The Wessex Basin is a

petroleum Petroleum, also known as crude oil, or simply oil, is a naturally occurring yellowish-black liquid mixture of mainly hydrocarbons, and is found in geological formations. The name ''petroleum'' covers both naturally occurring unprocessed crud ...

-bearing geological area located along the southern coast of England and extending into the

English Channel The English Channel, "The Sleeve"; nrf, la Maunche, "The Sleeve" (Cotentinais) or ( Jèrriais), (Guernésiais), "The Channel"; br, Mor Breizh, "Sea of Brittany"; cy, Môr Udd, "Lord's Sea"; kw, Mor Bretannek, "British Sea"; nl, Het Kana ...

. The onshore part of the basin covers approximately 20,000 km² and the area that encompasses the English Channel is of similar size. The basin is a

rift basin 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 wi ...

that was created during the

Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleoz ...

to early

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...

in response to movement of the

African plate The African Plate is a major tectonic plate that includes much of the continent of Africa (except for its easternmost part) and the adjacent oceanic crust to the west and south. It is bounded by the North American Plate and South American Plat ...

relative to the

Eurasian plate The Eurasian Plate is a tectonic plate that includes most of the continent of Eurasia (a landmass consisting of the traditional continents of Europe and Asia), with the notable exceptions of the Indian subcontinent, the Arabian subcontinent and ...

. In the late

, and again in the

Cenozoic The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configura ...

, the basin was inverted as a distant effect of the Alpine orogeny. The basin is usually divided into 3 main sub-basins including the Winterborne-Kingston Trough, Channel Basin, and Vale of Pewsey Basin. The area is also rich in

hydrocarbons 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 hydrophobic, and their odors are usually weak or ex ...

with several offshore wells in the area. With the large interest in the

hydrocarbon exploration Hydrocarbon exploration (or oil and gas exploration) is the search by petroleum geologists and geophysicists for deposits of hydrocarbons, particularly petroleum and natural gas, in the Earth using petroleum geology. Exploration methods Vis ...

of the area, data became more readily available, which improved the understanding of the type of inversion

tectonics Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents k ...

that characterize this basin.

Tectonic mechanisms

Lithospheric A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust and the portion of the upper mantle that behaves elastically on time scales of up to thousands of years or ...

thinning and crustal extension led to the subsidence and consequently the creation of asymmetric graben. These events happened in pulses starting in the Permian and concluding in the early Cretaceous period. 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. Inversion began starting in the late Cretaceous and carried on through Cenozoic times coinciding with the Alpine orogeny happening at the now

convergent boundary A convergent boundary (also known as a destructive boundary) is an area on Earth where two or more Plate tectonics, lithospheric plates collide. One plate eventually slides beneath the other, a process known as subduction. The subduction zone can ...

of the African and Eurasian plates.

Basement structure and stratigraphy

Starting at the

basement A basement or cellar is one or more floors of a building that are completely or partly below the ground floor. It generally is used as a utility space for a building, where such items as the furnace, water heater, breaker panel or fuse box, ...

of the structure, the area of interest overlies

Variscan The Variscan 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 ''Variscan'', comes f ...

externides consisting of

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...

and

Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...

sediments in imbricated

thrust sheets In geology, a nappe or thrust sheet is a large sheetlike body of rock that has been moved more than or above a thrust fault from its original position. Nappes form in compressional tectonic settings like continental collision zones or on the ...

. 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 In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

. 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. The sands created within this period lie in an

unconformable An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval ...

manner above the Devonian and Carboniferous basement. 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. 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. 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. Triassic sediments sit unconformably above the Permian layer with the makeup being mainly sands and silts. Like the former stage, the

Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...

sediments were first concentrated on the west side of the basin but also appeared in the southwest as a direct result from

marine transgression A marine transgression is a geologic event during which sea level rises relative to the land and the shoreline moves toward higher ground, which results in flooding. Transgressions can be caused by the land sinking or by the ocean basins filling ...

. The

Jurassic period The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...

followed with the same marine sedimentation, but by the end of the period,

sea-level Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datuma standardised g ...

began to fall leaving behind shallow marine sediments. The Cretaceous period marks a major transition as the area became unstable with the basement faults reactivating and additional deposition of 1000m of

brackish Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estuari ...

non-marine/freshwater sediments along with major uplift of the basin margins. The later Cretaceous consisted of deposition almost entirely of chalk with a basin wide unconformity delineating the period between the Paleogene and Late Cretaceous. These sediments are important as they represent the marker bed that show when the inversion of the basin began.

Basin inversion

The basin's inversion mechanism can be traced back to the movements of the Alpine orogeny that resulted in a series of

monocline A monocline (or, rarely, a monoform) is a step-like fold in rock strata consisting of a zone of steeper dip within an otherwise horizontal or gently-dipping sequence. Formation Monoclines may be formed in several different ways (see diagram) * ...

s along the boundary faults and the uplift of pre-Cenozoic sediments. Inversion can happen along reactivated fault lines and in this case the faults involved are the

Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceo ...

extensional

growth fault Growth faults are syndepositional or syn-sedimentary extensional faults that initiate and evolve at the margins of continental plates. They extend parallel to passive margins that have high sediment supply. Their fault plane dips mostly toward th ...

s. The reverse movements of the extensional faults in response reactivated the basement faults but this time as thrust faults. Former structural highs underwent inversions to become areas where large amounts of sediments began being deposited, while also slowly deepening to the basement faults. The amount of deepening of a former high structure correlates directly with the amount of over thrusting. 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 Paleogene is one of the first indicators, along with a regional change in the chalk composition to non-marine fluvial sediments. Major chalk fissures also became infilled with Paleocene sediments. There are east-west trending stylolites specifically in the Purbeck chalk formation that date the onset of the compression, the start of the inversion. Additionally,

Eocene The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...

paleosol In the geosciences, paleosol (''palaeosol'' in Great Britain and Australia) is an ancient soil that formed in the past. The precise definition of the term in geology and paleontology is slightly different from its use in soil science. In geolo ...

s created during the late Cretaceous south of the Purbeck- Isle of Wight fault zone is a direct contrast of the marine sediments of the same age to the north. Presently the basin is described with a series of east-west trending monoclines and

anticline In structural geology, an anticline is a type of fold that is an arch-like shape and has its oldest beds at its core, whereas a syncline is the inverse of an anticline. A typical anticline is convex up in which the hinge or crest is the ...

s that lie above the underlying Mesozoic growth faults. The Purbeck-Isle of Wight fault is one of the areas where inversion of the basin is distinctly seen with pre and post inversion structures.

Hydrocarbons

Hydrocarbon exploration led to immense seismic profiling in the areas of South

Dorset Dorset ( ; archaically: Dorsetshire , ) is a county in South West England on the English Channel coast. The ceremonial county comprises the unitary authority areas of Bournemouth, Christchurch and Poole and Dorset (unitary authority), Dors ...

Hampshire Hampshire (, ; abbreviated to Hants) is a ceremonial county, ceremonial and non-metropolitan county, non-metropolitan counties of England, county in western South East England on the coast of the English Channel. Home to two major English citi ...

, and the Isle of Wight. The petroleum geology is primarily confined to the Mesozoic strata and were first drilled in 1937. The

Kimmeridge Oil Field The Kimmeridge Oil Field is to the northwest of Kimmeridge Bay, on the south coast of the Isle of Purbeck, in Dorset, England. Location The small village Kimmeridge is about south of Wareham and about west of Swanage. The Kimmeridge oil fi ...

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.

Wytch Farm Wytch Farm is an oil field and processing facility in the Purbeck district of 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 ...

is the major onshore field that entered production during the 1970s. This accumulation stems from three major source rocks known as the Blue Lias Formation with Liassic clays, the

Oxford Clay The Oxford Clay (or Oxford Clay Formation) is a Jurassic marine sedimentary rock formation underlying much of southeast England, from as far west as Dorset and as far north as Yorkshire. The Oxford Clay Formation dates to the Jurassic, specifical ...

Formation, and the

Kimmeridge Clay Formation The Kimmeridge Clay is a sedimentary deposit of fossiliferous marine clay which is of Late Jurassic to lowermost Cretaceous age and occurs in southern and eastern England and in the North Sea. This rock formation is the major source rock for Nort ...

References

External links