Wealden Group
| Wealden Group Stratigraphic range: Berriasian-Aptian, 140–125 Ma | |
|---|---|
| Type | Group |
| Sub-units | |
| Underlies | Lower Greensand Group |
| Overlies | Purbeck Group |
| Thickness | Up to 850 m in Weald Basin, c. 500 m in Wessex basin, few m in marginal areas |
| Location | |
| Region | England |
| Country |
 |
| Type section | |
| Named for | Weald |
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]
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 2 Jackson (2008)
- ↑ 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.
- ↑ 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.
- ↑ 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