Becton Sand Formation

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Valley of the eponymous "Becton Bunny"; in the background the cliffs of are Isle of Wight recognizable

The Becton Sand or Becton Formation is a sedimentary formation in the Hampshire Basin in southern England . It is the uppermost formation of the Barton group and was deposited in the late Upper Middle Eocene ( Bartonian ).

Name and type locality

The Becton Sand Formation got its name from the "Becton Bunny", a small stream that flows into the English Channel about 1 kilometer east of Barton-on-Sea . The origin of the local name " Becton " is unclear. Eilert Ekwall derives place names such as “ Becton ” or “ Beckton ” from the old English terms “ bæce ” (“stream”, “river”) and “ tūn ” (“fence”, “homestead”, “settlement”) and combines them accordingly "Homestead in the valley". The stretch of coast east of Barton-on-Sea represents the type locality of the Becton Sand Formation. It forms a precipice that is exposed to relatively strong erosion with landslides arranged like stairs . The coast recedes here by an average of 1.0 (1931–1967) to 1.5 meters / year (1967–2001).

history

The Barton group was originally divided into 14 "Faunen Horizons" ("Bed A" (1-3) to "Bed L"). The Becton Sand Formation, or “Becton Sands” for short, was formerly also known as “Headon Hill Sand”, “Upper Bagshot Sand”, “Barton Sand” or “Upper Barton Beds”. The designation was introduced in 1986 by Jerry J. Hooker who thus summarized the sand-dominated, hanging parts of the Barton group (upper part from “Bed H” to “Bed K” and it corresponds to the clay dominated parts of the Barton Clay Formation (“Bed A1 “To the lower part of“ Bed H ”in the lying position .

Independently of Hooker, the term is also used in 1987 by Richard A. Edwards and Edward Cameron Freshney , who separate the lying parts of Hooker's Becton sand formation as an independent Chama sand formation (corresponds approximately to "Bed H"). This classification is partially rejected by later authors, since the Becton sand formation and the Chama sand formation hardly differ from one another lithologically .

stratigraphy

The coast east of Barton-on-Sea. The valley of the Becton Bunny in the foreground. The Becton Sand Formation lies above the Barton Clay Formation

The Becton sand formation, which is up to 70 meters thick , lies concordantly on the underlying Barton clay formation. It is in turn concordant from a lignite leading clayey palaeosol superimposed, the formerly known as "Bed L" has been assigned yet to the Barton group and today as the basis of Totland Bay formation ( "Totland Bay Member" of Headon-Hill- Formation ) of the Solent group .

The generally sandy formation was deposited in the inner neritic and sand-dominated coastal areas . It consists lithologically mainly of fine to coarse-grained sands and sandy clays. In the upper section, it shows a flattening of the deposit environment and brackish tendencies.

Christopher King divides the Becton-Sand-Formation into three members (from the lying to the hanging wall):

Barton Common Member

The sequence of glauconite-bearing , bioturbatic , sandy, clayey silts and silty sands roughly encompasses the layers "Bed H" and "Bed I" of the older literature. The Chama sand formation ("Bed H") proposed by Edwards & Freshney in 1987 essentially corresponds to the lying parts of this member with typical shell remains of the Chama squamosa mussel . In addition, more representative of occur trapezoidal shells glans (Venericardia) oblonga , Cardita and Crassatella and representatives of turritellidae Turritella and the roller screw Athleta , which is also a fully marine indicate depositional environment.

Becton Bunny Member

The strongly silty and sandy, brown clays with an increased proportion of organic matter correspond to the original "Bed J". This section was also known as the “Becton Bunny Bed” and, after the abundant gastropod Olivancillaria (Pseudolivella) branderi (formerly Olivella ventricosa or Oliva branderi ), also called the “Oliva branderi Zone”.

About 3 meters above the base, there is a horizon with round, up to 10 centimeters, siderite - concretions (with remains of decapods the genus Callianassa ). Against the hanging wall towards the bunny Becton member is marly brackish water fossils of estuarine area such as Asiatic clams ( Corbicula ) or snails of the genus Bayania and Potamides . Marine taxa in the Becton Bunny Member are glans (Venericardia) oblonga , olivancillaria (Pseudolivella) branderi , nut clams of the genus Nucula and clams of the genus Pitar . The Becton Bunny Member is also Stratum typicum (the found layer of the holotype) for the sea ​​urchin Echinopedina paucituberculata .

Long Mead End Member

Batillaria pleurotomoides

The mostly well-stocked, bioturbatic and glauconite-free fine sands are also known as "Long Mead End Bed" or "Bed K". In the area of ​​the type locality, a fossil-rich site with typical brackish water fauna occurs in the upper section of the member ("Batillaria pleurotomoides Bed"). In addition to the snail Batillaria pleurotomoides , the gastropods Olivancillaria (Pseudolivella) branderi and Bayania hordacea as well as the clams Cyrena gibbosula and Lentidium nitidum are also known.

Fossil content

In addition to the bivalvia and gastropods already mentioned, nautilus and remains of tortoises of the genus Puppigerus were also found in the Becton sand formation .

Age

The absolute age of the Becton Sand Formation is not known and biostratigraphic data only allow an approximate age. Dinoflagellates from the lying parts of the formation can be assigned to the Bartonian or the earliest Priabonian . However, there is no corresponding fossil evidence for a more precise delimitation of the hanging parts. The mammal fauna in the most lying parts of the overlying Solent group (early Priabonian) indicates a similar age. The original location of the “Chama Bed” (“Bed H”) in the nannoplankton zone NP17 was strongly doubted by later authors.

King suspects a deposit of the formation in the late Bartonian and proposes, as a provisional working hypothesis, to equate the hanging wall boundary of the Becton-Sand Formation with the turning Bartonian / Priabonian, which would correspond to an age of around 38 million years.

Individual evidence

  1. ↑ CN Waters, K. Smith, PM Hopson, D. Wilson, DM Bridge, JN Carney, AH Cooper, RG Crofts, RA Ellison, SJ Mathers, BSP Moorlock, RC Scrivener, AA McMillan, K. Ambrose, WJ Barclay & AJM Barron: Stratigraphical Chart of the United Kingdom: Southern Britain. British Geological Survey, 1 poster, 2007. (digitized version)
  2. ↑ a b J. J. Hooker: Mammals from the Bartonian (Middle / Late Eocene) of the Hampshire Basin, southern England. In: Bulletin of The British Museum (Natural History) Geology , Vol. 39, No. 4, pp. 191–478, 1986. (digitized version )
  3. ^ E. Ekwall: The Concise Oxford Dictionary of English Place-Names. 3rd edition, 530 pp., Oxford University Press, 1947. (digitized version)
  4. ^ S. Brown & ME Barton: Downdrift erosion and the frequency of coastal landsliding. In: R. McInnes, J. Jakeways, H. Fairbank & E. Mathie (Eds.): Landslides and Climate Change: Challenges and Solutions: Proceedings of the International Conference on Landslides and Climate Change, Ventnor, Isle of Wight, UK, 21–24 May 2007. pp. 429–433, Taylor & Francis, 2007. ISBN 978-0-415-88937-7 (reading sample)
  5. ↑ E. StJ. Burton: The horizons of Bryozoa (Polyzoa) in the Upper Eocene Beds of Hampshire. In: Quarterly Journal of the Geological Society , Vol. 85, pp. 223-239, 1929.
  6. ↑ a b E. StJ. Burton: Faunal horizons of the Barton Beds in Hampshire. In: Proceedings of the Geologists' Association , Vol. 44, pp. 131-167, 1933.
  7. ^ A b c d R. A. Edwards & EC Freshney: Geology of the country around Southhampton. In: Memoir for 1:50 000 geological sheet 315 (England & Wales) , 111 p., British Geological Survey, 1987. (digitized version)
  8. ↑ a b c d e f g h i j k C. King: A revised correlation of Tertiary rocks in the British Isles and adjacent areas of NW Europe. In: Geological Society of London - Special Report , No. 27, 724 pp., 2016. (excerpt)
  9. ↑ C. King: Paleogene and Neogene: uplift and a cooling climate. In: PJ Brenchley & PF Rawson (eds.): The Geology of England and Wales , 2nd edition, 559 pp., The Geological Society, 2006. ISBN 978-1-86239-200-7 (reading sample)
  10. ^ J. Le Renard: Notes de Nomenclature sur quelques Mollusques caractéristiques de l'Éocène du Bassin de Paris. In: Cossmanniana , Tome 1, No 2–4, pp. 1–14, 1992. (digitized version)
  11. ^ J. St. Gardner, H. Keeping & HW Monckton: The Upper Eocene, comprising the Barton and Upper Bagshot Formations. In: Quarterly Journal of the Geological Society , Vol. 44, pp. 578–635, 1888. (digitized version )
  12. ^ A b I. West: Barton and Highcliff Cliff Exposures: Eocene and Pleistocene Strata. Retrieved November 6, 2018 .
  13. ^ DN Lewis: Fossil Echinoidea from the Barton Beds (Eocene, Bartonian) of the type locality at Barton-on-Sea in the Hampshire Basin, England. In: Tertiary Research , Vol. 11, No. 1, pp. 1-47, 1989. (digitized version)
  14. ^ MJ Benton & PS Spencer: Fossil Reptiles of Great Britain. Volume 10 of the Geological Conservation Review Series, 386 pages, Springer Science & Business Media, 2012. ISBN 978-94-010-4231-4 (reading sample)