Bouldnor Formation | |
---|---|
Stratigraphic range: Priabonian-Rupelian ~ | |
Type | Geological formation |
Unit of | Solent Group |
Sub-units | Bembridge Marls Member, Hamstead Member, Cranmore Member |
Underlies | Alluvium |
Overlies | Bembridge Limestone |
Thickness | >50 m (160 ft) |
Lithology | |
Primary | Clay |
Location | |
Region | England |
Country | UK |
The Bouldnor Formation is a geological formation in the Hampshire Basin of southern England. It is the youngest formation of the Solent Group and was deposited during the uppermost Eocene and lower Oligocene.
The Bouldnor Formation was named after Bouldnor, a small hamlet east of Yarmouth, Isle of Wight. The formation is exposed along Bouldnor Cliff between Yarmouth and Hamstead occupying the core of the east-southeast-striking Bouldnor Syncline.
Yet the stratotype of the formation is found at Whitecliff Bay on the east side of the Isle of Wight.
The Bouldnor Formation was scientifically established 1985 by A. Insole and B. Daly, who also defined its members. [1] The paleogene strata on the Isle of Wight had already been described in 1853 by Edward Forbes. [2] Forbes was followed in 1921 by H.J.O. White, a geologist from the Geological Survey. [3]
The Bouldnor-Formation is the topmost formation of the Solent Group before the sea withdrew completely from the Hampshire Basin. The thickness of the formation can vary between 45 and 115 metres. After a long hiatus Pleistocene and Holocene sediments covered the formation discordantly. The Bouldnor Formation lies concordantly on desiccation cracks of the upper Bembridge Limestone (Bembridge Limestone Formation), a freshwater deposit.
The formation consists mainly of clays with some intercalated sands which were sedimented along a coastal plain in lagoonal and lacustrine/palustrine facies judging by the enclosed freshwater, brackish and marine biota. Marine conditions were only rarely achieved, examples being the Bembridge Oyster Bed, the Nematura Bed and sections of the upper Cranmore Member.
A very diversified and well preserved biota can be found within the Bouldnor Formation comprising molluscs, vertebrates (especially mammals), charophytes and vascular plants. The nonmarine layers are characterized by gastropods like Australorbis, Lymnaea/Galba and Viviparus and ostracods like Gandona, Cypridopsis and Moenocypris. In the middle section (i.e. in the Hamstead Member) the effects of the Grande Coupure on the biota are clearly noticeable and follow immediately after the negative oxygen excursion Oi-1 at the beginning of the Oligocene.
Stratigraphically the Bouldnor Formation is subdivided into three members (from top to bottom):
The basal, 20 to 23 metres, exceptionally-35 metres-thick Bembridge Marls Member is mainly composed of blueish to greenish-gray clays and marls. Interlaced are several mollusc-bearing horizons. The clays show a rhythmical, varve-like layering. The member overlies the summital mudcracks of the Bembridge Limestone Formation without any discontinuity. It correlates magnetostratigraphically with the upper part of chron C 13r and biostratigraphically with the calcareous nannoplanktonzone NP21. The member therefore belongs to the upper Priabonian and has an absolute age of 34.0 to 33.75 million years BP.
The Bembridge Marls Member was mainly sedimented in fresh or brackish water as indicated by cirripedia and gastropods like Terebia. The lower section of the member is of estuarine origin, whereas the upper section was laid down by rivers inhabited by prosobranchs like Viviparus. Relatively short-lived marine inraids are recognizable in horizons like the Bembridge Oyster Bed 1.5 metres above the base and a limestone band with bivalves like Corbicula and Nucula. Amongst the fish Amia sp. and other amiids have been found
The fossil contents of the Bembridge Marls Member are quite varied, with freshwater species like Lymnaea and Unio and marine taxa like Melanopsis, Meretrix and Ostrea. The Bembridge Insect Bed at the base of the member is a marly sand layer with a very rich insect fauna and many leaves. This layer constitutes a lagerstätte with very good preservation. Amongst the finds are coleoptera, diptera, hymenoptera and arthropods as for instance Aeschnophlebia andeasi , Oligoaeschna anglica and Vectaraneus yulei . Plant remains within the member include palm seeds and the fern Acrostichum.
Amongst the mammals are Anoplotherium latipes , Bransatoglis bahloi , Choeropotamus parisiensis , Ectropomys exiguus , Gesneropithex sp. , Glamys devoogdi , Haplomeryx zitteli , Heterohyus , Microchoerus edwardsi , Palaeotherium medium , Paroxacron sp. , Peratherium , Plagiolophus major , Plagiolophus minor , Saturninia gracilis , Suevosciurus ehingensis , Tarnomys schmidtkittleri , Theridomys bonduelli and Treposciurus .
The 20 to 70 meter thick Hamstead Member is divided in two by the Nematura bed (rich in Nematura parvula ).
The 10-meter-thick Lower Hamstead Member follows directly upon the Bembridge Marls Member with a 40-centimetre-thick olive to black seam, the Black Band. This layer is very rich in organic matter and was deposited under freshwater conditions. At its base it carries calcrete nodules and rootlets. The Black Band is overlain by roughly 4 metres of a greenish-greyish clay-silt interlayering. This changes to 3 metres of blue to brown, finely laminated clays including some shelly horizons. These clays are capped by a 1-metre-thick, blueish-greyish, clayey sand layer with ball-and-pillow-structure, contorted bedding and convolute bedding indicating dewatering of the sediment during diagenesis. This gravitationally unstable bed is known as the log bed for its up to 5-metre-long tree trunks. The log bed is clearly a freshwater deposit as it contains besides the tree trunks plenty of washed-up seeds of the species Potamogeton and Stratiodes, and also the leaves of monocotyledon and dicotyledon plants. Its faunal remains attest the last pre-Grande Coupure assemblage (MP20).
After a distinct hiatus follows unconformably the Nematura bed which closes the Lower Hamstead Member. This bed is almost 1 meter thick and is characterized by chocolate-brown ripple marks enclosed in alternating clays and sands. It attests brackish conditions with a lot of reworked wood debris. Besides molluscs like Nematura (now Stenothyra) and Polymesoda there are also marine dinoflagellates and the ostracode Hemicyprideis. Within the basal shell layer traces of an eroded soil horizon (paleosoil) were found. The hiatus underneath is estimated to have lasted 350.000 years.
The following mammal taxa were found in the Lower Hamstead Member:
Amphidozotherium cayluxi , Amphiperaterium exile , Anoplotherium latipes , Bransatoglis planus , Butselia biveri , Cryptopithecus , Eotalpa anglica , Glamys fordi , Palaeotherium curtum , Palaeotherium muehlbergi , Paradoxonycteris tobieni , Pseudoltinomys cuvieri , Ronzotherium sp. , Stehlinia minor , Suevosciurus ehingensis , Suevosciurus fraasi , Theridomys bonduelli and Xiphodon gracilis .
Amongst plants conifers start appearing, an example being Quasisequoia couttsiae and the pollen Inaperturopollenites magnus .
The Upper Hamstead Member can reach a thickness of 60 meters. It starts with a 3-meter thick interlayering of greenish-greyish clay and silt bearing decalcified Polymesoda shells. Roughly 10 metres above the base follows the Eomys bed and immediately above it the White Band also containing Polymesoda shells. After the Crocodile bed the member ends with 8 meters of turquoise, plastic clays with orange-red freckles. Intercalated are occasional brown, slickensided, laminated clays and some shell horizons. Worth mentioning is also the White lily bed in the upper third of the Upper Hamstead Member.
Mammal remains were also found in the Upper Hamstead Member. They belong to the following species:
Amphicynodon sp. , Amphiperatherium exile , Amphiperaterium minutum , Asteneofiber , Atavocricetodon atavus , Bothriodon velaunus , Butseloglis micio , Cryptopithecus , Elomeryx porcinus , Entelodon magnus , Eomys , Glamys fordi , Hyaenodon dubius , Isoptychus margaritae , Leptadapis sp. , Myxomygale antiqua , Paradoxonycteris tobieni , Pecora , Peratherium perriense , Pseudoltinomys gaillardi , Ronzotherium romani , Stehlinia gracilis , Tapirulus hyracinus and Tetracus .
Chronologically the Hamstead Member starts at the Priabonian/Rupelian boundary and reaches into the upper Rupelian. It comprises the chrons C 13n and the lower part of C 12r. In absolute age it covers the time span 33.75 to 32.5 million years BP.
The Cranmore Member on top of the Bouldnor Formation is merely 5 to 9 meters thick and consists mainly of blueish-greenish clays. It starts off as a brackish facies (Cerithium beds with Cerithium) but changes to marine in the Corbula beds (with Corbula pisum and Corbula vectensis ). The marine character is also underlined by the gastropods Hydrobia sp. , Pusillina turbinata , Sandbergeria vectiana , Strebloceras cornuides , Syrnola sp. and Teinostoma decussatum . [4] Also present are Viviparus lentus shells. The Cranmore Member belongs biostratigraphically to the calcareous nannofossil biozone NP23. The sedimentation stopped at the end of the member and the sea withdrew completely from the Hampshire Basin.
The Bouldnor-Formation consists of two second-order sequences, the sequence boundary (SB) being situated right underneath the Nematura bed. The first sequence already started at the base of the Bembridge Limestone Formation. The marine intervals within the Bouldnor Formation are interpreted as sea level highstands. The log bed was formed during retreating sea levels and are part of a falling stage systems tract (FSST). It is plausible that this fall in sea level correlates with the onset of glaciation in Antarctica at the beginning of the Oligocene.
The Eocene/Oligocene boundary most likely is situated below the sequence boundary in the Lower Hamstead Member or high in the upper Bembridge Marls Member.
Remark: This interpretation proposed by Hooker et al. (2009) is not accepted by all geologists. Gale et al. (2006) for instance place the sequence boundary much lower in the Bembridge Limestone Formation and also further subdivide the lower sequence into three sequences. [5]
The Grande Coupure within the Bouldnor Formation can be characterized by the enclosed biota as follows:
In the Upper Hamstead Member 16 new taxa appear for the first time and 11 disappear. Within the pre-Grand Coupure Lower Hamstead Member only 5 new appearances were registered, mainly European rodents like Butselia. Amongst the 16 newcomers at the Grande Coupure are 10 immigrant species from Asia. Noticeable is also a concurrent general reduction in diversity. Within the Bembridge Limestone Formation 47 taxa were present, whereas within the Upper Hamstead Member the number of species had reduced to 28. One should notice though that the minimum in diversity with 20 taxa was already reached within the Lower Hamstead Member. This argues for a much more drawn out process in the reduction of species setting in already before the Grande Coupure. The Grande Coupure itself is distinguished by the fairly rapid replacement of endemic species with immigrants from Asia.
The Chalk Group is the lithostratigraphic unit which contains the Upper Cretaceous limestone succession in southern and eastern England. The same or similar rock sequences occur across the wider northwest European chalk 'province'. It is characterised by thick deposits of chalk, a soft porous white limestone, deposited in a marine environment.
Dorset is a county in South West England on the English Channel coast. Covering an area of 2,653 square kilometres (1,024 sq mi); it borders Devon to the west, Somerset to the north-west, Wiltshire to the north-east, and Hampshire to the east. The great variation in its landscape owes much to the underlying geology, which includes an almost unbroken sequence of rocks from 200 to 40 million years ago (Mya) and superficial deposits from 2 Mya to the present. In general, the oldest rocks appear in the far west of the county, with the most recent (Eocene) in the far east. Jurassic rocks also underlie the Blackmore Vale and comprise much of the coastal cliff in the west and south of the county; although younger Cretaceous rocks crown some of the highpoints in the west, they are mainly to be found in the centre and east of the county.
The Purbeck Group is an Upper Jurassic to Lower Cretaceous lithostratigraphic group in south-east England. The name is derived from the district known as the Isle of Purbeck in Dorset where the strata are exposed in the cliffs west of Swanage.
The Hampshire Basin is a geological basin of Palaeogene age in southern England, underlying parts of Hampshire, the Isle of Wight, Dorset, and Sussex. Like the London Basin to the northeast, it is filled with sands and clays of Paleocene and younger ages and it is surrounded by a broken rim of chalk hills of Cretaceous age.
Anoplotherium is an extinct genus of herbivorous artiodactyl mammal, possibly belonging to or a close relative of the suborder Tylopoda, which lived in Europe from the Late Eocene to the earliest Oligocene. Fossils of Anoplotherium were first discovered in the gypsum quarries of Paris in 1804 and were subsequently described by French naturalist Georges Cuvier. One of the first Paleogene mammals to be described, 19th Century reconstructions of Anoplotherium can be seen at Crystal Palace Park.
Whitecliff Bay and Bembridge Ledges is a 131.6-hectare (325-acre) Site of Special Scientific Interest that lies around the coastline of the easternmost part of the Isle of Wight from the Bembridge harbour entrance in the north around Foreland to Whitecliff Bay to the south. The site was notified in 1955 for both its biological and geological features.
The Dongen Formation is a geologic formation in the subsurface of the Netherlands. The formation consists of early Eocene marine clay and sand. It is named after the town of Dongen in North Brabant.
The Pyrenees are a 430-kilometre-long, roughly east–west striking, intracontinental mountain chain that divide France, Spain, and Andorra. The belt has an extended, polycyclic geological evolution dating back to the Precambrian. The chain's present configuration is due to the collision between the microcontinent Iberia and the southwestern promontory of the European Plate. The two continents were approaching each other since the onset of the Upper Cretaceous (Albian/Cenomanian) about 100 million years ago and were consequently colliding during the Paleogene (Eocene/Oligocene) 55 to 25 million years ago. After its uplift, the chain experienced intense erosion and isostatic readjustments. A cross-section through the chain shows an asymmetric flower-like structure with steeper dips on the French side. The Pyrenees are not solely the result of compressional forces, but also show an important sinistral shearing.
The geology of the Isle of Wight is dominated by sedimentary rocks of Cretaceous and Paleogene age. This sequence was affected by the late stages of the Alpine Orogeny, forming the Isle of Wight monocline, the cause of the steeply-dipping outcrops of the Chalk Group and overlying Paleogene strata seen at The Needles, Alum Bay and Whitecliff Bay.
The Solent Group is a geological group in the Hampshire Basin of southern England. It preserves fossils ranging in age from Priabonian to Rupelian. The group is subdivided into three formations, the Headon Hill Formation, the Bembridge Limestone Formation and the Bouldnor Formation.
Ronzotherium is an extinct genus of perissodactyl mammal from the family Rhinocerotidae. The name derives from the hill of 'Ronzon', the French locality near Le Puy-en-Velay at which it was first discovered, and the Greek suffix 'therium' meaning 'beast'. At present 5 species have been identified from several localities in Europe and Asia, spanning the Late Eocene to Upper Oligocene.
The geology of Lebanon remains poorly studied prior to the Jurassic. The country is heavily dominated by limestone, sandstone, other sedimentary rocks, and basalt, defined by its tectonic history. In Lebanon, 70% of exposed rocks are limestone karst.
The geology of Tunisia is defined by the tectonics of North Africa, with large highlands like the Atlas Mountains as well as basins such as the Tunisian Trough. Geologists have identified rock units in the country as much as a quarter-billion years old, although most units date to the Mesozoic and Cenozoic, in the past 250 million years. Tunisia has a small but active mining industry and a significant oil and natural gas sector.
The island of Curaçao began to form within the past 145 million years, beginning in the Cretaceous, as part of the Lesser Antilles island arc. Because the island was submerged for large parts of its history, reef environments formed atop thick layers of mafic volcanic rock, producing carbonate sedimentary rocks.
The geology of Georgia is the study of rocks, minerals, water, landforms and geologic history in Georgia. The country is dominated by the Caucasus Mountains at the junction of the Eurasian Plate and the Afro-Arabian Plate, and rock units from the Mesozoic and Cenozoic are particularly prevalent. For much of its geologic history, until the uplift of the Caucasus, Georgia was submerged by marine transgression events. Geologic research for 150 years by Georgian and Russian geologists has shed significant light on the region and since the 1970s has been augmented with the understanding of plate tectonics.
The geology of Bosnia & Herzegovina is the study of rocks, minerals, water, landforms and geologic history in the country. The oldest rocks exposed at or near the surface date to the Paleozoic and the Precambrian geologic history of the region remains poorly understood. Complex assemblages of flysch, ophiolite, mélange and igneous plutons together with thick sedimentary units are a defining characteristic of the Dinaric Alps, also known as the Dinaride Mountains, which dominate much of the country's landscape.
The geology of Bahrain is poorly studied before the Cenozoic. Extensive sedimentary formations from the Eocene through recent times cover much of the island.
The geology of Mississippi includes some deep igneous and metamorphic crystalline basement rocks from the Precambrian known only from boreholes in the north, as well as sedimentary sequences from the Paleozoic. The region long experienced shallow marine conditions during the tectonic evolutions of the Mesozoic and Cenozoic, as coastal plain sediments accumulated up to 45,000 feet thick, including limestone, dolomite, marl, anhydrite and sandstone layers, with some oil and gas occurrences and the remnants of Cretaceous volcanic activity in some locations.
The geology of Kuwait includes extremely thick, oil and gas-bearing sedimentary sequences from the Mesozoic and Cenozoic. Kuwait is a country in Western Asia, situated in the northern edge of Eastern Arabia at the tip of the Persian Gulf.
The geology of Lithuania consists of ancient Proterozoic basement rock overlain by thick sequences of Paleozoic, Mesozoic and Cenozoic marine sedimentary rocks, with some oil reserves, abundant limestone, dolomite, phosphorite and glauconite. Lithuania is a country in the Baltic region of northern-eastern Europe.