Kimeridgian
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In geology, Kimeridgian or Kimmeridgean is the name of the basal division of the Upper Oolites in the Jurassic system.
The name is derived from the hamlet of Kimeridge or Kimmeridge near the coast of Dorsetshire, England. It appears to have been first suggested by T. Webster in 1812; in 1818, in the form Kimmeridge Clay, it was used by Buckland. From the Dorsetshire coast, where it is splendidly exposed in the fine cliffs from St Alban's Head to Gad Cliff, it follows the line of Jurassic outcrop through Wiltshire, where there is a broad expanse between Westbury and Devizes, as far as Yorkshire, there it appears in the vale of Pickering and on the coast in Filey Bay. It generally occupied broad valleys, of which the vale of Aylesbury may be taken as typical. Good exposures occur at Seend, Calne, Swindon, Wootton Bassett, Faringdon, Abingdon, Culham, Shotover Hill, Brill, Ely and Market Rasen. Traces of the formation are found as far north as the east ccast of Cromarty and Sutherland at Eathie and Helmsdale.
In England the Kimeridgian is usually divisible into:
- an Upper Series, 600-650 ft. in the south, dark bituminous shales, paper shales and clays with layers and nodules of cement-stones and septaria. These beds merge gradually into the overlying Portlandian formation.
- the Lower Series, with a maximum thickness of 400 ft., consists of clays and dark shales with septaria, cement-stones and calcareous doggers.
These lithological characters are very persistent. The Upper Kimeridgian is distinguished as the zone of Perisphinctes biplex, with the sub-zone of Discina latissima in the higher portions. Cardioceras alternans is the zonal ammonite characteristic of the lower division, with the sub-zone of Ostrea deltoidea in the lower portion. Exogyra virgula is common in the upper part of the lower division, and the lower part of the Upper Kimeridgian. A large number of ammonites are peculiar to this formation, including Reineckia eudoxus, R. Thurmanni, A spidoceras longispinus, etc.
Large dinosaurian reptiles are abundant, Cetiosaurus, Gigantosaurus, Megalosaurus, also plesiosaurs and ichthyosaurs, crocodilian and chelonian remains are also found. Protocardia striatula, Thraciadepressa, Belemnites abreviatus, B. Blainvillei, Lingula ovalis, Rhynchonella inconstans and Exogyra nana are characteristic fossils.
Alum has been obtained from the Kimmeridge Clay, and the cementstones have been employed in Purbeck; coprolites are found in small quantities. Bricks, tiles, flower-pots, etc., are made from the clay at Swindon, Gillingham, Brill, Ely, Horncastle, and other places. The so-called 'Kimeridge coal' is a highly bituminous shale capable of being used as fuel, which has been worked on the cliff at Little Kimmeridge.
The Kimeridgian of continental geologists is usually made to contain the three sub-divisions of A. Oppel and W. Waagen, viz.:
- I Upper (Virgulian) with Exogyra virgula Kimeridgien
- II Middle (Pteroceran) with Pteroceras oceani
- III Lower (Astartian) with Astarte supracorallina; but the upper portion of this continental Kimmeridgian is equivalent to some of the British Portlandian; while most of the Astartian corresponds to the Corallian.
A. de Lapparent now recognizes only the Virgulian and Pteroceran in the Kimeridgien. Clays and mans with occasional limestones and sandstones represent the Kimeridgien of most of northern Europe, including Russia. In Swabia and some other parts of Germany the curious ruiniform marble Felsenkalk occurs on this horizon, and most of the Kimeridgien of southern Europe, including the Alps, is calcareous.
Representatives of the formation occur in Caucasia, Algeria, Abyssinia, Madagascar; in South America with volcanic rocks, and possibly in California (Maripan beds), Alaska and King Charles's Land.
This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.
