NAMURIAN

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Abstract

Abstract. Carboniferous rocks had a great impact on the landscape and industrialisation of western Europe, hence a distinction between ‘Carboniferous limestone’ and ‘Coal Measures’ since the dawn of geological science. André Dumont (1852) and Omalius d’Halloy (1853) already distinguished a ‘Houiller sans houille’ (Coal Measures without coal), subsequently named ‘Namurian’ by Purves in 1883. Whereas the lower boundary was quite clear – the quick transition from carbonates to siliciclastics – the boundary of the Namurian with the overlying coal-rich Coal Measures was subject to different interpretations and miscorrelations. International status of the Namurian as a chronostratigraphical stage was acquired in 1927 at the first International Carboniferous Congress, as part of a general classification scheme for the West European Carboniferous. Boundaries were based on ammonoid biozonation, selected to coincide with major events affecting the regional lithological framework. The twofold subdivision of the Carboniferous, Dinantian - Silesian or Mississippian – Pennsylvanian Subsystems, has been a matter of debate since the second Carboniferous Congress in 1935. Dinantian and Silesian were ratified for the Lower and Upper Carboniferous in 1971 although the western European Upper Carboniferous was not considered very suitable for intercontinental correlation, due to its position in the Variscan closure zone of the Pangaea supercontinent. The definition in 1985 of a Mid-Carboniferous Boundary and GSSP at Arrow Canyon, Nevada paved the way to adoption of Mississippian and Pennsylvanian Subsystems for the Lower and Upper Carboniferous in 1999. As a consequence, the status of the Namurian stage has been reduced to a regional European Stage. This decision has also practical consequences as the Namurian straddles this Mid-Carboniferous Boundary. Actually, the time span for the Namurian is 326.4 – 315 Ma and the accepted GTS age for the Mid-Carboniferous boundary is 318.1 Ma. The lower part of the Namurian is equivalent to the Mississippian Serpukhovian stage, the upper part of the Namurian is equivalent to part of the Pennsylvanian Bashkirian stage. Application of the global time scale in western Europe is hindered, however, by poor correlation potential of the fossil record and by insufficient radiometric dating. Continued use of the Namurian stage is allowed, on condition that global equivalents are indicated, and that ambiguous terms such as Lower and Upper Carboniferous are avoided, except when related to historic concepts.

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A propos de : Michiel DUSAR