Sedimentology and magnetic susceptibility of Mississippian (Tournaisian) carbonate sections in Belgium

摘要

Magnetic susceptibility (MS) and biostratigraphy have been used to correlate better the reference sections of the Belgian Tournaisian, the Rivage road and railway sections and the Gendron-Celles railway section. These 200 m thick time-equivalent sections are about sixty kilometres apart and belong to two different sedimentation areas: a shallow ramp setting for Rivage (Condroz Sedimentation Area) and a subsiding area for Gendron (Dinant Sedimentation Area). The sedimentological model shows that both sections are characterized by a carbonate-dominated sedimentation (crinoids- peloids-algae assemblages), interrupted by more argillaceous facies related to rapid sea-level rises (crinoids- brachiopods-bryozoans assemblages). Accommodation space was significantly higher in the DSA and allowed the development of Waulsortian buildups during the Ivorian. Variations of magnetic susceptibility (MS) seem to be related to fluctuations in detrital input and carbonate productivity. MS evolution with palaeogeography can be integrated in the previously published model for the Devonian ramp system: external ramp settings have low carbonate productivity, low water agitation and high MS, whereas more proximal environments are characterized by higher carbonate productivity, higher water agitation and lower MS. MS curves are in general agreements with the 3rd-order sequence interpretation. Lowstand system tracts (LST) show the highest MS values while transgressive system tracts (TST) are characterized by decreasing values and highstand system tracts/falling stage system tracts (HST/FSST) by the lowest values.