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

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.