Magnetic susceptibility contrasts in Ordovician greywackes of the Southern Uplands of Scotland

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Abstract
Ordovician greywacke formations of the Southern Uplands are demonstrated to have consistent differences in magnetic susceptibility, which parallel differences in petrography. The observed susceptibilities range from a minimum of 0.16 X 10 −3 SI for the quartz-rich, feldspar-poor Glenwhargen Formation to a maximum of 13.81 X 10 −3 SI for the quartz-poor, feldspar-rich March-bum Formation, and are considered to be directly related to variation in the amount of detrital magnetite in the greywackes. Rapid measurement of magnetic susceptibility is demonstrated to be a valuable field technique for distinguishing otherwise uniform sedimentary sequences on the basis of differing magnetite content. Detailed field observations illustrate considerable variation in suscep-tibility within metre-scale sections perpendicular to bedding through graded sedimentary units, which may be caused by density and/or grain-size effects. Laboratory measurements of the intensity of natural remanent magnetisation in the petrographically contrasting Kirkcolm and Galdenoch forma-tions show a similar variation to that observed in susceptibility. Thermally distributed, scattered magnetization directions indicate that no primary remanence has survived the low-grade burial metamorphic event in the Southern Uplands.