Anisotropy of magnetic susceptibility in a Palaeozoic flysch basin: the Windermere Supergroup, northern England

The Windermere Basin of northern England was initiated in Late Ordovician times and developed into a foreland basin by collision of the Laurentian and Eastern Avalonian Plates during Silurian times. It was largely filled by axial turbidite input in mid to Late Silurian times. Rock magnetic studies suggest that paramagnetic minerals, probably mostly chlorite, dominate the magnetic susceptibility. Studies of anisotropy of susceptibility (AMS) identify a bedding-related fabric which has survived later Acadian deformation in all but the oldest (U. Ordovician) units. The magnitude of the fabric becomes amplified in the higher more-argillaceous parts of individual beds but the direction of maximum susceptibility ( k 1 ) remains aligned with palaeoflow direction throughout the massive and parallel-laminated divisions. AMS is best defined in those units with well developed planar and cross bedding. The mechanism responsible for rippling the upper parts of the beds involves rolling of detrital grains along the bed during traction and alignment of the long axis (the grain's rotation axis) normal to the current; consequently the intermediate axis of AMS ( k 2 ) is in the direction of the secondary current responsible for rippling. Some beds which are rippled throughout show the same feature. Hemipelagic facies comprising laminated silty mudstones have highly uniform fabrics to which detrital hematite is an important contributor. The k 1 directions in the bedding planes show cyclic swings in declination which may be related to some long-period (perhaps orbitally-forced) secular oceanographic variation. Since these sediments were deposited in optimum conditions for preserving grain alignment with the geomagnetic field, AMS and AMR (Anisotropy of Magnetic Remanence) study of such facies may provide methods for recovering secular variation in the remote geological past and for estimating rates of deposition.