High resolution palaeomagnetic analyses of Cretaceous shallow water carbonates
A sequence of 88 m of continuous borehole cores of Lower Cretaceous shallow water carbonates from Southern Italy was obtained and studied lithologically and palaeomagnetically. It was found that the rocks are organized in cyclic lithofacies representing depositional environments from subtidal to supratidal and their magnetic remanences are all measurable and stable even if very weak. Cyclicities in the Milankovitch band were found using F.F.T analyses in both the sedimentological and palaeomagnetic records, and these matched the astronomical periodicities predicted for the Early Cretaceous. From this link it is possible to date the absolute time represented by the sediments, and the absolute time of acquisition of the palaeomagnetic signal. The palaeomagnetic signal is independent of sedimentological characteristics and lithofacies organization. The mean palaeolatitude and the tectonic results are consistent with other Lower Cretaceous structures in the Southern Apennines. The palaeomagnetic data, although still being studied, also shows shorter wavelengths signals than the Milankovitch cycles, some of which are likely to be comparable with secular variations of the geomagnetic field in the Early Cretaceous. Polarity changes are present, which correlate to the Geomagnetic Polarity Scale. Longer Normal polarities were found at about 1,000,000 years and in them the secular variations were found longer than usual. A repeatable magnetic behaviour characterises the polarity changes each about 230,000 years. It is hypothesized that climatic variations and the sea level oscillations affect the thickness, type and cyclicities of sediments and geomagnetic variations control the palaeomagnetic data. This study confirms that ultra-high resolution magnetostratigraphy can be undertaken on shallow water carbonates.