In geology, a group is a lithostratigraphic unit consisting of a series of related formations that have been classified together to form a group. Formations are the fundamental unit of stratigraphy. Groups may sometimes be combined into supergroups .
19-588: The Wealden Group, occasionally also referred to as the Wealden Supergroup , 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
38-531: 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 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
57-463: 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 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
76-607: Is a species that has a stratigraphically problematic and geographically limited distribution. A global reference profile (a GSSP ) for 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
95-577: 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
114-648: 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 the Purbeck Group , which spans the Jurassic - Cretaceous boundary. Within
133-865: 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 the Mons Basin, including the Sainte-Barbe Clays Formation where large numbers of Iguanodon were found in the 19th century. Group (stratigraphy) Groups are useful for showing relationships between formations, and they are also useful for small-scale mapping or for studying
152-586: The Glenwoody Formation , other strata (particularly in the lower part of the group) remain undivided into formations. Some well known groups of northwestern Europe have in the past also been used as units for chronostratigraphy and geochronology . These are the Rotliegend and Zechstein (both of Permian age); Buntsandstein , Muschelkalk , and Keuper ( Triassic in age); Lias , Dogger , and Malm ( Jurassic in age) groups. Because of
171-945: 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 the Hastings Beds . In Oxfordshire , Buckinghamshire and Wiltshire ,
190-866: The Wingate Sandstone , the Moenave Formation , the Kayenta Formation , and the Navajo Sandstone . Each of the formations can be distinguished from its neighbor by its lithology , but all were deposited in the same vast erg . Not all these formations are present in all areas where the Glen Canyon Group is present. Another example of a group is the Vadito Group of northern New Mexico . Although many of its strata have been divided into formations, such as
209-655: 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
SECTION 10
#1732771773307228-715: 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 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
247-641: 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 a major eustatic (global) transgression of the sea. The Greensand ( Aptian / Albian in age) consists of marine deposits. The sequence in
266-531: 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
285-624: The beds of the English Purbeck Formation, named as 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
304-546: 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
323-529: The confusion this causes, the official geologic timescale of the ICS does not contain any of these names. As with other lithostratigraphic ranks, a group must not be defined by fossil taxonomy. 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
342-468: The stratigraphy of large regions. Geologists exploring a new area have sometimes defined groups when they believe the strata within the groups can be divided into formations during subsequent investigations of the area. It is possible for only some of the strata making up a group to be divided into formations. An example of a group is the Glen Canyon Group , which includes (in ascending order)
361-647: The time between 145.0 ± 4.0 Ma and 139.8 ± 3.0 Ma (million years ago). The Berriasian succeeds 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
#306693