THE MAGNETOSTRATIGRAPHY ESTABLISHED AT GUBBIO (ITALY)

Magnetostratigraphy was introduced at Gubbio on the sequence of pelagic carbonates of the Umbrian Series, through the profile of the Scaglia Formation exposed in the outskirts of this medieval town. The numerical dating system of chrons established by the Geomagnetic Polarity Time Scale (GPTS) was there tested on the stratigraphic series just revisited by the detailed micropaleontological biozonation, across its most prominent event at the Cretaceous/Tertiary (K/T) boundary.

The basic support to this magnetostratigraphic approach was provided by the global character of the geomagnetic field, for its dipolar distribution on the whole earth that makes coeval each event, being any residual magnetization globally fossilized as a polarity reversal. The major task at Gubbio was to testing the possibility of whether a reversal could have been recorded by such carbonates and also a continuous sequence of them, as well, could follow the continuous distribution of the sequence. The durations of biozonal intervals from the Gubbio profile were refined to providing apparent accumulation rates varying from more than 15 meters per million years (m/my), approaching the K/T boundary, to 2.3 m/my just after it.

Also the minor changes often observed to cyclically occurring in sedimentary series were dated in number of years. In the Piobbico core, from the bore hole drilled near Gubbio, in the Black Shales, the various cyclicity orders were measured for its nearly 70 m thickness. All of the sedimentary parameters recorded the full range of the astronomically driven cyclicities, from the color changes of the layer beds to the abundances of calcareous microfossils, the organic carbon content and the elements of the magnetic fossil vector. Cyclostratigraphy was thence established at Piobbico. For recent sequences, near Gubbio and on both flanks of the Apennine Belt, the cyclostratigraphic resolution of their magnetic record led calibrating the Pliocene/Pleistocene (P/P) boundary at the date of 2.58 Ma and the major tectonic activity that shortly interrupted the sedimentary process. Also the decisive change in the climate regime was detected. The steadily warm conditions that marked the Pliocene, moved to oscillating in increasingly larger amplitudes and longer periods. Its onset, from the precessional forcing to the obliquity, occurred within the narrow interval of 2.88-2.84 Ma and the short eccentricity became the predominantly forcing ciclicity of the insolation at nearly 1.0 Ma.