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The origin of a peculiar type of silicified limestone with nodular flints used in parish churches at Sluizen and Vreren, south of Tongeren (province of Limburg, Belgium), has now been elucidated by the discovery of the same rock type in its natural setting, namely the silicified top of the Cretaceous which underlies Clay-with-flints (‘flint eluvium’) and Oligocene sands in a disused quarry at Elst, municipality of Riemst (Limburg, Belgium). Co-operation between professional geologists and amateur palaeontologists has allowed to characterise this rock type, here referred to as ‘Elst tauw’, both petrographically and palaeontologically. The rich echinoid fauna has also been assessed, on the basis of a comparison with assemblages from the Clay-with-flints at Halembaye (Haccourt/Lixhe, province of Liège, Belgium) and at Zichen-Eben Emael in the Hesbaye region. P.J. Felder's ecozones, based on bioclast assemblages, substantiated by analyses of petrographical biofacies features of the original calcarenite, has allowed lithostratigraphic correlation of the ‘Elst tauw’ with the ‘Roosburg block’, which is a variety of ‘Maastricht stone’. Both methods indicate that the ‘Elst tauw’ developed in beds that can be assigned to the condensed Valkenburg-Schiepersberg interval of the lower Maastricht Formation. Petrographical analysis has shown the ‘Elst tauw’ to be quite distinctive; in addition, its natural occurrence at the Elst quarry matches the building stone records in rock type. The latter stem from the same small area, situated southwest of Maastricht. The major steps in its diagenetic history could be reconstructed, starting with pervasive pyritisation of the calcareous allochems, followed by silicification of the grains and pore spaces (different silica cements that became partially recrystallised), completed by oxidation of pyrite with transformation into limonite and, finally, dissolution of the non-pyritised or partially pyritised skeletal allochems creating a mouldic porosity. Silicification probably was achieved prior to the Oligocene. This particular mode of formation has generated a discussion on the proper use of the vernacular term ‘tauw’, a term used in a different sense by the industry, stratigraphers and students of building stones.
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