Sedimentological implications of magnetic fabric measurements of some Jurassic and late Triassic sediments

The anisotropy of magnetic susceptibility (magnetic fabric) has been used as an aid to investigate the sedimentary environment and palae oceanography of the Penarth Group, the Blue Lias Formation, the Bridport-Yeovil Sands and the Oxford Clay. Investigation of the magnetic mineralogy of these formations shows the induced magnetic properties to be largely the result of paramagnetic minerals, the most important of which is chlorite. This mineral gives rise to a primary style depositional magnetic fabric. In the Bridport-Yeovil Sands ferromagnetic ferrianilmenite also contributes to the magnetic fabric. In several cases a post-depositional magnetic fabric (not of a primary style) has been produced as a result of the diagnostic nucleation of siderite. The magnetic fabric due to chlorite is sensitive to the effects of weathering, due to the ease with which chloride is altered to other minerals. A model is proposed using competition as the principal element to explain the acquisition of the fabric due to chlorite. This model suggests that the lineation records any preferred quartz grain long axis orientation within the foliation plane. Despite intense bioturbation in many of the lithologies examined, a magnetic fabric has been preserved which appears to be no different to that in non-bioturbated lithologies. In the Bridport-Yeovil Sands mean lineation directions are both parallel and perpendicular to the mean cross-lamination dip direction. This grain orientation has been controlled by both wave and current processes. In the Westbury Member of the Penarth Group and the lineation directions are considered dominantly the result of wave-produced grain orientation. In the Blue Lias lineation directions differ between mudshale and mudstone lithologies, reflecting the different sedimentary environments. Lineation directions from the Oxford Clay show complex between-section (same subzone), and between-site (same section) variability. Instrumental errors can radically affect the sample susceptibility anisotropy, particularly if the samples are weakly anisotropic. A technique has been developed to quantify these errors and reject those samples whose anisotropy is significantly affected by such errors.