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22-761: 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

44-564: 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. 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

66-562: Is defined by the base of the Valanginian, which is fixed at the first appearance of calpionellid species Calpionellites darderi . This is just a little below the first appearance of the ammonite species Thurmanniceras pertransiens . Regional terms used in Russia include "Volgian"(which spans perhaps the latest Kimmeridgian, all the Tithonian and an uncertain amount of the lower Berriasian) and

88-501: Is of Early to Late Valanginian age. The Formation takes its name from the market town of Wadhurst 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. When they become exposed to

110-562: 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

132-476: 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

154-607: The Hastings Beds . 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 . In Yorkshire, the equivalently aged Speeton Clay Formation , a marine unit, is present. On top of the Wealden Group is the Lower Greensand Group . The difference between these two groups has been formed by

176-935: 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 Weald 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 . The lower three formations are sometimes collectively referred to as

198-653: The Tithonian (part of the Jurassic ) and precedes the Valanginian . The Berriasian Stage was introduced in scientific literature by Henri Coquand in 1869. It is named after the village of Berrias in the Ardèche department of France . The largely non-marine English Purbeck Formation is in part of Berriasian age. The first rocks to be described of this age were the beds of the English Purbeck Formation, named as

220-415: The Weald region of Kent , Sussex , Surrey and Hampshire . It has yielded many fossils, including dinosaurs like Iguanodon and Hypsilophodon . 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. Taxa included in the table below have an uncertain provenance and cannot be placed into one of

242-702: 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. The terms Wealden and Wealden facies are also used as generic terms referring to Early Cretaceous non-marine sequences elsewhere in Europe. The Wealden Group lies stratigraphically on top of

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264-581: 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 . The Wealden Group can be found in almost all Early Cretaceous basins of England: its outcrops curve from

286-1085: The Berriasian has been under active consideration by the Berriasian Working Group (ISCS) of IUGS since 2010. A range of contender GSSP localities has been studied in detail by the Working Group including localities as far apart as Mexico, Ukraine, Tunisia, Iraq and the Russian Far East. Several markers have been employed to refine correlations and to work towards defining a base for the Berriasian Stage. These include calcareous microfossils , such as Nannoconus , calpionellids , ammonites , palynological data and magnetostratigraphy , notably magnetozone M19n. The calibration of these markers, especially Nannoconus steinmannii minor , N. kamptneri minor , and Calpionella alpina , within precisely fixed magnetozones give greater precision in trying to identify

308-673: The Mons Basin, including the Sainte-Barbe Clays Formation where large numbers of Iguanodon were found in the 19th century. Berriasian In the geological timescale , the Berriasian is an age / stage of the Early/Lower Cretaceous . It is the oldest subdivision in the entire Cretaceous . It has been taken to span the time between 145.0 ± 4.0 Ma and 139.8 ± 3.0 Ma (million years ago). The Berriasian succeeds

330-618: The Purbeckian by Alexandre Brongniart in 1829 following description by Henry De la Beche , William Buckland , Thomas Webster and William Henry Fitton . The base of the Berriasian, which is also the base of the Cretaceous System , has traditionally been placed at the first appearance of fossils of the ammonite species Berriasella jacobi . But this is a species that has a stratigraphically problematic and geographically limited distribution. A global reference profile (a GSSP ) for

352-520: The base of the Alpina Subzone in the middle of magnetozone M19n.2n. This site proposal, of Tré Maroua, was subsequently unsuccessful in a vote of the ISCS (8 votes for and 8 against: 4 not voting); a new working group was formed in 2021. In the western part of the ocean of Tethys , the Berriasian consists of four ammonite biozones , from top to bottom (latest to earliest): The top of the Berriasian stage

374-529: The best position for a boundary. In 2016, the Berriasian Working Group voted to adopt Calpionella alpina as the primary marker for the base of the Berriasian Stage. In 2019, a GSSP for the Berriasian was nominated by a vote of the Berriasian Working Group of the Cretaceous Subcommission (ISCS): it is the profile of Tré Maroua in the Vocontian Basin (Hautes Alpes, France). The GSSP was defined at

396-577: The constituent formations, thus they are placed here. [REDACTED] The term "Wealden" and "Wealden facies" has been applied to other Lower Cretaceous sequences in Europe, including the "German Wealden", comprising the Berriasian aged Bückeberg Formation of the Lower Saxony Basin and in Belgium, where "Wealden facies" has been used as a term to refer to the Barremian-Aptian aged sequences of

418-575: The elements at the surface, the mudstones often degrade over a short period of time and weather to yellowish brown and greenish grey clays. 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 and in West Sussex up to 80m near Horsham . The base of the Wadhurst Clay

440-426: 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. 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. Natural slopes in the Wadhurst Clay, like those in

462-545: 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. When percolating groundwater in the permeable sandstones of the Tunbridge Wells Sands comes into contact with

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484-478: 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. Taken from Hastings Beds The Wealden Group, occasionally also referred to as the Wealden Supergroup , is a group (a sequence of rock strata ) in

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