Bouldnor-Formation

The Bouldnor Formation is a geological formation in the Hampshire Basin, southern England. She is the youngest formation of the Solent Group and was deposited in the Eocene and Lower Oligocene Supreme. Your Fauneninhalt documents the Grande Coupure.

• 7.1 Literature
• 7.2 External links
• 7.3 Notes and references

Type locality and occurrence

The Bouldnor formation was named after the east of Yarmouth (Isle of Wight ) located village Bouldnor. Along the coastal erosion between Yarmouth and Hamstead in the northwest of the island is the entire formation, which here in an east-southeast - trending open syncline - the Bouldnor Syncline - was folded, very nice open-minded.

However, type locality ( stratotype ) of the formation is the Whitecliff Bay on the east coast of the Isle of Wight.

History

The Bouldnor lineup was not until 1985 that established scientifically by A. Insole & B. Daly and her shift elements ( engl. member) defined. However, a first description of the Paleogene of the Isle of Wight has been going on E. Forbes back to the year 1853. He was followed H.J.O. White in 1921.

Stratigraphy

The Bouldnor lineup is the final formation of the Solent Group in the Hampshire Basin, then the pool was dry. The thickness of the formation ranges from 45 115 meters. After a long hiatus to Hüllsedimente the Pleistocene and the Holocene deposited unconformably over the formation. At its base the Bouldnor lineup follows conformably on dry cracks on the top Bembridge Limestone of Bembridge Limestone Formation, a Süßwasserkalk.

The Bouldnor lineup consists mainly of clays and subordinate sands along a coastal plain in various beach- Faziesräumen ( lagunär, lacustrine / palustrin and fluvial ) were deposited - to recognize the contained faunal communities from the fresh water, brackish water and marine area. Only rarely were marine conditions as in Bembridge Oyster Bed, Nematura Bed and realized in the upper Cranmore Members.

The lineup includes a diversified and well-preserved Faunengemeinschaft consisting of mollusks, vertebrates (especially mammals), charophyte and vascular plants. The non-marine sections are characterized by the gastropod Orbis Australasia, Lymnaea and Viviparus and ostracods Candona, Cypridopsis and Moenocypris from. In its central section ( Hamstead Members ), the Faunenschnitt the Grande Coupure reveals that correlates with the negative oxygen isotope anomaly Oi -1 of the lowermost Oligocene.

• Cranmore Members
• Hamstead Members
• Bembridge Marl Members

Bembridge Marl Members

The basal, about 20 to 23 meters ( some places even up to 35 meters) powerful Bembridge Marls member is constructed from blue -gray to green-gray, finely laminated clays and marls. It contains numerous horizons with Molluskenlagen. The clays show Warvenschichtung. The member shall be concordant on the Bembridge Limestone Formation, whose final horizon is crossed by dry cracks. The member magnetostratigraphisch largely correlated with the upper portion of Chron 13r C, and biostratigraphically with the lime nannoplankton zone NP21. It thus belongs to the top Priabonium and is expected to be 34.0 to 33.75 million years old.

The Bembridge Marl Members is mainly in freshwater and brackish water ( indicated by Cirripedia and gastropods Tarebia ) were sedimented. The lower section is from the estuarine area and the Upper is fluvial origin, recognized by the prosobranchen taxon Viviparus. Horizons such as the Bembridge Oyster Bed 1.5 meters above the base and about a Kalklage with corbiculiden Bivalvia and Nucula suggest, however also on relatively short-term marine incursions.

As contained fossils are mentioning the Frischwassertaxa Lymnaea and Unio and the marine species Melanopsis, Meretrix and Ostrea. Of fish are present Amia sp. and other indefinite Amiidae. The Bembridge Insect Bed at the base is a Kalksandlage that contains a very rich insect fauna and many leaf prints. The layer provides a fossil site with very good condition dar. Found Coleoptera, Diptera, Hymenoptera and arthropods such as Aeschnophlebia andeasi, Oligoaeschna anglica and Vectaraneus yulei.

Herbal discoveries in Bembridge Marl Members are palm pollen and fern Acrostichum.

At mammal remains to Anoplotherium latipes and Anoplotherium commune, 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 found gracilis, Suevosciurus ehingensis, Tarnomys schmidtkittleri, Theridomys bonduelli and Treposciurus.

Hamstead Members

The Mighty between 20 to maximum 70 meters Hamstead Members is the Nematura Bed (named after the gastropods Nematura parvula ) in two parts.

The 10 meter thick Lower Hamstead Members follows concordant with the basal 40 cm mighty Black Band - a olivschwarzen, organic silt Location - on the Bembridge Marls Members. The Black belt was deposited in fresh water and leads to his Bais Krustenkalkknollen (English calcrete nodules ) and Wurzelrhizome. In addition to nearly 4 meters wield massive, green-gray clay - silt mixed layer. Then sedimented 3 meters of blue to brown, feinlagigen clays with shell layers in the central region of the sequence. The clays are unconformably covered by up to a meter thick, pale, blue-gray, clayey sands, indicate the ball -and- pillow structure, changing stratification and Convolute bedding. This very unstable situation is called a log bed as it contains up to 5 meter logs. In the freshwater area clearly belonging log bed with washed seeds of Potamogeton and Stratiodes and on and zweikeimigen leaves also are the last faunal elements ( MP20 ) before the Grande Coupure.

The Lower Hamstead Members ends after the log bed and a significant hiatus bed with the discordant, almost 1 meter thick Nematura. The Nematura bed consists of characteristic chocolate brown ripples in alternating overlapping clays and sands. It is brackish and contains remanufactured wood residues. At fossils it contains the mollusks Nematura (now Stenothyra ) and Polymesoda the ostracods Hemicyprideis and marine dinoflagellates. In its basal shell layer evidence of an eroded paleosol were detected. For the hiatus, a period of 350,000 years is estimated.

From the Lower Hamstead Members taxa Amphidozotherium cayluxi, Amphiperaterium exile, Anoplotherium commune, Anoplotherium latipes, Bransatoglis planus, Butselia biveri, Cryptopithecus, Eotalpa anglica, Glamys Fordi, Palaeotherium curtum, Palaeotherium muehlbergi, paradoxonycteris tobieni, Pseudoltinomys cuvieri, Ronzotherium sp come. , Stehlinia minor, Suevosciurus ehingensis, Suevosciurus fraasi, Theridomys bonduelli and Xiphodon gracilis.

At plants now occur conifers, such as quasi Sequoia couttsiae with the pollen Inaperturopollenites magnus.

The up to 60 m thick Upper Hamstead Members starts again with three meters thick, greenish-gray clay - silt layers alternating with entkalzifizierten Palymesoda shells. About 10 meters above the base follows the Eomys bed and shortly after the White Band, also with Polymesoda shells. After the intermediary Crocodile bed the member ends with 8 meters of turquoise, plastic clays, which are mottled orange-red. Perform occasional brown, laminated pitches and some shell horizons. The plastic clays show Harnisch Trie munging. Worth mentioning also the White lily bed in the upper third.

Also in the Upper Hamstead Members found themselves mammalian residues whose ensemble reveals the impact of the Grande Coupure fauna well. The Grande Coupure must therefore be carried out between the Lower and Upper Hamstead Members, first faunal after the Grande Coupure appear 4 meters after the start of the Upper Hamstead - members.

The Upper Hamstead Members contains the following taxa: 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 antiquarian, Paradoxonycteris tobieni, Pecora, Peratherium perriense, Pseudoltinomys gaillardi, Ronzotherium romani, Stehlinia gracilis, Tapirulus hyracinus and Tetracus.

The Hamstead Members begins at the border Priabonium / Rupelium and dates back to the Upper Rupelium up. It includes the C Chron's 13n and the lower part of C 12r. The age of the member thus amounts to the period 33,75 bis 32.5 million years BP.

Cranmore Members

The final, 5-9 meters thick Cranmore member consists mainly of blue to blue- green toning. It comes initially to the brackish water area ( Cerithium Beds with Cerithium ), but with the nannofossils - leading Corbula Beds ( with Corbula pisum and Corbula vectensis and the gastropod Hydrobia sp., Pusillina turbinata Sandbergeria vectiana, Strebloceras cornuides, Syrnola sp., And Teinostoma decussatum ) then clearly marin. There are Viviparus lentus mussels to other fossils. It belongs to the biostratigraphically Biozone NP23. After completion of the sedimentation of the Cranmore Members to the sea he finally retired from the Hampshire Basin.

Sequence stratigraphic interpretation

The Bouldnor lineup is one of two second-order sequences whose limit ( engl. sequence boundary or SB) below the Nematura Beds runs. The beginning of the first sequence is still at the base of the Bembridge Limestone Formation. The marine intervals within the Bouldnor formation are interpreted as high stands of sea level. The log bed was at sea retreat ( engl. falling stage systems tract or FSST ) formed, caused by the glaciation in Antarctica.

The boundary of the Eocene / Oligocene is likely to be below the sequence boundary in the Lower Hamstead Members or possibly the top Bembridge Marl Members.

Note: This by J. J. Hooker et al ( 2009) interpretation was represented by AS Gale et al (2006 ) is not recognized. Place the sequence boundary much deeper into the Bembridge Limestone Members and share also the lower sequence into three sequences.

Grande Coupure

The Grande Coupure can be based on the mammalian fauna of the Bouldnor lineup characterized as follows:

In the Upper Hamstead Members 16 new taxa occur for the first time, 11 are dying out at the turn. In -off before the turn of the Lower - Hamstead Members comparatively only 5 first appearance busbars were recorded predominantly European rodents and Butselia. Among the 16 new releases after the Grande Coupure are 10 immigrant Asian taxa. In addition, a general reduction in biodiversity are found. Were still present 47 taxa in the Bembridge Limestone Members, then the upper - Hamstead Members show only 28. It should be remembered that the low point was in the biodiversity already in the Lower Hamstead Members with 20 taxa. This suggests therefore to no abrupt extinction towards, but a long process of decline starting in the Lower Hemstead Members. The real Grande Coupure of displacing endemic species by immigrants from Asia may have gone relatively rapid.