The Givetian- Frasnian boundary at Nismes parastratotype (Belgium): the magnetic susceptibility signal controlled by ferromagnetic minerals

A comparison between microfacies and magnetic susceptibility (MS) curves has been performed at the Givetian-Frasnian transition in southern Belgium. The MS curve is subdivided in four magnetic sequences. Most of the section has MS and MF curves anti-correlated. Hysteresis parameters reveal a low coercivity mineral of magnetite type phase and a high coercivity mineral (i.e. hematite) in all samples. Strong correlation between ferromagnetic and low-field magnetic susceptibilities suggests that ferromagnetic minerals control almost totally the MS signal with an increasing upsection abundance of paramagnetic grains (iron-bearing clay minerals and pyrite). A decreased proportion of superparamagnetic grains, probably of diagenetic origin, is observed upsection. The hematite grains increases in the Frasnian as attested by the higher hematite contribution to the IRM500mT. The anti-correlation between the magnetic viscosity coefficient and the hematite contribution to the IRM500mT suggest that hematite are not associated to the superparamagnetic fraction and has a detrital origin. The opposite evolutions could be partly explained by sedimentological parameters and by an increased amount of primary detrital magnetite and hematite upsection. This detrital influx highlights the drowning of the carbonate platform at the end of the Givetian illustrating the progressive change from a Givetian rimmed platform towards a Frasnian ramp setting.SCOPUS: ar.j