Palaeomagnetism of the Jurassic Asphaltkalk deposits, Holzen, northern Germany

Abstract

SUMMARY Palaeomagnetic and rock magnetic studies on the Kimmeridgian-Portlandian Asphaltkalk deposits of the Hils Syncline near Holzen in northern Germany suggest a connection between bitumen impregnation and magnetization in the limestones. A stable remanent magnetization is only found in the Asphaltkalk deposits, not in non-bitumen-impregnated limestones of the same age. The stable remanence was acquired after folding in this region, as revealed by a palaeomagnetic fold test. The timing of remanence acquisition is Late Cretaceous-early Tertiary, as derived from a comparison of the mean pole of the magnetic component with the European apparent polar wander path (APWP). The decay pattern of the natural remanent magnetization (NRM) during alternating field (AF) and thermal demagnetizaton indicates that the magnetization is carried by magnetite. Isothermal remanent magnetization (IRM) acquisition and thermal decay measurements, as well as hysteresis parameters, are consistent with this interpretation. This study suggests that the creation of authigenic magnetite during the lower Tertiary in the Asphaltkalk deposits might coincide with the timing of hydrocarbon migration into the unit.