SEDIMENTOLOGY, MAGNETIC SUSCEPTIBILITY AND CORRELATIONS OF MIDDLE FRASNIAN PLATFORM LIMESTONE (TAILFER AND AYWAILLE SECTIONS, BELGIUM)
Unité de Recherche de Pétrologie Sedimentaire, B20, Université de Liège, SartTilman B-4000 Liège .
Unité de Recherche de Pétrologie Sedimentaire, B20, Université de Liège, SartTilman B-4000 Liège .
Abstract
ABSTRACT. The Tailfer (northern border of the Dinant Synclinorium) and Aywaille sections (eastern border of the Dinant Synclinorium) expose Middle Frasnian shallow-water calcareous deposits corresponding to the Lustin Formation. A petrological, sequence stratigraphy and magnetic study of these sections has been carried out. The petrological analysis leads to the definition of three lithofacies and 14 microfacies, to the construction of a microfacies curve and to the identification of fourth order sequences grouped in sedimentological units and in systems tracts. The crinoidal lithofacies (A) was deposited in the storm wave zone. The biostromal lithofacies (B) is composed of an ideal vertical microfacies succession grading from the storm wave zone to the normal wave zone. Finally, the lagoonal lithofacies (C) is characterized by bedded and brecciated limestones and mudstone grading from the sub- to the supratidal zone. Both sections were divided into two main sedimentological units: the lower part, called biostromal unit is composed of biostromal and crinoidal lithofacies with episodic lagoonal interruptions. This biostromal unit is followed by a regressive surface and by the second unit, constituted by lagoonal lithofacies only. The magnetic susceptibility (MS) curve depends on the abundance of ferromagnetic minerals, which is related to lithogenic supplies and to the proximity of landmasses (Crick et ai, 1994). This proximity could be related to eustatism or to other parameters like tectonic or climatic variations. Magnetic susceptibility curves lead to fourth order correlation between the two sections and confirm the bathymetric reconstruction. The first unit shows low MS values related to the distal position of landmasses. After the regression, the susceptibility strongly increases due to a higher amount of lithogenic supplies, which is related to the relative proximity of the continent.