Abstract

The magnetic fabric of mudrock samples from the Westbury Member of the Penarth Group (Rhaetian), the Blue Lias Formation (Hettangian to Sinemurian) and the Oxford Clay (Callovian to Oxfordian) has been measured, using a high field torque magnetometer. The magnetic susceptibility anisotropy of these mudrocks originates from paramagnetic phyllosilicate minerals, of which the most important is considered to be detrital chlorite. The magnetic fabric preserves both a lineation and a foliation, which are of a primary style. This magnetic fabric is sensitive to the effects of recent weathering, due to the ease with which chlorite is oxidized during weathering. A model is proposed utilizing compaction, to explain the acquisition of this magnetic fabric. This model infers that the magnetic fabric represents, in both degree and direction, a measure of the preferred crystallographic orientation of the paramagnetic phyllosilicate minerals. The magnetic fabric data suggest that the Kmax direction corresponds with the quartz grain long-axis preferred orientation, and therefore may be used as a palaeocurrent indicator as in titanomagnetite-bearing sediments. Evidence is presented to suggest that paramagnetic susceptibility anisotropy may be common in other mudrocks.

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