Stratigraphical reinterpretation of Devonian strata underlying the Mons Basin based on cuttings from the Saint-Ghislain borehole, Hainaut, Belgium

1. Introduction and objectives Evaporitic formations have been observed in Givetian strata in Annappes, Tournai and Leuze deep boreholes, located on the northern border of the Brabant Parautochthon (Coen-Aubert et al., 1980) (Fig. 1). Delmer already formulated in 1972 the hypothesis that important volumes of evaporites, including halite, could be present in the deep basement of the Mons Basin. The dissolution of these evaporites could explain the peculiarities of the Upper Carboniferous and Meso-Cenozoic deposits in the Mons Basin. Figure 1 General geological map with location of Saint-Ghislain and other deep boreholes, and indication of the Givetian and Dinantian outcrops in the Brabant Parautochthon (modified after de Bethune, 1961). Note that the Conde, Douvrain and Ghlin boreholes have not reached the Devonian units. The Annappes (FR) borehole is located 16 km WNW from Tournai city. The main objective of the Saint-Ghislain borehole, drilled between 1972 and 1976, was to test this hypothesis. During the drilling, a total of several hundred metres of anhydrite (CaSO4) was recovered not from the Devonian, as expected, but from the Lower Carboniferous. Preserved anhydrite layers are located in Middle and Upper Visean limestones, between -1950 and -2750 m, representing about 50% of the series within this section (Dejonghe et al., 1976; Delmer, 1977; Groessens et al., 1979; de Magnee et al., 1986). This was the first direct evidence of the presence of evaporites in the French-