Abstract

Upper Visean reef-mound carbonates from the Campine Basin, northern Belgium, have a complex diagenetic history, which reflects the evolution of subsurface fluids in the basin. Primary pores became cemented by marine fibrous (stage A) and shallow burial (stage B, C and D) scalenohedral and blocky calcites. Tectonic deformation then created successive networks of fractures which acted as conduits for aggressive fluids and which were subsequently cemented by six stages of carbonate cement.

Fluid inclusion evidence suggests that the first cloudy columnar calcite cements in fractures (stage E) formed at or below 50°C. These calcites have a δ18O of - 13.3% PDB and they are interpreted to have grown in meteoric waters (δ18O = −6.7‰ to −7.4‰ SMOW). After a further period of fracturing and dissolution, white blocky calcites (stage F, δ18O = −9.1‰PDB) precipitated around 60°C from fluids with a δ18O = −0.9‰ SMOW. Later fractures contain brown columnar and blocky calcites (stage G) with a δ18O of −12.1‰ PDB and a minimum homogenization temperature (TH) of c. 90°C, giving a pressure-corrected precipitation temperature of 95°C. An oxygen isotopic composi­tion of +0.6‰ SMOW is indicated for the fluid. Granular to bladed ferroan calcites (stage H), blocky ferroan calcites (stage I, δ18O = 9.8‰ PDB and TH=128°C) and saddle dolomites (stage J, δ18O= −10.0‰ PDB and TH = 147°C) are the last diagenetic carbonate phases. Methane has been detected in the vapour phase of the fluid inclusions in the blocky ferroan calcites. The ferroan calcites and the saddle dolomites are interpreted to have formed respectively at 150°C and 200°C in 18O-rich fluids (+7.9‰ SMOW and +10.9‰ SMOW). The stable isotope values of the fluids indicate an abrupt change from a meteoric composition towards marine-derived compactional fluids and basinal brines. Enrichment in 18O is mainly attributed to the dewatering/dehydration of clay minerals in the adjacent Upper Carboniferous strata.

The paragenetic sequence of cements described here is comparable to that encountered in other Lower Carboniferous limestones where there is not such an extensive data base of fluid inclusion and stable isotopic data. It is suggested that the large change in the pore fluid composition documented here might be a common subsurface process and that the interpretation of diagenetic temperatures from isotopic data alone can be very misleading.

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