D-H evidence for the timing of kaolinization in Northeast Bavaria, Germany

The delta D-SMOW values of sedimentary kaolins from the western border of the Bohemian Massif, northeast Bavaria, that did not suffer a deep burial (less than 1000 m) nor a hydrothermal overprint, change systematically from Late Triassic (- 50 parts per thousand) to Mid-Jurassic and Late Cretaceous (-56 parts per thousand to -66 parts per thousand) to Upper Oligocene-Mid-Miocene (-77 parts per thousand to -90 parts per thousand). All analyzed clays are far from hydrogen isotope equilibrium with present-day meteoric waters. Combined oxygen and hydrogen isotope data of selected samples indicate low temperatures of formation (< 30 degrees C) and no evidence for preferential D/H exchange with younger waters. The hydrogen isotopic evolution of kaolins is interpreted as reflecting a systematic isotopic change of paleo-meteoric waters in that region. This can be related mainly to the northward drift of stable Europe after the break-up of Gondwana. Increasing continentality, surface uplift and global cooling are additional factors responsible for decreasing delta D-SMOW values since the Mid-Cretaceous. Kaolinite hydrogen isotope ratios of two large residual economic deposits (Tirschenreuth: delta D-SMOW = - 80 parts per thousand to - 76 parts per thousand; Hirschau-Schaittenbach: delta D-SMOW = - 70 parts per thousand to - 63 parts per thousand) can be used in combination with additional geological evidence to constrain the timing of weathering in these areas. A late Early Cretaceous kaolinization age is suggested for the Early Triassic sandstone-hosted deposits near Hirschau-Schnaittenbach, whereas a Late Oligocene to Mid-Miocene age is indicated for the Carboniferous granite-hosted Tirschenreuth deposits. (C) 2000 Elsevier Science B.V. All rights reserved.