CLAY MINERALOGICAL VARIATIONS AND EVOLUTIONS IN SANDSTONE SEQUENCES NEAR A COAL SEAM AND SHALES IN THE WESTPHALIAN OF THE CAMPINE BASIN (NEBELGIUM)

Mineralogical trends have been investigated on a detailed scale in two Westphalian fluvial sandstone sequences in contact with either a coal seam or shales. The evolution in an authigenic clay mineral assemblag e can be related to changes in the pH of the pore-water. Firstly, kaol inite formed early in diagenesis, mainly as a result of K-feldspar dis solution and alteration. This process, which took place under acidic c onditions, consumed protons. Subsequently the pH of the pore-water inc reased and after compaction, illitization of kaolinite occurred under near neutral conditions. Deep burial is marked by dickite formation, w hich again reflects acidic conditions. Distribution of clay minerals i s related to meteoric water flux through the sediments and the release of organic acids and CO2 from coalification of organic matter in the underlying strata.