The Chazy Group, St. Lawrence Lowlands: anisotropy of magnetic susceptibility

Abstract

Magnetic susceptibility (K) and anisotropy of magnetic susceptibility (AMS) were measured on 69 specimens from five sites in the Chazy Group of the St. Lawrence Lowlands in southern Quebec. The K values depict an isotropic to slightly anisotropic character. The AMS ellipsoid shapes range from oblate to prolate. The axes of minimum susceptibility (Kmin) are mainly perpendicular to the subhorizontal bedding, whereas the other two axes (Kmax) and (Kint) are subparallel to it and somewhat scattered. These observations suggest that the Chazy seafloor was a relatively stable sedimentary platform, with a rather flat bottom, and was a high-energy depositional environment. A cluster analysis indicates a predominant orientation of the Kmax axes in the north-northwest – south-southeast direction. Microscopic studies have shown that detrital magnetic minerals in some specimens align in that direction as well. It is thus inferred that this direction reflects the flow direction of paleocurrents. K was measured also at low temperature (in liquid nitrogen). The results show that the paramagnetic contribution to K is relatively well detected at very low temperature, but it is hampered by diamagnetic contributions to K at room temperature. The specimens contain very few primary ferromagnetic mineral carriers, and therefore the sedimentary rocks of the Chazy Group are not appropriate for standard paleomagnetic investigation.