Wadhurst Clay Formation

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Wadhurst Clay Formation
Stratigraphic range: Valanginian
Bexhill Brick Pit in East Sussex - geograph 5315822.jpg
Wadhurst Clay (grey) exposed at Bexhill Brick Pit
Type Geological formation
Unit of Wealden Group
Underlies Tunbridge Wells Sand Formation
Overlies Ashdown Formation
Thickness30 to 78 m
Lithology
Primary Shale, Mudstone
Other Siltstone, Sandstone, Limestone, Ironstone
Location
Region Europe
CountryFlag of the United Kingdom.svg  UK
Extent Weald Basin
Type section
Named for Wadhurst

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.

Contents

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. [1] The Formation takes its name from the market town of Wadhurst in East Sussex.

Lithology

The Top Ashdown Pebble Bed, which marks the base of the Wadhurst Clay in East Sussex Top Ashdown Pebble Bed.jpg
The Top Ashdown Pebble Bed, which marks the base of the Wadhurst Clay in East Sussex

The Wadhurst Clay comprises predominantly medium to dark bluish grey over-consolidated clays, silts, mudstones, and shales. These lithologies often occur with subordinate amounts of pale grey silty mudstones, laminated siltstones, sandstones, conglomerate, shelly limestones and clay-ironstones. [2] When they become exposed to the elements at the surface, the mudstones often degrade over a short period of time and weather to yellowish brown and greenish grey clays. [3]

The formation thickness ranges from 55m in the Tenterden area, to 30m near Lewes and varies in between. In Kent, the Wadhurst Clay has been proven to over 70m thick near Tunbridge Wells [2] and in West Sussex up to 80m near Horsham. [4]

The base of the Wadhurst Clay is taken at the bottom of the Top Ashdown Pebble Bed. The base of this marker horizon marks the formational change to the Ashdown Formation. Despite its name this thin and impressistent bed comprises a coarse grained to gravelly sandstone. This horizon is best exposed at Cliff End, East Sussex, but where it is encountered elsewhere, it is usually fairly distinctive and easily identified. The Top Ashdown Pebble Bed occurs mainly in the southern half of East Sussex and is often missing elsewhere. Where this is the case, the boundary is taken at a layer of disconnected ripples. [5]

The top of the Wadhurst Clay is marked by a distinct lithological change into the siltstones of the Tunbridge Wells Sand Formation. The uppermost clays of the Wadhurst Clay, closest to the boundary are often stained red. [1]

Translational landslip at the boundary of the Wadhurst Clay and Tunbridge Wells Sand near Etchingham Landslip.jpg
Translational landslip at the boundary of the Wadhurst Clay and Tunbridge Wells Sand near Etchingham

Engineering Geology

Natural slopes in the Wadhurst Clay, like those in the Ashdown Formation, are known for their instability, especially where impermeable clays and impermeable silts and siltstones are interbedded. Instability, resulting in landslips, often occurs along shear surfaces and weaknesses that originally developed during the Late Devensian glaciation. Other common features include cambering, valley bulging and solifluction lobes.

Landslips often occur at or close to the upper boundary of the Wadhurst Clay, which is shared with the overlying Tunbridge Wells Sand Formation. This is partly caused by the steep sided hill, valley and ravine topography of the High Weald and partly by the lithological variation between the formations and the presence of spring lines and seepages. [3]

When percolating groundwater in the permeable sandstones of the Tunbridge Wells Sands comes into contact with the upper impermeable clay beds of the Wadhurst Clay, it is forced to find alternative migration pathways to the surface. This results in the saturation and weakening of the upper portion of the Wadhurst Clay, increasing the chances of failure. [3]

Paleobiota

Taken from [6]

Fish

Fish reported from the Wadhurst Clay
GenusSpeciesLocationStratigraphic positionMaterialNotesImages
Egertonodus E. basanusAshdown Brickworks [7] Hybodont shark
Planohybodus P. ensis
Polyacrodus P. parvidens, P. brevicostatus
Ocloedus Pycnodontiform
Coccolepis Coccolepidid [8]
Semionotiformes,
Albuliformes

Dinosaurs

Dinosaurs reported from the Wadhurst Clay
GenusSpeciesLocationStratigraphic positionMaterialNotesImages
Altispinax A. dunkeriHollington QuarryToothAn carnosaurian theropod
Barilium B. dawsoni [9] Shornden Quarry, Old Roar Quarry,"[Two] partial skeletons." [10] An iguanodontian
Unnamed dromaeosaurid Indeterminate
Hypselospinus H. fittoni [9] Hollington Quarry, Shornden Quarry, Hare Farm Lane, BredePartial skeletonsAn iguanodontian Hypselospinus NT.jpg
Suchosaurus S. cultridensHare Farm Lane, BredeTeethA spinosaurid theropod
Unnamed maniraptoran IndeterminateAshdown BrickworksSpecimen BEXHM: 2008.14.1 [11] A single cervical vertebraPossibly an oviraptorosaur [11]
Unnamed spinosaurid Indeterminate
Unammed tyrannosauroid Indeterminate

Amphibians

Amphibians reported from the Wadhurst Clay
GenusSpeciesLocationStratigraphic positionMaterialNotesImages
Anura IndeterminateAshdown BrickworksTwo fragmentary iliaAt least 2 taxa [12]
Urodela IndeterminateAtlas and postatlas vertebrae, dentary, maxilla and vomer fragmentsAt least 3 taxa distinguishable by their atlas vertebrae [12]

Crocodyliformes

Crocodyliformes reported from the Wadhurst Clay
GenusSpeciesLocationStratigraphic positionMaterialNotesImages
Goniopholis G. crassidensHare Farm Lane, BredeA Goniopholid neosuchian Goniopholis BW.jpg
Hulkepholis IndeterminateAshdown Brickworks [7] A goniopholid neosuchian
Theriosuchus An atoposaurid neosuchian
Bernissartiidae

Mammals

Mammals reported from the Wadhurst Clay
GenusSpeciesLocationStratigraphic positionMaterialNotesImages
Laolestes L. hodsoniCliff End bonebed, Tighe FarmCypridea paulsgrovensis ostracod ZoneA dryolestid mammal
Spalacotherium S. tricuspidensCliff End bonebedA spalacotheriid mammal
Aegialodon A. dawsoniA cladotherian mammal
Loxaulax L. valdensisA eobataarid multituberculate

Plesiosaurians

Plesiosaurians reported from the Wadhurst Clay
GenusSpeciesLocationStratigraphic positionMaterialNotesImages
Hastanectes H. valdensisOld Roar Quarry, Hollington Quarry, Black Horse Quarry, Hastings, BrenchleyA leptocleidid plesiosaur

Pterosaurs

Pterosaurs reported from the Wadhurst Clay Formation
GenusSpeciesLocationStratigraphic positionMaterialNotesImages
? Istiodactylidae indet.IndeterminateCliff End Bone Bed, Cliff End. [13] 2 isolated tooth crowns (BEXHM: 2022.109.1 & BEXHM: 2022.109.2). [13] An istiodactylid.


See also

Related Research Articles

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<span class="mw-page-title-main">Weald</span> Area of South East England

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 Weald" in the centre, the clay "Low Weald" periphery and the Greensand Ridge, which stretches around the north and west of the Weald and includes its highest points. The Weald once was covered with forest and its name, Old English in origin, signifies "woodland". The term is still used, as scattered farms and villages sometimes refer to the Weald in their names.

<span class="mw-page-title-main">Weald Clay</span> Geological formation in England

Weald Clay or the Weald Clay Formation is a Lower Cretaceous sedimentary rock unit underlying areas of South East England, between the North and South Downs, in an area called the Weald Basin. It is the uppermost unit of the Wealden Group of rocks within the Weald Basin, and the upper portion of the unit is equivalent in age to the exposed portion of the Wessex Formation on the Isle of Wight. It predominantly consists of thinly bedded mudstone. The un-weathered form is blue/grey, and the yellow/orange is the weathered form, it is used in brickmaking.

<span class="mw-page-title-main">Purbeck Group</span> Stratigraphic Group in England

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<span class="mw-page-title-main">Wealden Group</span> Stratigraphic Group in England

The Wealden Group, occasionally also referred to as the Wealden Supergroup, is a group 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.

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<span class="mw-page-title-main">Greensand Ridge</span> Range of hills in south east England, UK

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<span class="mw-page-title-main">Wessex Formation</span> Early Cretaceous geological formation in England

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<span class="mw-page-title-main">Geology of East Sussex</span> Overview of the geology of East Sussex

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<span class="mw-page-title-main">Tunbridge Wells Sand Formation</span> Stratigraphic Unit in England

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.

<span class="mw-page-title-main">Ashdown Formation</span> Geological unit of the Lower Cretaceous

The Ashdown Formation is a geological unit, which forms part of the Wealden Group and the lowermost and oldest part of the now unofficial Hastings Beds. These geological units make up the core of the Weald in the English counties of East Sussex and Kent.

The geology of Kent in southeast England largely consists of a succession of northward dipping late Mesozoic and Cenozoic sedimentary rocks overlain by a suite of unconsolidated deposits of more recent origin.

The geology of West Sussex in southeast England comprises a succession of sedimentary rocks of Cretaceous age overlain in the south by sediments of Palaeogene age. The sequence of strata from both periods consists of a variety of sandstones, mudstones, siltstones and limestones. These sediments were deposited within the Hampshire and Weald basins. Erosion subsequent to large scale but gentle folding associated with the Alpine Orogeny has resulted in the present outcrop pattern across the county, dominated by the north facing chalk scarp of the South Downs. The bedrock is overlain by a suite of Quaternary deposits of varied origin. Parts of both the bedrock and these superficial deposits have been worked for a variety of minerals for use in construction, industry and agriculture.

The Whitchurch Sand Formation is a geological formation, in England. part of the Wealden Group, it is preserved as an inlier in hills in Buckinghamshire, Oxfordshire and Wiltshire. It was deposited in the Valanginian stage of the Early Cretaceous. The lithology largely consists of unconsolidated fine-medium grained sand with isolated bodies of limonite cemented sandstone, with localised beds of siltstone and mudstone.

The geology of the South Downs National Park in South East England comprises a gently folded succession of sedimentary rocks from the Cretaceous and early Palaeogene periods overlain in places by a range of superficial deposits from the last 2.6 million years. Whereas the South Downs are formed from the Late Cretaceous age chalk, the South Downs National Park extends into the Weald to the north of the range and thereby includes older rock strata dating from the Early Cretaceous including sandstones and mudstones. The youngest solid rocks are found on the southern fringes of the National Park in the eastern extension of the Hampshire Basin and include sand, silt and clay deposited during the Palaeocene and Eocene epochs.

References

  1. 1 2 Hopson, P.M., Wilkinson, I.P. and Woods, M.A. (2010) A stratigraphical framework for the Lower Cretaceous of England. Research Report RR/08/03. British Geological Survey, Keyworth.
  2. 1 2 Lake, R.D. & Shepard-Thorn, E.R. (1987) Geology of the country around Hastings and Dungeness: Memoir for 1:50,000 geological sheets 320 and 321. British Geological Survey, London.
  3. 1 2 3 Codd, J.W. (2007) Analysis of the distribution and characteristics of landslips in the Weald of East Sussex. MSc dissertation, University of Brighton.
  4. Reeves, G.M., Sims, I., & Cripps, J.C. (2006) Clay Minerals Used in Construction, Geological Society, London, Engineering Geology Special Publication, 21, 153-175.
  5. Lake, Shepard-Thorn, E.R., Smart, J.G.O., Bisson, G. & Edmonds, E.A. (1966) Geology of the country around Tenterden: Memoir for 1:50,000 geological sheet 304. British Geological Survey, London.
  6. "Wadhurst Clay Formation". Paleobiology Database.
  7. 1 2 Turmine-Juhel, Pernelle; Wilks, Richard; Brockhurst, David; Austen, Peter A.; Duffin, Christopher J.; Benton, Michael J. (2019-12-01). "Microvertebrates from the Wadhurst Clay Formation (Lower Cretaceous) of Ashdown Brickworks, East Sussex, UK". Proceedings of the Geologists' Association. 130 (6): 752–769. doi:10.1016/j.pgeola.2019.08.003. ISSN   0016-7878. S2CID   202904040.
  8. Peter and Joyce Austen Wealden fish (2011) – Based on a talk given by Peter Forey (Natural History Museum)
  9. 1 2 "10.19 East Sussex, England; 1. Hastings Beds" in Weishampel, et al. (2004). Page 559.
  10. "Table 19.1," in Weishampel, et al. (2004). Page 416.
  11. 1 2 Naish, D. and Sweetman, S.C. (2011). "A tiny maniraptoran dinosaur in the Lower Cretaceous Hastings Group: evidence from a new vertebrate-bearing locality in south-east England." Cretaceous Research, 32: 464-471. doi : 10.1016/j.cretres.2011.03.001
  12. 1 2 Sweetman, S.C; Evans, S. E. (2011). "Lissamphibians (frogs, salamanders and albanerpetontids)". English Wealden fossils. Palaeontological Association. ISBN   978-1-4443-6711-9. OCLC   777385514.{{cite book}}: CS1 maint: multiple names: authors list (link)
  13. 1 2 Sweetman, Steven C. (2023-06-28). "Pterosaur teeth from the Lower Cretaceous (Valanginian) Cliff End Bone Bed, Wadhurst Clay Formation, Wealden Supergroup of southern England, and their possible affinities". Cretaceous Research: 105622. doi: 10.1016/j.cretres.2023.105622 . ISSN   0195-6671.