Evaluation of sediment provenance using magnetic mineral inclusions in clastic silicates: comparison with heavy mineral analysis

摘要

Magnetic Fe-oxide inclusions within framework clastic grains in sediments provide an indication of the provenance of the enclosing host silicate particles. Magnetic mineral inclusion characterisation is performed using a variety of magnetic properties that are related to magnetic mineral abundance, magnetic grain size (domain state), oxidation state and magnetic grain interaction. The magnetic methodology extends that based on a conventional set of environmental magnetic measurements. Using a variety of recent sediments, transported from known rock sources (within the UK and Eire), it is demonstrated, using discriminant function analysis, that the magnetic properties of the Fe-oxide inclusions provide a clear distinction of primary provenance. This is both at the large scale in terms of various gneiss, schist and granite sources and also at a smaller scale when considering subdivisions of these sources. Using case studies from the marine Upper Jurassic (Piper Formation) and non-marine Triassic sediments (Otter Bank Sandstone and Foula Sandstone Formations), it is shown that provenance differences, clearly expressed by the results of heavy mineral analysis, are also sensitively displayed by the magnetic mineral inclusion data. The provenance classification is based around use of hierarchical cluster analysis and multidimensional scaling. Simple statistical tools are developed to determine a least-noisy subset of magnetic parameters, which are most suitable for stratigraphic provenance discrimination. The Triassic case study has indicated the magnetic mineral inclusion technique is most sensitive when using the sediment fractions larger than 150 μm.