The Callovo-Oxfordian argillites from the eastern Paris Basin: Magnetic data and petrofabrics
Résumé
A set of regularly spaced specimens, made up of dark-coloured calcareous argillites, were collected from borehole-cores from the Early Callovian up to the Middle Oxfordian, in the eastern Paris Basin. These specimens have been studied for their magnetic susceptibility, natural and artificial remanence, and for their anisotropy of magnetic susceptibility. Compared with sulfate, carbonate and iron contents, these magnetic properties provide information on both the nature and the relative amounts of the constitutive minerals. To a first approximation, these data outline the main end-members of these argillites, namely (i) carbonate-enriched and less susceptible, (ii) clay-enriched and more susceptible, and (iii) more remanent and partly due to the silt fraction. The middle of the Callovo-Oxfordian formation, called the maximum clay zone, is marked by the highest recorded susceptibility, mainly due to the paramagnetic iron-bearing clay minerals, and by a rather low remanence. The anisotropy of susceptibility provides information on the petrofabric of mostly the clay fraction. The fabric is highly planar, in agreement with the subhorizontal preferred orientation of the clay platelets, but the always present linear anisotropy of the argillites, although modest in strength, points to similar to north-south trending directions throughout most of the formation. These directions agree with the detrital fractions coming from the northern Rhineland, Bohemian and London-Brabant massifs.