Chalk Formation
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The Chalk Formations of Europe are thick deposits of chalk, a soft porous white limestone, deposited in a marine environment during the upper Cretaceous period. They appear most prominently in England. The formations are divided into three parts: The Upper Chalk, the Middle Chalk, and the Lower Chalk. The famous White cliffs of Dover, England are a good example of a Chalk Formation deposit. Another good example displaying the sequence of the Chalk Formation are the southern cliffs on the Isle of Wight, England and the quarries and motorway cutting at Blue Bell Hill, Kent, England (which has been classified as a Site of Special Scientific Interest).
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It has sometimes been stated that the Chalk Formation is composed almost entirely of microscopic fossils. This is however inaccurate, since the proportion of microscopic fossils varies, but is usually never more than 5 to 10 percent of the rock.
The Chalk Formation usually shows few signs of bedding, other than lines of flint nodules which become common in the upper part. Nodules of the mineral pyrite also occur and are usually a weathered brown on the outside.
The Lower Chalk is usually relatively soft and greyish in colour, it is also the most fossiliferous (especially for ammonite fossils). The Lower Chalk strata usually begins with marl called the Glauconitic or (Chloritic) Marl, named after the grains of the green mineral glauconite which it contains. The remainder of the Lower Chalk is normal chalk with few, if any, flint nodules present. The thickness of the Lower Chalk strata varies, averaging around 200 feet, depending upon the location. The Lower Chalk often contains fossils such as the ammonites Schloenbachia, Scaphites, and Mantelliceras, the belemnite Actinocamax, and the bivalves Inoceramus and Ostrea.
The Middle Chalk averages 200 feet in thickness. Fossils found in the Middle Chalk include the brachiopod Terebratulina and the echinoid Conulus. However, though fossils have been found, they are generally sparce.
The Upper Chalk by comparison is softer than the Middle Chalk and the flint nodules it contains are far more abundant, and may contain ammonite and gastropod fossils in some nodular layers. The thickness of the Upper Chalk strata varies greatly, often averaging around 300 feet. In the Upper Chalk, fossils may be abundant and include the bivalve Spondylus, the brachiopods Terebratulina and Gibbithyris, the echinoids Sternotaxis, Micraster, Echinocorys, and Tylocidaris, the crinoid Marsupites, and the small sponge Porosphaera.
The youngest beds of the Upper Chalk formation in England are found on the coast of Norfolk. Other fossils commonly found in the Chalk Formation include: solitary corals (such as Parasmilia), marine worm tubes (such as Rotularia), bryozoans, scattered fragments of starfish, and fish remains (including shark teeth such as Cretolamna and Squalicorax).
See also
References and further reading
AB Smith and DJ Batten, Fossils of the Chalk (Second Edition), The Palaeontological Association, 2002.
External links
Blue Bell Hill Picnic Site - Kent County Council. (http://www.kent.gov.uk/sp/countrysideaccess/parks/BlueBellHillPicnicSite.html)