See also: Cambrien (homonymy)

The Cambrien , which extends from -542  ±  0.3 with -488.3  ±  1.7  million years, is the first of the six period S of the Paléozoïque.

Origin of the denomination

The name Cambrien was introduced in geology by Adam Sedgwick in 1835. This name derived from Cambria , the Latin name of the Wales, where the grounds of this period level particularly well.

In 1831, Sedgwick and its collaborator, Roderick Murchison, began their work on the Stratigraphie from Scandinavian Wales. Sedgwick began its research by bottom from the section and Murchison the top. Sedgwick called the layer of lower Roche the cambrien and Murchison the Silurien the higher formations generally more fossiliferous. Thereafter their sections met, and each one proclaimed the mediosilic rocks for their systems, until in becoming a widespread polemic. They are only forty years later that, Charles Lapworth named Ordovicien the transition courses.

Biostratigraphy

Cambrien is the only system not having yet biostratigraphy established on a worldwide scale because of the strong endemism of fauna. By vote in December 2004 of the ISCS (International Sub-commission one Cambrian Stratigraphy), Cambrien was divided into four " séries" and ten stages in order to replace the old terms of lower, average and higher Cambrien. Divisions are in the course of definition, also the series and stages have temporarily a numéro  ; work should lead from here 2008.

The had a presentiment of limits, likely to be modified, are presented Ci-après :

  • series 1 and 2 correspond roughly to the old concepts of lower Cambrien and Géorgien. The base of Cambrien (542  My), and also of series 1 and stage 1, is marked by the first appearance (FAD, First appareance datum) of the fossil trace Trichophycus pedum , whose GSSP is taken in Newfoundland with the Canada. The étage  2 is marked by the appearance of the archéocyathes and the " Small shelly fossils". Series 1 represents about half of Cambrien in age. The base of the série  2 (524  My), and of the étage  3, is marked by the appearance of the Trilobite S, probably the Fallotaspididae family. The étage  4 is marked by the appearance of the trilobite Olenellus to the Laurentia or of Redlichiidae to the Gondwana.

  • series 3 corresponds roughly to the old concepts of average Cambrien and Acadien. Its base is fixed at 513  My. The étage  5 is based on the FAD of the trilobite Oryctocephalus indicus , the Drumien is based on the FAD of the trilobite Ptychagnostus (Acidusus) atavus , the étage  7 on the FAD of the trilobite Lejopyge laevigata .

  • series 4, now called Furongien, corresponds roughly to the old concepts of higher Cambrien and Potsdamien. Its base, as that of the stage Paibien is fixed by GSSP at 501  My by the FAD of the cosmopolitan trilobite Glyptagnostus reticulatus . Stage 9 is based on the FAD of the trilobite Agnostotes orientalis , and the name of Gonggirien is had a presentiment of. Lastly, the étage  10 is based on the FAD of the trilobite Lotagnostus americanus . The base of Ordovicien, the system which follows Cambrien, is marked by the FAD of the Conodonte Iapetognathus fluctivagus , whose GSSP is also taken with Newfoundland, with the Canada.

Cambrien in the World

The grounds cambriens level particularly well share the world, and are present on all the continents. in North America, one meets it with the Canada, Greenland, the United States and with the Mexico. In South America, one finds it in Argentine. In Africa, it is particularly well represented with the Morocco, in Algérie, and probably in Libya and Egypt by drilling. In Europe, it meets in Spain, France, the United Kingdom, Ireland, Belgium, Norway, Sweden, Denmark, Germany, Italy, Poland and in Tchéquie. In Asia, one meets it in Jordan, Saudi Arabia, Oman, Turkey, Iran, Afghanistan, Kazakhstan (and probably other states more in the east…), Russia (Siberia), China, Korea, India, Vietnam, Malaysia. One also meets it in Australia, New Zealand and on the continent the Antarctic.

Cambrien in France

Cambrien is well represented in France, in the Black Montagne, the the Pyrenees, but also in Normandy (Carteret, valley of the Orne), and in small outcrops in the countries of the Loire (Sillé-the-Guillaume, Sigournais, Cléré-on-Tailboard), but also in the the Ardennes. The discovery of fossils cambriens in France goes back to 1888, when J. Bergeron found remains of trilobites in Black Montagne. This discovery had a great repercussion in the French geological medium, because faunas cambriennes (the " fauna première" as called it Barrande), had then been discovered a little everywhere in Europe, except in France. As it was evoked previously, there is no established biostratigraphy in a world way, as exists it many local scales, while waiting for as all are correlated. A biostratigraphy exists for the south of Europe (France, Spain, Italy), which applies also partially in Germany, in Czech Republic, with the Morocco and in Turkey.

  • lower Cambrien: it contains the stages Cordubien, Ovétien, Marianien and Bilbillien, all defined in Spain.

  • average Cambrien: it contains the stages Léonien, Caesaraugustien and Languedocien.
Each one of these stages contains many biozones, based on the archéocyathes then on the trilobites.

The grounds cambriens of France belong to several fields having each one a paleogeographic history disctincte. The Black mountain and the Pyrenees are remainders belonging to the continental terrace of the Gondwana, whereas the outcrops more in north belong to a whole of microphone-continents wedged at the time of orogenesis hercynienne.

Stratigraphy of French Cambrien

In the Pyrenees, the grounds cambriens form a thick series of several km, constituting the major part of the chain. Stratigraphy is not final and besides has just been the revision object. The upper part of the Group of Canaveilles (Formation of Cabrils) constitutes the base of Cambrien. The Group of Jujols (Formations of Evol and Jujols) constitutes then Cambrien and probably the lower Ordovicien. The unit is a powerful schito-sandy detrital series, with carbonate intercalations.

In the Black Mountain, stratigraphy is particularly representative.

  • the Formation of Marcory, whose base is Protérozoïque final and summon it lower Cambrien, is consisted a thick series of schists and dark green-brown sandstones for a thickness close to 1000 Mr. Of the intercalations limestones are known, notammentdans Minervois. They correspond to a deltaic mode.
  • the Formation of Pardailhan is a mode of alternations between sandy benches, and limestone bars, these last becoming increasingly dominant. Its thickness is very variable, being able to go up to 250 Mr.
  • the Formation of Lastours watch the establishment of a carbonated platform. Thickness going up to 500 Mr.
  • the Formation of the Bridge of Poussarou precedes collapse by this platform, and sees the passage to average Cambrien.
  • the Formation of Tanque shows the collapse of the platform, and ennoiement by a mode of alternation schisto-limestone, with a characteristic wine-color color.
  • the Formation of Coulouma consists of vareigated schists (red, green, yellow, blue…). Thickness of 50  m approximately.
  • the Formation of Ferrals consists of quartzitic bars, corresponding to an important regressive event. Thickness going until 250  Mr.
  • the Formation of Gardie is made up at the base by schists then by sandstones.
  • the passage to the Formation of the Valley of Homs sometimes confused, but is marked by the intercalation of episodes limestones in a sometimes important schistous series. These two formations make several hundred meters.
The higher limit of Cambrien is not recognized with precision and is the subject of works in progress.

Paleobiology

The principal sediments of the cambrien reveal the extension of not very deep seas recovering continental shelfs. Its beginning was marked by the abrupt multiplication of new groups of animals and in particular of animals with hard parts. Their appearance remains one of the great mysteries of the fossil history and it gave place to many assumptions. One proposed the modifications of the climate and the increased activity of predatory or sea salts supporting the absorption of chemical substances and the deposit of hard layers on the skin. But it is also probable that the development of external skeletons was an adaptive reaction to the formation of new ecological niches. They could be organizations of the great depths acclimatizing itself to the habitats of deep water, but also of an evolution towards species able to exploit new food resources.

Tiny métazoaires with shells (approximately 1 to 5 mm) and with low diversity, SSF. (Small Shelly Fossils), are largely widespread in world (Australia, India, China, Mongolia, Siberia, Iran, North America, Europe, etc). This appearance and that of the fauna édiacarienne, from 565 My, represent 2 major pulsations which characterize the emergence of the macroscopic animals at the end of Néoprotérozoïque and lower Cambrien, followed by " the explosion cambrienne". In basal Cambrien appear Anabarites , Protohertzina and molluscs primitive. Quickly with the wire of time the organizations with skeletons and distinct members become increasingly abundant. In Tommotien (530-527  My), of new phyla are listed: Brachiopods and Porifères (reefs of Archéocyathes), Lapworthella , and of many phyla with soft body. On the floor Atdabanien (527-525  My), the Arthropods emerge (of which Trilobites which will dominate Cambrien) and of primitive Echinodermata. In average Cambrien, the Fauna of Burgess Shale (towards 520  My) will be made up of animals with soft body and animals with hard parts. The majority of the Phyla modern sailors are represented there (except the Bryozoaire S). One deducts of them approximately 37 among whom Porifère S, Annélide S, Priapulide S, Onychophore S, Mollusque S, Arthropode S, Coelentéré S, Echinoderme S, Chordé S and many phyla extinct with soft body. One regard can be to a hundred the number of phyla appeared in Cambrien. Diversification is at the level of the phyla and not of the kinds (approximately 400  kinds in Cambrien for 1500 in Ordovicien). Cambrien arises thus more like one trial period evolutionary intense, of radiation of the métazoaires and diversification of the lines that like one period of specialization. Radiation cambrienne of the métazoaires constitutes one of the events of most important of the history of the life with the appearance of the majority of the plans of organization associated with the most essential innovations with living organisms (skeletons mineralized, internal, jaws, gills, eyes, etc). Causes suggested of " the explosion cambrienne" can be classified in two categories:

  1. intrinsic causes: genetic flexibility, complexification of genes of the métazoaires, the existence of ecological niches, appearance of the predation.

  2. extrinsic causes: modifications of the environment, chemical composition of the oceans (carbon, sulfur), variations of the level of the nutrients, phosphogenèse, formations limestones, increase in the oxygen rate in the atmosphere and oceans.

beginnings of the fossil files

The hard shells fossilize better than the bodies mous  ; this is why the rocks of this time are filled with fossils. The animal life of the cambrien was extraordinarily varied.

In Burgess Shale, which was formerly under the sea and which forms today part of Rock Canadian, one realizes of what was the life. In this place, thousands of small odd animals and with shells were absorbed in a slip of muddy silt which preserved them perfectly. Still more surprising: the site also contains animals with soft body which seldom fossilizes.

In the rocks of Cambrien, one can observe that many ancestors of live animals today had just appeared. They was the molluscs with shell and tentacles which evolved/moved to our clams and our winkles, and the Arthropods with the articulated legs which became the crabs and the lobsters. The starfishes, the Sea urchin S, the Corals, the sponge S, also appeared at that time. They increased the number of the known families appreciably and modified the composition of the communities living on sea-beds.

Life on ground

The life on ground is represented only by marine micro-organisms, probably of the Lichen S (symbiosis of a Algue and a Champignon).

Subdivisions:

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