ENVIRONMENTAL MAGNETISM AS A STREAM SEDIMENT TRACER - AN INTERPRETATION OF THE METHODOLOGY AND SOME CASE-STUDIES

A comprehensive sediment sampling program was undertaken in the upper Molonglo catchment in south-eastern New South Wales to determine if mi neral magnetics could be used to estimate sidestream contribution at r iver confluences in this environment. Some 12 confluences were examine d over 1400 km(2) in 2 major basins and over 2 contrasting geological types. Sediment samples were divided into 7 size classes and the follo wing magnetic properties measured: magnetic susceptibility at 2 freque ncies, isothermal remanent magnetisation at 3 flux densities, and anhy steristic remanent magnetisation. The sidestream inputs were calculate d for each particle size class from the range of magnetic parameters. Significant discrepancies and differences appeared in the resultant si destream inputs, and this paper outlines the conclusions as to the rel iability of the different analytical procedures. It is shown that both the concentration and magnetic grain size of ferrimagnetic minerals i n the sediments must be taken into account. Where the difference in ma gnetic grain size between the upstream and sidestream sediments is sma ll, the use of parameter crossplots or bulked magnetic ratios is gener ally not appropriate. The use of mass (concentration) magnetic values may be better. The difference in the demands of the crossplots and mas s values methods is that crossplots require a wide range of mass magne tic concentrations in each branch, with the upstream and sidestream se diments having different magnetic grain sizes, whereas the mass values procedure does best with a very limited (but different) range of conc entrations at the upstream and sidestream branches, but similar magnet ic grain sizes. This paper provides an extensive discussion of the est imation technique using different parameter combinations, and uses 3 c ontrasting confluences as case studies.