Cédric Mabille

Sedimentology and magnetic susceptibility of the Upper Eifelian-Lower Givetian (Middle Devonian) in SW Belgium: insights into carbonate platform initiation

Abstract The major part of the Hanonet Formation is deposited on a mixed siliciclastic—carbonate detrital ramp, whereas the top is dominated by carbonate-rimmed shelf-related sedimentation. The transition corresponds roughly to the Eifelian—Givetian boundary. This work is based on two stratigraphic sections located in the southern part of the Dinant Synclinorium. Petrographic study leads to the definition of 11 microfacies, which demonstrate important sedimentological differences existing between the sections. A curve showing microfacies evolution is interpreted in terms of changing bathymetry. An environmental model depicts the lateral transition from a multiclinal carbonate ramp (to the east) to a fore-reef setting (to the west). Magnetic susceptibility was used to establish accurate stratigraphic correlations between the two sections. It also leads to an appreciation of the relative importance of eustatic sea-level change and local sedimentation rate. The combined interpretation of the microfacies curves and the magnetic susceptibility provides a new view of the sedimentary dynamics of the studied sections and, in a more general way, a better understanding of the processes responsible for magnetic susceptibility variations in carbonate rocks.

Magnetic susceptibility and its relationship with paleoenvironments, diagenesis and remagnetization: examples from the Devonian carbonates of Belgium

To better understand the origin of the initial magnetic susceptibility (χin) signal in carbonate sequences, a rock magnetic investigation that includes analysis of acquisition curves of the isothermal remanent magnetization (IRM) and hysteresis parameters, was undertaken on Devonian carbonates from the Villers and Tailfer sections, Belgium. Both sections are divided into a lower unit, dominated by biostromal and external ramp facies (biostromal unit) and an upper unit, only consisting of lagoonal facies (lagoonal unit). The variations in χin signal are mainly driven by magnetite variation, mostly pseudo-single-domain (PSD) magnetite. Clay minerals, pyrite, hematite and obviously calcite and dolomite are also present but their contribution to the χin pattern is not significant. There is a correlation between detrital proxies (Zr, Rb, Al2O3, TiO2) and χin for the Tailfer biostromal unit and the entire Villers section. The pervasive presence of fine-grained magnetite is interpreted as related to remagnetization. In absence of external fluids, the iron released during the smectite to illite transition remains in situ. In those situations χin may reflect an inherited primary synsedimentary signal. In the lagoonal unit of the Tailfer section, remagnetization appears to have obscured the original detrital information prompting the need for an evaluation of the composition of the susceptibility signal for individual case studies.