Diagenesis of Tertiary marlstones in the Vienna Basin

To investigate the diagenetic changes of Tertiary pelitic sediments in the Vienna Basin a suite of drill core samples from the OeMV-bore-hole Aderklaa 78 has been studied using mineralogical and chemical methods. The sediments generally correspond to shales and cover a section from 780–2812 m depth. Their mineralogical composition is complex and variable and mainly due to the initial variation in detrital constituents at the time of sedimentation and the local sedimentary environments. Quartz, feldspars (sodic-plagioclase, K-feldspar), carbonates (calcite, dolomite, siderite), phyllosilicates (mica, chlorite, kaolinite, illite-smectite mixed-layers), and pyrite to a minor extent are the essential components of the shales. The analytical data of bulk samples and the less than 0.2 micron fractions point out that mechanical compaction of the sediments was accom inied by isochemical changes within the shale sequence. The diagenetic overprint of the mineral constituents is revealed by a decrease with depth of K-feldspar and an increase of quartz respectively, but especially by the gradual illitization of smectite via mixed-layer intermediates. Burial diagenesis in this Vienna Basin profile therefore can be expressed by the mineral reaction: K-feldspar+smectite→illite+quartz. A local reversal in all these trends in the basal Tertiary section can be explained by unusually high magnesium contents of the pore waters which inhibited illitization by promoting the stability of smectite relative to illite.