THE SIGNIFICANCE OF VARIOUS SEDIMENT MAGNETIC MINERAL FRACTIONS FOR TRACING SEDIMENT SOURCES IN KILLIMICAT CREEK

The mineral magnetic properties of sediment are increasingly being use d to determine the sources of sediment, and associated nutrients and c ontaminants in drainage basins. This study was undertaken to measure t he relative contributions of the magnetic mineral components in sedime nt (i.e., components associated with surface bound Fe, the heavy miner al fraction, and as inclusions in particles) to determine the extent t o which they represent the bulk of the sediment. Deposited channel sed iment samples from the tributaries and downstream reach at the first m ajor confluence in the headwaters of Killimicat Creek, New South Wales , were sieved to separate 6 particle size fractions, and the mineral m agnetic properties measured to determine the relative contribution of sediment from the smaller tributary basin. The finest sand component ( 63-125 mu m) was then separated into Light and heavy mineral fractions , and magnetic measurements made on each fraction. Magnetic parameter data from the 63-125 mu m fraction, measured before density separation , show a dominance of sediment (70 +/- 12%) derived from the smaller t ributary basin. Measurements of the light sediment fraction give a sim ilar result (73 +/- 13%). The proportion of the heavy mineral fraction delivered from the smaller tributary is 78 +/- 38%. The Light mineral fraction of all of the samples were treated with HCl to remove surfac e bound Fe, and subsequent magnetic measurements show that 41-94% of t he mass specific magnetic susceptibility is attributable to magnetic m inerals associated with surface bound Fe. The heavy mineral component, while <4% by weight of the 63-125 mu m fraction, contributes 5-40% of the total magnetic susceptibility. The results show that most of the magnetic mineral component (>60%) is associated with sediment particle s, rather than the discrete heavy mineral component. (C) 1998 Elsevier Science B.V. All rights reserved.