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.[1]

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.[1]

Wealden Group
Stratigraphic range: Berriasian-Aptian, 140–125 Ma
TypeGroup
Sub-units
UnderliesLower Greensand Group
OverliesPurbeck Group
ThicknessUp to 850 m in Weald Basin, c. 500 m in Wessex basin, few m in marginal areas
Location
RegionEngland
Country UK
Type section
Named forWeald

Stratigraphy

The Wealden Group lies stratigraphically on top of the Purbeck Group, which spans the Jurassic-Cretaceous boundary. Within the Wessex Basin, the Wealden Group consists of two formations: the Wessex Formation and overlying Vectis Formation. In the Wealden Basin, the Wealden Group consists of four formations: the Ashdown Formation, the Wadhurst Clay Formation, the Tunbridge Wells Sand Formation and the Weald Clay Formation.[2] In Oxfordshire, Buckinghamshire and Wiltshire, the Wealden Group is only found as an outlier on top of hills and only consists of a single formation, the Whitchurch Sand Formation.

On top of the Wealden Group is the Lower Greensand Group. The difference between these two groups has been formed by a major eustatic (global) transgression of the sea. The Greensand (Aptian/Albian in age) consists of marine deposits.

The sequence in the Weald Basin has also been described as a supergroup, containing the Weald Clay Group and Hastings Group.[3]

Palaeontology

The Wealden Group forms outcrops covering a large part of south and south-eastern England including the Isle of Wight. It takes its name from the Weald region of Kent, Sussex, Surrey and Hampshire. It has yielded many fossils, including dinosaurs like Iguanodon and Hypsilophodon.[4] Apart from fossils, it shows many other signs of being deposited in a continental environment, such as mudcracks and -in some rare cases- dinosaur footprints.

References

  1. ^ a b Jackson (2008)
  2. ^ Hopson P.M.; Wilkinson I.P.; Woods M.A. (2008). "A stratigraphical framework for the Lower Cretaceous of England" (PDF). British Geological Survey Research Report , RR/08/03. British Geological Survey.
  3. ^ Benson R.B.J.; Brusatte S.L.; Hutt S.; Naish D. (2009). "A new large basal tetanuran (Dinosauria: Theropoda) from the Wessex Formation (Barremian) of the Isle Of Wight, England" (PDF). Journal of Vertebrate Paleontology. 29 (2): 612–615. doi:10.1671/039.029.0202.
  4. ^ Weishampel et al. (2004)
  • Jackson, A.A.; 2008: Bedrock geology UK south. An explanation of the bedrock geology map of England and Wales - 1:625,000 edition, Keyworth, Nottingham, British Geological Survey, ISBN 978-0-85272-586-3.
  • Weishampel, D.B.; Barrett, P.M.; Coria, R.A.; Le Loeuff, J.; Xu, X.; Zhao, X.; Sahni, A.; Gomani, E.M.P. & Noto, C.R.; 2004: Dinosaur distribution, in: Weishampel, D.B.; Dodson, P. & Osmólska, H. (eds.): The Dinosauria, University of California Press, Berkeley, p. 517–606.

See also

Blackhorse Quarry

Blackhorse Quarry is a 0.2-hectare (0.49-acre) geological Site of Special Scientific Interest south-east of Battle in East Sussex. It is a Geological Conservation Review site.This is the Type locality for the Wadhurst Clay Formation Telham Bond Bed, which dates to the Early Cretaceous and is part of the Wealden Group. It has yielded many fossils, including turtles, crocodiles, pterosaurs and dinosaurs.The site is private land with no public access.

Brede Pit and Cutting

Brede Pit and Cutting is a 0.6-hectare (1.5-acre) geological Site of Special Scientific Interest in Brede in East Sussex. It is a Geological Conservation Review site.This site shows the junction between two formations in the Wealden Group, dating to the Early Cretaceous. It exposes the top 2 metres of the Ashdown Formation and the bottom 1.5 metres of the Wadhurst Clay Formation. The environments change from shallow fluvial to deeper lakes and lagoons and there are fossils of plants, fishes and reptiles.The site is private land with no public access.

Chantry Mill

Chantry Mill is a 8.7-hectare (21-acre) geological Site of Special Scientific Interest in Storrington in West Sussex. It is a Geological Conservation Review site.This site provides the best exposure of the junction between the Gault and Folkestone Beds of the Wealden Group, dating to around 140 million years ago in the Early Cretaceous.A public footpath runs along the south-western edge of the site.

Freshfield Lane

Freshfield Lane is a 17-hectare (42-acre) geological Site of Special Scientific Interest east of Haywards Heath in West Sussex. It is a Geological Conservation Review site.This working quarry exposes rocks dating to formations in the Wealden Group of Lower Cretaceous age, around 140 to 113 million years ago. It is described by Natural England as "internationally important for palaeoenvironmental, provenance and palaeogeographical studies".There are footpaths around the boundaries of the two areas in the site.

Geology of the Isle of Wight

The geology of the Isle of Wight is dominated by sedimentary rocks of Cretaceous and Paleogene age. This sequence was affected by the late stages of the Alpine Orogeny, forming the Isle of Wight monocline, the cause of the steeply-dipping outcrops of the Chalk Group and overlying Paleogene strata seen at The Needles, Alum Bay and Whitecliff Bay.

Hastings Beds

The Hastings Beds is a geological unit that includes interbedded clays, silts, siltstones, sands and sandstones in the High Weald of southeast England. These strata make up the component geological formations of the Ashdown Formation, the Wadhurst Clay Formation and the Tunbridge Wells Sand Formation. The term 'Hastings Beds' has been superseded and the component formations are included in the Wealden Group.The sediments of the Weald, including the Hastings Beds, were deposited during the Early Cretaceous Period, which lasted for approximately 40 million years from 140 to 100 million years ago. The Hastings Beds are of Early Berriasian to Late Valanginian age. The Group takes its name from the fishing town of Hastings in East Sussex.

Dinosaur remains are among the fossils that have been recovered from the included formations.

Heterosuchus

Heterosuchus is an extinct genus of crocodylomorph that may have been a eusuchian. It is known only from neck and back vertebrae recovered from Early Cretaceous-age rocks of the Hastings Beds (Wealden Group) of Hastings, Sussex. These vertebrae are procoelous (ball-and-socket articulation with the socket in front and the ball on the back of individual vertebrae), which is a trait of eusuchians. Heterosuchus was described by Harry Seeley in 1887, with H. valdensis as the type species. It may be the same genus as the slightly younger Hylaeochampsa, inferred to have been of similar evolutionary grade, but there is no overlapping material as Hylaeochampsa is known only from a partial skull; Hylaeochampsa would be the name used for both in that case, because it is the older name (coined in 1874). Because of the sparse material and apparent lack of distinguishing characteristics, James Clark and Mark Norell (1992) considered Heterosuchus a dubious name.

Houghton Green Cliff

Houghton Green Cliff is a 0.14-hectare (0.35-acre) geological Site of Special Scientific Interest north of Rye in East Sussex. It is a Geological Conservation Review site.This site exposes the Cliff End Sandstone Member of the Wadhurst Clay Formation, part of the Wealden Group, which dates to the Lower Cretaceous between 140 and 100 million years ago. It is a key site for studies of sandstone bodies in the clay formation.Ths site is at the side of a public road.

Hylaeochampsa

Hylaeochampsa is an extinct genus of eusuchian crocodylomorph. It is known only from a partial skull recovered from Barremian-age rocks of the Lower Cretaceous Vectis Formation (Wealden Group) of the Isle of Wight. This skull, BMNH R 177, is short and wide, with a eusuchian-like palate and inferred enlarged posterior teeth that would have been suitable for crushing. Hylaochampsa was described by Richard Owen in 1874, with H. vectiana as the type species. It may be the same genus as the slightly older Heterosuchus, inferred to have been of similar evolutionary grade, but there is no overlapping material as Heterosuchus is known only from vertebrae. If the two could be shown to be synonyms, Hylaeochampsa would have priority because it is the older name. Hylaeochampsa is the type genus of the family Hylaeochampsidae, which also includes Iharkutosuchus from the Late Cretaceous of Hungary. James Clark and Mark Norell positioned it as the sister group to Crocodylia. Hylaeochampsa is currently the oldest known unambiguous eusuchian.

Northiam SSSI

Northiam SSSI is a 0.3-hectare (0.74-acre) geological Site of Special Scientific Interest in Northiam in East Sussex. It is a Geological Conservation Review site.This partly flooded former quarry is the type locality for the Northiam Sandstone Member of the Wadhurst Clay Formation, part of the Wealden Group which dates to the Early Cretaceous. It is important for the study of the paleogeography and paleoenvironments of the Wadhurst Clay Formation.The site is private land with no public access.

Philpot's and Hook Quarries

Philpot's and Hook Quarries is a 2.6-hectare (6.4-acre) geological Site of Special Scientific Interest south-west of Sharpthorne in West Sussex. It is a Geological Conservation Review site.These quarries expose the Ardingly Sandstone Member in the Tunbridge Wells Sand Formation, which is part of the Wealden Group, dating to the Lower Cretaceous between 140 and 100 million years ago. Philpot's Quarry has many dinosaur fossils and both quarries have debris dating to the Precambrian.The quarries are private land with no public access.

Stone Farm Rocks

Stone Farm Rocks or Stone Hill Rocks is a 0.6-hectare (1.5-acre) geological Site of Special Scientific Interest south of East Grinstead in West Sussex. It is a Geological Conservation Review site.This is typical of many sandstone crags in mid-Sussex which expose the Tunbridge Wells Sand Formation, part of the Wealden Group which dates to the Early Cretaceous between 140 and 100 million years ago. It displays a variety of sedimentary structures in three dimensions and is described by Natural England as "an important site for the study and interpretation of sedimentary structures in the upper Lower Tunbridge Wells Sand".The site has been owned by the British Mountaineering Council since 2001, who run it for the benefit of climbers. There are 74 identified climbs on the rocks. Bolt belays have been installed at the top of many of the climbs to reduce erosion of the site.

Tunbridge Wells Sand Formation

The Tunbridge Wells Sand Formation is a geological unit which forms part of the Wealden Group and the uppermost and youngest part of the unofficial Hastings Beds. These geological units make up the core of the geology of the Weald in the English counties of West Sussex, East Sussex and Kent.

The other component formations of the Hastings Beds are the underlying Wadhurst Clay Formation and the Ashdown Formation. The Hastings Beds in turn form part of the Wealden Group which underlies much of southeast England. The sediments of the Weald, including the Tunbridge Wells Sand Formation, were deposited during the Early Cretaceous Period, which lasted for approximately 40 million years from 140 to 100 million years ago. The Tunbridge Wells Sands are of Late Valanginian age. The Formation takes its name from the spa town of Tunbridge Wells in Kent.

Vectisuchus

Vectisuchus is a genus of goniopholidid mesoeucrocodylian, known from the Early Cretaceous-age Wealden Group of the Isle of Wight, England. It was a small, piscivorous crocodylomorph with a narrow, elongate snout, and relatively long forearms. The type specimen, SMNS 50984, was found in 1977. When discovered, it was complete and right-side-up, but the posterior portion was lost during excavation. Vectisuchus was described in 1980. The type species is V. leptognathus.

Wadhurst Clay Formation

The Wadhurst Clay Formation is a geological unit which forms part of the Wealden Group and the middle part of the now unofficial Hastings Beds. These geological units make up the core of the geology of the High Weald in the English counties of West Sussex, East Sussex and Kent.

The other component formations of the Hastings Beds are the underlying Ashdown Formation and the overlying Tunbridge Wells Sand Formation. The Hastings Beds in turn form part of the Wealden Group which underlies much of southeast England. The sediments of the Weald, including the Wadhurst Clay Formation, were deposited during the Early Cretaceous Period, which lasted for approximately 40 million years from 140 to 100 million years ago. The Wadhurst Clay is of Early to Late Valanginian age. The Formation takes its name from the market town of Wadhurst in East Sussex.

Weald Clay

Weald Clay or the Weald Clay Formation is a Lower Cretaceous sedimentary rock underlying areas of South East England. It is part of the Wealden Group of rocks. The clay is named after the Weald, an area of Sussex and Kent. It varies from orange and grey in colour and is used in brickmaking.

The un-weathered form is blue/grey, and the yellow/orange is the weathered form; they have quite different physical properties. Blue looks superficially like a soft slate, is quite dry and hard and will support the weight of buildings quite easily.

Because it is quite impermeable, and so dry, it does not get broken by tree roots. It is typically found at 750mm down below a layer of yellow clay. Yellow, found on the surface, absorbs water quite readily so becomes very soft in the winter.

The two different types make quite different bricks.

West Hoathly SSSI

West Hoathly SSSI is a 0.7-hectare (1.7-acre) geological Site of Special Scientific Interest in Sharpthorne in West Sussex. It is a Geological Conservation Review site.This working quarry exposes clays of the Wadhurst Clay Formation, which is part of the Wealden Group, dating to the Early Cretaceous between 140 and 100 million years ago. The site lies close to a postulated gap in the London-Brabant Massif through which the Boreal Sea is thought to have periodically flowed, and it is described by Natural England as "important for interpreting environmental conditions at the northwestern extremity of the Wadhurst Clay outcrop".The site is private land with no public access.

Winchelsea Cutting

Winchelsea Cutting is a 0.15-hectare (0.37-acre) geological Site of Special Scientific Interest on the southern outskirts of Winchelsea in East Sussex. It is a Geological Conservation Review site.This site exposes the top two metres of the Ashdown Sand Formation and the bottom four metres of the Wadhurst Clay Formation, dating to the Wealden Group of the Lower Cretaceous around 140 million years ago.The site is at the side of the A259 road.

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