The Tournaisian stage (Lower Mississippian) is known as the golden age of crinoids. In S Belgium, Upper Tournaisian crinoidal limestones, locally called "Petit Granit", are known in the Condroz area (Ourthe Formation) and in the Soignies area (Soignies Member) where tens of quarries expose the crinoidal facies. An isopach map shows that the Ourthe Formation thins northward and westward, probably because of synsedimentary block faulting. Despite its apparent monotony, five crinoidal and peloidal

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... microfacies are identified throughout the formation, and the size and preservation of the crinoid columnals vary vertically and laterally. The encrinite deposited as amalgamated tempestites that accumulated under fair-weather wave base. An estimation of the carbonate production rate was calculated and a value of c. 1200 cm³/m².y is proposed for the Ourthe Fm. The density in individuals was lower in the Hainaut area possibly due to its deeper situation. The analysis of disarticulated crinoid columnals reveals that the crinoidal meadows were diverse with several taxa following a vertical tiering model to maximise the capture of particles from the water column. Beside the crinoids, the fauna was dominated by suspension feeders (brachiopods, bryozoans, tabulate and rugose corals) adapted to a weakly-agitated environment and a relatively soft ground. Rare nektonic and benthic macrophages (holocephalan chondrychthians, actinoceratoid cephalopods, palaechinids, and trilobites) show that the ecosystem was relatively complex despite a simple appearance.