Genesis and significance of the silico-aluminous nodules in the Ordovician of the Montagne Noire and the Massif Armoricain (France)
Identifieur interne : 001320 ( Main/Exploration ); précédent : 001319; suivant : 001321Genesis and significance of the silico-aluminous nodules in the Ordovician of the Montagne Noire and the Massif Armoricain (France)
Auteurs : Jean-François Becq-Giraudon [France] ; Suzanne Bouillé [France] ; Jean-Jacques Chauvel [France]Source :
- Sedimentary Geology [ 0037-0738 ] ; 1992.
Abstract
Siliceous nodules of the Arenigian of the Montagne Noire (southern France) and the Massif Armoricain (western France) are made up of quartz, illite and chlorite with secondary apatite and, less frequently, dolomite. Diffractometrical and geochemical data indicate that the chlorite is a chamosite (trioctahedral 2:1 chlorite), whose development may be accompanied by that of apatite and diminution of illite.Three types of nodules are defined: Type 1, siliceous nodules, with more than 85% of SiO2, very frequent in the Montagne Noire; Type II, nodules made of a micaceous siltstone with chlorite matrix, abundant in the Massif Armoricain but present also in the Montagne Noire; Type III, chloritic nodules with a finely globular structure, especially developed in the Massif armoricain but observed in the Montagne Noire as well.The nodules are always poorer in illite than their host sediments and often show traces of the initial sediments. Their formation occurs after the deposition and prior to the development of foliation.The frequent presence of undistorted fossil remains suggests an early formation linked to the physico-chemical modifications occurring in an unconsolidated sediment around a decaying organism. The post-depositional mineralogical evolution should have started with an illite/dolomite assemblage and end up with a chamosite-dominant one, under cold climatic conditions.The simultaneous presence of the three types of nodules in the same formation, or even in the same beds, shows that this mineralogical differentiation is not strictly related to the diagenetic and metamorphic transformation of this material.
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DOI: 10.1016/0037-0738(92)90104-Y
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Diffractometrical and geochemical data indicate that the chlorite is a chamosite (trioctahedral 2:1 chlorite), whose development may be accompanied by that of apatite and diminution of illite.Three types of nodules are defined: Type 1, siliceous nodules, with more than 85% of SiO2, very frequent in the Montagne Noire; Type II, nodules made of a micaceous siltstone with chlorite matrix, abundant in the Massif Armoricain but present also in the Montagne Noire; Type III, chloritic nodules with a finely globular structure, especially developed in the Massif armoricain but observed in the Montagne Noire as well.The nodules are always poorer in illite than their host sediments and often show traces of the initial sediments. Their formation occurs after the deposition and prior to the development of foliation.The frequent presence of undistorted fossil remains suggests an early formation linked to the physico-chemical modifications occurring in an unconsolidated sediment around a decaying organism. The post-depositional mineralogical evolution should have started with an illite/dolomite assemblage and end up with a chamosite-dominant one, under cold climatic conditions.The simultaneous presence of the three types of nodules in the same formation, or even in the same beds, shows that this mineralogical differentiation is not strictly related to the diagenetic and metamorphic transformation of this material.</div></front></TEI><affiliations><list><country><li>France</li></country><region><li>Centre-Val de Loire</li><li>Région Bretagne</li><li>Région Centre</li></region><settlement><li>Orléans</li><li>Rennes</li></settlement><orgName><li>Université de Rennes 1</li></orgName></list><tree><country name="France"><region name="Centre-Val de Loire"><name sortKey="Becq Giraudon, Jean Francois" sort="Becq Giraudon, Jean Francois" uniqKey="Becq Giraudon J" first="Jean-François" last="Becq-Giraudon">Jean-François Becq-Giraudon</name></region><name sortKey="Bouille, Suzanne" sort="Bouille, Suzanne" uniqKey="Bouille S" first="Suzanne" last="Bouillé">Suzanne Bouillé</name><name sortKey="Chauvel, Jean Jacques" sort="Chauvel, Jean Jacques" uniqKey="Chauvel J" first="Jean-Jacques" last="Chauvel">Jean-Jacques Chauvel</name></country></tree></affiliations></record>Pour manipuler ce document sous Unix (Dilib)
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