Neocomian - Encyclopedia




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NEOCOMIAN, in geology, the name given to the lowest stage of the Cretaceous system. It was introduced by J. Thurmann in 1835 on account of the development of these rocks at Neuchhtel (Neocomum), Switzerland. It has been employed in more than one sense. In the type area the rocks have been divided into two sub-stages, a lower, Valanginian (from Valengin, E. Desor, 1854) and an upper, Hauterivian (from Hauterive, E. Renevier, 1874); there is also another local sub-stage, the infra-Valanginian or Berriasian (from Berrias, H. Coquand, 1876). These three sub-stages constitute the Neocomian in its restricted sense. A. von Koenen and other German geologists extend the use of the term to include the whole of the Lower Cretaceous up to the top of the Gault or Albian. Renevier divided the Lower Cretaceous into the Neocomian division, embracing the three sub-stages mentioned above, and an Urgonian division, including the Barremian, Rhodanian and Aptian sub-stages. Sir A. Geikie (Text Book of Geology, 4th ed., 1903) regards "Neocomian" as synonymous with Lower Cretaceous, and he, like Renevier, closes this portion of the system at the top of the Lower Greensand (Aptian). Other British geologists (A. J. Jukes-Browne, &c.) restrict the Neocomian to the marine beds of Speeton and Tealby, and their estuarine equivalents, the Weald Clay and Hastings Sands (Wealden). Much confusion would be avoided by dropping the term Neocomian entirely and employing instead, for the type area, the sub-divisions given above. This becomes the more obvious when it is pointed out that the Berriasian type is limited to Dauphine; the Valanginian has not a much wider range; and the Hauterivian does not extend north of the Paris basin.

Characteristic fossils of the Berriasian are Hoplites euthymi, H. occitanicus; of the Valanginian, Natica leviathan, Belemnites pistilliformis and B. dilatatus, Oxynoticeras Gevrili; of the Hauterivian, Hoplites radiatus, Crioceras capricornu, Exogyra Couloni and Toxaster complanatus. The marine equivalents of these rocks in England are the lower Speeton Clays of Yorkshire and the Tealby beds of Lincolnshire. The Wealden beds of southern England represent approximately an estuarine phase of deposit of the same age. The Hils clay of Germany and Wealden of Hanover; the limestones and shales of Teschen; the Aptychus and Pygope diphyoides marls of Spain, and the Petchorian formation of Russia are equivalents of the Neocomian in its narrower sense.

See CRETACEOUS, WEALDEN, SPEETON BEDS. (J. A. H.)

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