Gamma-ray measurements in the Kirchrode I and II boreholes

The downhole γ-ray logs in the Kirchrode I and II boreholes through the Albian southeast of Hannover in Northwest Germany were used (a) to correlate the two boreholes, (b) to calculate the clay content of the sediments, and (c) to detect cyclicity in changes in sediment composition. oss-correlation of the downhole γ-ray logs of the Kirchrode I and II showed that a shift of the Kirchrode II borehole downwards by 89 m provides the best correlation with the Kirchrode I borehole. A comparison of the γ-ray log of Kirchrode II with that of the nearby Prakla 2 borehole, for which biostratigraphic data is available, also yields a close correspondence. Using this correlation of the logs and adopting the stratigraphic subdivision of the Prakla 2 borehole by Kemper (1989), the Upper/Middle Albian and Middle/Lower Albian boundaries in Kirchrode II are at depths of 152 and 252 m, respectively. ray logs were used to calculate the clay content. It varies between 30 and 60%, indicating only small lithologic changes. This result is in agreement with values derived from analysis of the quartz and CaCO3 contents of core samples. The montmorillonite, illite, and kaolinite contents of the clay fraction determined by Kühn et al. (2001), are the basis for an independent calculation of the clay contents of the core using mean literature values for the thorium and potassium contents of these three clay minerals. riodicities identified by spectral analysis of the γ-ray measurements made every 5 cm throughout the recovered Albian sequence, can be interpreted in terms of Milankovitch cycles. Sliding window spectral analysis shows that the sedimentation rates were relatively constant during the Late Albian (8–12 cm/ka). Sedimentation rates of 4 cm/ka were determined for the Middle Albian.