Magnetic fabric characteristics of bioturbated wave-produced grain orientation in the Bridport-Yeovil Sands (Lower Jurassic) of Southern England



There is little visible primary hydrodynamic lamination preserved in the Bridport-Yeovil Sands as a result of intense bioturbation. Where lamination is present, it exhibits wave-produced characteristics, although current ripple lamination is also found. The grain orientation of a variety of bioturbated and non-bioturbated fine-grained sandstones has been determined by measuring the magnetic susceptibility anisotropy. The magnetic fabric is of a primary style and preserves two lineation directions approximately 90° apart in azimuth. These lineation directions are interpreted as the result of grain long-axis orientations produced by wave and current processes. The magnetic fabric is dominantly carried by a small proportion of paramagnetic minerals, thought to be largely detrital chlorite and micas. This magnetic fabric has been acquired by depositional alignment of the detrital phyllosilicates and by reorientation of the phyllosilicates during the early stages of compaction. The magnetic fabric of the intensely bioturbated sandstone is not significantly different in magnitude characteristics or in the preservation of lineation directions from that of the non-bioturbated sandstone.