Wealden Formation
Encyclopedia
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 rock
s of Valanginian
to Barremian
age and thus forms part of the English Lower Cretaceous. It is composed of alternating sand
s and clay
s. The sandy units were deposited in a flood plain of braided river
s, the clays mostly in a lagoon
al coastal plain
.
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 meters.
on top of the Purbeck Group, which spans the Jurassic
-Cretaceous
boundary. The Wessex Formation
and overlying Vectis Formation
are the two formations within the Wealden Group.
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.
s 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 dinosaur
s like Iguanodon
and Hypsilophodon
. Apart from fossils, it shows many other signs of being deposited in a continental environment, such as mudcrack
s and -in some rare cases- dinosaur footprints.
Group (stratigraphy)
A group in stratigraphy is a lithostratigraphic unit, a part of the geologic record or rock column that consists of defined rock strata. Groups are divided into formations and are sometimes themselves grouped into "supergroups"....
(a sequence of rock strata) in the lithostratigraphy
Lithostratigraphy
Lithostratigraphy is a sub-discipline of stratigraphy, the geological science associated with the study of strata or rock layers. Major focuses include geochronology, comparative geology, and petrology...
of southern England
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
. The Wealden group consists of paralic to continental
Continent
A continent is one of several very large landmasses on Earth. They are generally identified by convention rather than any strict criteria, with seven regions commonly regarded as continents—they are : Asia, Africa, North America, South America, Antarctica, Europe, and Australia.Plate tectonics is...
(freshwater) facies sedimentary rock
Sedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....
s of Valanginian
Valanginian
In the geologic timescale, the Valanginian is an age or stage of the Early or Lower Cretaceous. It spans between 140.2 ± 3.0 Ma and 136.4 ± 2.0 Ma...
to Barremian
Barremian
The Barremian is an age in the geologic timescale between 130.0 ± 1.5 Ma and 125.0 ± 1.0 Ma). It is a subdivision of the Early Cretaceous epoch...
age and thus forms part of the English Lower Cretaceous. It is composed of alternating sand
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
s and clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
s. The sandy units were deposited in a flood plain of braided river
Braided river
A braided river is one of a number of channel types and has a channel that consists of a network of small channels separated by small and often temporary islands called braid bars or, in British usage, aits or eyots. Braided streams occur in rivers with high slope and/or large sediment load...
s, the clays mostly in a lagoon
Lagoon
A lagoon is a body of shallow sea water or brackish water separated from the sea by some form of barrier. The EU's habitat directive defines lagoons as "expanses of shallow coastal salt water, of varying salinity or water volume, wholly or partially separated from the sea by sand banks or shingle,...
al coastal plain
Coastal plain
A coastal plain is an area of flat, low-lying land adjacent to a seacoast and separated from the interior by other features. One of the world's longest coastal plains is located in eastern South America. The southwestern coastal plain of North America is notable for its species diversity...
.
The Wealden Group can be found in almost all Early Cretaceous basins
Basin (geology)
A structural basin is a large-scale structural formation of rock strata formed by tectonic warping of previously flat lying strata. Structural basins are geological depressions, and are the inverse of domes. Some elongated structural basins are also known as synclines...
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
Wales
Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...
, 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
Essex
Essex is a ceremonial and non-metropolitan county in the East region of England, and one of the home counties. It is located to the northeast of Greater London. It borders with Cambridgeshire and Suffolk to the north, Hertfordshire to the west, Kent to the South and London to the south west...
. Offshore, the Wealden Group can reach a thickness of 700 meters.
Stratigraphy
The Wealden Group lies stratigraphicallyStratigraphy
Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....
on top of the Purbeck Group, which spans the Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
-Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
boundary. The Wessex Formation
Wessex Formation
The Wessex Formation is an English fossil site and geological formation that dates to the Barremian stage of the Early Cretaceous. It is part of the Wealden Group and underlies the younger Vectis Formation.-Invertebrates:...
and overlying Vectis Formation
Vectis Formation
The Vectis Formation is a geological formation on the Isle of Wight, England whose strata date back to the Early Cretaceous . The Vectis Formation is composed of three geological members: the Shepherds Chine member, the Barnes High Sandstone member, and the Cowleaze Chine member...
are the two formations within the Wealden Group.
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
Transgression
Transgression may be:*a Biblical transgression, violation of God's ten commandments; sin *a legal transgression, a crime usually created by a social or economic boundary*a social transgression, violating a norm...
of the sea. The Greensand (Aptian
Aptian
The Aptian is an age in the geologic timescale or a stage in the stratigraphic column. It is a subdivision of the Early or Lower Cretaceous epoch or series and encompasses the time from 125.0 ± 1.0 Ma to 112.0 ± 1.0 Ma , approximately...
/Albian
Albian
The Albian is both an age of the geologic timescale and a stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early/Lower Cretaceous epoch/series. Its approximate time range is 112.0 ± 1.0 Ma to 99.6 ± 0.9 Ma...
in age) consists of marine deposits.
Palaeontology
The Wealden Group forms outcropOutcrop
An outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth. -Features:Outcrops do not cover the majority of the Earth's land surface because in most places the bedrock or superficial deposits are covered by a mantle of soil and vegetation and cannot be...
s covering a large part of south and south-eastern England
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
including the Isle of Wight
Isle of Wight
The Isle of Wight is a county and the largest island of England, located in the English Channel, on average about 2–4 miles off the south coast of the county of Hampshire, separated from the mainland by a strait called the Solent...
. It takes its name from the Weald
Weald
The Weald is the name given to an area in South East England situated between the parallel chalk escarpments of the North and the South Downs. It should be regarded as three separate parts: the sandstone "High Weald" in the centre; the clay "Low Weald" periphery; and the Greensand Ridge which...
region of Kent
Kent
Kent is a county in southeast England, and is one of the home counties. It borders East Sussex, Surrey and Greater London and has a defined boundary with Essex in the middle of the Thames Estuary. The ceremonial county boundaries of Kent include the shire county of Kent and the unitary borough of...
, Sussex
Sussex
Sussex , from the Old English Sūþsēaxe , is an historic county in South East England corresponding roughly in area to the ancient Kingdom of Sussex. It is bounded on the north by Surrey, east by Kent, south by the English Channel, and west by Hampshire, and is divided for local government into West...
, Surrey
Surrey
Surrey is a county in the South East of England and is one of the Home Counties. The county borders Greater London, Kent, East Sussex, West Sussex, Hampshire and Berkshire. The historic county town is Guildford. Surrey County Council sits at Kingston upon Thames, although this has been part of...
and Hampshire
Hampshire
Hampshire is a county on the southern coast of England in the United Kingdom. The county town of Hampshire is Winchester, a historic cathedral city that was once the capital of England. Hampshire is notable for housing the original birthplaces of the Royal Navy, British Army, and Royal Air Force...
. It has yielded many fossils, including dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
s like Iguanodon
Iguanodon
Iguanodon is a genus of ornithopod dinosaur that lived roughly halfway between the first of the swift bipedal hypsilophodontids and the ornithopods' culmination in the duck-billed dinosaurs...
and Hypsilophodon
Hypsilophodon
Hypsilophodon is an ornithopod dinosaur genus from the Early Cretaceous period of Europe. It was a small bipedal animal with an herbivorous or possibly omnivorous diet...
. Apart from fossils, it shows many other signs of being deposited in a continental environment, such as mudcrack
Mudcrack
Mudcracks are sedimentary structures formed as muddy sediment dries and contracts.- Formation :...
s and -in some rare cases- dinosaur footprints.