Diagenesis and remanence acquisition in the Lower Pliocene Trubi marls at Punta di Maiata (southern Sicily): palaeomagnetic and rock magnetic observations

Abstract

Three new profiles (LCM, PMD and CMD) through the lower Cochiti polarity zone enable comparison of the effects of weathering and diagenesis on the magnetic properties of the Early Pliocene Trubi marls These marls have been well dated, by means of astronomical calibration, and are of particular interest in terms of polarity zonation and regional tectonics The freshest exposures (LCM profile) carry a remanence associated with pyrrhotite, which is particularly enhanced in some grey layers This is the first time that occurence of pyrrhotite as the mam remanence carrier in the Trubi marls is documented (SD magnetite was previously recognised as the fundamental ferromagnetic phase) The mean ChRM direction from the LCM record does not show the expected 30-35° clockwise rotation found in other studies, and does not record the normal Cochiti interval either (i. e. is reverse throughout the sampled interval), suggesting a relatively late origin for the NRM Standard rock magnetic techniques are used to investigate the nature of the magnetization in the three laterally equivalent profiles at Punta di Maiata The actual data set seems to be better explained by a complex diagenetic history involving a multistage redox history that invokes a late' anoxia event The previously postulated iron migration model (van Hoof et al. 1993) was put forward for the 'suboxic situation' and does not take into account the presence of magnetic sulfides, which are clearly demonstrated in the zone under study The 'extended diagenetic model' inferred here proposes a pathway that accounts for most of the new observations The salient feature is the postulated existence of a diagenetic phase that has altered the original NRM in some areas (dissolution of magnetite) resulting in a later stage acquisition of chemical remanence that does not date to the timing of deposition of the sediments The extension and the detailed mechanisms by which such a process has taken part deserve further research Nonetheless, the present investigation accounts for previously unrecognised features in the Trubi marls from Sicily and outlines how intricate the nature of the mechanisms contributing to the blocking of remanence in sediments can be