CHOKIERIAN

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Abstract

ABSTRACT. The Chokier alum shales, exposed in the Meuse valley between Namur and Liège (Belgium), were defined by d’Omalius d’Halloy in 1853 as the lower unproductive unit of the Carboniferous coal measures. From this original concept, both a lithostratigraphic and a chronostratigraphic unit were defined. The Formation of Chokier returned to d’Omalius d’Halloy’s original ‘houiller sans houille’ concept (Paproth et al., 1983). The intermediate ‘assise de Chokier’ (Stainier, 1901) reflected a hybrid litho-bio construction by using synchronous marine incursions or goniatite bands for subdivision and basin-wide correlation. The ‘assise de Chokier’ became equivalent to the Namurian A, encompassing the Eumorphoceras and Homoceras goniatite zones, defined as the lower subdivision of the Namurian Stage at the first Carboniferous Congress in Heerlen, 1927. The Chokierian chronostratigraphic stage - with type section located in the UK - was defined by Hodson (1957), splitting the Sabdenian (or Homoceras) stage of Bisat (1928). The Chokierian formed a step in a coherent system of goniatite stages, whose applicability on regional, northwest European scale has not been questioned. The Chokierian received the status of an international faunal stage – necessarily as a substage to the Namurian - at the Sheffield Carboniferous Congress in 1967. Chokierian and Alportian are transitional stages in the faunal turnover that characterises the difference between the lower and upper Carboniferous. Following the definition of a Mid-Carboniferous Boundary GSSP in 1985 and acceptance of the Mississippian – Pennsylvanian as the basic subdivision of the Carboniferous, the Chokierian is used as a Regional Stage on current ICS Geological Time Scales. The Chokierian was initially correlated with the Misissippian Subsystem and Serpukhovian stage. Re-assessment of the duration of the Namurian substages and goniatite – conodont correlation suggest to shift the Chokierian to the Pennsylvanian Subsystem and Bashkirian Stage. Exact position of the Mid-Carboniferous Boundary is probably very near the base of the Chokierian. According to recommendation by SCCS, the preferred use of the Chokierian is as a European Substage of the Namurian.

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A propos de : Michiel DUSAR