Stable isotopic composition of an evaporite dissolution breccia in the Lower Viséan limestones of SE Belgium

Abstract An evaporitic dissolution breccia occurs in the Lower Visean limestones of southeastern Belgium. Three periods of brecciation have been recognized, the first of which took place early in the diagenetic history. The breccia matrix consists of infiltrated marine sediments. The fragments and the sedimentary matrix subsequently neomorphosed and lithified in meteoric waters. Multiple water-rock interactions in an open system caused an oxygen isotopic composition of the limestones in equilibrium with meteoric water. This composition varies between −8.9 and −5.5‰ depending on the palaeogeographical position. The early diagenetic interaction led also to a spread in the carbon isotopic composition, which ranges between an Early Visean marine ( δ 13 C = +4‰ pbd ) and a palaeosol ( δ 13 C = −3‰ pdb ) isotopic composition. Due to the migration of fluids along fractures in the subsurface, a second dissolution, brecciation and cementation period occurred. During this period, breccia fragments and the early diagenetic matrix locally neomorphosed a second time. The oxygen isotopic composition of the limestone constituents decreased due to the interaction with burial fluids. The δ 18 O composition of the breccia fragments and matrix moved towards those of dull orange-brown luminescent calcite cements ( δ 18 O = −20 to −11‰ pdb ), filling the secondary cavities. A third period of brecciation and neomorphism caused the development of orange-yellow zoned and bright luminescent microsparites to sparites and of granular to blocky calcites.