MAGNETOSTRATIGRAPHIC DATING OF THE MIDDLE AND LATE PLIOCENE SEQUENCE IN THE MARCHEAN APENNINES, CENTRAL ITALY

Abstract

The Periadriatic Basin of the external Apennines is reported for its Fermo sector, containing a long sequence of pelites interbedding two major conglomerate complexes, in order to calibrate it to the geomagnetic polarity time scale. The studied succession begins with the 283 meters of Mid Pliocene pelites in the Fosso Morignano section, dated from the Mammoth magnetochron (3.3 Ma) to the latest
Gauss chron, and calculated by the cyclostratigraphic distribution of its continuous magnetic signal to last until 2.63 Ma. The boundary
with the Matuyama chron (2.58 Ma) is placed in the middle of the overlying conglomerate complex of Mt. Ascensione. The shift from the marl to gravel facies suddenly occurs 50 ky before the Gauss/Matuyama boundary and is followed in continuity by a new marly sequence lasting nearly 100 ky in the Rotella section during the earliest reversed Matuyama. A similar depositional cycle, but of shorter duration, is repeated with the Offida profile: the 20 m thick basal pelites exposed in the Fornace section are dated earlier than the beginning of the Olduvai (1.95 Ma), while the upper pelites contain almost fully the Olduvai split end (1.815-1.785 Ma), there included the Pleistocene boundary which was dated 1.796 Ma in the Vrica stratotype. Both profiles focused the calibration of the Late Pliocene and Pleistocene boundaries delineated by the microfaunal content, as the Globorotalia gr. crassaformis and G. inflata zones were always present throughout the Fermo sector, even if their extremes were both missing. This prevented quantifying the stratigraphic relationships between the two sequences, from either the stratimetric or biostratigraphic viewpoints. The present dating revealed a missing time span of roughly 300 ky, including the short normal chron of Reunion (2.15-2.14 Ma), which may be though represented in other portions of the Fermo sector. The correlation of their sequences to one another may be now used as a calibrated signal applied to individual biozones, from the date of nearly 3.3 Ma in the Mid Pliocene through the Pleistocene boundary.