Late Cretaceous geomagnetic polarity sequence: detailed rock and palaeomagnetic studies of the Scaglia Rossa limestone at Gubbio, Italy

Abstract

Summary. A magnetic polarity sequence has been developed for the Late Cretaceous from about 500 stratigraphic samples of the Scaglia Rossa limestone from a 250-m thick section near Cubbio, Italy. Although a large soft component is present in all samples, it is possible to remove it by alternating field magnetic cleaning. A modification of the minimum dispersion technique was used to determine the representative direction for each sample. The mean cleaned directions of samples with negative and positive polarities are slightly different indicating incomplete removal of a stable remanence component. Isothermal remnanence acquisition curves indicate that magnetite is present in all samples, and haematite also in most of them. The NRM directions of samples containing magnetite only are not significantly different from those containing both magnetite and haematite. It is proposed that the haematization was either contemporaneous with the primary magnetization or occurred very soon afterwards. The magnetizations respond positively to a fold test and therefore pre-date the Late Tertiary folding. The sedimentation in the section was continuous, although the rate in the Maastrichtian was almost double the rate earlier in the Late Cretaceous. The palaeontologically dated Cubbio section has been proposed as a type section for the Late Cretaceous (Alvarez er aZ.). Detailed sampling for the present study locates each clearly defined reversal within about 0.2 m, establishes the existence of a previously overlooked polarity zone and corrects the polarity sequence for local minor tectonic effects. The Gubbio magnetic polarity sequence shows a one-to-one correspondence with the polarity sequences derived from oceanic magnetic anomalies 29-34. A palaeontological age can therefore be assigned to each of the corresponding Late Cretaceous geomagnetic reversals.