Smectite to illite diagenesis in early Miocene sediments from the hyperthermal western Pannonian Basin

The smectite to illite transformation in early Miocene sediments was studie d in wells and outcrops in the western Pannonian Basin, where magmatic acti vity caused very high Miocene heat flows (250-400 mW/m(2)). Although the th ermal history is similar, large differences in smectite to illite diagenesi s were observed. (1) The boundaries between early/middle and middle/late di agenesis in two wells (Pichla, Radkersburg) and the Maribor area correspond to vitrinite reflectance values of 0.4-0.8 and 1.1-1.5% Rr, Anchimetamorph ism starts at similar to 2.1% Rr. In spite of magmatic heating, the smectit e to illite transformation can be modelled using kinetic data proposed for areas with a 'normal' burial diagenesis. (2) Smectite to illite reactions a re advanced compared to vitrinite reflectance in the Ribnica-Selnica Trough . This is probably related to fluids with elevated K+ concentrations. (3) C lay mineral alterations lag behind coalification significantly in the Mitte rlabill well. These different correlations indicate that clay mineral therm ometry should be used with caution and that factors other than temperature and time can influence clay mineral reactions.