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.

Wadhurst Clay Formation Stratigraphic range: Valanginian PreꞒ Ꞓ O S D C P T J K Pg N
Wadhurst Clay (grey) exposed at Bexhill Brick Pit
Type	Geological formation
Unit of	Wealden Group
Underlies	Tunbridge Wells Sand Formation
Overlies	Ashdown Formation
Thickness	30 to 78 m
Lithology
Primary	Shale, Mudstone
Other	Siltstone, Sandstone, Limestone, Ironstone
Location
Region	Europe
Country	UK
Extent	Weald Basin
Type section
Named for	Wadhurst

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

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

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]

Dinosaurs

Dinosaurs reported from the Wadhurst Clay
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Altispinax	A. dunkeri	Hollington Quarry		Tooth	Originally known as Megalosaurus dunkeri
Barilium	B. dawsoni[7]	Shornden Quarry, Old Roar Quarry,		"[Two] partial skeletons."[8]	An iguanodontian
Hypselospinus	H. fittoni[9]	Hollington Quarry, Shornden Quarry, Hare Farm Lane, Brede		Partial skeletons	An iguanodontian
Suchosaurus	S. cultridens	Hare Farm Lane, Brede		Teeth	A spinosaurid
Unnamed maniraptoran	Indeterminate	Ashdown Brickworks		Specimen BEXHM: 2008.14.1[10] A single cervical vertebra	Possibly an oviraptorosaur[10]

Crocodyliformes

Crocodyliformes reported from the Wadhurst Clay
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Goniopholis	G. crassidens	Hare Farm Lane, Brede			A Goniopholid Neosuchian.

Mammals

Mammals reported from the Wadhurst Clay
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Laolestes	L. hodsoni	Cliff End bonebed, Tighe Farm	Cypridea paulsgrovensis ostracod Zone
Spalacotherium	S. tricuspidens	Cliff End bonebed
Aegialodon	A. dawsoni
Loxaulax	L. valdensis

Plesiosaurians

Plesiosaurians reported from the Wadhurst Clay
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Hastanectes	H. valdensis	Old Roar Quarry, Hollington Quarry, Black Horse Quarry, Hastings, Brenchley

See also

• Hastings Group
• Ashdown Formation
• Tunbridge Wells Sand Formation
• Geology of East Sussex

References

1. 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. 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. 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. "10.19 East Sussex, England; 1. Hastings Beds" in Weishampel, et al. (2004). Page 559.
8. "Table 19.1," in Weishampel, et al. (2004). Page 416.
9. "10.19 East Sussex, England; 1. Hastings Beds" in Weishampel, et al. (2004). Page 559.
10. 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

External links

• British Geological Survey lexicon