PALEOMAGNETISM AND ROCK MAGNETISM IN THE UPPER PLIOCENE-VALLE-RICCA (ROME, ITALY) SECTION

A revision of the magnetostratigraphy in the Valle Ricca (Monterotondo , Rome) Plio-Pleistocene section was carried out. This section, consis ting mainly of blue-grey marry clays, was previously investigated for palaeomagnetic effects by Arias et al. (1980, 1990), who detected a no rmal polarity interval, close to a volcanic ash layer, which was corre lated to the Reunion subchron. This interpretation conflicts, however, with new biostratigraphic data and palaeomagnetic observations. In or der to investigate the possible origin of these discrepancies, we perf ormed a detailed sampling for palaeomagnetic and rock-magnetic analyse s in the controversial interval around the volcanic ash layer. The pre sent results show strong variations in the rock-magnetic parameters an d suggest significant changes in the magnetic mineralogy. Two levels w ith a high-intensity, low-temperature, normal component of magnetizati on were distinguished. At these two levels, the rock-magnetic paramete rs are typical of greigite, and a widespread occurrence of framboids o f micron-sized iron sulphide grains was found by mineralogical analyse s. A high-temperature reversed component of magnetization, carried by magnetite, was isolated for the whole interval investigated. We demons trate that the previously recognized normal polarity interval is the e ffect of changes in the magnetic mineralogy and it does not represent a true reversal of the Earth's magnetic held. The origin of the low-te mperature normal component of magnetization is discussed in. light of the present knowledge about greigite formation and remanence acquisiti on.