Headon Hill Formation
The Headon Hill Formation is a geological formation in the Hampshire Basin in southern England . It is the lowest formation of the Solent group and was deposited in the Upper Eocene . The Headonium , a European land mammal zone of the Upper Eocene, was named after her .
Type locality and occurrence
The Headon Hill Formation got its name from Headon Hill (also Headon Warren ), a hill southwest of Totland on the Isle of Wight . The hill separates Totland Bay from Alum Bay and represents the type locality for the formation. From this type locality, the Headon Hill Formation then stretches in a narrow band to the east across the Isle of Wight to at Whitecliff Bay again to meet the sea. The formation is also pending in the north of the island, on the coast near Ryde , near Cowes and in the east of Newtown Bay . The Headon Hill Formation is not restricted to the Isle of Wight, but continues on the opposite mainland at Milford on Sea , Hordle and in the New Forest .
history
The first fossil finds from Mammal Bed date back to 1844 and were described by Wood. A first detailed scientific description of the Headon Hill Formation as part of the Solent Group comes from Edward Forbes from 1853, who divided the current formation into three (Lower, Middle and Upper Headon Beds). He was followed by HJO White on behalf of the Geological Survey in 1921. It was not until 1985 that A. Insole & B. Daley established the Headon Hill Formation as an independent formation within the Solent Group. In 1999 B. Daley undertook an extensive revision of all paleogenic formations on the Isle of Wight .
stratigraphy
The Headon Hill Formation, around 90 meters thick on average, is the lowest formation in the Solent Group. It follows concordantly on a final, clayey paleo-soil of the underlying Becton sand formation from the Barton group . The marly hanging wall of the formation is concordantly overlaid by the Lower Limestone Band of the Bembridge Limestone Formation .
The Headon Hill Formation was deposited primarily in fresh and brackish water . The depositional environment consisted of behind sand bars located lagoons , in the smaller rivers flowed and the farther inland from Lake were lined. A temporary marine incursion occurred only in the lower Colwell Bay member .
In terms of lithology , the formation consists mainly of clays , sandy clays and silts , with sands and freshwater limestone as a subordinate feature .
Lithological sequence
The sequence of Headon-Hill formation is due to significant thickness changes and facies relatively complex changes and is composed (from bottom to top ends slope) of many layer members ( engl. Members with distinctive horizon):
- Seagrove members
- Osborne member
- Fishbourne member
- Lacey's Farm Member
- Nettlestone member
- Cliff-end member
- Colwell Bay Member
- Totland Bay Member
Totland Bay Member
The 10 to 15 meter thick Totland Bay Member , named after the Totland Bay in the west of the Isle of Wight, is the lowest member of the Headon Hill Formation. It is equivalent to the former Lower Headon Beds . The first 5 meters consist of light green clays that contain iron concretions in the base layer. Slightly above the base is the Mammal Bed , where mammal remains were found. Above this again light green clays with switched-in white sand layers and remains of peel from Viviparus lentus follow . About 5 meters of purple and brown sands lay over the clays (they contain the Leaf Bed and the Crocodile Bed ). In its upper section the member becomes clay again and closes with a light brown marl layer, the Rodent Bed (this marl layer is formed as a 3 meter thick limestone band on Headon Hill, the Howe Ledge Limestone ). The final tones are initially greenish-blue and become increasingly deep gray in the hanging wall; they run the Chara Bed , a Lignithorizontal and the Unio Bed . To the overlapping Colwell Bay member there is a layer gap, the underside of which is riddled with thalassinoides structures.
Colwell Bay Member
The Colwell Bay Member , named after Colwell Bay , consists mainly of sandy clays and sandy silts that were deposited in shallow marine and mollusc-rich, marine facies. The maximum between 20 and 30 meters thick member is identical to the former Middle Headon Beds . The lower section of the Colwell Bay member represents the only fully marine stratification of the entire Solent group, the member only becomes brackish again at the hanging wall. It contains up to 5 layer gaps ( engl. Omission surfaces ) against which it in parasequences can be divided. The Colwell Bay member shows clear, lateral facies changes and the thicknesses also fluctuate strongly (for example, it is only 5 meters thick on the mainland and in the west of the Isle of Wight).
The sequence begins with the silty, glauconite Brockenhurst Bed , which rests on a marine transgression surface. It contains a steno- to polyhaline faunal community with bivalves such as Crassostrea ventilabrum , Cardita deltoidea and Pseudocominella semiscostata , gastropods such as Athleta dunkeri and ostracods from the Hazelina community. The Brockenhurst Bed could be assigned to the nannofossil zone NP19 / NP20 due to its marine microfossils . This is followed by silts with sidereal mudstone concretions. The marine mollusc fauna of the Silte is dominated by Varicorbula gibba . Included in the lower section is the Ditrupa horizon , a very valuable location for the lateral correlation. It consists of worm structures of the taxon Ditrupa ( Serpulidae ). The subsequent Roydon Zone is finer-grained (clay) and is separated in places by a layer gap. The marine gastropod Neoathleta geminata is characteristic of them . The marine sedimentation ends shortly before the brackish Venus Bed , which has three distinctive shell layers and two layer gaps. This sandy-silty section contains the characteristic shells of Pelecyora suborbicularis (or Sinodia suborbicularis ). The three shell layers of the Venus Bed are each characterized by Sinodia (lower layer), Galba (middle layer) and Corbicula obovata (uppermost). Colwellia flexuosa also occurs in the Venus Bed . The lower of the two layer gaps lies over a paleo soil , the upper over green, pedogenic clays. Above the Venus Bed are gray silts and green clays, which are highlighted by two sidereal concretions, the East Cowes Siderite Nodules . After a renewed green-gray, pedogenic tone, the Colwell Bay member closes with a brackish, fine-grained, yellowish sand layer, the Batillaria Concava Bed , which is very rich in oysters and mollusks ( Batillaria concava and Sinodia). The Colwell Bay member is usually concordantly overlaid by the cliff end member , but in the west of the sedimentation area discordantly by the Linstone Chine member , which is inserted between the two members.
Linstone Chine Member
The Linstone Chine Member , named after the hanging valley Linstone Chine , is a lenticular sand layer with an erosive base. It was formerly the base of the Upper Headon Beds that ended with the cliff-end member. The member has been deposited in the fresh water and oligohaline area and contains in its lower portion abundant shell residues of Potamomya . The sand is often traversed by fibrous clay membranes. The hanging wall is followed by the limestone plaque of Hatherwood Limestone , which is often viewed as an independent Hatherwood Limestone member . The Hatherwood Limestone remains limited to the western part of the deposit area, so that the subsequent cliff-end member usually follows directly on the Linstone-Chine member.
Cliff-end member
The Cliff-End-Member , named after the headland Cliff End north of Totland Bay, is 20 to 30 meters thick. The member, once the upper section of the Upper Headon Beds , consists mainly of brightly colored clays, claystones and marls with subordinate sand inclusions. At its base it is formed as a gray, silty clay that was drawn into the underlying worm structures of the Linstone Chine member. The base clay contains siderite concretions and is quite rich in Corbicula obovata and Ptychopotamides . Above that there are clear layers of potamomya pods . The member ends in yellow-gray, spotted, silty tones that are of pedogenic origin.
Nettlestone member
The 6 meter thick Nettlestone member , named after the town of Nettlestone east of Ryde , lies concordantly over the cliff end member with a thin layer of limestone. The Osborne Beds began with him earlier . It is made up of fine-grain, cemented sandstones , which are initially formed in layers (with filled worm structures), but then merge into flat-angled sloping bodies. Switched on are conglomeratic flint layers as well as cavities that are lined with Viviparus lentus shell remains. The member becomes very calcareous on its last meter and ends with a paleocarst , the Seaview paleocarst .
Lacey's Farm Member
The Lacey's Farm Member , often also Lacey's Farm Limestone Member , is between 3 and 7 meters thick. It is extremely calcareous and contains greenish, sandy marls , marl limestone, sandy limestone and calcareous quartz sandstone . Depressions up to 1 meter deep in the Seaview Paleocarst below are filled with pale green clay. Shallower depressions are sedimented by shell limestone. Then thin, pale yellow marl limestone layers of the actual member follow. The gastropods Viviparus and Galba are present as fossils . The member ends with pale green clays.
Fishbourne member
The Fishbourne Member , only around 4 meters thick , named after the village of Fishbourne west of Ryde, lies concordantly on the Lacey's Farm Member and is made up of dark gray to bluish tones. The characteristic fossil is the quite common genus Tarebia . The member closes with the Chapelcorner Fish Bed , a finely laminated pitch.
Osborne member
The Osborne Member , which is very incompetent and prone to landslides , was named after Osborne Bay or Osborne House , Queen Victoria's country residence , and was previously viewed as an independent unit overlying the Headon Beds. The concordant member is subject to strong fluctuations in thickness and can be between 3 and 20 meters thick. It is composed almost exclusively of brightly colored (red, gray, bluish), silty clays, some of which are of educational origin.
Seagrove members
The more than 8 meters powerful and quite competent Seagrove member (or Seagrove Bay member), named after Seagrove Bay , transgresses with erosive contact via the underlying Osborne member and in places reaches very deeply into the underlying member. The Seagrove member wedges to the west. It is primarily a yellow, solid, fine-grained sandstone, which has thin ribbed layers at its base and is interspersed with concretions and thin layers of clay. The marly slope end of the member, and in places as Paläokarst over a palaeosol formed ( Horestone Paläokarst ), closes with a Flint conglomerate, the washed coarse sediments an abandoned river channel (engl. Channel lay deposit). The Seagrove member is then concordantly overlaid by the Bembridge Limestone Formation .
Fossil content
A very rich plant community is present in the leaf bed of the Totland Bay member. In addition to up to 1 meter long tree trunk remains and tree stumps in their original position (taxon Glyptostroboxylon ), it contains a great deal of pollen material, on the basis of which 76 plant families with 100 genera can be reconstructed. The vegetation found suggests a predominantly moist location. In the Colwell Bay Member and the Linstone Chine Member, floral remains (here with conifers) of a total of 38 genera have also been described, including 8 endemic taxa.
Also charophytes are Headon Hill Formation quite frequently, then the Totland Bay member characterized by Gyrogona , the Cliff-end member by Harrisichara (Tectochara) that Lacey's farm Members by Stephanochara and Harrisichara and Osborne -Member out by Nitellopsis and Harrisichara.
In addition to the already mentioned bivalves and gastropods, which often occur in layers, ostracodes should be mentioned. Ostracods are very sensitive to salinity , so ostracodal associations can be used to determine salinity. The following associations occur in the Headon Hill formation:
- Candona / Cypridopsis and Moenocypris in the fresh water area (0 - 3 ‰)
- Cytheromorpha in the mesohaline range (3 - 9 ‰)
- Neocyprideis in the mesohaline range (9 - 16.5 ‰)
- Haplocytheridea in the polyhaline range (16.5 - 33 ‰)
- Hazelina in the fully marine area (> 33 ‰)
To fish finds in particular that's Chapel Corner Fish Bed mention in Fishbourne member. It contains Amia sp. , Potamoschistus bleicheri and Vectichthys vectensis . In the green pitches directly above the Chapelcorner Fish Bed, in addition to numerous bones and scales from Leidosteus , reptile remains came to light, including alligators , snakes ( Boidae ), frogs and above all turtles such as Chelone , Emys and Trionyx . The alligator Diplocynodon hantoniensis had already been described by Wood in 1844.
The Headon Hill Formation also carries numerous mammalian taxa , including various Palaeotherium ( Palaeotherium curtum , Palaeotherium medium , Palaeotherium muehlbergi ). Amphirhagatherium edwardsi , an early cloven-ungulate was discovered in the Crocodile Bed of the Totland Bay Member, the Hatherwood Limestone Member and the Linstone Chine Member. Plagiolophus annectens and Plagiolophus minor , two other Palaeotheridae, come from the Osborne and Seagrove members. The Totland Bay member contained the insect eater Eotalpa anglica . With Dichodon from the Seagrove member and Xiphodon , two Xiphodontidae are also to be mentioned.
Noteworthy is Microchoerus erinaceus , an early primate after which the Microchoerus Bed was named in the Linstone Chine member.
Recent finds in the Hordle Cliff (Totland Bay Member) expand the list of Säugetiertaxa around the Proteutherium Opsiclaenodon major , the marsupial Amphiperatherium that Apatotherium Heterohyus nanus , the cloven pumilum Acotherulum , Dichodon cervinus , Dichodon cuspidatus , Pseudamphimeryx hantoniensis , Tapirulus perrierensis and Xiphodon gracilis , the Lipotyphla Gasneropithex grisollensis , Saturninia grisollensis , Saturninia aff. tobieni , Saturninia gracilis and Scraeva woodi that rodents Glamys priscus , Gliravus daamsi , Isoptychus euzetensis , Paradelomys quercyi vectisensis , sciuroides ehrensteinensis , Suevosciurus bosmae , Thalerimys fordi and Treposciurus mutabilis and other primates like Leptadapis magnus , Protoadapis ulmensis and Pseudoloris parvalus .
Age
Absolute ages for the Headon Hill Formation are unknown. The formation can, however, be delimited biostratigraphically . The Colwell Bay member belongs to the nannofossil zone NP 19/20 and the concordant Bembridge Limestone Formation to the land mammal horizon MP 20 . The formation is believed to have been deposited between 36.5 and 34.1 million years BP . It covers almost the entire Priabonium .
literature
Individual evidence
- ^ Forbes, E .: On the fluvio-marine Tertiaries of the Isle of Wight . In: Quarterly Journal of the Geological Society of London . tape 9 , 1853, pp. 259-270 .
- ↑ White, HJO: A Short Account of the Geology of the Isle of Wight . In: Memoirs of the Geological Survey of Great Britain . 1921.
- ↑ Insole, A. & Daley, B .: A revision of the lithostratigraphical nomenclature of the Late Eocene and Early Oligocene strata of the Hampshire Basin, southern England . In: Tertiary Research . tape 7 , 1985, pp. 67-100 .
- ^ Daley, B .: Palaeogene sections on the Isle of Wight. A revision of their description and significance in the light of research undertaken over recent decades . In: Tertiary Research . tape 19 , 1999, p. 1-69 .
- ^ Fowler, K., Edwards, N. & Brett, DW: In situ coniferous (taxodiaceous) tree remains in the Upper Eocene of Southern England . In: Paleontology . tape 16, 1 , 1973, pp. 205-217 .
- ↑ Holman, JA, DL Harrison and DJ Ward: Late Eocene snakes from the Headon Hill Formation, southern England . In: Cainozoic Research . tape 5 (1-2) , 2006, pp. 51-62 .
- ↑ Holman, JA and DL Harrison: A new helmeted frog of the genus Thaumastosaurus from the Eocene of England . In: Acta Palaeontologica Polonica . tape 48 (1) , 2003, pp. 157-160 .
- ↑ Hooker, Jerry and Katherine M. Thomas: A new species of Amphirhagatherium (Choeropotamidae, Artiodactyla, Mammalia) from the late Eocene Headon Hill Formation of Southern England and phylogeny of endemic European 'Anthracotherioids'. In: Palaeontology 44 (5), 2001, pp. 827-853
- ↑ Hooker JJ and Harrison, DL: A new clade of Omomyid primates from the European Paleogene . In: Journal of Vertebrate Palaeontology . tape 28 (3) , 2008, p. 826-840 .