Paléoécologie des prairies à crinoïdes et corrélation stratigraphique des "Petits Granits" de l'Ourthe et de Soignies (Tournaisien supérieur)

Abstract

The Mississippian and specially the Tournaisian is known as the golden age of crinoids. In Belgium it includes the crinoidal limestones, localy called «Petit Granit», which crop out in the Condroz area (Ourthe Fm.) and in the Soignies area (Soignies Mbr.). Relatively well-known from a historical point of view, the tournaisian encrinite has not been studied regarding the palaeoenvironment and the fossil assemblage. Suspension feeding organisms dominated this ecosystem and they were distributed following a tiering model i.e. a vertical spreading to maximize the distribution of particles in the water column. The Belgian encrinite from the Ourthe Fm. and the Soignies Mbr. results from similar crinoidal meadows equivalent in term of diversity. They both were complex ecosystem: crinoids and other suspension feeding organisms (brachiopods, bryozoan, tabulates and rugosa corals) were adapted to a weakly-agitated environment and a relatively soft ground. Regarding the carbonated product rate, the crinoidal meadows were different, probably due to the deeper position of the Soignies area. Although contour map shows a deepening from West to East. There is no lateral and few vertical changes in the facies but the fossil assemblages which show some peaks of diversity, varies laterally. It allows to differenciate each group of beds for correlations of the the Ourthe Fm. and the Soignies Mbr. Conversely, no sedimentological or ecological partitioning has been highlighted within the sedimentary area.