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 .
16-570: The Lambeth Group is a stratigraphic group , a set of geological rock strata in the London and Hampshire basins of southern England . It comprises a complex of vertically and laterally varying gravels, sands, silts and clays deposited between 56-55 million years before present during the Ypresian age (lower Eocene ). It is found throughout the London Basin with a thickness between 10m and 30m, and
32-738: A group must not be defined by fossil taxonomy. Vadito Group The Vadito Group is a group of geologic formations that crops out in most of the Precambrian -cored uplifts of northern New Mexico . Detrital zircon geochronology and radiometric dating give a consistent age of 1700 Mya for the group, corresponding to the Statherian period . The Vadito Group is an extensive sequence of supracrustal rocks dominated by feldspathic gneisses , feldspathic schists , and metamorphosed sandstones and conglomerates . Its lower section includes amphibolites , though mafic beds make up less than 10% of
48-578: Is usually covered by the Harwich Formation , the oldest formation of the Thames Group . Group (stratigraphy) Groups are useful for showing relationships between formations, and they are also useful for small-scale mapping or for studying 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
64-850: The Glenwoody Formation in the Picuris Mountains. The Burnt Mountain Formation probably correlates with the Glenwoody Formation while the Big Rock Conglomerate corresponds to a disconformity in the Picuris Mountains. Detrital zircon dating indicates that the Marquenas Formation , formerly assigned to the Vadito Group, has a maximum age of 1435 Mya and is not part of the Vadito Group. The Vadito Group lies structurally above
80-591: The Hampshire Basin with a thickness between 50m and less than 25m. Although this sequence only crops out in these basins, the fact that it underlies 25% of London at a depth of less than 30m means the formation is of engineering interest for tunnelling and foundations. The formation was first known as the Plastic Clay by Thomas Webster in 1816 after the Argile plastique of Georges Cuvier and Alexandre Brongniart . It
96-512: The Hondo Group in the Picuris Mountains . However, both groups have been severely deformed and metamorphosed, and the Vadito Group is thought to actually be the older of the two groups. Cross-bedding indicates that the Vadito Group has been overturned. Three metamorphic episodes are recognized in the group. Only the third is well constrained in time, occurring around 1420 Mya and reaching
112-527: The Reading Formation appears on the coast at Studland Bay and at other points inland. The Hertfordshire puddingstone is a distinctive rock from near the base of the formation; a flint pebbly conglomerate in a siliceous matrix. The fossils, estuarine, freshwater and marine, include Corbicula cuneiformis , C. tellinella , Ostrea bellovacina , Viva parus lentus , Planorbis hemistoma , Melania (Melanatria) inquinata , Neritina globulus , and
128-922: The amphibolite facies. This is likely associated with the Picuris orogeny . The Vadito Group is interpreted to have been deposited in deep water in a volcanic back-arc basin (the Pilar basin) that opened during the Yavapai orogeny . The Vadito Group is intruded by the pegmatite of the Harding Pegmatite Mine , an historically important source of lithium , beryllium , and tantalum . The Vadito Group has also been mined for kyanite on Mesa La Jara ( 36°32′39″N 106°04′55″W / 36.5443°N 106.082°W / 36.5443; -106.082 ). Some 1500 tons of kyanite were extracted from Big Rock in 1928, and further prospecting took place in 1948–1949. However, there
144-695: 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) 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
160-531: 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 the confusion this causes, the official geologic timescale of the ICS does not contain any of these names. As with other lithostratigraphic ranks,
176-719: The remains of turtles , crocodiles , sharks , birds ( Gastornis ) and the mammal Coryphodon . Bricks, tiles and coarse pottery and occasionally firebricks have been made from the clay beds in this formation. Except in the Hampshire Basin, the Lambeth Group usually rests on the Thanet Formation of the Montrose Group , but is found on the Chalk Group near Bromley , Charlton , Hungerford , Hertford and Reading . It
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#1732845280065192-551: 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 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
208-646: The sediments that became the Vadito Group had ages of 1765 to 1704 Mya . The absolute age of the Burned Mountain Formation constituting the upper part of the Vadito Group in the Tusas Mountains is 1700 Mya, based on uranium-lead dating . The Vadito Group is divided into the Big Rock Formation and Burned Mountain Formation in the Tusas Mountains , and the upper Vadito Group is assigned to
224-627: The unit. It is found in the Picuris Mountains , the Tusas Mountains , the Truchas Range , the Mora River area, the Rincon Mountains ( 36°04′42″N 105°21′01″W / 36.0784234°N 105.3502279°W / 36.0784234; -105.3502279 ), the northern Taos Range , and in the Cimarron Range . Detrital zircon geochronology establishes that the principal source regions for
240-641: Was called the Mottled Clay by Joseph Prestwich in 1846, but in 1853 he proposed the name Woolwich-and-Reading Beds to emphasise the differing local aspects of the series. This name received widespread usage, but it was in turn deprecated in 1994 in favour of the Lambeth Group by the British Geological Survey , to conform with new standards and to allow scope for more detailed subdivisions. The Lambeth Group consists of three formations : In Dorset
256-538: Was no further development of the deposits. The kyanite pods have been interpreted as either metamorphosed pelitic silt lenses, as unusual pegmatites, or as hydrothermal systems. The unit was designated as the Vadito Formation in 1953 by Arthur Montgomery, who had first taken interest in the geology of the Picuris Mountains as a result of his wartime operation of the Harding Pegmatite Mine . P.E. Long recommended raising it to group rank in 1976 and this
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