Original Articles

Products and timing of diagenetic processes in Upper Rotliegend sandstones from Bebertal (North German Basin, Parchim Formation, Flechtingen High, Germany)

CORNELIUS FISCHER^a1a4 c1, ISTVÁN DUNKL^a1, HILMAR VON EYNATTEN^a1, JAN R. WIJBRANS^a2 and REINHARD GAUPP^a3

^a1 Geowissenschaftliches Zentrum, Abteilung Sedimentologie/Umweltgeologie, Georg-August-Universität, Göttingen, Germany

^a2 Department of Isotope Geochemistry, Vrije Universiteit, Amsterdam, The Netherlands

^a3 Institut für Geowissenschaften, Friedrich-Schiller-Universität, Jena, Germany

^a4 Department of Earth Science, Rice University, Houston, Texas, USA

Abstract

Aeolian-fluvial Upper Rotliegend sandstones from Bebertal outcrops (Flechtingen High, North Germany) are an analogue for deeply buried Permian gas reservoir sandstones of the North German Basin (NGB). We present a paragenetic sequence as well as thermochronological constraints to reconstruct the diagenetic evolution and to identify periods of enhanced mesodiagenetic fluid–rock reactions in sandstones from the southern flank of the NGB. Bebertal sandstones show comparatively high concentrations of mesodiagenetically formed K-feldspar but low concentrations of illite cements. Illite-rich grain rims were found to occur preferentially directly below sedimentary bounding surfaces, i.e. aeolian superimposition surfaces, and indicate the lowest intergranular volume. Illite grain rims also indicate sandstone sections with low quartz and feldspar cement concentrations but high loss of intergranular volume due to compaction. ⁴⁰Ar–³⁹Ar age determination of pronounced K-feldspar grain overgrowths and replacements of detrital grains indicates two generations: an early (Triassic) and a late (Jurassic) generation. The latter age range is similar to published diagenetic illite ages from buried Rotliegend reservoir sandstones. The first generation suggests an early intense mesodiagenetic fluid flow with remarkably high K⁺ activity synchronous with fast burial of proximal, initial graben sediments on the southern flank of the NGB. Accordingly, zircon fission-track data indicate that the strata already reached the zircon partial annealing zone of approximately 200°C during early mesodiagenesis. Zircon (U–Th)/He ages (92 ± 12 Ma) as well as apatite fission-track ages (~ 71–75 Ma) indicate the termination of mesodiagenetic processes, caused by rapid exhumation of the Flechtingen High during Late Cretaceous basin inversion.

(Received June 21 2011)

(Accepted November 21 2011)

(Online publication January 31 2012)

Keywords:

• North German Basin;
• Rotliegend;
• Flechtingen High;
• diagenesis;
• K-feldspar overgrowth;
• geochronology

Correspondence:

^c1 Author for correspondence: cornelius.fischer@geo.uni-goettingen.de