to sever morsels from large carcasses in the manner of a cookie cutter. The wings of Istiodactylus may have been adapted for soaring, which would have helped it find carcasses before terrestrial carnivores. Istiodactylus is known from the Wessex Formation and the younger Vectis Formation, which represent river and coastal environments that were shared with various pterosaurs, dinosaurs, and other animals.

London Basin

The London Basin is an elongated, roughly triangular sedimentary basin approximately 250 kilometres (160 mi) long which underlies London and a large area of south east England, south eastern East Anglia and the adjacent North Sea. The basin formed as a result of compressional tectonics related to the Alpine orogeny during the Palaeogene period and was mainly active between 40 and 60 million years ago.

Lower Greensand Group

The Lower Greensand Group is a geological unit, which forms part of the underlying geological structure of southeast England. South of London in the counties of West Sussex, East Sussex, Surrey and Kent, which together form the wider Weald, the Lower Greensand can usually be subdivided to formational levels with varying properties into the Atherfield Clay Formation, the Hythe Formation, the Sandgate Formation, and the Folkestone Formation. In areas north and west of London, including Cambridgeshire, Bedfordshire and Buckinghamshire the Lower Greensand is referred to as the Woburn Sands Formation.

Purbeck Group

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 the cliffs west of Swanage.

The Purbeck Group is famous for its fossils of reptiles and early mammals. This sequence of rocks has gone by various names in the past including amongst others the Purbeck Beds, Purbeck Formation, Purbeck Limestone Formation and Purbeck Stone.Rocks of this age have in the past been called the Purbeckian stage by European geologists. The Purbeckian corresponds with the Tithonian to Berriasian stages of the internationally used geologic timescale.

Southampton District Energy Scheme

The Southampton District Energy Scheme is a district heating and cooling system in Southampton, United Kingdom. The system is owned and operated by ENGIE.

Upper Greensand Formation

The Upper Greensand Formation is a Cretaceous formation of Albian to Cenomanian in age. It is found within the Wessex Basin and parts of the Weald Basin in southern England. It overlies the Gault Clay and underlies the Chalk Group. It varies in thickness from zero to 75 m. It is predominantly a glauconitic fine-grained sandstone, locally becoming silty.

Weald Basin

The Weald Basin () is a major topographic feature of the area that is now southern England and northern France from the Triassic to the Late Cretaceous. Its uplift in the Late Cretaceous marked the formation of the Wealden Anticline. The rock strata contain hydrocarbon deposits which have yielded coal, oil and gas.

Wealden Group

The Wealden Group is a group (a sequence of rock strata) in the lithostratigraphy of southern England. The Wealden group consists of paralic to continental (freshwater) facies sedimentary rocks of Berriasian to Aptian age and thus forms part of the English Lower Cretaceous. It is composed of alternating sands and clays. The sandy units were deposited in a flood plain of braided rivers, the clays mostly in a lagoonal coastal plain.The Wealden Group can be found in almost all Early Cretaceous basins of England: its outcrops curve from the Wessex Basin in the south to the Cleveland Basin in the northeast. It is not found in northwest England and Wales, areas which were at the time tectonic highs where no deposition took place. The same is true for the London Platform around London and Essex. Offshore, the Wealden Group can reach a thickness of 700 metres.

Wessex Formation

The Wessex Formation is a fossil-rich English geological formation that dates from the Berriasian to Barremian stages (about 145–125 million years ago) of the Early Cretaceous. It forms part of the Wealden Group and underlies the younger Vectis Formation and overlies the Durlston Formation. The dominant lithology of this unit is mudstone with some interbedded sandstones.

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